sequencing batch reactors

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Page 1 SBRs 16Mar2007 N Slater.doc

SEQUENCING BATCH REACTORS: COST EFFECTIVE WASTEWATER TREATMENT

Alberta Water & Wastewater Operators Association32nd Annual Operators Seminar - 16th March 2006, Banff, Alberta

Presented by Nigel Slater, EarthTech (Canada) Inc., Calgary (email [email protected]).

Background

Activated sludge is the most widely used biological wastewater treatment process treating both municipasewage and a variety of industrial wastewaters.

During the early 1900s, the basic principles of biological degradation processes using activated sludgewere established by Ardern, Lockett and Fowler amongst others. These researchers operated fill-and-draw processes on crude sewage at Manchester in the UK and established the concept of sequencingbatch reactors (SBRs) operating a single biological reactor basin using repetitive cycles of aerationsettlement and discharge of the treated effluent. These original fill-and-draw variable-volume SBRsystems were capable of achieving excellent treated effluent quality but suffered many operationadifficulties which favored the development of fixed volume continuous-flow activated sludge processeswhich incorporate two separate units, one for aeration and one for settlement.

Further developments of the SBR process did not occur until the 1950s when Pasveer and co-workersincorporated interrupted and continuously fed batch treatment principles in their variable volume activatedsludge system. Further development took place in the 1970s mainly in Australia and the United Statesand with grant aid from the EPA and publication of the EPAs SBR Design Manuals in 1986 and 1992, ledto the wide scale application of the technology world wide. The earlier operational difficulties have beenresolved by technological improvements, particularly reliable microprocessor control systems, aerationequipment and mechanically actuated valves. Through process performance monitoring and variations /modifications of the original SBR process, the modern generation of SBRs have found application in largescale municipalities (up to 1 million population equivalent), as well as the modular expansion and up-ratingof existing wastewater treatment facilities.

SBR Operating Principles

Conventional activated sludge systems requireseparate tanks for the unit processes of biologicalreactions (aeration of mixed liquor) and solids-liquidseparation (clarification) and also require processmixed liquor solids (return activated sludge) to bereturned from the final clarification stage to the aerationtanks.

In contrast, SBR technology is a method of wastewatertreatment in which all phases of the treatment processoccur sequentially within the same tank. Hence, the

main benefits of the SBR system are less civilstructures, inter-connecting pipework, and processequipment and the consequent savings in capital andoperating costs.

Comparison of Conventional and SBRProcess

All SBRs operate on a time-based process cycle to achieve the process conditions necessary forcarbonaceous oxidation, nitrification, de-nitrification and biological phosphorus removal. In additionsolids-liquid separation, treated effluent removal, and solids wasting are also incorporated to complete theprocess cycle.


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The various phases in a typical SBR process cycle usually comprise the following:-

Fill - Wastewater enters the SBR tank and mixes with activated sludge mixedliquor solids within the tank.

Mixed Fill - Influent wastewater and activated sludge are mixed together to produceanaerobic / anoxic conditions in biological nutrient removal (BNR) systems.

React - Aeration of the tank contents. Biological reactions occur until the desireddegree of treatment has been achieved.

Settle - Aeration is stopped and the activated sludge solids settle to form a blanket onthe base of the reactor vessel, leaving an over-layer of treated effluent.

Decant - Clarified treated effluent (supernatant) is removed (decanted) from the tankwithout disturbing the sludge blanket.

Idle - Unexpired time between cycles. Wasting of excess activated sludge occurs.

Completion of these phases constitutes a cycle, which is then repeated.

Typical SBR Process Cycle

SBR technology has the advantage of being much more flexible than conventional activated sludgeprocesses in terms of matching reaction times to the concentration and degree of treatment required for aparticular wastewater. For example, the SBR process allows for the following adjustments to be made inaddition to those (such as sludge age and operating mixed liquor solids concentration) that can be madein an equivalent conventional process:

total cycle duration

duration of each phase within the process cycle pattern of inflow dissolved oxygen profile during aeration operating top water level operating bottom water level

Hence changes in wastewater characteristics over time may be readily accommodated in the SBRprocess.


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SBR Process Configuration

The essential components of SBRs are:-

reactor basin waste sludge draw-off mechanism aeration equipment effluent decanter

process control system

To accommodate continuous inflow of wastewater, the SBR system generally comprises either a storage /equalization tank and a single SBR tank or a minimum of two tanks. As with conventional activatedsludge treatment systems, conventional screening and grit removal are usually provided as preliminarytreatment. A primary sedimentation stage is not usually required with SBR processes unless the influentsuspended solids are excessive. Settled sewage may also be treated if the SBR is installed downstreamof existing primary settlement tanks.

