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WATER POLLUTION (S SENGUPTA, SECTION EDITOR)

Sequencing Batch Reactor for Wastewater Treatment:Recent Advances

Aparna Dutta1 & Sudipta Sarkar1

Published online: 8 September 2015# Springer International Publishing AG 2015

Abstract Sequencing batch reactors (SBRs), due to its oper-ational flexibility and excellent process control possibilities,are being extensively used for the treatment of wastewaterwhich nowadays is fast becoming contaminated with newerand more complex pollutants. It is also possible to includedifferent expanding array of configurations and various oper-ational modifications to meet the effluent limits which are alsocontinuously getting upgraded. This article provides basic de-scription of SBR process along with its functional and phys-ical variants that lead to improved the removal of nutrients andemerging contaminants. The significance of selectors and var-ious recent advancements in the application of SBR has beendiscussed along with the possibilities held by SBR process inthe treatment of wastewater of different origins and composi-tion to produce effluent of reusable quality.

Keywords Selectors . SND . Short-cut nitrogen removal .

ANAMMOX . EBPR . Process control . SBBR . EDCs .

CASS . UNITANK . ICEAS

Introduction

As the environmental discharge standards are getting moreand more stringent, the traditional continuous flow-based bi-ological wastewater treatment process faces severe chal-lenges. The sequencing batch reactor (SBR) technology is a

modification of the much popular activated sludge process(ASP). Such a conversion of the continuous nature of theASP-based treatment process to a batch process as in SBRhelps introduce various process flexibilities and alternativesin process controls and design so as to better achieve the latesteffluent discharge standards. The term SBR was originallycoined by R.L. Irvine [1]. Opposed to the common belief ofSBR being a new technology, the SBR-like fill and draw pro-cesses were popular during 1914–1920. The revival of interestin SBR technology in its present form occurred during the late1950s and early 1960s due to the improvement in technologyrelated with aeration and process control. In its initial years,SBR technology was mainly used by small communities forsewage treatment and for the treatment of high strength indus-trial wastes. Due to the design flexibility and better processcontrol that can be achieved by the modern technology, theuse of the SBR process has not been limited to the field ofsewage treatment only; it has also found wide acceptance inbiological treatment of industrial wastewater containingdifficult-to-treat organic chemicals. As the SBR process canbe effectively automated, it is known to save more than 60 %of the operating expenses required for a conventional ASP andis able to achieve high effluent quality in a very short aerationtime. In densely populated countries such as India and regionssuch as Europe, SBR is being considered as a preferable tech-nology due to its low requirement of area as well as manpowerfor operation. The SBR process is often preferred over contin-uous flow process (CFP) due to reduction in energy consump-tion and enhancement in the selective pressures for BOD,nutrient removal, and control of filamentous bacteria. Due tothese reasons, SBR process is gaining immense popularity inthe recent years. The SBR technology has been undergoingseveral minor and major modifications over the past few yearsto be able to effectively treat the exponentially increasingnumber of new pollutants in wastewater. This article provides

This article is part of the Topical Collection on Water Pollution

* Sudipta [email protected]

1 Department of Civil Engineering, Indian Institute of TechnologyRoorkee, Roorkee 247667, Uttarakhand, India

Curr Pollution Rep (2015) 1:177–190DOI 10.1007/s40726-015-0016-y

http://crossmark.crossref.org/dialog/?doi=10.1007/s40726-015-0016-y&domain=pdf

an insight into the technology as well as reviews the recentadvances in the design and application of SBR technology.

The Process

In a CFP such as an activated sludge process, all the unitprocesses are in work in tandem at a given point of time. Onthe other hand, in an SBR process, all these unit processes takeplace within a single tank for their specific duration and inter-vals, sequentially spaced over a span of time. Thus, SBRprovides in time the same treatment what the CFP providesin space. The SBR technology basically incorporates a fill-and-draw type biological wastewater treatment process, func-tionally resembling to an activated sludge process. Dependingon the scale of operation, the SBR system, along with itsvariants and hybrids, may involve single or multiple tanks,each of which features five basic operating modes, namely,Fill, React, Settle, Draw, and Idle. Being a batch process, thetime duration of each mode within a tank can be adjusted tomeet the different treatment needs, such as low COD in theeffluent, biological nutrient removal, etc. Figure 1 schemati-cally shows various modes of operation of an SBR system.The figure also informs about few alternative arrangementspossible during each of the individual steps so that specifictreatment objectives are met.

During the Fill phase, the tank receives the raw wastewaterthat comes in contact with the active biomass left inside thetank at the end of the previous cycle. There are three variationswhich may be incorporated singly or combined, depending onthe wastewater characteristics, the target organics and biolog-ical nutrient removal: static fill, mixed fill, and aerated fill.During static fill, influent wastewater is added to the biomassalready present in the SBRwithout mixing, resembling almosta plug flow situation creating a high food to microorganisms(F/M) ratio, similar to a selector compartment used in an ASP,

promoting the growth of floc-forming bacteria by suppressingthe filamentous ones, which provides good settling character-istics for the sludge. Additionally, static fill conditions create aBfeast^-like situation in which phosphate accumulating organ-isms (PAO) are favored; as discussed later, these are respon-sible for biological phosphorus removal.

The React phase is intended for the completion of the bio-logical reactions responsible primarily for the degradation oforganics. Further, the React phase is often designed to provide ahigh degree of nutrient removal as well. The treatment is con-trolled by air, either on or off, to produce anaerobic, anoxic, oraerobic conditions. Variations such as mixed react and aeratedreact modes may be adopted. During aerated react, the aerobicreactions initialized during aerated fill are completed. Designsoften include conversion of ammonia-nitrogen to nitrite-nitrogen and ultimately to nitrate-nitrogen, a process knownas nitrification. In mixed react mode, apart from aerobic con-ditions, there may be combinations of anoxic and anaerobicconditions created within the reactor. Anoxic conditions canachieve denitrification, a process in which nitrate-nitrogen isconverted into nitrogen gas. Anaerobic conditions shall createa Bfamine^ phase that promotes phosphorus removal.

