136 Int. J. Environmental Engineering, Vol. 1, No. 2, 2009

Copyright © 2009 Inderscience Enterprises Ltd.

Aerobic sludge granulation: state-of-the-art

José Luis Campos*, Mónica Figueroa, Anuska Mosquera-Corral and Ramón Méndez Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, Rua. Lope Gómez de Marzoa s/n, E-15782, Santiago de Compostela, Spain Fax: +34 981 528050 E-mail: joseluis.campos@usc.es E-mail: monica.figueroa@usc.es E-mail: anuska.mosquera@usc.es E-mail: ramon.mendez.pampin@usc.es *Corresponding author

Abstract: Recently, the research focused on the development and operation of aerobic granular biomass has increased in interest, due to the advantages of this option as compared to the conventional activated sludge systems traditionally used for the biological treatment of wastewater. The aerobic granular sludge is produced in Sequential Batch Reactors (SBR) where a large amount of biomass is accumulated by means of the improvement of its settling properties. These compact systems allow the reduction of the required space for implantation and favour the treatment of large loads of contaminants and the performance of the nitrification and denitrification processes simultaneously.

Keywords: aerobic granules; industrial effluents; SBR; sequencing batch reactor; shear stress; SND; simultaneous nitrification denitrification.

Reference to this paper should be made as follows: Campos, J.L., Figueroa, M., Mosquera-Corral, A. and Méndez, R. (2009) ‘Aerobic sludge granulation: state-of-the-art’, Int. J. Environmental Engineering, Vol. 1, No. 2, pp.136–151.

Biographical notes: José Luis Campos is an Assistant Professor of the Department of Chemical Engineering, University of Santiago de Compostela (Spain). He has been involved in several European projects related to biofilms and Anammox process. His most recent work focused on the Anammox process, partial nitrification and sludge reduction by ozonation.

Mónica Figueroa is pursuing her PhD student in the Department of Chemical Engineering, University of Santiago de Compostela (Spain). Her research is focused on the identification of micro-organisms in industrial wastewater treatment systems and aerobic granulation. She has published papers in several conferences proceedings and specialised journals.

Anuska Mosquera-Corral is an Assistant Professor in the Department of Chemical Engineering, University of Santiago de Compostela (Spain). Her research interests include issues related to aerobic granulation and nitrogen removal from industrial wastewaters. She has participated in more than 30 papers related to different topics on wastewater treatment.

Aerobic sludge granulation: state-of-the-art 137

Ramón Méndez is a Professor in the Department of Chemical Engineering, University of Santiago de Compostela (Spain). His research fields of interest include fish cannery by-products recovery, new technologies of wastewater treatment and clean technologies for the tannery industry.

1 Introduction

The biological treatment of wastewater in the Wastewater Treatment Plants (WWTPs) is often accomplished by means of the application of conventional activated sludge systems. These systems generally require large surface areas for implantation of the treatment and biomass separation units due to the usually poor settling properties of the sludge. The increase in the number of inhabitants, in many cases concentrated in urban areas, raises the need for upgrading already existing wastewater treatment plants or building up new ones to cope with the proportional increase of amounts of wastewater production. Often, the space available for these constructions is restricted. This means that the new treatment systems must occupy as small a surface as possible. One possible means to reduce the volume of the required units is the development of systems based on the improvement of biomass retention; for example, by performing actions to improve the settleability properties of the activated sludge. In aerobic systems the growth of biomass adhering to support materials has been widely studied in airlift or fluidised bed reactors (Heijnen et al., 1990; Tijhuis et al., 1995).

In the 1990s the first studies of aerobic granular sludge were performed. Sludge granulation in aerobic conditions has been firstly reported in continuously operated systems where the removal of organic matter and/or ammonia nitrogen occurs. Mishima and Nakamura (1991) used the so called Aerobic Upflow Sludge Blanket (AUSB) to treat a synthetic wastewater previously aerated with pure oxygen to oxidise the organic matter and obtained granules with diameters ranging from 2 mm to 8 mm.

