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Wastewater Technology Fact Sheet: Sequencing Batch Reactors

United StatesEnvironmental ProtectionAgencyOffice of WaterWashington, 932-F-99-073 September 1999 WastewaterTechnology Fact SheetSequencing Batch ReactorsDESCRIPTIONThe Sequencing Batch reactor (SBR) is a fill-and-draw activated sludge system for wastewatertreatment. In this system, Wastewater is added to asingle Batch reactor, treated to removeundesirable components, and then , aeration, and clarification can all beachieved using a single Batch reactor. To optimizethe performance of the system, two or more batchreactors are used in a predetermined sequence ofoperations. SBR systems have been successfullyused to treat both municipal and industrialwastewater.

the electron acceptor. In a conventional biological nutrient removal (BNR) activated sludge system, mixed fill is comparable to the anoxic zone which is used for denitrification. Anaerobic conditions can also be achieved during the mixed fill phase. After

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Transcription of Wastewater Technology Fact Sheet: Sequencing Batch Reactors

1 United StatesEnvironmental ProtectionAgencyOffice of WaterWashington, 932-F-99-073 September 1999 WastewaterTechnology Fact SheetSequencing Batch ReactorsDESCRIPTIONThe Sequencing Batch reactor (SBR) is a fill-and-draw activated sludge system for wastewatertreatment. In this system, Wastewater is added to asingle Batch reactor, treated to removeundesirable components, and then , aeration, and clarification can all beachieved using a single Batch reactor. To optimizethe performance of the system, two or more batchreactors are used in a predetermined sequence ofoperations. SBR systems have been successfullyused to treat both municipal and industrialwastewater.

2 They are uniquely suited forwastewater treatment applications characterized bylow or intermittent flow Batch processes similar to the SBRare not a recent development as commonly 1914 and 1920, several full-scale fill-and-draw systems were in operation. Interest in SBRswas revived in the late 1950s and early 1960s, withthe development of new equipment and in aeration devices and controls haveallowed SBRs to successfully compete withconventional activated sludge unit processes of the SBR and conventionalactivated sludge systems are the same. A 1983 report, summarized this by stating that theSBR is no more than an activated sludge systemwhich operates in time rather than in space.

3 Thedifference between the two technologies is that theSBR performs equalization, biological treatment,and secondary clarification in a single tank using atimed control sequence. This type of reactor does,in some cases, also perform primary clarification. Ina conventional activated sludge system, these unitprocesses would be accomplished by using modified version of the SBR is the IntermittentCycle Extended Aeration System (ICEAS). In theICEAS system, influent Wastewater flows into thereactor on a continuous basis. As such, this is nota true Batch reactor, as is the conventional SBR. Abaffle wall may be used in the ICEAS to buffer thiscontinuous inflow. The design configurations of theICEAS and the SBR are otherwise very of a Wastewater Treatment PlantUsing an SBRA typical process flow schematic for a municipalwastewater treatment plant using an SBR is shownin Figure 1.

4 Influent Wastewater generally passesthrough screens and grit removal prior to the Wastewater then enters a partially filled reactor,containing biomass, which is acclimated to thewastewater constituents during preceding the reactor is full, it behaves like aconventional activated sludge system, but without acontinuous influent or effluent flow. The aerationand mixing is discontinued after the biologicalreactions are complete, the biomass settles, and thetreated supernatant is removed. Excess biomass iswasted at any time during the cycle. Frequentwasting results in holding the mass ratio of influentsubstrate to biomass nearly constant from cycle tocycle. Continuous flow systems hold the mass ratioof influent substrate to biomass constant byadjusting return activated sludge flowratescontinually as influent flowrates, characteristics, andsettling tank underflow concentrations vary.

