The sequencing batch reactor (SBR) is perhaps the most promising and viable of the proposed activated sludge modifications today for the removal of organic carbon and nutrients. In a relatively short period, it has become increasingly popular for the treatment of domestic and industrial wastewaters, as an effective biological treatment system due to its simplicity and flexibility of operation. Mechanism and Design of Sequencing Batch Reactors for Nutrient Removalhas been prepared with the main objective to provide a unified design approach for SBR systems, primarily based on relevant process stoichiometry. Specific emphasis has been placed upon the fact that such a unified design approach is also by nature the determining factor for the selection of the most appropriate cyclic operation scheme, the sequence of necessary phases and filling patterns for the particular application. The proposed basis for design is developed and presented in a stepwise approach to cover both organic carbon and nutrient removal, domestic and industrial wastewaters, strong and specific wastes. The merits of model simulation as an integral complement of process design, along with performance evaluation of SBR models are also emphasized. Scientific and Technical Report No. 19
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1.1. HISTORICAL PERSPECTIVE
Activated sludge is the most ingeniously conceived process today for the biological treatment of domestic and industrial wastewaters. For almost a century, it has been the traditional scheme for the effective removal of organic carbon. In the last few decades it has been successfully explored and tested for its potential of nitrogen and phosphorus removal. It has also served as the platform for the most remarkable and productive research effort in environmental sciences, constantly improving and updating the understanding of the fundamental microbial mechanisms involved, which readily translated into process improvement.
Today, continuous-flow activated sludge systems almost always guarantee satisfactory performance with minimal supervision and maintenance for conventional wastewaters. Despite substantial practice and accumulated experience however, more and more stringent effluent quality requirements often imposing removal of nutrients together with organic carbon and the increasing complexity of wastewaters seriously challenged their efficiency. The difficulties encountered in the operation of the conventional process configuration have been the basic motivation of extensive efforts to review existing design criteria and operation practices as well as to develop process modifications likely to offer improved treatment potential. While the problems were recognized, they were not always thoroughly and scientifically defined, so that rational corrective measures could not always be taken. Therefore, the majority of plant improvements and modifications had to rely on experience and engineering judgement. New ideas were mostly developed by trial and error for their particular situation. Several new concepts and process modifications emerged from these individual efforts and found acceptance in wastewater treatment practice.
The sequencing batch reactor (SBR) is perhaps the most promising and viable of the proposed activated sludge modifications today for the removal of organic carbon and nutrients. In a relatively short period, it has become increasingly popular for the treatment of domestic and industrial wastewaters, as an effective biological treatment system due to its simplicity and flexibility of operation.
The SBR is basically a single tank that serves both as a biological reactor and settler in a temporal sequence; whereas aeration and settling are simultaneous but in a spatial sequence in the continuous-flow activated sludge, they are carried out in the same reactor but in a temporal sequence in the SBR systems. SBR inherently involves a cyclic operation, each cycle incorporating the same pattern of successive selected phases. As contrasted to continuous-flow systems, wastewater feeding is provided during the desired portions of the cycle, which then secures a batch wise, fill-and- draw type of an operation.
The fill-and-draw scheme, constituting the basic principle of the SBR system was the essential instrument that initiated the development of the original activated sludge, an excellent review of the historical evolution of the activated sludge process was recently presented by Wilderer et al. (2001). The experiments of Arden and Lockett (1914) should be recognized as the major pioneering work leading to this most widely used process in wastewater treatment. Their work was essentially a follow-up of the earlier experiments in Massachusetts, which demonstrated that aeration of sewage for a short time in a tank containing slabs of slate about 2-3 cm apart produced a "compact brown growth" on the slate and a clear effluent. In their fill-and-draw experiments, they saved the flocculent solids which they called activated sludge and reported that their repeated use in sewage treatment greatly improved the purification potential of simple aeration; the improvement was observed to be closely related upon the ratio of activated sludge to the sewage treated. They also concluded that a clear, well-oxidized effluent could be obtained after aeration of sewage in contact with activated sludge, for a period of six to nine hours – a period adopted as a design parameter in actual practice. Similar fill-and-draw experiments conducted the same year at the University of Illinois (Mohlmann, 1917) confirmed the function of the activated sludge as an indispensable factor for sewage stabilization and a six-hour aeration period for a satisfactory treatment. It was also reported that the aeration rate should be between 7 to 11m3 of air/m3 of sewage – another operational value that has survived the years as a key design parameter.
The discovery of activated sludge found immediate recognition in practice, both in England and in the US. Shortly after the publication of the results obtained by Arden and Lockhett, a treatment plant operated as a fill-and-draw system was installed for Daryhulme, Manchester (Orhon and Artan, 1994). The first similar fill-and-draw plant in the US was put into operation in 1915, in Milwaukee, Wisconsin. From 1914 to 1920 several full-scale fill-and-draw systems of different size were placed into operation in England and in the US Wilderer et al. 2001).
