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Under U.S. Coast Guard Regulations, if your boat has an installed toilet, you are required to have a certified Marine Sanitation Device (MSD). Type I or II MSDs treat sewage and discharge it into the water. Type III MSDs include holding tanks that retain and discharge untreated human body waste.
The sewage disposal system you select will depend upon your boat's design, space configuration, and electrical system. Other considerations are cost and how often you boat in No Discharge Areas, where even treated sewage can't be dumped. Remember that any MSD you select must be certified by the U.S. Coast Guard.
Boaters can choose a treatment device, a holding tank, or a combination of the two. Holding tank capacity depends on the number of people customarily aboard, usual trip length, and whether you anchor or use marinas offering toilet and/or sewage disposal facilites. Other options include: manual, electrical, or vacuum toilets; pipes or hoses; and flexible or rigid tanks. Many owners of smaller craft can opt for less costly unregulated portable toilets.