the performance enhancements of upflow anaerobic sludge blanket (uasb) reactors for domestic sludge...
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Review
The performance enhancements of upflow anaerobic sludgeblanket (UASB) reactors for domestic sludge treatment e AState-of-the-art review
Siewhui Chong a,*, Tushar Kanti Sen a, Ahmet Kayaalp b, Ha Ming Ang a
aDepartment of Chemical Engineering, Curtin University, GPO Box U1987, Perth 6845, AustraliabWater Corporation of Western Australia, West Leederville 6007, Australia
a r t i c l e i n f o
Article history:
Received 20 November 2011
Received in revised form
24 March 2012
Accepted 31 March 2012
Available online 10 April 2012
Keywords:
Anaerobic
Post-treatment
UASB
Domestic sludge
Granulation
Review
* Corresponding author. Tel.: þ61 8 9266 9 20E-mail addresses: [email protected]
0043-1354/$ e see front matter Crown Copyrdoi:10.1016/j.watres.2012.03.066
a b s t r a c t
Nowadays, carbon emission and therefore carbon footprint of water utilities is an important
issue. In this respect, we should consider the opportunities to reduce carbon footprint for
small and large wastewater treatment plants. The use of anaerobic rather than aerobic
treatment processes would achieve this aim because no aeration is required and the gener-
ation of methane can be used within the plant. High-rate anaerobic digesters receive great
interests due to their high loading capacity and low sludge production. Among them, the
upflowanaerobic sludge blanket (UASB) reactorshave beenmostwidelyused.However, there
are still unresolved issues inhibiting the widespread of this technology in developing coun-
tries or countrieswith climate temperature fluctuations (such as subtropical regions). A large
number of studies have been carried out in order to enhance the performance of UASB
reactors but there is a lack of updated documentation. In face of the existing limitations and
the increasing importanceof this technology, theauthorspresent anup-to-date reviewon the
performance enhancements of UASB reactors over the last decade. The important aspects of
this article are: (i) enhancing the start-up and granulation in UASB reactors, (ii) coupling with
post-treatment unit to overcome the temperature constraint, and (iii) improving the removal
efficiencies of the organic matter, nutrients and pathogens in the final effluent. Finally the
authors have highlighted future research direction based on their critical analysis.
Crown Copyright ª 2012 Published by Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34352. The UASB technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3437
2.1. The UASB reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34372.2. The uniqueness of UASB reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34372.3. The challenges and performance enhancements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3437
2.3.1. Start-up and granulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34372.3.2. Performance enhancement by modifying reactor configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3439
2; fax: þ61 8 9266 2681..au, [email protected] (S. Chong).ight ª 2012 Published by Elsevier Ltd. All rights reserved.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3435
2.3.3. Post-treatment of UASB effluent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34423. Recent advances in wastewater treatment technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3452
3.1. Membrane bioreactors (MBRs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34523.2. Advanced oxidative processes (AOPs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34563.3. Microbial fuel/electrolysis cells (MFCs/MECs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3456
4. Summary and Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34575. Future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3459
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3460References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3460
Nomenclature
Al3þ aluminium ion
C carbon
Ca2þ calcium ion
CH4 methane
Fe2þ ferrous ion
H2O2 hydrogen peroxide
N nitrogen
NH4þ/NH4eN ammonium ion/ammonium nitrogen
O3 ozone
P phosphorus
PO3�4 phosphate ion
S2� sulphide ion
TiO2 titanium dioxide
Ti/PteIrO2 titanium/platinum-irridium oxide
AEF aerated filter
AF fixed filter
AH anaerobic hybrid
AOP advanced oxidative processes
AS activated sludge
ASP activated-sludge process
BF biofilter
BOD biochemical oxygen demand
CET chemically enhanced primary treatment
COD chemical oxygen demand
CODcol colloidal COD
CODss suspended COD
CSR cascade-sponge reactor
CSTR continuous-stirred-tank reactor
CW constructed wetland
DAF dissolved-air flotation
DOC dissolved organic carbon
DHS downflow hanging sponge
EC E. coli
EGSB expanded granular sludge blanket
ESR enhanced sludge retention
FC faecal coliforms
FWS free water surface
GLS gaseliquidesolid
HPB hydrogen-producing biofermentor
HRT hydraulic retention time
HUSB hydrolytic upflow sludge blanket
inhab inhabitant
IC inorganic carbon
MBR membrane reactor
MEC microbial electrolysis cell
MFC microbial fuel cell
org/100 mL organisms/100 mL
OLR organic loading rate
PAVB passively aerated vertical beds
PE polyethylene
PUR polyurethane
PVA polyvinyl-alcohol
RBC rotating-biological contactor
RPF reticulated polyurethane foam
SBR sequential-batch reactor
SF surface flow
SMA specific methanogenic activity
SP stabilising pond
SRT solids retention time
SS suspended solids
SSF subsurface flow
T temperature
TC total coliforms
TF trickling filter
TKN total Kjeldahl nitrogen
TN total nitrogen
TOC total organic carbon
TP total phosphorus
TSS total suspended solids
UAFB upflow anaerobic fixed bed
UASB upflow anaerobic sludge blanket
UV ultraviolet
VS volatile solids
WEMOS water extract of the Moringa Oleifera seeds
ZVI zero-valent iron
1. Introduction depletion of fossil fuel and the increase in greenhouse gas
Intensive deforestation for industrialization and urbanization
is a substantial source of global warming and depletion of
resources such as fossil fuel, fresh water and topsoil. The
emission have resulted in an urgent demand for clean energy.
Although fossil-fuel fired power stations serve as the major
source of electricity for Australia, in the year of 2010, the
uptake of clean energy reached 8.67% of the total electricity
Table 2 e Advantages and disadvantages of anaerobicsewage treatment processes over aerobic processes, withspecial emphasis on high-rate digesters (Reprinted fromSeghezzo et al. (1998). Copyright (1998), with permissionfrom Elsevier).
Advantages High efficiency. Good removal efficiency can
be achieved in the system, even at high loading
rates and low temperatures.
Simplicity. The construction and operation of these
reactors is relatively simple.
Flexibility. Anaerobic treatment can easily be
applied on either a very large or a very small scale.
Low space requirements. When high loading
rates are accommodated, the area needed for
the reactor is small.
Low energy consumption. As far as no heating of the
influent is needed to reach the working
temperature and all plant operations can be
done by gravity, the energy consumption of the
reactor is almost negligible. Moreover, energy is
produced during the process in the
form of methane.
Low sludge production. The sludge production is
low, when compared to aerobic methods, due
to the slow growth rate of anaerobic bacteria.
The sludge is well stabilized for final disposal
and has good dewatering characteristics. It can
be preserved for long periods of time without a
significant reduction of activity, allowing its use
as inoculum for the start-up of new reactors.
Low nutrients and chemicals requirement. Especially
in the case of sewage, an adequate and stable
pH can be maintained without the addition
of chemicals. Macronutrients (nitrogen and
phosphorus) and micronutrients are also
available in sewage, while toxic compounds
are absent.
Disadvantages Low pathogen and nutrient removal. Pathogens
are only partially removed, except helminth
eggs, which are effectively captured in the
sludge bed. Nutrients removal is not complete
and therefore a post-treatment is required.
Long start-up. Due to the low growth rate of
methanogenic organisms, the start-up takes
longer as compared to aerobic processes,
when no good inoculum is available.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03436
generated, in which the bioenergy accounted for 11.5%
(Clean Energy Council, 2010). As shown in the statistical data
(Table 1) from the Clean Energy Council Australia, among the
renewable energy sectors installed, wind energy occupied the
most, and hydro energy had the largest number of projects
that became operational in 2010. It is worth pointing out that
among the renewable resources, the installed capacity of the
sewage-gas energy (energy generated from the anaerobic
digestion of sewage) occupied about 1% in 2010, and was
ahead of energy generated from food and agricultural waste,
landfill and large-scale solar photovoltaic, indicating its
increased potential as a feasible renewable energy.
Compared to other sludge treatment methods, such as
incineration, land application, composting and aerobic treat-
ment, anaerobic digestion, despite its potential of producing
a useful fuel, is more beneficial due to its small land require-
ment and low excess sludge production. The overall advan-
tages and disadvantages of anaerobic digesters have been
presented in Table 2. Till date, a large number of anaerobic
digesters have been built and studied. Among the anaerobic
digesters, high-rate digesters are popularly used in sewage
treatment. This is because, unlike the conventional low-rate
anaerobic digesters such as anaerobic ponds and septic
tanks, high-rate anaerobic reactors are designed to operate at
short hydraulic retention times (HRT) and long solids reten-
tion times (SRT) to incorporate large amounts of high-activity
biomass, thus allowing improved sludge stabilisation and
higher loading capacity (von Sperling and Chernicharo, 2005).
The common high-rate digesters are: upflow anaerobic sludge
blanket, expanded granular sludge bed, anaerobic filter,
anaerobic baffled, anaerobic migrating blanket, sequencing
batch, anaerobic hybrid, hybrid upflow anaerobic sludge
blanket, as well as fully mixed liquid digesters such as
continuous-stirred tank digesters. The upflow anaerobic
sludge blanket (UASB) reactors are by far the most robust
high-rate anaerobic reactors for sewage treatment and there
have been more than 1000 UASB reactors installed worldwide
(Tiwari et al., 2006). Nevertheless, as listed in Table 2, there are
still unresolved issues in the anaerobic treatment technology.
One of the major drawbacks of anaerobic digesters has been
the requirement of long solids retention time which is not
Possible bad odours. Hydrogen sulphide isproduced during the anaerobic process,
especially when there are high concentrations
of sulphate in the influent. A proper handling
of the biogas is required to avoid bad smell.
Necessity of post-treatment. Post-treatment of
the anaerobic effluent is generally required
to reach the discharge standards for organic
matter, nutrients and pathogens.
Table 1 e Total capacity of new renewable energyprojects in 2010 inAustralia, by technology (only includesprojects larger than 100 kW) (Clean Energy Council, 2010).
Fuel source Installed capacity(MW)
Number ofprojects
Wind 167 3
Hydro 36 7
Sewage gas 2.2 2
Food and agriculture
wet waste
2.0 2.2
Landfill gas 1.8 2
Large-scale solar
photovoltaic
0.6 1
Total: 210 17
associated with the increasing volume of sludge produced
from industrialization and human activities. Other drawbacks
have been the long start-up period, impure biogas and
incomplete or insufficient removal of organic matters, path-
ogens and nutrients in the final effluent, thereby failing to
comply with the local standards for discharge or reuse.
Therefore, researchers are trying to find out new technologies
to enhance the performance of anaerobic digesters, especially
on the effluent quality, start up and biogas purification, in
order to develop a global sustainable wastewater treatment
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3437
technology. Recent reviews mostly focused on anaerobic
digestion (Aiyuk et al., 2006; Gomec, 2010), granulation (Tiwari
et al., 2006) and post-treatment technology (Kassab et al., 2010;
Chan et al., 2009; Khan et al., 2011). There is still a lack of
reviews and documentation on the enhancements of high-
rate anaerobic digestion with collective information. There-
fore, the goal of this review article is to compile the various
advances made in addressing the performance enhancement
technologies on high-rate anaerobic digesters, with emphasis
on the UASB reactors, to serve as a database for the prelimi-
nary selection of the anaerobic digesters (type and/or
method), as well as to provide perspectives for future
improvement on domestic sludge treatment processes.
2. The UASB technology
2.1. The UASB reactor
The schematic diagram of a UASB reactor is shown in Fig. 1.
The reactor consists of two parts: a cylindrical or rectangular
column and a gaseliquidesolid (GLS) separator (Lettinga and
Hulshoff Pol, 1991). The UASB reactor is initially seeded
(with inoculum such as digested, anaerobic, granular, floccu-
lent and activated sludge). Sludge enters from the bottom of
the reactor. Under appropriate conditions, light and dispersed
particles will be washed out while heavier components will
retain, thus minimizing the growth of finely dispersed sludge
whilst forming granules or flocs consisting of the inert
organic, inorganic matters and small bacterial aggregates in
the seed sludge (Hulshoff Pol et al., 2004). After a certain period
(usually 2e8 months), depending on the operating conditions
and the characteristics of the wastewater and seed sludge,
a very dense sludge bed which may be granular or flocculent
Fig. 1 e Schematic of a UASB reactor.
in nature with high settling properties develops. Above the
dense sludge bed, there is a sludge blanket zone with a much
diffused growth and lower particle setting velocities (Aiyuk
et al., 2006). The biological reactions take place throughout
the highly active sludge bed and blanket zone. As the flow
passes upward, the soluble organic compounds in the influent
are converted to biogas consisting of mainly methane and
carbon dioxide. The produced biogas and the sludge buoyed
by the entrapped gas bubbles are then separated from the
effluent by the immersed GLS separator, in which the baffles
prevent as efficiently as possible the wash-out of the viable
bacterial matter or floating granular sludge by sliding the
settled solids back to the reaction zone (Lettinga and Hulshoff
Pol, 1991; Hickey et al., 1991).
2.2. The uniqueness of UASB reactors
The uniqueness and the challenges of UASB reactors are
outlined in Table 3. In addition to the listed advantages shown
in Table 1, the main feature of a UASB reactor which makes it
the popular high-rate anaerobic digester worldwide (espe-
cially in tropical countries) is the availability of granular or
flocculent sludge, allowing it to achieve high chemical oxygen
demand (COD) removal efficiencies without the need of
a support material. Furthermore, the natural turbulence
caused by the rising gas bubbles which buoy the sludge,
provides efficient wastewater and biomass contact. Therefore
mechanical mixing is not required, thus significantly reducing
the energy demand and its associated cost. Most importantly,
due to the granulation/blanketing in a UASB reactor, the solids
and hydraulic retention times can be manipulated indepen-
dently and effectively, thus permitting the design to be based
upon the degradative capacity of the biomass, resulting in the
reduction of treatment times from days (typical for conven-
tional digesters) to hours (Hickey et al., 1991).
2.3. The challenges and performance enhancements
Due to the inherent limitations of anaerobic treatment and
the UASB technologies, there is a need to focus on the
improvement of these drawbacks, thus challenging the
designers and engineers. This section confers the common
shortcomings of the UASB reactors in the aspect of start-up/
granulation, temperature limitation and effluent quality, by
deliberating the researchwork carried out so far on enhancing
the respective deficiencies.
2.3.1. Start-up and granulationThe effectiveness and stability of a UASB reactor depends
strongly on the initial start-up, which in turn is mainly
affected by numerous physical, chemical and biological
parameters (Ghangrekar et al., 1996), such as the type of
wastewaters, the operating conditions and the characteristics,
availability and growth of active microbial populations in the
seed sludge or inoculum. An acclimatization period is
required before the full design organic loading rates can be
applied to inoculate the seed sludge to the operating condi-
tions. This period which is typically 2e8 months (Vlyssides
et al., 2008) is rather long and has been the major hitch of
the industrial applications of UASB reactors. Typical granular
Table 3 e The uniqueness and challenges of UASB reactors.
Uniqueness Challenges
Availability of granular or flocculent sludge,
thus no requirement of a support mediuma,b
High biomass content, enabling a wide range of
loading rates and high COD removal efficiencya,c
Blanketing of sludge, enabling short hydraulic
retention time and high solids retention time
Rising gas bubbles produced, eliminating the need
of mixing and thus lower energy demand
Long experience in practicea
Start-up is susceptible to temperature and organic shock loadsa
Difficulties in controlling the bed expansions, thus
limiting the applicable organic loading ratesa,d
Wash-out, flotation and disintegration of granular sludgee
Performance deteriorates at low temperaturesf
High sulphate concentrationg
Necessity of post-treatment to reach the discharge
standards for organic matter, nutrients (eg NHþ4 , PO
3�4 , S2) and pathogensh
Purification of biogas
a Weiland and Rozzi (1991).
b Zoutberg and de Been (1997).
c Hickey et al. (1991).
d Lettinga and Hulshoff Pol (1991).
e Li et al. (2008).
f Lew et al. (2011).
g Heffernan et al. (2011).
h Mahmoud (2008).