The performance of the pioneer activated sludge plants in their early years of operation was quite satisfactory. They provided a number of basic criteria for the principal design features of new plants and firmly established the activated sludge process. After 1920 however, taking advantage of the flocculent properties of activated sludge along with its "clarifying power", efforts were then directed towards the adaptation of the process to operate under continuous-flow conditions. Successful experience with this new approach, together with the promotion of the diffused air process as a feasible means of air provision consolidated the continuous-flow principle as the major practical method for activated sludge operation.
After the choice for continuous-flow processes, the interest for batch-fed systems was revived in the 1960s with the development of new technology and equipment. The system in its newly devised scheme with a periodic influent feeding and periodic discharge was called the sequencing batch reactor. The following research mainly focused on exploring and stressing the advantages of the SBR over the conventional continuous-flow system. Among major examples of this research effort is the study by Dennis and Irvine (1979) where sludge settleability characteristics were found to vary markedly with different fill/react ratios. Hopker and Schroeder (1979) showed that the lower feed strength resulted in better effluent quality, and semi-batch treatment was more suitable in minimizing dispersed growth. Irvine et al. (1979) demonstrated the feasibility of nitrification-denitrification, given proper design and operation. Ketchum and Liao (1979) explored the potential of the SBR for phosphorus removal. The results of these studies secured the credibility of the SBR as a reliable process contributed to its acceptance in practice as a viable and resourceful alternative for the biological treatment of domestic and industrial wastewaters.
1.2. CURRENT EXPERIENCE
The most striking feature of the SBR process is the system flexibility offered with a very simple physical structure. The operation alternatives inherently associated with SBR has been the basis of its promotion both in full-scale application and as the perfect tool for fundamental and applied research.
1.2.1. Basic and Applied Research
The SBR process has the mechanistic capability to serve as a perfect experimental instrument for the exploration of the intricate array of microbial mechanisms associated with different processes involved in biological wastewater treatment. Conceptually, it offers a much better opportunity to follow the functional relationships between the relevant parameters of a selected process as compared to a continuous-flow, completely mixed reactor, (CSTR). In the latter, each parameter can only be observed as a single value at a given steady-state operation; this value reflects the combined resulting effect of all the biochemical mechanisms involved. The relative importance or even the presence of any of these mechanisms cannot possibly be identified. However, the SBR reflects during each cycle the transient responses of all the observed parameters, such as COD, nitrogen forms, biochemical storage products, oxygen uptake rate, etc. Moreover, the SBR is also much better suited for this purpose compared with a simple batch reactor, mainly because it is operated at steady-state with identical responses in each cycle, reflecting this way the microbial culture history for selected operating conditions, a very important factor often overlooked in batch experiments.
Appropriate interpretation and modelling of a number of fundamental issues dealing with different aspects of biological treatment had to rely on the SBR process for experimental support. The study of Orhon et al. (1986: 1989) may be remembered as one of the first examples highlighting the merit of the SBR as a valuable research tool, where a model proposed for the formation of soluble residual microbial products, an important parameter especially for the treatment of industrial wastewaters, was calibrated and verified for a set of experimental data, accurately predicting COD accumulation at the end of each cycle. Among recent research efforts is the work by Karahan et al. (2003), which studied the biodegradation of different types of starch as typical hydrolysable substrates, through observation and modelling of glycogen concentration and oxygen uptake rate profiles during a cycle at steady-state; they suggested a new model structure involving both simultaneous growth and biochemical storage. Similarly, based on extensive experimental data collected from an SBR system fed with acetate as the sole carbon source at different COD/P ratios, Yagci et al. (2003) proposed a new metabolic model for acetate uptake by a mixed culture of phosphate and glycogen accumulated microorganisms under anaerobic conditions.
The SBR has also been extensively utilized, both at laboratory and pilot-scale for the experimental assessment of the level of removal efficiencies achieved in the treatment of domestic and industrial wastewaters, an elaborate review of various applications of the SBR technology was presented by Mace and Mata-Alvarez (2002). In these studies, system flexibility of the cyclic operation offering additional system parameters such as cycle time, fill time, etc., mostly selected at random, have been elaborated as substantial competitive advantages of the SBR against the continuous-flow configuration. Most of the studies were conducted with a pre-selected set of operating parameters and almost always reported satisfactory removal performance for the particular case of application.
Application of the SBR technology for improved treatment of domestic sewage has been a major focus of attention. The wide spectrum of the operating parameters selected for this purpose should be underlined. Imura et al. (1993) chose to adopt a 4 cycles/day (6-h cycles) operation for complete nitrification and BOD, SS and total phosphorus removals of over 95%. Umble and Ketchum (1997) obtained similar results with a 12-h cycle operation for effluent nutrient recovery through agriculture. Bernardes and Klapwijk (1996) reported satisfactory nutrient removal with a set of two SBRs in series, each operated at 6 cycles/day and a different sequence of arbitrarily selected phases within the cycle. The SBR was also tested as quite a successful biological treatment option for landfill leachates, again with a wide array of recommended operating parameters involving a range of 1.5 – 10 days for the hydraulic retention time, 20 – 50 days for the solids retention time and 12 – 24 h for cycle time (Dollerer and Wilderer, 1996; Zaloum and Abbot, 1997; Timur and Ozturk, 1997; Yilmaz and Ozturk, 2001).