Although granular sludge from the selected nitrifying sludge is not frequently reported, some works are available, such as the initial one of van Benthum et al. (1996), who proposed a dynamic model to explain the formation of nitrifying granules in a Biofilm Airlift Suspension (BAS) reactor through the breaking-up of biofilms which form on basalt particles. The removed patches of biofilms grew in the form of individual granules. Later, Campos et al. (2000) reported the formation of nitrifying granular sludge in an airlift system with average particle diameters ranging from 0.22 µm to 0.36 µm, treating loads up to 4.1 g NH4

+-N/(L·d) using synthetic wastewater. The common feature of these works was that no control of the formation of the nitrifying granules was achieved and granules were obtained either in special conditions in non-repeated experiments or using carrier material as the initiator.

Recent research has showed that it is possible to grow granular sludge in Sequencing Batch Reactors (SBR) without carrier at large Dissolved Oxygen (DO) concentrations using either a synthetic wastewater (Dangcong et al., 1999; Beun et al., 1999, 2002a) or industrial effluents (Arrojo et al., 2004). From this point onwards, an increasing number and diversity of studies reflecting research on aerobic granular sludge were developed making it necessary to establish a definition to discern between an aerobic granule and a simple floc with relatively good settling properties.

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This definition of ‘aerobic granule’ emerged from the discussions which took place at the “1st IWA-Workshop Aerobic Granular Sludge” in Munich (2004) and literally stated that:

“Granules making up aerobic granular activated sludge are to be understood as aggregates of microbial origin, which do not coagulate under reduced hydrodynamic shear and which settle significantly faster than activated sludge flocs.” (de Kreuk et al., 2005a)

In this paper a revision of the different works based on aerobic granulation is performed, which is focused on the operational conditions needed to obtain these aggregates, the capacity to carry out the Simultaneous Nitrification Denitrification (SND), applications to the treatment of industrial wastewater and pilot scale experiences.

2 Formation of aerobic granules

Aerobic granulation can be considered as the gathering together of cells through cell-to-cell immobilisation to form a stable, contiguous, multi-cellular association. Evidence shows that aerobic granulation is a gradual process from seed sludge to compact aggregates, further to granular sludge and, finally, to mature granules (Tay et al., 2001a). Obviously, for cells in a culture to aggregate, a number of operational and environmental conditions have to be fulfilled and some of them are described in the following sections.

2.1 Reactor operational conditions

2.1.1 Feast: famine regime

From previous research it is known that to promote the formation of aerobic granular sludge, in systems where oxidation of organic matter takes place, short feeding periods must be selected (Beun et al., 1999, 2000). With this procedure high substrate concentrations are reached in the reactor in short periods of time. The reaction phase comprises, in these cases, two different periods: the feast and famine periods characterised by the presence, or absence, of organic matter in the liquid media, respectively. During the feast period the organic matter is oxidised and stored inside the bacteria cells as glycogen, lipids or Poly-β-Hidroxyalkanoates (PHA) (van Loosdrecht et al., 1997), while during the famine period the bacteria grow on the stored compounds (Beun et al., 2002b).

With this feeding strategy the selection of the appropriate micro-organisms to form granules is achieved. The use of this selection pressure was based initially in the hypothesis of Chudoba et al. (1973) who claimed that the filamentous micro-organisms are slow-growing micro-organisms characterised by maximum specific growth rates and affinity constants lower than the floc-forming micro-organisms. This peculiarity makes them gain in the competition with the floc formers when the substrate concentrations in the bulk liquid are low. On the other hand, the floc formers are at an advantage when the substrate concentration is high. Recently, Martins et al. (2003) found out that this hypothesis is not always applicable. They proposed a new hypothesis to explain this competition based on diffusion limitations inside the flocs caused by low substrate concentrations in the media. In this case filamentous bacteria which grow in one or two dimensions overtake the floc formers in the access to the substrate. On the other hand,

Aerobic sludge granulation: state-of-the-art 139

other authors (Bossier and Verstraete, 1996; Liu et al., 2004) also observed that bacteria become more hydrophobic under periodic feast-famine conditions which facilitate microbial aggregation.

What seems clear is that the presence of large substrate concentrations in the liquid media during the aeration phase helps the formation of granules. This periodic feast-famine regime, which acts as a kind of microbial selection pressure condition, is easily achieved in discontinuous reactors such as the SBR type.