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03438
inoculums are granules from a full-scale UASB reactor treat-
ing brewery wastewater; and non-granular inoculums are:
anaerobic digested sludge andwaste activated sludge. In order
to select the best inoculum source for a specific type of
wastewater, the toxicity and biodegradability tests as reported
in the literature can be used (Ghangrekar et al., 1996; Sarria
et al., 2003). Although a UASB reactor can perform efficiently
without granules, granule formation during start-up lends
a decided advantage for its ability to provide high COD
removal efficiency within a much shorter period, allowing
treatment for larger volumes of wastewater. The importance
of granules in the operation of UASB reactors has led to an
increasing number of studies on the relevant theories and
mechanisms of anaerobic granulation (Hulshoff Pol et al.,
2004; Schmidt and Ahring, 1996; Liu et al., 2002). According
to Schmidt and Ahring (1996), the initial development of
granules can be divided into four steps: (1) Transport of cells to
a substratum (i.e. an un-colonized inert material or other
cells); (2) Initial reversible adsorption to the substratum by
physicochemical forces; (3) Irreversible adhesion of cells to the
substratum by microbial appendages and/or polymers; (4)
Multiplication of the cells and development of granules.
Previous studies have suggested that the presence of
divalent and trivalent cations exert positive impact on gran-
ulation process by neutralizing negative charges on bacterial
surfaces, serving as cationic bridges between bacteria (Liu
et al., 2002). Investigators (Yu et al., 2000, 2001a, 2001b)
successively studied the roles of a few specific multivalent
cations on granulation using the same UASB reactor of 7.3 L in
volume for treating synthetic wastewater with influent COD
concentration of 4000mg/L. They found that with an optimum
concentration of iron (Fe2þ) at 300 and 450 mg/L, the granu-
lation process was accelerated; higher Fe2þ concentration in
the feed (more than 450mg/L) resulted in excessive deposition
of minerals on the granules, therefore deteriorating the
bacterial specific activity (Yu et al., 2000). As with the addition
of calcium (Ca2þ), the optimum concentration of Ca2þ at
150e300 mg/L enhanced biomass accumulation and granula-
tion process by accelerating the adsorption, adhesion and
multiplication of the granulation process (Yu et al., 2001a). On
the other hand, the addition of aluminium (Al3þ) at 300 mg/L
resulted in the formation of large granules and shortened the
granulation time by approximately one month (Yu et al.,
2001b). However, Sondhi et al. (2010), who examined the
effect of adding aluminium chloride on the treatment of low-
strength (650e750 mg/L) synthetic wastewater, in contradic-
tion to the earlier study (Yu et al., 2001b), found that the
addition of Al3þ at 200e300 mg/L (either continuously
throughout the operation or during the start-up) adversely
affected COD removal efficiency and growth of agglomerates.
A smaller concentration of Al3þ at 50 mg/L on the other hand,
did not affect the COD removal efficiency but adversely
affected the growth of agglomerates (Sondhi et al., 2010).
These studies (Yu et al., 2001b; Sondhi et al., 2010) have criti-
cally suggested that the optimumconcentration of the studied
multivalent cations is dependent on the influent COD
concentration. Consequently more research studies are
needed to determine their relationships and roles in the
granulation process.
In addition to the use ofmultivalent cations, the addition of
natural polymers, such as water extract of theMoringa Oleifera
seeds (WEMOS) (Kalogo et al., 2001), Chitosan (Tiwari et al.,
2004, Tiwari et al. 2005), Reetha extract (Tiwari et al., 2004,
2005) and powdered bamboo-charcoal (Cao et al., 2010), as
well as commercial and synthetic polymers, such as the
commercial cationic polymer “AA 180H” (Wang et al., 2004;
Show et al., 2004) and organic-inorganic hybrid polymers
(Jeong et al., 2005), also showed promising results in
enhancing the start-up and granulation in UASB reactors. The
details of these additives are as follows:
� Water extract of the Moringa Oleifera seeds (WEMOS):
WEMOS is a pan-tropical, multipurpose tree from regions of
north-west India and indigenous to many parts of Asia,
Africa and South America, whose seeds contain a high
quality edible oil (up to 470% by weight) and water
soluble proteins that act as effective coagulants for water
and wastewater treatment. (Bhuptawat et al., 2007;
Ndabigengesere and Subba Narasiah, 1998; Vieira et al.,
2010). Due to its properties, WEMOS has been shown to
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3439
remove suspended solids effectively and thus acts as a good
pre-treatment unit for anaerobic digesters to obtain higher
biogas production (Folkard and Sutherland, 2002; Bhatia
et al., 2007). In addition, Kalogo et al. (2001) showed that,
by continuously supplying WEMOS to the UASB reactor, not
only was the granule formation enhanced in terms of
improved aggregation of coccoid-like bacteria and growth of
filamentous cells, but also the specific gas production from
the UASB reactor was increased by 1.6 fold.
� Chitosan and Reetha extract (bulk, cationic fraction, and
anionic fraction): Chitosan is a modified polysaccharide,
mostly produced by alkaline deacetylation of chitin.
Thaveesri et al. (1995) reported that in a UASB reactor,
granules start to form with the aggregation of acidogens,
and with methanogens enclosed inside, leading to the
formation of an elastic hydrophilic layer and a hydrophobic
inner core, thus decreasing the wash-out of methanogens.
Due to this nature, Chitosan has shown positive results in
enhancing sludge granulation and has exceeded Percol 763
(El-Mamouni et al., 1998), as well as both the cationic and
anionic fraction of Reetha extract in treating synthetic
wastewater (Tiwari et al., 2004, 2005). In the latter, large
granules were formed and their stabilities were shown to be
stable, indicating the low possibility of substrate-diffusion
limitations with low organic loading. Apart from the
formation of large granules (and thus longer SRT), the
addition of Chitosan also enhanced COD removal and biogas
production rate in treating wastewater from a tropical fruit-
processing industry (Lertsittichai et al., 2007), and palm oil
mill effluent (POME) (Khemkhao et al., 2011).
� Commercial cationic polymer “AA 180H”: Wang et al. (2004)
and Show et al. (2004) demonstrated the impact of
a commercial cationic polymer “AA 180H” at concentrations
5e20 mg/L on granulation and organics removal efficiency
in treating synthetic sludge with COD 5000 mg/L. With
dosage of 20 mg/L, the organic loading capacity of the UASB
reactor was increased from 19.2 to 25.6 g COD/L/d (Wang
et al., 2004); whilst with dosage of 80 mg/L, the start-up
period was shortened and the granules in UASB reactors
were strengthened and exhibited the best settleability at all
studied organic loading rates (OLRs) (2e40 g COD/L/d),
leading to increased organic removal efficiency and loading
capacity of the UASB system (Show et al., 2004).
� Organic-inorganic hybrid polymers: Jeong et al. (2005)
synthesized organic-inorganic hybrid polymers and added
to the UASB reactors. It was found that granular sludge was
formed within 5 min. The granules were stable throughout
the operation and the COD removal efficiencywas as high as
90% at even up to 18 g COD/L/d of OLR.
The studies mentioned above have shown the positive
impacts of polymer addition. Nevertheless, Bhunia and
Ghangrekar (2008) elucidated that the addition of polymer is
not necessary for thick inoculum to reduce compactness of
sludge bed. They showed that when using inoculum which
was thick in nature (SS > 110 g/L), at both high and low OLRs
(<1.0 and >1.7 kg COD/m3/d), the COD removal efficiency of
the UASB without polymer addition was better than that with
polymer addition. Alternatively, other enhancement methods
as follows provided efficient start-up and granulation:
� Inert material e polyvinyl-alcohol (PVA)-gel beads and
polyethylene (PE) cubes: As shown in Zhang et al. (2008),
when the granules in the UASB reactor treating high-
strength synthetic wastewater (using corn steep liquor as
the substrate) was seeded along with PVA-gel beads,
a higher biomass attachment (0.9 g/cm3 inert material) with
a density of 1.3 g/mL (most similar to that of the natural
granules of 1.03e1.08 g/mL) was obtained, compared to the
case when clay, foam, granular activated carbon or
powdered activated carbon was used. Unlike others, Meth-
anosarcina was found to be the dominant organism, replac-
ing the structural function of Methanosarta (Zhang et al.,
2008). Zhang et al. (2009) on the other hand, used a hybrid
UASB and anaerobic fixed filter reactor (UASB-AF) packed
with PE cubes to treat the wastewater from the extrusion
process of a fruit juice factory (with an influent COD of
8000e1000 mg/L). It was shown that application of both
internal hydraulic and external excess sludge recirculation
(recycling both the effluent from the top of the reactor and
the sediments collected at the effluent-sedimentation tank
to the bottom of the reactor) resulted in good contact
between the substrate and sludge, low loss of sludge and
increased granule size, thus yielding a high and stable COD
removal over the range of OLRs studied (1.4e15.4 kg COD/
m3/d). However when the recirculation ratio was increased
from 19 to 28, the COD removal dropped abruptly due to the
excessive mutual collision among sludge granules which
resulted in fracturing of the granular sludge (Zhang et al.,
2009).
� Zero-valent-iron (ZVI) bed: Recently, Zhang et al. (2011)
integrated a ZVI-bed consisting of stainless steel mesh and
scrap iron in the UASB reactors and compared its perfor-
mance after incorporation with the bi-circulation method
proposed in Zhang et al. (2009), as well as with a UASB
reactor alone. Among these reactors, the ZVI-UASB reactor
with bi-circulation provided the fastest granule growth and
the best COD removal efficiencies. Thereafter, Liu et al.
(2011) demonstrated that when a voltage of 1.4 V was
supplied to the ZVI bed, the COD removal efficiency
increased to over 90% and the granule size reached 696 mm
within 38 days, a large improvement when compared to
using the ZVI-UASB reactor without electric field and the
UASB reactor (the granule size was 189 mm on day 30 and
113 mm on day 80 respectively). These studies showed
that the ZVI bed is expected to create a favourable envi-
ronment for methanogenesis. An electric field in addition,
enhanced the buffer acidity, maintained low oxidation-
reduction potential in the reactor and further benefited
methanogenesis.
Table 4 provides an overall comparison in terms of the
positive, negative or neutral impacts of the above mentioned
additives applied during start-up and/or operation of UASB
reactors.
2.3.2. Performance enhancement by modifying reactorconfigurationsTemperature is a key parameter affecting the initial hydro-
lysis rate of sludge. At 35 �C, methanogenic bacteria (the core
component of UASB granules) are expected to be generated
Table 4 e Recent studies on the enhancement of start-up and granulation in UASB reactors.
Ref.a Enhancements(e.g. additives, reactor
modifications)
Digestersize (L)
T (�C) Substrate InfluentCOD (mg/L)
Dose(mg/L)
SMA (g CH4-COD/g VSS/d)b
Approx. meanagglomeratesize (mm)b
Start-up/granulationc
COD removalefficiencyc
Multivalent cations:
[1] Ferrous chloride
tetrahydrate (FeCl2.4H2O)
7.3 35 Synthetic 4,000 150 1.26 �1.8 0 0
300 1.14 þ 0
450 1.07 þ 0
600 0.81 e 0
800 0.74 e 0
[2] Calcium chloride
dehydrate (CaCl2.2H2O)
7.3 35 Synthetic 4,000 150e300 1.04e0.58 �2.3 þ 0
[3] Aluminium chloride (AlCl3) 7.3 35 Synthetic 4,000 300 1.10 �1.8 þ 0
[4] Aluminium chloride (AlCl3) 1.3 26 Synthetic 665e738 50 <0.1 e 0
200 e e
300 e e
Natural polymers:
[5] WEMOS (2.5%) 2.3 29 Domestic 320 2 ml/Ld 0.22 þ 0
[6] Reetha extract: Cationic 3.25 Synthetic 750e850 25e 0.114 þ 0
Reetha extract: Anionic 25e 0.111 þ 0
25e 0.107 0 0Reetha extract: Bulk
Chitosan 25e 0.129 þ 0
Commercial and synthetic polymers:
[7] Commercial cationic
AA 184 H
4.4 35 Synthetic 4,000 5 0.71e1.76 0.1e2.4 þ þ10 0.71e2.54 0.1e2.0 þ þ20 0.71e2.36 0.1e2.6 þþ þ
[8] Commercial cationic
AA 184 H
4.4 35 Synthetic 5,000 20 1.3e2.5 � 2.4 þ 0
40 � 2.2 þ 0
80 � 2.65 þþ þ160 � 2.65 þ 0
320 � 2.92 þ 0
[9] Synthetic granular
sludge, A and B
0.84 37.5 Synthetic >250 2e3.84 þ þ
[10] Commercial cationic polymer 12.57 22e31 Synthetic 300e630 0.083f 0.29e0.59 0.3e3.03 e e
Others:
[11] PVA-gel beads 12.5 35 Synthetic 768e10,910 þ þ[12] PE cubes, bi-circulation 5.9 35 Wastewater from
fruit juice factory
8,000e12,000 0.26 þ
[13] ZVI 5.9 35 Synthetic 1,000e8,000 0.1e0.25 þ þZVI-bicirculation 0.25e0.84 þþ þþ
[14] ZVI 18.5 35 Synthetic 1,400e8,000 0.1e0.21 þ þZVI-electric field 0.1e0.73 þþ þ
a References: [1] Yu et al. (2000); [2] Yu et al. (2001a); [3] Yu et al. (2001b); [4] Sondhi et al. (2010); [5] Kalago et al. (2001); [6] Tiwari et al. (2004, 2005); [7] Wang et al. (2004); [8] Show et al. (2004); [9] Jeong
et al. (2005); [10] Bhunia and Ghangrekar (2008); [11] Zhang et al. (2008); [12] Zhang et al. (2009); [13] Zhang et al. (2011); [14] Liu et al. (2011).
b At the end of the experiment.
c Compared to control: � Unfavourable, þ Favourable (when granule formation exceeding 30%, or in the case of no comparison, COD removal >90%), þþ Very favourable, 0 no effect/no direct
correlation.
d In 1% acetic acid solution.
e mg/g TSS.
f mg/g of inoculum SS.
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wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3441
within 3 days; whereas at a much lower temperature of 10 �C,the generation time can be as high as 50 days (Bhuptawat
et al., 2007). Higher temperatures are known to enhance
methanogenesis, but at the expense of lower effluent quality
due to sludge wash out (Liu and Tay, 2004). Therefore, justi-
fication and selection of the operating temperature is impor-
tant to achieve high treatment efficiencies. Generally,
anaerobic digestion can be categorized into psychrophilic
(�20 �C), mesophilic (30e40 �C), thermophilic (55e58 �C), andsometimes extreme thermophilic (70 �C) digestions. At low
temperatures (�20 �C), due to the relatively high accumulation
of suspended solids in the sludge bed which results in insuf-
ficient solids retention time, the performance of UASB reac-
tors was found to reduce (Lew et al., 2011, 2004; Halalsheh
et al., 2005a; Elmitwalli et al., 1999; Mahmoud, 2008). For
instance, Agrawal et al. (1997a) showed that the gas produc-
tion rate and the COD removal dropped by 78% and 25%
respectively when the temperature was decreased from 27 �Cto 10 �C in treating dilute sludge. For this reason, UASB reac-
tors are pioneered in tropical countries where the climate
temperatures are 20e40 �C. In order to widely apply the UASB
technology, one key challenge is to overcome the problems
caused by the local climate change conditions. Recently,
a number of studies have reported on the effect of modifying
the reactor configurations to overcome the temperature
constraint:
� Anaerobic filter (AF) and anaerobic hybrid (AH) reactors: A
UASB reactor followed by two anaerobic filters (packed with
blast furnace slag) operating in parallel were able to achieve
86% removal of COD and 85% removal of total suspended
solids (TSS) at the local temperatures of 13e28 �C(Chernicharo and Machado, 1998). An anaerobic hybrid (AH)
reactor consisting of vertical reticulated polyurethane foam
(RPF) sheets on top of the gas-liquid-solid separator yielded
a slightly better total COD removal than that of a UASB
reactor alone at the steady state (64 and 60% respectively) in
treating pre-settled domestic sewage at a temperature of
13 �C (Elmitwalli et al., 1999). Thereafter, the anaerobic
hybrid reactor was utilized as a post-treatment unit to an
anaerobic filter reactor (packed with RPF sheets). With
a hydraulic retention time (HRT) of 12 h, this two-step
anaerobic filter-hybrid (AF þ AH) system provided 71%
removal of COD which is comparable to the one-step UASB
reactors in tropical countries (Elmitwalli et al., 2002, 2001).