Reactors are usually simple circular, square or rectangular tanks and may be constructed from concrete osteel. Lagoon structures can also be used and existing tanks, for example, primary sedimentation tankscan be retrofitted. Since the tank serves as an aeration tank and a final clarifier, fewer structures are used

for the treatment plant as a whole and a more compact layout for the site can be obtained. Extensions tothe plant by the addition of modular basins using common wall construction can easily be designed fofuture loading conditions.

Circular SBR tanks Rectangular SBR tanks and aerobic digester tanks

The volume between the design bottom water level and top water level represents the volume treated pebatch or hydraulic volume. These volumes are typically up to thirty percent of the designated top watelevel volume and the overall basin depth is generally sized around 5 to 7 m. The volume of liquid a

bottom water level is sized to provide sufficient mass of activated sludge to complete the biologicatreatment processes.

Waste activated sludge (WAS) is typically withdrawn by pump or actuated valves from the settled sludgeblanket during the decant or idle phase of the cycle. Typical WAS concentrations are usually in the range8,000 15,000 mg/l and sludge production (yield) will be similar to conventional activated sludgeprocesses operating under the same process loading rates (and sludge age). Waste activated sludge canbe further treated by aerobic or anaerobic processes, or thickened and dewatered before disposal.

Most SBR processes use air blowers to provide aeration air to biologically degrade the organic


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components in the wastewater. Many SBR facilities in North America operate with low maintenancecoarse bubble diffused aeration, although jet aeration systems are in operation, particularly in industriafacilities. Jet aeration systems can also be used without air to provide a formal anoxic mixing phase inbiological nutrient removal plants. Surface aeration equipment has also been used successfully. The highefficiency of flexible membrane, self-sealing, fine bubble diffusers has led to their adoption in SBR plantswhere energy usage is important.

Fine bubble flexible membrane diffusers

provide high efficiency in terms of processoxygen per unit of energy and also allowthe flow of air to be interrupted duringprocess air off (settling and decanting)phases without fouling of the diffusers orflooding the air distribution pipework.Removable fine bubble aerationequipment may be used to facilitatemaintenance of the diffusers, where thetank cannot easily be drained

Fine Bubble Membrane Aeration System

Operating any SBR process, the air can only be supplied during the fill, aerate and react periodsTherefore, for a cycle comprising 50% aeration, the process air must be supplied to the SBR tank in a 12hour period every day. For a two tank system, this translates to continuous blower operation for the totasystem, with each tank being provided with an aeration grid capable of taking the total airflow.

Air flow is directed to the correct tank by motorized valves controlled by the process control centre. Amajor advantage of the time based sequence of operations is the ability to vary the aeration intensity andduration. A large turndown capability can be achieved so that over-aeration does not occur at plant start-up, or during periods of off-peak loading. Additional fine-tuning of the aeration system can be achievedthrough installing a dissolved oxygen probe within the basin, and controlling the air output with a variablespeed blower.

Nitrification and denitrification can be achieved through turning the air on and off during the filling andreact phases, and can also be simultaneously achieved during the aerobic phases of the cycle bycontrolling the aeration intensity (and hence the process dissolved oxygen concentration) to ensure macroanoxic conditions within the activated sludge flocs. Denitrification also takes place within the sludgeblanket during the air-off phase, but rising sludge is rarely a problem because of the low concentrations ofnitrate present. Typically, biological phosphorus removal can be achieved by incorporating an anaerobicphase within the process cycle, usually at the beginning during filling.

Treated effluent can be removed from the SBR tank after the settle period. Enough time must be allowedduring the settle phase to enable the solids interface to reach a low enough position in the tank to avoidentrainment and scouring during the effluent decanting. Since hydraulic surges are equalized within thereactor and inflow to the tank is normally interrupted during decant, mixed liquor suspended solids cannot

be washed out with the treated effluent.

A variety of different effluent removal systems have been developed for SBRs: Fixed decanters including submerged outlet pipes with automated siphon control valves, and air-

locked multiple pipe arrangements. Moving devices including weir troughs, floating weirs and pipes connected to flexible couplings,

tilting weirs, and floating submersible pumps.

Some decanters suffer from solids loss by trapping mixed liquor suspended solids during the aerationphase or in the submerged pipework.