During the Settle phase, the entire reactor tank acts as abatch clarifier, without any inflow or outflow. In a CFP pro-cess, on the contrary, the quiescent settling is often impairedby the continuous inflow and outflow of liquid, giving rise toinefficient settling that causes poor effluent quality.

The Draw phase uses a decanter, either fixed or floating, todecant the treated supernatant after the settlement of the bio-mass generated after the React phase.

Idle phase is the time between the draw and fill phase. Theneed for such a phase is often necessitated when there areseveral reactors operating in parallel operation, acting as abuffer in time. During this phase, mixing of the biomass tocondition the reactive contents, and wasting of excess sludge,may be taken up, depending on the operating strategy.

a) Receiving of raw waste; Static fill (high F:M, no mixing or aeration, suitable for BPR), Mixed fill (mixing of organic influent with biomass, anoxic environment for denitrification) and Aerated Fill (Aeration to begin reactions occurring in React phase)

b) Desired bio-chemical reactions occur; Aerated react (Aerobic reactions initiated during aerated fill are completed, Nitrification), Mixed react (Anoxic condition for nitrogen removal and Anaerobic condition for Phosphorus removal)

c) Microorganisms are separated from treated effluent under gravity

d) Discharge of treated effluent

e) Time between discharging and refilling of effluent

Fig. 1 Different phases of SBRoperation cycle with descriptionsand possible process variations

178 Curr Pollution Rep (2015) 1:177–190

The complete cycle time spans the duration between begin-ning of Fill and end of Idle phase for a single tank SBR sys-tem. The multiple tank system consists of tanks in serieswhere it is ensured that the Draw of a tank is completed bythe time another tank completes Fill. Single tank operation issuitable for low population localities or in industries with var-iable flow conditions. The wasting of microorganisms is doneonce per cycle during the react phase in high yielding multipletank system while the frequency of wasting may be as low asonce every 2 weeks for low yielding single tank operation [2].The simultaneous aeration, mixing, reaction, and settling oc-curring within the SBR tank obviate the requirement of aseparate clarifier unit. The duration of Fill and React phasescan be adjusted to impart the SBR system a CSTR-like orideal PFR-like treatment characteristics. The SBR system pro-vides major operational flexibilities like internal equalizationand control of biological reactions through regulation of aer-ation. The presence of microorganisms in high concentrationsright from the Fill phase reduces the treatment duration sig-nificantly. The ability to control the substrate availability byvarying the aeration duration during Fill provides a high de-gree of flexibility in controlling the filamentous organism pop-ulation and concentration of nitrogen. Anoxic period duringthe React phase is useful in nitrogen removal from the system.

In ASP, reactor operating conditions such as low dissolvedoxygen (DO), low F/M ratio, and completely mixed operationare responsible for the growth of filamentous bacteria thathave poor settling characteristic. It causes the effluent to havehigh suspended solids content, a situation known as sludgebulking that result in poor efficiency of the treatment plant.Due to the operational similarity of ASP and SBR, sludgebulking is a common problem in SBR processes, too. In orderto overcome such problem, a variation of design in SBR pro-cess is often made in the form of providing special bioreac-tor(s), known as bio-selector or simply, selector that favors thegrowth of floc-forming heterotrophic bacteria over the fila-mentous one. The bio-selective mechanism for floc-formersis to contact the return activated sludge with the influentwastewater in a separate initial contact zone which is termedas selector zone. The initial contact zone typically consists ofthree or four completely mixed zones with a gradual high tolow F/M values and having limited or no molecular oxygenpresent, where heterotrophs remove the majority (75–90%) ofthe low molecular weight, soluble substrates from the waste-water within first 5 to 10min, only to utilize the absorbed foodfor later when molecular oxygen is available. Some hetero-trophs such as denitrifiers can even use combined oxygensuch as nitrate or nitrite for metabolic purposes. Filamentousbacteria, on the other hand, neither can compete with floc-formers at high F/M ratio nor are able to store substrate forsuch later use. Thus, they get suppressed by the floc-formersin the selector zone as well as inside the SBR during subse-quent aeration, anoxic, or anaerobic stages. The selectors can

be made either anoxic or anaerobic, by varying the degree ofmixing at low or no oxygen supply, depending on whetherdenitrification and biological phosphorus removal are targetedin the SBR. For effective working of selectors, it is requiredthat aeration in the subsequent SBR tank is complete so thatreturn sludge to be fed into the reactor does not have any un-oxidized substrate.

Biological Nutrient Removal in an SBR

With increasing water demand, it has become inevitable toinclude tertiary treatment units for nutrient removal fromwastewater apart from the conventional pollutants like COD,BOD, and suspended solids and pathogens. An SBR-basedtreatment plant can easily address this requirement withoutaddition of any new infrastructure, only by optimizing thesequence of aerobic, anoxic, and anaerobic phases duringthe different stages of SBR process.

Biological Nitrogen Removal Process

The operation of SBR cycle phases in time rather than space; itprovides a greater degree of flexibility for nutrient removalthrough alteration of aeration and mixing regimes to createalternating aerobic and anoxic environment during BReact^phase. The biological nutrient removal process mainly in-volves two steps, nitrification and denitrification. The organicnitrogen is first converted into ammonia-nitrogen during theoxidation of the organics or chemical oxygen demand (COD)present in the wastewater. Complete nitrification means oxi-dative conversion of ammonia-nitrogen to nitrate-nitrogen bychemoautotrophic bacteria. It consists of two steps: nitritation(from ammonia to nitrite) and nitratation (from nitrite to ni-trate), catalyzed by two groups of autotrophic bacteria: ammo-nia oxidizing bacteria (AOB) and nitrite oxidizing bacteria(NOB), respectively. Both use CO2 as carbon source and ox-ygen as terminal electron acceptor. Nitrification requires strict-ly aerobic environment which is achieved through aeration inan SBR tank. The second step, denitrification, involves theheterotrophic bacteria which are anaerobic and utilize com-plex organic compounds for their carbon requirements andnitrate serves as the electron acceptor under anoxic or anaer-obic conditions, which in turn forms nitrogen gas that leavesthe aqueous phase.

