Features
4 Challenge to Energy-Independent Treatment forLivestock and Food Wastes
Yukimasa Ogawa

Obayashigumi Co.Ltd,.

Introduction

In the current standard of living, livestock waste, food waste, and raw garbage are increasingly generated. The said waste materials have been bringing about various ecological problems. However, they can be converted to biomass in order to become reproducible resources. By the application of methane fermentation process, the attention to energy recovery as biomass is becoming a trend.

1. Outline of Yagi Bioecology Center

1.1 Outline of the plants

The Yagi Bioecology Center, established in March 1998, is located in Yagi city, Kyoto. The plants belonging to the center are classified based into two equipments being used in the processing of waste materials. The methane equipment processes materials from livestock waste and tofu refuse. Meanwhile, the compost equipment treats waste materials from cattle slaughterhouses and from the production of dehydrated cake.

The methane equipment began its operation in April 1998. In July of the same year, the generation of biogas power was started. Since then, livestock farmers disposed cattle manure through the plant. Consequently, livestock farming increased the productivity growth.

The second methane fermentation tank was built in March 2002. The second tank was then used for handling concentrated effluent, such as wasted milk from dairy farms. The system flow and specification of the main plants are shown in Fig. 1 and Table.

Fig.1 System flow.
Fig.1 System flow.


Table Plant specification of Yagi Bio-Ecology Center

Table Plant specification of Yagi Bio-Ecology Center


1.2 Outline of each equipment

Livestock waste and Tofu refuse are quantified and then sent to the receiving tank. Long waste materials, such as straw in livestock waste, are crushed into smaller pieces before placing in the raw water tank. The waste materials are pumped into the two methane fermentation tanks from the raw water tank. The two fermentation tanks operate in parallel. The first fermentation tank operates in a medium temperature at 37 degrees C (first construction: capacity 2100m3) while the other tank is a high-temperature fermentation tank at 55 degrees C (second construction: capacity 600m3). Fermentation under high temperature has a high disinfection effect on pathogenic bacteria. Since the rate of sprout of a seed is controlled, part of this digestive juice of methane fermentation can be used as liquid fertilizer for paddy fields.

The biogas generated in the methane fermentation tank is stored temporarily in a gas holder and pressurized with a blower. Subsequently, the stored biogas will be sent to the three gas-engine type generating equipments. The generated output of biogas is used for the operation of the center and surplus electric power is sold to an adjoining sewage disposal plant (supply started in April 2002) and electric power company. The heat released by the power generation equipment is recovered in warm water, which is used for warming the fermentation tank, as well as the management office, and provides the supply of hot water for the plant.

Regarding digestive juices after methane fermentation, solid matters are separated with a dehydrator. Dehydration filtrate sent to the wastewater treatment plants undergo biotic-denitrification, film separation, condensation precipitation, and ozone and chlorine treatments after which organic matter, nitrogen, and phosphorus are discharged. The solid separated with a dehydrator is carried to the compost equipment. It is mixed with livestock waste from cattle slaughterhouses or from raising of cows. The mixture is subjected to primary fermentation where agitation is carried out for 25 days with a rotary type churning machine. In secondary fermentation, deposition of the compost is allowed to occur for about 65 days. Then the fully ripened compost, as a product, is shipped for agriculture use.

2. Operation track record of Yagi Bioecology Center

The track record value, such as cattle manure, received by this center is shown in Fig. 2. Average waste received per day between October 2003 and September 2004 is 3.7t of pigs, 51.0t of dairy cow manure, 5.1t of tofu refuse, 9.7t of refuse from cattle slaughterhouses, and 1.6t of concentrated effluent. The methane equipment processes an average value of 60t/day of livestock waste and Tofu refuse, with 85% dairy cow manure among those wastes. The input to a fermentation tank is about 80t/day including about 20t/day of bacterial sludge for wastewater treatment. The generated biogas reaches an average value of 2,340m3/day as shown in Fig. 2. The methane concentration of biogas is 53-60% and is used for power generation. Furthermore, an input of about 40% organic matter into the fermentation tank is decomposed and recovered as biogas.

Fig.2 Received amount and biogas volume
Fig.2 Received amount and biogas volume

 As shown in the balance of electrical energy in this center in Fig. 3, the output generated in the fermentation process covers most of the electrical energy needed for the operation of the plant. A comparison of electric energy consumption per month indicates that electric load for the center is provided by power generation after 2003 and sale of electrical power to utilities outside the center shows 7-29% of the generated output. The three gas engine type power generators hardly have seasonal variations. No. 1 and No. 2 generators show 30.0% of power generation efficiency, 49.3% of heat recovery efficiency, and 79.3% of total efficiency. Power generation efficiency of No. 3 generator is 27.2%. Heat recovery efficiency is lower than generators No. 1 and 2 at 36.6% due to exhaust heat recovery only from jacket water, not from an exhaust gas. Total efficiency remains at 63.8%.

Fig.3 Monthly electricity power balance (2002.10-2004.9)
Fig.3 Monthly electricity power balance (2002.10-2004.9)

3. Evaluation as energy production plants

An example of energy balance experimental measurement during the winter in this center shows that, except for about 5% receiving electricity out of the total electric power, about 75% of generated electricity is used in this center and the remaining 25% is sold. As regards heat recovery, about 43% salvaged by heated water is used as a warming heat for methane fermentation. The remaining 56% of heat is radiated to the atmosphere as surplus, except for the minor use in heating air and for the provision of hot water. During the summer, more heat is released to the air because less heat is needed for methane fermentation. Regarding the rate of electric power usage, the methane fermentation tank consumes about 1/3, while the dehydration/wastewater treatment equipment/composting equipment spends two thirds.

On the other hand, if all digestive juices from methane fermentation can be used as liquid fertilizer, dehydration/wastewater treatment equipment/composting equipment will become unnecessary. In result, the saleable electric power ratio will rise drastically. An estimate indicates that about 70% of electric power production can be sold. The use of recovered heat for warming a greenhouse and compost, and various drying techniques can also improve heat utilization efficiently. Furthermore, since more chemicals are used in the dehydration/wastewater treatment equipments, employing liquid fertilizer reduces the chemical expense. Consequently, a double effect is acquired. While treatment of livestock and food wastes is being imposed, providing carbon-neutral biogas as energy required for treatment can contribute greatly to the control of CO2 emissions.

4. Conclusion

Based on the operation performance of this facility where methane is fermented from livestock waste and Tofu refuse, an energy-independent treatment system is established.Approximately 7-29% surplus energy from the production of electricity is generated. Since about half of heat recovered from waste heat can be utilized as surplus energy even in winter, the effective use of heat is a future problem. In addition, if all methane fermentation digestive juices can be used as a liquid fertilizer in this center, about 70% of generated output can be supplied to outside facilities.


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