Introduction

As the environmental discharge standards are getting more and more stringent, the traditional continuous flow-based biological wastewater treatment process faces severe challenges. 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 the ASP-based treatment process to a batch process as in SBR helps introduce various process flexibilities and alternatives in process controls and design so as to better achieve the latest effluent discharge standards. The term SBR was originally coined by R.L. Irvine [1]. Opposed to the common belief of SBR being a new technology, the SBR-like fill and draw processes were popular during 1914–1920. The revival of interest in SBR technology in its present form occurred during the late 1950s and early 1960s due to the improvement in technology related with aeration and process control. In its initial years, SBR technology was mainly used by small communities for sewage treatment and for the treatment of high strength industrial wastes. Due to the design flexibility and better process control that can be achieved by the modern technology, the use of the SBR process has not been limited to the field of sewage treatment only; it has also found wide acceptance in biological treatment of industrial wastewater containing difficult-to-treat organic chemicals. As the SBR process can be effectively automated, it is known to save more than 60 % of the operating expenses required for a conventional ASP and is able to achieve high effluent quality in a very short aeration time. In densely populated countries such as India and regions such as Europe, SBR is being considered as a preferable technology due to its low requirement of area as well as manpower for operation. The SBR process is often preferred over continuous flow process (CFP) due to reduction in energy consumption and enhancement in the selective pressures for BOD, nutrient removal, and control of filamentous bacteria. Due to these reasons, SBR process is gaining immense popularity in the recent years. The SBR technology has been undergoing several minor and major modifications over the past few years to be able to effectively treat the exponentially increasing number of new pollutants in wastewater. This article provides an insight into the technology as well as reviews the recent advances in the design and application of SBR technology.

The Process

In a CFP such as an activated sludge process, all the unit processes are in work in tandem at a given point of time. On the other hand, in an SBR process, all these unit processes take place within a single tank for their specific duration and intervals, sequentially spaced over a span of time. Thus, SBR provides in time the same treatment what the CFP provides in space. The SBR technology basically incorporates a fill-and-draw type biological wastewater treatment process, functionally resembling to an activated sludge process. Depending on the scale of operation, the SBR system, along with its variants 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, the time duration of each mode within a tank can be adjusted to meet the different treatment needs, such as low COD in the effluent, biological nutrient removal, etc. Figure 1 schematically shows various modes of operation of an SBR system. The figure also informs about few alternative arrangements possible during each of the individual steps so that specific treatment objectives are met.

Fig. 1
figure 1

Different phases of SBR operation cycle with descriptions and possible process variations

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During the Fill phase, the tank receives the raw wastewater that comes in contact with the active biomass left inside the tank at the end of the previous cycle. There are three variations which may be incorporated singly or combined, depending on the wastewater characteristics, the target organics and biological nutrient removal: static fill, mixed fill, and aerated fill. During static fill, influent wastewater is added to the biomass already present in the SBR without mixing, resembling almost a 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 suppressing the filamentous ones, which provides good settling characteristics for the sludge. Additionally, static fill conditions create a “feast”-like situation in which phosphate accumulating organisms (PAO) are favored; as discussed later, these are responsible for biological phosphorus removal.

The React phase is intended for the completion of the biological reactions responsible primarily for the degradation of organics. Further, the React phase is often designed to provide a high degree of nutrient removal as well. The treatment is controlled by air, either on or off, to produce anaerobic, anoxic, or aerobic conditions. Variations such as mixed react and aerated react modes may be adopted. During aerated react, the aerobic reactions initialized during aerated fill are completed. Designs often include conversion of ammonia-nitrogen to nitrite-nitrogen and ultimately to nitrate-nitrogen, a process known as nitrification. In mixed react mode, apart from aerobic conditions, there may be combinations of anoxic and anaerobic conditions created within the reactor. Anoxic conditions can achieve denitrification, a process in which nitrate-nitrogen is converted into nitrogen gas. Anaerobic conditions shall create a “famine” phase that promotes phosphorus removal.

During the Settle phase, the entire reactor tank acts as a batch clarifier, without any inflow or outflow. In a CFP process, on the contrary, the quiescent settling is often impaired by the continuous inflow and outflow of liquid, giving rise to inefficient settling that causes poor effluent quality.

The Draw phase uses a decanter, either fixed or floating, to decant the treated supernatant after the settlement of the biomass generated after the React phase.

Idle phase is the time between the draw and fill phase. The need for such a phase is often necessitated when there are several reactors operating in parallel operation, acting as a buffer in time. During this phase, mixing of the biomass to condition the reactive contents, and wasting of excess sludge, may be taken up, depending on the operating strategy.

