SBR-ICEAS a proven treatment solution today and for the future

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1 SBR-ICEAS a proven treatment solution today and for the future M. Hallberg*,1, S. Sankaramanchi* 2, J. Newman*, A. Pasculea** 3 and F. Andrei** *ITT Water and Wastewater, SE , Sundbyberg, Sweden **Danex Consult, RO , Bucharest, Romania 1 Corresponding author, magnus.hallberg@itt.com 2 Corresponding author, siva.sankaramanchi@itt.com 3 Corresponding author, alexandru.pasculea@danex.ro ABSTRACT The development of the SBR (Sequencing Batch Reactor) technique started for more than 100 years ago. The Fill and draw technique was from the start very promising but was considered complex in operation compared to the conventional activated sludge processes. However, with the extensive development of Programmable Logic Control (PLC) units, the SBR technique have now for several years provided a very competitive alternative to the conventional activated sludge technique. There has also been a substantial development of the SBR, namely the development of the SBR-ICEAS (Intermittent Continous Extended Aeration System) process solution. The ICEAS system uses a continous inflow which allows for at time based operational control as opposed a volume based control of a classical SBR. For small plants a single basin can be used, whereas with a common SBR two basins is a minimum. For larger plants the ICEAS system is accordingly easily expandable. The flexibility of the ICEAS system gives the operators a robust system which easily can be upgraded to meet stringent discharge demands and energy consents in the future. Dezvoltarea tehnicii de epurare SBR a început cu mai bine de 100 de ani in urma. Conceptul Umple-Extrage a fost inca de la început foarte promiţător dar considerat complex din punct de vedere operaţional in comparaţie cu procesul de epurare cu nămol activ convenţional. Dezvoltarea si modernizarea unitarilor de Control Logic Programabil (PLC) din ultimi ani a favorizat tehnica de epurare SBR aceasta fiind azi considerata ca o soluţie competitiva in cea ce priveşte epurarea biologica. Concomitent a evoluat si tehnica SBR si anume dezvoltare soluţiei de epurare biologica SBR-ICEAS (Intermittent Continous Extended Aeration System). Acest sistem permite un debit influent continuu si foloseşte un sistem de control bazat pe timp si nu pe volum ca si in cazul unui SBR clasic. Pentru staţii de epurare mici este suficient un singur bazin ICEAS in schimbul a cel puţin doua bazine SBR clasice. Pentru staţii de epurare mai mari sistemul ICEAS este usor de extins. Flexibilitatea ICEAS oferă clienţilor un sistem robust, uşor de extins si eficientizat pentru a îndeplini condiţiile de deversare si restricţiile de consum energetic din viitor. Keywords Advanced biological treatment; biological nutrient removal, footprint, operator friendly, reuse Hallberg et al 1

2 INTRODUCTION The development of the wastewater treatment and the SBR technique started as early as the late 19 th century. The original activated sludge system was based on a fill and draw mode which later was named SBR. After the start of wastewater treatment era, the SBR technique lived a life in the shadows for nearly five decades. Today the SBR technology is a well known and a state of the art technology [1]. Notably, as late as 1999 SBR systems was perceived as more suitable for flows less than 19,000 m 3 /d (5 MGD) [2]. However, today it is common knowledge that the SBR process is an fits all size technique e.g. the Chon Buri plant in Thailand (200,000 m 3 /d) and Cardiff WWTP located in United Kingdom (286,200 m 3 /d). One of the key advantages with the SBR system is the possibility to change the process cycles to meet wastewater quality variations (e.g. [3]). This also makes the SBR extremely flexible to adapt to regulatory changes for effluent parameters such as nutrient removal [2]. The SBR system is also considered very cost effective if further treatment is required, such as filtration [2] for tertiary treatment for re-use of water (e.g. [4]). The main advantages with a SBR system compared to a conventional activated sludge system are [2]: Equalisation, primary clarification (in most cases), biological treatment, and secondary clarification can be achieved in a single reactor vessel Operating flexibility and control Minimal footprint Potential capital cost savings by eliminating clarifiers and other equipment The drawbacks with a conventional SBR system according to [2] are the increased maintenance associated sophisticated controls, automated switches and valves conventional activated sludge systems. However with the use of modern PLC systems and development of more reliable on-line meters, the early shortcomings of the SBR technology have been remedied [1]. When discussing complexity it is important to stress, that with a SBR system there is no need for primary clarification or secondary clarification. Problems that can be encountered with primary clarifiers are poor solids removal and low solids concentration in the primary sludge and for secondary clarifiers bulking or rising sludge [5]. These problems may involve physically changes of a treatment unit and/or providing a process control algorithms to solve the problems. The risk for sludge bulking problem is accordingly decreased with a SBR system. A development of the conventional SBR technique is the ICEAS system. The ICEAS-SBR system is a continous inflow SBR. The aim of this paper is to (1) give a short overview of the conventional activated sludge process, conventional SBR process and the ICEAS-SBR process and (2) comparison between the conventional activated sludge techniques and SBR techniques. 2 SBR-ICEAS a proven treatment solution today and for the future

