The biological treatment of wastewater in the waste water treatment plant 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. In recent years, new technologies are being developed to improve this system. The use of aerobic granular sludge is one of them.
Aerobic granular biomass
A definition to discern between an aerobic granule and a simple floc with relatively good settling properties came out 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. 2005)"
Formation of aerobic granules
Granular sludge biomass is developed in Sequencing Batch Reactors (SBR) and without carrier materials. These systems fulfil most of the requirements for their formation as:
Feast - Famine regime: short feeding periods must be selected to create feast and famine periods (Beun et al. 1999), characterized by the presence or absence of organic matter in the liquid media, respectively. With this feeding strategy the selection of the appropriate micro-organisms to form granules is achieved. When the substrate concentration in the bulk liquid is high, the granule-former organisms can storage the organic matter in form of poly-β-hydroxybutyrate to be consumed in the famine period, being in advantage with the filamentous organisms.
Short settling time: This hydraulic selection pressure on the microbial community allows retaining granular biomass inside the reactor while flocculent biomass is washed-out. (Qin et al. 2004)
Hydrodynamic shear force : Evidences show that the application of high shear forces favours the formation of aerobic granules and the physical granule integrity. 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., 2001 ).
Advantages
The development of biomass in the form of aerobic granules is being recently under study for its application to the removal of organic matter, nitrogen and phosphorus compounds from wastewater. Aerobic granules in aerobic SBR present several advantages compared to conventional activated sludge process such as:
Stability and flexibility: the SBR system can be adapted to fluctuating conditions with the ability to withstand shock and toxic loadings
Excellent settling properties: a smaller secondary settler will be necessary, which means a lower surface requirement for the construction of the plant.
Good biomass retention: higher biomass concentrations inside the reactor can be achieved, and higher substrate loading rates can be treated.
Presence of aerobic and anoxic zones inside the granules to perform simultaneously different biological processes in the same system (Beun et al.. 1999)
The cost of running a wastewater treatment plant working with aerobic granular sludge can be reduced by at least 20% and space requirements can be reduced by as much as 75% (de Kreuk et al.., 2004).
Treatment of industrial wastewater
Synthetic wastewater was used in most of the works carried out with aerobic granules. These works were mainly focussed on the study of granules formation, stability and nutrient removal efficiencies under different operational conditions and their potential use to remove toxic compounds. The potential of this technology to treat industrial wastewater is under study, some of the results:
* Arrojo et al. (2004) operated two reactors that 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 of 80%.
* Schwarzenbeck et al. (2004) treated malting wastewater which 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 at 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 protozoa population covering the surface of the granules.
* 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.
* 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.
* 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.
* Usmani et al. (2008) high superficial air velocity, a relatively short settling time of 5-30 min, a high ratio of height to diameter (H/D=20) of the reactor and optimum ogranic load facilitates the cultivation of regular compact and circular granules.
* Figueroa et al. (2008), treated wastewater from a fish canning industry. Applied OLR were up to 1.72 kg COD/(m3·d) with fully organic matter depletion. Ammonia nitrogen was removed via nitrification-denitrification up to 40% when nitrogen loading rates were of 0.18 kg N/(m3·d). The formation of mature aerobic granules occurred after 75 days of operation with 3.4 mm of diameter, SVI of 30 mL/g VSS and density around 60 g VSS/L-granule
* Farooqi et al. (2008), Wastewaters from fossil fuel refining, pharmaceuticals, and pesticides are the main sources of phenolic compounds. Those with more complex structures are often more toxic than the simple phenol. This study was aimed at assessing the efficacy of granular sludge in UASB and SBR for the treatment of mixtures of phenolics compounds. The results indicates that anaerobic treatment by UASB and aerobic treatment by SBR can be successfully used for phenol/cresol mixture, representative of major substrates in chemical and petrochemical wastewater and the results shows proper acclimatization period is essential for the degradation of m - cresol and phenol. Moreover, SBR was found as a better alternative than UASB reactor as it is more efficient and higher concentration of m cresols can be successfully degraded.
Pilot research in aerobic granular sludge
Aerobic granulation technology for the application in wastewater treatment is widely developed at laboratory scales. The large-scale experience is still limited but different institutions are making efforts to improve this technology:
* Since 1999 DHV Water, Delft University of technology (TUD), STW (Dutch Foundation for Applied Technology) and STOWA (Dutch Foundation for Applied Water Research) have been cooperating closely on the development of the aerobic granular sludge technology (Nereda). Based on the results obtained, a pilot plant was started up in September 2003 in Ede (Netherlands). The heart of the installation consists of two parallel biological reactors with each a height and diameter of 6 m and 0.6 respectively and a volume of 1.5 m3.
* From the basis of the aerobic granular sludge but using a contention system for the granules, a sequencing batch biofilter granular reactor (SBBGR) with a volume of 3.1m3 was developed by IRSA (Istituto di Ricerca Sulle Acque, Italy). Different studies were carried out in this plant treating sewage at an Italian wastewater treatment plant.
* The use of aerobic granules prepared in laboratory, as a starter culture, before adding in main system, is the base of the technology ARGUS (Aerobic Granules Upgrade System) developed by EcoEngineering Ltd.. The granules are cultivated on-site in small bioreactors called propagators and fill up only 2 to 3% of the main bioreactor or fermentor (digestor) capacity. This system is being used in a pilot plant with a volume of 2.7 m3 located in one Hungarian pharmaceutical industry.
* The Group of Environmental Engineering and Bioprocesses from the University of Santiago de Compostela is currently operating a 100 L pilot plant reactor.
The feasibility study showed that the aerobic granular sludge technology seems very promising (de Bruin et al., 2004. Based on total annual costs a GSBR (Granular sludge Sequencing Batch Reactors) 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 municipal wastewater, mainly because of exceeding the suspended solids effluent standard caused by washout of not well settleable biomass.
From http://en.wikipedia.org/
