deliverable-1.10 guideline design granular sludge reactor

7/25/2019 Deliverable-1.10 Guideline Design Granular Sludge Reactor

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Sustainable and Integrated Urban Water System ManagementDeliverable Nr : 10.1

Deliverable Title: Guideline for granular sludge reactor design

SANITAS

SUSTAINABLE AND INTEGRATED URBAN WATER SYSTEM MANAGEMENT

Marie Curie Network for Initial Training

Seventh Framework Programme

Grant Agreement Nr. 289193

Guideline for granular sludge reactor design

Deliverable reference 1.10

Partner in charge UGent

Authors C. M. Castro-Barros

Revised by E. I. P. Volcke

Target dissemination PU

Coordinator institution UNIVERSITAT DE GIRONA

Date of delivery 30/08/13

The research leading to these results has received funding from the People Program (Marie

Curie Actions) of the European Unions Seventh Framework Programme FP7/2007-2013,

under REA agreement 289193.

This publication reflects only the authors views and the European Union is not liable for any

use that may be made of the information contained therein.


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INDEX

1. INTRODUCTION ........................................................................................................3

1.1. Biological nitrogen removal from wastewater.................................................................... 3

1.2. Granular sludge ................................................................................................................... 5

2. DEVELOPING AEROBIC GRANULES ................................................................................8

2.1. How to start granulation? ................................................................................................... 8

2.2. Hydraulic and operational factors during granulation ........................................................ 9

3. SBR REACTOR DESIGN.............................................................................................. 10

3.1. SBR operation .................................................................................................................... 11

3.2. Fundamental design parameters ...................................................................................... 11

4. MODELLING BIOFILM SYSTEMS.................................................................................. 12

4.1. Models for biofilm and granular sludge systems .............................................................. 13

4.2. Controlling partial nitritation-anammox in granular sludge reactors ............................... 14

5. CONCLUSIONS........................................................................................................ 15

REFERENCES................................................................................................................. 16


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Abstract

The partial nitritation-anammox pathway is an innovative alternative for nitrogen removal

from wastewater compared with conventional nitrification-denitrification over nitrate.

Granular sludge reactors are suitable systems to develop partial nitritation-anammox that

present several advantages compared with floc-based systems such as lower footprint and

higher settleability. A review on granular sludge technology is given to provide a guide for

reactor design, focusing on aerobic granular sludge systems to carry out the partial nitritation-

anammox pathway. Microbial kinetic factors as well as hydrodynamic and operational

parameters involved in aerobic granular sludge systems are described. Fundamentals of

sequencing batch reactor design for aerobic granular systems are provided and modelling isput forward as a useful tool for biofilm system design. The outcome of the review shows that

an appropriate selection pressure is essential to develop proper granules, mainly short sludge

settling times and relatively high shear stress. Sequencing batch reactors are appropriate

systems to develop granules since their operational flexibility allows establishing suitable

selection pressures. Modelling granular sludge has to take into account physical-chemical and

biological aspects. Modelling granular sludge partial nitritation-anammox systems allows the

assessment of important parameters that influence the reactor design and operation. Granule

size and oxygen concentration are key factors for granular sludge partial nitritation-anammox

reactor design.

Keywords:anammox, control, granular sludge, modelling, partial nitritation, reactor design


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1.INTRODUCTION

1.1. Biological nitrogen removal from wastewater

Due to the potential problems that nitrogen compounds can cause in natural resources,

effective nitrogen removal in wastewater treatment plants is required. High concentrations of

ammonium, nitrite and nitrate are toxic for life. Free ammonia can cause severe problems in

aquatic life such as fish mortality or impacts on reproduction. Moreover, an excess of nitrogen

and phosphorus enhances the eutrophication in water bodies. These nutrients stimulate an

excessive algae growth and these algal blooms impede the penetration of sunlight with the

subsequent death of the aquatic flora. The death plants promote a depletion of oxygen, which

is highly necessary for aquatic life and its lack can hamper the development of the ecosystem

and also cause bad odours.

Nitrogen in wastewater is present mostly in the form of ammonium. Its biological

removal implies a series of reactions performed by different bacteria to obtain nitrogen gas,

which is innocuous to the environment. The conventional strategy to remove ammonium

from wastewater is based on the nitrification-denitrification over nitrate pathway. In the last

decade, an innovative method that allows important savings in energy and carbon source was

and is being developed and implemented: the coupling of a partial nitrification and the

anammox reaction.

