Conventional aerobic treatment technologies based on activated sludge processes are predominantly applied for the treatment of domestic wastewater. However, there is a need for more efficient and cost-effective technologies to coincide with the financial constraints on the expansion of sewage treatment coverage in developing countries. Sequential Batch Reactor (SBR) is a potential technology which is flexible and effective for biological wastewater treatment for removal of biological oxygen demand (BOD) and also nutrient removal. SBR systems are composed of one or more tanks which operate in batches. Biological Nutrient removal in SBR systems makes it different from conventional activated sludge process, which are effective in the removal of BOD. SBR involves stages such as fill, react, settle, decant and idle. This study emphasizes the evaluation of sequencing batch reactors technically. 1. IntroductionQuality of water is of major concern to humankind as it is concerned with the welfare of humans. Sources of pollution include domestic wastes, industrial wastes and agricultural wastes. Industrial wastewater is the foremost concern for treatment as the wastewater pollutants are high, hence very difficult to treat. There are many technologies present in the market for treating various organic and inorganic pollutants in the wastewater. The common unit processes include preliminary screening, primary clarification for physical separation of particles, secondary biological treatment for degradable organics removal and tertiary treatment for further removal of pollutants which is optional. The most commonly adopted technique for secondary biological treatment is either aerobic activated sludge process or anaerobic treatment. Sequential Batch reactors (SBR) is a technology-based on activated sludge process operating in batches. A 1983 U.S.EPA report, summarized this by stating that “the SBR is no more than an activated sludge system which operates in time rather than in space”. The difference between the two technologies is that the SBR performs equalization, biological treatment, and secondary clarification in a single tank using a timed control sequence. Many modifications in the system have been developed later with the anaerobic system, anaerobic-aerobic, intermittently operated, continuous flow etc. [1]. Sequential Batch Reactor (SBR) is a potential technology which is flexible and effective for biological wastewater treatment for removal of BOD and also nutrient removal. SBR systems are composed of one or more tanks which operate in batches. Biological nutrient removal in SBR systems makes it different from a conventional activated sludge process, which is effective in the removal of BOD. SBR involves stages such as fill, react, settle, decant and idle. Different design configurations have been developed with specific cycle times with different lengths. It is an ideal system to control bioreaction inside the reactor. Therefore selecting different reaction condition like HRT, cycle times could be used as a tool to get higher removal efficiencies [2]..2. Technical Evaluation2.1. Stages in SBRSequencing batch reactors are technically different from conventional activated sludge process (ASP). The difference lies in conducting the experiments from the same basin, unlike having solids removal systems in conventional ASP. The stages of treatment and the cycle times may depend on the type of treatment planned. It can be aerobic or anaerobic or both in the same basin which cannot be a case in conventional systems.Typical SBR contains five stages in the treatment process, naming a. Fill, b. React, c. Settle, d. Decant, e. Idle.These cycle steps run in a sequence depending on the problem. It may be a single tank or multiple tank process. The time of the complete cycle is between fill to end of the idle cycle in a single tank system. In multiple tank system, the complete cycle is between beginnings of the fill phase in the first tank to end of the idle phase in the last tank. The complete process is summarized below. The overall process is shown in the Fig-1. a. Fill:The fill phase is where the reactor is filled with wastewater. Fill can occur under aerated, unaerated, mixed or unmixed conditions. Quantity of wastewater to be filled depends on loading rate, F/M ratio, HRT, and settling characteristics of the organisms. Duration of fill depends on the type of design and the pollutants targeted to be removed.b. React:React phase starts when the fill phase is complete. In this phase there is no flow of wastewater in the tank. It includes mixing, aeration of the influent and sludge is wasted. Aeration process helps in oxidizing organic carbon, nitrifying ammonia and promote uptake of phosphorous by microbes. When aeration is done for consistent time aeration is stopped, during this condition denitrification happens. Duration of this phase is more than any other phases. In this phase, alternating conditions of low dissolved oxygen concentrations and high dissolved oxygen concentrations may be required. Liquid level is maintained to the maximum in this phase.c. Settle:Settle phase starts when react phase terminates. Sludge is not wasted in this phase. This phase serves for settlement of MLSS after aeration is done. Clearwater