A Type 3 marine sanitation device (MSD) is a self-contained holding tank that treats sewage using either chemical or physical means. MSDs are required by the U.S. Environmental Protection Agency (EPA) to be used on all boats that have toilets, but no direct connection to a sewer system.
There are three main types of MSDs:
Type I – Treats sewage using chemicals only
Type II – Treats sewage using physical and chemical processes
Type III – Treats sewage using physical processes only (i.e. no chemicals are used in the treatment process)
The most common type of MSD is the Type II MSD, which uses both chemicals and physical processes to treat sewage. Physical processes include holding the sewage in the tank long enough for solids to settle out (called “sludge”), and then pumping the remaining liquid (called “effluent”) through a filter before discharging it overboard. Chemical processes usually involve adding chlorine to kill bacteria and other pathogens in the effluent before it is discharged overboard.
It is important to note that even with an MSD onboard, boaters are still required to follow local laws and regulations regarding dumping of waste material into waterways. Many states have specific requirements regarding when and where you can discharge your effluent, so it is important to check with your local authorities before setting out on your trip.