Takuma KUMO

Highly Efficient Circulating Wasabi Cultivation Using Sewage Heat

Shuji HIMENO, Toshiya KOMATSU

Sewage heat refers to the thermal energy contained in sewage and is characterized by temperature fluctuations that are less than those of outside air temperatures. Sewage heat can be recovered as cold heat in summer and warm heat in winter. This study was conducted in a sewage treatment plant in Niigata City. Sewage heat was recovered from a chlorine mixing pond, and 540 wasabi plants were cultivated. Sewage heat was recovered from the effluent by immersing a coil-type heat exchanger in the chlorine mixing pond. The maximum heat recovery was 6640[MJ/day] in cold operation and 3712[MJ/day] in warm operation. The heat pump (HP) then produced cold and hot water at 7[°C] during cold operation and 40[°C] during warm operation. The produced cold and warm water was used to cool and heat the wasabi cultivation water, which was adjusted to 12~16[°C] and supplied at 0.15[L/min/seedling]. 
　Heat extraction from sewage heat was performed using the immersion method and the draw-in method. As for the maintenance performance against sewage-derived contamination, the performance of the immersion method did not deteriorate for more than 3 years, while the performance of the retraction method deteriorated by 11% in 8 months. The heat recovery efficiency per unit of power required for heat extraction was 1.7 times higher for the immersion type. In order to reduce the amount of heat consumed in wasabi cultivation, the aeration tank was removed from the circulation cultivation system. Within 4 months of removal, a 22% reduction from the previous year was confirmed. In addition, the cultivation plant of this facility is formed above ground. In the cultivated field where the cultivation plant is formed underground, the reduction was confirmed to be 10% lower than that of this facility. In addition, the area of wasabi cultivation was reduced by 70% per wasabi plant. As of the 8th month of growth, there was no delay in the growth of the wasabi plants with the reduced cultivation area, so it is expected to be about 3.2 times larger. 
In order to reduce the amount of electricity used when wasabi is cultivated in a sewage treatment plant using sewage heat, we examined the amount of electricity that would normally be reduced if the entire roof of the greenhouse were covered with solar power generation. The expected amount of electricity generated was 65% of the electricity consumed by the facility. In addition, a trial calculation on the scale of a sewage treatment plant showed that plant cultivation at a sewage treatment plant with a digestion process resulted in a reduction of 9422 [t-CO2/year], achieving a carbon negative effect.