If a vessel has an installed toilet (technically referred to as a marine sanitation device (MSD), it must be equipped with one of three types of MSDs. The MSDs (Type I, Type II, Type III) are designed to meet different needs and effluent level requirements. Since portable toilets can be moved on and off a vessel, they are not considered installed toilets; therefore, vessels that have portable toilets are not subject to MSDs regulations.
The Sewage Treatment Device (Standard Vessel Length) Type I, is a flow-through device (maceration and disinfection) for vessels less than 65 feet in length. The effluent produced must not have a fecal coliform bacteria count greater than 1000 per 100 milliliters and have no visible floating solids. Type I MSDs rely on maceration and disinfection for treatment of the waste prior to its discharge into the water.
Type II Flow-through devices are for vessels greater than 65 feet in length. The effluent produced must not have a fecal coliform bacteria count greater than 200 per 100 milliliters and suspended solids not greater than 150 milligrams per liter. Type II MSDs are similar to the Type I, however, the Type II devices provide an advanced micro-biological treatment and discharges wastes with lower fecal coliform counts and reduced suspended solids. It operates as/like a septic system.