3 Conventional Activated Sludge Process A typical conventional activated sludge process consists of separate tanks to accomplish unit processes of primary clarification, biological treatment, and secondary clarification with recycle pumping and piping (Fig. 1). Figure 1 Schematic layout of a conventional activated sludge system for BOD removal. The wastewater from the plant head works is received in the primary clarifier. The primary clarifier is typically equipped with a sludge collection mechanism and an effluent overflow weir. In this unit process, solids (or sludge) with a higher density settles to the bottom of the clarifier and partially treated primary settled effluent is discharged over the weirs to the aeration basin. The sludge settled in the primary clarifier is sent to the sludge handling facilities. The aeration basin is typically equipped with diffusers installed on the floor of the basin. The blowers located in a building near the basins are used to supply the air to the basins via the diffusers. The effluent received from the primary clarifier is continuously mixed and aerated in this basin with return sludge from the secondary clarifier. The mixed liquor discharge from the aeration basin enters the secondary clarifier through the feed well. Similar to the primary clarification, the liquid solids separation occur in the clarifier where sludge is settled to the bottom and the treated effluent is discharged over the weirs to the downstream facilities. A major portion of the sludge that settled in the secondary clarifier is recycled back as RAS (Return Activated Sludge) to the aeration basin and the remainder is wasted to the sludge handling facilities. Hallberg et al 3

4 Conventional SBR Process The Sequential Batch Reactor (SBR) process is a variant of the Activated Sludge process and does not need primary nor secondary clarifiers. It uses the fill and draw principal in which unit processes occur sequentially in five cyclic phases (Fig 2). If denitrification is needed, six phases must be used. Figure 2 Schematic layout of the five phases for BOD removal with a conventional SBR system (TWL = Top Water Level) During the filling phase raw wastewater, that has been screened and degritted, flows into the basin and mixes with the mixed liquor settled during the previous phase. After the fill phase, the influent valve is closed and the influent is routed to the other basin. For the react phase the basin is aerated and biological oxidation takes place as in a conventional activated sludge process. Aeration is stopped and the settle phase occurs. During the settle phase the solids settle to the bottom of the basin leaving the clear water on the top. The decant phase commences immediately after the settle phase. The clear water is discharged using a decant mechanism. After the decant phase the basin goes into the idle phase. During this phase the sludge is wasted from the bottom of the basin using pumps. The SBR must be designed with a minimum of either two reactors or an equalization/storage tank in conjunction with a single reactor (Fig. 3). The configurations are required to allow continuous acceptance and treatment of the influent. During the react, settle and decant phases of the cycle, flow is diverted to the other basin or to the storage tank. 4 SBR-ICEAS a proven treatment solution today and for the future

5 Figure 3 Schematic layout of a typical conventional SBR layout with two basins The ICEAS-SBR Process The ICEAS process is a modification and enhancement of the conventional SBR. The ICEAS allows for a continuous inflow of wastewater to the basin during all phases. By utilising a continous inflow the ICEAS-SBR can reduce the cycles from five down to three (Fig. 4). If denitrification is required an anoxic phase is added to the cycles. Figure 4 The three phases for BOD removal for the ICEAS-SBR system. In the Aerate phase raw wastewater from screening and grit removal flows into the basin and mixes with the mixed liquor. The basin is aerated while filling and biological oxidation takes place simultaneously. The Aeration is stopped and the settling phase starts. The solids settle to the bottom of the basin leaving clear water on top. The basin continuously receives the influent. Hallberg et al 5

6 After the settling phase the decant phase begins. The clear water is discharged from the top of the basin, while the basin continuously receives the influent. Typically, sludge is wasted during this phase of the cycle DISCUSSION The primary and secondary clarification needed for the conventional activated sludge process occupies a significant land area. In the conventional activated sludge process, a limited amount of flexibility can be exercised by adjusting the rate of RAS and waste activated sludge or through varying the rate of air introduced in the aeration basin. In a SBR, all unit processes occur sequentially in one basin. This in turn results in less land use, or smaller footprint, for the SBR compared to that of a conventional activated sludge process and also increases the flexibility of the SBR system. The SBR process is automated through the use of a control system ranging in sophistication from simple timers to PLC or PC based systems. The control system automatically coordinates equipment operation through various phases of the SBR cycle. The use of PLC for automating the process control means that full use can be taken of the high degree of flexibility naturally built in a SBR system compared to conventional activated sludge process. The possibility to combine the automated process control with e.g. on line measurement of dissolved oxygen allows for easy adaptation of the SBR process cycle to meet the changing influent conditions through simple changes in control set-points. However, In regard to the process control of conventional activated sludge process and the conventional SBR process there are some concerns that must be considered. For most municipal treatment facilities and some industrial applications, flow and loadings to a plant vary according to a diurnal cycle. With a conventional SBR system, this results in unequal mass and hydraulic loadings to each reactor in a multi-reactor facility. The loadings to a specific reactor are dependent on when it is receiving flow during the diurnal flow variation. The variation in loadings causes differences in the biomass and oxygen demand of the individual reactors. Independent of the modern tools for process control, this complicates the operational control of the treatment plant resulting in the need for additional testing, a more intensive instrumentation/control system and greater operator attention to the system. Furthermore, the conventional SBR commonly incorporates a water level based control system. Thus, the duration of the daily process cycles are subject to change based on the specific inflow to a reactor. Since diurnal flow variations occur, the cycling results in different actual aeration times for the biological reactions. This can lead to difficulty in controlling the process and cycling/switchover of the blowers. For Biological Nutrient Removal (BNR) systems, a continuous carbon source is beneficial in maintaining consistent performance. Organic compounds in the raw influent to such secondary treatment systems are typically used as the source of the carbon. Conventional SBR systems however periodically interrupt this food source especially during the react phase. This lowers the removal of nitrogen and phosphorus and may necessitate expensive chemical additions to enhance biological nutrient removal. 6 SBR-ICEAS a proven treatment solution today and for the future