is found as a supernatant. Duration of this phase depends on settle ability of the sludge. The major advantage of this system lies in settling sludge in the same aeration tank where volumes are much higher than conventional clarifier systems.d. Decant:Treated wastewater from the reactor is decanted once the settle phase is completed. The decanting process is carried out until a consistent depth of supernatant disappears. Decant process is done from the upper part of the reactor with the help of automatic valves. Decanter mechanisms are available where mechanical floating weirs are used for decanting.e. Idle:The idle phase is an optional phase where settled sludge is wasted. When two or more tanks are employed, this can be eliminated. Based on the design and operation of SBR, sludge wastage can occur at any stage during the react phase, during the decant process, or during the idle phase. This phase takes place daily, weekly or every cycle [3,4,4,5]..2.2. Components of SBRTypical sequencing batch reactor consists of the following components. 1. Aeration system 2.Decanter system 3.Waste sludge pump2.2.1. Aeration Systems:Aeration systems are provided as an oxygen source for microorganisms for biological reactions to happen to degrade the organic content. Aeration is considered to be the most energy intensive process at wastewater treatment plants as it consumes up to 65% of a plant’s total energy need. There are two different kinds of aeration devices used in Sequential batch reactors.a. Diffused aeration system: Diffused aeration is done by inducing air into the fine bubble diffusers through blowers from outside the tank. These diffusers can be made of the membrane, PVDF, ceramic material. There are tube diffusers and disc diffusers. Fig-2(a) shows the photograph of disc diffusers. Fig-2(b) represents tube diffusers. These diffusers have the finest holes on the material covered where they pass air bubbles from [6]..b. Jet Aeration System: To provide the oxygen source for the biological reaction, a jet aeration manifold with multiple jet nozzle assemblies is connected to a pump and a blower. Each jet has a primary mixing nozzle and an outer secondary nozzle. Jet aeration provides high oxygen transfer and complete mixing to provide both anoxic and aerobic conditions as required to promote biological nutrient removal. Jets, with their large solids handling capability and robust design, are ideal for the constant on/off operation common with SBRs [7]..2.2.2. Decanter SystemsDecanters are used for removal of clean water from the top of the SBR tank after settle phase of operation. There are 3 types of decanting systems available.1.Valve arrangement 2.Floating decanters 3.mechanically operated decanters a. Valve Arrangement: Valves are positioned at different levels in the tank’s side, and these are opened sequentially from top to bottom. The outlet quality is poor and inconsistent as the decanting rate is not controlled. Generally used in small installations. Mechanical problems related to valve and actuators require regular maintenance. This is of low cost. These are used in small installations. It is represented in Fig-3 [8]..b. Floating Decanters: The decanter floats on water due to buoyancy and the treated water leaves the tank through a flexible rubber hose. The outlet quality is poor and inconsistent as the decanting rate is not controlled. Buoyancy problems lead to decanter sinking. These are used in small and medium installations. They are higher cost than valve arrangement. It is represented in Fig-4[8]..c. Mechanically operated Decanters: The decanter is pushed down by a top-mounted motor-driven gearbox in a precise manner by PLC. Outlet quality is very good and consistent as the decanter descent rate is controlled. It is used in small and large installations. Good plant life and lesser maintenance problems. Cost-wise it is very high. It is represented in Fig-5 [8]. In mechanically operated decanters there are various types like moving weir decanters, crown decanters, etc..2.3. Advantages of SBR over other technologiesCompletely automaticThe regular time-based changes in operation necessitate automationAutomation includes-Opening and closing of Gates, Valves o Operation of Pumps, Decanters dissolved Oxygen control through VFD based blower operationThe plant can be remotely monitoredPlant operation is independent of operator capabilityConventional plants like ASP, MBBR, UASB are normally manually operated. Lack of qualified manpower affects performance.Electrical consumption will be lower when compared to conventional systems as blowers will not run continuously, and they run only in designated intervals. .2.4. Performance EvaluationSequential batch reactors compete with other technologies with their promising removal of pollutants like BOD, COD and nutrient removal. Besides the removal of BOD with the aerobic system, if anaerobic conditions are also maintained COD removal will be significant. Nitrification, denitrification and biological phosphorous removals are also proven in SBR. COD and BOD removals were >95% in most of the studies with SBR. However, for nitrogen and phosphorus removals were below 80% in most of the studies as shown in the Fig-6(a-d). Table-1 shows the comparison of SBR with other