PURPOSE: To effectively remove the chromaticity components and COD components of a secondary treatment liquid of night soil by passing the secondary treatment liquid of night soil through a synthetic adsorbent resin.

CONSTITUTION: The secondary treatment liquid formed by the biological treatment of the night soil by activated sludge is adjusted to 3W5pH and is passed through an adsorption column in which the synthetic adsorbent resin such as styrene/divinyl benzene copolymer is packed. The chromaticity components and COD components in the secondary treatment liquid of the night soil are effectively removed by such simple equipment. The above-mentioned synthetic adsorbent resin can be easily regenerated by bringing a 1W5% caustic soda soln. or aq. 5W50% alcohol soln. into contact with the synthetic adsorbent resin layer.