Sequencing Batch Reactor Systems

Sequencing Batch Reactor Model Diagram 1

Sequencing Batch Reactor Model Diagram 2

The sequencing batch reactor (SBR) process is a sequential suspended growth (activated sludge) process in which all major steps occur in the same tank in sequential order (figure 1). There are two major classifications of SBRs: the intermittent flow (IF) or "true batch reactor," which employs all the steps in figure 1, and the continuous flow (CF) system, which does not follow these steps. Both have been used successfully at a variety of  installations. SBRs can be designed and operated to enhance removal of nitrogen, phosphorus, and ammonia, in addition to removing TSS and BOD. The intermittent flow SBR accepts influent only at specified intervals and, in general, follows the five-step sequence. There are usually two IF units in parallel. Because this system is closed to influent flow during the treatment cycle, two units may be operated in parallel, with one unit open for intake while the other runs through the remainder of the cycles. In the continuous inflow SBR, influent flows continuously during all phases of the treatment cycle. To reduce short-circuiting, a partition is normally added to the tank to separate the turbulent aeration zone from the quiescent area.

Figure 1. Sequencing batch reactor (SBR) design principle


The SBR system is typically found in packaged configurations for onsite and small community or cluster applications. The major components of the package include the batch tank, aerator, mixer, decanter device, process control system (including timers), pumps, piping, and appurtenances. Aeration may be provided by diffused air or mechanical devices. SBRs are often sized to provide mixing as well and are operated by the process control timers. Mechanical aerators have the added value of potential operation as mixers or aerators. The decanter is a critical element in the process. Several decanter configurations are available, including fixed and floating units. At least one commercial package employs a thermal processing step for the excess sludge produced and wasted during the "idle" step. The key to the SBR process is the control system, which consists of a combination of level sensors, timers, and microprocessors. Programmable logic controllers can be configured to suit the owner's needs. This provides a precise and versatile means of control.
Typical applications
SBR package plants have found application as onsite systems in some states and counties where they are allowed by code. They are normally used to achieve a higher degree of treatment than a continuous-flow, suspended-growth aerobic system (CFSGAS) unit by eliminating impacts caused by influent flow fluctuations. For discharge to surface waters, they must meet effluent permit limits on BOD, TSS, and possibly ammonia. Additional disinfection is required to meet effluent fecal coliform requirements. For subsurface discharge, they can be used in situations where infiltrative surface organic loadings must be reduced. There are data showing that a higher quality effluent may reduce soil absorption field area requirements. The process may be used to achieve nitrification as well as nitrogen and phosphorus removal prior to surface and subsurface discharge.
Design assumptions
Typical IF system design information is provided in table 1. With CF-type SBRs, a typical cycle time is 3 to 4 hours, with 50 percent of that cycle devoted to aeration (step 2), 25 percent to settling (step 3), and 25 percent to decant (step 4). With both types, downstream or subsequent unit processes (e.g., disinfection) must be designed for greater capacity (because the effluent flow is several times the influent flow during the decant period) or an equalization tank must be used to permit a consistent flow to those processes.

Table 1. Design parameters for IF-type SBR treatment systems


SBR systems


Septic tank or equivalent

Mixed liquor suspended solids (mg/L)

2,000 - 6,500

F/M load (lb BOD/d/ML VSS)

0.04 - 0.20

Hydraulic retention time (h)

9 - 30

Total cycle times (h)a

4 - 12

Solids retention time (days)

20 - 40

Decanter overflow ratea (gpm/ft2)


Sludge wasting

As needed to maintain performance

Cycle times should be tuned to effluent quality requirements, wastewater flow, and other site constraints.