The complete cycle time spans the duration between beginning of Fill and end of Idle phase for a single tank SBR system. The multiple tank system consists of tanks in series where it is ensured that the Draw of a tank is completed by the time another tank completes Fill. Single tank operation is suitable for low population localities or in industries with variable flow conditions. The wasting of microorganisms is done once per cycle during the react phase in high yielding multiple tank system while the frequency of wasting may be as low as once every 2 weeks for low yielding single tank operation [2]. The simultaneous aeration, mixing, reaction, and settling occurring within the SBR tank obviate the requirement of a separate clarifier unit. The duration of Fill and React phases can be adjusted to impart the SBR system a CSTR-like or ideal PFR-like treatment characteristics. The SBR system provides major operational flexibilities like internal equalization and control of biological reactions through regulation of aeration. The presence of microorganisms in high concentrations right from the Fill phase reduces the treatment duration significantly. The ability to control the substrate availability by varying the aeration duration during Fill provides a high degree of flexibility in controlling the filamentous organism population and concentration of nitrogen. Anoxic period during the React phase is useful in nitrogen removal from the system.

In ASP, reactor operating conditions such as low dissolved oxygen (DO), low F/M ratio, and completely mixed operation are responsible for the growth of filamentous bacteria that have poor settling characteristic. It causes the effluent to have high suspended solids content, a situation known as sludge bulking that result in poor efficiency of the treatment plant. Due to the operational similarity of ASP and SBR, sludge bulking is a common problem in SBR processes, too. In order to overcome such problem, a variation of design in SBR process is often made in the form of providing special bioreactor(s), known as bio-selector or simply, selector that favors the growth of floc-forming heterotrophic bacteria over the filamentous one. The bio-selective mechanism for floc-formers is to contact the return activated sludge with the influent wastewater in a separate initial contact zone which is termed as selector zone. The initial contact zone typically consists of three or four completely mixed zones with a gradual high to low F/M values and having limited or no molecular oxygen present, where heterotrophs remove the majority (75–90 %) of the low molecular weight, soluble substrates from the wastewater within first 5 to 10 min, only to utilize the absorbed food for later when molecular oxygen is available. Some heterotrophs such as denitrifiers can even use combined oxygen such as nitrate or nitrite for metabolic purposes. Filamentous bacteria, on the other hand, neither can compete with floc-formers at high F/M ratio nor are able to store substrate for such later use. Thus, they get suppressed by the floc-formers in the selector zone as well as inside the SBR during subsequent aeration, anoxic, or anaerobic stages. The selectors can be made either anoxic or anaerobic, by varying the degree of mixing at low or no oxygen supply, depending on whether denitrification and biological phosphorus removal are targeted in the SBR. For effective working of selectors, it is required that aeration in the subsequent SBR tank is complete so that return 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 to include tertiary treatment units for nutrient removal from wastewater apart from the conventional pollutants like COD, BOD, and suspended solids and pathogens. An SBR-based treatment plant can easily address this requirement without addition of any new infrastructure, only by optimizing the sequence of aerobic, anoxic, and anaerobic phases during the different stages of SBR process.

Biological Nitrogen Removal Process

The operation of SBR cycle phases in time rather than space; it provides a greater degree of flexibility for nutrient removal through alteration of aeration and mixing regimes to create alternating aerobic and anoxic environment during “React” phase. The biological nutrient removal process mainly involves two steps, nitrification and denitrification. The organic nitrogen is first converted into ammonia-nitrogen during the oxidation of the organics or chemical oxygen demand (COD) present in the wastewater. Complete nitrification means oxidative conversion of ammonia-nitrogen to nitrate-nitrogen by chemoautotrophic bacteria. It consists of two steps: nitritation (from ammonia to nitrite) and nitratation (from nitrite to nitrate), catalyzed by two groups of autotrophic bacteria: ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB), respectively. Both use CO2 as carbon source and oxygen as terminal electron acceptor. Nitrification requires strictly aerobic environment which is achieved through aeration in an SBR tank. The second step, denitrification, involves the heterotrophic bacteria which are anaerobic and utilize complex organic compounds for their carbon requirements and nitrate serves as the electron acceptor under anoxic or anaerobic conditions, which in turn forms nitrogen gas that leaves the aqueous phase.

