Boat holding tank odor and treatment

A smelly holding tank is one of the most common complaints on a boat with a head. The fix is usually a mix of regular pump-outs, simple cleaning and a look at the hoses.

Where boat holding tank odor comes from

A holding tank, which the EPA calls a Type III marine sanitation device, stores sewage until it can be pumped out. It does not treat the waste. The longer waste sits, the more it smells. The usual sources are:

How to clean a boat holding tank

Florida Sea Grant’s tips for boaters are simple and cheap:

• Rinse with fresh water regularly. A rinse after a pump-out helps clear what is left in the tank and hoses.
• Clean with vinegar after emptying. Use a vinegar solution right after the tank has been emptied and before you add a treatment. It reduces scale buildup, keeps the hose walls clean and extends hose life.
• Deodorize gently. Florida Sea Grant suggests about ½ cup of borax per gallon of water, and baking soda and water for cleaning the bowl, with baking soda sprinkled around the rim.
• Mind the paper. Use toilet paper that dissolves quickly, and never flush wipes, even ones sold as flushable.

The rag test for hoses

Sanitation hoses can absorb odor over time, and then no amount of tank cleaning helps. Wet a rag with hot water, wrap it around the hose for a few minutes, then remove it and smell it. If the rag smells, the hose needs replacing. A marine service technician can replace hoses and check the vent and fittings.

Choosing a holding tank treatment

Florida Sea Grant recommends choosing gentler treatments over harsh chemical deodorizers. The EPA notes that not every marine sanitation device needs chemicals for day-to-day use, and that the devices that do use them usually rely on chlorine. If your boat has a Type I or Type II treatment device, follow its manual. For a plain holding tank, use a product made for marine holding tanks and follow the label.

A treatment does not change the rules: a holding tank still has to be emptied at a pump-out station or, beyond 3 miles from shore and outside a No Discharge Zone, at sea.

US no-discharge rules for boaters

Federal law sets clear limits on vessel sewage:

• Within 3 miles of shore, untreated sewage cannot be discharged, according to the EPA’s vessel sewage FAQ. Treated discharge in that area requires a Coast Guard-approved Type I or Type II device. A holding tank is the other option.
• In a No Discharge Zone (NDZ), both treated and untreated sewage discharges are prohibited. Boaters must keep sewage aboard and empty it at a pump-out facility, or at sea beyond 3 miles from shore where that is allowed.
• Secure the overboard valves. In an NDZ, the EPA’s no-discharge zone guidance lists approved ways to secure a holding tank’s overboard discharge valve: close it and remove the handle, padlock it closed, or hold it closed with a non-releasable wire-tie.
• Some inland waters also bar treated and untreated discharges, including certain freshwater lakes, reservoirs and non-navigable rivers.

NDZs are set by state, and they cover different waters in each one. Check the EPA’s No-Discharge Zones by State list before you go out, and confirm local rules with your state boating agency.

Finding a pump-out station

The U.S. Fish and Wildlife Service’s Clean Vessel Act program funds state boating agencies to build, renovate, operate and maintain pump-out stations and to teach boaters where they are. To find one:

• Ask your marina manager or dockmaster. Some pump-outs are for members or customers only.
• Check your state boating agency, which often lists pump-out locations, as the EPA notes.
• Look for the national pump-out symbol at marinas.
• In Florida, try the free Pumpout Nav app, recommended by Florida Sea Grant, which maps public pump-outs and mobile pump-out vessels.

How to use a pump-out station

Here are Florida Sea Grant’s steps:

1. Unscrew the deck waste fitting and open the holding tank’s air vent.
2. Uncoil the hose, close the nozzle valve and turn on the pump.
3. Insert the nozzle into the deck fitting, then open the valve.
4. Watch the sight glass. If nothing flows within a minute, dip the nozzle in water for 10 seconds. If it still won’t flow, check for an air leak or a clog.
5. When the tank is empty, close the nozzle and let it drain.
6. Pump a bucket of water through the system to rinse it, then turn off the pump and return the hose for the next boater.

Wear gloves, wash your hands afterward, and never lean into an open holding tank or put your face near the deck fitting. The EPA notes that sewage poses risks to human health. If you see an illegal discharge, Florida Sea Grant advises reporting it to the Coast Guard or the National Response Center.

For the RV side of holding tanks, see how to treat an RV black tank and RV black tank smell.