An overview of these biological nitrogen removal pathways is given below.

- Nitrification-denitrification

The most common strategy to remove ammonium from wastewater is nitrification-

denitrification over nitrate. Nitrification concerns the oxidation of ammonium to nitrite

(nitritation) and then to nitrate (nitratation) under aerobic conditions. First, ammonium is

aerobically converted to nitrite by ammonium oxidizing bacteria (AOB) (Eq. 1) and further,

nitrite is oxidized to nitrate by nitrite oxidizing bacteria (NOB) (Eq. 2).

[1]

[2]

Both AOB and NOB are chemolithoautotrophic microorganisms. These bacteria use

inorganic compounds as electron donor and their carbon source is carbon dioxide, or in

practice, bicarbonate.

During denitrification, nitrate and/or nitrite is reduced to nitrogen gas by heterotrophic

microorganisms under anoxic conditions. This transformation implies several subsequent

reduction steps (Eq. 3).


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[3]

The electron donor in these reactions is a carbon source, that normally has to be added

to the wastewater (usually methanol or ethanol), while the nitrogen compounds act as

electron acceptors.

- Partial nitritation-anammox

The partial nitritation-anammox pathway constitutes a more sustainable alternative for

nitrogen removal from wastewater than the conventional nitrification-denitrification over

nitrate.Partial nitrification is the oxidation of ammonium only to nitrite (nitritation) (Eq. 1).

Further oxidation to nitrate can be avoided by imposing limited oxygen concentrations, taking

advantage of the higher oxygen affinity of ammonium oxidizers compared to nitrite oxidizers

(Garrido et al., 1997). Temperature and sludge retention time (SRT) can also be used to

promote the partial nitrification. Ammonium oxidation has higher activation energy than the

oxidation of nitrite. Therefore, working at relative higher temperatures (above 25 C) would

allow the removal of NOB, which would grow slower than AOB (Hellinga et al., 1998).

The anammox process is a shortcut in the nitrogen cycle where ammonium is combined

with nitrite to yield nitrogen gas in the absence of carbon source (Strous et al., 1998).

Anammox bacteria are autotrophic microorganisms belong to the genus Plantomycetes. They

have a slow growth rate and productivity and their optimal temperature of operation is 35 C

and pH 8. The research on anammox technology started two decades ago. Enrichments at lab-

scale were realized by Strous et al. (1999) and Van de Graaf et al. (1996). About 10 years later,

the technology would be successfully implemented at full-scale in the wastewater treatment

plant of Rotterdam, The Netherlands (van der Star et al., 2007).

During partial nitritation half of the ammonium in wastewater is oxidized to nitrite. This

reaction can be followed by the anammox conversion under anoxic conditions, in which aboutequimolar amounts of ammonium and nitrite are converted to nitrogen gas (Eq. 4). This is the

reaction between the nitrite formed during the partial nitritation and the remaining

ammonium that was not converted.

[4]

Compared with conventional nitrification-denitrification for nitrogen removal, the partial

nitritation-anammox pathway, also termed completely autotrophic nitrogen removal, requires

up to 62.5% less oxygen. This implies important saving in aeration, one of the main costs inwastewater treatment plants. Moreover, since the whole process is autotrophic, organic

carbon is not required, which means 100% savings in carbon source addition. Also, due to the


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autotrophic reactions CO2 emissions are minimal. Furthermore, owing to slow anammox

growth rate, the sludge production is expected to be lower.

Biofilm reactors are suitable systems to carry out partial nitritation-anammox pathway.

Granular sludge can be used to develop partial nitritation-anammox systems. The following

section will have a look to granular sludge technology and also will go with more detail into

the application of granular sludge to perform partial nitritation-anammox process.

1.2. Granular sludge

Granular sludge is a special type of biofilm in which biomass grows in compact aggregates(granules) without any carrier material. Granular sludge technology started to be developed

about 40 years ago at Wageningen University. At that time, granules were implemented for

anaerobic treatment in upflow anaerobic sludge bed (UASB) reactors (Lettinga et al., 1983).

- Why granular sludge technology?

