Apparatus designed to be interposed between the discharge of a marine toilet and the overboard discharge line of a vessel comprising a treating chamber provided with inlet and discharge openings through which the effluent is delivered to the chamber for treatment and pumped therefrom following treatment, an impeller supported in the chamber for rotation therein which, in conjunction with the hemi-toroidal formation of the chamber, produces vortexlike movement of the effluent which reduces the particle size by hydraulic attrition and mixes it with chemicals deposited therein, a dispenser for depositing the chemicals in the chamber, a pump for introducing flush water to the chamber, a pump for discharging the treated effluent and the flush water and controls for timing the cycle of operation from the time it is initiated by flushing the toilet until it is timed out by a timer in the control circuit, together with suitable indicators for indicating the condition of the apparatus at any given time, and to prevent operation in the event that the chemicals are depleted.