Type III marine sanitation devices have the least impact on the environment. Most are holding tanks which retain sewage until it can be pumped into a local sewage system at a marina. Type III MSDs also include incineration and composting toilets, vacuum collection systems and recirculation systems.
The U.S. Coast Guard certifies two other classes of MSDs. A Type I device may be installed only on boats no more than 65 feet in length and usually uses maceration and disinfection for sewage treatment. Its discharge or effluent may contain no more than 1000 fecal coliform bacteria per 100 milliliters and may not include any visible floating solids. A Type II MSD may use biological treatment and disinfection as well as the maceration and disinfection processes used by Type I devices. Its effluent may contain no more than 200 fecal coliform bacteria per 100 milliliters and 150 milligrams of total suspended solids per liter.