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The most important aspect to the decanter design is to ensure that effluent is withdrawn uniformly fromwithin the tank. Point discharges suffer from scouring suspended solids from the settled sludge blankeand have no flexibility for process changes, e.g. higher sludge blanket through increased tank solidsconcentration. Decanter designs usually incorporate guard mechanisms to prevent scum and othefloating material from causing deterioration in effluent quality.

SBR processes are usually fully automated, using microprocessor programmable logic controllers (PLCs)with operator changeable set points. Process cycles and phases can easily be changed to optimize plant

performance for actual loading conditions.

SBRs have been used on thousands of plants worldwide and are generally appropriate for all municipaand industrial wastewater where activated sludge processes are utilized. Typically, the small footprintsimple structures, and high flexibility in process operations makes them cost-effective solutions.

There are many SBR process equipment suppliers on the market, all with different equipment and withvarious operational claims, including treated effluent quality and waste sludge yield. Generally, SBRperformance has been well-documented for municipal wastewater and effluent permits of 10 mg/L BOD,15 mg/L TSS, 1mg/L NH3N, 10 mg/L TN and 1 mg/L P (as 30-day averages) are typical.

Modifications of the Sequencing Batch Reactor

Large scale applications of SBRs aregenerally based on the Cyclic ActivatedSludge System (CASS), or IntermittentCycle Aerated Extended System (ICEAS).These systems have been used on municipalwastewater treatment plants treatingpopulations over 1 million. For example, theDublin Bay WWTP in Ireland utilizes theICEASprocess in 24 SBR basins stacked intwo levels, for an average daily flow of320,000 m3/d and a wet weather peak flow of

960,000 m

3

/d

Dublin Bay WWTP, Ireland

Further examples of the compact design of the SBR process can be found in Bangkok, Thailand. Twoseparate CASS SBR facilities, each with an average daily flow of 200,000 m3/d and peak flow o500,000 m3/d, utilize tanks stacked on 4 levels to achieve a treatment plant footprint of 6,000 m2

Interestingly, both these facilities operate as BNR systems and equivalent conventional activated sludgesystems would have required either more footprint (which as not available due to existing buildings), oradditional levels (and higher capital and operating costs)

Four levels of stacked SBR basins treating200,000 m3/d at BMA4, Bangkok, Thailand

Inside-building view of SBR Tanks


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The essential features of the CASS SBR technology are the unique combination of plug flow initiareaction conditions and complete-mix SBR tank configuration. Each SBR tank is divided by baffle wallsinto three zones and sludge is continuously recycled within the zones to maintain high FM loadingconditions to remove the readily degradable soluble substrate and favour the growth of floc-forming micro-organisms. As a result, the total process cycle time can be shorter than the typical SBR process cycletime when treating domestic wastewater and this permits smaller tanks and equipment to be utilized.

The ICEAS process incorporates a pre-react zone within the SBR tank, where the influent wastewateis continuously received, thereby eliminating influent flow control equipment.

The CASS SBR system operates with the following simple repeated time-based process cycle:

FILL - AERATION (for biological reactions, two hours duration).

FILL - SETTLEMENT (for solids-liquid separation, one hour duration).

DECANT (to remove treated effluent, one hour duration total).

For treating domestic wastewater, this constitutes a four hour cycle which is then repeated six times perday.

The reactors are usually simple rectangular tanks, constructed from concrete or steel, although lagoonstructures and retrofits of existing primary sedimentation tanks have also been used. Large scaleapplication of circular structures have also been used where space is not limited.

Circular CASSTMSBR Process Tanks, Ireland 4 tank SBR system using one circulastructure, Australia

For large scale SBR facilities, treated effluent is removed from the SBR tank after the settle period by amoving weir decanter.


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Typical Moving Weir Effluent Decanter

The weir trough of the decanter is situated abovetop water level for both aeration and settlingphases to prevent the accidental discharge ofmixed liquor suspended solids. When operatedduring the decant phase of the cycle, the decanteris driven by an electro-mechanical actuatormechanism into the liquid at a uniform rate and aconstant flow of clarified effluent is discharged from

the tank. Upon reaching the pre-set designatedbottom water level, the decanter then returns to itsout-of-liquid rest position. The decanter is fittedwith a scum guard that positively excludes surfacescum and other floating material.

Large scale facilities are normally operated as a minimum of two side-by-side basins (with commondividing wall) to ensure influent can be continuously accepted by the treatment plant. More basins (e.g. 34, 8 etc.) are generally provided on larger plants to increase the overall plant flexibility and facilitatemaintenance.

2 tank lagoon SBR process, Australia 4 tank SBR, with circular storm water clarifiers, Scotland

sequencing batch reactors

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