Onsite package units should be constructed of noncorrosive materials, such as coated concrete, plastic, fiberglass, or coated steel. Some units are installed aboveground on a concrete slab with proper housing to protect against local climatic concerns. The units can also be buried underground as long as easy access is provided to all mechanical parts, electrical control systems, and water surfaces. All electric components should meet NEC code and should be waterproofed and/or sheltered from the elements. If airlift pumps are used, large-diameter pipes should be provided to avoid clogging. Blowers, pumps, and other mechanical devices should be designed for continuous heavy-duty use. Easy access to all moving parts must be provided for routine maintenance. An effective alarm system should be installed to alert homeowners or management entities of malfunctions. The area requirements for SBR package plants are similar to those in Fact Sheets 1 and 2.
With appropriate design and operation, SBR plants have been reported to produce high quality BOD and TSS effluents. Typical ranges of CBOD5 (carbonaceous 5-day BOD) are from 5 to 15 mg/L. TSS ranges from 10 to 30 mg/L in well-operated systems. FC removal of 1 to 2 logs can be expected. Normally, nitrification can be attained most of the time unless cold temperatures persist. The SBR systems produce a more reliable effluent quality than CFSGAS or FFS owing to the random nature of the wastewater generated from an individual home. The CF/SBR is also capable of meeting secondary effluent standards (30 mg/L of CBOD and TSS), but more subject to upset by randomly generated wastewaters than the IF/SBR  if short-circuiting cannot be minimized.
Management needs
Long-term management (including operation and maintenance) of SBRs through homeowner service contracts or local management programs is an important component of the operation and maintenance program. Homeowners do not typically possess the skills needed or the desire to learn to perform proper operation and maintenance. In addition, homeowner neglect, ignorance, or interference (e.g., disabling alarm systems) has contributed to operational malfunctions. No wasting of biomass should be practiced until a satisfactory concentration has developed. Intensive surveillance by qualified personnel is desirable during the first months of startup.
Most operating parameters in SBR package systems can be controlled by the operator. Time clock controls may be used to regulate cycle times for each cycle, adjusted for and depending on observed performance. Alarm systems that warn of aerator system failure and/or pump failure are essential.
Inspections are recommended three to four times per year; septage pumping (solids wasting) is dependent upon inspection results. Routine maintenance requirements for onsite SBRs are given below. Operation and maintenance requires semiskilled personnel. Based on field experience, 5 to 12 person-hours per year, plus analytical services, are required. The process produces 0.6 to 0.9 lb TSS/lb BOD removed and requires between 3.0 and 10 kWh/day for operation. Operating personnel prefer these systems to CFSGAS for their simplicity of O/M tasks. The key operational components are the programmer and the decanter, and these must be maintained in proper working order. The primary O/M tasks are provided in table 2.

Table 2. Suggested maintenance for sequencing batch reactor package plants

Systems component

Suggested maintenance tasks

Reaction tank

Check for foaming and uneven air distribution; check for floating scum; check decanter operation and adjust as required; adjust cycle time sequences as required to achieve effluent target concentrations; check settled sludge volume and adjust waste pumping to maintain target MLVSS levels.

Aeration system-diffused air

Check air filters, seals, oil level, and backpressure; perform manufacturer's required maintenance.

Aeration system-mechanical

Check for vibrations and overheating; check oil level, and seals; perform manufacturer's required maintenance.

Septic tank (primary clarifier)

Check for accumulated solids and order pumping if required.


Check functions of all controls and alarms; check electrical control box.

Sludge wasting

Pump waste solids as required to maintain target MLVSS range (typically 500 to 4,000 mg/L).


Measure aeration tank grab sample for MLVSS, pH, and settle ability; collect final effluent decant composite sample and analyze for water quality parameters as required (BOD, TSS, pH, N, P, etc.).

Risk management issues
With proper management, a package SBR system is reliable and should pose no unacceptable risks to the homeowner or the environment. If neglected, however, the process can result in environmental damage through production of poor quality effluent that may pose public health risks and can result in the premature failure of subsurface systems. Odor and noise may also create some level of nuisance. SBRs are less susceptible to flow and quality loading changes than other aerobic biological systems, but they are still not suitable for seasonal applications. They are similarly susceptible to extreme cold and should be buried and/or insulated in areas subjected to these extremes. Local authorities can provide guidance on climatic effects on equipment and how to prevent them. The controller should be located in a heated environment. Long power outages can result in odors and effluent degradation, as is the case with other aerobic biological systems.

