water reclamation and sustainability || sustainability of activated sludge processes

16
Sustainability of Activated
Sludge Processes

Water Rec

Copyright

Amir Mohaghegh Motlagh, Ramesh K. Goel
CIVIL & ENVIRONMENTAL ENGINEERING, UNIVERSITY OF UTAH,
SALT LAKE CITY, UTAH, USA

1. IntroductionFor a century, an activated sludge process has been used for removing organic matter

and nutrients in the water resource recovery facilities. The Federal Clean Water Act

passed in 1972 introduced the National Pollutant Discharge Elimination System, which

is a permit system for regulating point sources of pollution. Since then, the level of

regulated limits on pollutants in treated wastewater has become increasingly stringent.

Furthermore, loads on existing plants are increasing because of growth in urban areas.

Therefore, the needs for more efficient treatment procedures for wastewater are greater

than ever. One alternative to improve treatment efficiency could be to construct new and

larger basins, but in addition to the high cost of the construction, it is not possible in

many places because of the lack of land. Another alternative would be the introduction

of more advanced control and operating systems. This is expected to reduce the need for

larger volumes, improve the effluent wastewater quality, decrease the use of chemicals,

and save energy and operational costs. Sustainable solutions to the problems of

wastewater treatment will require the development of adequate information systems for

control and supervision of the process and reduce concentration of pollutants and

consequently the loading in the treatment processes.

On the environmental aspect, sustainable wastewater treatment needs to protect

drinking water sources, aquatic life, and recreational uses of waterways. The wastewater

industry must also minimize greenhouse gas emissions during the treatment process

and mitigate other impacts resulting from energy consumption and chemical use. The

importance of optimization of energy consumption, energy recovery processes, effi-

ciency of equipment and technology operations, and good management of energy costs

is growing in the field of wastewater treatment systems because of numerous factors

such as population and pollution growth, as well as increasing stringent regulations for

effluent quality and residual water reuse.

In a water resource recovery facility, organic matters and nutrients should be

removed from the wastewater before being discharged back into receiving water bodies.

The treatment process is essentially divided into mechanical, biological, and in some

lamation and Sustainability. http://dx.doi.org/10.1016/B978-0-12-411645-0.00016-X 391© 2014 Elsevier Inc. All rights reserved.

http://dx.doi.org/10.1016/B978-0-12-411645-0.00016-X

392 WATER RECLAMATION AND SUSTAINABILITY

cases chemical treatment. In the first stage, large debris and suspended solids are me-

chanically removed from the wastewater using bar screen, grit chamber, and in some

cases, primary clarification. After that, biodegradable waste including nutrients and

organic matters can be broken down biologically by a mass of microorganisms. Although

different biological processes exist, most water resource recovery facilities make use of

the activated sludge process.

Under more controlled conditions, the activated sludge process removes the biode-

gradable organics and nutrients from wastewater by maintaining large numbers of

aerobic and in some cases facultative bacteria. These bacteria use the components of

wastewater as a food source and also take part in biochemical reactions that convert the

soluble and colloidal material portion to disposable microbial aggregates (flocs) in the

aeration tank. Formation of flocculant settleable solids that can be removed by gravity

settling is an important feature of the activated sludge process.

The activated sludge process, which was devised in 1913 by Ardern and Lockett,1

consists of three basic components including a reactor to keep the microorganisms in

contact with the organic matters, a clarifier for the liquid–solid separation, and a sludge

recycling system for returning activated sludge to maintain the bacterial population and

active biomass, i.e., return activated sludge (RAS). Also, portions of settled biomassmust be

wasted from the system to prevent overloading of sludge, i.e., waste activated sludge (WAS).

Five-day biochemical oxygen demand (BOD5) is a surrogate for the strength of

wastewater. It is the amount of oxygen required by the bacteria in utilizing organic

carbon compounds during a 5-day period, and therefore, it is often referred to as the

carbonaceous BOD. Biological oxidation of ammonia nitrogen (NH4–N) to nitrite (NO2�)

and nitrate (NO3�) also requires oxygen demand, which is referred to as nitrogenous

BOD. As bacteria also require phosphorus for their metabolism, a small quantity of

phosphorus is also removed from the wastewater in the conventional activated sludge

process. In general, the activated sludge process can be a carbon (BOD5) removal pro-

cess with the possibilities of nitrogen conversion and phosphorus removal.

The activated sludge process is the most widely studied and used form of secondary

wastewater treatment and efficient removal of BOD, chemical oxygen demand (COD),

and nutrients can be achieved through the process. The process requires less space than

the biological filter and has the flexibility to be modified to meet specific requirements

such as nitrogen and phosphorus removal.

As mentioned earlier, in a conventional activated sludge system, a primary clarifier is

usually employed ahead of the aeration basin to decrease the bioreactor loading with

removing large inert solids and settleable organic matter in the raw sewage. In some

cases, the mixed liquor activated sludge developed in the aeration tank is settled in the

secondary clarifier and removed for further treatment along with the settled sludge from

the primary clarifier in an anaerobic digester prior to the disposal for beneficial use.

Microorganisms in the aeration tank utilize organic substances for respiration and syn-

thesis of new cells. In order to maintain constant mixed liquor suspended solids, the net

cell production must be removed with wastage biomass (Qw). A schematic diagram of

FIGURE 1 Conventional activated sludge process configuration.