Another feature was the use of RPF media at the upper part
of the hybrid reactor which enhanced the entrapment of
colloidal COD (CODcol). On the other hand, Sawajneh et al.
(2010) incorporated an anaerobic filter reactor as a pre-
treatment unit to a UASB reactor in treating strong sludge
at 15e21 �C. This anaerobic filter-UASB (AF þ UASB) system
showed satisfactory COD removal despite the fact that the
system was operated for a short period of time, most of
which was during winter (Sawajneh et al., 2010). However,
as suggested by the authors, due to the excess sludge
produced, frequent removal of the excess sludge from the
anaerobic filter reactor and post-stabilisation for the
anaerobic filter-hybrid system are necessary despite its
improved settleability and dewaterability (Elmitwalli et al.,
1999, 2002).
To assist biomass attachment, a hybrid UASB-filter, in
which the gas-liquid-solid separator was replaced by plastic
filter rings, was used to treat domestic sludge at 10e28 �C (Lew
et al., 2004). However, this reactor was restricted by low
temperatures. At 10 and 14 �C, the COD and TSS removal
efficiencies of the hybrid UASB-filter were lower than those of
the UASB reactor by 17% and 26% respectively due to better
solids retention in the latter. In another study (Gao et al., 2011),
an upflow anaerobic fixed bed (UAFB) reactor, in which
randomly packed polyethylene ring-shaped matrix pieces
were fixed at the bottom half of the UASB reactor, was used to
treat domestic sludge at 15e35 �C. Decreasing the temperature
from 35 to 15 �C resulted in the decrease of the COD removal
efficiency by 32%. However, interestingly, the maximum
methane production occurred at 20 �C, yielding 7 L methane
per day, which was 40% higher than that operating at 35 �C.This study showed that with a good start-up strategy (i.e.
gradual lowering the temperatures from mesophilic to
psychrophilic), due to the protecting functions and the buff-
eringmechanism provided by the UAFB reactor, psychrophilic
sewage treatment with UAFB reactor is feasible (Gao et al.,
2011).
� Combined UASB-digester systems: Various studies have
suggested using a digester (also known as a continuous-
stirred-tank reactor) to post-treat UASB effluent in order to
improve the reactor performance subject to climate
temperature fluctuations (Lettinga and Hulshoff Pol, 1991;
Mahmoud et al., 2004). Mahmoud et al. (2004, 2008) reported
that by incorporating a post digester which was maintained
at 35 �C to a UASB reactor in treating sewage at 15 �C, with
a HRT of 6 h, the COD and CODcol removal efficiencies
reached 66% and 44% respectively, compared to 44% and 3%
in a UASB reactor alone. Furthermore, Mahmoud et al. (2008)
compared the performance of the UASB reactor and the
UASB-digester system operated during the late summer
time in Palestine. During the summer time with tempera-
tures 29e34 �C, the UASB-digester system outperformed the
UASB reactor in terms of COD removal (by 33%) and effluent
TSS concentrations. Mahmoud et al. (2008) also proposed
that the UASB-digester volume corresponding to an HRT of
8.6 h might suffice for sewage treatment in Palestine
(temperatures 15e35 �C). Although this system required
a temperature raise for the digester unit and to maintain of
the sludge conveyed to the digester from 15 to 35 �C, thesystem provided better COD removal efficiencies, lower
excess sludge production and well dewatered and stabilised
sludge.
� Single- and two-staged UASB systems: Halalsheh et al.
(2005b) showed that with HRTs three to four times higher
than those in tropical countries, the operation of UASB
reactors in treating strong sewage in Jordan (at average
ambient temperatures of 18 and 25 �C during winter and
summer respectively) was still feasible. They compared
a single-staged and a two-staged UASB system, and found
that there was no significant improvement on COD removal
efficiency, suggesting that a single-stage UASB reactor
operated at relatively long HRT is preferred at the Jordanian
climate conditions (Halalsheh et al., 2005b). Alvarez et al.
(2008) on the other hand, suggested that at low
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03442
temperatures of 17e21 �C, using an overall HRT ranging
from 9.3 to 17.3 h, a two-step UASB system, in which one
served as a hydrolytic unit, thus known as the hydrolytic
upflow sludge blanket (HUSB) reactor, the other served as
a methanogenic unit (the UASB reactor), would provide
better performance than a UASB reactor alone in terms of
TSS and COD removal efficiencies, due to the self-regulating
mechanism in the HUSB reactor which allowed rather
independent TSS removal from the temperature influence
(Alvarez et al., 2008).
� Combined UASB-septic tank systems: Al-Shayah and
Mahmoud (2008) utilized a UASB-septic tank system
(which is an improved design of the septic tank by applying
upward flow and placing a gas-liquid separator at the top of
the tank) for strong sewage sludge treatment at 24 �C and
a HRT of 2 days. They obtained total COD and suspended
COD (COD and CODss) removal efficiencies of 56 and 87%
respectively. Thereafter, Al-Jamal and Mahmoud (2009)
used the same system to treat strong sewage at a much
lower temperature of 17.4 �C and obtained similar COD
removal efficiencies (51 and 83% of COD and CODss). Their
results implied that UASB-septic tank systems can be
adequately designed in Palestine with HRT of 2 days. One of
the major advantages of this system was the reduced
sludge-handling cost due to long sludge hold-up time in the
system. The authors also suggested that the removal of
large amount of suspended solids by the UASB-septic tank
system would enable simpler operations and longer oper-
ating life of the possible post-treatment units for removing
nutrients and pathogens (Al-Jamal and Mahmoud, 2009;
Luostarinen et al., 2007).
� Modified design: A few research teams have focused on
modifying the design of the UASB reactor for the same
purpose.More recently, the use of lamella settlers to achieve
better digestion in UASB reactors without the need of
increasing the HRT was proposed in Halalsheh et al. (2010).
They investigated two reactor configurations e UASB-ESR1
and UASB-ESR2. The difference in these two reactors is
that in the former, four lamella settlers were installed in the
settling zone whereas in the latter, three lamella settlers
were installed underneath the gas-liquid-solid separator.
UASB-ESR2 obtained 2e3% higher COD removal tha UASB-
ESR1. After 6 months of operation during winter time with
an average temperature of 16.4 �C, both modified reactors
achieved approximately 38% and 61% of COD and CODss
removals respectively, which were 1.52 and 1.74-fold higher
than those in conventional UASB reactor under similar
operating conditions, due to increased sludge retention time
after incorporated with lamella settlers. The group sug-
gested that the modified systems are expected to signifi-
cantly improve when arriving at steady-state conditions
(Halalsheh et al., 2010).
Table 5 presents a compilation of various reactor configu-
rations used to overcome the temperature constraint problem
on UASB reactors.
2.3.3. Post-treatment of UASB effluentTable 6 provides an outline of the typical effluent quality with
various levels of treatment. The requirement of post-
stabilization or effluent polishing to meet the stringent
discharge standards has led to the development of two-step
processes. This section discusses the common post-
treatment options coupled with the UASB reactors. In each
subsection, the important experimental results, revealing the
advantages and disadvantages are provided.
2.3.3.1. UASB-activated sludge (AS). As shown in Fig. 2,
a UASB-activated sludge (AS) system consists of a UASB
reactor, a continuous-flow aeration tank and a settler. The
aerated solids collected in the settler are returned to the
UASB reactor and the aeration tank to allow sludge thick-
ening and further digestion. The uniqueness of this system is
that the UASB reactor replaces the primary sedimentation
tank as in the conventional activated-sludge system
(Chernicharo, 2006). As seen in Table 7, the studied sequen-
tial UASB-AS systems were capable of achieving high
COD, BOD and TSS removal efficiencies (von Sperling et al.,
2001; La Motta et al., 2007; Tawfik et al., 2008; Cao and Ang,
2009).
At the temperatures of 15e30 �C, with an overall HRT of at
least 5 h, an one-year pilot-scale UASB-AS system was able to
produce an effluent which meets the secondary-effluent
water quality requirements (with COD and TSS concentra-
tions 46 and 8 mg/L in the final effluent respectively), signi-
fying the satisfactory independence of the system from
climate conditions (La Motta et al., 2007). In treating strong
sewage such as the combined dairy and domestic wastewater
with COD concentration 4500 mg/L at 20 �C, using an HRT of
26 h, the UASB-AS system was able to achieve excellent
removal efficiencies of COD (98.9%), BOD (99.6%) and oil and
grease (98.9%). The final effluent complied with the standards
given by the Egyptian law for discharge into agricultural
drains (COD ¼ 80 mg/L, BOD ¼ 60 mg/L, TSS ¼ 60 mg/L, and
faecal coliform ¼ 105/100 mL). However, the removal of coli-
formswas limited (Tawfik et al., 2008). This conforms with the
results in Mungray and Patel (2011) who showed that even
with an extensive aerobic treatment like AS, the finally treated
effluents still contained significant number of total coliforms
and faecal coliforms, thus requiring an additional post-
treatment or disinfection step.
Similar lab-scale experiments were carried out by Cao and
Ang (2009) with a UASB-modified Ludzack-Ettinger (MLE) AS
system in treating dilute domestic sludge (COD 376 mg/L).
They obtained similar results as shown by von Sperling et al.
(2001), however due to inhibition, they pointed out that the
pH (which was less than 6 in the final effluent) remains an
issue of concern if nitrification is to be considered. They sug-
gested other alternatives, such as recycling nitrate-containing
stream to the UASB reactor, which are effectively appreciable.
2.3.3.2. UASB-sequencing-batch reactor (SBR). A sequencing-
batch reactor (SBR), as shown in Fig. 3, is a variant of the AS
system. The difference is that instead of continuous flow, an
intermittent flow activated-sludge system, which undergoes
five steps in sequencee fill, react, settle, decant and idlee can
be adopted and incorporated sequentially within a single
reactor, thereby eliminating the need for primary and
secondary clarifiers (Kassab et al., 2010; Guimaraes et al., 2003;
Moawad et al., 2009).
ble 5 e Recent studies and comparison of various coupled systems at low temperatures.
.a T (�C) Digester type Size (m3) HRT (h) Influent COD(mg/L)
Concentration in final effluent(mg/L) [Average removal efficiency (%)]b
COD removalefficiencyc
CODt CODss TSS NH4eN EC
h a fixed-bed reactor:
15e35 UASB þ AF (�2) 0.416 þ 0.204 5 þ (1.5 e 24) 640 84 [86] 31 [85] þ13 AF þ AH 0.06 þ 0.65 4 þ 8 506 147 [71] 18 [91] 50 [�7] 2.2 � 106 [0.48] þ
3 þ 6 187 [63] 43 [79] e
2 þ 4 207 [59] 59 [71] e
10e28 UASB-AF 0.0053 3 � 24 200e1300 201 [63] 68 [70] e
15e21 AF þ UASB 56.5 þ 113 4 þ 8 962e1627 545 [58] 152 [81] 0
15e35 UAFB 0.0059 3.4 272 139 [49] 0
h a continuous-stirred-tank reactor:
15, 35 UASB þ CSTR 0.14 þ 0.106 6 h þ 21.2 d (SRT) 460 151 [66] 32 [87] [�18] þ29e34 UASB þ CSTR 0.14 þ 0.106 10 h þ 20 d (SRT) 1186 332 [72] þ
-stage UASB:
18e25 UASB þ UASB 60 þ 33 5e6, 8e10 1531 55 0
14
17
18.5
20.6
HUSB þ UASB 25.5 þ 20.36 5.7 þ 11.6 118 142 [65] 31 [87] 0
5 þ 9.3 201 103 [64] 21 [89] þ2.8 þ 6.5 281 102 [49] 18 [86] þ3 þ 13.9 401 55 [53] 19 [81] þ
h a septic tank:
24 UASB þ Septic tank 0.8 48
96
1189 523 [56]
499 [58]
[87]
[90]
þ
17.3 UASB þ Septic tank 0.8 48
96
905 433 [51]
408 [54]
[83]
[87]
89 [74]
73 [78]
36 [12]
36 [13]
þ
h a lamella settler:
16.4 UASB-ESR1
UASB-ESR2
3 23 h
24 h
1447 897 [38]
854 [41]
271 [61]
550 [62]
þþ
eferences: [1] Chernicharo andMachado (1998); [2] Elmitwalli et al. (2001), Elmitwalli et al. (2002); [3] Lew et al. (2003), Lew et al. (2004); [4] Sawajneh et al. (2010); [5] Gao et al. (2011); [6] Mahmoud et al.
4), Mahmoud et al. (2008); [7] Mahmoud (2008); [8] Halalsheh et al., 2005a,b; [9] Alvarez et al. (2008); [10] Al-Shayah and Mahmoud (2008); [11] Al-Jamal and Mahmoud (2009); [12] Halalsheh et al.
0).
ll concentrations are given in mg/L, except EC (in org/100 mL); average removal efficiencies are given in percentage, except EC (in log units).
OD removal efficiency compared to UASB reactors at the particular operating condition: � Unfavourable, þ Favourable, þþ Very favourable, 0 no effect/no direct correlation.
water
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(2012)3434e3470
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Ta
Ref
Wit
[1]
[2]
[3]
[4]
[5]
Wit
[6]
[7]
Two
[8]
[9]
Wit
[10]
[11]
Wit
[12]
a R
(200
(201
b A
c C
Table 6 e Typical effluent quality following various levels of treatment (Australian Guidelines for Sewerage Systems -Effluent Management, 1997).
Treatment BOD(mg/L)
TSS(mg/L)
TN(mg/L)
TP(mg/L)
EC(org/100 mL)
Anionic surfactants(mg/L)
Oil and grease(mg/L)
Raw sewage 150e500 150e450 35e60 6e16 107�108 5e10 50e100
Class A e Pre-treatment 140e350 140e350
Class B e Primary treatment 120e250 80e200 30e55 6e14 106�107
Class C e Secondary
treatment
20e30 25e40 20e50 6e12 105�106 <5 <10
Class D e Nutrient removal 5e20 5e20 10e20 <2 <5
Class E � Disinfection <103
Class F e Advanced
wastewater treatment
2e5 2e5 <10 <1 <102 <5
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03444
As shown in Table 8, the recent studies on UASB-SBR
systems were carried out under subtropical climate condi-
tions and the results were comparable with those using
UASB-AS systems. The aeration time in the SBRs favoured
nitrification and did not significantly affect the COD and TSS
removal efficiencies (Moawad et al., 2009; Torres and Foresti,
2001). Unfortunately, although the UASB-SBR systems were
able to remove ammonia almost completely but the systems
were not able to simultaneously remove total nitrogen and
total phosphorus/phosphate (Moawad et al., 2009; Torres and
Foresti, 2001). Guimaraes et al. (2003) pointed out that despite
its advantages (such as simple reactor configuration and
mechanical equipment compared to a UASB-AS system), the
major drawback of a UASB-SBR system was the difficulty in
start-up due to the abundant formation of foam and
that initial nitrification was inefficient due to the toxic
effect of sulphide which was present in a concentration of
4e6 mg/L.