Compared to biomass growing in flocs, granular biomass presents several advantages that

make it very attractive for wastewater treatment purposes. Granules are denser and have a

stronger microbial structure than flocs. Thus, granular biomass has very high settling velocity,

while the typical settling velocity for flocs is at least three times lower. These excellent settlingproperties allow the use of high hydraulic loads to the reactors without the wash out of the

biomass. Besides, more compact clarifiers can be used or in some cases, an extra unit is not

required to separate the biomass. This implies a lower footprint and important savings in

construction. The high biomass retention also enables to enhance the performance of the

reactor and a faster removal of the different contaminants.

From a microbiological point of view, granules consist of different layers where diverse

microorganisms can be present as well as different reactions can take place. In conventional

wastewater treatment plants with biomass growing in flocs, different units and recycling are

required to perform the aerobic and anaerobic conversions. However, in granular sludge,

anaerobic and aerobic reactions can occur at the same granule, since the stratification allows

different conditions along the biomass. For instance, in an aerobic system, the outer part of

the granule, where oxygen is available, nitrifiers can grow, while in the inner part, denitrifiers,

anammox bacteria or phosphate accumulating organisms (PAOs) can develop themselves

under anaerobic and anoxic conditions.Figure 1 shows the differences in the structure of a

floc and an aerobic granule.


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Figure 1. Microbial distribution in A) a sludge floc and B) a heterotrophic aerobic granule.

Source: Winkler (2012)

Taking advantage of the stratification of the biomass in granular sludge, completely

nitrogen removal is feasible in one unit as for example in the CANON reactor operated at the

Olburgen sewage treatment plant in The Netherlands (Abma et al., 2010) or more ambitious,

the simultaneous removal of organic matter and nutrients in one unit, as it was demonstrated

with the Nereda technology applied in a full-scale plant in The Netherlands (van der Roest et

al., 2011).

By applying granular sludge technology, important savings in space and energy can be

achieved, leading to a more cost-effective wastewater treatment plant.

- Anaerobic and aerobic granular sludge

Anaerobic granular sludge technology has been extensively studied and is widely applied in

wastewater treatment plants to remove biodegradable organic matter. The aggregation and

interaction of many microorganisms such as methanogenic bacteria and acetogens growing in

anaerobic granules allow the conversion of organic matter to CO2, CH4, fatty acids and H2.

UASB reactor has been and is the most usual reactor configuration to grow anaerobic granules

for COD removal. Many modifications and improvements have been developed in UASB

reactor to enhance their efficiency.

Disadvantages of anaerobic granulation include the long start-up time and the relative

high operation temperature needed. Moreover, this technology only established for COD

removalno nutrient removaland is not suitable for the treatment of low-strength organic

wastewater and cannot remove nutrients.

These drawbacks are overcome through aerobic granulation technology. Mishima and

Nakamura (1991) established aerobic granules in an upflow sludge blanket reactor. The

technology was further developed in sequencing batch reactors (SBRs) (van Loosdrecht and

Heijnen, 1993; Morgenroth et al. 1997; Beun et al., 1999; Tay et al., 2002a).


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- Heterotrophic granules

The application of aerobic granular technology to wastewater treatment allows removing

organic matter, nutrients and toxic substances. The so-called Nereda technology is based on

aerobic heterotrophic granules that are able to remove simultaneously COD, nitrogen and

phosphorus in one unit (de Kreuk et al., 2005). This technology was implemented successfully

at full-scale in Epe, The Netherlands in 2011 (van der Roest et al., 2011). The technology is

based on SBR mode operation with cycles of feeding under anaerobic conditions followed by

two aeration periods, a settling period and an effluent discharge. The influent is fed from the

bottom of the reactor in a plug-flow regime with the subsequent effluent withdrawal.

Anaerobic feeding is used to obtain high concentrations of organic carbon and to grow PAOs.During the aeration period heterotrophs grow and PAOs take up phosphate. Regarding

ammonium, this is oxidized to nitrite and nitrate in the aerobic outer layer of the granules,

while denitrification takes place in the anoxic inner zone of the granules by PAOs, which use

stored PHB as electron donor to reduce the nitrate.

