Optimization of Removal Efficiency and Operational of Mbbr | Sewage Treatment

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Ingeniería ambiental
   OPTIMIZATION OF REMOVAL EFFICIENCY AND OPERATIONAL COSTS IN URBAN WASTEWATER TREATMENT PLANTS BY USING BIOMASS CARRIERS E. Görgün 1 , G. Insel 1 , S. Tabak 2  K. Ünal 2 and A.O. Erdogan 2   1 Istanbul Technical University, Environmental Engineering Department, 34469, Maslak, Istanbul, Turkey 2 io Environmental Solutions, Research and Development Ltd., Istanbul Technical University, Ari Teknokent Ari 1 Building, No:3, 34469, Maslak, Istanbul Turkey. E-mail: egorgun@ins.itu.edu.tr ABSTRACT In this study, the use of biomass carriers was found to be economically feasible option in terms of investment and operational costs together with maintaining organic pollutant removal. A real high-rate WWTP having an over designed capacity was extended with the use of biomass carriers in current plant. Finally, after the extension, the system is able to handle approximately 3.4 times COD load of existing wastewater treatment plant. The main advantages of biomass carriers to be used for the purpose of the extension of existing urban WWTPs can be summarized as: (i) low operational cost, (ii) no additional land requirements, (iii) easy to be integrated into nutrient removal plants and (iv) short start-up period for the construction of the system. 1  1. INTRODUCTION In sensitive areas, appropriate wastewater management is the key issue for maintaining the quality of the receiving water. The European directives enforces strict effluent nutrient discharge criteria especially in coastal areas to prevent receiving waters from eutrophication problem. In the past, the most favorable treatment choice was the conventional activated sludge system for municipal wastewaters. As a common practice, the activated sludge plants have been designed for organic carbon removal considering the future capacity extension of the wastewater treatment plant in  parallel to population increase. In this respect, without additional construction of biological stage, the process extensions can be applied upon existing activated sludge systems that were previously designed for organic carbon removal. The promulgation of strict discharge standards requires additional processes such as nitrification-denitrification for nitrogen in addition to organic carbon removal. In parallel to the pollution increase, the coupling the nitrification-denitrification process alternatives should be considered in the context of process performance and cost optimization. However, higher investment costs are required for upgrading or extension of existing activated sludge systems which have exceeded maximum capacity. Considering available process alternatives for the extension of existing system to reach the optimal investment (Capex) and operation cost (Opex) providing sustainable effluent quality, first issue is to handle the exceeded amount of pollutant loads (organic carbon). The above mentioned process alternatives might include mainly (i) building of additional activated sludge modules and (ii) the use of biomass carriers within the biological reactor units. Biomass carriers are fashionable tools to be used within the biological reactors to maintain organic carbon removal and alternatively nitrification. In this way, the biomass can grow on the attached media (polyethylene etc.) without any adjustment of actual sludge age. In this respect, the Moving Bed Biological Reactor (MBBR™) technology aims to remove pollution with minor modifications made in the aeration basin. In this study, MBBR™ technology is analyzed and compared with the conventional activated sludge system, considering the capacity extension for a real municipal wastewater treatment plant which discharges into an inner-lake of Turkey. In this regard, the efficiency and overall costs (Capex, Opex) were investigated for the application of MBBR™ technology that serves as an optimum extension option. 2. DESIGN PARAMETERS OF MBBR™ PROCESS The MBBR™ technology is based on specially designed plastic biofilm carriers that are suspended and completely mixed within reactor of specified volume. The MBBR™ system design is critical due to the requirements for efficient mass transfer of substrate and dissolved oxygen. In addition, the design of aeration system, diffuser pattern, sieves, spray nozzles and other integral parts to the reactor are of great importance in design [1]. Diffuser grid located at the bottom of the reactor supplies oxygen to the biofilm along with the mixing energy required to keep the biocarriers suspended and completely mix within the reactor. Most important design parameters of the system are material filling ratio and accordingly, the specific biofilm surface area, hydraulic retention time, organic loading rate (i.e. COD, BOD) and nitrogen. However, the shape and dimensions of  biocarriers do not play significant role in removal efficiency [2]. 