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Sequential Batch Reactor for Bio Degradation of Organic Wastewater: A Review

Wastewater treatment is challengeable in today’s scenario, as it contains many types and varying physical and chemical pollutants which enormously effect the environment and its living beings. The current review elaborates treatment of various organic effluents using sequential batch reactor (SBR). Reactor operating conditions like anaerobic, anoxic and aerobic in single or mixed forms have been covered in the review. Literatures say that SBR can be used to treat many organic, industrial and municipal wastewater (MWW) successfully. Strict effluent characteristics from government force the individuals to treat the effluent to such extent so that it can match the discharge norms of wastewater.


Introduction
To serve the life, water is one of the foremost essential thing. Without water there is no life on earth. Out of 71% water available in earth,5% water is contemporary. It is primarily impure by phylogeny sources like industrial, agricultural and household activities. Owing to growing industries and urbanization, contaminants in water is increasing day by day. Discharging untreated wastewater to water bodies lead to several environmental problems like (i) depletion of dissolved oxygen (DO) of the water stream, which is required to aquatic life (ii) untreated wastewater contains large no of pathogens, or disease causing microorganisms, that causes various epidemic diseases, (iii) it can contain toxic compounds which have harmful effects on human health. Therefore, the treatment of wastewater is very important before leaving it in the natural water bodies.
Wastewater can be treated by using different physical, chemical and biological treatment methods including biological degradation, chemical precipitation, adsorption, ion exchange, reverse osmosis, coagulation, flocculation, etc. All these treatment methods have different performance characteristics and also different direct impacts on the environment.
Conventional activated sludge (CAS) is most commonly used method for treatment of wastewater. This process generally not give water discharge quality, therefore, further treatment is required. Moreover, excessive amount of sludge produced in this process is one of the major drawbacks. The sludge produced is highly tough to stabilize and dewater 1 . Due to this, effective methods are needed that can produce low sludge to reduce the prices related to sludge dewatering and sludge production. SBR and MBR is advance process to that of CAS, which give better water quality and low sludge generation. Regulation in operating conditions like DO and sludge retention time (SRT) and addition of chemicals to reduce biomass growth are some options to reduce biomass production 2 .SBR is a biological treatment method and its application has increased significantly than other biological treatment methods is due to (i) inherent flexibility in cycle time can be regulated and different organic load can be treated, (ii)ease of operation and less area is required (iii) low treatment cost (iv) low sludge generation [3][4][5] . SBR treatment system consists of five sequencing operation: filling of wastewater to the reactor (fill phase); degradation of organic matter (react phase); settling of biomass (settle phase); withdrawing of treated water from reactor (draw phase) and keeping the reactor to inoperative condition (idle phase)for about 15 -30 minutes for preparation of next cycle 6 . SBR in various forms have been originally used for reduction of COD and BOD from different types of wastewaters [7][8][9][10] . The SBR has several benefits to the activated sludge. The aim of this paper is to highlight the operation and use of SBR for treatment of wastewater.

Sequential Batch Reactor
A SBR is sort of activated sludge method applied for the wastewater treatment. According to 1999 U.S. EPA report 11 , SBR operates based on space and ASP based on time. The operation of SBR has been described by Irvine and Davis 12 . SBR can treat wastewater that is biodegradable, which can be directly generated from process or it can be pretreated by anaerobic digestion. To reduce organic load i.e COD and BOD the air is bubbled through wastewater and activated sludge mixture. The treated effluent could also be appropriate for discharge to water receiving bodies like river, pond, surface waters or presumably to be used towards land. There are many configurations of SBRs, the fundamental method is analogous. SBR installation may consist of one or additional tanks which is operated mainly as fully mixed reactors. The raw waste (influent) enters the one end and treated water (effluent) goes out the opposite. In multiple tank system one tank is operated as settled and decant mode while opposite in aerating and filling mode. This helps to mix the incoming influent and the returned activated sludge. High amount of pollutants like BOD, COD, TS, Total Kjeldahal Nitrogen (TKN), phosphorous, oil and grease removal have been observed in treatment of various effluents like Tannery 13 , Paper mill 14  The SBRs are used for (i) Anaerobic (ii) Aerobic treatment of wastewater which is described below.