Temperature, pH, DO level, and solids retention time (SRT) are important parameters in nitrification kinetics. The optimum temperature is between 25 and 35 °C, whereas the rate of nitrification drops to 50 % of the optimum if the pH lies beyond the range of 7.5 to 9.8. The nitrification reaction destroys alkalinity, thereby tending to lower the pH. The maximum rate of nitrification has been observed to occur at a DO level greater than 2.0 mg/L. The nitrification rate can be increased with increasing concentration of nitrifiers. This is achieved by increasing the mixed liquor suspended solids (MLSS) concentration within the reactor, which is possible when SRT is increased. Increase in SRT can be achieved by lowering the flow rate of waste activated sludge (WAS). Denitrification process is effective at anoxic conditions when DO level falls below 0.5 mg/L and at greater than 1.0 mg/L, the denitrification process is inhibited. The optimum pH is between 6.5 and 9.

Simultaneous Nitrification-Denitrification Process

SBR provides a viable alternative to the CFP systems for biological nutrient removal (BNR) by introducing anaerobic, anoxic, and aerobic reactions within a single tank during a treatment cycle. Hence, SBR can provide simultaneous nitrification-denitrification (SND) when only nitrogen removal is targeted. Apart from other controlling factors, carbon to nitrogen (C/N) ratio usually governs the SND process within a SBR. It was demonstrated that it is possible to achieve complete COD and NH4 +-N removal without even having any residual NO2 −-N in the effluent from an SND–SBR system by adjusting the ratio to 11.1. A low COD/NH4 +-N ratio may result in unbalanced SND, causing little or no denitrification [3]. SND has slower ammonia and nitrate utilization rates as compared to separate basin designs because only a fraction of the total biomass is participating in either the nitrification or the denitrification steps. Typical nitrification efficiency is close to 100 %, while the total nitrogen removal is about 90–95 % under stable operation conditions. The excellent nitrogen removal performance of SBR has led to setting up of a number of plants for landfill leachate treatment. Logically, the inclusion of anoxic phase right after the aerobic phase enhances the nitrogen removal efficiency; however, this will necessitate carbon supplementation from external source for low-strength wastewater. This need may get reduced to 5 % of the above in case of wastewater having high organic concentration. With better process control, as has been discussed later, step feed system with an optimized intelligent regime control in terms of DO and ORP may help in elimination of such external carbon supplementation.

Short-cut Nitrogen Removal Process

Another promising treatment is partial nitrification and denitrification process. It is based on the partial nitrification (nitritation) up to nitrite followed by the reduction of nitrite to nitrogen (denitritation). This process is popularly known as short-cut nitrogen removal [4]. Short-cut nitrogen removal reduces the aeration requirement by 25 % and also the external carbon-supplementation by 40 % as compared to conventional SND 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 nitrogenous wastewater, N removal via this process showed promising results for the process optimization [6, 7]. These features are typical 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 to nitrite which is later used by the anammox (anaerobic ammonia oxidizing) bacteria as an electron acceptor to react with ammonia-N to produce nitrogen gas and nitrate [8]. The process eliminates the requirements of aeration and exogenous organic carbon sources compared to the traditional nitrification–denitrification process of nitrogen removal. However, slow growth rate of the anammox bacteria and toxicity issues forced the early development of the process with biofilms, granulation, and suspended biomass reactors [9–11]. The process requires the combination of two processes, nitritation and anammox reaction, which, in engineered systems, can either be separated in time or space. Spatial separation is implemented by the use of two-stage reactor systems whereas temporal separation, as in SBRs, requires strict control strategy and operation regime for successful implementation. It was concluded that ORP correlates well with different stages of activity during different feeding and aeration strategies and, so, interval feeding with interval aeration is the best strategy for process performance in terms of ammonia-N removal, nitrate-N production, and pH stability [12••]. In SBR, only a part of the water is withdrawn as effluent, thus nitrate as well as total nitrogen concentration continuously rises during long-term operation. Ideally, a denitrification process is expected to be included in anammox SBR system to solve the problem. Denitrification requires addition of extraneous carbon source; it was reported that COD concentration is a control variable for process selection between anammox reaction and denitrification so that anammox bacterial growth is significantly suppressed by the presence of high concentration of COD [13, 14]. However, it was demonstrated that at low COD concentration, activity is not diminished for neither anammox nor denitrifiers, and total nitrogen removal efficiency is improved [15••]. It was shown that good efficiency with combined anammox and denitrification is possible in SBR for wastewater with low organic carbon and high nitrogen content. Although the anammox activity was suppressed at high organic content such as shock load, the activity was recovered quickly when such a shock load was withdrawn [16]. It was reported that ammonium oxidation rates of up to 500 gN m−3day−1 with greater than 90 % conversion to N2 have been achieved in a pilot study using online ammonia sensor, with continuous aeration at dissolved oxygen concentrations <1 mg O2 L−1 [17]. The nitrite oxidation and the anammox reaction occurred simultaneously, allowing increased overall nitrogen performance and simplified process control compared to separate aerobic and anaerobic phases.

