Ask SAIL: Holding Tank Blockage

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Q: I recently purchased a 34ft cruising boat that has been sitting unused for a couple of years. Everything is great except for one thing: the holding tank is about two-thirds full with, uh, material that has presumably been sitting there unattended the entire time and is now largely solid. What can I do about this?

Lisa Hoffman, Chicago, IL


Go to the laundry section of your local supermarket or big-box retailer and buy some Calgon Water Softener—powder or liquid. This is water softener, not fabric softener. Mix two cupfuls into a gallon of hot water and pump the mixture into your holding tank. Add a cup of liquid laundry detergent to a second gallon of hot water and pump that into the tank as well. Allow the mixture to sit in the tank for several hours (or overnight) and then fill the tank completely with freshwater to give the softener mix an opportunity to also release deposits on the sides of the tank. After a few more hours, pump the tank out. The softened and emulsified waste should come out in the discharge. Rinse the tank and empty it again. If the tank is still not acceptably clean, you can try using a tank-rinse wand that feeds through the pump-out line—available from RV supply stores. Alternatively, repeat the softener/detergent application, at the same time taking the boat out for a vigorous sail. To avoid this condition in the future, pump the tank out regularly and use an aerobic tank treatment. Note that aerobic bacteria require a generous vent to thrive. Before storing your boat for the off season, clean the tank thoroughly and leave it empty.

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