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%.
Trang 1ZERO 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
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
maintenance cost (personnel expenses not included) For these reasons, it is important to restructure the wastewater treatment system
Trang 2OUTLINE OF WASTEWATER TREATMENT SYSTEM
(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
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
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
Pelletizing-Drying Equipment
Drainage Basin Heating
Reuse Pellet
Scum
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
Pelletizing-Drying Equipment
Drainage Basin Heating
Reuse Pellet
Scum
Trang 3FLOATATION 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
P
Scraper
P Pressure
Tank
Receiving Well
Traditional Root
P
Scraper
P Pressure
Tank
Receiving Well
Traditional Root
Trang 4Organism 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
-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
Sludge Temp ( O C)
-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
Sludge Temp ( O C)
-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
Sludge Temp ( O C)
y = -1.1x + 160
R 2 = 0.49
0 20 40 60 80 100 120 140 160 180
Sludge Temp (C O )
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
Sludge Temp (C O )
Trang 5Figure 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
Pelletizing-Drying Equipment
Reuse Pellet
Steam
Kneading Machine
Gas Co-Generation System
Moisture 30～40%
Moisture 60%
Cake
Storage
Pelletizing-Drying Equipment
Reuse Pellet
Steam
Kneading Machine
Gas Co-Generation System
Moisture 30～40%
Moisture 60%
Cake
Trang 6Reduction 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