A horizontal flow biofilter to be buried in the backyard of private homes and residential areas was developed to treat effluents of septic tanks. Pilot and full scale experiments have been conducted to study the efficiency of the biofilter. A tank volume of about 0.6 m3 per capita is needed to achieve advanced COD removal, complete nitrification and up to 60 per cent nitrogen removal. The bacterial count was reduced by 2 to 3 orders of magnitude. The treated effluent may be used for irrigation or toilet flushing. Percolation is possible without negative effects on groundwater quality.

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