- Autotrophic granules

Complete autotrophic nitrogen removal can be carried out in granules using anammox

bacteria. The process can be developed in two reactor units, one to oxidize part of the

ammonium to nitrite in aerobic conditions and a subsequent reactor where anammoxbacteria grow in anaerobic granules, as in the SHARON-anammox configuration (Van Dongen

et al., 2001). Alternatively, complete autotrophic nitrogen removal can be developed in a

single reactor unit containing aerobic granules. In this case, limited oxygen conditions have to

be established to enable the coexistence of ammonium oxidizing bacteria (AOB) and

anammox bacteria at the same granule. More specifically, AOB grow in the outer part of the

granules, while anammox bacteria develop inside of the granules, where anoxic conditions are

present, since anammox reaction is inhibited by oxygen (Strous et al., 1997). Figure 2 shows a

representation of a granule that develop partial nitritation-anammox pathway.

Figure 2. Partial nitritation-anammox in a granule

Nowadays, the anammox process developed in granules is already implemented at full-scale to treat reject water with high ammonium concentrations, both in two-reactor

configurations (Van der Star et al., 2007) and in a single reactor (Abma et al., 2010). This

AOB

ANAMMOX

BACTERIA


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process further fits in innovative process schemes for energy-efficient treatment of municipal

wastewater (Kartal et al., 2010).

2.

DEVELOPING AEROBIC GRANULES

Several factors influence the formation, structure and developing of aerobic granular sludge.

Aerobic granule formation depends of microbial aspects as well as hydraulic and operational

conditions. As it was proposed by van Loosdrecht et al. (1995), the biofilm structure is the

result of a balance between the biomass surface production rate (growth) and detachment. In

general, aerobic granulation is a complex reaction process that has not been well established

yet and that requires more research to overcome the limitations for industrial and

wastewater treatment application.

This section shows an approximation of the mechanisms and factors involved in the

formation of aerobic granular sludge as well as the different parameters that affect the

development and structure of aerobic granules.

2.1. How to start granulation?

Microbial aspects have to be taking into account to answer this question. Many

microorganisms are present in the aerobic sludge of a wastewater. Filamentous bacteria canhinder the compaction of the sludge, decrease the settleability of this and promote the

bulking of the biomass. To develop a granular sludge system, filamentous bacteria need to be

avoided. The kinetic selection theory (Chudoba et al., 1973) proposes a selective method for

biomass based on the substrate uptake rate of the microorganisms in aerated systems. High

macro-gradients of substrate along the system would prevent the growth of filamentous

bacteria while improving the development of floc-forming microorganisms, with good settling

properties and which would be the precursors in the formation of granules (Figure 3). Systems

that provide low substrate concentrations such as continuously fed completely mixed reactors

enhance the dominance of filamentous microorganisms. However, plug-flow reactors or SBR

allow having high substrate concentrations, which improves the growth of good settling

sludge.

The alternation of feast and famine conditions presents competitive advantages for the

selection of bacteria that form granules capable of combined COD, N- and P-removal. Recall

that these bacteria are favoured by high substrate concentrations, being able to accumulate

polyhydroxyalkanoate (PHA) during these feast periods and to consume the storage

material during the periods without substrate. The filamentous bacteria can only be

competitive during low substrate concentrations and therefore the feast and famine cyclebenefits the selection of granule-former organisms.
http://en.wikipedia.org/wiki/Polyhydroxyalkanoateshttp://en.wikipedia.org/wiki/Polyhydroxyalkanoates


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Figure 3. Relation between substrate uptake rate and substrate concentration for filamentous andgranule formers according to the kinetic selection theory (Chudoba et al., 1973)

For the stability and cohesion of the aggregates, extracellular polymer substances (EPS)

play an important role. EPS can be polysaccharides, proteins, nucleic acids, phospholipids or

humic substances. They participate in the stability of granules through London forces

(hydrophobic character of proteins), electrostatic interactions (Ca2+

ions) and hydrogen bonds

(hydroxyl groups -hydrophilic polysaccharides- and water). Hydrophobic bacteria in the seed

sludge also contribute to the faster growth of aerobic granules providing stability (Wilen et al.,

2008).

2.2. Hydraulic and operational factors during granulation

Apart from the abovementioned microbial factors and chemical aspects in the stability of

granule formation, the selection of appropriate hydraulic and operational pressures on

different parameters such as shear stress, organic loading rate (OLR) or sludge settling time

are highly important for the development and formation of good aerobic granular sludge. The

most important influencing factors are detailed in the next subsections.

