Keisuke IIZUKA

Development of High-Quality Wasabi Circulation Cultivation Technology that Utilizes Sewage Heat

Shuji HIMENO

In recent years, sewage treatment plants are expected to serve as district heating points because of the stable and abundant inflow of sewage. Most of the use cases of sewage heat are air conditioning in urban areas, and many of them are unused because there are few heat consumers in rural areas. Therefore, in this study, we focused on the heat demand of agriculture, which is a primary industry in rural areas, and carried out plant cultivation. The target of cultivation was wasabi, which requires cool spring water and mountain stream water and has unevenly distributed cultivation points. Heat is collected from the sewage by immersing a coil heat exchanger in a chlorine mixing pond where the treated discharged water collects, and using a heat pump to cool the temperature to 7 [℃] in summer and 40 [℃] in winter. Produced water. The produced cold and hot water was used for cooling and heating wasabi cultivation water, adjusted to 12 to 14 [℃], and then supplied at 0.15 [L / min, seedlings]. The cultivated water was circulated without draining the cultivated water by cooling and heating it again.
　As for the sewage heat recovery results, a maximum of 6.5 [GJ / day] was recovered when the daily average effluent temperature was 26.3 [℃] for cold heat, and a maximum of 4.3 [GJ / day] was recovered when the daily average effluent temperature was 17.4 [℃] for hot heat. did. The amount of heat recovered from the chlorine mixing pond is estimated from the amount of discharged water at the Nishikawa Purification Center 23,000 [m3 / day], the number of heat exchanger units installed 81, and the total heat exchange area 1620 [m2]. Cold heat was 0.049 [W / m3 ・ m2] and hot heat was 0.049 [W / m3 ・ m2], and the amount of water and the basic unit per heat exchange area were obtained. For the purpose of energy-saving cultivation of wasabi cultivation, it was verified whether the required amount of heat could be reduced by laying a heat insulating material on the medium soil. As a result of comparison with 2019 without heat insulating material, the power consumption per cultivated area was 232 [kWh / m2 ・ year], and in 2020 it was 190 [kWh / m2 ・ year], confirming a 22% reduction effect. .. On the other hand, as a result of harvesting the green that had finished its growing period from mid-September 2020 in order to evaluate the cultivated wasabi, the yield per cultivated area was 1.8 [kg / m2]. In the analysis of allyl isothiocyanate, which is a pungent component contained in wasabi, the content of this study wasabi was 1.4 [mg / g-dry] and that of wasabi on the market was 1.3 [mg / g-dry]. Subsequently, as a result of requesting a wasabi farmer to teach us the impression of color, shape, and taste, we secured the same quality as the wasabi that was highly evaluated and distributed in the market. Finally, the business profitability in wasabi cultivation using sewage heat was evaluated from the yields of power consumption and yield obtained in the experiment. We also compared the expenditures when the energy production required for cultivation was carried out with an air-cooled chiller. As a result, when sewage was used as the heat source, the amount of electricity was reduced by 47%, demonstrating the energy-saving effect of using sewage heat. In terms of business profitability, the annual cost was recorded, and the profit was 54% of the expenditure, suggesting that it is difficult to recover the investment.