Enhanced Biological Phosphorus Removal (EBPR)

Phosphorus (P) removal, now an integral component of wastewater treatment plants, uses a special group of organisms known as polyphosphate accumulating organisms (PAO) that under alternative anaerobic and aerobic conditions, incorporate the influent P into the cell mass. The sludge is subsequently removed during sludge wasting. Under anaerobic conditions, 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 mainly through hydrolysis of the intracellular stored poly-P, resulting in the release of ortho-phosphate into water. Under aerobic or anoxic conditions, PAOs are able to take up excess phosphorus in intracellular poly-P formation, biomass growth, and glycogen replenishment by using stored PHAs as the energy source. The phosphate released in the anaerobic stage is less than that absorbed in the aerobic or anoxic stage; the net removal of phosphorus can be achieved through wasting of sludge which is enriched in poly-P. Sequencing batch reactors (SBRs) can achieve alternating anaerobic and aerobic conditions by controlling the operational process, and consequently, biological phosphorus removal using SBRs has drawn increasing attention worldwide [18, 19]. The P-removal efficiency as high as 90 % have been reported in SBR, whereas in conventional activated sludge systems, maximum efficiency achieved is only 10–20 % [20].

Another group of organisms, known as glycogen accumulating organisms (GAOs), closely resemble to and compete with PAOs in their metabolism. GAOs have no contribution to the P removal and their proliferation is known to cause failures in P removal in reactors. Finding optimal conditions that favor PAOs to GAOs are necessary for success in biological-P removal. The controlling parameters are pH, temperatures, and more importantly, substrate type. The cold temperature seems to favor PAOs [21]. Increasing pH is known to provide an advantage to PAOs over GAOs, and optimum pH was reported to be between 7.2 and 8 for effective GAO control [22].

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

Recently, a study on EBPR from abattoir wastewater showed a possibility of achieving high level of Bio-P removal at a much lower SRT of 2–4 days [26••]. Due to the short SRT, nitrification/denitrification could not be achieved and hence post treatment of SBR effluent might be required. The PAOs demonstrate a high requirement of organic carbon and hence application of short SRT for Bio-P removal in low COD domestic wastewater can be challenging. The idle time of a SBR had potential impact on biological phosphorus removal, especially when the influent phosphorus concentration increased [27••]. A new configuration of SBR with sludge tank halved (STH-SBR) has been successfully designed to address the main drawback associated with the extended-idle phase being much longer than the anaerobic phase in the anaerobic/oxic (A/O) regime. It demonstrated higher P removal the conventional A/O-SBR [28••].

Simultaneous Removal of Nitrogen and Phosphorus in an SBR Process

It is expected that nitrification, denitrification, and EBPR should often take place simultaneously in an SBR. For the simultaneous removal of nitrogen and phosphorus, the interaction among the processes, if not optimally controlled, may give rise to the failure of the treatment plant. Among the reactants and intermediate products, toxicity of nitrite and its acidic counterpart, free nitrous acid (FNA), is important as they are known to provide a competitive disadvantage to PAOs over GAOs in the EBPR systems. They are a key selection factor in the PAOs/GAOs competition, severely inhibiting PAO activity at a concentration as low as 2 mg/L nitrite-N and complete inhibition at 6 mg/L nitrite-N [29]. Although earlier studies pointed out to poor phosphorus removal under nitrate-rich conditions in the anaerobic zone, it is more attributed to the disruption of anaerobic conditions by nitrate [30], consumption of fatty acids by denitrifying non-polyphosphate heterotrophs [31], and inhibition of PAOs by nitrite, as a result of incomplete denitrification [32–34].