Chapter 16 • Sustainability of Activated Sludge Processes 393

conventional activated sludge is illustrated in Figure 1. Conventional activated sludge is a

process in which influent and returned activated sludge enters at the head of the aeration

tank and travels through the tank at a constant rate to the point of discharge. The solid

retention time (SRT) is generally less than 15 days, usually best between 3 and 10 days.2

2. Nutrients in the EnvironmentAccording to the Liebig’s law of the minimum, the yield of plants can be limited by the

nutrients present in the environment in the least quantity of the plant’s demands for the

growth. Although many mineral resources are required for plant growth, however, it has

been found that inorganic nitrogen and phosphorus are the two principal nutrients that

can limit the growth of terrestrial plants.3 On the other hand, the supply rate of nitrogen

and phosphorus can considerably affect the growth of algae in freshwater and marine

ecosystems.4

Ecosystems can be classified on the basis of their supplies of growth-limiting nutri-

ents. Waters having poor supplies of nutrients are termed oligotrophic, intermediate

nutrient supplies are termed mesotrophic, and those having large supplies of nutrients

are termed eutrophic. In addition, hypertrophic is the term used for ecosystems having

greatly excessive amounts of nutrients input. As described in Table 1, different water

bodies including lakes,5 streams,6 and marines7 have different nutrient thresholds for

being categorized in eutrophic state.

Table 1 Threshold Concentration of Total Nitrogen andTotal Phosphorus for Assessing Eutrophic Status

Total Nitrogen(mg/m3)

Total Phosphorus(mg/m3)

Lakes 650–1200 30–100Streams >1500 >75Marines 350–400 30–40


Eutrophication is the process by which water bodies become increasingly rich in plant

biomass as a result of increase in nutrient supplies, mainly nitrogen and phosphorus.

This excessive growth of plant biomass, which is mainly phytoplankton species and

aquatic macrophytes, can block the sunlight, eventually leading to the loss of light-

dependent photosynthetic organisms. Furthermore, accumulation and decomposition

of dead organic matter results in the depletion of oxygen and can generate harmful gases

such as methane and hydrogen sulfide because of the anoxic condition.

3. Nutrient Removal in Wastewater TreatmentBased on a recent study, two-thirds of US coastal systems are moderately to severely

impaired because of nutrient loading8 and therefore, decreasing the nutrients in the

effluent of water resource recovery facility, which is discharged into water bodies, is one of

the critical means to control and minimize eutrophication issues. Nitrogen and phos-

phorus are essential components to the growth and sustenance of microorganisms and

therefore, some of the nutrients are removed naturally in any biological system. In con-

ventional biological wastewater treatment, 20–30% of influent nitrogen and phosphorus

can be naturally biodegraded and removed.9 Taking advantage of this fact, the treatment

system can be engineered for biological nutrient removal (BNR) to remove nutrients

greater than that required for the bacterial metabolism and growth. Biological nitrogen

and phosphorus removal are two major processes for BNR, which will be discussed next.

3.1 Biological Nitrogen Removal

According to the US Environmental Protection Agency,10 nitrogen in the ammonia form

is toxic to certain aquatic organisms. In addition, organic and inorganic forms of ni-

trogen may cause eutrophication issues in nitrogen-limited freshwater lakes, estuarine,

and coastal water.10 Nitrogen in the wastewater can be present in different forms

including ammonium (NH4þ), nitrate (NO3

�), and nitrite (NO2�) and as organic com-

pounds. Most of the nitrogen in the influent wastewater to the treatment plant is present

in the form of ammonium (NH4þ) or ammonia (NH3) depending on the pH. Gaseous

(NH3) and aqueous (NH4þ) ions are in equilibrium at a pH of 9, and ammonium is more

ubiquitous at pH lower than 9.

Biological nitrogen removal processes comprise a two-step procedure. In the first

step, by a specific group of autotrophic microorganisms, ammonium is oxidized to ni-

trite and then nitrate in an aerated zone. This process is called nitrification and can be

described by the following two chemical reactions:

NHþ4 þ 1:5O2 ¼ NO2 þH2Oþ 2Hþ

NO� þ 0:5O2 ¼ NO�
2 3
The first step of nitrification, which is the process of oxidation of NH4þ into NO2

�, is callednitritation. Nitratation is the second part of nitrification, which involves oxidation of

nitrite (NO2�) to nitrate (NO3

�).


As autotrophic bacteria are involved in this process, the carbon source for nitrifica-

tion for the cell growth is obtained from carbon dioxide, and presence of organic sub-

strate (BOD) is not required.

In the second step, the nitrate produced under aerobic conditions in the nitrification

process is reduced to nitrogen gas by certain heterotrophic bacteria in a process called

denitrification. The denitrification reaction requires anoxic conditions where the bac-

teria responsible for denitrification respire with nitrate instead of oxygen. A source of

readily biodegradable organic carbon is also required to be present as the carbon source.

The process can be summarized by the following chemical reaction:

4NO�3 þ CODþ 4Hþ ¼ N2 þ CO2 þH2OþOH�

By using these two biological processes, i.e., nitrification and denitrification, nitrogen is

biologically removed from wastewater. As described previously, an anoxic zone is

necessary for denitrification reaction. Anoxia is the presence of combined oxygen as

nitrate and nitrite and absence of free or dissolved oxygen (DO). Anoxic zones can be

placed either in the beginning of the process as predenitrification or in the end of the

process as postdenitrification (Figure 2).

As illustrated in Figure 2(A) in a predenitrification system, an extra recirculation

flow is usually used to transport the nitrate-rich wastewater to the anoxic zone. In a

predenitrification process, presence of oxygen in the anoxic zone because of internal

recycling makes denitrification less efficient, and supplemental carbon source is

required for the process. An additional anoxic zone can be included at the end of the

bioreactor to reduce the oxygen concentration before recirculating to the denitrifica-

tion zone.