2.3.3.3. UASB-biofilter (BF). A biofilter (BF), also known as
a trickling filter (TF) or an aerated filter (AEF) is a fixed bed
consisting of highly permeable packing media, and in which
aerobic condition is maintained via diffusion, forced aera-
tion, natural convection or splashing. UASB effluent, under
Fig. 2 e Typical configuratio
the form of drops or jets, percolates towards the bottom
drain, resulting in the formation of a fixed film, so called
biofilm on the surface of the packing media which then
degrades the organic matter as the wastewater passes
(Chernicharo, 2006). As shown in Table 9, the performance
of UASB-TF systems was inferior to that of UASB-AS systems
in treating domestic sludge. Nevertheless, UASB-TF, unlike
the UASB-AS systems, can be designed with short HRTs,
allowing compact treatment, low energy consumption,
labour and running costs (Chernicharo and Nascimento,
2001; de Almeida et al., 2009). In addition, it was found in
various studies that the return of excess aerobic sludge from
the TF unit to the UASB reactor did not affect its perfor-
mance significantly; instead, it slightly decreased the
stability of the system (Goncalves et al., 2002; Pontes et al.,
2003; Pontes and Chernicharo, 2011). Goncalves et al. (2002)
pointed out the need of high sequential hydraulic washing
of the biofilter to eliminate the excess of fixed biomass in
the system.
Fig. 4 shows one of the compacts UASB-TF system used in
Pontes, (2011), in which the trickling filters and the secondary
clarifiers/settlers are coupled to the UASB reactor as one unit.
The UASB effluents were passed through the trickling filters,
the settler which was equipped with lamellar plates and the
n of a UASB-AS system.
Table
7e
Rece
ntstudiesandco
mpariso
nofUASB-A
ctivatedsludgesy
stem
s.
Ref.a
T(�C)
UASB
AS
Conce
ntrationin
finaleffl
uent(m
g/L)
[Averagerem
ovalefficiency
(%)]b
Size(m
3)
HRT(h)
OLR(kgCOD/m
3/d
)COD
(mg/L)
Size(m
3)
HRT(h)
F/M
cCOD
BOD
TSS
NH
4eN
FC
Inf.
Eff.
[1]
30
0.416
42.3e4.4
558
115
0.023
2.8
0.6e0.9
55[90]
15[91]
[2]
15e30
0.396
3.2
2.6
341
225
0.3
2.5
1.2
46[87]
8[92]
[3]
20
0.005
24
1.9e4.4
4500
1385
0.002
235[99]
7[100]
14[98]
9�
104[2.5]
[4]
30
0.0063
61.5
376
125
0.0066
6.3
51[86]
13[95]
3[92]
[5]
20e34
35,113
8.5
2.1
803
455
5344
6.3
127[84]
16[94]
89[62]
3�
105[5.8]
Averaged:
0.0063e0.416
3.2e24
1.5e4.4
341e4500
115e1385
0.0066e0.3
2e6.3
0.6e1.2
47[91]
7[100]
13[94]
3[92]
9�
104[2.5]
aReference
s:[1]vonSperlingetal.(2001);[2]LaMottaetal.(2007);[3]Tawfiketal.(2008);[4]CaoandAng(2009);[5]MungrayandPatel(2011).
bAllco
nce
ntrationsgivenin
mg/L,exce
ptFC(inorg/100mL);averageremovalefficienciesare
givenin
percentage,exce
ptFC(inlogunits).
cFood-to-m
icro
org
anism
ratioin
kgCOD
kgS/S/d.
dAveragevaluesare
calculatedbase
donth
esy
stem
size
andth
ese
lectedoperatingco
nditions.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3445
aerobic sludge was returned to UASB reactor. Various
packing media were studied e blast furnace slag, Rotopack,
Rotosponge, downflow hanging sponge (DHS), random
plastic rings and pieces of corrugated plastic tubing. Among
them, the best packing medium was found to be blast
furnace slag, followed by DHS. Their studies suggested that
when the BOD loading rate was kept at 0.4 kg BOD/m3/d and
HLR at 10 m3/m2/d, the UASB-TF system was able to produce
an effluent in compliance with Brazilian local discharge
standards in terms of BOD, COD and TSS concentrations.
However, none of the systems were able to remove
ammonia; in fact the ammonia removal was only 13e27%,
which was possibly due to the dominant presence of
heterotrophs (de Almeida et al., 2009; Missagia et al., 2008).
Therefore, in order to overcome the problem of limited
ammonia and organic matter removals, de Almeida et al.
(2009) suggested the OLR of the shallow UASB-TF system
should be less than 0.25 kg BOD/m3/d. Alternatively, in order
to enhance organic matter removal, higher-height TFs can be
used, thus eliminating the use of secondary settlers, offering
operational simplicity and compensation to the additional
cost of having larger filters. The use of sponges can reduce
the size of TF compartments (de Almeida et al., 2009), and in
some cases, as suggested by Tandukar et al. (2007), enhance
nitrification.
In a recent study carried out by Tawfik et al. (2010), UASB
effluent was treated in a moving-bed biofilm reactor (MBBR)
consisting of 1158 polyethylene cylindrical carriers with an
effective specific surface area of 363 m�1. Their study showed
that nitrification rate was strongly dependent on the CODss/N
ratio. At a HRT of 13.3 h, the system effectively reduced the
COD fraction in the final effluent to 54 mg/L. As pointed out in
their earlier study using a DHS for post-treating UASB effluent
(Ndabigengesere and Subba Narasiah, 1998), the removal of E.
coli (EC) in the colloidal form was the limiting step in the bio-
film system. Therefore, the authors suggested using a longer
HRT for the removal of faecal coliforms (FC) in the colloidal
form.
2.3.3.4. UASB-downflow hanging sponge (DHS). The UASB-
downflow hanging sponge (DHS) systems were developed by
a research team led by Professor Harada of Nagaoka Univer-
sity of Technology in Japan. The first generation developed by
Agrawal et al. (1997b) and Machdar et al. (1997) consists of
a UASB reactor and several 2e3 m long downflow hanging
modules which are made from vertically diagonally-
connected polyurethane (PUR) sponge cubes (1e2 cm in
size). The UASB effluent is supplied at the top of each module
and trickled down the module by gravity. The effluent coming
out from a sponge unit comes into contact with air prior to
penetrating to the next sponge unit, raising the dissolved
oxygen content of the sponge units which enhances aerobes
growth. As an improvement to the first generation, the group
developed the second through to fourth generation of the
UASB-DHS systems (Machdar et al., 2000; Tandukar et al.,
2005). A schematic diagram of the fourth generation is
shown in Fig. 5. The main advantage of the UASB-DHS
systems is that the system employs simple and economic
packingmaterial (PUR foams), therefore significantly reducing
the energy demand, maintenance and operating costs,
Fig. 3 e Typical configuration of a UASB-SBR system.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03446
making the technology a viable option for developing
countries.
As shown in Table 10, the studied UASB-DHS systems
were able to achieve near perfect BOD removal, high coli-
forms removal and satisfactory organics removal. Unlike a TF
system, the DHS post-treatment enabled simultaneous
nitrification and denitrification in which the former occurred
in the aerobic zone and the latter took place at the inner
anoxic zone of the sponge material (Tandukar et al., 2005).
Tracer studies suggested that the mixing intensity in the DHS
system was very low, resembling a plug-flow system
(Tandukar et al., 2006a, 2006b). In addition, the temperature
effect on the digester performance was shown to be minute
(Tandukar et al., 2007; Takahashi et al., 2011). When
compared with the activated-sludge process (ASP), despite
the cost benefit, the UASB-DHS systems out-competed the
ASP in terms of pathogen removal, SRT, HRT and the amount
of excess sludge produced (Tandukar et al., 2007, 2005;
2006b). This technology was picked up and tested in
a demonstration-scale second-generation UASB-DHS system
(of 1000 m3/d capacity) in India and the system has been
operated continuously since 2003 (Tandukar et al., 2005). This
technology however, requires further studies on the dura-
bility of the sponge material, susceptibility to shock loads
and the scale-up issues.
2.3.3.5. UASB-stabilising pond (SP). Stabilisation ponds are
usually used to recover nutrients from the UASB effluents.
Results from the recent studies are compiled in Table 11. As
shown in von Sperling et al. (2002, 2003), therewas virtually no
helminth eggs found in the final effluent after post-treated in
baffled or unbaffled ponds systems. In addition, Cavalcanti
et al. (2002) suggested that, despite the poor hygienic quality
of the sludge, the excess sludge removal (or de-sludging) from
the UASB reactor and the polishing ponds could be optional
during the lifespan of the polishing ponds, especially in
tropical regions, due to the low rate of sludge accumulation in
the system.
Von Sperling and Mascarenhas (2005) showed the effec-
tiveness of shallow ponds system (of 0.4 m in depth) in
removing the BOD and EC contents of the UASB effluent to
comply with the urban wastewater and World Health Orga-
nisation (WHO) guidelines for unrestricted irrigation.
However, as shown in several studies (Sato et al., 2006; El-
Shafai et al., 2007; Walia et al., 2011), the final effluents were
not compliant with the set standards. The major drawback
has been the sensitivity of the system towards climate
temperatures. As shown in El-Shafai et al. (2007) who studied
UASB-duckweed system, although the COD, BOD and TSS of
the final effluent were satisfactory during the whole year, the
recoveries of nutrients and faecal coliforms were ineffective
during winter time. During summer time however, the main
part of the total nitrogen removal (in total 78.5%) contributed
to the recovery by the duckweed (80.5%), denitrification
(14.8%) as well as sedimentation (4.7%).
Sato et al. (2006) and Walia et al. (2011) showed that ponds
retention time of 1e1.6 days was not sufficient to treat UASB
effluent, probably due to longer retention time for algae
growth which usually acquires 2e2.25 days or high sulphide
concentration in the effluent. The latter study showed that
with an additional aeration system, the COD and BOD in the
final effluent were decreased by approximately 50% (Walia
et al., 2011). As shown in von Sperling and de Andrada
(2006), for the UASB-ponds-rock filter system, the UASB
reactor was responsible for BOD removal; the ponds-in-series
provided excellent coliform removal; whereas the coarse rock
filters (of diameter 3e8 cm) for polishing pond effluent played
an important role in removing significant suspended solids
amount, leading to the potential use of the final effluent for
unrestricted irrigation.
2.3.3.6. UASB-rotating-biological contactor (RBC). As shown in
Fig. 6, a rotating-biological contactor (RBC) consists of ten
polystyrene (or polyurethane) foam disks mounted on a steel
shaft and submerged in the effluent solution. The foam disks
were 0.6 m in diameter with a thickness of 0.2 m apart, spaced
Table 8 e Recent studies and comparison of UASB-Sequencing batch reactor systems.
Ref.a T (�C) UASB SBR Concent ion in final effluent (mg/L) [Averageremoval efficiency (%)]b
Size (L) HRT (h) OLR (kg COD/m3/d)
COD (mg/L) Size (L) Aeration (h) qc (d) COD BOD TSS NH4eN TKN TP (PO3�4 )
Inf. Eff.
[1] 21 150 6 2.1 569 228 90 22 46 [92] 16 [88] 0 [100] 5.5 [87] (13) [22]
10 46 [92] 18 [86] 0 [100] 4.6 [89] (10) [29]
4 46 [92] 21 [84] 0.5 [98] 8.4 [80] (8) [44]
2 51 [91] 21 [84] 33 [69] 15 [64] (4) [72]
[2] 25 10 4 3.8 587 215 7 1 9 49 [92] 18 [92] 7 [97] 7.8 [83]
11 48 [92] 10 [96] 6 [98] 1.7 [96]
15 43 [93] 12 [95] 6 [98] 1.6 [97]
[3] Subd 47 4 5.3e1.3 541 227 31 2 33 [84] 8 [93] 8 [86] 9.5 [60] 22 [49] 1.2 [64]
3 5.1e1.9 389 5 23 [86] 6 [90] 3 [96] 2 [90] 19 [63] 1.1 [61]
3 5.1e1.9 416 9 22 [89] 4 [96] 4 [94] 0 [100] 13 [77] 1.5 [61]
Averagee: 49e150 3e6 1.9e5.1 500 223 31e90 4e22 37 [90] 5 [93] 12 [90] 0.5 [98] 10 [79] 1.3 [61]
a References: [1] Torres and Foresti (2001); [2] Guimaraes et al. (2003); [3] Moawad et al. (2009).
b All concentrations are given in mg/L, except FC (in org/100 mL); average removal efficiencies are given in percentage, except FC (in log units)
c Sludge age.
d Subtropical climate temperatures.
e Average values are calculated based on the system size and the selected operating conditions, with at least two data.
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rat
.
Table 9 e Recent studies and comparison of UASB-Biofilter systems.
Ref.a T (�C) PT unitb UASB Post-treatment unit Concentration in final effluent (mg/L)[Average removal efficiency (%)]c
Size (m3) HRT (h) OLR (kg COD/m3/d)
COD (mg/L) Size (m3) HRT (h) COD BOD TSS NH4eN TKN FC
Inf. Eff.
[1] 25 SAB 0.046 10 2.3 556 156 0.0063 0.28 50 [91] 10 [96] 10 [94] 27 [30]
[2] 25 SAB 35 9 1.6 523 186 12 0.346 80 [85] 26 [88]
175 [74] 31 [88] 15 [88]
[3] Subd TF 0.416 4 3.8 521 148 0.06 3 80 [86] 32 [90] 11 [91]
1.5 72 [83] 23 [92] 21 [83]
0.8 113 [78] 55 [81] 33 [73]
[4] 26 TF 0.416 5.6 1.9 437 108 0.106 1.5 82 [81] 27 [87] 17 [89]
[5] 20e25 TF 22 7.7 0.46e 303 107 15 3.6 63 [79] 23 [84] 14 [90] 19 [27]
17 8.5 0.71e 532 174 3.87 2.5 85 [84] 27 [90] 18 [91] 28 [14]
[6] 20e27 TF 232 62 30 [85] 71 [78] 22 [84]
AEF 459 124 66 [80] 31 [87] 54 [79]
[7] 22e35 MBBR 8 1.5 699 203 0.008 5.3 54 [92] 13 [59] 15 [60] 8.9 � 104 [2.3]
0.01 6 2.4 733 244 4 95 [86] 18 [28] 26 [37] 4.9 � 105 [1.4]
3 5.8 803 293 2 142 [80] 21 [14] 29 [31] 9.4 � 105 [0.7]
[8] 25 TF 22 7.7 1.4 497 164 18.75 2.7 88 [82] 31 [90] 19 [91]
Averagef: 0.01e35 4e10 1.5e3.8 303e733 62e244 0.006e18.75 0.28e5.3 70 [85] 31 [88] 21 [88] 20 [32] 20 [49] 2.9 � 105 [1.9]
a References: [1] Goncalves et al. (1998); [2] Goncalves et al. (2002); [3] Chernicharo and Nascimento (2001); [4] Pontes et al. (2003); [5] de Almeida et al. (2009); [6] Oliveira and von Sperling (2009); [7]
Tawfik et al. (2010); [8] Pontes and Chernacharo (2011).
b Post-treatment unit: SAB: submerged-aerated-biofilter; TF: trickling filter; AEF: aerated-filter.
c All concentrations are given in mg/L, except FC (in org/100 mL); average removal efficiencies are given in percentage, except FC (in log units).
d Subtropical climate temperatures.
e Organic loading rate in kg BOD/m3/d.
f Average values are calculated based on the system size and the selected operating conditions, with at least two data.
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Fig. 4 e Schematic configuration of a compacts UASB-TF
system. 1: Inlet distribution points; 2: Tri-phase separator
device; 3. UASB sedimentation chamber; 4: UASB effluent
outlets; 5: TF effluent inlets; 6: TF packing media; 7: TF
lamellar sedimentation chamber; 8: TF effluent collection
box; 9: TF sludge accumulation box; 10: pump (reprinted
from Pontes and Chernicharo (2011). Copyright (2011), by
permission of the publisher (Taylor & Francis Group, http://
www.informaworld.com)).