Denitrifiers as well as the PAOs both require organic substrate, and quite naturally at low oxygen concentrations they are likely to compete. A long-term anaerobic exposure to nitrate was demonstrated to diminish the number of certain group of PAOs showing that it may inhibit PAOs activity or activate 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 simultaneously uptake phosphorus using nitrate as a terminal electron acceptor using stored PHAs in the anoxic zone [37, 38]. The DPAOs are desirable because they, as they are able to remove nitrate and phosphorus simultaneously and also, have lower cell yield and sludge production [39]. The PAOs that use 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 conventional anaerobic–aerobic processes incorporating an anoxic zone for denitrification have been used for N and P removal in full-scale wastewater treatment plants [41]. Significant P removal can also be attained by using DPAOs in a single sludge system coexisting with nitrifiers [42]. Studies were carried out using an anaerobic-aerobic-anoxic-aerobic system for simultaneous removal of nitrogen and phosphorus, and some important control strategies were suggested so that the DPAO are not inhibited [43]. Another suggestion was to use anaerobic-aerobic-anoxic process which showed successful simultaneous removal of nitrogen and phosphate [44]. A novel scheme using anaerobic co-digestate of waste activated sludge and organic fraction of municipal solid waste in SBR, that can be integrated into municipal anaerobic co-digestion plants for denitrifying biological phosphorus removal via nitrite. This scheme can be employed in future for the side stream treatment of sludge reject water, enhancing nutrient removal, and reducing footprint and energy requirement of the new plants [45]. Some of the work regarding treatment of wastewater from different sources using SBR process and their corresponding treatment efficiencies has been summarized in Table 1.

Table 1 Removal efficiencies of C, N, and P from wastewater from different sources using SBR process
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Different Variants of SBR Technology

Cyclic Activated Sludge System

Cyclic Activated Sludge System (CASS) incorporates a single basin with variable volume operating in an alternating mode. It provides a unique combination of a plug flow in the initial zone followed by completely mixed reactor basin with secondary and main aeration zones. The activated sludge from the main aeration zone is recirculated into a selector zone located ahead of the complete-mix unit where it gets mixed with the raw wastewater entering the plant. The inclusion of a suitably designed high rate plug-flow selector facilitates stable and relatively uniform level of metabolic activity of the sludge in the complete-mix aeration tank leading to faster digestion of the organic contents and better settleability of the flocs. The operation is therefore generally indifferent to any variations in the flow rate and organic concentration of the influent raw water. Apart from these advantages, superior degree of simultaneous nitrification and denitrification as well as biological phosphorus removal is achieved using a CASS as compared to conventional SBR process [55]. This system can be employed for both industrial and municipal wastewater treatment systems [56].

UNITANK Technology

The UNITANK systems incorporate the advantages of SBR, three ditch oxidation treatment and normal aeration tank. The basic UNITANK configuration consists of a single tank divided into three hydraulically connected compartments in series. Each compartment has an aeration system and no provision for external sludge recirculation. The outer compartments alternately act as aeration and sludge settling tank while the middle one acts as aeration unit only. A single operation cycle consists of two main stages which have three basic steps which are performed in a symmetrical manner beginning from either of outer compartments in each stage. There is no separate sedimentation tank with scraper but the outer compartments have sludge slots and fixed effluent weirs. For removal of N and P, an advanced variant of UNITANK is used. This configuration possesses additional anaerobic/ anoxic compartments with internal recirculation of mixed liquor. UNITANK is more suitable for small- to middle-sized wastewater treatment plants with the advantages of simple structure, less land occupation, cost-efficient, and reliable operation. The UNITANK system is being used in different countries like China, Mexico, Argentina, Brazil, Vietnam, etc [57, 58].

Intermediate Cycle Extended Aeration System (ICEAS)

A further enhancement of the standard SBR batch process is Intermediate Cycle Extended Aeration System (ICEAS) process which processes continuous inflow of the wastewater. Variable inflow is handled by a distributor box which distributes flow evenly among all the tanks so as to avoid overloading in any single tank. A pre-react zone with high F/M acts as a selector. Thus, enhanced settling of sludge and inhibition of the filamentous growth can be achieved. The main-react zone located after the pre-react zone is operated in three basic operation modes, Aeration, Settle, and Draw. The equal loading of all the basins during continuous inflow simplifies the operation and process control. It also makes maintenance easier. There is significant capital cost reduction as compared to conventional SBR process as only a single set of tanks is required. The complex process control associated with conventional SBR process is overcome as at any given point of time all the basins receive equal loading and flow. The ICEAS is gaining popularity in China, US, UK, Peru, Qatar, etc. for replacing the old STPs or for new plants where limited space is available or enhanced effluent quality is required [59, 60].

Process Control Strategies

Unlike CFP system, the SBR process can be used under steady or unsteady state conditions. SBR process wins over CFP or ASP processes due to its superiority in many aspects such as better effluent quality in terms of COD and nutrient, better control of filamentous bacteria as well as low energy consumption. These feats were possible due to superior process control in SBR. Over the past 30 years, control technologies for the SBR process have continuously evolved, leading to the development of a wide variety of control systems to offset the complexity of the SBR process.