2.1.2 Short settling time

Strategies to promote the formation of aerobic granules include the use of short settling times. This hydraulic selection pressure on the microbial community allows retaining granular biomass inside the reactor while the flocculent biomass is washed-out. Qin et al. (2004a, 2004b) studied the effect of this parameter on aerobic granulation in SBR systems and found that aerobic granules were successfully cultivated and became dominant only in SBRs operating at settling times shorter than 5 min, while a mixture of aerobic granules and suspended sludge developed in SBRs run at longer settling times. In aerobic granulation research, a short settling time has been commonly used to enhance aerobic granulation in SBR (Jiang et al., 2002; Lin et al., 2003; Liu et al., 2003; Yang et al., 2003; Wang et al., 2004; Hu et al., 2005). In fact, when long settling times are applied, poorly settling sludge flocs cannot be effectively withdrawn; and they may overtake granule-forming micro-organisms. As a result, aerobic granulation can fail in SBR systems run at too long settling times. This seems to indicate that aerobic granules can be formed only at short settling times below a critical level, the settling time being a decisive factor in the formation of aerobic granules in SBR.

It is accepted that the selection pressure imposed by short settling times is important in fully aerobic systems, but in anaerobic-aerobic systems with Phosphate Accumulating Organisms (PAOs), the settling criteria seem less important because of the inherent tendency of PAOs to aggregate.

2.1.3 Hydrodynamic shear force

Evidence shows that the application of high shear forces favours the formation of aerobic granules and physical granule integrity (Shin et al., 1992; Tay et al., 2001b; Di Iaconi et al., 2006). It was found that aerobic granules could be formed only above a threshold shear force value in terms of superficial upflow air velocity above 1.2 cm/s in a column SBR and more regular, rounder and more compact aerobic granules were developed at high hydrodynamic shear forces (Tay et al., 2001b; Wang et al., 2005). The density and strength of the granules were also proportionally related to the applied shear force (Tay et al., 2003). These observations seem to imply that the physical structure of aerobic granules is mainly determined by the hydrodynamic shear force present in a bioreactor. Extra-cellular polysaccharides can mediate both cohesion and adhesion of cells and play a crucial role in maintaining the structural integrity in a community of immobilised cells. Tay et al. (2001b) reported that the shear force stimulated bacteria to secrete more extra-cellular polysaccharides and the stability of aerobic granules was found to be related to the production of extra-cellular polysaccharides (Tay et al., 2001c) as was previously observed in biofilms (Ohashi and Harada, 1994). Consequently, the combination of high shear hydrodynamic

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conditions and the enhancement of extra-cellular polysaccharides production contribute to the compact and strong structure of aerobic granules. Some effects of shear on micro-organisms and aggregates have been discussed further elsewhere (Chisti, 1999).

Tay et al. (2004a), working with a Sequencing Batch Airlift Reactor (SBAR), observed that low superficial air velocities caused an outbreak of filamentous micro-organisms, which gave rise to poorly settling sludge and eventual biomass washout, but high superficial velocities significantly improved the settling characteristics of the biomass granules.

In most of the discussions about shear and the difficulty of measuring the value of applied shear to a certain system makes the comparison between different systems difficult. In systems where the mixture is achieved by air flow (airlift reactors, bubble columns) shear is related to the DO which is a parameter that has been found to seriously affect the stability of the granules when it is limited.

2.2 Environmental conditions

2.2.1 Substrate composition

Aerobic granules have been successfully grown using both synthetic and industrial wastewater, granule microstructure and species diversity being related to the type of carbon source (Beun et al., 1999; Peng et al., 1999; Tay et al., 2004b; Moy et al., 2002; Schwarzenbeck et al., 2005; Arrojo et al., 2004). Those granules obtained in glucose-fed SBRs exhibited filamentous structures, while those obtained in acetate-fed SBRs presented a non-filamentous structure in which rod like species of bacteria were predominant. More recently, aerobic granules were also successfully developed in laboratory-scale SBRs for treating particulate organic matter-rich wastewater (Schwarzenbeck and Wilderer, 2005). From these researches it can be stated that almost any wastewater with enough biodegradable organic matter content is suitable for treatment in an aerobic granular SBR. Aerobic granules, comprising mainly nitrifying bacteria, have been also cultivated in SBRs (Tsuneda et al., 2003, 2006; Mosquera-Corral et al., 2005a).