Components of the SBR
  • Bar Screen or Septic Tank (Optional): Preliminary Treatment, such as a bar screen, in- line grinder or Septic Tank, is typically provided to accept raw sewage from the building services to intercept and remove rags, plastics and debris which would impair the process or clog the pumps. Restaurant or other special wastewater should have additional grease traps or preliminary treatment as appropriate.
  • Influent Batch Equalization (EQ) Tank: Effluent from Preliminary Treatment discharges into the Influent EQ Tank. The Influent EQ Tank stores the wastewater until the volume of one batch is reached. The Influent EQ Tank is aerated intermittently to maintain sufficient dissolved oxygen to prevent the generation of odours and the settling of solids.
  • Influent Pumps: The wastewater is pumped from the Batch EQ Tank to the SBR Tank. Influent Pumps are controlled by the PLC and a water level input from a float or pressure transducer in the Influent Tank.
  • SBR Tanks: Upon completion of the Filling Stage, the Influent Pump shuts off and the Reaction Stage begins. The SBR Tank is aerated by diffusers mounted near the bottom of the tank, and supplied by an air blower in the control building. During this stage the microorganisms in the SBR mix with the wastewater and, through biological activity, break down and consume the organics and nutrients in the wastewater. After the Reaction Stage is completed, the blower is either turned off or redirected to aerate another SBR Tank. Suspended solids in the SBR are allowed to settle in the Settling Stage.
  • Aeration System: Aeration of the Influent EQ and Sludge Tanks is accomplished by coarse bubble diffusers for maintenance-free effective mixing and oxygen transfer. Aeration of the SBR Tanks is accomplished by fine bubble diffusers with superior oxygen transfer capacity. Blowers are typically regenerative type for low energy consumption and low noise generation.
  • Decant Pumps: Decanting of the SBR Tanks can be by pumping or by gravity. Depending on the configuration, the Decant System would consist of either Decant Pumps or Decant Valves (actuated pinch valves). Following the Settling Stage, the PLC signals the decant mechanism to start decanting. The Decant Pump turns on or the Decant Valve opens to transfer the process effluent to the next stage of treatment such as filtration, disinfection, effluent storage (if reuse is proposed), or outfall.
  • Sludge Pumps: The population of microorganisms will increase over time. In order to maintain an optimum population, some of the settled sludge must be removed from the SBR Tanks. A Sludge Pump is provide in each SBR Tank to transfer some of the settled sludge into the Sludge Tank.
  • Sludge Management System: The Sludge Tank has an overflow at the normal operating water level which leads to the Influent EQ Tank. This maintains the level of this tank full under normal operating conditions. As the waste sludge is pumped into the Sludge Tank, this displaces the same volume of supernatant into the Influent EQ Tank. The Sludge Tank is aerated intermittently by diffusers which are supplied by an air blower in the control building. Accumulated sludge should be pumped out of the Sludge Tank on a regular basis. By observation of the sludge blanket level with a "Sludge Judge" or a Secchi disk, the operator would arrange for the removal of the settled sludge as required.
  • Tanks (FRP Option) : Heavy duty fiberglass vessels are typically supplied. The vessels have internal baffles to separate the various functions. Tanks are complete with access hatches for operation and maintenance.
  • Tanks (Concrete Option): Tanks are constructed of reinforced concrete including a top slab with access hatches for maintenance. Tanks are to be watertight including sealing around pipe penetrations. The top slab of the tanks may serve as the floor of the control building. This arrangement has been incorporated successfully in three of our SBR facilities. ABL prepares dimensional drawings for structural design of the tanks by others.
  • Disinfection System (Optional): If disinfection is required, the plant can be equipped with either sodium hypochlorite or ultraviolet disinfection. The disinfection system is sized to suit the discharge rate of the plant.

Control System

The program logic for the PLC for the ABL SBR© was developed by ABL. The PLC program automatically adjusts the number of cycles based on the flow rate through the plant. The number of cycles is variable (typically ranging from 4 to 8 cycles per day per SBR Tank), and as the number of cycles increases, the duration of the react stage decreases. The program is written in standard ladder logic and controls the plant equipment (actuated valves, blowers and pumps) based on input signals from field instruments such as float switches, tank levels, current draws, alarms, hour meters readings, pump and blower running status, and actuated valve status. Key process controls are adjustable by the plant operator through the PLC interface to allow changes to process and alarm setpoint values.