FIGURE 2 (A) Biological nitrogen removal with predenitrification process. (B) Biological nitrogen removal withpostdenitrification process.

FIGURE 3 Basic principles of predenitrifaction with dynamics of BOD, NH4þ, and NO3

�.


A sufficient supply of carbon is a critical parameter for achieving efficient nitrogen

removal. If the influent wastewater carbon source is inadequate to support denitrifica-

tion, an external readily biodegradable organic carbon such as methanol, ethanol, ace-

tate, or glycerol can be added to enhance the process. If the postdenitrification process is

in place, a supplemental carbon source needs to be added. The basic principles of the

predenitrification process and dynamics of carbon, ammonium, and nitrate concen-

trations are illustrated in Figure 3.

Biological nitrogen removal through combination of nitrification–denitrification is

highly beneficial for domestic wastewater treatment in terms of lower carbon re-

quirements, reduced oxygen demand, and less biomass production.11

3.2 Biological Phosphorus Removal

As phosphorus is an important limiting nutrient, more stringent environmental regu-

lations are enforced to reduce the input of phosphorus in water bodies and to control

eutrophication. Phosphorus in the wastewater can be present in three forms: ortho-

phosphate, polyphosphate, and organically bound phosphorus.12 Orthophosphate is the

most stable form of inorganic phosphorus. Polyphosphate is the condensed form of

FIGURE 4 Typical configuration of Phoredox (A/O) process for biological phosphorus removal.


inorganic phosphorus in the wastewater, which cannot be precipitated by coagulants.

Organic phosphorus is bound to plant or animal tissue, formed by biological processes,

and generally precipitated and removed with the sludge.

Phosphorus in the wastewater can be about 90% removed by chemical precipitation

with divalent and trivalent salts such as alum and ferric chloride. However, a special

configuration of the activated sludge process can be carried out for achieving sustainable

biological phosphorus removal.

The metabolic process for biological phosphorus removal requires conditions that

enhance the amount of released phosphorus by a selection of microorganisms in an

anaerobic zone along with a sufficient supply of volatile fatty acids (VFAs). This group of

bacteria having a high capacity of polyphosphate accumulation stores the released

orthophosphate in the form of polyphosphate within their cells in an oxygen-rich

environment. Eventually, physical removal of biomass with WAS removes the phos-

phorus from the activated sludge system. The typical configuration of enhanced bio-

logical phosphorus removal (EBPR) is shown in Figure 4.

3.3 Nitrogen and Phosphorus Removal

Biological nitrogen and phosphorus removal requires that the activated sludge process

be modified to enhance the environment of the activated sludge to accomplish the

biological uptake and conversion of phosphorus and nitrogen. The separation of

anaerobic, anoxic, and aeration zones in the activated sludge process in conjunction

with carefully controlled internal sludge recycles is required to meet the needs of both

systems. Different activated sludge configurations will produce varying reliabilities of

nitrogen and phosphorus removal efficiencies. When removal of both nitrogen and

phosphorus is required, additional factors such as the need to add external carbon

source and the separation of anoxic and anaerobic zones must be considered.

Although both nitrogen and phosphorus removal systems use aerobic, anoxic, and

anaerobic environments, they have very different metabolic functions, process se-

quences, and different design and operating parameters. Sustainable removal process


configurations for both nitrogen and phosphorus are discussed in detail in the

following sections.

4. Microbiology of Wastewater TreatmentAs discussed earlier, two main bacterial groups are responsible for the activated sludge

processes. Aerobic heterotrophic bacteria are the main activated microorganisms that

obtain energy from carbonaceous organic matter in influent wastewater for their

metabolism and the synthesis of new cells. On the other hand, autotrophic bacteria

reduce oxidized carbon compounds as their carbon source for cell growth and synthesis

of new cells.

4.1 Nitrifying Bacteria

As previously mentioned, biological nitrogen removal is a two-step process comprising

nitrification and denitrification. Nitrification also consists of a two-step process of

nitritation and nitratation, which are performed by ammonia-oxidizing bacteria (AOB)

and nitrite-oxidizing bacteria (NOB), respectively. Nitrifiers are generally classified as

obligate chemolithotrophs and their sole carbon source is obtained through fixation of

dissolved inorganic carbon such as carbon dioxide.13 Energy is acquired by oxidizing

inorganic substrates such as ammonium and nitrate ions.

Because of low energy yield from the oxidation reaction, nitrifying bacteria have very

low reproductive rate. Therefore, long sludge retention is required to maintain the

number of the nitrifying community. Although the primary genera of nitrifying bacteria

are Nitrosomonas and Nitrobacter, there are several bacterial genera that belong to the

nitrifying bacteria group (Table 2). In wastewater treatment systems and activated sludge

processes, Nitrosomonas europeae and Nitrobacter agilis are the principle species of

nitrifying bacteria for the oxidation of ammonium and nitrite ions.14

Culture-independent studies on nitrifying bacteria revealed that ammonia mono-

oxygenase (AMO) is a key enzyme for ammonia oxidation. Furthermore, the gene

Table 2 List of AOB and NOB in the Activated Sludge

Energy Substrate Oxidized Product Bacterial Genus

NH4þ NO2

� NitrosococcusNitrosocystisNitrosolobusNitrosomonasNitrosospira

NO2� NO3

� NitrobacterNitrococcusNitrospira


encoding AMO for all three subunits of AMO, i.e., amoA, amoB, and amoC, can be a

function-specific target for detecting AOB in the wastewater treatment processes.