Anaerobic Process
Anoxic SBR are often used for anaerobic treatment processes. During this case, the reactors are purged of oxygen by flushing with nitrogen. As the microorganisms multiply and die, the sludge at intervals in the tank increases with time which is removed using sludge pump. The mass or age of sludge in the tank is closely monitored at time intervals, because it has a vigorous impact on the treatment method. The sludge is allowed to settle till clear water is obtained at the top of the reactor as supernatant, mostly 20-30% of the tank contains sludge. The clear liquid can be further treated or it can be used as water source to vegetables.

Aerobic Process
When oxygen is added to the SBR, it enhances the multiplication of aerobic microorganism so that they can consume the nutrients and hydrocarbons. The method converses ammonia to chemical group and nitrate forms is referred to as nitrification. COD and BOD also reduced by oxidation bacteria. The sludge attached with microorganisms is allowed to settle in the tank. The aerobic microorganism still multiply till the dissolved oxygen is virtually spent. The schematic diagram of SBR process is presented in Figure 1.The utilization of SBR in wastewater treatment is presented in Table 1.

Figure 1: Schematic diagram of SBR
The hydraulic retention time (HRT) and fill time have been found to major effect on SBR operation which is discussed below.

Effect of HRT
HRT play an important role during wastewater treatment in SBR. HRT is defined as the time required by the wastewater to pass through the system. Effect of HRT on degradation of pollutants from coking wastewater (CWW) was studied in a pilot plant SBR 37 . Average values of COD = 1100 -1700 mg/dm 3 , Phenol = 185 -253 mg/dm 3 , thiocyanide = 210 -485 mg/dm 3 , ammonia nitrogen = 532 -567 mg/dm 3 contained in CWW. For HRT = 58 -225 h, COD removal was in between 69 to 81 %, phenol removal was 97 to 99 %, SCNremoval was 90 to 98 %, NH 3 -N removal was 41 to 85 %. In the study, 58 h HRT was found to be optimum. A study done by Kushwaha et al. 38 for the treatment of diary waste water showed 96.5 % COD removal and 64.61 % TKN removal at HRT = 24 h. Similar studies were performed by Thakur et al. 39 to treat petroleum refinery wastewater (PRW) which had a COD = 350 ± 25 mg/dm 3 and TOC = 70 ± 10 mg/dm 3 . HRT was varied in between 0.56 to 3.33 days. Among these studies, HRT = 0.83 d gave maximum 80 % COD and 83 % TOC removal. The reason for lower COD and TOC removal at HRT> 0.83 was due to lower growth rate of microorganisms and accumulation of older cell.