SCADA System

Access to the control system is typically through a graphical computer interface Supervisory Control And Data Acquisition (SCADA) interface, such as Visual Tag System (VTS TM) operator interface, running on a dedicated PC computer. This enables process adjustments and logging data/trends of levels and alarms. Operator adjustable process variables are accessible through the computer interface. The interface also enables access to logged information on float switch positions, tank levels, alarms, hour meters readings, pump and blower running status, etc. The levels in the reactors are monitored by pressure transducers mounted in each reactor tank. The VTS provides accurate metering of the flow through the plant eliminating the need for a plant flowmeter.
The control system can be accessed from virtually anywhere in the world using a computer, software, modem link and telephone access. By this method the operator and support personnel can remotely check plant status and operational trends. This is particularly useful for alarm "call outs" so the operator can check the priority of the call and determine before leaving home (or a remote office) the type of response required. Also if the operator is away for a period of time, the operator can check the plant status by a modem link from anywhere in the world. The data acquisition is particularly useful for trouble shooting the plant. The system will incorporate a dialer for alarms


The Sequencing Batch Reactor (SBR) process has been extensively used in Europe and the United States in the past two decades. Its use in India has been limited to date, although within the last three years. One of the obstacles in the acceptability of SBR process has traditionally been the need for precise, automated and reliable control of various stages of the process. Recent developments in the programmable logic controller (PLC) technology, however, have made the control of an SBR process readily achievable. The SBR process is an activated sludge process in which the sewage is introduced into a Reaction Tank (or SBR Tank), one batch at a time. Wastewater treatment is achieved by a timed sequence of operations which occur in the same SBR Tank, consisting of filling, reaction (aeration), settling, decanting, idling, and sludge wasting. The various stages in the sequence are as follows:
Stage 1: Filling
During this stage the SBR Tank is filled with the influent wastewater. In order to maintain suitable F/M (food to microorganism) ratios, the wastewater should be admitted into the tank in a rapid, controlled manner. This method functions similarly to a selector, which encourages the growth of certain microorganisms with better settling characteristics.
Stage 2: Reaction
This stage involves the utilization of biochemical oxygen demand (BOD) and ammonia nitrogen, where applicable, by microorganisms. The length of the aeration period and the sludge mass determines the degree of treatment. The length of the aeration period depends on the strength of the wastewater and the degree of nitrification (conversion of the ammonia to a less toxic form of nitrate or nitrite) provided for in the treatment.
Stage 3: Settling
During this stage, aeration is stopped and the sludge settles leaving clear, treated effluent above the sludge blanket. Duration for settling varies from 45 to 60 minutes depending on the number of cycles per day.
Stage 4: Decanting
At this stage of the process effluent is removed from the tank through the decanter, without disturbing the settled sludge.
Stage 5: Idling
The SBR Tank waits idle until it is time to commence a new cycle with the filling stage.
Stage 6: Sludge Wasting
Excess activated sludge is wasted periodically during the SBR operation. As with any activated sludge treatment process, sludge wasting is the main control of the effluent quality and microorganism population size. This is how the operator exerts control over the effluent quality by adjusting the mixed liquor suspended solids (MLSS) concentration and the Mean Cell Residence Time (MCRT).
In this process, the SBR Tank acts as the equivalent of several components in the conventional activated sludge treatment process, as follows:
1. Aeration Tank: the SBR Tank acts as an aeration tank during the reaction stage where the activated sludge is mixed with the influent under aerated conditions.
2. Secondary Clarifier: the SBR Tank acts as a secondary clarifier during the settling and decanting stages where the mixed liquor is allowed to settle under quiescent conditions, and the overflow is discharged to the next stage of treatment.
3. Sludge Return System: the activated sludge, following settling in the SBR Tank, is mixed with the influent similar to the sludge return system, except that the feed is transferred to the sludge rather than the sludge being transferred to the front end of the plant.

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