2.2.2 Presence of divalent cations

Another parameter to favour the formation and stability of granular biomass in aerobic conditions is the presence of divalent cations in the treated media such as iron and calcium (Tsuneda et al., 2005). Ions such as Ca+2 have been proposed to act as a bridge to bind negatively charged groups present on bacterial surfaces and/or extracellular polysaccharides molecules to adhere individual bacteria to each other. From experimental work it has been observed that the addition of Ca+2 in concentrations of 100 mg Ca+2/L to the feeding media allowed the faster formation of aerobic granules (16 days) than in cultures without Ca+2 additions (32 days), which also showed better settling and strength characteristics and had higher polysaccharides content (Jiang et al., 2003).

All the different parameters previously discussed seemed to play a role in granule stability. There is no conclusive agreement on which of these parameters is the crucial one and this is most probably due to the fact that it is a combination of all of them. Results obtained from aerobic granulation must be compared to those obtained from

Aerobic sludge granulation: state-of-the-art 141

bulking sludge in order to be able to better understand the granulation procedure and the main parameters involved.

3 Simultaneous Nitrification Denitrification in aerobic granules

A SBAR, in which aerobic granular sludge can be grown, is a suitable system for simultaneous Chemical Oxygen Demand (COD) and nitrogen removal SND. Special circumstances, depending on granules size, oxygen concentration and COD/N ratio, can lead to a substrate rich aggregate with an oxygen free zone in the interior (diffusion limitation of oxygen), in which denitrification takes place (Table 1).

Table 1 Efficiencies of nitrification (Enitrif) and of nitrogen removal (ENrem) in aerobic granular systems

COD/N OD

(mg/L) T (°C) dparticle (mm) Enitrif (%) ENrem (%)

OLR (g COD/(L·d))

NLR (g N/(L·d)) Authors

21 ≥2 25 1.0–2.0 97 97 2.5 0.12 Kim et al. (2004) 4.4–13.3 ≥4 25 0.8–4.6 ≥99 24–32 2 0.15–0.45 Qin and Liu

(2006) 8.3 8 20 1.6 100 16 1.6 0.19 Mosquera-Corral

et al. (2005b) 0 ≥2 – 0.36 100 – 0 Up to 1.5 Tsuneda et al.

(2003) 0 ≥2 20–25 0.3–0.5 100 – 0 Up to 2.5 Kim and Seo

(2006)

In the granular sludge SBAR system, the substrate (acetate) is supplied in a short period of time. High substrate concentrations in the bulk liquid cause a penetration of substrate deeper into the granule than oxygen. During the period in which the external substrate is available (feast period), oxygen is quickly consumed in the outer layers by growth, substrate storage and nitrification. Oxygen has a limited penetration depth in this period. Acetate storage (as poly-β-hydroxybutyrate or PHB) and growth occur aerobically in the outermost layer or anoxically inside the granule (Figure 1). During the period without external substrate (famine period), growth takes place on the internally stored PHB at much lower growth rates (Beun et al., 2001). The oxygen penetration depth during the famine period will be higher, because of the decrease of the heterotrophic respiration rate, but will still be limited due to increased nitrification relative to the feast phase (Third et al., 2003). The produced nitrate can be simultaneously denitrified inside the granule using the stored PHB as electron donor. The applied oxygen concentration in combination with the size of the granule or biofilm is closely related to the size of the anoxic zone: the lower the oxygen concentration or the bigger the granule, the larger the anoxic zone and thus, the larger the nitrogen removal capacity.

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Figure 1 Theoretical concentration profiles of acetate, PHB, NH4+, NOx

– and O2 during the feast (a) the famine and (b) period in a SBR (external mass transfer neglected)

(a) (b)

The distribution of biomass inside the granule affects the performance of the processes taking place in the reactors. In aerobic granule systems, where COD removal and nitrification take place simultaneously, heterotrophic and autotrophic micro-organisms compete inside the granules for space and oxygen (Figure 2). Fast growing micro-organisms (heterotrophs) will be located in the outer layers while slow growing micro-organisms (autotrophs) will be confined to the deeper zones of the granule with lower oxygen availability (van Loosdrecht et al., 1995). After the feast period the nitrifiers get sufficient oxygen, because of the mentioned increased oxygen penetration depth due to the lower oxygen requirement of heterotrophs growing on PHB (Beun et al., 2001).

