In order to evaluate performance of on-site treatment facilities which can be provided for environment protection in coastal areas, effluent qualities and removal efficiencies were surveyed in actual treatment facilities for wastewater from households, hotels and restaurants. On-site treatment facilities in Japan are fundamentally built according to the structural standards. They have a pretreatment process (sedimentation separation tank, anaerobic filter or equalization tank with screens) followed by an aerobic process (contact aeration, activated sludge, etc.). Small-scale facilities for individual household wastewater showed good performance of BOD removal with their effluent BOD below 20mg 1−1. They also exhibited nitrogen removal efficiency when they were operated in mixed liquor recycle mode. The facilities applied to wastewater from hotels, restaurants and stores showed good performance when the influent oil (hexane extracts) concentration had been decreased below 30mg 1−1 by using pretreatment. Nitrogen removal performance was high in the facilities which treated wastewater from a residential area or a condominium when they were operated in intermittent aeration mode. But resort condominiums of which influent BOD load was extremely low showed low performance of nitrogen removal even though they were operated in intermittent aeration mode because of the low BOD/N ratio in the influent. An equation was proposed to estimate the amount of methanol to be added in facilities in which the influent BOD/N ratio is low.

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