CYCLIC ACTIVATED SLUDGE TECHNOLGY Ms. Aparna Kapoor, Ion Exchange (I) Ltd
1.
ABSTRACT
Cyclic activated sludge process has gained increasing importance in wastewater treatment, especially on account of the design flexibility to handle varying daily flow and load fluctuations along with operation simplicity. The practice of manipulating activated sludge reaction environments to obtain maximum nitrogen and biological phosporus removal has been optimized using cyclic activated sludge technology. In its simplest form, the sequences of fill, aeration, settle and decant are consecutively and continuously operated in a compartment reactor. Low Nitrogen concentrations < 5 gm/lit by co-current nitriflcation denitrification mechanism and phosphorus < 5 gm/lit by co-current nitrification denitrification mechanism and phosphorus < 5 gm/lit without any chemical addition is achievable with this technology. A detailed technical description, salient features & advantages of the technology is described in this paper. 2.
HISTORICAL BACKGROUND
The C-TECH – System specifically refers to the use of variable volume treatment in combination with a biological SELECTOR and BIORATE control, which is operated in a fed-batch reactor mode. The Cyclic Activated Sludge Technology is a parallel development of sequencing batch reactor technology (SBR) and was invented in Australia. It was later developed for North American climates and extended to meet local climate and discharge requirements of Central Europe and Asia (Goronszy, 1979, 1985; Demoulin et al. 1997; Demoulin and Goronszy, 1999). Since then many applications have been implemented with one of the largest applications being for approximately 4,50,000 population equivalent (210 MLD). 3.
EXPLANATION OF CYCLIC OPERATION
A basic cycle comprise: • Fill-Aeration • Settlement • Decanting
(F/A) (S) (D)
These phases in a sequence constitute a cycle, which is then repeated. During the period of a cycle, the liquid volume inside the tank increases from a set minimum operating bottom water level. Aeration ends at a predetermined period of the cycle to allow the biomass to flocculate and settle under quiescent conditions. After a specific settling period the treated supernatant is removed (decanted), using a moving weir DECANTER. The liquid level in the vessel is so returned to the bottom water level after which the cycle is repeated. Solids are wasted from the tanks during the decanting phase.
Fig: The C-Tech Cycle •
Fill-Aeration (F/A)
Aeration refers to the air-on time during which influent is received into the basin. Multiple basin facilities can be operated with different sequences of air-on during the nominal aeration sequence. Less than design load operation typically requires the interruption of the aeration sequence. Sub-aeration sequences are available to maximize short term mixing without causing major response to the in-basin dissolved oxygen concentration or BIORATE. During the fill-aeration sequence mixed liquid from the aeration zone is recycled into the SELECTOR. The overall recycle rate (20-30% compared to the daily influent) is therefore significantly smaller than in conventional systems.
•
Settlement (S)
This sequence refers to the initial part of the air-off sequence where setting conditions are created to provide solids-liquid separation. The sludge mass forms a definitive sludge level interface, which progressively falls as the solids compact. The activated sludge floc particles adhere together and the mass settles within the confines of a blanket leaving an over layer of clear supernatant. The settled sludge layer has a mean biomass concentration of around 10 g/l in typical domestic waste water facilities. •
Decanting
During decanting there is no inflow to the basin. The moving weir DECANTER is motor driven and travels slowly from its out of liquid rest position to a designated bottom water level. The weir is returned to its rest position through reversal of the drive.
Fig: Decanter of the C-Tech Process 4.
SPECIAL FEATURES a) CO-Current Nitrification & denitrification Cyclic process makes it possible to process the waste water with Nitrification/Dentrification (N/DN) without recirculation of liquids and anoxic mixing. Conventionally, nutrient removal reactions in the activated sludge process require favorable chemical, biological and biochemical environments
brought about by oxic, anoxic and anaerobic conditioning. Several nutrient removal processes are based on continuous flow through principles using separate basins for each reaction condition with significant recycle flows between the separate basins. To favour the required reaction for N/DN, it is necessary to be able adjust the following parameters: pH, dissolved oxygen, ORP, intrate substrate and various recycle rates. Also the required process flexibility also needs to consider basin construction, mechanical equipment and interconnecting piping & mixers. Whereas co-current N/DN simplifies the necessary recycle and recirculation flows required by conventional systems. A significant benefit of discontinuous process is the magnitude of the process flexibility that is derived through the ability to simply adjust the time cycles of operation in cases of extended load deviation. Nitrification rates are about 35% higher than the conventional system. Co-current N/DN means operation under reaction conditions which permit nitrification and denitrification to take place in same time in the same reaction basin without any specific anoxic mixing sequences for nitrate reduction is achieved by providing a biological selector. Complete nitrification and denitrification pathways take place in same basin with nitrification taking place external to the activated sludge flocs and denitrification-taking place within the interior of the flocs. This denitrification pathway is not bound to the absence of dissolved oxygen in the liquid phase but requires diffusion of nitrate into the anoxic parts of the floc with a probable use of intracellular stored or adsorbed organic carbon for denitrification. The storage of intracellular carbon is favoured by the combination of a biological selector at high organic loading. b) Biological SELECTOR zone The incorporation of a biological SELECTOR within the C-TECH- Systems distinguishes it from all other technologies. This is with this feature of the process there is no need for an ANOXIC-MIXING sequence and is therefore replaced by a simple FILL-AERATE sequence. The SELECTOR simplifies the operation of the process and insures biological selection of predominantly floc-forming microorganisms over all loadings, particularly at less than design load. The biological SELECTOR allows process operation at time cycles that are less than other SBR systems with the direct result of a more economical process design, smaller basins and less cost. The biological SELECTOR operates under essentially anoxic to anaerobic reaction conditions whereby the readily degradable soluble organic fraction of the waste water is rapidly removed by enzymatic transfer mechanisms. These soluble organics, principally acetates, are quickly transformed to intracellular storage products. The biological SELECTOR combines to assist with the selection of poly P microorganisms. Using 4 hr cycles and recirculating activated sludge to this zone exposes the biomass to appropriate reaction conditions. This favours the selection of phosphate accumulating micro-organisms and floc forming micro-organisms. The biological SELECTOR is very effective in containing all of the known low F/M bulking microorganisms. The special method of operation referred to above has been proven effective in the containment of Nocardia and Microthrix Parvicella which often are the cause of nuisance surface foaming. Diffusers are located in this zone for periodic flash mixing if required. The figure below shows the experience with SVI developments in parallel operations with a wastewater that traditionally favours sludge bulking.