Classical SBR control is performed with fixed time cycle which has a disadvantage that it does not allow for the adaptation of length cycle to compensate the effect of process deviations and variations in the influent composition. Real-time control, on the other hand, should provide better flexibility for adaptation of optimized control in varied conditions. Precise real-time process control requires feedback on at least the start and end of various biological reactions taking place within an SBR. Real-time monitoring of direct parameters such as COD or BOD, TSS, and various forms of nitrate and phosphate may not be accurately possible with the currently available technology. Online monitoring of indirect parameters such as pH, dissolved oxygen (DO), and oxidation-reduction potential (ORP) can successfully indicate the reaction processes that occur during carbon and nitrogen removal in SBR processes. Figure 2 shows time-dependent profile of pH, DO, and ORP during one typical cycle of a conventional SBR and also, corresponding concentration profile of COD and different species of nitrogen.

Fig. 2
figure 2

Typical variations of DO, pH, and ORP value and concentrations of NH4-N, NO2-N, and NO3-N during nitrification and denitrification process in a conventional SBR [adopted from ref. 64]

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ORP has a direct correlation with nitrification rates and other biological reactions in anoxic conditions [61]. In normal condition, ORP is positive and increases during aeration phase and negative during anoxic stage. The normal range of values of ORP is 0 to 50 mV in aerobic stages and 0 to −300 mV in anoxic stages. In the anoxic stage, ORP has a continuous dropping profile with respect to time; a steep drooping profile, known as nitrate knee, occurs that signifies the end of denitrification so that it is safe to stop anoxic phase and start the next step.

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

  1. (a)

    Ammonia valley: As nitritation produces acid, pH tends to decrease gradually at the beginning of nitrification. When all the ammonia has been oxidized to produce peak nitrite concentration, there is no further acid production due to ammonia conversion. pH profile shows a concomitant minimum which is known as ammonia valley.

  2. (b)

    Nitrate Apex: During the anoxic stage, the pH rises and produces a continuously rising profile. A maximum is reached when the entire nitrate is converted to nitrogen, indicating an end of denitrification stage. Nitrate apex exactly corresponds with nitrate knee as observed through ORP profile.

Researchers argue that pH profile is the best indicator of the changes in the microbes profile occurring inside a SBR reactor [64]. However, the background alkalinity present in the wastewater often provides a strong buffering capacity that minimises noticeable variation in the pH.

During aeration, when COD is getting depleted, there is a consumption of DO. As the COD is consumed at constant DO supply rate, the reactor DO profile continuously increases because the COD level is fast decreasing. When nitritation occurs, the DO profile rises sharply because nitritation only required 25 % as much of the oxygen as that of nitrification; this point of inflexion is called DO breakpoint. DO breakpoint and ammonia valley correspond with each other in time [65]. At the end of the aeration, DO profile falls sharply to zero and maintains it until the end of anoxic phase, and therefore, does not provide effective information; as a result, DO profile alone cannot be reliably used to get feedback and control denitrification.

Oxygen uptake rate (OUR)-based controls are becoming popular [66]. OUR is the DO consumption per unit time in unit volume of the reactor and can be calculated by a PC or a PLC (programmable logic controller) from sensor-based DO measurements. OUR has been applied in the control of new SBR 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 derivative of OUR), from negative to positive, indicated the endpoint of phosphate uptake. OUR-based control has the same disadvantage as DO-based control.

Many studies on real-time control strategies have been employed 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 strategies based on specific process parameter patterns have thus been proposed [74]. In one study, optimal control of anoxic and aerobic phases by the indirect parameters was done through an algorithm that switched phases at some characteristics points on the profile curve obtained by filtering and processing primary data on the parameters, resulting in reduction of energy [75]. Due to highly nonlinear nature and time variations associated with the SBR process, along with fluctuations in hydraulics and components and possible equipment unreliability, a single control strategy based on multiple indirect parameters may not be successful, rather a strategy based on multivariate statistical process control analysis may be developed through further research [76, 77]. Intelligent control strategy (ICS) such as fuzzy logic [78], expert system [79], and also artificial neural network (ANN) based model [80] has been used to effectively optimize the SBR processes. Intelligent control being the advanced form of real-time control strategy, it is going to be the future of SBR real-time control.