The SBR is also widely explored and tested for the treatment of industrial wastes. Satisfactory and reliable performance is reported for winery wastewaters (Torrijos and Moletta, 1997), brewery wastewaters (Ling and Lo, 1999), food industry wastewaters (Raper and Green, 2001) dairy wastewaters (Mohseni and Bazari, 2000) slaughterhouse wastewaters (Belanger et al., 1986) piggery wastewaters (Lee et al., 1997) pulp and paper mill effluents (Franta and Wilderer, 1997) and tannery effluents (Carucci et al., 1999). Similar to the experience with domestic sewage, very little evidence, if any, is available in related literature to justify the specific operating parameters and design criteria selected to ensure the desired performance of the SBR system for the particular industrial wastewater. A remarkable exception to this statement is the comprehensive evaluation of the SBR treatment of tannery wastewater by Murat et al. (2002) involving appropriate wastewater characterization and COD fractionation of the wastewater for modelling, experimental assessment of model coefficients, mass balances for carbon and nitrogen removal, performance prediction of SBR operation based on process stoichiometry and model calibration of system performance. Effects of temperature were investigated in a following study (Murat et al., 2004),which established the nitrogen balance of the SBR system treating tannery wastewater for a wide temperature range between 9 and 30o C and evaluated the experimental results by means of model calibration of COD, nitrate and ammonia nitrogen concentration profiles during cyclic operation.
1.2.2. Full-scale Application
The urge for simpler and more reliable treatment systems, the comfort of system flexibility and additional operating parameters and extensive applied research highlighting SBR as a novel process with competitive advantages against the conventional continuous-flow activated sludge have been the major ingredients for the increasing success of this process in full-scale application, both for domestic and industrial wastewaters.
The development of the Intermittently Decanted Extended Aeration, (IDEA), a system with continuous influent feeding but intermittent aeration and decanting, between 1965 and 1975 in Australia (Goronszy, 1979), together with the conversion of the full-scale continuous-flow activated sludge treatment plant at Culver, Indiana, USA in 1980 (Irvine et al. 1983) may be cited as pioneering efforts that have contributed a lot to the success of the SBR process. The ability of the system to easily adapt to nutrient removal by simple incorporation of an anoxic/anaerobic phase has been an equally important factor contributing to the increasing popularity of the SBR systems. Today, full-scale application of the SBR process is wide spread with more than 1200 full-scale plants in North America; more than 700 plants in Japan, mainly as small installations in rural agricultural areas; around 150 plants in Germany with a large portion for industrial applications; more than 100 plants in Australia (Wilderer et al. 2001). In Turkey, close to 1000 sewage treatment applications exist, mostly package and standardized plants for effluents from holiday resorts and small communities, in addition to some 50 plants for a variety of industrial wastewaters. More than 20 of these industrial applications were reported (Artan et al., 1996).
The application of the SBR process has not been limited with suspended growth systems. SBR alternatives using biofilm systems have also been developed with the generic name of sequencing batch biofilm reactor, (SBBR) and found application in the treatment of domestic and industrial wastewaters (Wilderer, 1992; Pujol et al., 1998).
1.3. NEED FOR MODELING AND A UNIFIED BASIS FOR DESIGN
The development of the SBR process took place in a period that could also be associated with remarkable achievements in the interpretation of fundamental microbial mechanisms, which greatly affected the modelling, and the design of the activated process. Introduction of a number of substrate components with different rates of biodegradation, the concept of residual microbial products, the ensuing detailed COD fractionation in terms of different substrate and biomass parameters, incorporation of hydrolysis as a major process for slowly biodegradable substrate, identification of microbial storage products and their utilization under a sequence of feast and famine conditions may be cited among revolutionary landmarks drastically changing the conventional understanding of the system.
The promotion of the SBR however did not benefit from these conceptual achievements as it basically developed as a polishing effort of the original, sixty years old pioneering engineering idea, mainly for two reasons: Firstly, related applied research chose to concentrate mainly upon the competitive advantages offered by the operation of the SBR system against the conventional continuous-flow scheme, such as the elimination of the often problematic settling tank and equipment, ability for better operation control, etc.; it emphasized new features of the system such as intermittent influent feeding, cyclic operation, etc., and manipulation of these parameters like fill time, cycle time, fill/react ratio, etc., in different combinations, mainly as a supporting evidence of the advantages of the system. In short, the common attribute of most SBR studies was perhaps the effort to acquire the required engineering experience.
Excerpted from "Mechanism and Design of Sequencing Batch Reactors for Nutrient Removal"
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Table of Contents
The Authors, ix,
1 Introduction, 1,
2 Process description, 9,
3 Process design for carbon removal, 23,
4 Process design for nutrient removal, 46,
5 Performance evaluation by simulation models, 69,
6 Concluding Remarks, 92,