Fig. 5 e Schematic configuration of a pilot-scale fourth-
generation UASB-DHS system (reprinted from Tandukar
et al. (2007). Copyright (2007), with permission from
Elsevier).
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3449
at 0.02 m distance and operated at 5 rpm (Tawfik et al., 2002a,
b, Tawfik et al., 2003). The main benefits of using RBC are the
low-shear environment, easiness for scaling up, resistance to
high hydraulic and organic loadings (Tawfik and Klapwijk,
2010). As shown in Table 12 for a series of studies carried
out by Tawfik’s group, single RBC and RBCs in series have been
investigated for the treatment of UASB effluent. At relatively
higher temperatures (>22 �C), anaerobic treatment of raw
sewage in a UASB reactor followed by a single-stage RBC
equipped with polystyrene disks (RBC operating at HRT of
2.5 h and OLR of 14.5 g COD/m2/d) provided a good COD
removal (with 76 mg/L in the final effluent), and partial
removals of ammonia and EC (Tawfik et al., 2003, 2005). Based
on their recent study (Tawfik and Klapwijk, 2010), RBC with
polyurethane disks would perform much more efficiently
compared to polystyrene disks especially in terms of
ammonia and EC removal.
If it is subtropical climate (10e22 �C) or if nitrification is the
main objective, a single RBC with polyurethane disks or two
RBCs in series can be used to obtain good removal of COD and
ammonia from the final effluent. As shown in Tawfik and
Klapwijk (2010), after post-treating the UASB effluent with
a RBC with polyurethane disks operating at HRT of 2.5 h and
OLR of 10.5 g COD/L/d, the final effluent contained 70mg COD/
L, 6 mg NH4eN/L, 8 mg TKN/L and 1.8 � 104 counts of EC/
100 ml. During winter time (around 4 �C), when the perfor-
mance of the UASB reactor deteriorated, the RBC can be
operated with a longer HRT (i.e. >5 h) and shorter OLR
(<13 g COD/m2/d). Additionally, for excellent removal of EC or
good nitrification efficiency, a three-stage RBC system can be
used. Tawfik et al. (2002a) showed that operating a three-stage
RBC at HRT of 10 h and OLR of 15, 6.5 and 0.5 g COD/m2/d in
each stagewas possible. An addition of recirculation (at a ratio
of 1) of the final effluent to the first RBC stage was able to
provide the final effluent with 980 counts of EC/100 mL, which
complied with the WHO (1989) for reuse in unrestricted
irrigation.
2.3.3.7. UASB-constructed wetland (CW). A constructed
wetland is an artificial wetland, which serves as a biofilter
allowing natural processes (physical, chemical and microbi-
ological) for the degradation of organic matter and the
removal of nutrients, pathogens, sediments and pollutants
from the wastewater. In order to emulate these features,
a constructed wetland usually consists of ponds, basins or
shallow canals (usually less than 1 m deep) and is planted
with aquatic vegetation providing substrates (such as roots,
stems, and leaves) for microorganism growth to degrade
organic matters. Depending on the design, the constructed
wetland can contain biofilm-support medium such as gravel,
sand or stone and engineered structures to emulate its
features. The most commonly used types of constructed
wetlands are the subsurface flow (horizontal or vertical) (SSF)
and free-water-surfacewetlands (FWS, or simply surface flow,
SF). SSF wetlands allow effluent to flow through the gravel, or
sand-filled channel in which the roots are planted. The porous
medium filters out particles, and the microorganisms degrade
Table 10 e Recent studies and comparison of UASB-downflow-hanging sponge systems.
Ref.a T (�C) UASB DHS Concentration in final effluent (mg/L) [Averageremoval efficiency (%)]b
Size (m3) HRT (h) OLR (kg COD/m3/d)
COD (mg/L) Size (L)c HRT (h) Gen.d COD BOD TSS NH4eN TN FC
Inf. Eff.
[1] 7e30 0.047 7 1.03 300 71e179 1.215s 0.5e0.7 1st 25 [92] 3.8 5 [71e100] 6.5 � 102 [2.57]
[2] 25 0.155 7 2.3 672 144 1.215p 1.3 1st 42 [94] 2 [99] 0 [100] 7 [75]
[3] 25 0.155 6 1.57 393 165 51p 2 2nd 65 [84] 4 [97] 28 [79] 20 [52] 40 [25]
51s 68 [81] 10 [94] 46 [63] 15 [61] 38 [31]
[4] 25 0.155 6 1.57 393 165 51s 2 2nd 65 [84] 8 [95] 12 [70] 33 [39]
6 1.49 51p 2 65 [83] 4 [97] 28 [79] 20 [50] 40 [34]
[5] 25 0.155 6 1.49 373 170 51s 2 2nd 60 [80] 8 [95] 43 [70] 12 [70] 37 [39]
4 2.24 51s 1.3 69 [82] 9 [94] 40 [77] 10 [73] 34 [45]
[6] 20e25 1.148 6 2.15 532 195 375 2 4th 46 [91] 9 [96] 17 [93] 18 [28] [31] 1.9 � 104 [3.45]
[7] 9e32 1.148 6 2.4 600 227 480 2.5 4th 62 [90] 17 [94] 18 [95] 9 [60] 18 [56] 3.8 � 104 [4]
Averagee: 0.047e1.148 4e7 1.03e2.3 300e600 115e227 1.215e480 0.5e2.5 57 [86] 8 [96] 28 [94] 13 [62] 34 [38] 1.9 � 104 [3.3]
a References: [1] Agrawal et al., 1997a,b; [2] Machdar et al. (1997); [3] Machdar et al. (2000); [4] Uemura et al. (2002); [5] Tandukar et al. (2006a); [6] Tandukar et al. (2005, 2006b); [7] Tandukar et al. (2007).
b All concentrations are given in mg/L, except FC (in org/100 mL); average removal efficiencies are given in percentage, except FC (in log units).
c Size. Superscript s: DHS falls in series; superscript p: DHS falls in parallel.
d Generation of DHS. 1st: first generation; 2nd: second-generation.
e Average values are calculated based on the system size and the selected operating conditions, with at least two data.
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ble 11 e Recent studies and comparison of UASB-Stabilising pond systems.
f.a T (�C) UASB SP Concentration in final effl nt (mg/L)[Average removal effici cy (%)]b
EC Helmintheggs
Size (m3) HRT (h) COD (mg/L) Typec Size (m3) HRT (d) Depth (m) COD BOD TSS NH4-N TN TP FC
Inf. Eff.
] 23 9 5.5 Baffled pond 32 6.3 1 3.9 � 106 [3] 0.1
] 25 1.5 3 175 Baffled pond 32.5 6.25 0.65 45
5 Baffled 8 1 0
5 Baffled 6.4 1 0.49
7.5 Baffled 6.1 1 0
] 17e26 9 7.5 Baffled 32 9.3 0.6 0
5 Unbaffled 8 1 0
5 Unbaffled 6.4 1 0.31
7.5 Unbaffled 7.7 1 0
7.5 Unbaffled 8.9 0.6 0
] 23 436 159 Shallow4S 52.5e85.3 1.4e2.5 0.4e0.65 170 [61] 44 [88] 113 [60] 7.3 [67] 11.4 [63 .8 [28] 3.8 � 102
[9.23]
Shallow2�2S 2.9e6.2 0.4e0.8 130 [70] 29 [92] 52 [82] 12.3 [50] 15 [51] .9 [51] 1.4 � 105
[9.23]
] 17e28 14.2 0.3 235d 45d Shallow
ponds-rock
filter
175 12.4 0.4e0.8 27 [88] 26 [85] 4.5 � 102
[5.68]
] 17e23 13,600 9.1 363e1194 149e510 Baffled 43733 1.1 1.25e2 238 [68] 96 [63] 262 [36] 34 [0] 4.6 � 105 [0.7]
] 16 0.04 6 871 257 Duckweed 1.44 15 0.48 73 [92] 25 [93] 31 [91] 10.4 [40] 10.8 [38 .69 [56] 4.7 � 105 [3.4]
41 749 151 49 [93] 14 [96] 32 [92] 0.4 [98] 1.3 [94] .11 [79] 4 � 103 [4.9]
364d 55d Facultative 55 [84] 48 [82]
] 20e27 744d 59d Facultative 59 [92] 59 [91]
219d 30d Maturation 30 [83] 61 [51]
] 24e29 4477 9.8 318e443 185e337 Ponds 49333 1.1 1.3e2 130 [50] 40 [50] 120 [60]
eragee: 0.04e4477 0.3e9.8 318e871 151e337 1.44e49333 1.1e15 0.4-2 110 [73] 33 [85] 60 [77] 7.6 [64] 9.6 [62] .1 [54] 2.4 � 105 [4.15] 4.7 � 104 [8.05] 0
References: [1] von Sperling et al. (2002); [2] Cavalcanti et al. (2002); [3] von Sperling et al. (2003); [4] von Sperling andMascarenhas (2005); [5] von S ling and de Andrada (2006); [6] Sato et al. (2006); [7]
-Shafai et al. (2007); [8] Oliveira and von Sperling (2009); [9] Walia et al. (2011).
All concentrations are given in mg/L, except FC (in org/100 mL) and Helminth eggs (in counts/L); average removal efficiencies are given in pe ntage, except FC (in log units).
Type of the polishing ponds. Subscript 4S: 4 ponds in series; 2 � 2S: 2 sets of 2 ponds in series.
BOD (mg/L).
Average values are calculated based on the system size and the selected operating conditions, with at least two data.
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Ta
Re
[1
[2
[3
[4
[5
[6
[7
[8
[9
Av
a
El
b
c
d
e
ueen
] 2
1
] 2
1
2
per
rce
Fig. 6 e Schematic of a two-stage RBC treating anaerobically pre-treated domestic sewage (reprinted from Tawfik et al. (2002)
a,b. Copyright (2002), with permission from Elsevier).
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03452
the organic matters. Because the flow is not at the water
surface, there is less odour generation, less hospitable to
mosquitoes and rats and lower land requirement compared to
FWS wetlands. Unlike the SSF wetlands, FWS (or SF) wetlands
allow effluent to flow above the topsoil, in which the sedi-
mentation of particles, removal of pathogens and uptake of
nutrients by the vegetation and microorganisms take place.
FWS wetlands require more land and if it is not operated
correctly it produces increased odour and favours mosquito
breeding.
As shown in Table 13, in the studied range of sludge
loading rate (SLR) (6.6e60 g COD/m2/d) and HRT (1.2e10.8 d),
very good removals of COD, BOD and TSS were attained,
despite less favourable nutrient and pathogen removal. With
SLR less than 11.65 g COD/m2/d, the faecal coliforms in the
final effluent can be reduced to less than 1000 counts/100 mL
(de Sousa et al., 2001; de Sousa et al., 2003; El Khateeb et al.,
2009). As shown in El Khateeb et al. (2009), with a series
(FWS þ SSF) wetland unit, the final effluent was free from any
parasitic stages. Vegetated SSF wetlands performed better
than FWS wetlands and un-vegetated wetlands, especially in
terms of nutrient and pathogen removal (de Sousa et al.,
2001; de Sousa et al., 2003; El Khateeb and El-Gohary, 2003;
Kaseva, 2004; Mbuligwe, 2004; Dornelas et al., 2009).
However, the unplanted wetlands still provided good
removals of COD and TSS. Therefore, depending on the
compatible usage of the final effluent, the requirement of
vegetation in the constructed wetlands can be justified
(Dornelas et al., 2009).
There were very little differences in terms of COD, BOD,
TSS, nutrient and pathogen removals when different plant
species were adopted (e.g. Phragmites, Colocasia and Typha
spp.) (Kaseva, 2004; Mbuligwe, 2004). The main problem of
the constructed wetlands is the clogging of the support
medium and subsequent ponding. This is especially in the
first metre in the vicinity of the inlet, which is largely
dependent on the TSS loading rate (Dornelas et al., 2009; Ruiz
et al., 2008; Green et al., 2006). The operating period of the
current studies is not sufficient to investigate this clogging
problem and there is a lack of studies on the solids accu-
mulation or hydraulic conductivity evolution in the system
(Ruiz et al., 2008). The use of UASB as a pre-treatment unit for
the constructed wetlands is beneficial due to the good
removal of TSS prior to entering the constructed wetlands,
thus reducing the solids accumulation at the gravel medium.
However, adequate practices such as the removal of surplus
sludge from the UASB reactor are necessary, especially
during the winter time to avoid the wash-out of suspended
solids with its effluent.
2.3.3.8. Other post-treatment units. Table 14 presents the
results from other post-treatment units coupled to UASB
reactors in treating synthetic and domestic sludge. The
dissolved-air-flotation (DAF) process employs a primary
coagulant which is usually a cationic polymer such as alum
and ferric salts, and/or a coagulant aid, which is usually a non-
ionic or anionic polymer, to separate suspendedmaterials and
floc particles from the wastewater. The DAF process attaches
micro-bubbles to the flocculated particles which rise to the
surface of the tank, forming a dense foam which is removed
periodically (Crossley and Valade, 2006). Reali et al. (2001a)
showed that the addition of ferric chloride (at 65 mg/L)
resulted in better performance than using cationic polymer
(at 7 mg/L) in terms of COD, TSS and TP removal efficiencies.
Also, the simultaneous addition of polymer (1mg/L cationic or
non-ionic polymers) and ferric chloride (30 mg/L) in the DAF
process resulted in flocs formation with much greater uplift
rates, yielding satisfactory removal efficiencies of COD,
phosphate, turbidity removal efficiencies (Reali et al., 2001b).
The phosphate removal was shown to depend strongly on the
dosage of ferric chloride (Reali et al., 2001b). On the other
hand, Marchioretto and Reali (2001) showed that pre-
ozonation to DAF permitted the removal of iron and manga-
nese (which constitutes the apparent colour) and the turbidity
of anaerobic effluent. Despite its great versatility, DAF has the
disadvantages of poor ammonium and faecal coliforms
removal efficiencies (Chernicharo, 2006). In light of this limi-
tation, Tessele et al. (2005) used a two-staged flotation fol-
lowed by ultraviolet (UV) disinfection for the treatment of
UASB effluent. The first-stage flotation was a flocculation-
flotation system utilizing 5e7.5 mg/L of cationic flocculant
and removed 99% of the suspended solids and the second-
stage flotation which used 5e25 mg/L of ferric chloride
removed the residual fine solids and completely recovered
Table 12 e Recent studies and comparison of UASB-Rotating biological contactor systems.
Ref.a UASB RBC Concentration in final effluent (mg/L) [Averageremoval efficiency (%)]b
T (�C) COD (mg/L) Typec Size (L) T (�C) HRT (h) SLR (g COD/m2/d) COD NH4eN TKN EC
Inf. Eff.