4.2 Denitrifying Bacteria

Denitrifying bacteria are a group of microorganisms that reduce nitrite or nitrate to

gaseous nitrogen compounds such as NO, N2O, and N2. Nitrate has become a critical

pollutant in the groundwater and surface water and in recent years, more research has

focused on the denitrification process. In addition, nitrous oxide (N2O) has up to

300-fold more efficient deteriorating effects on the ozone layer than carbon dioxide and

therefore, the N2O balance is critical to the natural environment.15

Denitrification metabolism is not limited to a specific microbial group and culture-

independent studies target the relevant functional gene clusters that encode enzymes

involved in the denitrification pathway. Nitrate reductase (nar), nitrite reductase (nir),

nitric oxide reductase (nor), and nitrous oxide reductase (nos) are classified in this

functional gene cluster.16,17 The respiratory system of denitrification process for utilizing

nitrate, nitrite, nitric oxide, and nitrous oxide is represented in Figure 5. All four modules

must be activated for the complete denitrification process to be accomplished.

Although a broad variety of bacterial groups have the capability of denitrification,

most denitrifiers belong to the alpha and beta classes of Proteobacteria. Most de-

nitrifiers are heterotrophic organisms that use organic carbon for their metabolism and

growth. However, there are autotrophic denitrifiers that utilize inorganic compounds

such as sulfur, hydrogen, ammonia, and nitrite. Culture-dependent research on acti-

vated sludge from different municipal wastewater treatment plants revealed the

FIGURE 5 Modular organization ofdenitrification.


phylogenetic diversity of denitrifiers.18 Rhodobacteraceae, Comamonadaceae, and

Pseudomonadaceae are the initial denitrifying Proteobacteria that were recognized.

Later on, much more important and diverse denitrifiers such as Epsilonproteobacteria,

Firmicutes, and Bacteroidetes were found. Wagner and Loy19 recognized the Azoarcus-

Thauera group of the Rhodocyclaceae as the most prominent denitrifier in industrial

wastewater treatment plants.

4.3 Anammox Bacteria

It is common to find wastewater characterized by high ammonium concentration and

low biodegradable organic matter content (low C/N ratio), which is difficult to be

treated by conventional nitrification–denitrification processes. An anaerobic ammo-

nium oxidation (anammox) process can be a more sustainable alternative because of

reduced energy requirement (about 63% reduction in oxygen demand) for aeration and

no additional organic carbon (nearly 100% reduction in carbon demand as ammonia is

used as electron donor) that is needed for the denitrification process.20 An anammox

process is an autotrophic process that combines ammonium and nitrite within the

phylum Planctomycete under anoxic condition to generate nitrogen gas.21 This process

can be performed either in two sequentially operating units22 or in a single aerobic

biofilm unit.23 The biological nitrogen cycle including an anammox process is illustrated

in Figure 6.

As mentioned earlier, prior to the anammox process, the ammonium is partially

oxidized to nitrite by ammonium oxidizing bacteria (AOB). In order to have optimal

nitrogen removal by the anammox bacteria, nitrate-oxidizing bacteria (NOBs) and

FIGURE 6 Biological nitrogen cycle.


biodegradable organic matter should be avoided.24 As shown in the stoichiometry

equation below, anammox bacteria use nitrite as the electron acceptor to form dini-

trogen gas as the final product.

NHþ4 þ 1:32NO�

2 þ 0:066HCO�3 þ 0:13Hþ ¼ 1:02N2 þ 0:26NO�

3 þ 2:03H2Oþ 0:066CH2O0:5N0:15

In addition to ammonium, other organic and inorganic compounds including propio-

nate, acetate, and formate can be used as alternative electron donors.25,26 Although

recent research indicated that the anammox process can be successfully operated at

room temperatures of about 20 �C to treat effluents from anaerobic digesters,27 since

anammox bacteria grow slowly with generation times of 10–12 days at 35 �C, the

anammox process is still limited to treat warm wastewater with a high ammonium

content.28

As the anammox bacteria are identified as deep-branching monophyletic group of

bacteria within Planctomycete and are strictly anaerobic and chemolithoautotrophic,

they have not been cultured yet. However, culture-independent molecular techniques

have identified 13 species that belong to five different Candidatus genera including

Candidatus brocadia, Candidatus kuenenia, Candidatus scalindua, Candidatus anam-

moxoglobus, and Candidatus jettenia.29

Anammox bacteria grow relatively slowly compared to other microorganisms in

activated sludge systems, and they compete for substrates with other nitrogen-cycling

microorganisms such as nitrifiers, complicating bioreactor design and operation. They

can thrive in an environment with warm or low C/N side streams such as postanaerobic

digestion dewatering centrate. In a coupled process called deammonification (DEMON),

these systems convert half of the influent total Kjeldhal nitrogen into nitrite and prior to

anammox’s producing nitrogen gas from the ammonia and nitrite in a side-stream

process.

The potential for mainstream DEMON as an innovative technology presents an op-

portunity to develop a more sustainable nutrient-removal configuration. In an A/B

configuration, wastewater with a high-rate carbon stage and a low-rate nitrogen stage

operates with a low SRT and limited aeration to remove a significant portion of both the

particulate and soluble COD, producing wastewater with a decreased C/N ratio more like

the side-stream process. Furthermore, the high-rate, carbonaceous A-stage WAS can be

directed to an anaerobic digestion process to produce methane gas and energy.

