An Efficient Alternative to Wastewater Treatment
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
[1].
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 [1]. 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:
1. Anoxic Fill
2. Aerated Fill
3. React
4. Settle
5. Decant
6. Idle.
2. Aerated Fill
3. React
4. Settle
5. Decant
6. Idle.
Anoxic Fill
The
influent wastewater is distributed throughout the settled sludge through the
influent distribution manifold to provide good contact between the microorganisms
and the substrate [1]. 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.
Figure: 1
Aerated Fill
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 [1]. 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.
Figure: 2
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 [1], [2].
Figure: 3
Figure: 4
Settle
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.
Figure:5
Figure:6
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 [1]. This removal must be done without disturbing the
settled sludge.
Figure:7
Idle
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.
Figure:8
Aeration Equipment
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.
Figure:9
B. Decanter
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 [1]. This decanter is sustained
about sixteen inches below the scum by a float therefore avoiding the decanting
of floating matter.
Figure:10
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.
References
1. Norcross, K.L. "Sequencing
Batch Reactors-An Overview". Water Science and Technology. vol. 26, no.
9-11. 1992.
2. Chambers, B. "Batch Operated Activated Sludge Plant for
Production of High Effluent Quality at Small Works". Water Science and
Technology. vol. 28, no. 10. 1993.