In recent years, most night soil treatment plants constructed in Japan have the function of nitrogen removal. This report describes three representative night soil treatment plants adopted for a nitrified liquor recycling process (we call this the Denipac process) constructed in the second half of the 1970's.
The Matsue City plant uses coagulation to remove phosphorus and color in addition to BOD and nitrogen. The removal of both BOD and phosphorus is nearly 100%. Inorganic nitrogen and color of the effluent are under 6 mgl−1 and 60 units, respectively. The Shiroishi City plant which was reconstructed from an anaerobic digestion system removes over 95% of BOD and inorganic nitrogen without any use of chemicals. At the Hiroshima City plant, night soil is evaporated by multiple effect evaporation, and condensate is treated by the nitrified liquor recycling process. The temperature of mixed liquor is controlled to about 40°C by adding sea water to lower the temperature (over 55°C) of the condensate before the biological treatment step. Nitrogen oxides in nitrified liquor consist almost entirely of nitrite because of controlling the temperature. The removal of BOD and total nitrogen is over 99% in this plant.