Đang tải... (xem toàn văn)
The Hanshin Water Supply Authority (HWSA) has introduced effective wastewater treatment system to its New Amagasaki water treatment plant (WTP) with the objective of strengthening water quality management, dewatering process improvement, and cake volume reduction. As for the water quality management, turbidity particles including microorganisms in filter backwash water are separated by dissolved air floatation (DAF), and removed from the washwater reclamation system. The exhaust heat energy from the natural gas co-generation system installed into the WTP is used for heating sludge and drying dewatered cake. Heating sludge improves dewaterability and reduces power consumption. It is possible to extend re-utilization field of the cake by heating the dewatered cake into pelletized one, which leads to complete recycling; zero emission. Wastewater treatment system at the plant could reduce microbial risk and environmental load, while decreasing operation and maintenance cost by about 15%.
- 55 - ZERO EMISSION; AN OPTIMUM WASTEWATER TREATMENT SYSTEM Toshiaki HASHIMOTO, Takashi HANAMOTO, Daiji NAGASHIO Hanshin Water Supply Authority, 3-20-1 Nishi-okamoto, Higashinada-ku, Kobe, 658-0073 Japan ABSTRACT The Hanshin Water Supply Authority (HWSA) has introduced effective wastewater treatment system to its New Amagasaki water treatment plant (WTP) with the objective of strengthening water quality management, dewatering process improvement, and cake volume reduction. As for the water quality management, turbidity particles including microorganisms in filter backwash water are separated by dissolved air floatation (DAF), and removed from the washwater reclamation system. The exhaust heat energy from the natural gas co-generation system installed into the WTP is used for heating sludge and drying dewatered cake. Heating sludge improves dewaterability and reduces power consumption. It is possible to extend re-utilization field of the cake by heating the dewatered cake into pelletized one, which leads to complete recycling; zero emission. Wastewater treatment system at the plant could reduce microbial risk and environmental load, while decreasing operation and maintenance cost by about 15%. KEYWORDS Zero emission; wastewater treatment; dissolved air floatation (DAF); sludge heating; dewatered cake; INTRODUCTION In response to the increasing demand for safe and high-quality drinking water, water utilities have been upgrading their water treatment technology. However, recent issue of diversifying chemical substances and infective microorganisms calls for water management as a total system, including wastewater treatment technology. Under the global environmental problems, restructuring recycling society toward less environmental burdens is being promoted. Also in the field of drinking water supply, effective utilization of wastes produced from its plant is required. Looking at the cost, wastewater treatment accounts for about 25% of operation and maintenance cost (personnel expenses not included). For these reasons, it is important to restructure the wastewater treatment system. - 56 - OUTLINE OF WASTEWATER TREATMENT SYSTEM New Amagasaki water treatment plant (WTP) (373,000m 3 ･d -1 ) of the Hanshin Water Supply Authority (HWSA) has introduced advanced water treatment system with ozonation and granular activated carbon adsorption aiming at enhanced water quality management of finished drinking water. In regard to environmental protection, the WTP adopted a co-generation system with gas engine generator, for the purpose of securing emergency power and saving energy. The co-generation system provides stable thermal energy which makes heat utilization easy (Sasaki, et al., 2000) . In the WTP, from the standpoint of following effluent standard on Water Pollution Control Law and utilizing water resource effectively, washwater is reclaimed as raw water. Figure 1 shows wastewater treatment process at the WTP. The wastewater treatment system consists of reclamation of filter backwash water into raw water, and disposal of sedimentation sludge as dehydrated cake, further, its effective re-utilization. Main features of the system are as follows; First, filter backwash water is reclaimed into raw water through dissolved air floatation (DAF). Turbidity particles in filter backwash water circulate