biological treatment for removal of organic content and Nutrient removal [9-47]..2.5. Market Status Sequencing batch reactors are operated using different aeration systems and decanters. Installations in India are mostly found to be applied for sewage treatment applications. The existing plants in India are known to operate with diffused aeration systems and mechanically operated decanters. SBR operation is in a double tank process with the automatic operations are seen most in India. The application of SBRs in India has been demonstrated in Fig-7. SBRs have been applied mostly in sewage treatment plants (STP) in India. SBR is very widely applied internationally using various types of decanters, types of SBR and applications. SBR has been applied to different kinds of wastewater like a tannery, textile, and chemical treatment as well as for sewage application. Most of the SBRs are single tank process. Aeration systems of both jet aeration and diffused aeration are used equally. Decanter systems like floating aerators, fixed aerators, and mechanical weir operated decanters are used [48-59]..3. ConclusionsSequential batch reactors are found to be the most efficient technology for the removal of organic matter, along with nitrogen and phosphorus. A technical evaluation has been carried out based on various reactors present in the literature. SBR, being a single tank system unlike the conventional aerobic treatment technologies, occupies lesser space, operationally flexible and cost-effective. Sequential Batch Reactor (SBR) is a potential technology which can be applied for the treatment of domestic wastewater in the developing countries, which offers variable advantages particularly in the cities, where decentralized sewage treatment plants are mandatory. Space is the major constraint in the cities for the treatment of wastewater. Operational viability is another important constraint. In order to address such issues, sequential batch reactors will be a good option. This study emphasized on the operating principle, types of aerators and decanters used in the research and performance evaluation of the SBR technologies. Application of technology in the market was also successful, but in India, it was limited to sewage application as low strength wastewater. Sequencing Batch Reactor, however as a technology can be explored more in India on high strength wastewaters for removing pollutants from complex natured wastewater as future scope of research. There is a critical need for the industry to take up the patented work on SBR and implement in the market for industrial effluents. For article references contact author
Conventional aerobic treatment technologies based on activated sludge processes are predominantly applied for the treatment of domestic wastewater. However, there is a need for more efficient and cost-effective technologies to coincide with the financial constraints on the expansion of sewage treatment coverage in developing countries. Sequential Batch Reactor (SBR) is a potential technology which is flexible and effective for biological wastewater treatment for removal of biological oxygen demand (BOD) and also nutrient removal. SBR systems are composed of one or more tanks which operate in batches. Biological Nutrient removal in SBR systems makes it different from conventional activated sludge process, which are effective in the removal of BOD. SBR involves stages such as fill, react, settle, decant and idle. This study emphasizes the evaluation of sequencing batch reactors technically. 1. IntroductionQuality of water is of major concern to humankind as it is concerned with the welfare of humans. Sources of pollution include domestic wastes, industrial wastes and agricultural wastes. Industrial wastewater is the foremost concern for treatment as the wastewater pollutants are high, hence very difficult to treat. There are many technologies present in the market for treating various organic and inorganic pollutants in the wastewater. The common unit processes include preliminary screening, primary clarification for physical separation of particles, secondary biological treatment for degradable organics removal and tertiary treatment for further removal of pollutants which is optional. The most commonly adopted technique for secondary biological treatment is either aerobic activated sludge process or anaerobic treatment. Sequential Batch reactors (SBR) is a technology-based on activated sludge process operating in batches. A 1983 U.S.EPA report, summarized this by stating that “the SBR is no more than an activated sludge system which operates in time rather than in space”. The difference between the two technologies is that the SBR performs equalization, biological treatment, and secondary clarification in a single tank using a timed control sequence. Many modifications in the system have been developed later with the anaerobic system, anaerobic-aerobic, intermittently operated, continuous flow etc. [1]. Sequential Batch Reactor (SBR) is a potential technology which is flexible and effective for biological wastewater treatment for removal of BOD and also nutrient removal. SBR systems are composed of one or more tanks which operate in batches. Biological nutrient removal in SBR systems makes it different from