- Substrate composition and organic loading rate (OLR)

Aerobic granular sludge can be developed on various organic substrates such as glucose andacetate (Tay et al., 2002b) as well as on real wastewater (Arrojo et al., 2004). Low organic

loading rates promote the formation of small and compact granules. In contrast, high organic

loading rates enhance the formation of large but flyaway granules. Very high organic loading

rate can lead the disintegration and breakdown of the granules (Tay et al., 2004; Adav et al.,

2010).

- Hydraulic shear stress

The shear stress exerted on the granules is an important factor in the formation and structure

of the granules. The shear forces depend on the surficial gas velocity in the reactor andinfluence the sludge settling velocity and the diameter of the granules. The higher the surficial

gas velocity, the higher the stress forces on the granules, yielding denser and more compact


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granules, resulting in an improved sludge settling velocity. However, when the shear stress is

too high, the granules are subject to disintegration. At low surface gas velocities, small

granules could be formed, but microscopic observation showed the growth of filamentous

bacteria and the appearance of flocs (Zhu et al., 2013). Shear stress due to collisions with the

mechanical parts of the reactor and between particles may be considered as well.

In general, the shear forces in an aerobic granular sludge system need to be balanced to

avoid the breakdown of the granules (too high shear stress) on the one hand and to allow and

enhance the formation and good properties of the granules (above of a threshold shear

stress) on the other hand.

-

Sludge settling time

Various studies have reported the sludge settling time as one of the most important

parameters to grow and select aerobic granules (Beun et al., 1999; Beun et al., 2002; Qin et

al., 2004). Long sludge settling times allow a lower settling velocity of the sludge, which is

reflected in a lower sludge volume index (SVI). Reducing the sludge settling time, more

compact granules are formed and the sludge with bad settling properties is washed out. More

suspended sludge is left over when applying long settling times. Thus, aerobic sludge

granulation is favoured by imposing short settling times.

-

Hydraulic retention time (HRT)

Short HRTs enhance the wash-out of suspended biomass and decrease its growth, improving

the granulation in the system (Beun et al., 1999). The cycle time a SBR can be used as a

hydraulic selection pressure. For instance, with short cycle times, nitrifying sludge would be

washed out.

3.SBR REACTOR DESIGN

Sequencing batch reactor (SBR) is one of the most common reactors to develop aerobicgranules. Its flexibility allows the change of parameters and the modification of the operation

easily. Regarding granule development, SBRs are suitable to establish the feast and famine

cycle, to select the granules by setting short sludge settling times and also to modify other

parameters such as the HRT or the substrate loading rate as in other reactor configurations. In

general, systems with intermittent operation were found more favourable to develop aerobic

granular sludge than continuous reactors (Beun et al., 2002). Moreover, these authors found

that the performance of thefeeding as pulses allows larger substrate penetrations into the

granules compared with systems based on continuous feeding, which also benefits the

granulation.


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3.1. SBR operation

Figure 4 shows the cycle operation of a SBR. The different steps can vary in function of the

function and operation conditions of the reactor. For instance, the feeding can be added in

pulses or in continuous at the same time that the reaction takes place. For granular sludge,

short feeding times are preferable.

Figure 4. Cycle of sequencing batch reactor operation

3.2. Fundamental design parameters

A basic guideline is given in this section for the geometric and operational design of an aerobic

SBR. Design parameters are selected (Table 1)and then some steps and calculations can be

done to obtain a first approach of the SBR design.

Table 1. Parameters for SBR design

Symbol Units Description Calculation

VR m3 Total volume of the reactor V0+ Vf

Vf m3 Volume fed (water) -

V0 m3 Volume that remains from the previous

cycle (water + sludge)

-

Vw m3 Volume purged (sludge) -

Vd m3 Volume decanted (water) -

tc min Total time of the cycle ti

ti* min Time of each phase -

FTR - Feeding time ratio tf/tc

VER - Volumetric exchange ratio Vf/VR

HRT h Hydraulic retention time VR/Q

*ti: tf (feeding time), ta(active time, for the reaction), ts(settling time), td(decanting time)

The steps in the design of SBR are listed below:

1. Sludge retention time (SRT)

[5]


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[6]

2. Effective fraction of the cycle (EFC)

[7]

3. Total sludge mass (MX)

[8]

Where CS is the difference between the substrate concentration in the influent and in the

effluent. Depending on the purpose, the substrate can be COD, nitrogen, etc.