Temperature, pH, DO level, and solids retention time(SRT) are important parameters in nitrification kinetics. Theoptimum temperature is between 25 and 35 °C, whereas therate of nitrification drops to 50% of the optimum if the pH liesbeyond the range of 7.5 to 9.8. The nitrification reaction de-stroys alkalinity, thereby tending to lower the pH. The maxi-mum rate of nitrification has been observed to occur at a DOlevel greater than 2.0 mg/L. The nitrification rate can be

Curr Pollution Rep (2015) 1:177–190 179

increased with increasing concentration of nitrifiers. This isachieved by increasing the mixed liquor suspended solids(MLSS) concentration within the reactor, which is possiblewhen SRT is increased. Increase in SRT can be achieved bylowering the flow rate of waste activated sludge (WAS). De-nitrification process is effective at anoxic conditions when DOlevel falls below 0.5 mg/L and at greater than 1.0 mg/L, thedenitrification process is inhibited. The optimum pH is be-tween 6.5 and 9.

Simultaneous Nitrification-Denitrification Process

SBR provides a viable alternative to the CFP systems forbiological nutrient removal (BNR) by introducing anaerobic,anoxic, and aerobic reactions within a single tank during atreatment cycle. Hence, SBR can provide simultaneousnitrification-denitrification (SND) when only nitrogen remov-al is targeted. Apart from other controlling factors, carbon tonitrogen (C/N) ratio usually governs the SND process within aSBR. It was demonstrated that it is possible to achieve com-plete COD and NH4

+-N removal without even having anyresidual NO2

−-N in the effluent from an SND–SBR systemby adjusting the ratio to 11.1. A low COD/NH4

+-N ratio mayresult in unbalanced SND, causing little or no denitrification[3]. SND has slower ammonia and nitrate utilization rates ascompared to separate basin designs because only a fraction ofthe total biomass is participating in either the nitrification orthe denitrification steps. Typical nitrification efficiency isclose to 100 %, while the total nitrogen removal is about90–95 % under stable operation conditions. The excellent ni-trogen removal performance of SBR has led to setting up of anumber of plants for landfill leachate treatment. Logically, theinclusion of anoxic phase right after the aerobic phase en-hances the nitrogen removal efficiency; however, this willnecessitate carbon supplementation from external source forlow-strength wastewater. This need may get reduced to 5 % ofthe above in case of wastewater having high organic concen-tration. With better process control, as has been discussedlater, step feed system with an optimized intelligent regimecontrol in terms of DO and ORP may help in elimination ofsuch external carbon supplementation.

Short-cut Nitrogen Removal Process

Another promising treatment is partial nitrification and deni-trification process. It is based on the partial nitrification(nitritation) up to nitrite followed by the reduction of nitriteto nitrogen (denitritation). This process is popularly known asshort-cut nitrogen removal [4]. Short-cut nitrogen removalreduces the aeration requirement by 25% and also the externalcarbon-supplementation by 40% as compared to conventionalSND process [5], cutting down considerably the energy-related expenditure. Higher denitrification rate and lower

wasted sludge production can also be obtained by this process.In the presence of low C/N ratio, and strong nitrogenouswastewater, N removal via this process showed promisingresults for the process optimization [6, 7]. These features aretypical of liquid effluents from anaerobic digestion of bio-waste, leachate from landfills, and digestate of sewage sludge.

Anammox Process

In this process, first half of the ammonia-N is oxidized tonitrite which is later used by the anammox (anaerobic ammo-nia oxidizing) bacteria as an electron acceptor to react withammonia-N to produce nitrogen gas and nitrate [8]. The pro-cess eliminates the requirements of aeration and exogenousorganic carbon sources compared to the traditional nitrifica-tion–denitrification process of nitrogen removal. However,slow growth rate of the anammox bacteria and toxicity issuesforced the early development of the process with biofilms,granulation, and suspended biomass reactors [9–11]. The pro-cess requires the combination of two processes, nitritation andanammox reaction, which, in engineered systems, can eitherbe separated in time or space. Spatial separation is implement-ed by the use of two-stage reactor systems whereas temporalseparation, as in SBRs, requires strict control strategy andoperation regime for successful implementation. It was con-cluded that ORP correlates well with different stages of activ-ity during different feeding and aeration strategies and, so,interval feeding with interval aeration is the best strategy forprocess performance in terms of ammonia-N removal, nitrate-N production, and pH stability [12••]. In SBR, only a part ofthe water is withdrawn as effluent, thus nitrate as well as totalnitrogen concentration continuously rises during long-termoperation. Ideally, a denitrification process is expected to beincluded in anammox SBR system to solve the problem. De-nitrification requires addition of extraneous carbon source; itwas reported that COD concentration is a control variable forprocess selection between anammox reaction and denitrifica-tion so that anammox bacterial growth is significantly sup-pressed by the presence of high concentration of COD [13,14]. However, it was demonstrated that at low COD concen-tration, activity is not diminished for neither anammox nordenitrifiers, and total nitrogen removal efficiency is improved[15••]. It was shown that good efficiency with combinedanammox and denitrification is possible in SBR for wastewa-ter with low organic carbon and high nitrogen content. Al-though the anammox activity was suppressed at high organiccontent such as shock load, the activity was recovered quicklywhen such a shock load was withdrawn [16]. It was reportedthat ammonium oxidation rates of up to 500 gN m−3day−1

with greater than 90 % conversion to N2 have been achievedin a pilot study using online ammonia sensor, with continuousaeration at dissolved oxygen concentrations <1 mg O2L

−1

[17]. The nitrite oxidation and the anammox reaction occurred


simultaneously, allowing increased overall nitrogen perfor-mance and simplified process control compared to separateaerobic and anaerobic phases.

