Soil-covered wastewater treatment systems have progressively been used during this decade both in the rural and urban areas in Japan. In order to evaluate the influences of extremely low air temperatures on the covered soil and sewage temperatures, the pressure of contact aeration tank and the treatment of wastewater, we compared the efficiencies of three types of contact aeration apparatus (sewage surface level, 30, 50, 70 cm from the soil surface; top shape of gravel layer, convex, flat; soil-sewage boundary, contact, separate) being packed with gravels as a contact medium and set in the temperature controlled test laboratory. At extremely low air temperatures, the covered soils and ice columns on the surface soils are important as a heat insulator. Freezing depths of soils were inversely correlated with sewage surface levels. The influences of soil freezing on the pressure of aeration tanks were negligibly small and thus soil freezing does not affect aeration systems. No significant changes in the sewage treatment functions among the three apparatus were found. The soil-covered wastewater treatment systems must be useful in cold districts.

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