4. Volume of the settled sludge (V0)

[9]

Where SVI is the sludge volume index and SFV is a safe factor for design (around 25%).

5. Select the VER or tC

A good starting point is VER=0.5

6. If VER is chosen, calculate tC

[10]

7. Exchange volume of the reactor (VER)

8. Total volume of the reactor

9. Choose the number of reactors required

10.Check the values of tsand td11.Determine the geometry of the reactor

4.

MODELLING BIOFILM SYSTEMS

Mathematical modelling is a useful tool to design and optimize systems, to acquire knowledge

and predict the behaviour of processes. Biofilm modelling is a complex task that involves a

wide number of parameters. When modelling granules, reaction kinetics and physicochemical

aspects may be addressed simultaneously.


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4.1. Models for biofilm and granular sludge systems

Two different situations can be considered when modelling biofilms: the granulation phase

and the mature system (also dynamic scenario). Granulation modelling involves granule size

variation, selection of pressures to develop the granules (see section 2) and biological aspects

as well. Facing modelling of granular systems implies to consider physicochemical processes,

such as mass transfer of substrates, products and oxygen (aerobic systems) or detachment of

the biofilm. Also, biological conversions have to be considered, which will depend on the

processes occurring in the biofilm. In general, a model should be as simple as possible. It

should just to try to meet the objective for which was required.

Table 2. Examples of modelling studies with biofilm and granular sludge systems

Description BiomassModel

informationReference

Nitrogen removal in granular sludge

reactorGranular 1-d model Beun et al. (2001)

Sensitive analysis of CANON process Biofilmmodification

ASM3Hao et al. (2002a)

Influence of temperature and inflow

variations in partial nitrification-anammoxBiofilm

modification

ASM3Hao et al. (2002b)

COD on partial nitritation-anammox Biofilmmodification

ASM3Hao et al. (2004)

Oxygen consumption in partial

nitrification-anammoxBiofilm

modification

ASM3Hao et al. (2005)

Study of bactarial community structure of

nitrifying granulesGranular

1-d and 2-d

modelsMatsumoto et al. (2010)

Heterotrophic and autotrohphic granules Granularmodification

ASM3Ni et al. (2007)

Anammox reactor Granularmodification

ASM1Ni et al. (2009a)

Aerobic granulation of activated sludge

with low-strength municipal wastewaterGranular

modification

ASM3Ni et al. (2009b)

Multi-population biofilm for completely

autotrophic nitrogen removalBiofilm 1-d model Terada et al. (2006)

Granule size in partial nitritation-

anammox granulesGranular 1-d model Volcke et al. (2010)

Granule size distribution in an anammox-

based granular sludge reactor Granular 1-d model Volcke et al. (2012)

The existing ASM models proposed by the International Water Association (IWA) for floc-

based systems can be used as a starting point in granular sludge systems to describe the

biological conversions. However, the aggregation in granules creates substantial differences

since there is a separation between the bulk liquid and the granules. Concentration gradients

of substrates are present in the biological phase, which are influence by diffusion coefficients,

conversion rates, granule size, density, porosity, etc. To describe the processes inside the

granules, ASM models are not enough. Wang and Zhang (2010) give a review of differentmathematical models to face the dynamics of biofilm systems. One-dimensional or

multidimensional models can be used, depending on the simplicity of the model. Beun et al.


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(2001) describe the simultaneous removal of COD and nitrogen in one dimensional model.

Multidimensional modelling of biofilms is showed by Xavier et al (2005). In particular, for

granular sludge models, discrete particle models should be used instead of continuous.Table

2 shows some examples of models developed for biofilm systems and some in particular for

granular sludge.

Granular sludge models can be implemented in different software. MATLAB allows to

develop the whole model by introducing all the equations and parameters. The program

AQUASIM simplifies the modelling since it has systems already implemented.

4.2. Controlling partial nitritation-anammox in granular sludge reactors

Partial nitritation-anammox (section 1.1) can be developed in granular sludge by growing

granules where AOB and anammox bacteria coexist (section 1.2). This section will describe

some aspects related to operation and control of granular systems performing partial-

nitritation anammox in one stage.

