How to Deep Clean a Boat Holding Tank

Holding tank treatments come in several types (enzyme, bacterial, oxidizing and chemical). Practical Sailor prefers enzyme and bacterial products for routine use. The EPA says each sanitation device should be maintained as its own manual describes. Read your tank, toilet and treatment labels for compatibility and dose, and pick one product for the job. Don't stack different chemicals in the tank.

Open hatches and ports, run a fan if you have one, and put on gloves, eye protection and an apron. Have rags and a sealed bag ready for anything you wipe up. Make sure the head's seacocks and Y-valve are set so nothing can go overboard.

At a pump-out station, empty the tank completely. Let the station pump run a moment after flow stops to clear the hose.

A deep clean means two or three pump-outs in a row, so plan it at a station you can tie up at for a while, or book a pump-out boat. Many public pump-outs are free or cheap because the federal Clean Vessel Act funds state boating agencies to build, run and maintain them, paid for by excise taxes on fishing gear and boat fuel. Your state boating agency's website usually lists locations.

Fill the tank partway with fresh water. You can flush it in through the head (if it can use fresh water) or carefully add it through the deck fitting with a hose used only for sanitation. Pump out again. Many owners repeat this once more before the cleaning soak.

Lanier's method for a tank of about 20 gallons, which you can scale up or down:

• Dissolve 2 cups of water softener in 1 gallon of hot water and pump it into the tank through the head.
• Mix 1 cup of biodegradable, bleach-free liquid laundry detergent with 1 gallon of hot water and pump that in too.

Dometic/SeaLand, quoted by Practical Sailor, suggests filling the tank about halfway with fresh water and adding about 8 ounces of the same type of detergent. If you use a commercial "shock" or cleaning treatment instead, follow its label exactly.

Top off the tank with fresh water and let it sit for a couple of hours (Lanier's method). Better still, go for a run in choppy water: the motion scrubs the tank walls. For extra scouring, both BoatUS and SeaLand suggest adding a couple of buckets of ice through the tank's access port before you go. With a commercial shock treatment, makers typically want it to slosh around for three days to a week; follow the label.

Pump out the cleaning solution. Right away, refill the tank with fresh water (about half full is enough), and pump that out too. Lanier recommends repeating the fill-and-pump step a couple of times if you can. Any loosened residue leaves with the rinse water instead of settling back.

A tank needs air. While it is empty, check the vent fitting on the hull for insect nests, salt or corrosion, and confirm air moves freely. Back-flushing the vent line with fresh water is easier now. Replace a vent filter that has reached the end of its life. See the holding tank vent filter guide.

With a flashlight, look at:

• The tank's outside for weeps, stains, cracks or bulges, and the straps or chocks for tightness
• Hose connections at the tank, head and deck fitting for leaks, rusty clamps and stiff, smelly hose (do the warm damp cloth test on the hose)
• The deck pump-out cap O-ring; replace it if flattened or cracked
• On aluminum or stainless tanks, signs of corrosion at welds or fittings

Any leak from the tank itself is a job for a pro.

A deep clean often frees a sticky float sensor. If your gauge still reads wrong, see the holding tank level monitor guide; in-tank senders sometimes need to be removed and cleaned by hand.

If the buildup has hardened, chemical treatments won't do much. BoatUS and Practical Sailor agree the options then are living with it, cleaning by hand, or replacing the tank. To clean by hand through an access plate: empty and rinse the tank, remove the plate, spray the inside with a high-pressure hose, and loosen stubborn spots with a disposable stick. Lanier suggests doing it in cold weather to keep the smell down and wearing a raincoat, gloves and a face shield, and keeping your mouth closed. Never climb in or lean far into a tank. If your tank has no access plate, ask a pro before cutting one.

• Flush the tank with fresh water every four or five pump-outs (Practical Sailor's advice), or more often.
• Add water to the bowl before use, and use little paper or none. Lanier keeps a lidded, lined bin in the head for paper.
• Never flush wipes, even ones sold as "flushable."
• Use a regular enzyme or bacterial treatment as its label directs.
• Lubricate the head only with what its maker specifies.
• Pump out before the tank is full.

Note the date and products used in BoatMend so you can keep a twice-a-year rhythm and spot changes (like faster odor return) that hint at a hose or vent problem.