Enhanced Biological Phosphorus Removal (EBPR)

Phosphorus (P) removal, now an integral component of waste-water treatment plants, uses a special group of organismsknown as polyphosphate accumulating organisms (PAO) thatunder alternative anaerobic and aerobic conditions, incorpo-rate the influent P into the cell mass. The sludge is subsequent-ly removed during sludge wasting. Under anaerobic condi-tions, PAOs take up carbon source such as volatile fatty acids(VFAs) and store them in the form of polyhydroxyalkanoates(PHAs). The energy for this process is obtained mainlythrough hydrolysis of the intracellular stored poly-P, resultingin the release of ortho-phosphate into water. Under aerobic oranoxic conditions, PAOs are able to take up excess phospho-rus in intracellular poly-P formation, biomass growth, andglycogen replenishment by using stored PHAs as the energysource. The phosphate released in the anaerobic stage is lessthan that absorbed in the aerobic or anoxic stage; the net re-moval of phosphorus can be achieved through wasting ofsludge which is enriched in poly-P. Sequencing batch reactors(SBRs) can achieve alternating anaerobic and aerobic condi-tions by controlling the operational process, and consequently,biological phosphorus removal using SBRs has drawn in-creasing attention worldwide [18, 19]. The P-removal effi-ciency as high as 90 % have been reported in SBR, whereasin conventional activated sludge systems, maximum efficien-cy achieved is only 10–20 % [20].

Another group of organisms, known as glycogen accumu-lating organisms (GAOs), closely resemble to and competewith PAOs in their metabolism. GAOs have no contributionto the P removal and their proliferation is known to causefailures in P removal in reactors. Finding optimal conditionsthat favor PAOs to GAOs are necessary for success inbiological-P removal. The controlling parameters are pH, tem-peratures, and more importantly, substrate type. The cold tem-perature seems to favor PAOs [21]. Increasing pH is known toprovide an advantage to PAOs over GAOs, and optimum pHwas reported to be between 7.2 and 8 for effective GAO con-trol [22].

The PAOs and GAOs accumulate storage products thatrequire carbon alone for synthesis [23]. A higher COD-to-phosphorus ratio (mg/mg) such as above 40 in raw waterhelp achieve low effluent phosphorus concentration and highprocess stability in full-scale plants [24]. The form of theCOD is also a crucial factor for selection of PAOs. If theinfluent COD has a sufficient portion of volatile fatty acids(VFAs) or readily biodegradable COD that can getfermented into VFAs, PAOs can outcompete GAOs andachieve low phosphorus levels in effluent [25].

Recently, a study on EBPR from abattoir wastewatershowed a possibility of achieving high level of Bio-P removalat a much lower SRTof 2–4 days [26••]. Due to the short SRT,nitrification/denitrification could not be achieved and hencepost treatment of SBR effluent might be required. The PAOsdemonstrate a high requirement of organic carbon and henceapplication of short SRT for Bio-P removal in low COD do-mestic wastewater can be challenging. The idle time of a SBRhad potential impact on biological phosphorus removal, espe-cially when the influent phosphorus concentration increased[27••]. A new configuration of SBR with sludge tank halved(STH-SBR) has been successfully designed to address themain drawback associated with the extended-idle phase beingmuch longer than the anaerobic phase in the anaerobic/oxic(A/O) regime. It demonstrated higher P removal the conven-tional A/O-SBR [28••].

Simultaneous Removal of Nitrogen and Phosphorusin an SBR Process

It is expected that nitrification, denitrification, and EBPRshould often take place simultaneously in an SBR. For thesimultaneous removal of nitrogen and phosphorus, the inter-action among the processes, if not optimally controlled, maygive rise to the failure of the treatment plant. Among the re-actants and intermediate products, toxicity of nitrite and itsacidic counterpart, free nitrous acid (FNA), is important asthey are known to provide a competitive disadvantage toPAOs over GAOs in the EBPR systems. They are a key se-lection factor in the PAOs/GAOs competition, severelyinhibiting PAO activity at a concentration as low as 2 mg/Lnitrite-N and complete inhibition at 6 mg/L nitrite-N [29].Although earlier studies pointed out to poor phosphorus re-moval under nitrate-rich conditions in the anaerobic zone, it ismore attributed to the disruption of anaerobic conditions bynitrate [30], consumption of fatty acids by denitrifying non-polyphosphate heterotrophs [31], and inhibition of PAOs bynitrite, as a result of incomplete denitrification [32–34].

Denitrifiers as well as the PAOs both require organic sub-strate, and quite naturally at low oxygen concentrations theyare likely to compete. A long-term anaerobic exposure to ni-trate was demonstrated to diminish the number of certaingroup of PAOs showing that it may inhibit PAOs activity oractivate the competition between PAOs and denitrifiers [35,36]. It was also found that a significant proportion of PAOs,known as denitrifying PAOs (DPAO), has the ability to simul-taneously uptake phosphorus using nitrate as a terminal elec-tron acceptor using stored PHAs in the anoxic zone [37, 38].The DPAOs are desirable because they, as they are able toremove nitrate and phosphorus simultaneously and also, havelower cell yield and sludge production [39]. The PAOs thatuse nitrite but not nitrate also have been reported [33]. So,there are three types of PAOs depending on the electron


acceptors who use: (a) oxygen only; (b) oxygen and nitrate;and (c) oxygen, nitrate, and nitrite [40]. The conventionalanaerobic–aerobic processes incorporating an anoxic zonefor denitrification have been used for N and P removal infull-scale wastewater treatment plants [41]. Significant P re-moval can also be attained by using DPAOs in a single sludgesystem coexisting with nitrifiers [42]. Studies were carried outusing an anaerobic-aerobic-anoxic-aerobic system for simul-taneous removal of nitrogen and phosphorus, and some im-portant control strategies were suggested so that the DPAO arenot inhibited [43]. Another suggestion was to use anaerobic-aerobic-anoxic process which showed successful simulta-neous removal of nitrogen and phosphate [44]. A novelscheme using anaerobic co-digestate of waste activated sludgeand organic fraction of municipal solid waste in SBR, that canbe integrated into municipal anaerobic co-digestion plants fordenitrifying biological phosphorus removal via nitrite. Thisscheme can be employed in future for the side stream treat-ment of sludge reject water, enhancing nutrient removal, andreducing footprint and energy requirement of the new plants[45]. Some of the work regarding treatment of wastewaterfrom different sources using SBR process and their corre-sponding treatment efficiencies has been summarized inTable 1.

