LITERATURE REVIEW
D. Sequencing Batch Reactor (SBR)
2.3.5.2 Attached growth system
In attached growth systems microbes exists in an anchored form, either on the surface of an inert carrier or attached to one another. The carrier could be the wall of the reactor, baffles provided for this purpose etc (Ødegaard et al. 2006). Bioreactors implying biofilm systems play important roles in detoxification of hazardous organic contaminants such as volatile aromatic hydrocarbons, chlorinated solvents, phenolics and chlorinated aromatics (Hosseini and Borghei, 2005). In comparison with the suspended-growth wastewater treatment systems, the advantages of the attached growth systems are: (1) the treatment plant require less space and compactness due to the availability of biofilm media with high specific surface area; (2) the high biomass hold up in biofilm enables the process to be operated significantly at higher hydraulic/organic loading and the final treatment results is less dependent on biomass separation since the biomass concentration to be separated is 10 times lower than suspended growth system and (3) the attached biofilm acts as buffer to reduce the concentration of toxic chemicals thereby providing advantage for the treatment of low biodegradable industrial wastewater containing recalcitrant compounds and also (4) provides lower sensitivity and better recovery from shock loadings and supposed to be robust for toxic and changing wastewater stream where slowly growing organisms with special metabolic capabilities are to be protected from washout (Wilderer et al. 1993;
Zhang et al. 1998; Ødegaard et al. 2006). Biofilm processes have proved to be reliable for organic carbon and nitrogen removal without some of the problems of activated sludge processes (Yang et al. 2009). Various biofilm processes such as trickling filter, rotating biological contactor, packed bed reactor, fluidized bed reactor and moving bed biofilm
reactor etc are in use for industrial wastewater treatment process and have been studied to improve the treatment performance of the industrial and domestic wastewater removing organics and improving nitrification process (Li et al. 2003; Jeong and Chung, 2006a; Lai et al. 2008).
Tziotzios et al. (2005) studied phenol removal using same inoculums in two systems, one operated as suspended growth system and other as a packed bed system. The later was capable of higher phenol removal almost 12.65 times than the former resulting in less reaction time requirement. Ramos et al. (2007) found submerged fixed film reactor having good capacity for eliminating high concentration of phenol (1000 mg/L) which also implied high COD removal in presence of total nitrogen 400 mg/L at HRT 1 day with air flow and recirculation. The fixed film system also exhibited advantages over suspended growth system like higher concentration of active biomass on immobilized media.
However the system could not remove total nitrogen more than 63%. Zaiat et al. (2001) reported utilization of biofilm, where microorganisms were immobilized and this technology seems to be promising, since it eliminates uncertainty as to the granulation phenomenon, solves the problem of solids retention and may eliminate the sedimentation step, hence reducing total cycle period and extending the range of possible uses of sequencing batch biofilm reactor (SBBR). Goh et al. (2009) reported SBBR with polyurethane sponge cubes showed better performance than sequencing batch reactor (SBR) as SBBR achieved almost complete removal of ammonia nitrogen removal while SBR achieved average 86% when both were used in simultaneous removal of p- nitrophenol and ammonia-nitrogen. Dictor et al. (1997) observed maximum thiocyanate biodegradation rate 5.3 g/L.day using heterotrophic microorganisms and packed-bed reactor charged with pumice stone and zeolite. Mudliar et al. (2008) reported almost 98%
of pyridine removal in a rotating rope bioreactor from pyridine loading of 0.78 g/L.day that was higher than completely mixed activated sludge process reported by Padoley et al.
(2006). Banerjee (1996) studied four- stage across the flow laboratory scale RBC reactor for treatment of simulated steel plant wastewater containing phenol and thiocyanate. The treatment revealed that the toxicants were removed in sequence. Phenol was mostly removed in the earlier (first and second) stages and thiocyanate was removed in the later (third and fourth) stages. Mohan et al. (2007) studied SBBR for low biodegradable
composite chemical wastewater (low COD/BOD ratio ~ 0.3) with fixed packed bed system operated at anoxic – aerobic –anoxic microenvironment. The reactor was capable of removing 88-55% COD and 89-75% BOD at 1 day operational HRT and organic loading of 0.90-4.76 g COD/L.day, which was more than suspended growth system operated at same conditions.