- Control through low oxygen concentration

Good performance of a partial nitritation-anammox system depends on the suppression of

NOB. Partial nitritation-anammox is only achieved when NOB are outcompeted by anammox

bacteria. Controlling the oxygen concentration in the bulk liquid can be used to meet this

purpose. Ammonium oxidizers have a higher oxygen affinity than nitrite oxidizers, thus

limiting the oxygen concentration helps in developing AOB and avoiding the growth of NOB.

Modelling studies confirm this fact (Volcke et al., 2010). Also, full-scale implementations of

partial nitritation-anammox in one stage were reached by controlling the aeration and

keeping low oxygen concentrations inside the reactor, as in the CANON reactor at the

Olburgen sewage treatment plant in The Netherlands (Abma et al., 2010).

- Granule size

The granule size is determined by a balance between growth and detachment. Volcke et al.

(2010) carried out a modelling study assessing the influence of granule size in reactors

performing partial nitritation-anammox with biomass growing in granules. When the particle

size increases, higher oxygen concentrations are required to achieve complete ammonium

removal. Besides, the larger the granules, the lower the aerobic fraction in the granules,

resulting in a relatively higher anammox activity. This outcome was also observed by Volcke et

al. (2012). Higher nitrogen gas production is obtained with simulation studies with larger

granules due to the improvement of anammox activity, while nitrite or nitrate are

accumulated in systems with smaller granules. These results are in agreement with the

findings obtained in experimental studies performed by Vlaeminck et al. (2010). Winkler et al.(2011) also found the presence of NOB in smaller granules, while anammox bacteria govern

big granules. The increased ammonium surface load owing larger granules is linked with these


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results. Higher ammonium surface load results in smaller oxygen penetration depth and as a

consequence less aerobic fraction and more anoxic part for anammox bacteria is feasible.

Furthermore, the oxygen concentration range in the bulk liquid to obtain good anammox

conversion is higher with bigger granules. In general, systems with large granules are less

sensitive to changes in the bulk oxygen concentration, giving easier systems for controlling.

5.

CONCLUSIONS

- Granular sludge reactors are suitable systems for biological nitrogen removal from

wastewater through the innovative partial nitritation-anammox pathway, i.e. completely

autotrophic nitrogen removal. This pathway involves important savings in aeration energy,

external carbon source addition and sludge handling costs compared to conventional

nitrification-denitrification over nitrate, at the same time showing low CO2-emissions.

- The granular sludge technology presents important advantages compared with floc-based

systems. It allows a lower plant footprint and thus results in more cost-effective

wastewater treatment plants.

- Appropriate operational pressures are required to develop good aerobic granular sludge

systems. In general, low organic loading rates, relatively high shear forces, short sludge

settling time and hydraulic retention times lead to good granulation.- Sequencing batch reactors are suitable units to develop aerobic granular sludge, in

particular in view of combined COD, N and P removal. Their operational flexibility allows

parameter modification easily and the establishment of appropriate selection pressures to

develop aerobic granules.

- Modelling is a useful tool for granular sludge reactor design. It allows investigatingthe

combined effect of physicochemical parameters (diffusion coefficients, granule size, among

others) and biological reactions, which results in complex systems.

- Controlling the aeration rate and/or thedissolved oxygen concentration is essential to

establish partial nitritation and anammox reaction in one unit. The granule size and thegranule size distribution also constitute important parameters in the design of granular

sludge reactors for partial nitritation - anammox. In general, low dissolved oxygen

concentrations need to be imposed to achieve partial nitritation-anammox in a one-stage

granular sludge reactor, while the granules should be large enough to allow the growth of

anammox bacteria, but not too large to prevent large inactive zones.


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Abma, W.R., Driessen, W., Haarhuis, R., van Loosdrecht, M.C.M., 2010. Upgrading of sewage treatment

plant by sustainable and cost-effective separate treatment of industrial wastewater. Water

Science and Technology 61(7), 1715-1722.

Adav, S.S., Lee, D.J., Lai, J.Y., 2010. Potential cause of aerobic granular sludge breakdown at high

organic loading rates. Appl Microbiol Biotechnol 85, 16011610.

Arrojo, B., Mosquera-Corral, A., Garrido, J.M., Mendez, R., 2004. Aerobic granulation with industrial

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