5 Afterthe SBR, the Batch of Wastewater may flow to anequalization basin where the Wastewater flowrate toadditional unit processed can be is controlled at adetermined rate. In some cases the Wastewater isfiltered to remove additional solids and illustrated in Figure 1, the solids handling systemmay consist of a thickener and an aerobic SBRs there is no need for return activatedsludge (RAS) pumps and primary sludge (PS)pumps like those associated with conventionalactivated sludge systems. With the SBR, there istypically only one sludge to handle. The need forgravity thickeners prior to digestion is determinedon a case by case basis depending on thecharacteristics of the SBR serves as an equalization basin when thevessel is filling with Wastewater , enabling the systemto tolerate peak flows or peak loads in the influentand to equalize them in the Batch reactor.

6 In manyconventional activated sludge systems, separateequalization is needed to protects the biologicalsystem from peak flows, which may wash out thebiomass, or peak loads, which may upset thetreatment should also be noted that primary clarifiers aretypically not required for municipal wastewaterapplications prior to an SBR. In most conventionalactivated sludge Wastewater treatment plants,primary clarifiers are used prior to the biologicalsystem. However, primary clarifiers may berecommended by the SBR manufacturer if the totalsuspended solids (TSS) or biochemical oxygendemand (BOD) are greater than 400 to 500 data should be evaluated and the SBRmanufacturer consulted to determine whetherprimary clarifiers or equalization are recommendedprior to an SBR for municipal and may be required after the SBR,depending on the downstream process.

7 Ifequalization is not used prior to filtration, the filtersneed to be sized in order to receive the Batch ofwastewater from the SBR, resulting in a largesurface area required for filtration. Sizing filters toaccept these Batch flows is usually not feasible,which is why equalization is used between an SBRand downstream filtration. Separate equalizationfollowing the biological system is generally notrequired for most conventional activated sludgesystems, because the flow is on a continuous andmore constant are typically used at flowrates of 5 MGD orless. The more sophisticated operation required atlarger SBR plants tends to discourage the use ofthese plants for large these systems have a relatively small footprint,they are useful for areas where the available land islimited.

8 In addition, cycles within the system can beeasily modified for nutrient removal in the future, ifit becomes necessary. This makes SBRs extremelyflexible to adapt to regulatory changes for effluentparameters such as nutrient removal. SBRs are alsovery cost effective if treatment beyond biologicaltreatment is required, such as AND DISADVANTAGESSome advantages and disadvantages of SBRs arelisted below:INFLUENTSCREENING/GRINDINGSBRDISIN FECTIONEFFLUENTDIGESTIONTO SOLIDS HANDLING,DISPOSAL, ORBENEFICIAL REUSETHICKENINGEQUALIZATIONFILTRATIONS ource: Parsons Engineering Science, 1 PROCESS FLOW DIAGRAMFOR A TYPICAL SBRA dvantagesCEqualization, primary clarification (in mostcases), biological treatment, and secondaryclarification can be achieved in a single flexibility and capital cost savings by eliminatingclarifiers and other higher level of sophistication is required(compared to conventional systems), especiallyfor larger systems, of timing units and level of maintenance (compared toconventional systems)

9 Associated with moresophisticated controls, automated switches, andautomated of discharging floating or settled sludgeduring the DRAW or decant phase with someSBR plugging of aeration devices duringselected operating cycles, depending on theaeration system used by the requirement for equalization after theSBR, depending on the downstream CRITERIAFor any Wastewater treatment plant design, the firststep is to determine the anticipated influentcharacteristics of the Wastewater and the effluentrequirements for the proposed system. Theseinfluent parameters typically include design flow,maximum daily flow BOD5, TSS, pH, alkalinity, Wastewater temperature, total Kjeldahl nitrogen(TKN), ammonia-nitrogen (NH3-N), and totalphosphorus (TP).

10 For industrial and domesticwastewater, other site specific parameters may alsobe state regulatory agency should be contacted todetermine the effluent requirements of the proposedplant. These effluent discharge parameters will bedictated by the state in the National PollutantDischarge Elimination System (NPDES) parameters typically permitted for municipalsystems are flowrate, BOD5, TSS, and FecalColiform. In addition, many states are movingtoward requiring nutrient removal. Therefore, totalnitrogen (TN), TKN, NH3-N, or TP may also berequired. It is imperative to establish effluentrequirements because they will impact the operatingsequence of the SBR.


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