[1] 14 527 295 2S 120 12.3 10 6.45 49 [91] 3 [94] 3.5 � 104 [2.4]
5 14.2 61 [88] 12 [76] 0.5 � 105 [2.2]
2.5 22 70 [87] 29 [41] 3.4 � 105 [1.4]
[2] 2S-SPUR 3 47e25e1.8 51 [90] 12 [76] 14 [76] 1.3 � 105 [1.82]
14 527 274 2S-SPUR 155 14 10 15e6.5e0.5 43 [92] 2.2 [96] 2.2 [96] 2.0 � 103 [3.63]
2S-SPUR’ 10 15e6.5e0.5 51 [90] 4 [92] 9.8 � 102 [3.94]
11 276 13 2.5 27 100 [81] 36 [27] 38 [34] 7.9 � 105 [1.0]
11 276 13 5 13 76 [86] 34 [31] 37 [36] 6.5 � 105 [1.1]
[3] 20 527 225 S 60 21 2.5 20 85 [84] 40 [18] 42 [28] 2.5 � 105 [1.5]
30 164 30 2.5 14.5 72 [86] 24 [51] 1.2 � 105 [1.9]
12 288 2S 120 25 65 [88] 25 [49] 1.5 � 104 [2.8]
12 288 S 60 25 100 [81] 36 [27] 3.8 � 104 [2.3]
[4] 15 527 223 S 60 21 2.5 20 92 [83] 40 [18] 2.5 � 105 [1.5]
30 165 S 60 14 72 [86] 18 [63] 1.1 � 105 [1.9]
SPUR 2.5 10.5 70 [86] 6 [87] 8 [87] 1.8 � 104 [2.5]
S 2.5 10.5 76 [85] 34 [28] 37 [40] 6.5 � 105 [1.0]
[5] 21 508 225 SPUR 55 21 5 4 67 [87] 5 [89] 3.6 � 104 [2.2]
SPUR 2.5 11 67 [87] 6 [87] 1.8 � 104 [2.5]
SPUR 1.25 23 129 [75] 38 [19] 2.7 � 104 [2.3]
Averaged:
Single RBC with polystyrene foam disks 12- 2.5e5 10.5e20 79 [85] 32 [32] 39 [35] 3.4 � 105 [1.48]
Single RBC with polyurethane rotating disks 21 2.5e5 4e11 68 [87] 6 [88] 2.4 � 104 [2.4]
Series RBCs (2� or 3�) 2.5e10 6.5e47.25 56 [89] 13[75] 8 [86] 8.2�104 [2.6]
a References: [1] Tawfik et al. (2002a); [2] Tawfik et al. (2002b); [3] Tawfik et al. (2003); [4] Tawfik et al. (2005); [5] Tawfik and Klapwijk (2010).
b All concentrations are given in mg/L, except EC (in org/100 mL); average removal efficiencies are given in percentage, except EC (in log units).
c RBC type. S: 1 � RBC with polystyrene foam disk; SPUR e 1 � RBC with polyurethane rotating disks; 2S: 2 � RBCs with polystyrene foam disk in series; 2S-SPUR: 2 � RBCs with polystyrene foam disk in
series followed by 1 � RBC with polyurethane rotating disk; symbol ’: recirculation of the final effluent to the first stage of RBC.
d Average values are calculated based on the system size and the selected operating conditions, with at least two data.
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Table 13 e Recent studies and comparison of UASB-Constructed wetland systems.
Ref.a T(�C)
UASB CW Concentration in finaleffluent (mg/L) [Averageremoval efficiency (%)]b
Size(m3)
HRT(h)
OLR(kg COD/m3/d)
COD (mg/L) CW type(plant)c
Size(m3)
SLR(g COD/m2/d)
HRT(d)
COD BOD TSS NH4eN TN TKN TP FC
Inf. Eff.
[1] 19e33 1.5 3e6 2.8e5.6 304 SSF (unplanted) 6 11.65 10 52 [830] 144 [710] 38 [190] 45 [230] 4.3 [370] 3.3 � 103 [3.90]SSF (Juncus) 22.8 5 64 [790] 259 [480] 26 [450] 31 [480] 2.6 [610] 1.3 � 103 [3.30]SSF (Juncus) 16.72 7 56 [820] 218 [560] 26 [450] 31 [470] 2.4 [640] 4.1 � 104 [4.80]SSF (Juncus) 11.65 10 57 [810] 174 [650] 14 [700] 18 [700] 0.7 [890] 1.0 � 103 [4.50]SSF (unplanted) 6.64 10 96 [710] 194 [610] 36 [180] 4 [280] 1 � 104 [30]
[2] 19e33 1.5 6 2.6 333 SSF (Juncus) 6 9.53 7 77 [770] 169 [660] 25 [440] 3 [470] 2 � 103 [3.70]SSF (Juncus) 6.64 10 60 [820] 149 [700] 14 [690] 2.8 [500] 1 � 103 [40]
[3] 15e30 1.3 8 1.86 621 241 FWS (Typha) 0.9 19.28 10.8 73 [88] 20 [93] 26 [86] 16 [48] 32 [48] 2.4 [54] 7.2 � 104 [4.5]
SSF (Typha) 0.8 21.69 5 53 [91] 22 [92] 12 [94] 24 [23] 40 [34] 2.1 [60] 8.3 � 104 [4.5]
[4] 26e29 106 SSF (unplanted) 3.53 34.5 1.85 71 [340] 18 [110] 5.5 � 106 [0.40]SSF (Phragmites) 32.6 1.96 47 [560] 15 [270] 4.1 � 106 [0.50]SSF (Typha) 32.1 1.99 42 [610] 16 [230] 3.6 � 106 [0.60]SSF (unplanted) 1.03 14.5 1.2 38 [650] 12 [630] 2.8 [510]
[5] 18e33 117 SSF (Typha) 22 [790] 8 [740] 1.5 [690]SSF (Colocasia) 28 [750] 8 [750] 1.4 [750]
[6] 12e30 7 1050 525 PAVB ( � 2) +
SSF (Phragmitis)
78 14 (to CW) 4.95 100 [32] 11 [76] 11 [63] 20 [60] 38 [53] 2.8 [51] 1.3 � 104 [2.1]
[7] 14 25.5 11 0.72 315 166 SSF + SF (Juncus) 75 60 < 2.8 54 [81] 31 [83] 12 [92]
[8] 5e21 3.6 54 0.218 516 175 SSF + SF (Juncus) 18 7.2 5 28 [95] 15 [94] 20 [95] 17 [52]
[9] 20 7.2 6 2.2 528 145 SSF (unplanted)
SSF (Typha)
43.4 52.8 1.2 64 [88] 19 [88] 5 [97] 29 [-4] 33 [-14] 1.5 [-6] 4.6 � 105 [2.3]
42 [92] 15 [90] 3 [98] 25 [9] 27 [8] 1.2 [32] 1.3 � 105 [2.9]
[10] 30e38 1.3 8 1.86 492 152 FWS + SSF (Typha) 1.7 11 3 21 [96] 6 [97] 3 [98] 1.6 � 102 [9.2]
Averaged: 14e38 1.3e25.5 6e11 0.7e2.2 315e1050 117e525 0.8e78 7.2e52.8 1.2-10.8 47 [82] 17 [89] 12 [89] 17 [38] 33 [31] 36 [41] 1.9 [49] 6 � 104 [4.6]
a References: [1] de Sousa et al. (2001): [2] de Sousa et al. (2003); [3] El Khateeb and El-Gohary (2003); [4] Kaseva (2004); [5] Mbuligwe (2004); [6] Green et al. (2006); [7] Ruiz et al. (2008); [8] Barros et al. (2008); [9]
Dornelas et al. (2009); [10] El Khateeb et al. (2009).
b All concentrations are given in mg/L, except FC (in org/100 mL); average removal efficiencies are given in percentage, except FC (in log unit). Symbol ‘: calculated based on the UASB effluent.
c SSF: Subsurface horizontal flow; SF: Superficial horizontal flow; FWS: Free-water surface; PAVB: Passively aerated vertical beds.
d Average values are calculated based on the system size and the selected operating conditions, with at least two data.
Table 14 e Recent studies and comparison of other post-treatment units coupled to UASB reactors.
Ref.a Post-treatment unit Concentration in final effluent (mg/L) [averageremoval efficiency (%)]b
Type Additive, dosage(mg/L)
Size (L) T (�C) COD (mg/L) COD BOD TSS NH4-N TN (TKN) TP (PO43-) FC Helminth
eggs
[1] DAF FeCl3, 65 5.1 23 133 20 2 (0.6)
Cationic polymer,7 5.1 23 133 45 14 (2.5)
[2] DAF FeCl3þcationic/non-
ionic polymer, 30 þ 1
5.1 23 150 23 (0.9)
[3] Ozonation - DAF FeCl3, 45e65,
activated carbon
259 51 23 (33) 0.7 3.5
[4] Two-staged flotation-UV 17e33 40 16 [78] 17 [60] 0.1 [98] 10 [5.8]
[5] CEPT-UASB-Zeolite column 1.5 33 55 45 [91] 0.3 [99] (0.5) [99] 0.5 [94] 105 [2]
[6] Thermal reactor 5 70 540 0c
[7] Cascade-sponge reactor 30 160 10 [99] 0.2 [99] 4 [99] 2.2 [89] 0.1 [99] 23 [9.4]
[8] Aerated fixed bedd 4320 9.7 103 65 [82] 25 [85]\ 32 [82] 43 [10] 1 [75]
21 107 66 [79] 18 [86] 15 [91] 39 [15] 2 [33]
27 173 54 [83] 11 [93] 10 [94] 30 [21] 3 [67]
[9] DHS-sand filtrationd 14870 7 216 70 [84] 9 [95] 49 [79] 34 [38]
23 250 68 [88] 8 [97] 45 [88] 25 [56]
a References: [1] Reali et al. (2001a); [2] Reali et al. (2001b); [3] Marchioretto and Reali (2001); [4] Tessele et al. (2005); [5] Aiyuk et al. (2004, 2006); [6] Borges et al. (2005); [7] Patel and Mungray (2011); [8]
Sumino et al. (2007); [9] Takahashi et al. (2011).
b All concentrations are given in mg/L, except FC( in org/100 mL) and Helminth eggs (in counts/L); average removal efficiencies are calculated based on the UASB effluent.
c Ascaris lumbricoides eggs as indicator.
d With a pre-denitrifying reactor.
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phosphate ions either by precipitation as ferric phosphate
with the soluble ferric ions or adsorption by chemical inter-
action with the ferric surface sites (Tessele et al., 2005). After
flotation, the effluent was disinfected with a low pressure UV
lamp, with a theoretical UV dosage of 25 mJ/cm2, achieving
low coliforms concentrations that were below the emission
standards (Tessele et al., 2005).
Aiyuk et al. (2006, 2004) used a chemically enhanced
primary treatment (CEPT) unit, in which ferric chloride and
polyelectrolyte (E10 polymer) were added, to pre-treat the
wastewater prior to introducing to a UASB reactor. The CEPT
was able to remove 80%, 85% and 73% of the phosphate, TSS
and COD concentrations of thewastewater. The CEPT effluent
with a low COD of 140 mg/L was then passed to the UASB
reactor which removed about 55% of the COD, resulting in
approximately 45 mg/L of COD in the UASB effluent. A zeolite
column was used to treat the UASB effluent and achieved an
almost complete removal of the ammonium nitrogen in the
effluent. The major drawbacks of this concept were ineffec-
tive removal of the faecal coliforms, generation of large
sludge volume and higher costs associated with the use of
chemicals. The authors justified that, in comparison to the
conventional activated sludge system, the total costs of
chemicals and additives were favourably low due to the
simplicity, low technology and thus low investment and
operating costs of the system (Aiyuk et al., 2006, 2004). On the
other hand, Borges et al. (2005) showed that the biogas
produced from a 416-L UASB reactor was sufficient to
accomplish the thermal hygienisation of all excess anaerobic
sludge produced, leading to a simplified and fully self-
sustainable solution for sludge hygienisation, especially for
reuse in agricultural purposes.
Similar to the operation of the DHS unit, Patel andMungray
(2011) used a cascade-sponge reactor (CSR) to treat UASB
effluent and obtained excellent removal efficiencies of COD,
BOD, TSS, TN, TP, TC and FCwhichmet the local standards for
surface discharge inmost of the developing countries andwas
superior to the performance of the UASB-DHS system. In the
CSR, a number of polyurethane sponges were cascaded and
UASB effluent was allowed to flow through the cascaded
sponges by gravity, permitting natural aeration (Patel and
Mungray, 2011). Takahashi et al. (2011) also extended the idea
of DHS units to integrate a sulphur-redox reaction for
enhanced organic removal, particularly under psychrophilic
condition, which is also amodification of theirwork in Sumino
et al. (2007). The authors employed a denitrifying reactor (DN)
packed with PUR sponge sheets, a UASB reactor, a DHS and
finally a sand filtration bed. The sludge from the bottompart of
the DHS system was recirculated at a ratio of to the influent
point of the DN reactor fromwhere the sand-filtrated washing
water containing suspended solids and the reproduced
sulphate were returned. The results showed that with HRT of
12 h and recirculation ratio of 2, the sulphide produced in the
UASB was completely oxidized to sulphate by the sulphur-
oxidizing bacteria and the sulphur-reducing bacteria contrib-
uted to organic matter degradation in the UASB reactor, thus
making the sulphureredox reaction feasible for operation at
low temperatures. These systems however require further
investigations on the durability of the sponges, including any
clogging, cleaning or disposal issues.
3. Recent advances in wastewater treatmenttechnologies
3.1. Membrane bioreactors (MBRs)
Recently, membrane bioreactors (MBRs) have been receiving
increasing applications due to their capability of producing
high-quality effluents that comply with most water recla-
mation standards. Membrane bioreactors use membrane
instead of a final clarifier to separate solids, including
microorganisms and liquid. There are two types ofmembrane
bioreactor design e one uses a side-stream mode outside the
aeration tank, and the other has the membrane unit
submerged in the aeration tank (Forster, 2003); the latter
being more favourable in MBR application due to its more
compact configuration and less energy consumption (Huang
et al., 2010). Membranes of different materials such as
ceramic or polymeric (e.g. polyvinylidene fluoride (PVDF),
polyethylene, polyether sulfone (PES), polyvinyl chloride
(PVC)) generally have pore sizes of 0.1e0.5 mm, and may be
configured as plate and frame, hollow fibre or tubular units
(Huang et al., 2010; Gander et al., 2000). Membrane fouling due
to the accumulation of retained matter at the membrane
surface and high cost of membranes have been the major
obstacles for the widespread of this technology. Recent
reviews have comprehensively discussed the recent progress,
fouling mechanisms and abatement strategies, applications
and research needs of MBRs in treating different wastewaters
(Huang et al., 2010; Meng et al., 2009).
Table 15 presents a few experimental studies that have
used MBRs to treat UASB effluent. MBRs have been shown to
achieve excellent removal efficiencies of turbidity, SS, COD
and ammonium nitrogen. Therefore present research studies
have been focussing more on TN, TP and pathogen removal.
As shown in An et al. (2008, 2009), a UASB-MBR system
enabled simultaneous nitrogen removal and methanogenesis
when treating low-strength synthetic wastewater enriched
with organic carbon and ammonium chloride. At a low C/N
ratio, the TN removal efficiency increased from 48.1% to 82.3%
when the sludge recirculation ratio was increased from 50% to
800%, via shortcut biological nitrogen removal process during
with the ammonium nitrogen was oxidized to the form of
nitrite instead of nitrate, consuming less TOC (or COD)
(An et al., 2008, 2009). A recycling ratio of 400% was recom-
mended to obtain high carbon and nitrogen removal effi-
ciencies over 56.3% of methane in the biogas produced
(An et al., 2009). It has been shown in the lab-scale and pilot-
plant studies for the treatment of olive mill wastewater that
a hybrid system consisting of an electro-Fenton reaction fol-
lowed by anaerobic digestion and ultrafiltration as a post-
treatment was able to completely detoxify the anaerobic
effluent, and satisfactorily remove COD, TSS, polyphenols and
lipids (Khoufi et al., 2009, 2006). Herrera-Robledo et al. (2011)
also showed the effectiveness of a UASB-MBR in producing
an effluent with COD, SS, pathogen (faecal coliforms) and
parasite ova contents that met Mexican municipal waste-
water reclamation criteria. Their investigations also indicated
that a partial or mild cleaning procedure using chlorine
(NaClO at 300 ml/L, for 30 min) resulted in 13% removal of
Table 15 e Recent studies and comparison of some advanced treatment units coupled to anaerobic digesters.
