Sequencing Batch Reactor - STP

8/13/2019 Sequencing Batch Reactor - STP

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Sequencing Batch Reactor - Writeup Process

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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 settlementboth 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 ofinfluent volumes whereas the continuous system is based upon a fixed influent flowrate. Thus, thereis 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 intothe aeration in a controlled manner. Controls range from a simplified float and timer based systemwith a PLC to a PC based SCADA system with color graphics using either flow proportional aerationor dissolved oxygen controlled aeration to reduce aeration to reduce energy consumption andenhance the selective pressures for BOD, nutrient removal, and control of filaments . An appropriatelydesigned SBR process is a unique combination of equipment and software. Working with automatedcontrol reduces the number of operator skill and attention requirement.

The majority of the aeration equipment of sequencing batch reactors consist of jet, fine bubble, andcoarse bubble aeration systems. The main focus of this report is a jet aerated sequencing batchreactor activated sludge system.

Sequencing Batch Reactor Process Cycles

The operating principles of a batch activated sludge process, or SBR, are characterized in six discreteperiods:

Anoxic Fill Aerated Fill React Settle Decant Idle

Anoxic Fil l

The influent wastewater is distributed throughout the settled sludge through the influent distributionmanifold to provide good contact between the microorganisms and the substrate . The influent can beeither pumped in allowed to flow in by gravity. Most of this period occurs without aeration to create anenvironment that favors the procreation of microorganisms with good settling characteristics. Aerationbegins at the beginning of this period.




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Aerated Fil lMixed liquor is drawn through the manifold, mixed with the influent flow in the motive liquid pump, anddischarged, as motive liquid, to the jet aerator. This initiates the feast period. Feast is when themicroorganisms have been in contact with the substrate and a large amount of oxygen is provided tofacilitate the substrate consumption. Nitrification and denitrification occurs at the beginning of thisstage. This period ends when the tank is either full or when a maximum time for filling is reached

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ReactDuring 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 diebecause of the lack of food and will help reduce the volume of the settling sludge. The length of theaeration period determines the degree of BOD consumption.

Settle Aeration is discontinued at this stage and solids separation takes place leaving clear, treated effluentabove the sludge blanket. During this clarifying period no liquids should enter or leave the tank toavoid turbulence in the supernatant.




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DecantThis period is characterized by the withdrawal of treated effluent from approximately two feet belowthe surface of the mixed liquor by the floating solids excluding decanter . This removal must be donewithout disturbing the settled sludge.

IdleThe time in this stage can be used to waste sludge or perform backwashing of the jet aerator. Thewasted sludge is pumped to an anaerobic digester to reduce the volume of the sludge to bediscarded. The frequency of sludge wasting ranges between once each cycle to once every two tothree months depending upon system design.

Aeration Equipment

A. Jet Aeration Header

Jet aeration offers significant advantages in the SBR process due to its flexibility, good contactbetween substrate and microorganisms, and efficient oxygen transfer. One of its main features is thatit can mix without aerating.Therefore it can provide for aerated and anoxic mix periods. The header inconjunction with a computer controlling for flow proportional aeration makes more oxygen available athigher flows than at lower flows by measuring the rate of change in the flow level in reactor.




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B. Decanter

Decanting is best achieved through solids excluding decanters. The floating decanter is one of themost efficient and contains a spring loaded plug valve operated by hydraulic differential . Thisdecanter is sustained about sixteen inches below the scum by a float therefore avoiding the decantingof floating matter.

Conclusion

Wastewater treatment has been a challenge throughout the years due to varying influent chemicaland physical characteristics and stringent effluent regulations. Treatment systems using activatedsludge have been able to handle many of these difficulties. Given the lack of on-line computercontrols, continuous flow systems have been mostly used for these purposes versus sequencingbatch processes. The availability of artificial intelligence has now made the option of a SBR processmore attractive thus providing better controls and results in wastewater treatment. This is coupled bythe 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 microorganismsand substrate, small floor space, and good removal efficiency.

Sequencing batch reactors operate by a cycle of periods consisting of fill, react, settle, decant, andidle. The duration, oxygen concentration, and mixing in these periods could be altered according tothe needs of the particular treatment plant. Appropriate aeration and decanting is essential for thecorrect operations of these plants. The aerator should make the oxygen readily available to themicroorganisms. The decanter should avoid the intake of floating matter from the tank. The manyadvantages offered by the SBR process justifies the recent increase in the implementation of thisprocess in industrial and municipal wastewater treatment.

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Sequencing Batch Reactor - STP