4.4 Phosphorus-Accumulating Bacteria

A biological phosphorus-removing process can be achieved through periodic

anaerobic–aerobic cycles with a specific group of bacteria known as polyphosphate-

accumulating organisms (PAOs). This group of bacteria has a high capacity of accu-

mulating large amounts of orthophosphate in the form of polyphosphate in an aerobic

environment, known as the luxury uptake.30 In the anaerobic cycle, a readily available,

biodegradable carbon source such as acetate is converted into polyhydroxyalkanoates

(PHAs). Also, internal polyphosphate and glycogen are hydrolyzed to supply energy


and reduce power to the cells.31 In the subsequent aerobic cycle, utilization of

intracellular carbon and PHAs are accompanied by the uptake of dissolved

orthophosphate.

In 1975, initial research on the microbiology of PAOs using traditional culturing

techniques resulted in isolation of Acinetobacter-related organisms as the dominant PAO

in the EBPR process.32 However, the advent of culture-independent molecular tools in

recent years indicate that b-proteobacteria in the Rhodocyclaceae family are the

dominant PAOs in acetate- and propionate-fed lab-scale reactors and in full-scale EBPR

plants32–35 and the PAOs identified in this group have been termed Candidatus

Accumulibacter phosphatis.36

Molecular studies on the responsible enzyme identified polyP kinase (ppk) for cata-

lyzing polyphosphate synthesis in Escherichia coli as the model organisms.37 Although

polyP kinase 1 (ppk1) and smaller polyP kinase 2 (ppk2) have reversible functions in

polyP catalysis, both enzymes are present in polyP transformation.38 The ppk1 gene that

encodes the polyP kinase enzyme can be used as a genetic marker to target and quantify

C. Accumulibacter phosphatis in molecular-scale population structure. Based on ppk1

sequence information, the abundance and relative distribution of C. Accumulibacter

phosphatis can be categorized into five distinct clades.39

5. Sustainable Removal Process Configurationsin Activated Sludge Processes

Conventional activated sludge process requires the continuous operation of oxygen

blowers and sludge pumps for the aeration basins, and a constant energy supply is a key

requirement that increases the capital and operation and maintenance costs. In order to

increase the efficiency of biological nitrogen and phosphorus removal, the conventional

activated sludge process must be modified to enhance biological nitrogen removal and

phosphorus uptake. Control and modification of internal recycles and separation of

reduction–oxidation zones (aerobic, anoxic, and anaerobic) in the activated sludge

process are the most important parameters employed for advanced biological nitrogen

and phosphorus removal. In this section, several activated sludge processes with

different configurations and recycling rates are discussed for achieving high efficiency of

nitrogen and phosphorus removal.

5.1 Modified Bardenpho Process (5-Stage)

The Bardenpho process was developed by James Barnard in the 1970s. Modified

Bardenpho process with an additional anaerobic zone in the beginning of the process

can remove high levels of BOD, suspended solids, phosphorus, and nitrogen through

advance modification of the conventional activated sludge process without any

addition of chemicals. In the first step, the fermentation stage, influent wastewater is

mixed with the returned activated sludge from the clarifier to produce the appropriate

FIGURE 7 Modified Bardenpho process for nitrogen and phosphorus removal. ANR, anaerobic zone; ANX, anoxiczone; AER, aerobic zone.


stress conditions in the absence of DO and nitrate. In the second step, wastewater is

transported to the first anoxic zone where the recycled nitrate from the following

nitrification stage is converted to nitrogen gas in the denitrification process. In the

third step, nitrification occurs in the aerobic zone, BOD is converted to carbon dioxide,

and luxury uptake of phosphorus takes place. In the fourth step, residual nitrate is

converted to nitrogen gas in the second anoxic zone. Finally, in the last step, as the

mixed liquor can contain 5–6% phosphorus, the sludge is subjected to reaeration to

ensure that it remains aerobic and to prevent phosphorus release in the final clarifier

(Figure 7).

5.2 Anaerobic/Anoxic/Oxic (A2O) process

Combined phosphorus and nitrogen removal can be achieved if the conventional

Phoredox (A/O) process (Figure 8(A)) that is being used for biological phosphorus

removal is combined with Modified Ludzack–Ettinger (MLE) process (Figure 8(B)) that

consists of anoxic (denitrification zone) and aerobic (nitrification zone) stages

(Figure 8(C)). The A2O process is not as reliable as the Modified Bardenpho process, but

it can achieve nitrogen and phosphorus removal with proper design and plant optimi-

zation. The SRT in each zone must be controlled to allow complete phosphorus release

or uptake.

5.3 University of Cape Town Process

The University of Cape Town (UCT) process is adapted from the Bardenpho process and

consists of anaerobic, anoxic, and aerobic zones. There are two internal recycles in this

process. The first recycle line returns internal nitrates from the aerobic zone to the

anoxic zone and the second one returns mixed liquor from the anoxic zone to the

anaerobic zone. In order to minimize the amount of nitrates in the anaerobic zone, RAS

is transported to the anoxic zone. This configuration will let the PAOs to consume VFAs

as the carbon source in the anaerobic zone without competition from denitrifiers using

VFAs (Figure 9).

FIGURE 8 (A) Conventional Phoredox (A/O) process for phosphorus removal. (B) Modified Ludzack–Ettinger (MLE)process for nitrogen removal. (C) A2O process for simultaneous phosphorus and nitrogen removal. ANR, anaerobiczone; ANX, anoxic zone; AER, aerobic zone.


5.4 Modified UCT process

The modified UCT process has an additional anoxic zone in series with the first one to

minimize the amount of returned nitrate to the anaerobic zone. The nitrate from the

final aerobic zone is returned to the second anoxic zone and the second internal

recycle returns mixed liquor from the first anoxic zone to the anaerobic zone

(Figure 10).