in the water treatment process unless removed. For this reason, DAF equipment has been installed between drainage basin and receiving well for the purpose of improving the reclaimed water quality to raw water level. Secondly, the plant has heating equipment of sedimentation sludge and pelletizing-drying device of dewatered cake using exhaust heat from natural gas co-generation system. The exhaust gas (420℃) from gas engine is used to generate steam with gas boiler. About 25 to 40% of the generated steam energy is used for heating the sludge, and 60%, for drying the dewatered cake. Thus, dewaterability improvement and reuse of cake are achieved. Figure 1 Wastewater treatment process at New Amagasaki WTP Clear Well Source Sludge Waste Washwater Reclaimed Water C/F/ES: coagulation / flocculation / enhanced sedimentation, GAC-FB: granular activated carbon fluidized bed, C/RF: coagulation / rapid filtration Dissolved Air Floatation Thickener Storage Tank Dehydrator Pelletizing-Drying Equipment Drainage Basin Heating Reuse Pellet Scum C/F/ES Ozone GAC-FB C/RF Clear Well Source Sludge Waste Washwater Reclaimed Water C/F/ES: coagulation / flocculation / enhanced sedimentation, GAC-FB: granular activated carbon fluidized bed, C/RF: coagulation / rapid filtration Dissolved Air Floatation Thickener Storage Tank Dehydrator Pelletizing-Drying Equipment Drainage Basin Heating Reuse Pellet Scum C/F/ES Ozone GAC-FB C/RF - 57 - FLOATATION EQUIPMENT Wastewater is mainly composed of filter backwash water. Therefore, it contains light-weight particles which was not separated by coagulation / sedimentation process, such as suspended solids, microorganisms including algae, and so on. Because of its water quality characteristics, it is more effective to remove the particles by DAF than re-sedimentation. Suspended solids floats to the water surface with minute bubbles generated by releasing pressured-dissolved air in the water. Finally, they are carried out of the washwater reclamation system as scum (Photo 1; DAF equipment). Photo 1 DAF equipment Figure 2 shows diagram of DAF equipment. Both scum and sediment scum are collected by the scraper and pumped up to thickener. The volume of the scum produced by DAF is 0.3% of sedimentation sludge volume. Therefore, the scum from DAF has no influence on dewaterability. Moreover, drainage basin could also function as a storage tank, which enables constant water reclamation to the receiving well. Figure 2 Diagram of DAF equipment PP Drainage Basin Scraper P Pressure Tank Receiving Well Scum Thickener Traditional Root PP Drainage Basin Scraper P Pressure Tank Receiving Well Scum Thickener Traditional Root - 58 - Organism removal depends on coagulation / sedimentation and filtration during water treatment process. Therefore, reliable separation of microorganisms by DAF leads to enhanced microbial risk management (Sasaki, 2000). DEWATERING PROCESS Sludge heating equipment To utilize exhaust heat from co-generation system for heating the sludge, heating system is installed at the sludge tank placed before dehydration process. Heating the sludge improves dewaterability (Nezu, et al., 1996). Dewatering time of short-time presssurized dehydrator (electroosmotic type) adopted in the WTP is largely dependant on filtration, compression, and discharge performance. Therefore, improving these capabilities brings cycle time reduction. Figure 3 and 4 show the results of sludge heating experiment conducted by HWSA. Figure 3 shows relationship between sludge temperature and dewatering speed. Here, dewatering speed could be calculated as follows; [cake volume (kg-DS)] / ([dewatering filter area (m2)]×[dewatering time (h)]) Figure 3 indicates that the dewatering speed increases with the rise of temperature. Temperature rise from 10℃ to 40℃ improves the dewatering speed by about 40%. Raising the temperature to 60℃ increases dewatering speed by about 60%. This is because the drop of sludge viscosity improves dewaterability. Figure 3 Dewatering speed vs. sludge temperature Figure 4 Electric power consumption dewatering vs. sludge temperature Dewatering Speed (kg-DS･m -2 h -1 ) y = 0.011x + 0.82 R 2 = 0.42 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 10 20 30 40 50 60 70 Sludge Temp. ( O C) Dewatering Speed (kg-DS･m -2 h -1 ) y = 0.011x + 0.82 R 2 = 0.42 