Ref.a Post-treatment unit Concentration in final effluent (mg/L) [averageremoval efficiency (%)]b
Substrate,COD (mg/L)
Type Size (L) T (�C) COD BOD TSS NH4eN TN TP FC Helmintheggs
[1] Synthetic, 150c Membrane reactor 6 28e32 3c [98] 0.6 [98] 7e19 [48e82]
[2] Olive mill wastewater, 95 Membrane reactord 5.3 [94] 1.8 [91] 0 [100]
[3] Raw sewage, 445 Membrane reactor 22 33 [93] 0 [99] 14 3 [73] 0 [6.6] <1 [99]
[4] Pig excreta and
kitchen garbage, 2549cElectrochemical cell 0.3 25 1200c [53] 0 [100]
[5] Poultry manure
wastewater, 1840
Electrocoagulation 0.8 32 184 [90]
[6] Dairy, 5000 Solar photocatalysis 1 234 [95] 124 [96]
[7] Dye, 176e Ozonation 0.2 23 35e [80]
Photo-Fenton 72e [59]
[8] Poultry manure
wastewater, 1760
Fenton 0.5 32 88 [95]
[9] By-product water
from mechanical
thermal expression, 958
Ozonation 0.06 345 [64]
Coagulation 0.05 307 [68] 8 [97]
a References: [1] An et al. (2008, 2009); [2] Khoufi et al. (2009); [3] Herrera-Robledo et al. (2011); [4] Lei andMaekawa (2007); [5] Yetilmezsoy et al. (2009); [6] Banu (2008); [7] Garcıa-Montano et al. (2008); [8]
Yetilmezsoy and Sakar (2008); [9] Artanto et al. (2009).
b All concentrations are given in mg/L, except FC (in org/100 mL) and Helminth eggs (in counts/L); average removal efficiencies are calculated based on the UASB effluent.
c TOC in mg/L.
d Pre-treated with electro-Fenton process prior to UASB reactor.
e dissolved organic carbon (DOC).
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fouling mass per unit area, and the remnant fouling layer was
caused in part, by biologically-induced mineralization mate-
rials which were synthesized in response to cleaning proce-
dure and may be the basis of irreversible membrane fouling
(Herrera-Robledo et al., 2011). More studies are on-going to
develop cost-effective membranes, membrane-fouling
control and optimisation, including thorough investigations
of the fouling mechanisms, as well as operational optimisa-
tion and design.
3.2. Advanced oxidative processes (AOPs)
In recent years, advanced oxidation processes (AOPs) have
been studied extensively for different wastewaters due to
their abilities to treat contaminated wastewater (including
chlorinated solvents, pesticides, polychlorinated biphenyls,
phenolics, fuel hydrocarbons, cyanides and other organic
compounds ranging from a few thousand milligrammes per
litre to less than 1 mg per litre). AOPs utilise the highly reactive
intermediates, especially hydroxyl radicals (OH�) to degrade
even the most recalcitrant molecules into biodegradable
compounds through oxidation or mineralization under near
ambient temperature and pressure, yielding carbon dioxide
and inorganic ions (Malato et al., 2009; Matilainen and
Sillanpaa, 2010). The versatility of AOPs is featured by its
non-selective attack, based on various methods, such as the
photolysis of hydrogen peroxide (H2O2) and ozone (O3), the
heterogeneous photocatalysis and the homogeneous photo-
Fenton, electrochemical oxidation, catalytic oxidation and
ultrasound radiation, as listed in Table 16. A number of
comprehensive reviews of published oxidation and AOPs
studies are conducted on different types of wastewaters
(Malato et al., 2009; Matilainen and Sillanpaa, 2010; Pang et al.,
2011; Chong et al., 2010; Oller et al., 2011). High operational
costs have always been the major problem in all AOPs.
Therefore, several studies have focused on coupling AOPs
with other treatment units, such as anaerobic digesters. As
shown in Banu (2008), solar titanium dioxide-photocatalysis
was able to remove 62% of COD from the primary anaerobic
treatment, resulting in 95% of global COD removal in dairy
wastewater. Ozonation post-treatment has also been shown
to allow great mineralization and COD removal efficiency
Table 16 e The common advanced oxidation processes(AOPs).
Recent intensive review papers Reviewed technologies
Matilainen and Sillanpaa (2010) O3/H2O2
Oller et al. (2011) O3/UV
Malato et al. (2009) UV/H2O2
Pang et al. (2011) TiO2/UV
Chong et al. (2010) Solar photocatalysis
Kim and Ihm (2011) H2O2/catalyst
Mahamuni and Adewuyi (2010) Fenton
Photo-Fenton
Solar photo-Fenton
Electrochemical oxidation
Ultrasound
Wet air oxidation
(Garcıa-Montano et al., 2008; Artanto et al., 2009). Garcıa-
Montano et al. (2008) showed that ozonation at pH 10.5 out-
competed photo-Fenton catalytic process in treating dye
(Cibacron Red FN-R) wastewater. Almost complete COD
removal (99.3%) was obtained when Fenton’s oxidation
(400 mg/L Fe2þ/L and 200 mg H2O2/L) was coupled to a UASB
reactor to treat raw poultry manure wastewater (Yetilmezsoy
and Sakar, 2008).Lei and Maekawa (2007) showed the feasi-
bility of applying electrochemical treatment, one of the AOPs,
using a titanium/platinum-iridium oxide (Ti/PteIrO2) elec-
trode to treat anaerobic effluent. It was shown that using an
electric current of 1 A and a sodium chloride dosage of 1%, the
effluent produced after 5 h was free of ammonia (with some
nitrate produced), and good removal efficiencies in TOC, IC
and turbidity were achieved (51%, 74% and 96% respectively).
More studies are in demand to reduce the costs, investigate
full-scale applications, and develop degradation kinetics,
economic models and dynamics of the initial attack on
primary contaminants and intermediate species generation
(Oller et al., 2011).
3.3. Microbial fuel/electrolysis cells (MFCs/MECs)
Recently in wastewater treatment, microbial fuel cell (MFC)
technologies have gained intense interests due to their good
temperature tolerance and requirement of shorter retention
times compared to the anaerobic digestion processes. Most
importantly, unlike the anaerobic digestion processes in
which the biogas produced has to go through the scrubbing
process (to remove hydrogen sulphide) and the combustion
process in a gas motor, MFCs can simultaneously capture
energy in the form of electricity or hydrogen gas (Logan, 2008;
Rismani-Yazdi et al., 2008). A microbial fuel cell usually
consists of two chambers e an anode chamber and
a cathodic chamber, separated by a proton exchange
membrane (PEM), as shown in Fig. 7. Typical MFCs used in
wastewater treatment are the flat-plate MFCs (Min and
Logan, 2004), upflow MFCs (He et al., 2005) and stacked
MFCs (Aelterman et al., 2006a). Substrate enters the anode
Fig. 7 e Schematic of a double-chamber microbial fuel cell
(reprinted from Rismani-Yazdi et al. (2008). Copyright
(2008), with permission from Elsevier).
Fig. 8 e Important parameters for anaerobic digestion of
domestic sludge (adapted from Weiland and Rozzi (1991)).
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3459
chamber and comes into contact with the bacteria grown on
the anode. The bacteria, which act as a catalyst, degrade the
organic matter and directly convert the chemical energy into
electrical energy, generating electrons (e�) and protons (Hþ)(Logan, 2008; Rismani-Yazdi et al., 2008; Zhou et al., 2011).
Apart from the double- and single-chamber MFCs, one of the
recent developments is the microbial fuel cells with a power
supply to the chamber, thus known as the microbial elec-
trolysis cells (MECs), have also gained increased interests,
due to their ability to produce hydrogen gas (instead of
electricity in a MFC) by electrically connecting the oxidation
of organic material at a biological anode to the reduction of
protons at the cathode (Rozendal, 2007; Logan et al., 2008). As
outlined in many reviews, unlike anaerobic digesters which
have already been well established (although the effluent
from the anaerobic digesters still needs polishing before it is
discharged to a receiving body), there is a limited number of
applications of MFCs in wastewater treatment, in another
word, the MFC technology is still in its infancy (Logan, 2008;
Rozendal, 2007; Aelterman et al., 2006b). In addition, many
authors have suggested that the combination of the waste-
water treatment units with MFC/MEC would be beneficial in
terms of COD removal and energy recovery (Logan, 2008;
Aelterman et al., 2006b; Yang et al., 2011). This is because not
only can the MFC/MEC helps in generating extra energy,
post-treating the effluent from an anaerobic digester with
a MFC/MEC can further remove the undesired wastewater
components such as the remaining organic matter and
nutrients. As shown in Sharma and Li (2010) in which
a single-chamber MFC (SFMFC) was used to treat the effluent
from a hydrogen-producing biofermentor (HPB), the combi-
nation of HPB and SFMFC improved the energy conversion
efficiency (ECE) and COD removal with the maximum energy
recovery and COD removal efficiency from MFC projected to
be 559 J/L and 97% respectively. MFC was also used to recover
phosphate from digested sludge as struvite (Fischer et al.,
2011). Many challenges remain, preventing one to exploit
the maximum power production possible by MFCs, such as
the material costs, especially the cathodes which currently
appear to be the ultimate elements controlling the cost of the
system, control of microbial community to shift towards
electrogenesis, testing with actual wastewaters and pilot-
scale studies (Logan, 2008; Rozendal et al., 2008).
4. Summary and Discussions
Fig. 8 shows the important parameters affecting the anaerobic
digestion of domestic sludge. Start-up is a very important step
affected by various factors e environment, operation, reactor,
inoculum and substrate. All these parameters stay in strong
interactions. High start-up efficiency leads to increased
product value, yielding biogas, biosolids and effluent of higher
qualities. The ultimate goal of the anaerobic treatment of the
domestic sludge is to produce biosolids and effluent which
comply with the local standards for reuse or discharge, at the
same timemaximize the economic benefits. The success of an
anaerobic digester, in this case, a UASB reactor, thus is
a function of the environment, operation, reactor, inoculum
and substrate. The importance of start-up has led to increased
research, particularly on the improvement of reactor config-
uration and optimisation of the environmental and opera-
tional factors. Additionally, due to the limitations of UASB
reactors in producing a good-quality effluent which satisfies
the local standards, the post-treatment options evaluated in
the previous sections are also included in Fig. 8. Fig. 8 can be
expanded to incorporate other processes, such as the dew-
atering of biosolids, pre-treatment of sludge and purification
of the biogas produced. In this context, a few concluding
remarks are drawn, outlining the future research needs on
enhancing the start-up efficiency of UASB reactors, enlarging
the temperature constraints, and improving the effluent
quality for facilitating compliance with the specific treatment
requirements.
As shown previously in Table 4, the addition of multivalent
cations, polymers and the integration of packing media, bi-
circulation and zero-valent iron bed (with and without an
electric field) had positive impacts on the start-up of UASB
reactors in terms of granulation time, granule size and set-
tleability. Due to the high dependence of start-up efficiency on
substrate characteristics and temperature, more studies are
essential to determine the optimum loading rate, dosing
necessity and frequency in treating real domestic sludge
under the local climate conditions. Calculation of the zeta-
potential of bacterial surface as suggested by Yu Liu et al.
(2002) could be used to determine the optimum dosage and
the use of sedimentation theories (Bhunia and Ghangrekar,
2007) could also be useful in determining the minimum
granule size for the improvement of granulation and frac-
tional size distribution of granules in UASB reactors. Other
important factors such as the operational conditions (pH,
alkalinity and inoculum), nutrients, trace elements, heavy
metal contents, shear due to upflow and gas production
should also be considered and optimised to suit the local
wastewater and environmental conditions. The general
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03460
optimum environmental parameters are: pH 7.2e7.6, reactor
temperature 33e37 �C (mesophillic) or 50e55 �C (thermophil-
lic), COD:N:P ratio 100:(10-1):(5-1), ammonium nitrogen less
than 1000 mg/L (Weiland and Rozzi, 1991).
From the microbial point of view, the efficiency of reactor
start-up is markedly reliant upon the actions and interactions
of the microbial communities, for instance, growth rate,
dominating bacterial groups (hydrolytic, acidogenic, aceto-
genic/methanogenic) and shape (of the methanogenic
species), biomass yield coefficient, half velocity constant,
adaption rate and ability to excrete polysaccharides (Weiland
and Rozzi, 1991). Although the anaerobic digesters are broadly
implemented, there is still a lack of understanding and
inventories on the diverse microbial communities in the
anaerobic digestion processes. Thanks to the rapid growth of
technology efficacy, the gap between the macro- and micro-
scale optimisation can be bridged via the increasingly
advanced microbiological techniques developed over the
years (Table 17). Molecular techniques have been shown to
well predict the granular sludge disintegration and process
disturbance, visualise yet-to-be cultured microbial pop-
ulations within anaerobic sludge, therefore allowing the early
recognition of potential operational problems and speculation
of their in situ function (McHugh et al., 2005; Sekiguchi et al.,
1999). The familiarisation and development of novel
molecular-based techniques would certainly lead to a better
understanding of the microbial ecology and therefore much
improved monitoring of microbes in the environment.
Previous studies (compiled in Tables 5, 7e13) have shown
that most of the two-staged systems are reasonably tolerant
to climate temperature conditions in the subtropical regions.
Table 17 e Advanced microbiological techniques involved in w2006).
Microbiological techniques Type
� Amplified rDNA Restriction
Analysis (ARDRA)
� 16S rDNA cloning and
sequence analysis
� Single-Strand Conformation
Polymorphism (SSCP) analysis
� Denaturing Gradient Gel
Electrophoresis (DGGE)
� Temperature Gradient Gel
Electrophoresis (TGGE)
� Terminal Restriction Fragment
Length Polymorphism (TRFLP)
� Length heterogeneity PCR
� Ribosomal intergenic spacer
analysis (Latif et al.)
� Random amplified polymorphic
DNA (RAPD)
Nucleic-acid b
� Confocal Laser Scanning
Microscopy (CLSM)
Fluorescent in
hybridisation
� FISH combined with
microautoradiography (MAR)
� Nuclear magnetic resonance (NMR)
As shown in Table 5, the coupling of an anaerobic hybrid,
anaerobic filter, CSTR, septic tank, lamella settlers with
a UASB reactor, or the use of two-stage UASB reactors yielded
higher organic matter removal efficiencies compared to
a UASB reactor alone under temperatures of 15e35 �C. In
addition, as shown in Tables 7e13, the use of the post-
treatment units e AS, SBR, DHS or BF system also achieved
satisfactory performance independent of subtropical climate
temperature fluctuations. However, there is a lack of studies
on the capability of the systems under low temperatures of
less than 15 �C, which is often the winter season. As shown in
Lettinga et al. (2001), stable methanogenesis was observed at
temperatures as low as 4e5 �C in an expanded granular sludge
blanket (EGSB) which resembles the UASB reactor with
a higher upflow velocity. Some studies have also shown that
more diverse methanogenic Archaea were present at
psychrophilic temperatures than at mesophilic temperatures,
with an abundance of Methanosaeta well established in low
temperatures and utilise only acetate compared to Meth-
anosarcina which consume both acetate and hydrogen at
mesophilic temperatures (Gomec et al., 2008; Peng et al., 2008;
Dhaked et al., 2010; Kotsyurbenko, 2005). In other words, at
low temperatures, homoacetogens out-compete hydro-
genotropic methanogens, forming acetate first, followed by
methane production, as opposed to the direct conversion from
H2/CO2 to methane at high temperatures. Dhaked et al. (2010)
indicated that biodigesters (made of fibre-reinforced plastics
and with polyurethane insulation) with immobilised matrices
that were inoculated with low temperature adapted inocula
(adapted to 10 �C) have been showing promising competency
in treating humanwaste in a glacier for the last 12 years in the
astewater treatment (McHugh et al., 2003; O’Flaherty et al.,
Function
ased Describe microbial population
structures and dynamics
situ
(FISH)
Establish quantitative abundance and
spatial distribution of community groups
and individuals within granules
Determine the specific uptake of organic
and inorganic radiolabeled substrate,
allowing supposition of function
Observe transport processes within
a biofilm or granule slice
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3461
Himalayas, producing biogas with a methane content of
68e73%, and achieved 90% reduction in volatile solids (VS) in
the odour-free effluent. Therefore, in light of the eco-friendly
nature of psychrophilic high-rate anaerobic digesters without
heating, and in order to enlarge the temperature constraints,
more studies are required to investigate the archeal pop-
ulation and their possible mechanisms involved for anaerobic
wastewater treatment at low temperatures.
