Aeration, a component of SBR 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.
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.
Quality 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. . 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. Technical Evaluation
2.1. Stages in SBR
Sequencing 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.
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.
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.
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.
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.
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].