a conventional activated sludge process, which is effective in the removal of BOD. SBR involves stages such as fill, react, settle, decant and idle. Different design configurations have been developed with specific cycle times with different lengths. It is an ideal system to control bioreaction inside the reactor. Therefore selecting different reaction condition like HRT, cycle times could be used as a tool to get higher removal efficiencies [2]..2. Technical Evaluation2.1. Stages in SBRSequencing batch reactors are technically different from conventional activated sludge process (ASP). The difference lies in conducting the experiments from the same basin, unlike having solids removal systems in conventional ASP. The stages of treatment and the cycle times may depend on the type of treatment planned. It can be aerobic or anaerobic or both in the same basin which cannot be a case in conventional systems.Typical SBR contains five stages in the treatment process, naming a. Fill, b. React, c. Settle, d. Decant, e. Idle.These cycle steps run in a sequence depending on the problem. It may be a single tank or multiple tank process. The time of the complete cycle is between fill to end of the idle cycle in a single tank system. In multiple tank system, the complete cycle is between beginnings of the fill phase in the first tank to end of the idle phase in the last tank. The complete process is summarized below. The overall process is shown in the Fig-1. a. Fill:The fill phase is where the reactor is filled with wastewater. Fill can occur under aerated, unaerated, mixed or unmixed conditions. Quantity of wastewater to be filled depends on loading rate, F/M ratio, HRT, and settling characteristics of the organisms. Duration of fill depends on the type of design and the pollutants targeted to be removed.b. React:React phase starts when the fill phase is complete. In this phase there is no flow of wastewater in the tank. It includes mixing, aeration of the influent and sludge is wasted. Aeration process helps in oxidizing organic carbon, nitrifying ammonia and promote uptake of phosphorous by microbes. When aeration is done for consistent time aeration is stopped, during this condition denitrification happens. Duration of this phase is more than any other phases. In this phase, alternating conditions of low dissolved oxygen concentrations and high dissolved oxygen concentrations may be required. Liquid level is maintained to the maximum in this phase.c. Settle:Settle phase starts when react phase terminates. Sludge is not wasted in this phase. This phase serves for settlement of MLSS after aeration is done. Clearwater is found as a supernatant. Duration of this phase depends on settle ability of the sludge. The major advantage of this system lies in settling sludge in the same aeration tank where volumes are much higher than conventional clarifier systems.d. Decant:Treated wastewater from the reactor is decanted once the settle phase is completed. The decanting process is carried out until a consistent depth of supernatant disappears. Decant process is done from the upper part of the reactor with the help of automatic valves. Decanter mechanisms are available where mechanical floating weirs are used for decanting.e. Idle:The idle phase is an optional phase where settled sludge is wasted. When two or more tanks are employed, this can be eliminated. Based on the design and operation of SBR, sludge wastage can occur at any stage during the react phase, during the decant process, or during the idle phase. This phase takes place daily, weekly or every cycle [3,4,4,5]..2.2. Components of SBRTypical sequencing batch reactor consists of the following components. 1. Aeration system 2.Decanter system 3.Waste sludge pump2.2.1. Aeration Systems:Aeration systems are provided as an oxygen source for microorganisms for biological reactions to happen to degrade the organic content. Aeration is considered to be the most energy intensive process at wastewater treatment plants as it consumes up to 65% of a plant’s total energy need. There are two different kinds of aeration devices used in Sequential batch reactors.a. Diffused aeration system: Diffused aeration is done by inducing air into the fine bubble diffusers through blowers from outside the tank. These diffusers can be made of the membrane, PVDF, ceramic material. There are tube diffusers and disc diffusers. Fig-2(a) shows the photograph of disc diffusers. Fig-2(b) represents tube diffusers. These diffusers have the finest holes on the material covered where they pass air bubbles from [6]..b. Jet Aeration System: To provide the oxygen source for the biological reaction, a jet aeration manifold with multiple jet nozzle assemblies is connected to a pump and a blower. Each jet has a primary mixing nozzle and an outer secondary nozzle. Jet aeration provides high oxygen transfer and complete mixing to provide both anoxic and aerobic conditions as required to promote biological nutrient removal. Jets, with their large solids handling capability and robust design, are ideal for the constant on/off operation common with SBRs [7]..2.2.2. Decanter SystemsDecanters are used for removal of clean water from the top of the SBR tank after settle phase of operation. There are 3 types of decanting