Fig. 9 gives an indication of the effectiveness of the
common post-treatment units in achieving the indicated
averaged levels of effluent quality, summarized from the
Fig. 9 e Performance comparisons of various UASB-post treatm
decade in achieving the indicated levels of effluent quality (not
operating conditions and only those systems achieved better pe
BOD; (d) TSS; (d) NH4eN; (e) TN; (f) TP; (g) FC; (h) EC.
studies over the recent decade. The coupling of a post-
treatment unit to a UASB reactor is effective at removing the
residual organic matter, suspended solids and pathogens
under certain conditions. However, the substantial reduction
of the organicmatter in the effluent often causes difficulties in
nitrogen and phosphorus removal (Chernicharo, 2006).
Among the eleven UASB-enhancement units of interest
(including the variants) shown in Fig. 9, it is apparent that
there is still a lack of studies on the removal of nutrients
(TN and TP concentrations) and pathogen counts (TC, FC and
Helminth eggs). Most two-staged systemswere able to achieve
ent systems summarised from the studies over the last
e that the average values were based on the selected
rformances than UASB reactors were included). (a) COD; (b)
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03462
excellent removal efficiencies of COD, BOD and TSS. However,
only six out of the total studied systems e two-stage UASBs
and UASB reactors coupled with AS, SBR, DHS, DAF and CW
managed to produce an effluent that was compliant with
Class D standard of the organic matter (COD or BOD and TSS)
from the Australian Guidelines for Sewerage Systems
(Australian Guidelines for Sewerage Systems - Effluent
Management, 1997) (Table 6). More specifically, SBR and DAF
systems showed the best organic matter removal efficiencies
among all, followed by AS, two-stage UASB, DHS and CW
systems. Relatively complete information has been presented
in the studies evaluating the feasibilities of CW, pond andDHS
systems. CW systems were able to obtain outstanding
performance not only in removing organic matter, but also
nutrients and pathogens in the UASB effluent. DHS and pond
systems also provided a good effluent quality with improved
nutrient and pathogen removal efficiencies.
In addition to the removal efficiencies, others factors such
as temperature, land requirement, associated costs, volume of
sludge generated and sustainability need to be considered and
optimised. In order to provide a preliminary reference in
evaluating, justifying and enhancing the potential application
of each system, an overall comparison and the benefits and
drawbacks of the reviewed systems are given in Tables 18 and
Table 18 e An overall relative comparison of the reviewed cou
Coupled system In compliance with Australianguidelines (Class D, Table 6)?a
Effect otemperat
BOD, TSS TN TP FC
AF þ AH/UASB N L
UASB þ CSTR N L
HUSB/UASB þ UASB Y L
UASB þ AS Y N L
UASB þ SBR Y Y Y N L
UASB þ BF N Y N M
UASB þ DHS Y N Y L
UASB þ Pond N Y Y N H
UASB þ RBC N Y L
UASB þ CW Y N Y Y M
UASB þ DAF Y N Y N L
a Y: yes; N: no; blank: data not available.
b Reference: von Sperling Chernicharo (2005).
Scale:0 0.2 0.4
Average land requiremen(m2/inhab)
L M H
c Reference: Sperling and Chernicharo (2005), Khan et al. (2011).
Scale:0 30 60
Average sludge for disposa(L/inhab/year)
L M H
d Costs are based on Brazilian experience (Basis: year 2002), adapted fro
Scale:0 30 60
Average construction costs (US$/inhab)
L M H
e Costs are based on Brazilian experience (Basis: year 2002), adapted fro
Scale:0 1.75 3.5
Average operational & maintenance costs (US$/inha
L M H
19 respectively. Table 18 shows that among the reviewed
coupled systems, SBR, DAF and DHS systems have been
found to be the preeminent post-treatment candidates in
terms of contaminant removal efficiency, temperature, land
requirement, sludge quantity and associated costs. However,
challenges as outlined in Table 19 remain to be tackled. More
studies are required to address the limitations for improved
functionality of the coupled systems in achieving an effluent
quality that is in compliance with the local stringent obliga-
tions for reuse or discharge. The comparisons made here are
forcibly superficial and notwithstanding. There is no ideal
system applicable to all conditions. Therefore, to feasibly
apply a suitable treatment method, the parametric studies
considering the important factors outlined in Fig. 8 should be
considered on a case-by-case basis.
5. Future prospects
It has been shown that most coupled systems consisting of
a UASB reactor and a post-treatment unit are feasible for
domestic sludge treatment in tropical and subtropical regions,
although further improvement on nutrient and pathogen
removal is necessary to meet the local reclamation or
pled systems.
fure
Full-scale operation
Landrequirementb
Sludgequantityc
Constructioncostsd
Operational &maintenance
costse
L L L L
L M M M
L M M H
M H M H
L M L M
H L L M
M H H H
H M M
L M L M
t
l
m Sperling and Chernicharo (2005).
m Sperling and Chernicharo (2005).
b)
Table 19 e Summary of pros and cons for various post-treatment options.
Post-treatment unit Advantage Disadvantage Ref.
Sequencing-batch
reactors (SBR)
1. High efficiency and operational
flexibility (variation of cycles)
2. Effective separation of solid and
liquid phases due to
non-interaction of sedimentation
and liquid movement, thereby
resulting in lower TSS and VSS,
lower production of excess sludge
3. Simple reactor configuration due
to the need of only three reactors
4. Simple mechanical equipment
as only SBR has moving parts
5. Satisfactory independence
from climate conditions
6. Low land requirement
7. Satisfactory nutrient removal
1. Start-up is not very easy
2. The desired microbial
populations requires appropriate
control of anaerobic and
aerobic residence times, thus
the need of high sophistication
in the control units for efficient
organic removal
3. Low coliforms removal
4. Need of more studies on
nutrient and pathogen removal.
Especially for simultaneous
removal of nitrogen and
phosphate
5. Great installed power than
the other activated sludge
systems
6. Treatment and disposal
of sludge is required
von Sperling (1996); Torres
and Foresti (2001); Guimaraes
et al. (2003); von Sperling and
Chernicharo (2005);
Chan et al. (2009);
Moawad et al. (2009)
Downflow-hanging
sponge (DHS)
1. Low cost and easy maintenance
2. Satisfactory independence
from climate conditions
3. No external aeration input
4. No withdrawal of excess
sludge required
5. Excellent settleability
6. Favourable for simultaneous
nitrification and denitrification
7. DHS systems very close to
plug-flow, favouring coliforms
removal, although further
disinfection may be needed
to meet standards
1. Need of proper
influent-distribution system
design for full-scale plants
for the first generation
2. Less favourable in the case
of shock loads especially in
terms of nitrogen removal
3. Need of further studies on
the durability of the packing
material and the physical and
biological interaction for
coliforms removal
4. Scale-up problem
.
Agrawal et al. (1997)a,b;
Machdar et al. (1997);
Machdar et al. (2000); Uemura
et al. (2002); Tandukar et al.
(2005, 2006a); Tandukar et al.
(2006b); Tandukar et al. (2007);
Sumino et al. (2007); Oliveira
and von Sperling (2009);
Takahashi et al. (2011)
Activated
sludge (AS)
1. High efficiency and
operational flexibility
2. High efficiency in BOD removal
3. High resistance to variable
flow and toxic loads
4. Satisfactory independence
from climate conditions
5. Consistent nitrification
6. Sludge stabilisation in the
reactor itself
1. High mechanisation level
2. High construction and
operational costs
3. High energy consumption
4. Sophisticated operation due
to the need for treating a
substantial amount of sludge
(although stabilization is
not required)
5. Problems of bulking and
production of stable foam
6. Limited coliforms removal
7. Requires complete treatment
and final disposal of the sludge
8. Possible environmental
problems with noise
and aerosols
von Sperling et al. (2001); von
Sperling and Chernicharo
(2005); La Motta et al. (2007);
Tawfik et al. (2008); Cao and
Ang (2009); Mungray and
Patel (2011)
Trickling filter (TF) 1. Flexible for short HRTs
2. High efficiency in BOD removal
3. Simpler than activated sludge
4. Low land requirements
5. Low operational cost
1. Less operational flexibility
than activated sludge
2. Need of packing media
3. Ammonia removal not
satisfactory
4. Higher construction costs
5. Need of complete sludge
treatment and disposal
6. High head loss
von Sperling (1996);
Chernicharo and Nascimento
(2001); Goncalves et al.
(2002); Pontes et al. (2003);
de Almeida et al. (2009);
Oliveira and von Sperling
(2009); Tawfik et al. (2010)
(continued on next page)
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3463
Table 19 e (continued )
Post-treatment unit Advantage Disadvantage Ref.
Stabilising pond (SP) 1. Simple, low maintenance,
suitable for rural communities
2. No energy requirement
3. High COD, BOD removals
regardless of climate
temperature fluctuations
4. High SS production (algae)
5. Low rate of sludge accumulation
especially in tropical regions,
if efficient anaerobic treatment
(e.g. with a UASB reactor)
is applied
1. Large land requirement
2. Need of de-sludging (some
authors have shown if coupled
with efficient anaerobic
treatment, the de-sludging
of ponds becomes optional)
3. May subject to high evaporation
rates, therefore water loss
and increased salinity
4. High SS concentration
5. Recovery of nutrients and
removal of faecal coliforms
affected by temperatures
Cavalcanti et al. (2002);
von Sperling et al. (2002);
von Sperling et al. (2003); von
Sperling and Mascarenhas
(2005); von Sperling and de
Andrada (2006); Sato et al.
(2006); El-Shafai et al. (2007);
Oliveira and von Sperling
(2009); Walia et al. (2011)
Rotating-biological
contactor (RBC)
1. Low-shear e no distribution
problems, no recirculation
required
2. Relatively easy scale-up
3. Resistance to high
hydraulic and organic loadings
4. Low energy requirement
5. Low maintenance
6. Good nitrification efficiency
7. Possible for complete removal
of E.coli
1. More units or higher HRTs
required for treatment at
lower temperatures
2. Excess sludge removal not
studied
3. Needs frequent motor and
bearing maintenance, problem
of excessive film build up on
disc after power failure,
leading to the possibility of
motor failure
Tawfik et al. (2002a);
Tawfik et al. (2002b);
Tawfik et al. (2003);
Tawfik et al. (2005);
Tawfik and Klapwijk
(2010); Kassab et al. (2010)
Constructed
wetland (CW)
1. Simple construction,
operation and maintenance,
suitable for rural communities
2. Enhanced removal of SS
and COD improves the overall
performance and reduce the
gravel-bed clogging problem
3. Utilization of natural processes
4. Low surplus sludge
1. High land requirement
2. Problems of clogging in the
gravel over time, especially
near the inlet, shortening the
lifespan of wetland
3. Contingent upon microbial
activity, HRT, loading rate,
temperature
4. Limited nutrient and
pathogen removal
5. May require support media/
engineered structures
6. May subject to high evaporation
rates, therefore water loss and
increased salinity
7. Nutrient removal is susceptible
to the ageing phase of the plants
8. Need of appropriate
practices in monitoring the
excess sludge removal from
the UASB reactor
de Sousa et al. (2001);
de Sousa et al. (2003);
El Khateeb and El-Gohary
(2003); Kaseva (2004);
Mbuligwe (2004); Green
et al. (2006); Ruiz et al.
(2008); Barros et al. (2008);
Dornelas et al. (2009);
El Khateeb et al. (2009)
Dissolved-air
flotation (DAF)
1. Compact, high loading rates,
small flocculation tanks
2. Low detention time
3. Great operational versatility
4. Partial striping of the volatile
gases
5. Resistant to shock discharges
and rapid flow oscillations
6. Permits high quality effluent
with low coagulant consumption
(low phosphorus, TSS and
organic matter concentrations)
7. Good to excellent removal
of Crytosporidium and Giardia
8. Works with already thickened
sludge, thus requires less
post-processing
1. Need of coagulant and/or
flocculant
2. Auxiliary operating costs
caused by the recycle water
which requires pumping of
up to 10% of the feed flow
to between 400 and 700 kPa
3. Needs to be protected from the
weather to prevent float
freezing leading to settling
of previously floated solids
caused by snow and rain
4. Poor removal of ammonium
and faecal coliforms
Reali et al. (2001a);
Reali et al. (2001b);
Chernicharo (2006);
Crossley and Valade (2006)
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03464
Fig. 10 e A feasible future wastewater treatment process train for optimising energy recovery from domestic wastewater.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 0 3465
discharge standards. MBRs and AOPs have shown excellent
performances and almost complete removal of contaminants
can be achieved in the former. Unlike aerobic post-treatment
units, MFC/MEC systems capture energy from the wastewater
and transform into electricity or hydrogen gas. In order to
meet the Australia Renewable Energy Target (RET) scheme
which is designed to deliver on the Government’s commit-
ment to ensure that 20% of Australia’s electricity supply will
come from renewable sources by 2020, the operation of
a wastewater treatment plant that can rekindle power/elec-
tricity from biogas whilst achieve the local discharge stan-
dards via sustainable means becomes an important mission.
The authors hereby suggest a sustainable technology for
future domestic wastewater treatment in tropical and
subtropical regions. By combining the benefits from various
Fig. 11 e Estimated capital costs of microbial electrolysis cells (
volume). The costs were either estimated based onmaterials cur
capital costs assuming less expensive substitute materials (b) (r
permission from Elsevier).
enhancement unitseMBR, AOP andMFC/MEC, a process train
is suggested as in Fig. 10 for optimising energy recovery from
domestic wastewater.
Raw domestic wastewater, after the screening and grit-
removal preliminary treatment, enters the primary clarifier.
The primary effluent is then subject to biodegradability tests
which can be carried out by (i) analysing global parameters,
such as BOD, COD, DOC and oxygen uptake, (ii) estimating the
ratio of BOD/CODor theaverageoxidation state; (iii)measuring
the bacterial growth rate; (iv) assessing toxicity by measuring
theECvaluebyMicrotox test; or (v) usingkineticmodels (Sarria
et al., 2003). The biodegradable fraction of the primary effluent
is allowed to flow to the MFC/MEC(s) for energy recovery into
electricity or hydrogen gas. On the other hand, the non-
biodegradable fraction is treated in AOP(s), which is known
single-cell design; current density: 1000 A/m3 reactor
rently used in laboratory systems (a), or on predicted future
eprinted from Rozendal et al. (2008). Copyright (2008), with
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 4 3 4e3 4 7 03466
for its effective performance in breaking down the refractory
organics into simpler and biodegradable molecular forms,
beforebeing returned to theMFC/MEC(s). Theprimarysludge is
treated in anaerobic digester(s) for methane generation and
the anaerobic effluent is returned to the MFC/MEC(s) for
further utilisation of the carbon sources. The effluent from the
MFC/MEC(s) is subsequently directed to MBR(s) and AOP(s) to
allow complete removal of contaminants (including solids,
nutrients and pathogens). The complex interactions and thus
challenges involving all microbiological, physicochemical,
technical, and economical aspects remain to be resolved and
optimised in order to able to test the compatibility of the
proposed idea. As estimated by Rozendal et al. (2008), it is very
likely that the material costs of MFC/MECs, which have been
one of the major hurdles for practical implementation, will
drop at a steady pace due to development of advanced tech-
nologies, as delineated in Fig. 11. In viewof the growingprofiles
of the mentioned technologies, the suggested process train
would be a feasible, self-sustainable option for domestic
wastewater treatment in the near future.
Acknowledgement
The authors would like to thank Water Corporation of
Western Australia for the financial support of the project.
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