The sequencing batch reactor (SBR) system is designed
for treating wastewater. Undesirable components from the water is
removed by adding the wastewater to a single "batch" reactor.
Using single batch reactor equalization, aeration, and clarification
processes can be achieved easily. For more better results two or more
batch reactors are used in a sequence of operations. SBR systems are
used worldwide and that too successfully for treating both municipal and
industrial wastewater. Wastewater characterized by low or intermittent
flow conditions can be treated easily using SBR.
SBR
is used since early fifties. The development of new equipment and
technology like aeration devices and controls helped SBR to compete
successfully with the traditional activated sludge systems. The unit
processes of the SBR and conventional. Using time control sequence, SBR
performs equalization, biological treatment and secondary clarification
in a single tank. In some cases it also perform primary clarification.
Whereas in conventional activated sludge system, all the processes are
carried out in separate tanks.
Mechanism of SBR
Before moving to SBR, Influent wastewater generally passes through
screens and grit removal prior to the SBR. The second step includes the
passage of wastewater into a partially filled reactor which contains
biomass, which is acclimated to the wastewater constituents during
preceding cycles. The reactor so filled acts like a conventional
activated sludge system. However, there is no continuous influent or
effluent flow. Once the biological reactions are completed the aeration
and mixing is discontinued. The biomass gets settled and the treated
supernatant is removed. The biomass which is in excess is wasted at any
time during the cycle.
After the treatment is over in SBR, wastewater flows to an equalization
basin. In this basin flow rate of the wastewater to additional unit
processed is controlled at a determined rate. In some cases additional
solids are removed through filtration and the wastewater is then
disinfected. The system that handles solids consists of a thickener and
an aerobic digester. Unlike conventional activated sludge treatment
system, SBRs needs no return activated sludge (RAS) pumps and primary
sludge (PS) pumps.
Working of SBR
An SBR acts like an equalization basin whereas in a conventional
activated sludge system, separate equalization is required for
protecting biological system from peak flows because it can result in
the washing of biomass, or peak loads thereby upsetting treatment
process. Most importantly in SBR systems, no primary clarifiers are
required. But exception is always there primary clarifiers are
generally used if the total suspended solids (TSS) or biochemical oxygen
demand (BOD) are greater than 400 to 500 mg/L. Depending on the
downstream process, equalization may be required after the SBR. However
if equalization is not utilized before filtration, the the filters
should be sized for receiving the batch of wastewater from the SBR.
Benefits
The benefits of the SBR are:
- Using single reactor vessel equalization, primary clarification,
biological treatment and secondary.
- clarification can be achieved.
- The operation is flexible.
- Footprint is less.
- Cost effective as it does not requires clarifiers and other
equipment.
Design Parameters
Before installing any wastewater treatment plant following
considerations should be kept in mind. Firstly the characteristics of
the anticipated influent, wastewater and the effluent requirements for
the proposed system should be determined. Paramet4ers that are included
in influent parameters are design flow, alkalinity, ammonia-nitrogen
(NH3-N), phosphorus (TP), temperature of wastewater. For effluent
requirements state regulatory agency should be approached. For municipal
system, the parameters includes are flowrate, BOD5, TSS, and Fecal
Coliform. Apart from this TKN, NH3-N, or TP may also be required.
Effluent requirements should be established as it may impact the
operating sequence of the SBR.
After this design parameters are decided other factors like number of
cycles and basins per day, volume of decant, size of the reactor,
detention times can be calculated. The aeration equipment, decanter, and
associated piping can be sized accordingly.
Fill and Draw principle determines the operation of an SBR that consist
of following steps-Idle, Fill, React, Settle, and Draw. Idle steps take
place between the Draw and the Fill steps. It is the idle step in which
treated effluent is removed and wastewater is added. The flow rate and
the operating strategy determines the length of the Idle step. If
variable idle times are used the equalization is achieved in this step.
Depending on the operating strategy, the biomass and sludge wasting can
also be performed during the Idle step.
The next step is fill step in which the influent wastewater is added to
the reactor. There are different variants of the fill step like static
fill, mixed fill and aerated fill which can be used either singularly or
in combination depending upon the operative strategy.
In static fill, biomass and influent waste water is mixed in the SBR.
Since here aeration is absent therefore it results in high substrate
(food) concentration. This creates favorable environment for the
formation of floc, hence the good settling characteristics for the
sludge.
In mixed fill step influent organics and biomass are mixed thereby
initiating the biological reactions. Whereas in aerated steps, aerobic
reaction is accomplished. Whole process can be summarized as follows:
Configuration
Construction of SBR system is easy. It requires small footprint as it
eliminates the need for primary clarifiers and don't use the secondary
clarifiers. The size is determined by the area of the site but its
advantage can be felt in limited space. The tank and the equipment of
the SBR is simple to construct. If SBR is used in Biological Nutrient
Removal (BNR) plants it does not uses return activated sludge (RAS)
pumps and pipes. However, the control system of the SBR is complex and
consists of automatic switches, automatic valves, and instrumentation.
The tank of SBR are constructed out of either steel or concrete. Steel
tanks are used for industrial applications whereas for municipal
applications tanks of concrete is preferred. For handling wastewater
level variations in the reactor positive displacement blowers are used.
There are two types of decanters used in SBR-Floating and fixed.
Operation And Maintenance
Since in many municipal systems SBR does not uses separate primary and
secondary clarifiers the operation and maintenance requirement is
reduced. Apart from this RAS pumps are not required. This makes both the
operation and the maintenance very easy. Different processes can be
achieved in single reactor. However, the core of the SBR system is the
control system which includes automatic valves, and automatic switches
hence this requires more maintenance than a conventional activated
sludge system.
External Resource
Functional analysis of microbial communities in SBR:
Yun
H. Kong, Michael Beer, Gavin N Rees, Robert J. Seviour 