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 10 20 30 40 50 60 70 Sludge Temp. ( O C) Dewatering Speed (kg-DS･m -2 h -1 ) y = 0.011x + 0.82 R 2 = 0.42 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 Sludge Temp. ( O C) y = -1.1x + 160 R 2 = 0.49 0 20 40 60 80 100 120 140 160 180 0 10203040506070 Sludge Temp. (C O ) Electric Power (kWh) y = -1.1x + 160 R 2 = 0.49 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 0 102030405060700 10203040506070 Sludge Temp. (C O ) Electric Power (kWh) - 59 - Figure 4 shows relationship between sludge temperature and electric power consumption in electroosmosis. Filterability improvement by rise in sludge temperature decreases electric energy required for electroosmosis. Heating the sludge raises processing capacity per-unit, leading a reduction of electric power consumption, which contributes to a reduction of annual dehydrator running cost by about 25%. Pelletizing-drying equipment Figure 5 shows the process flow for dewatered cake. Cake stored in the hopper is put into kneading machine to make it uniform in property. Then, at the steam tube drier of indirect-heating type drying machine with pelletizing-drying function (drying temperature; about 170℃), water content is reduced to 30 or 40% from 60%. At the same time, the grain size is pelletized to about 3 to 5 mm. Weed seeds and bacteria in the dry cake are extinct by heating. In addition, the cake has suitable hardness. These enables a variety of applications, including the uses for athletic field, farming and local replanting, which allows complete reuse of the cake. As for the protection against Cryptosporidium, it is possible to completely inactivate oocysts by heating the sludge in the storage tank nearly to 70 ℃ (Iseki, 1996). This is an effective measurement against Cryptosporidium that could be carried out of the plant and circulate the environment, as well as drying cake. Figure 5 Dewatering process flow CONCLUSION As for wastewater treatment, it is important to efficiently eliminate turbidity particles in filter backwash water and sludge produced during water treatment. At New Amagasaki WTP, additional installation of DAF, sludge heating equipment, and pelletizing-drying device assures effective wastewater treatment. Particularly, the sludge heating equipment and pelletizing-drying device, which utilize exhaust heat from co-generation system, enable about 15% cost reduction of operation and maintenance. Storage Tank Dehydrator Pelletizing-Drying Equipment Reuse Pellet Steam Kneading Machine Gas Co-Generation System Moisture 30～40% Moisture 60% Cake Storage Tank Dehydrator Pelletizing-Drying Equipment Reuse Pellet Steam Kneading Machine Gas Co-Generation System Moisture 30～40% Moisture 60% Cake - 60 - Reduction of dewatered cake from the plant and extending its re-utilization field lead to complete recycling; zero emission. Because of the recent years’ environmental problems, building of recycling society is in urgent desire. Also in the field of drinking water supply, addressing toward less environmental loading is necessary. The water treatment technology at the WTP could produce safe and high-quality drinking water, while contributing to environmental measurement through materializing resource and energy saving with the optimum wastewater treatment. REFERENCES Iseki M. (1996). Outbreaks of waterborne cryptosporidiosis: occurrences and control measures. Japanese Journal of Water Treatment Biology, 32(2), 67. Nezu H., Kawano S., Nishimura T. (1996). Improving dehydration efficiency in winter season by heating the suluge. Proceedings of 47th JWWA Annual Conference and Symposium, 264-265. Sasaki T., Nagashio D., Hanamoto T. (2000). Renewal with state-of-the-art technology: Amagasaki water treatment plant. Proceedings of 5th International Symposium On Water Supply Technology, 131-139. Sasaki T. (2000). Microbial Risk Management During Drinking Water Treatment System of the Hanshin Water Supply Authority. Proceedings of CREST Workshop on Integrated Water Quality Management, 135-149. . ) y = 0. 011 x + 0.82 R 2 = 0.42 0 0.2 0.4 0.6 0.8 1. 0 1. 2 1. 4 1. 6 1. 8 2.0 0 0.2 0.4 0.6 0.8 1. 0 1. 2 1. 4 1. 6 1. 8 2.0 0 10 20 30 40 50 60 70 0 10 20 30 40. ( O C) y = -1. 1x + 16 0 R 2 = 0.49 0 20 40 60 80 10 0 12 0 14 0 16 0 18 0 0 10 203040506070 Sludge Temp. (C O ) Electric Power (kWh) y = -1. 1x + 16 0 R 2 = 0.49