A marine vessel waste treatment system includes a tank for receiving successive charges of liquid and solid waste material. A water conduit conducts heated water from the vessel's engine through the tank to heat the contents thereof above the level at which coliform bacteria can survive, thereby reducing the count of such bacteria to a level at which it is safe and acceptable to discharge the material into a waterway. A macerator pump is mounted in the tank for operation only after the temperature within the tank is sufficiently high. An electrically operated heating element is also mounted in the tank to accelerate the treatment of waste and can be used alone when desired. A thermal switch at the waste material input inhibits discharge if a fresh charge of material is received when the system is ready to discharge, thereby preventing discharge into the environment of untreated material.