All boats operating in US waters with permanently installed toilets are required by federal law to have on board a Marine Sanitation Device (MSD) that either stores human waste until it can be transferred ashore, or reduces the coliform count to such low levels that discharged sewage poses no public health hazard...even in populated harbors. While the MSD must satisfy Coast Guard regulations, the boat owner still has a lot of choices of product types and overall system design.
First, you need to familiarize yourself with the laws as they apply to your local boating area.
More than three miles from the coast it is legal to discharge raw (untreated) waste overboard, either directly from the toilet or by emptying the holding tank. We think the most sensible sanitation system design gives you the choice of both a dockside pump–out and the ability to empty the tank yourself when offshore (see illustrations below). Inside the three–mile limit, it is illegal to dump raw sewage. In these areas, boaters may discharge waste only if it has been treated by an onboard treatment device like the Raritan Electro Scan (Type I or II MSD). Otherwise, it must be contained on board in a Type III MSD–a holding tank–and transferred ashore at a pump–out station (which, in many cases, sadly, means it will get a modest amount of treatment before finding its way back into the water).
A rapidly increasing number of coastal areas have been designated as No Discharge Zones. To qualify under the federal Clean Water Act, states must show that sufficient pump–out facilities exist for boaters to empty holding tanks. In Michigan, New Mexico, Rhode Island and Vermont all waters are no discharge. There remains considerable opposition to NDZs from boaters who feel both inconvenienced and unfairly singled out as sources of pollution (miniscule compared to typical sewage treatment overflow following rainstorms, runoff, industrial, etc.), but the political tide definitely favors the proliferation of NDZs.
While pumping raw sewage is not illegal offshore, you need an approved MSD for inshore and inland use. Direct discharge is ugly, and inside the three–mile limit, illegal. Don't operate your head without a means to contain or treat waste on board, and in foreign countries don't pump your waste overboard within 100 yards of the beach. This is harmful to swimmers and those who eat the local fish and shellfish. Just because the hundreds of Caribbean charter boats have little, if any, sewage treatment systems on board doesn't mean it's okay for the rest of us to dump in the otherwise pristine waters regularly used by snorkelers and swimmers.
What are the waste storage options?
There are a variety of ways to deal with waste on board, including recirculating, composting, and incinerating toilets, but by far the most common are toilets plumbed to holding tanks. These range from the simple and inexpensive self–contained heads (portable potties) to fairly complicated systems incorporating multiple valves, pumps and hoses. Any way you look at it, carrying around sewage is an unsavory business.
Regardless of how much boaters really contribute to water pollution, the inescapable truth is we need to do our part, if not for impact, then at least as a symbolic gesture. Creating a good, odor free sanitation system isn't that hard. Start with an intelligent design, buy quality components, and maintain them.
Thick–walled (1/4" or thicker) high–density polyethylene tanks are the most sensible choice. They are light, won't corrode and are much less expensive than metal or fiberglass tanks. While thinner wall tanks rarely burst, they can bulge so much that fittings are stressed to the point of leaking. Your nose will alert you, but it's safer to buy a quality tank in the first place. West Marine sells tanks in various wall thickness. Taller, narrower tanks can be emptied more completely but are more difficult to secure. Plumbing attachments should be as low and as high on the tank as possible. Flexible tanks can be used when spaces are oddly shaped or inaccessible but they lack the odor resistance and strength of rigid tanks so we don't recommend them for waste. Taller, narrower tanks are less subject to "free surface effect.