FIGURE 9 University of Cape Town (UCT) process.

FIGURE 10 Modified University of Cape Town (UCT) process. ANR, anaerobic zone; ANX, anoxic zone; AER,aerobic zone.


5.5 Virginia Initiative Process

The Virginia Initiative Process (VIP) is very similar to the modified UCT process with the

exception of having nitrate recycle from the aerobic zone to the first anoxic zone instead

of the second anoxic zone in the modified UCT. The second mixed liquor recycle is also

returning wastewater from the second anoxic zone to the head of the anaerobic zone.

These modifications in recycling allow additional denitrification and minimize the ni-

trate concentration in the anaerobic zone that can interfere with phosphorus release. As

some phosphorus is also taken up in the anoxic zone, enhancing phosphorus release in

the anaerobic zone can maximize the phosphorus uptake in the subsequent anoxic and

aerobic zones (Figure 11).

In Table 3, a summary of the described processes with their nitrogen and phosphorus

removal efficiencies is shown.

FIGURE 11 Virginia Initiative Process. ANR, anaerobic zone; ANX, anoxic zone; AER, aerobic zone.

Table 3 Summary of Nitrogen and Phosphorus Removal Processes

Treatment Process Total Nitrogen Concentration Total Phosphorus Concentration

Modified Bardenpho (5-stage) 3–6 mg/l Less than 1 mg/lBardenpho (4-stage) 7–9 mg/l NoneA2O 7–9 mg/l Less than 1 mg/lUCT / VIP 7–9 mg/l Less than 1 mg/lPhoredox (A/O) None Less than 2 mg/lModified Ludzack-Ettinger (MLE) 7–9 mg/l None


6. Optimization of Activated Sludge Processand Design Considerations

Optimization of the complex BNR processes requires achieving and maintaining a dy-

namic equilibrium among the biological, chemical, and physical processes. The treat-

ment process should be designed to have adequate operational flexibility to allow the

treatment plant to respond to fluctuation in the influent flow rate and characteristics and

possible adverse operating conditions. Some of the key design considerations for an

optimized activated sludge process are discussed in this section.

Since the nutrient removal processes are very sensitive to influent characteristics and

the influent variability can upset the process, a minimum of 2 years of plant data must be

used for designing the bioreactor. Sludge recycling rate and its location can modify the

influent characteristics significantly and should be considered in the design procedures.

For instance, an increased recycle rate (underflow) will result in greater clarifier capacity,

with better movement of solids to the bottom and less concentrated WAS, which requires

more recycle pumping. On the other hand, a decreased recycle rate (overflow) leads to

more concentrated RAS and less clarifier capacity, because of decreased movement of

solids to the bottom.

In conventional biological nitrogen removal, nitrification is a controlling process and

a prerequisite for the denitrification, which requires lots of energy for aerobic nitrifiers.

The presence of sufficient supplies of readily biodegradable carbon compounds is a

critical factor for the denitrifying bacterial population to increase denitrification rates

and compensate for deficiencies in the influent carbon/nitrogen (C:N) ratio. In addition,

applying high carbon dosage increases the sludge production, which affects the nitrifi-

cation capacity. Therefore, nitrification and then denitrification processes should be

optimized to achieve total nitrogen removal. The DEMON process for nitrogen removal

is one of the significant energy-saving technologies for ammonia removal from high-

strength wastewater. The DEMON process is a nitrification/deammonification process

using AOB and anaerobic ammonium-oxidizing bacteria (anammox) in which ammonia

and nitrite are simultaneously converted to nitrogen gas, without the use of organic

carbon. This results in 50% reduction of energy for nitrification and 100% reduction of

carbon source, when compared to conventional nitrification–denitrification processes.40

Temperature is one the most critical factors in the design of biological nitrogen

removal systems. For the margin of safety, the lowest monthly average temperature is

used for the nitrification process. In addition, to account for the influent variability and

fluctuation, a safety factor of 1.5–2.5 must be applied to determine the design nitrifi-

cation SRT.

The internal recycle rate to the anoxic zone must be controlled to minimize DO

concentration in the denitrification process. In addition, for handling the internal recycle

nitrate load, a practical denitrification rate needs to be considered for designing the

anoxic zone. Typically, the anoxic volume is 25–40% of the total bioreactor volume.41


In general, constant and even flow split to the bioreactors, proper mixing of the

bioreactor influent with the return sludge, and providing variable speeds of internal

recycle will significantly improve the nutrient removal efficiency of the system.

7. Sludge MinimizationAs discussed in previous sections, the activated sludge process is the most widely used

biological treatment for domestic and industrial wastewater. One of the disadvantages of

conventional activated sludge is an excess amount of sludge production, which accounts

for up to 60% of the total cost of wastewater treatment.42 In addition, large carbon

footprint and huge land requirements in the sludge dewatering process and handling

results in a less sustainable process. Increasing stringent regulations on land application

of sewage sludge (as class B biosolids) because of potential toxic chemicals in the sludge

provide considerable reasons to develop technologies for sludge minimization.

The ideal method for sludge minimization is to reduce sludge production in the

treatment process rather than post-treatment of the produced sludge. As was mentioned

in the nitrogen removal alternatives, partial nitrification needs 40% external carbon

source and produces less biomass compared to complete nitrification.43 Applying less

external carbon in the denitrification process also has an increased effect on reducing

the biomass production. Generally, in order to reduce the biomass production in

wastewater treatment processes, substrate assimilation must be diverted from biosyn-

thesis to nongrowth activities.44 Lysis-cryptic growth, uncoupling metabolism, and

maintenance metabolism are among the theoretical considerations that are currently

being developed for sludge reduction.