Different Variants of SBR Technology

Cyclic Activated Sludge System

Cyclic Activated Sludge System (CASS) incorporates a singlebasin with variable volume operating in an alternating mode.It provides a unique combination of a plug flow in the initialzone followed by completely mixed reactor basin with sec-ondary and main aeration zones. The activated sludge from

the main aeration zone is recirculated into a selector zonelocated ahead of the complete-mix unit where it gets mixedwith the raw wastewater entering the plant. The inclusion of asuitably designed high rate plug-flow selector facilitates stableand relatively uniform level of metabolic activity of the sludgein the complete-mix aeration tank leading to faster digestion ofthe organic contents and better settleability of the flocs. Theoperation is therefore generally indifferent to any variations inthe flow rate and organic concentration of the influent rawwater. Apart from these advantages, superior degree of simul-taneous nitrification and denitrification as well as biologicalphosphorus removal is achieved using a CASS as compared toconventional SBR process [55]. This system can be employedfor both industrial and municipal wastewater treatment sys-tems [56].

UNITANK Technology

The UNITANK systems incorporate the advantages of SBR,three ditch oxidation treatment and normal aeration tank. Thebasic UNITANK configuration consists of a single tank divid-ed into three hydraulically connected compartments in series.Each compartment has an aeration system and no provisionfor external sludge recirculation. The outer compartments al-ternately act as aeration and sludge settling tank while themiddle one acts as aeration unit only. A single operation cycleconsists of two main stages which have three basic stepswhich are performed in a symmetrical manner beginning fromeither of outer compartments in each stage. There is no sepa-rate sedimentation tank with scraper but the outer compart-ments have sludge slots and fixed effluent weirs. For removalof N and P, an advanced variant of UNITANK is used. Thisconfiguration possesses additional anaerobic/ anoxic compart-ments with internal recirculation of mixed liquor. UNITANKis more suitable for small- to middle-sized wastewater

Table 1 Removal efficiencies ofC, N, and P fromwastewater fromdifferent sources using SBRprocess

Wastewater source SRT(d) Percentage removal Reference

C N P

Synthetic wastewater 10 94 90 57 [46]



Domestic wastewater plant supplemented withorganics and phosphate

15 91 98 98 [49]

Domestic wastewater obtained from the gritchamber effluents of the Pasakoy WastewaterTreatment Plant (Istanbul)

28 91 78 87 [50]

Domestic wastewater 23 95 96 84 [51]

Abattoir (slaughterhouse) effluent 15 95 97 98 [52]

Piggery wastewater 1 70.2 99.7 97.3 [53]

Piggery wastewater 15 98.8 98.6 98 [54]


treatment plants with the advantages of simple structure, lessland occupation, cost-efficient, and reliable operation. TheUNITANK system is being used in different countries likeChina, Mexico, Argentina, Brazil, Vietnam, etc [57, 58].

Intermediate Cycle Extended Aeration System (ICEAS)

A further enhancement of the standard SBR batch process isIntermediate Cycle Extended Aeration System (ICEAS) pro-cess which processes continuous inflow of the wastewater.Variable inflow is handled by a distributor box which distrib-utes flow evenly among all the tanks so as to avoid overloadingin any single tank. A pre-react zone with high F/M acts as aselector. Thus, enhanced settling of sludge and inhibition of thefilamentous growth can be achieved. The main-react zone lo-cated after the pre-react zone is operated in three basic opera-tion modes, Aeration, Settle, and Draw. The equal loading ofall the basins during continuous inflow simplifies the operationand process control. It also makes maintenance easier. There issignificant capital cost reduction as compared to conventionalSBR process as only a single set of tanks is required. Thecomplex process control associated with conventional SBRprocess is overcome as at any given point of time all the basinsreceive equal loading and flow. The ICEAS is gaining popu-larity in China, US, UK, Peru, Qatar, etc. for replacing the oldSTPs or for new plants where limited space is available orenhanced effluent quality is required [59, 60].

Process Control Strategies

Unlike CFP system, the SBR process can be used understeady or unsteady state conditions. SBR process wins overCFP or ASP processes due to its superiority in many aspectssuch as better effluent quality in terms of COD and nutrient,better control of filamentous bacteria as well as low energyconsumption. These feats were possible due to superior pro-cess control in SBR. Over the past 30 years, control technol-ogies for the SBR process have continuously evolved, leadingto the development of a wide variety of control systems tooffset the complexity of the SBR process.

Classical SBR control is performed with fixed time cyclewhich has a disadvantage that it does not allow for the adap-tation of length cycle to compensate the effect of process de-viations and variations in the influent composition. Real-timecontrol, on the other hand, should provide better flexibility foradaptation of optimized control in varied conditions. Precisereal-time process control requires feedback on at least the startand end of various biological reactions taking place within anSBR. Real-time monitoring of direct parameters such as CODor BOD, TSS, and various forms of nitrate and phosphate maynot be accurately possible with the currently available tech-nology. Online monitoring of indirect parameters such as pH,

dissolved oxygen (DO), and oxidation-reduction potential(ORP) can successfully indicate the reaction processes thatoccur during carbon and nitrogen removal in SBR processes.Figure 2 shows time-dependent profile of pH, DO, and ORPduring one typical cycle of a conventional SBR and also,corresponding concentration profile of COD and differentspecies of nitrogen.

ORP has a direct correlation with nitrification rates andother biological reactions in anoxic conditions [61]. In normalcondition, ORP is positive and increases during aeration phaseand negative during anoxic stage. The normal range of valuesof ORP is 0 to 50 mV in aerobic stages and 0 to −300 mV inanoxic stages. In the anoxic stage, ORP has a continuousdropping profile with respect to time; a steep drooping profile,known as nitrate knee, occurs that signifies the end of denitri-fication so that it is safe to stop anoxic phase and start the nextstep.

The pH increases during denitrification and decreases dur-ing the nitrification reaction [62, 63]. There are two importantbreakpoints in pH profile with respect to time:

(a) Ammonia valley: As nitritation produces acid, pH tendsto decrease gradually at the beginning of nitrification.When all the ammonia has been oxidized to producepeak nitrite concentration, there is no further acid produc-tion due to ammonia conversion. pH profile shows aconcomitant minimum which is known as ammoniavalley.