systems available.1.Valve arrangement 2.Floating decanters 3.mechanically operated decanters a. Valve Arrangement: Valves are positioned at different levels in the tank’s side, and these are opened sequentially from top to bottom. The outlet quality is poor and inconsistent as the decanting rate is not controlled. Generally used in small installations. Mechanical problems related to valve and actuators require regular maintenance. This is of low cost. These are used in small installations. It is represented in Fig-3 [8]..b. Floating Decanters: The decanter floats on water due to buoyancy and the treated water leaves the tank through a flexible rubber hose. The outlet quality is poor and inconsistent as the decanting rate is not controlled. Buoyancy problems lead to decanter sinking. These are used in small and medium installations. They are higher cost than valve arrangement. It is represented in Fig-4[8]..c. Mechanically operated Decanters: The decanter is pushed down by a top-mounted motor-driven gearbox in a precise manner by PLC. Outlet quality is very good and consistent as the decanter descent rate is controlled. It is used in small and large installations. Good plant life and lesser maintenance problems. Cost-wise it is very high. It is represented in Fig-5 [8]. In mechanically operated decanters there are various types like moving weir decanters, crown decanters, etc..2.3. Advantages of SBR over other technologiesCompletely automaticThe regular time-based changes in operation necessitate automationAutomation includes-Opening and closing of Gates, Valves o Operation of Pumps, Decanters dissolved Oxygen control through VFD based blower operationThe plant can be remotely monitoredPlant operation is independent of operator capabilityConventional plants like ASP, MBBR, UASB are normally manually operated. Lack of qualified manpower affects performance.Electrical consumption will be lower when compared to conventional systems as blowers will not run continuously, and they run only in designated intervals. .2.4. Performance EvaluationSequential batch reactors compete with other technologies with their promising removal of pollutants like BOD, COD and nutrient removal. Besides the removal of BOD with the aerobic system, if anaerobic conditions are also maintained COD removal will be significant. Nitrification, denitrification and biological phosphorous removals are also proven in SBR. COD and BOD removals were >95% in most of the studies with SBR. However, for nitrogen and phosphorus removals were below 80% in most of the studies as shown in the Fig-6(a-d). Table-1 shows the comparison of SBR with other biological treatment for removal of organic content and Nutrient removal [9-47]..2.5. Market Status Sequencing batch reactors are operated using different aeration systems and decanters. Installations in India are mostly found to be applied for sewage treatment applications. The existing plants in India are known to operate with diffused aeration systems and mechanically operated decanters. SBR operation is in a double tank process with the automatic operations are seen most in India. The application of SBRs in India has been demonstrated in Fig-7. SBRs have been applied mostly in sewage treatment plants (STP) in India. SBR is very widely applied internationally using various types of decanters, types of SBR and applications. SBR has been applied to different kinds of wastewater like a tannery, textile, and chemical treatment as well as for sewage application. Most of the SBRs are single tank process. Aeration systems of both jet aeration and diffused aeration are used equally. Decanter systems like floating aerators, fixed aerators, and mechanical weir operated decanters are used [48-59]..3. ConclusionsSequential batch reactors are found to be the most efficient technology for the removal of organic matter, along with nitrogen and phosphorus. A technical evaluation has been carried out based on various reactors present in the literature. SBR, being a single tank system unlike the conventional aerobic treatment technologies, occupies lesser space, operationally flexible and cost-effective. Sequential Batch Reactor (SBR) is a potential technology which can be applied for the treatment of domestic wastewater in the developing countries, which offers variable advantages particularly in the cities, where decentralized sewage treatment plants are mandatory. Space is the major constraint in the cities for the treatment of wastewater. Operational viability is another important constraint. In order to address such issues, sequential batch reactors will be a good option. This study emphasized on the operating principle, types of aerators and decanters used in the research and performance evaluation of the SBR technologies. Application of technology in the market was also successful, but in India, it was limited to sewage application as low strength wastewater. Sequencing Batch Reactor, however as a technology can be explored more in India on high strength wastewaters for removing pollutants from complex natured wastewater as future scope of research. There is a critical need for the industry to take up the patented work on SBR and implement in the market for industrial effluents. For article references contact author