Effect of Fill Time
The reactor needs to give feed which has to be-treated. Amount of total feed given in particular time is known as fill time. Thakur et al. 39 studied the fill time variation for COD and TOC removal of PRW. In fill time of 0.5, 1 and 2 h, respectively, COD removal efficiencies were 58 %, 68 % and 74 %,and TOC removal were 28 %, 51 % and 59 %. Pollutants removal rate was low initially for higher fill time, which was increased when time proceeded. Effect of fill time was also performed by Kushwaha et al. 38 . Forfill time = 0 to 2 h, they have also found increase in COD reduction with increase in fill time. DO was found to increase with increase in fill time. Yu andGu 40 , didfill time studies for treatment of synthetic phenolic wastewater in the two SBRs which was operated at aerated fill and un-aerated fill conditions. When phenol concentration was low (< 400 mg/dm 3 ), the SBR was operated at un-aerated fill condition performed better to that SBR operated with aerated fill condition. It was also noted that at higher phenol concentration (> 800 mg/dm 3 ), accumulation of phenol during fill period had became inhibitory to microorganisms causes low phenol removal efficiency and low growth of dispersed biomass. The studies show,fill strategies should be selected according to wastewater composition, biodegradability and concentration of toxic substances in wastewater.
As we know that SBR has been utilized for treatment of various effluents. The work done by various researchers are presented below: Sarfaraz et al. 41 conducted anoxic treatment for degradation of phenol in SBR using granular denitrifying sludge. The different cycle lengths and influent phenol concentration was main variable parameters. In the processupto 80% phenol was degraded from its initial value of 1050 mg/ dm 3 at cycle length of 6 h, which was corresponded to 6.4 g COD/dm 3 .d. When phenol concentration was increased, the phenol and COD removal efficiencies was decreased. Tomei et al. 42 performed the biodegradation of 4-nitrophenol (4NP) in a SBR. In the experiments, both long feed phase and high biomass concentration showed much effective to reduce the 4 NP. Sahinkaya and Dilek 43 investigated the biodegradation kinetics of 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) separately in batch reactors and in mixed SBR. Fang et al. 44 investigated removal efficiency of phenol from synthetic wastewater using anaerobic thermophilic condition (55°C). Maximum phenol removal 99% was achieved at HRT of 40 h. Shariati et al. 45 have treated synthetic petroleum wastewater in a SBR at different HRT, similarly,Kutty et al. 46 also used six different SBR to treat PRW having COD concentration in the range of 500-750 mg/dm 3 . Experiments were performed at anaerobic and aerobic modes with a 24 h cycle in 2 dm 3 reactor. The process gave COD removal of 91%, 91%, and 88% respectively, for aerobic reactor, combined anaerobic-aerobic reactors and aerobic mixed.
Derlon et al. 47 studied the formation of aerobic granular sludge in SBR for MWW treatment.Granular sludge formation was possible at low upflow velocities during anaerobic feeding phase. In a study by Alvarez et al. 48 for treatment of DSin a two stage anaerobic pilot plant technique, total COD removal of 49% -65% obtained with a 35.1% methane conversion from influent COD.
Gutiérrezet al. 49 did the lab scale removal of carbon and nitrogen from dairy wastewater using SBR. They used a 15dm 3 reactor for treatment and found the aeration time 4.5 h to optimum. During operation, the HRT was 4 days and 20 days. In the process, COD reduction reached to 97% and total nitrogen to 90%. Similarly,studied byKushwahaet al. 38 for treatment of dairy wastewater, the optimization of parameters like fill time, HRT, sludge disposal was done.Up to 97.05 % COD removal and 63.08 % TKN removal was observed.Rajabet al. 50 investigated the performance of a lab scale anaerobic/aerobic SBR for poultry slaughterhouse wastewater. The anaerobic reactor of volume 12 dm 3 was used but the aerobic reactor volume varied according to the flow rate. Experiments were performed at room temperature of 26-28°C. The results obtained were overall COD removal of 97% ± 2%, NH 3 -N removal 98% ± 1.3%, oil and grease removal 90% ± 11% and total suspended solids (TSS) removal 96% ± 3%.
Gonza´lez et al. 51 worked on the photo fenton oxidation and sequential batch biofilm reactor. For 200 mg/ dm 3 of antibiotic sulfamethoxazole containing water, the 75.7% TOC removal obtained. Biodegradation of organic compounds Dichlorodiethyl ether (DCDE) was performed in SBR. For this, removal of organic was 92% in term of COD and 95% in term of TOC. Miqueletoet al. 52 analysed the performance of anaerobic SBR for COD removal of synthetic glucose solution. At optimum condition, 93-97% COD removal was seen for 500 mg/dm 3 glucose solution. Evaluation and characterization of granular formation was performed by Jang et al. 53 in aerobic and anoxic conditions. After 50 days of operation, the size of granules was found to be 1 ± 0.35 to 1.39 ± 0.45 mm. COD removal and nitrification efficiency was 95% and 97% respectively.
Some of the literatures are listed in Table 2.

Conclusions
Following conclusions are drawn from the review 1. SBR can be applied for the treatment of almost all the industrial and MWW with wide variety of organic (COD) contents. 2. It is better to conventional activated sludge process in terms of space requirement and extent of pollutant removal. 3. SBR can bear shock load of organics, thus it can be used at low and high organic contents in wastewater. 4. It can be used at anaerobic, anoxic and aerobic conditions. 5. Sludge generation is less so management of sludge is easy.