(b) Nitrate Apex: During the anoxic stage, the pH rises andproduces a continuously rising profile. A maximum isreached when the entire nitrate is converted to nitrogen,indicating an end of denitrification stage. Nitrate apexexactly corresponds with nitrate knee as observedthrough ORP profile.

Researchers argue that pH profile is the best indicator of thechanges in the microbes profile occurring inside a SBR reactor[64]. However, the background alkalinity present in the waste-water often provides a strong buffering capacity that mini-mises noticeable variation in the pH.

During aeration, when COD is getting depleted, there is aconsumption of DO. As the COD is consumed at constantDO supply rate, the reactor DO profile continuously increasesbecause the COD level is fast decreasing. When nitritationoccurs, the DO profile rises sharply because nitritation onlyrequired 25 % as much of the oxygen as that of nitrification;this point of inflexion is called DO breakpoint. DObreakpoint and ammonia valley correspond with each otherin time [65]. At the end of the aeration, DO profile fallssharply to zero and maintains it until the end of anoxic phase,and therefore, does not provide effective information; asa result, DO profile alone cannot be reliably used to getfeedback and control denitrification.


Oxygen uptake rate (OUR)-based controls are becomingpopular [66]. OUR is the DO consumption per unit time inunit volume of the reactor and can be calculated by a PC or aPLC (programmable logic controller) from sensor-based DOmeasurements. OUR has been applied in the control of newSBR processes, such as the short-cut nitrification process [67,68] and the enhanced biological phosphorus removal (EBPR)[69]. The breakpoint of the dOUR signal curve (first deriva-tive of OUR), from negative to positive, indicated the end-point of phosphate uptake. OUR-based control has the samedisadvantage as DO-based control.

Many studies on real-time control strategies have beenemployed in research on effect of parameters taken in pairs,such as pH and ORP [70], ORP and OUR [71], ORP and DO[72], and pH and DO [73]. Various real-time control strategiesbased on specific process parameter patterns have thus beenproposed [74]. In one study, optimal control of anoxic andaerobic phases by the indirect parameters was done throughan algorithm that switched phases at some characteristicspoints on the profile curve obtained by filtering and process-ing primary data on the parameters, resulting in reduction ofenergy [75]. Due to highly nonlinear nature and time varia-tions associated with the SBR process, along with fluctuationsin hydraulics and components and possible equipment unreli-ability, a single control strategy based on multiple indirectparameters may not be successful, rather a strategy based onmultivariate statistical process control analysis may be devel-oped through further research [76, 77]. Intelligent controlstrategy (ICS) such as fuzzy logic [78], expert system [79],

and also artificial neural network (ANN) based model [80] hasbeen used to effectively optimize the SBR processes. Intelli-gent control being the advanced form of real-time controlstrategy, it is going to be the future of SBR real-time control.

The mathematical modeling for operational optimizationand effective control of nutrient removal in an SBR by simu-lation instead of performing trial and error experiments at fullscale gained popularity towards the end of the twentieth cen-tury. Several models are available for dynamic simulation ofcombined biological processes for nutrient removal in activat-ed sludge systems. The IAWQ ASM2 model with furthermodifications could be employed for modeling of long-termnutrient removal in SBR, with better phosphorus dynamics byconsidering the DPAOs activity [81].

Recent Developments in the Application of SBR

The SBR technology is being applied in a large number oftreatment processes owing to the operational flexibility it of-fers. The ability of SBR to perform flow equalization, biolog-ical treatment, and secondary clarification within a single tankby varying the duration of each phase and aeration period [82]makes it a versatile treatment technology. In the recent years,the combination of different treatment technologies has beentested at lab scale to extend the application of SBR technologyfurther.

A variation of SBR is a sequencing batch biofilm reactor(SBBR), which is a combination of suspended and attached

Fig. 2 Typical variations of DO,pH, and ORP value andconcentrations of NH4-N, NO2-N, and NO3-N during nitrificationand denitrification process in aconventional SBR [adopted fromref. 64]


growth (CSAG) system. Biofilm grows at a solid–fluid inter-face by attachment to a support material. It provides a chanceto slow growing microorganisms to proliferate, irrespective ofthe HRT, and sedimentation characteristics of the bio-aggre-gates. The selection of support material and its size depend onthe characteristics of the wastewater and the treatment objec-tives. The reactor may be packed with the support material orit may be suspended in the reactor fluid. A typical SBBR cyclehas Fill, React, and Draw phases only. Plug flow conditionsexist within an SBBR. The time required for washing of thesupport media may be considered analogous to the settlingtime in an SBR. Due to the excessive head loss and sloughingoff risk, the SBBR systems are unsuitable for influent withhigh TSS and when excessive microbial growth is expected.There have been numbers of installations after the first pilotscale SBBR was used to treat leachate from the Georgswerderlandfill, Germany [83].

The immobilization of microorganisms on a carrier mediareduces the microbial washout, protects them from toxicconstituents, pH, and temperature extremes [84]. As the me-dia is retained, a shorter HRT is possible that results insmaller reactor size or higher treatment capacity with thesame size reactor compared to conventional SBR. Biofilmconfigured systems are more resilient and are well suited fortreating wastewater with highly variable quality with lowsludge production [85, 86]. When chosen judiciously, themedia may help absorb shock loads, for example, activatedcarbon for high organic load or zeolite for high ammonia ininfluent. These buffers temporarily adsorb the shock load-causing constituent and later gradually desorb the pollutantsalong with their simultaneous or subsequent biodegradation[87]. The powdered activated carbon (PAC) for treatment ofraw landfill leachate demonstrated better NH3-N, color, andCOD removal than conventional SBR [88]. The use of in-telligent dynamic control systems over the conventional timecontrol system has shown to improve the COD, TP, and TNremoval efficiencies with considerable energy savings [89].

