Sequencing Batch Reactors
Sequencing Batch Reactor (SBR) is a Fill-and-Draw-principled activated sludge (AS) wastewater treatment process. It has been used since the 1920s and has recently begun growing in popularity around the world. This system can be used to treat municipal and industrial wastewaters and is ideal for both treatment of continuous flow and low or varying flow patterns. Industries such as dairies, meat packing, pulp and paper, tanneries and textiles are using SBRs as a practical wastewater treatment option.
Improvements in equipment and technology have made SBRs a viable choice over conventional activated-sludge systems for a number of reasons. Since the treatment takes place in a single basin, these plants can have a much smaller footprint than other activated-sludge plants. Effluents with TSS levels of less than 10 mg/l can be met consistently using effective decanters, eliminating the need for a separate clarifier.
The treatment cycle can be adjusted to allow aerobic, anaerobic or anoxic conditions to achieve biological nutrient removal, including nitrification, denitrification and phosphorous removal. The treatment process can yield effluent with Biochemical Oxygen Demand (BOD) of less than 5 mg/l consistently. Through aerobic conversion of ammonia to nitrates and the anoxic conversion of nitrates to nitrogen gas within the same tank, total nitrogen limits of less than 5 mg/l can also be achieved. Phosphorous limits of 2 mg/l or less can be met using a combination of anaerobic biological treatment with PAOs (phosphorous absorbing organisms) and chemical agents (aluminum or iron salts) in the same tank.
The process may be set up in many existing wastewater plants since the tank is already in place, making the SBR process a cost-effective retrofit. Additional pumps and blowers may need to be added to the existing tanks.
SBR is a variation of the activated-sludge process. The difference is all treatment steps and processes are carried out in a single basin, whereas, conventional AS plants must have multiple basins. A Sequencing Batch Reactor is no more than an activated-sludge plant that OPERATES IN TIME RATHER THAN SPACE.
The operation of an SBR is based on the fill-and-draw principle and consists of five steps––fill, react, settle, decant and idle. Each step can be altered for different operational applications.
FILL – During the fill phase, the basin receives influent wastewater containing food for the bacteria in the activated sludge. Mixing and aeration can be varied to create the following scenarios.
Static Fill – No mixing or aeration occurs while the influent enters the basin. Static fill can be used at plants that do not need to nitrify/denitrify or during low flow times to save energy.
Mixed Fill – This occurs when the mixers are active while the influent is entering the basin but the aerators remain turned off. This will allow a uniform mixture of food and microorganisms. Since there is no aeration, an anoxic condition, which promotes denitrification, is present. Anaerobic conditions may also be achieved during a mixed fill promoting phosphorous removal by PAOs.
Aerated Fill – As the influent is entering the basin, the mixers and aerators are both on. This converts the anoxic condition to an aerobic condition that allows for the reduction of organic load and for nitrification. If nitrification does take place, the aerators must be shut down to create an anoxic condition to allow denitrification. By cycling the aerators on and off, both aerobic and anoxic conditions are created allowing for both nitrification and denitrification. Dissolved Oxygen (DO) content in the basin must be monitored so it does not go over 0.2 mg/l to assure an anoxic condition will exist during the idle phase.
REACT – No more influent enters the basin during this phase. The mixers and aerators are turned on allowing for a dramatic increase in the rate of organic removal in the water. Most of the carbonaceous BOD removal takes place during the React phase of operation. Further nitrification does take place, but the majority of nitrification takes place during the mixed fill phase.
SETTLE – At the end of the react phase, the aeration and mixing equipment are turned off allowing the activated-sludge to settle as a flocculent mass. A distinct interface between the clean water and biomass can be observed. The biomass settles as a blanket further cleaning the water. The sludge blanket must settle rapidly enough as to not interfere with the decant phase. If the sludge settles too slowly, the biomass may be drawn off degrading effluent quality.
DECANT – Once the solids have settled, a decanter is used to remove the clear supernatant effluent. Floating or fixed-arm decanters can be used. Floating decanters maintain the inlet pipe slightly below the water surface to minimize solids’ removal during the decant phase. Fixed-armed decanters are cheaper and can be designed to allow the operator to change the level of the decanter. Optimal operation of the decanter assures the decanted volume is equal to the volume of influent that enters the basin during the fill step. It is also important to assure no surface foam or scum is decanted. Typically, the volume decanted is no more than one-third of the basin volume.
IDLE – This step occurs between the decant step and the fill step. During this time, the operator should measure the sludge depth in the basin and waste sludge if necessary to a sludge digester or holding tank. It is important to maintain adequate sludge levels and quality to assure BOD and nutrient removal.
A flow diagram for a sample SBR program is shown below.
SBR systems typically consist of two or more treatment basins and some sort of preliminary treatment, such as bar screens to remove rags, sticks and other debris before the influent enters the treatment basin. It is also recommended that the system have an influent-flow equalization tank. This tank will allow for smaller SBR-basin size since influent can be stored until the process cycle is complete. It will also allow for one basin to be taken off-line for maintenance or in low-flow situations to save on electricity use. The influent tank will also allow scum and grease removal from a single point prior to entry to the SBR. Wet-weather flow increases can also be handled with greater efficiency.
The effluent from the SBR should also go into a flow equalization tank. This will allow the effluent to be metered, reducing the effects on disinfection treatment or other downstream processes caused by surging flow. Metering of effluent flow can also allow downstream processes to be sized smaller, saving capital dollars during construction.
As with any activated-sludge system, process control testing is essential. DO, pH and alkalinity testing must be done to assure the process is running correctly and efficiently. Oxidation reduction potential is also a good tool to use to control your process. Any online instrument should be tied into the SCADA system for your plant, allowing the operators to verify and make changes as needed to the SBR control parameters. Location of the probes for online instruments should be placed to allow the operators easy access. These probes must be maintained and calibrated. If they are not easily accessible to the operator, good maintenance, such as cleaning, may be hampered.
This has been a brief introduction to the use of sequencing batch reactors for wastewater treatment. As a treatment system, SBRs are easily adjusted to allow for nutrient removal, and they can handle high organic loads effectively and efficiently. They can be modified to treat wastewater from a wide variety of industries.