The mathematical modeling for operational optimization and effective control of nutrient removal in an SBR by simulation instead of performing trial and error experiments at full scale gained popularity towards the end of the twentieth century. Several models are available for dynamic simulation of combined biological processes for nutrient removal in activated sludge systems. The IAWQ ASM2 model with further modifications could be employed for modeling of long-term nutrient removal in SBR, with better phosphorus dynamics by considering the DPAOs activity [81].

Recent Developments in the Application of SBR

The SBR technology is being applied in a large number of treatment processes owing to the operational flexibility it offers. The ability of SBR to perform flow equalization, biological treatment, and secondary clarification within a single tank by 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 been tested at lab scale to extend the application of SBR technology further.

A variation of SBR is a sequencing batch biofilm reactor (SBBR), which is a combination of suspended and attached growth (CSAG) system. Biofilm grows at a solid–fluid interface by attachment to a support material. It provides a chance to slow growing microorganisms to proliferate, irrespective of the HRT, and sedimentation characteristics of the bio-aggregates. The selection of support material and its size depend on the characteristics of the wastewater and the treatment objectives. The reactor may be packed with the support material or it may be suspended in the reactor fluid. A typical SBBR cycle has Fill, React, and Draw phases only. Plug flow conditions exist within an SBBR. The time required for washing of the support media may be considered analogous to the settling time in an SBR. Due to the excessive head loss and sloughing off risk, the SBBR systems are unsuitable for influent with high TSS and when excessive microbial growth is expected. There have been numbers of installations after the first pilot scale SBBR was used to treat leachate from the Georgswerder landfill, Germany [83].

The immobilization of microorganisms on a carrier media reduces the microbial washout, protects them from toxic constituents, pH, and temperature extremes [84]. As the media is retained, a shorter HRT is possible that results in smaller reactor size or higher treatment capacity with the same size reactor compared to conventional SBR. Biofilm configured systems are more resilient and are well suited for treating wastewater with highly variable quality with low sludge production [85, 86]. When chosen judiciously, the media may help absorb shock loads, for example, activated carbon for high organic load or zeolite for high ammonia in influent. These buffers temporarily adsorb the shock load-causing constituent and later gradually desorb the pollutants along with their simultaneous or subsequent biodegradation [87]. The powdered activated carbon (PAC) for treatment of raw landfill leachate demonstrated better NH3-N, color, and COD removal than conventional SBR [88]. The use of intelligent dynamic control systems over the conventional time control system has shown to improve the COD, TP, and TN removal efficiencies with considerable energy savings [89].

A modified SBR system with bio-floc technology (BFT) has found interesting application in aquaculture where protein feed for fish as well as treatment of wastewater are considered to be cost-inhibiting [90••, 91]. Biofloc refers to a special kind of macro aggregate of microorganisms which are able to take up nitrogenous compounds present in wastewater and to convert it to microbial protein. Biofloc organisms can be used as food to the fishes. It has been shown at the lab-scale that SBR envisaged as external growth reactor for bio-floc was able to attain nitrogen removal efficiency of up to 98 % when an optimal C/N ratio between 10 and 15 was maintained [92••]. It has also been demonstrated that the BFT in SBR reactor also enabled conversion of nitrogen in aquaculture suspended solids into bacterial biomass [93], which could potentially be used to feed fish, thereby increasing the efficiency of nitrogen—nearly reaching 100 % nitrate removal within 6 h.

SBR and SBBR were used to treat industrial wastewater containing phenolic compounds, such as p-nitrophenol (PNP) which is a hazardous chemical widely used in agricultural, pharmaceutical, and dye industries as a synthetic intermediate in the manufacturing process. Complete removal was demonstrated for PNP removal up to 350 mg/L influent concentration (loading rate of 0.368 kg/m3day−1) by SBR and SBBR (with polyethylene rings) [94]. However, the average NH3-N removal efficiency for the SBR and SBBR was only slightly compromised; it reduced to 86 and 96 %, respectively.

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

These days, hybrid systems like the Porous biomass carrier SBR (PBCSBR) are being investigated to achieve improved nutrient removal efficiency using time-sequenced anoxic/oxic phases and high biomass [97]. In another study, a new biomass retention strategy using natural fibers as biofilm carriers was utilized to treat dairy manure. The concept, evaluated for treating 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 of uncoupling HRT with SRT for biomass retention. Additionally, a particular sequence of operation of ASBR was used to exert selection pressures on microbes for immobilization [98–100]. Aerobic SBR process, coupled the photo-fenton process and reverse osmosis (RO), was used to reclaim wastewater from textile industry enabling complete internal reuse of water [101•].

