Sequencing Batch Reactors in Wastewater Treatment
Sequencing Batch Reactors:
An Efficient Alternative to Wastewater Treatment
Luis H. Abreu and Saribel Estrada
Introduction The Sequencing Batch Reactor (SBR) is an activated sludge process designed to operate under non-steady state conditions. An SBR operates in a true batch mode with aeration and sludge settlement both occurring in the same tank. The major differences between SBR and conventional continuous-flow, activated sludge system is that the SBR tank carries out the functions of equalization aeration and sedimentation in a time sequence rather than in the conventional space sequence of continuous-flow systems. In addition, the SBR system can be designed with the ability to treat a wide range of influent volumes whereas the continuous system is based upon a fixed influent flowrate. Thus, there is a degree of flexibility associated with working in a time rather than in a space sequence .
SBRs produce sludges with good settling properties providing the influent wastewater is admitted into the aeration in a controlled manner. Controls range from a simplified float and timer based system with a PLC to a PC based SCADA system with color graphics using either flow proportional aeration or dissolved oxygen controlled aeration to reduce aeration to reduce energy consumption and enhance the selective pressures for BOD, nutrient removal, and control of filaments . An appropriately designed SBR process is a unique combination of equipment and software. Working with automated control reduces the number of operator skill and attention requirement.
The majority of the aeration equipment of sequencing batch reactors consist of jet, fine bubble, and coarse bubble aeration systems. The main focus of this report is a jet aerated sequencing batch reactor activated sludge system.
Sequencing Batch Reactor Process Cycles The operating principles of a batch activated sludge process, or SBR, are characterized in six discrete periods:
The influent wastewater is distributed throughout the settled sludge through the influent distribution manifold to provide good contact between the microorganisms and the substrate . The influent can be either pumped in allowed to flow in by gravity. Most of this period occurs without aeration to create an environment that favors the procreation of microorganisms with good settling characteristics. Aeration begins at the beginning of this period.
Mixed liquor is drawn through the manifold, mixed with the influent flow in the motive liquid pump, and discharged, as motive liquid, to the jet aerator . This initiates the feast period. Feast is when the microorganisms have been in contact with the substrate and a large amount of oxygen is provided to facilitate the substrate consumption. Nitrification and denitrification occurs at the beginning of this stage. This period ends when the tank is either full or when a maximum time for filling is reached.
React During this period aeration continues until complete biodegradation of BOD and nitrogen is achieved. After the substrate is consumed famine stage starts. During this stage some microorganisms will die because of the lack of food and will help reduce the volume of the settling sludge. The length of the aeration period determines the degree of BOD consumption , .
Aeration is discontinued at this stage and solids separation takes place leaving clear, treated effluent above the sludge blanket. During this clarifying period no liquids should enter or leave the tank to avoid turbulence in the supernatant.
This period is characterized by the withdrawal of treated effluent from approximately two feet below the surface of the mixed liquor by the floating solids excluding decanter . This removal must be done without disturbing the settled sludge.
The time in this stage can be used to waste sludge or perform backwashing of the jet aerator. The wasted sludge is pumped to an anaerobic digester to reduce the volume of the sludge to be discarded. The frequency of sludge wasting ranges between once each cycle to once every two to three months depending upon system design.
A. Jet Aeration Header
Jet aeration offers significant advantages in the SBR process due to its flexibility, good contact between substrate and microorganisms, and efficient oxygen transfer. One of its main features is that it can mix without aerating.Therefore it can provide for aerated and anoxic mix periods. The header in conjunction with a computer controlling for flow proportional aeration makes more oxygen available at higher flows than at lower flows by measuring the rate of change in the flow level in reactor.
Decanting is best achieved through solids excluding decanters. The floating decanter is one of the most efficient and contains a spring loaded plug valve operated by hydraulic differential . This decanter is sustained about sixteen inches below the scum by a float therefore avoiding the decanting of floating matter.
Conclusion Wastewater treatment has been a challenge throughout the years due to varying influent chemical and physical characteristics and stringent effluent regulations. Treatment systems using activated sludge have been able to handle many of these difficulties. Given the lack of on-line computer controls, continuous flow systems have been mostly used for these purposes versus sequencing batch processes. The availability of artificial intelligence has now made the option of a SBR process more attractive thus providing better controls and results in wastewater treatment. This is coupled by the flexibility of a SBR in the treatment of variable flows, minimum operator interaction required, option for anoxic or anaerobic conditions in the same tank, good oxygen contact with microorganisms and substrate, small floor space, and good removal efficiency.
Sequencing batch reactors operate by a cycle of periods consisting of fill, react, settle, decant, and idle. The duration, oxygen concentration, and mixing in these periods could be altered according to the needs of the particular treatment plant. Appropriate aeration and decanting is essential for the correct operations of these plants. The aerator should make the oxygen readily available to the microorganisms. The decanter should avoid the intake of floating matter from the tank. The many advantages offered by the SBR process justifies the recent increase in the implementation of this process in industrial and municipal wastewater treatment.