In fixed bed attached growth system the carrier material is kept fixed to the system and thus microbes get attached and remain in contact with the substrate with the flow of influent whereas in case of moving bed biofilm system the carrier material along with microbes keep moving through out the reactor due to influent flow or external mixing. The moving bed biofilm reactor (MBBR) process was introduced about 30 years ago and it has since become popular in Europe (Ødegaard, 2006). The basic idea of the MBBR is to have a continuous operated biofilm reactor with a high density of biomass and without backwashing or sludge return. Moving bed reactor is a combination of conventional activated sludge process and fluidized bed system, where biomass is grown on small carrier elements like sponge etc. having density less than water and some inert material like sand, basalts, granulated activated carbon, kaldane paricles and particles of polymers etc (Ødegaard et al. 1994). Chu and Wang (2011) reported better performance by reactor with sponge cube than that of biodegradable polymer as carrier. MBR is less prone to clogging; biomass in the effluent is less than suspended growth system and is suitable for slow growing nitrifying bacteria (Chen et al. 2008). Contrary to most biofilm reactors, the moving bed reactor utilizes the whole tank volume for biomass growth, and would be more efficient and stable to remove toxic pollutants from the wastewater. MBRs are reported for treatment of wastewater from poultry processing, pulp and paper industry, refinery and slaughterhouse, coke industry and also landfill leachate etc. (Johnson et al. 2000; Sigrun et al. 2002; Rusten et al. 2003; Chen et al. 2008). Moussavi et al. (2009) used a moving-bed sequencing batch reactor (MSBR) for treatment of high concentration of phenol (50-3000 mg/L) from synthetic wastewater. The system revealed efficient phenol and COD removals when operated at a single reactor with 30% carrier material without any recycle. The activity of the biofilm involved in the phenol degradation was almost eight times more than that of the suspended biomass. The inhibition concentration of phenol was found to be 3000 mg/L. The optimum HRT for MSBR was 40 h at which removal efficiency of phenol
and COD were greater than 99% and reactor was resistant to shock loading at various operational conditions. Borghei et al. (2008) studied performance of moving bed biofilm reactor (MBBR) in terms of phenol-COD to total-COD ratio. Results showed that the system performance was significantly affected by the ratio and system HRT. However the system was stable against hydraulic and toxic load and it can recover to steady state condition after 24 h. The system performance was better at HRT higher than 1 day.
Canziani et al. (2006) reported 30-40% saving of the carbon required for denitrification when MBBR as denitrification unit was installed after membrane bioreactor treating landfill leachate as the nitrification was partially completed to nitrite saving oxygen requirement. Li et al. (2011) used a laboratory-scale moving bed biofilm reactor (MBBR) with a volume of 4 L to study the biodegradation of coal gasification wastewater.
Maximum removal efficiencies of 81%, 89%, 94% and 93% were obtained for COD, phenols, SCN- and NH4+ -N, respectively. They observed that NO2- -N accumulation induced increase of effluent COD concentration when the hydraulic residence time (HRT) decreased. Phenols removal was not affected when the HRT decreased from 2-1.5 days.
Effluent SCN- and NH4+ -N concentration increased with the decrease of the HRT, and decreased gradually when the HRT returned to 2 days. Yang et al. (2009) studied a moving bed membrane bioreactor filled with carriers instead of activated sludge for simultaneous removal of organic carbon and nitrogen in wastewater and compared its performance with a conventional membrane bioreactor at various influent COD/TN ratios of 8.9-22.1. The moving bed membrane bioreactor system demonstrated better performance on nitrogen removal at different COD/TN ratios compared to conventional membrane bioreactor. Also, multifunctional microbial reactions in the carrier, such as simultaneous nitrification and denitrification (SND), play important roles in nitrogen removal. They observed from the specific oxygen utilization rate that the biofilm has a better microbial activity than an activated sludge. Nevertheless, the membrane fouling behavior was more severe in the moving bed membrane bioreactor than in the other due to a thick and dense cake layer formed on the membrane surface.