Figure 2 Processes combination into the granule

Recent works were focused on the capacity of PHB as internal carbon source for denitrification (Third et al., 2003; Zeng et al., 2003; Qin and Liu, 2006; Qin et al., 2005). Qin et al. (2005) carried out different batch tests to study the PHB storage behaviour in granules under anaerobic conditions. They observed that the specific denitrification rate with the supply of external carbon source is almost ten times higher than that calculated with PHB as internal carbon source. These authors also found that under unbalanced growth conditions (no nitrogen, but with the presence of organic carbon), PHB production would be the relatively main mechanism for removing organic carbon as microbial growth was suppressed by nitrogen limitation. Whereas under

Aerobic sludge granulation: state-of-the-art 143

balanced growth conditions (with organic carbon and nitrogen supply), external organic carbon was consumed simultaneously for denitrification, PHB storage as well as for cell growth and maintenance. Qin and Liu (2006) also observed, under anaerobic conditions, a slight denitrification when external carbon source was not available and a complete denitrification was only achieved when an external carbon source was available. They concluded that the potential of PHB for denitrification would be limited and PHB could not serve as a readily available internal carbon source for denitrification as expected. Micro-organisms would preferentially utilise PHB for cell functions prior to denitrification when the PHB pool falls below a critical level.

Removal of nitrogen was carried out by assimilation and SND, the predominance of each mechanism being related to the COD/N ratio in the feeding media (Table 2) (Mosquera-Corral et al., 2005a).

Table 2 Percentages of nitrogen assimilated and denitrified referred to the inlet ammonia concentration

COD/N (g/g) Nitrogen assimilated (%) Nitrogen denitrified (%)

0 0 0 1.25 0 0 2.5 4 29–37 5.0 8 40–46 15.0 39 14–16

Source: Mosquera-Corral et al. (2005a)

The SND processes can be optimised by the control of DO in the bulk, the optimal DO value being around 40% saturated air (Beun et al., 2001). Mosquera-Corral et al. (2005b) evaluated the long-term effects of decreased oxygen concentrations at saturation levels of 100 and 40% and observed an increase in the nitrogen removal from 8% to 45% when the oxygen saturation was reduced to 40%. Nevertheless, these authors could not maintain stable operation under these conditions due to the disintegration of the granules and the biomass washout. They also evaluated the short-term effects at saturation levels between 100% and 10%, observing an increase of the nitrogen removal when the DO level decreased. The maximum nitrogen removal was 34% at a saturation level of 10%. Contrary to the full aerobic pulse-fed operation carried out by Mosquera-Corral et al. (2005b) and de Kreuk et al. (2005b) introduced an anaerobic feeding period in the cycle of the aerobic granular reactor which allowed a stable granule formation at low oxygen levels and COD, N and P removal simultaneously. These authors observed that the nitrogen removal percentage increased from 34% to 94% when oxygen saturation was decreased from 100% to 20% while phosphate removal maintained constant around 95%. They also remark that nitrogen removal efficiency not only depends on the DO level but also on granule diameter because both parameters influence the percentages of oxic and anoxic zones. Therefore, it is necessary to find an optimum value between granule and oxygen supply to obtain high nitrogen removal efficiency.

Temperature also has an important effect on nitrogen removal efficiency because its influence on the consumption rates causes variations in DO levels along the granule. de Kreuk et al. (2005c) observed that the decrease of temperature from 20°C to 8°C

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caused a reduction on the nitrogen removal from 75% to 44% during a long-term experiment due to decrease of the anoxic zone volume.

On the other hand, since nitrification is the limiting step of nitrogen removal due to the low growing rate of nitrifiers, granulation of these micro-organisms would be useful in order to improve their retention within the systems. Nitrifying granules were obtained by Kim and Seo (2006) in a sequencing batch airlift reactor by shortening the settling time and by Tsuneda et al. (2003) in a continuous system (an upflow fluidised bed) by reducing hydraulic retention time. Kim and Seo (2006) observed that nitrite was the final product when the inlet ammonia concentration was increased up to 600 mg NH4

+-N/L. These authors explained the wash-out of the nitrite oxidisers due to their inhibition by free ammonia and nitrous acid concentrations. In these sequencing systems free ammonia concentrations were high at the beginning of the aeration cycle. Once ammonia was consumed by ammonia oxidisers its inhibitory effect decreased but nitrite started to accumulate and, then, the concentration of nitrous acid was enough to stop the nitrite oxidizing activity. However, it must be taken into account that during the initial period the reactor operated at a DO concentrations higher than 4 mg/L but when the inlet ammonia concentration was increased from 400 mg to 600 mg NH4

