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Marine Sanitation Device - MSD

Preventing Pollution

A Marine Sanitation Device (MSD) is designed to keep untreated sewage out of the water. Every boat with an installed marine toilet must have it connected to an operable Coast Guard approved MSD. Most boats have one of three basic types of MSDs. (Type III MSDs are the most common type found on boats.)

Type I and II MSDs

These types macerate the sewage and then treat it with chemicals or other means to reduce the bacterial count before it is discharged overboard.

A Type I MSD must macerate the sewage to no visible solids, and then reduce the bacteria count to less than 1,000 per 100 milliliters.

A Type II MSD macerates the sewage even finer so that the discharge contains no suspended particles and the bacteria count must be below 200 per 100 milliliters.


Type III MSDs are holding tanks. This is the most common type of MSD found on boats. These systems are designed to retain or treat the waste until it can be disposed of at the proper shoreside facilities.

Portable toilets are the simplest type of MSDs. They represent the easiest solution to marine sanitation on small boats because they require minimal space, and are inexpensive, reliable and easy to operate.

From the perspective of environmental impact, a Type III MSD -- when used correctly -- may be best, because it conveys boat waste into a local advanced sewage treatment system and reduces the need for on-board use of potentially toxic tank treatment chemicals.

What's The Law?

It's illegal to discharge untreated sewage into any of California's lakes, rivers, reservoirs, or coastal waters within the three-mile U.S. territorial limit.
There are 11 federal 'No Discharge Areas' in California where it is illegal to discharge any wastes, treated or untreated.

You are not required to have an installed marine toilet on your boat, but if there is one, it must be connected to a Coast Guard-approved MSD.
Boats 65 feet and under may use a Type I, II, or III MSD. Boats over 65 feet must have either a Type II or III MSD.

Certification: Make sure that your Type I or II MSD meets Coast Guard requirements by looking for a certification label. A Type III MSD is not required to have a label if it simply stores sewage at ambient pressure and temperature.
The Coast Guard can issue fines of up to $2,000 for the illegal discharge of sewage.

The Y Valve

'Y' valves are used as part of some MSD systems to direct waste overboard. If your sewage system is equipped with a Y valve for overboard discharge, and you are operating on inland waters, in a 'No Discharge Area' (see back panel), or within the three-mile U.S. territorial limit, you must secure the Y valve in the closed position. The preferred method is to use a padlock or non-releasable wire tie. If you have a thru-the-hull seacock, it must also be secured.

Additive Advice

The chemical disinfectants and deodorizers used in many MSDs can contain chlorine, quaternary ammonia, or formaldehyde. All harmful to aquatic life. When shopping for treatment products, read labels carefully and take advantage of the many environmentally friendly products now available without these ingredients. Be sure to follow the directions for applying a sufficient amount of chemical to ensure adequate treatment.