Abstract: A marine holding tank receives an excrement and water charge from a toilet and converts such charge into vapor and ash components for efficient and hygienic storage and/or removal. The holding tank is provided both with a rotary mixer to form a liquid-solid mixture from the excrement and water and with a heating means to reduce the mixture by vaporizing the liquid and incinerating the solids, thereby to complete the charge conversion. The heating means consists of two heating elements with selective energization so that one element may be independently operated for low heating or the two elements may be simultaneously operated for high heating. The heating elements are part of an energizing circuit that is controlled by a D.C. circuit, with such control circuit being automatically operative to discontinue the heating process when the temperature in the tank has reached a preselected point and to maintain the energizing circuit closed irrespective of A.C.