Cell lysis, which is the collapse of the cell and its integrity, will release cell content and

provide substrates that contribute to the organic loading of the bioreactor. This indig-

enous organic substrate will be reused in microbial metabolism resulting in an overall

reduced biomass production. Since the biomass growth that occurs from this indigenous

organic substrate cannot be distinguished from growth on the original substrate, this is

called cryptic growth. An overall reduction of sludge production can be achieved with

sludge disintegration. Thermal treatment; chemical treatment using acids or alkalis;

mechanical treatment using ultrasound, homogenization, and repetitive freeze-thaws;

biological treatment with hydrolysis enzyme; and oxidation processes using hydrogen

peroxide (H2O2) and ozone are some of the alternatives that have been used for cell

disintegration in the sludge minimization process.

An uncoupling metabolism is another sustainable alternative for reducing sludge

production. Metabolism is a set of biochemical transformations that includes catabo-

lism and anabolism. Catabolism is the series of metabolic processes that break down

large molecules, reduces the complexity of organic compounds, and produces free

energy. On the other hand, anabolism is the set of constructive metabolic pathways that

uses free energy to synthesize complex molecules required by the cells. In most aerobic

bacteria, energy is generated in the form of adenosine triphosphate where electrons

FIGURE 12 Relationship betweencatabolism and anabolism in cellularmetabolic reactions.


are transported from an electron donor as substrate to the electron acceptor as free

oxygen (Figure 12). However, an uncoupling metabolism would occur when catabolism

is allowed to continue unhindered and respiratory control does not exist, while anab-

olism of biomass is restricted and rate limiting to achieve biomass yield reduction.45

Uncoupled metabolism has been observed under some conditions such as the presence

of inhibitory compounds, excess energy source, unfavorable temperatures, and mini-

mal nutrient.

Maintenance respiration is a metabolism where part of the energy source is used for

maintaining living functions of microorganisms. Maintenance energy includes energy

for cell materials turnover and resynthesis of compounds, energy for maintenance of

metabolites across cellular membranes that are necessary for cellular integrity, and

energy for metabolic processes involved in physiological adaptation. Maintenance

metabolism is important in reducing the sludge production since the maintenance-

associated substrate consumption does not result in new cellular mass synthesis.

Therefore, the maintenance energy requirement can be achieved through endogenous

metabolism and the incoming substrate can finally be converted to carbon dioxide and

water, resulting in a lower biomass production.

8. Resource Recovery in Activated Sludge ProcessesAs discussed earlier, all BNR practices provide the benefit of removing nutrients with

little or no reliance on chemical use. Furthermore, BNR technologies can generate side-

stream loads with very high nutrient concentrations in anaerobic digester that are

suitable for nutrient recovery. Different key factors are important in determining the

BNR process from the nutrient recovery point of view. The efficiency of using influent

COD and ability of the process to meet effluent requirements without using chemical

dosing, the operational costs and energy requirements, the capital costs, tank volume

requirements, and associated carbon footprint are the main criteria that should be

considered in process decision making.

In the last two decades, a lot of focus has been placed on decreasing operational

costs by improving energy efficiency and increasing sustainability by energy and


nutrient recovery in water resource recovery facilities. Domestic wastewater is an

important carrier medium for nutrients, particularly nitrogen and phosphorus cycles.

Nitrogen is a ubiquitous element in human diet, which results in a key role in an

anthropogenic nitrogen cycle. Therefore, municipal wastewater has a substantial po-

tential to be used as an agricultural fertilizer. Studies show that an average excretion of

13 g N/capita/day results in an annual excretion of 4.75 kg N/capita.46 It has been

estimated that recovery of nitrogen present in domestic wastewater can cover up to

30% of the current agricultural nitrogen demand. Ammonia stripping or precipitation

with magnesium ions and sodium hydroxide are among recovery technologies for

ammonia from the activated sludge.

In addition to nitrogen, phosphorus is present at substantial levels in domestic

wastewater. Phosphorus can be recovered from phosphorite (rock phosphate) for agri-

cultural fertilizer. However, phosphorite is a limited resource concerning quantity and

quality. Mining of phosphate has a heavy environmental impact; therefore, recycling

phosphorus from wastewater would be a more sustainable alternative. A significant

amount of phosphorus ends up in domestic wastewater, including an average excretion

of 2 g P/capita/day46 besides phosphorus originating from detergent, food waste, and

other products.

The methods for phosphorus recovery may be categorized into two main groups

including recovery in the wastewater treatment and recovery from the produced sludge.

Phosphate can be removed from the mainstream in the wastewater after biological

treatment as calcium phosphate or magnesium ammonium phosphate. It is also

possible to recover phosphorus by treatment of a fraction of the returned activated

sludge with enhanced biological phosphorus removal. For instance, PhoStrip is a side-

stream anaerobic treatment in which a part of the return sludge flows into a stripper.

Through the addition of acetic acid and because of the anaerobic environment, a sig-

nificant release of phosphate can occur. After separation of the sludge phase through a

dewatering process, special treatment with lime will precipitate phosphate from the

phosphorus- and ammonium-rich supernatant.