A modified SBR system with bio-floc technology (BFT)has found interesting application in aquaculture where proteinfeed for fish as well as treatment of wastewater are consideredto be cost-inhibiting [90••, 91]. Biofloc refers to a special kindof macro aggregate of microorganisms which are able to takeup nitrogenous compounds present in wastewater and to con-vert it to microbial protein. Biofloc organisms can be used asfood to the fishes. It has been shown at the lab-scale that SBRenvisaged as external growth reactor for bio-floc was able toattain nitrogen removal efficiency of up to 98 % when anoptimal C/N ratio between 10 and 15 was maintained [92••].It has also been demonstrated that the BFT in SBR reactor alsoenabled conversion of nitrogen in aquaculture suspendedsolids into bacterial biomass [93], which could potentially beused to feed fish, thereby increasing the efficiency of nitro-gen—nearly reaching 100 % nitrate removal within 6 h.

SBR and SBBR were used to treat industrial waste-water containing phenolic compounds, such as p-nitrophenol (PNP) which is a hazardous chemical wide-ly used in agricultural, pharmaceutical, and dye indus-tries as a synthetic intermediate in the manufacturingprocess. Complete removal was demonstrated for PNPremoval up to 350 mg/L influent concentration (loadingrate of 0.368 kg/m3day−1) by SBR and SBBR (withpolyethylene rings) [94]. However, the average NH3-Nremoval efficiency for the SBR and SBBR was onlyslightly compromised; it reduced to 86 and 96 %,respectively.

SBR has been successfully applied for wastewater withhigh nitrogen content and low COD such as anaerobic SBR(ASBR)-based simultaneous partial nitrification, anaerobicammonium oxidation, and denitrification (SNAD) system thatwas applied to treat the opto-electronic industrial wastewaterwith C/N ratio of nearly 0.2 [95••]. A similar study was per-formed by [96] for the treatment of wastewater from produc-tion of thin-film transistor liquid crystal display (TFT-LCD)which contained chemicals like dimethyl sulfoxide (DMSO),monoethanolamine (MEA), and tetra-methyl ammonium hy-droxide (TMAH). Two different SBR systems, aerobic andanoxic/oxic (A/O), were used. Effective MEA degradationcould be easily achieved under all three conditions examined,while efficient DMSO and TMAH degradation could beattained only under anaerobic and aerobic conditions,respectively.

These days, hybrid systems like the Porous biomass carrierSBR (PBCSBR) are being investigated to achieve improvednutrient removal efficiency using time-sequenced anoxic/oxicphases and high biomass [97]. In another study, a new bio-mass retention strategy using natural fibers as biofilm carrierswas utilized to treat dairy manure. The concept, evaluated fortreating flushed dairy manure in a psychrophilic ASBR,showed higher methane yield despite short HRT (6 days)and low temperature. ASBRs are known to be capable ofuncoupling HRTwith SRT for biomass retention. Additional-ly, a particular sequence of operation of ASBR was used toexert selection pressures on microbes for immobilization[98–100]. Aerobic SBR process, coupled the photo-fentonprocess and reverse osmosis (RO), was used to reclaim waste-water from textile industry enabling complete internal reuse ofwater [101•].

SBR operating conditions such as cyclic feast andfamine regimes, high shear stress, and short settlingtime promote formation of floc granules which are noth-ing but dense microbial consortia consisting of differentbacterial species that perform different roles indegrading the complex wastes. Alternating anoxic/oxiccondition combined with step-feeding mode (AASF)was proved to be an efficient method for nitrogen re-moval in granular SBR (GSBR) [102]. Aerobic granular


sludge presents several advantages over activatedsludge, such as excellent settling properties, high bio-mass retention and biosorption, and ability to deal withhigh organic loading rates and to perform diverse bio-logical processes simultaneously, such as COD, N, andP removal [103••, 104–106]. The utility of aerobic gran-u la r s ludge in SBR for degrada t ion of tox icorganofluorine compounds such as 2-fluorophenol (2-FP) has been demonstrated [107]. The fluoroaromaticcompounds are usually biodegraded via halo-catechols[108, 109]. The maintenance of a good population ofhalo-aromatics degraders in bioreactors is highly desir-able due to the low concentration discontinuous natureof these compounds [107]. The GSBR provides highbiomass retention and thus is extremely promising forthe treatment of effluents containing toxic compounds.Conventional SBRs treating wastewaters with flocculat-ing sludge can be converted to granular SBRs by reduc-ing the settle time [110].

A study with wastewater containing azo dyes attempting forsimultaneous bio-decolorization and COD removal in SBRhaving a combination of anoxic-aerobic React phase revealedthe following: (a) longer anoxic React phase promotes decol-orization, while (b) COD removal was better with shorter an-oxic phase [111•]. The granular-activated carbon-SBR (GAC-SBR) has shown promising results for the treatment of textilewastewater containing dyes where the dye was removed byGAC via physical adsorption mechanism. Addition of extrane-ous organic carbon increased the removal efficiency of directdye [112].

In recent years, the presence of endocrine disruptorcompounds (EDCs) in surface water, public water suppliesas well as in wastewater has generated much public con-cern. EDCs are a group of different chemical substancesthat even in low concentrations such as sub-μg/L level inwater may interfere with the normal functioning of humanendocrine system and animals [113]. SBR process presentsan attractive avenue for the removal of EDCs from waste-water due to its ability to provide anoxic/anaerobic/aerobicconditions within the same basin. Maintenance of suchdynamic environmental conditions inside the SBR processtank provides ample scope for the microorganisms capableof degrading EDCs to utilize them. Longer SRT and HRThave been observed to cause greater degree of removal ofEDCs primarily because such conditions allow for theproper growth of the slow growing microorganisms capa-ble of utilizing the EDCs. Table 2 summarizes brief detailsof some of the recent studies performed on the efficienciesof different SBR configurations for the removal of EDCscommonly occurring in wastewater. Ozonation and otherpolishing steps are suggested after proper study of finalproducts in critical cases where significant dilution ofSTP effluent does not occur [118]. T

able2

Percentage

removalof

differentE

DCsusingSB

Rprocesseswith

differentconfiguratio

n

Configuratio

nReference

SRT(d)

Nam

eof

EDCs

Rem

oval(%

)Rem

arks

SBRwith

cycletim

eof

4h

[114]

1017-β

Estradiol

(E2),bisphenol

A(BPA

),Estriol

(E3),

17α-ethynilestradiol

(EE2),estrone

(E1),4-octylphenol

(4-O

P),4-nonylphenol

(4-N

P),bisphenol

A(BPA

),nonylp

henolethoxylate(N

PnEO)

>95

forE2,

E3,4-OP,

andBPA

Mem

brane-basedseparatio

nof

sludge

andeffluent,as

comparedto

sedimentatio

n,canrelativ

elyim

provethe

elim

inationof

targetEDCshaving

lower

removalrates.