SBR operating conditions such as cyclic feast and famine regimes, high shear stress, and short settling time promote formation of floc granules which are nothing but dense microbial consortia consisting of different bacterial species that perform different roles in degrading the complex wastes. Alternating anoxic/oxic condition combined with step-feeding mode (AASF) was proved to be an efficient method for nitrogen removal in granular SBR (GSBR) [102]. Aerobic granular sludge presents several advantages over activated sludge, such as excellent settling properties, high biomass retention and biosorption, and ability to deal with high organic loading rates and to perform diverse biological processes simultaneously, such as COD, N, and P removal [103••, 104–106]. The utility of aerobic granular sludge in SBR for degradation of toxic organofluorine compounds such as 2-fluorophenol (2-FP) has been demonstrated [107]. The fluoroaromatic compounds are usually biodegraded via halo-catechols [108, 109]. The maintenance of a good population of halo-aromatics degraders in bioreactors is highly desirable due to the low concentration discontinuous nature of these compounds [107]. The GSBR provides high biomass retention and thus is extremely promising for the treatment of effluents containing toxic compounds. Conventional SBRs treating wastewaters with flocculating sludge can be converted to granular SBRs by reducing the settle time [110].

A study with wastewater containing azo dyes attempting for simultaneous bio-decolorization and COD removal in SBR having a combination of anoxic-aerobic React phase revealed the following: (a) longer anoxic React phase promotes decolorization, while (b) COD removal was better with shorter anoxic phase [111•]. The granular-activated carbon-SBR (GAC-SBR) has shown promising results for the treatment of textile wastewater containing dyes where the dye was removed by GAC via physical adsorption mechanism. Addition of extraneous organic carbon increased the removal efficiency of direct dye [112].

In recent years, the presence of endocrine disruptor compounds (EDCs) in surface water, public water supplies as well as in wastewater has generated much public concern. EDCs are a group of different chemical substances that even in low concentrations such as sub-μg/L level in water may interfere with the normal functioning of human endocrine system and animals [113]. SBR process presents an attractive avenue for the removal of EDCs from wastewater due to its ability to provide anoxic/anaerobic/aerobic conditions within the same basin. Maintenance of such dynamic environmental conditions inside the SBR process tank provides ample scope for the microorganisms capable of degrading EDCs to utilize them. Longer SRT and HRT have been observed to cause greater degree of removal of EDCs primarily because such conditions allow for the proper growth of the slow growing microorganisms capable of utilizing the EDCs. Table 2 summarizes brief details of some of the recent studies performed on the efficiencies of different SBR configurations for the removal of EDCs commonly occurring in wastewater. Ozonation and other polishing steps are suggested after proper study of final products in critical cases where significant dilution of STP effluent does not occur [118].

Table 2 Percentage removal of different EDCs using SBR processes with different configuration
Full size table

Conclusions

Need for recycling of treated wastewater in many parts of the world has necessitated the introduction of newer stringent standards for treated wastewater. Unlike the conventional wastewater treatment plants, SBR-based wastewater treatment plants can achieve better treated water quality with no or minor modification in the installed infrastructure, only by simple alteration of the process control parameters in one or more of the phases of the treatment cycle. The SBR process offers smaller foot-print area, lower investment cost, lower operation complexity, and significant control performance as compared to conventional treatment process. If properly designed, the process may achieve significant degree of biological nutrient removal too. Although the SBR process is well developed, and different variants are continuously evolving, there are issues that need to be addressed further.

Ensuring process reliability for simultaneous N and P removal in SBR requires further work towards clear understanding of the microbial diversity of the system with an emphasis on its dynamics under different changing process situations. The study and improved design may follow the principles of ecologically engineered processes that derive stability from the presence of multiple species that accumulate phosphorus (functional richness). This may make the system more resilient with each species showing differential sensitivity to variations in the environmental conditions such as temperature and pH swings, toxic pollutants, presence of nitrite and nitrate, prevalence of VFAs, etc.

Appropriate process control is the heart of the SBR process as it important in ensuring removal of the target contaminants from the wastewater. PLC-based pre-programed control strategies are popular. Introduction of real-time control strategies can enable SBR process to achieve better robustness, reliability, and optimized operation. This will enhance energy efficiency and also shall help widen the areas of application of the SBR process. Future studies on SBR control strategies should include the development of intelligent control system, which is a real-time control strategy working on feedback-based control. This shall make the SBR process adaptive to changing environmental conditions and to varying wastewater quality so that optimum effluent quality is maintained with high degree of reliability.