c) BIORATE control BIORATE control uses dissolved oxygen (DO) measuring to provide a full-scale process respirometer. In this way, the metabolic activity of the biomass is measured within the actual process basin and is subsequently used as a control parameter to automatically regulate the duration of the aeration sequence and the rate of aeration. This methodology provides a true in-basin method for the efficient use of energy. A minimum of 15% energy saving (maximum 80%) is realised through the BIORATE operation. The DO concentration profile is automatically ramped to operate at lightly higher DO concentrations at the end of the cycle. This feature prevents low dissolved oxygen type filaments from proliferating. Cycles are set to make sure there is a sufficient degree of oxygen supply to properly meet process requirements. Process oxygen requirements in a facility can be easily measured by this oxygen utilisation rate (BIORATE) of the biomass. Continued operation with more oxygen input than is necessary to meet process needs will result in minimal denitrification and a loss of biological phosphorus removal. Turbid effluents are frequently caused by over – aeration. d) Biological Phosphorus removal During settlement and decant the biomass oxidation reduction potential depletes from a positive value of around +50 to+100 mV to a negative value of around –150 to – 200 mV. This reaction circumstance is essential for the biological phosphorus uptake mechanism to take place. During anaerobic conditions, all phosphorus that is released to the liquid phase is totally
contained within the biosolids layer. When operated for biological phosphorus removal there is no diffusional release of phosphorus into the overlying liquid phase. e) Operational Simplicity The plant control is essentially automatic which is a major factor in reducing operating costs. The cycling of each batch process and the regulation of each sequence interval is governed by a Programmable Logic Control (PLC).Control for the operation of each sludge pump, each decanter etc. is also included. The automatic basin air distribution system also regulated by the PLC. This means that optimum operation and performance are synonymous; management of high and low load circumstances is easily maintained providing for steady state performance. 5.
COMPARISON TO CONVENTIONAL SBR SYSTEMS By comparison with other conventional SBR (sequencing batch reactor) processes, the use of the CTECH – System offers significant economies in capital cost, operation and maintenance costs and land area requirements. The advantages are:
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Simultaneous N/DN allows operating without mixing equipment. Proper oxygen demand/supply operation control by BIORATE in combination with the SELECTOR allows controlling this process.
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BIORATE reduces aeration time during nighttime automatically. This increases the lifetime of blowers and diffusers and reduces energy demand.
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A clear water withdrawal system for high rate decanting allows to draw off up to 2.5 m of solids free effluent without complex valve arrangements. SBR systems are limited to about 1 m of decant depth.
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“Normal” and “High flow” operating protocols are standard features whereas SBR systems typically have only one cycle with confusing adjustment possibilities for the operator.
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Equalisation basins for influent or effluent equalisation are not necessary.
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C-TECH provides a better BIO-P using the SELECTOR effect.
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Filamentous sludge bulking in SBR systems can only be avoided if shock filling is carried out which requires influent equalisation and pumping.
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Good references of existing large scale plants.
6.
COMPARISON TO CONVENTIONAL ACTIVATED SLUDGE SYSTEMS Conventional activated sludge systems have the following disadvantages compared to C-TECH. •
As systems have dedicated volumes for biological treatment and sedimentation. Only very limited flexibility is available in the case that predicated design flows or loadings deviate from the reality (sludge bulking).
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As systems cannot operate with in basin BIORATE control which leads to increased operation cost, reduced treatment efficiency and potential of over aeration of the activated sludge.
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As systems show extreme sensibility against peak flow and loadings. Peak flow transfers activated sludge into the final settling tanks and finally into the effluent. Peak loadings show up in the effluent without delays as AS systems are typically designed as completely stirred tank reactors (CSTR).
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As systems do need relatively high amounts of recirculation flows which makes the design of effective SELECTOR basins impossible. The high initial F/M loading can not be achieved so that AS systems show a tendency to produce bulking sludge.
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AS systems need a high amount of Electro-mechanical equipment such as pumps, mixing devices, scraping bridges etc. which are subject to maintenance, repair and replacement. C-TECH avoids these disadvantages by a reliable and simple automation and control system with BIORATE control.
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As systems cannot detect toxicity of influent until effluent measurements are carried out. At the time toxic influents are reaching the effluent the conventional AS biomass is either damaged or destroyed already.
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C-TECH detects toxicity at the beginning of the cycle and does in this case not allow other basins to be filled.
Reference book: BTRA & FAITMA Seminar on Conservation of Utilities in Indian Textile Industry November 26, 2002