SBRwith

Cl 2disinfectio

n[115]

31.9

BPA

,estrone,17-βestradiol,Estriol,

17-α-ethnylestradiol

>91,>

82,≈

100

forrest,

respectiv

ely

The

studyshow

sno

significantd

ifferencebetweenrA

2O,

oxidationditchandSB

Rtechnology

inremovalof

EDCs.

Kowvalues

abovethreefavorsadsorptio

nover

biodegradatio

npathway

forremovalof

EDCs.

Lab

scaleanaerobic/aerobic

SBRunit

[116]

5Carbamazepine(CBZ),Diltiazem

(DTZ),

estrone,butylb

enzylp

hthalate,

progesterone,A

cetaminophen

>77,>

91,>

90,

≈100

forrest,

respectiv

ely

Biodegradationisthemostimportantrem

ovalmechanism

,except

forCBZ,w

hich

accumulated

largelyin

thesludge.

DTZshow

edlower

biodegradability

SBRwith

twobasins

with

anoxic

andaerobiczonesfollo

wed

bytertiary

treatm

entsystem

[117]

16estrone,estradiol

85,96

SBRisgenerally

effectivefortheremovalof

theestrogensbut

theexactrem

ovalmechanism

needsto

beinvestigated.


Conclusions

Need for recycling of treated wastewater in many parts of theworld has necessitated the introduction of newer stringentstandards for treated wastewater. Unlike the conventionalwastewater treatment plants, SBR-based wastewater treatmentplants can achieve better treated water quality with no or mi-nor modification in the installed infrastructure, only by simplealteration of the process control parameters in one or more ofthe phases of the treatment cycle. The SBR process offerssmaller foot-print area, lower investment cost, lower operationcomplexity, and significant control performance as comparedto conventional treatment process. If properly designed, theprocess may achieve significant degree of biological nutrientremoval too. Although the SBR process is well developed,and different variants are continuously evolving, there are is-sues that need to be addressed further.

Ensuring process reliability for simultaneous N and P re-moval in SBR requires further work towards clear understand-ing of the microbial diversity of the system with an emphasison its dynamics under different changing process situations.The study and improved design may follow the principles ofecologically engineered processes that derive stability fromthe presence of multiple species that accumulate phosphorus(functional richness). This may make the system more resil-ient with each species showing differential sensitivity to var-iations in the environmental conditions such as temperatureand pH swings, toxic pollutants, presence of nitrite and nitrate,prevalence of VFAs, etc.

Appropriate process control is the heart of the SBR processas it important in ensuring removal of the target contaminantsfrom the wastewater. PLC-based pre-programed control strat-egies are popular. Introduction of real-time control strategiescan enable SBR process to achieve better robustness, reliabil-ity, and optimized operation. This will enhance energy effi-ciency and also shall help widen the areas of application of theSBR process. Future studies on SBR control strategies shouldinclude the development of intelligent control system, whichis a real-time control strategyworking on feedback-based con-trol. This shall make the SBR process adaptive to changingenvironmental conditions and to varying wastewater qualityso that optimum effluent quality is maintained with high de-gree of reliability.

Compliance with Ethics Guidelines

Conflict of Interest Sudipta Sarkar and Aparna Dutta declare that theyhave no conflict of interest.

Human and Animal Rights and Informed Consent This article doesnot contain any studies with human or animal subjects performed by anyof the authors.

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http://www.watertoday.org/Article%20Archieve/Aquatech%2012.pdf

http://www.watertoday.org/Article%20Archieve/Aquatech%2012.pdf

http://www.xylem.com/Assets/Resources/1710-Sanitatire-ICEAS-BiologicalTreatmentProcess.pdf



http://www.xylemwatersolutions.com/scs/usa/en-us/Resource%20Library/Brochures/Documents/Sanitaire/Sanitiare%20ICEAS%20SBR%20Brochure.pdf



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90.•• Emerenciano M, Gaxiola G and Cuzon G. Biofloc Technology(BFT): A Review for Aquaculture Application and Animal FoodIndustry. In: Matovic DM, editor. Biomass Now – Cultivation andUtilization. InTech; 2013. 301-328. This chapter gives a com-prehensive idea about the emerging BFT and its applicationand scope

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92.•• Luo G, Avnimelech Y, Pan Y, Tan H. Inorganic nitrogen dynamicsin sequencing batch reactors using biofloc technology to treataquaculture sludge. Aquac Eng. 2013;52:73–9. This work repre-sents an initial research concerning the pretreatment of aqua-cultural SS and the quantification of NO3−-N pathways inSBR to treat aquacultural sludge using BFT.

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100. Wang ZW, Ma J, Chen S. Bipolar effects of settling time on activebiomass retention in anaerobic sequencing batch reactors digestingflushed dairy manure. Bioresour Technol. 2011;102(2):697–702.

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111.• Al-Amrani WA, Lim PE, Seng CE, Ngah WSW. Factors affectingbio-decolorization of azo dyes and COD removal in anoxic–aero-bic REACT operated sequencing batch reactor. J Taiwan InstChem Eng. 2014;45:609–16. This study investigates the effectsof duration of operating periods, azo dye molecular structureand presence/absence of co-substrate on the performances ofthe anoxic–aerobic REACT operated sequencing batch reac-tors (SBRs).

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116. MuzM, Aka S, Komesliab OT, Gokcaya CF. Removal of endocrinedisrupting compounds in a labscale anaerobic/aerobic sequencingbatch reactor unit. Environ Technol. 2014;35(9):1055–63.

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