+-N/L, the DO concentration decreased to a value around 2 mg/L, which can limit the activity of nitrite oxidisers (Garrido et al., 1997; Lazarova et al., 1998). Tsuneda et al. (2003) analysed the microbial community during the operation of their nitrifying granular SBR and found that the main ammonia-oxidising populations were Nitrosococcus mobilis and Nitrosomonas marina. They also observed that colonies of ammonia-oxidising bacteria existed in the outer layers of the granules (up to a depth of approximately of 100 µm) while nitrite-oxidising bacteria (Nitrobacter) were also observed in the granule surface but their number was quite small. However, Kim and Seo (2006), after an operational period of 100 days, found that Nitrosomonas spp. was the dominant ammonia oxidiser population (64% of the total bacteria) and the nitrite oxidisers, Nitrobacter and Nitrospira, gradually disappeared.

4 Treatment of industrial wastewater

Synthetic wastewater was used in most of the works carried out with aerobic granules. These works were mainly focused on the study of granules stability and nutrient removal efficiencies under different operational conditions and their potential use in the removal of toxic compounds (Tay et al., 2005a; Zheng et al., 2005; Xu et al., 2006). Up to now there are only few research works where aerobic granulation is applied to treat industrial wastewater.

In lab-scale reactors nitrogen and COD removal from industrial wastewater was successfully carried out by Arrojo et al. (2004) and Cassidy and Belia (2005) who also studied phosphorus removal. Arrojo et al. (2004) operated two lab-scale granular reactors, one always under aerobic conditions and other with an initial anoxic phase. Both reactors were fed with industrial wastewater produced in a laboratory for analysis of dairy products (Total COD: 1500–3000 mg/L; soluble COD: 300–1500 mg/L; total nitrogen: 50–200 mg/L). These authors applied organic and nitrogen loading rates up to 7 g COD/(L·d) and 0.7 g N/(L·d) obtaining removal efficiencies for both units of 70 and 80%, respectively. They observed similar specific denitrifying activity in both units but the specific nitrifying activity in the reactor operated always under aerobic conditions

Aerobic sludge granulation: state-of-the-art 145

was 50% higher than in the reactor with an anoxic phase. Cassidy and Belia (2005) obtained removal efficiencies for COD and P of 98% and for N and VSS over 97% operating a granular reactor fed with slaughterhouse wastewater (Total COD: 7685 mg/L; soluble COD: 5163 mg/L; TKN: 1057 mg/L and VSS: 1520 mg/L). To obtain these high removal percentages, they operated the reactor at a DO saturation level of 40%, which is the optimal value predicted by Beun et al. (2001) for N removal and with an anaerobic feeding period which helped to maintain the stability of the granules when the DO concentration was limited (de Kreuk et al., 2005b).

The TSS removal efficiency is a crucial aspect for the application of aerobic granular sludge technology since major fractions of the pollutants are present in the form of colloids or particles. The influent used by Arrojo et al. (2004) contained a TSS concentrations of 200–1200 mg/L and only effluent concentrations lower than 200 mg/L were achieved when the withdrawal time was decreased and the particulate COD/VSS applied was limited. Schwarzenbeck et al. (2004) treated malting wastewater, which also had a high content of particulate organic matter (0.9 g TSS/L). They found that particles with average diameters lower than 25–50 µm were removed at 80% efficiency, whereas particles bigger than 50 µm were only removed with 40% efficiency. These authors observed that the ability of aerobic granular sludge to remove particulate organic matter from the wastewaters was due to both incorporation into the biofilm matrix and metabolic activity of the protozoa population covering the surface of the granules. On the other hand, Inizan et al. (2005) treated industrial wastewaters from pharmaceutical industry and observed that the suspended solids in the inlet wastewater were not removed in the reactor.

Recently, Tsuneda et al. (2006), when treating wastewater from metal-refinery process (1.0–1.5 g NH4

+-N/L and up to 22 g/L of sodium sulphate), removed a nitrogen loading rate of 1.0 kg-N/m3·d with an efficiency of 95% in a system containing autotrophic granules.