2   Figure1. Pictures of various biomass carriers (AnoxKaldnes-K1, IFAS-Agar) After the primary treatment stage, the municipal wastewater is fed to the MBBR™ system where  biofilm growing attached on the biocarriers. Treated water flows out from the reactor through a sieve, which retains the biocarriers in the reactor. The biofilm area is the key parameter in design and therefore the design of the process is based on effective carrier area (g/m 2carrier_area .d 2 ). At standard filling ratio of 67%, the total specific carrier area is around 465 m 2 /m 3  [2]. The biofilm carrier elements are made of polyethylene (density 0.95 gram/cm 3 ) and shaped like small cylinders with a cross inside the cylinder and longitudinal fins on the outside (Figure 1) [3]. In order to keep them in the reactor, a sieve (net opening about 7 mm) is placed at the outlet of the reactor. In design, the total organic lading should not exceed 65-85 gCOD/m 2 .day. The typical biomass concentration is in order of 2-5 kgSS/m 3  as the same as in activated sludge systems. Since the volumetric removal rate, however, has been demonstrated to be several times higher in the moving bed process, the biomass of this process must be much more viable than in similar activated sludge processes [4][5]. In addition, the advantages of MBBR  TM  technology are (i) resistant to environmental conditions, (ii) allows necessary oxygen transfer and (iii) does not need return activated sludge. 3. EVALUATION OF CURRENT WWTP The full scale municipal wastewater treatment plant under study was built in 1999, as a traditional high rate activated sludge system [6]. Biological treatment units consist of two aeration tanks with the volume of 2,800 m 3 with 5 aerators each and two secondary clarifiers with activated sludge return to aeration tanks. Figure 2 gives the current flow scheme of the mentioned WWTP. Population and design and operational parameters of the WWTP are given in Table 1 and Table 2. Furthermore the unexpected population growth and development of the city, the capacity extension time planned in design has already been reached and current wastewater treatment plant is operated with overcapacity. According to the srcinal design data given in Table 1, the existing WWTP has to be extended in 2005 and 2015. TABLE 1. Populations and design parameters for periods Years Parameter 2005 2015 Population 160,000 231.000 Maximum daily flowrate(m 3 /d) 37,500 51.000 Design flowrate(m 3 /d) 26,500 BOD 5  charge(kg/d) 8,500 TSS charge(kg/d) 11,100 3  Coarse & Fine Screen Grit Chamber Primary Sedimentation Aeration Tanks Final Sedimentation Tanks Sludge Thickeners Anaerobic Sludge Digester Sludge Dewatering Sludge Storage Tanks Return Sludge Figure 2. Flow scheme of current WWTP TABLE 2. Influent wastewater characterization used in design periods Years Effluent Quality (mg/L) 2005 2015 BOD 5  230 300 COD 470 570 TSS 200 250 4. ALTERNATIVE CONFIGURATIONS FOR EXTENSION Four different configurations are compared for the extension of existing municipal WWTP. First, a traditional suspended growth activated sludge system is considered, and then at the rest of three alternatives, different combinations of the biocarriers system are investigated thoroughly. 4.1 Configuration 1 In this alternative, existing process (suspended growth activated sludge) will be applied for the extension. Based on the same design assumptions, three additional aeration basins are calculated to  be constructed and equipped with 15 more low speed surface aerators. Figure 3 gives the process flow schematically. The necessary amount of recirculation flow will be provided by additional recycle pumps. On the other hand there is a land limitation for this application. The allocated land for the extension is not sufficient for more than 5 aeration tanks. So that when this alternative accepted there will be no chance for further capacity increase after the year 2015. 4.2 Configuration 2 The system is designed for two phases. In the first phase (2005-2010), 26% of total aeration tank volume will be separated with a steel plaque to fill with biofilm material (Figure 4a). This part will  be aerated with blowers and diffuser grid system. The construction of any additional aeration tank is avoided, in this regard. As depicted in Figure 4a, the remaining part of the tanks are used as conventional suspended growth activated sludge process as before. The current system will guarantee the necessary removal efficiency for organic carbon. 4
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