7 Many of the practical shortcomings that may occur with the conventional SBR process have been addressed by development of the ICEAS-SBR system. The ICEAS-SBR applies a continous inflow during all cycles. The continous inflow allows the ICEAS-SBR process to be controlled on a time, rather than flow basis and ensures equal loading and flow to all basins. Use of a time-based control system in the ICEAS process facilitates simple changes to the process control program. The duration of each cycle and segment of each operating cycle are the same among all basins in a time-based system. Therefore, changes to the process are made simply by changing the duration of individual segments. In the flow-based conventional SBR, cycle times and individual segments of each cycle may be different among basins due to diurnal flow variations. Thus, it is not possible to simply affect a change to the entire system. In essence, separate control must be maintained over each basin in the SBR system as opposed to the ICEAS-SBR system. The simple operation of the ICEAS-SBR system is emphasised and much appreciated by the operators of ICEAS plants. In a two tank operation, the fill and react often become a single aerated fill phase that can lead to low food concentrations in the basin and problems with filamentous bulking. The continous feed in the ICEAS provides a large food source for BNR at the beginning of the react period, and helps prevent filamentous bulking. When conventional SBR systems are considered for smaller treatment plant applications, two basin designs are typically evaluated. However, due to the batch nature of the process, one basin can not be readily taken out of service for maintenance purposes. In addition, a single tank operational mode cannot be easily utilized for low flow situations (e.g. [3]). With the ICEAS-SBR system it is possible to use a single basin, which can provide a very pragmatic and efficient solution for smaller treatment plants. The ICEAS-SBR basins can be fed by gravity, without any automated valves to the individual basins. This will reduce the investment cost and the risk for mechanical failures compared to the conventional SBR system. The continous inflow also makes the ICEAS-SBR system very easy to expand independent if an addition of one, or more basins are needed to meet future needs [3]. According to [2] there is a risk of discharging floating or settled sludge during the decant phase. The ICEAS-SBR is using a decanter controlled by an actuator with a VFD (Variable Frequency Control). The actuated control of the decanter gives the opportunity to handle peak flows up to six time of the average flow in the same basin. The decanter itself is constructed with scum protection that minimises the risk of decanting floating sludge (Picture 1). Hallberg et al 7

8 A B Picture 1 A = Clear water decanted independent of floating scum. B = Example of typical ICEAS decanter CONCLUSIONS The ICEAS-SBR system has the advantages and flexibility as the conventional SBR system compared to the conventional activated sludge process. In addition, the ICEAS-SBR process addresses many of the drawbacks of the conventional SBR system. The ICEAS-SBR allows for easy operation in adverse process conditions to an overall optimised operation and energy cost. The system is operator friendly and well adapted to meet the more stringent discharge demands as well as for the ever increasing demand for re-use of water in the future. REFERENCES [1] Morling S. (2009). SBR Technology Use and Potential Application for Cold Wastewater, Doctoral Thesis, TRITA LWR PHD Thesis 1050, Royal Institute of Technology, Stockholm, Sweden. [2] USEPA (1999). Wastewater Technology Fact Sheet, Sequencing Batch Reactor, EPA 832-F [3] ITT WWW (2009). The ICEAS - SBR Biological Treatment System. Brief Notes on the Mako WWTP ICEAS-SBR System, EEE, Magnus Hallberg [4] ITT WWW (2008). Proyeto: Planta de Tratamiento de Aguas Residuales de de Manchay Distrito de Pachamac, Case Story Competition. [5] Metcalf and Eddy (2002) Tchobanoglous G., Burton F., Stensel H.D., Wastewater Engineering, McGraw-Hill Education, Europe, United Kingdom 8 SBR-ICEAS a proven treatment solution today and for the future