5 Pilot research in aerobic granular sludge

A large pilot research project was started in The Netherlands in order to demonstrate the applicability of the aerobic granular sludge technology for the treatment of municipal wastewater. Several operational philosophies were tested to learn the conditions under which granulation occurs, with municipal wastewater as a substrate. Fast formation of granules was observed under conditions of extensive COD-removal, extensive biological phosphate removal and low nitrate effluent concentrations. The potential of the technology is very promising since complete granulation with municipal wastewater as substrate was shown and extensive nutrient removal seems well feasible (de Bruin et al., 2005).

The feasibility study showed that the aerobic granular sludge technology seems very promising (de Bruin et al., 2004). Based on total annual costs, a Granular sludge Sequencing Batch Reactors (GSBR) with pre-treatment and a GSBR with post-treatment proves to be more attractive than the reference activated sludge alternatives (6–16%). A sensitivity analysis shows that the GSBR technology is less sensitive to land price and more sensitive to rain water flow. Because of the high allowable volumetric load the footprint of the GSBR variants is only 25% compared to the references. However, the GSBR with only primary treatment cannot meet the present effluent standards for

146 J.L. Campos et al.

municipal wastewater, mainly because it exceeds the standards for suspended solids effluent caused by washout of not well settleable biomass.

Moreover, municipal wastewater treatment plants are going to be faced with more stringent effluent standards. In general, activated sludge plants will have to be extended with a post treatment step (e.g., sand filtration) or be transformed into a membrane bioreactor. In this case a GSBR variant with primary treatment as well as post treatment can be attractive alternatives.

Tay et al. (2005b) also studied the development of aerobic granules in a pilot-scale sequencing batch reactor seeded with aerobic granules pre-cultured in a small column reactor. It was observed that the seed granules disintegrated in the first few days of operation, but the disintegrated biomass would re-form stable granules later. On the contrary, the seed granules could be successfully maintained in the laboratory-scale reactor without significant disintegration. The different hydrodynamic patterns encountered in the pilot and lab-scale reactors might be the reason for the observed phenomena. Factors such as reactor diameter and wall effect, as well as size and placement of air diffusers, would influence the hydrodynamic conditions in the reactors which, in turn, determine the properties of the granules.

6 Future application

The interest in applying aerobic granular systems is mainly related to the compactness of its design in comparison to conventional activated sludge wastewater treatment systems. The reduction in the system volume is possible mainly due to the following reasons

• external settling is not required

• large biomass concentrations are achieved

• nitrogen and phosphorus removal can be carried out in a single unit.

Studies performed with aerobic granular systems treating industrial wastewater indicated that this technology is suitable to obtain large efficiencies in terms of COD and nutrients removal. However, a pre- or post-treatment is recommended to fulfil the disposal requirements when important suspended solids concentrations are present.

In spite of the recent intensive research carried out in the aerobic granulation field, there is still lack of information about some aspects which could limit the widespread use of aerobic granulation in wastewater treatment:

• Most of the works were carried out at laboratory scale and only a few at the pilot scale (de Bruin et al., 2005; Tay et al., 2005b). The scale-up of granular systems leads to a modification of the hydrodynamic conditions which are very important in order to promote the formation and stability of aerobic granules; but further information is required on this particular issue.

• Little information is available regarding the application of aerobic granular technology to the treatment of low-strength domestic wastewater (de Kreuk et al., 2006; Liu et al., 2007), which is expected to complicate or lengthen the granulation process (Tay et al., 2004c). Thus, it is necessary to know the effects of the low COD loads on the integrity of the granular structures.

Aerobic sludge granulation: state-of-the-art 147

• The concentration gradients of oxygen determined the distribution and performance of the different populations of micro-organisms inside the granules (Li and Liu, 2005). These profiles are the result of factors such as diffusion coefficients values, conversion rates, granule sizes, pore sizes distribution, permeability and density (Zheng and Yu, 2007). All of these factors tightly influence each other; thus, the individual effects of each factor cannot be studied experimentally. In this sense, the development of mathematical modelling would be very a useful tool to provide significant insights into the most important factors that affect the nutrient removal rates and in the distribution of different microbial populations in the granules (Ni et al., 2007). Furthermore, the model could also be used for process optimisation and for the scale-up, design and control of a full-scale reactor (de Kreuk et al., 2007).

Acknowledgements

The authors would like to thank to the Spanish Government (Biogramem project CTQ2005-04935/PPQ and ‘From STP to STREP’-CONSOLIDER CSD2007-00055).

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