Ion exchange is another treatment alternative that can effectively recover a wide

range of chemicals including nutrients from the wastewater. A recent study has

confirmed that selective anion exchange resins such as indion NSSR resin47 can remove

nitrate, and cation exchange resin and natural zeolite48 can be used for ammonia re-

covery from wastewater. Polymeric anion exchange and polymeric-inorganic hybrid

anion exchange are the selective resins that have been used for phosphorus recovery in

recent years.49

Struvite precipitation is also considered a promising method in nutrient recovery

in wastewater activated sludge. Struvite (magnesium ammonium phosphate),

MgNH4PO4$6H2O, is a phosphate mineral that can simultaneously recover phosphorus

and ammonium nitrogen at equal molar bases, which results in slow release of mag-

nesium ammonium phosphate fertilizer.50 Struvite precipitates after contacting mag-

nesium ions such as magnesium chloride and ammonium ions such as ammonium


chloride with the wastewater containing phosphate ions. The pH is an important factor

in struvite precipitation; an increase in pH reduces the struvite solubility and increases

its precipitating potential.51

Anaerobic digestion, the most important process for sludge stabilization and energy

recovery, is employed worldwide. The process of anaerobic digestion results in digested

products from wastewater, which are rich in readily biodegradable substances that are

potentially renewable carbon sources, such as VFAs. The VFAs can be oxidized by slowly

growing acetogenic bacteria into acetate, molecular hydrogen, and carbon dioxide,

which are suitable as substrates for the methanogenic bacteria. These recovered re-

sources produce biogas, generate electricity, synthesize PHAs, and can also be used as

preferred external carbon source for nitrogen and phosphorus removal.

Anaerobic digestion can also be advantageous for minimizing the volume of pro-

duced sludge. The net sludge yield for most wastewaters in anaerobic digesters is less

than 0.1 kg volatile suspended solid (VSS) per kilogram COD removed, compared to

approximately 0.5 kg of VSS produced in aerobic activated sludge treatment. This

decrease in sludge production requires much less energy-intensive processing, and

thereby reduces the capital costs and carbon footprints proportionately. Van Lier52

estimated that about 13.5 MJ methane energy can be produced per kilogram COD

removed from wastewater. Assuming 40% electric conversion efficiency, 1.5 kWh electric

output would be obtained.

9. ConclusionsAs evident, the sustainability of activated sludge processes depends upon several factors

including those set up by regulatory agencies, local environmental needs, sludge

disposal options available, and the need to reuse treated water and resource recovery.

The trend has definitely moved from seeing activated sludge systems merely as

“contaminant removal” technologies to “contaminant removal and resource recovery”.

New technologies and continued research will definitely help the wastewater community

run these processes carbon- and energy-neutral. For example, employing codigestion at

a treatment plant will result in greater methane gas generation. The use of anaerobic

ammonia oxidation to treat reject liquid from an anaerobic digester belt press will help

achieve low nitrogen in the effluent and help save energy. Nutrient recovery, especially

P recovery, has the promise to meet our future P demands, especially in the light of

depleting P resources.

GlossaryActivated sludge process: A biological wastewater treatment process in which flocs of variety of mi-

croorganisms are allowed to break down organic compounds and nutrients in an aerobic, anoxic, or

anaerobic environment. The activated sludge is subsequently separated from the treated wastewater

(mixed liquor) by settling, wasted, and a portion is returned to the process as needed.

Ammonification: Bacterial decomposition of nitrogenous organic matter to ammonia.


Biochemical oxygen demand (BOD): The amount of oxygen used in the biochemical oxidation of

organic matter at certain temperature over a specific time period and under specified conditions.

The BOD value is usually measured during 5 days of incubation at 20 �C. BOD is used as a surrogate

of the level of organic pollution of treated wastewater.

Chemical oxygen demand (COD): A measure of the oxygen-consuming capacity of inorganic and

organic compounds in wastewater. Since COD includes biologically available, inert organic and

inorganic matter, COD values are always greater than BOD values.

Denitrification: Biological conversion of nitrate (NO3�) to nitrogen gas through an anaerobic respiration

reaction in which nitrate is reduced.

Dewatering: The process of separating solids from the liquid wastewater by using of solid–liquid sep-

aration processes such as sludge-drying beds, rotary drum vacuum filter, centrifuge, and the belt

filter press in order to reduce volume of the wastewater solids before disposal.

Hydraulic retention time (HRT): The average residence time that wastewater remains in the

bioreactor.

Inorganic matter: Chemical substances of mineral origin, which lack carbon and hydrogen atoms.

Nitrification: The biological process of oxidation of ammonia with oxygen and its conversion to nitrites

(NO2�) and then nitrates (NO3

�).Organic matter: Chemical substance that originates from once-living organisms such as plants and

animals and their waste products. In other words, carbon structure comprising compounds con-

sisting of hydrocarbons and their derivatives.

Readily biodegradable organic matter: Soluble organic compounds consisting of simple molecules that

can be directly metabolized by heterotrophic bacteria as the carbon source for their metabolism.

Return activated sludge (RAS): The settled activated sludge from the secondary clarifier, which is

aged and stressed and returned to the bioreactor to mix with incoming raw or primary settled

wastewater.

Sludge: The thick mixture of liquid and solids removed from the wastewater in thevarious sedimenta-

tion and separation processes.

Solid retention time (SRT): The average time that the activated sludge solids and microorganisms are

retained in the bioreactor.

Volatile fatty acids (VFA): A class of short-chain carboxylic acids with a carbon chain of six carbons or

less (e.g., acetic, propionic, and butyric acids), which are usually referred to as short-chain fatty

acids.

Waste activated sludge (WAS): The excess amount of microorganisms that must be removed from the

process to keep the biological system in balance and prevent overloading of sludge.

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