How do I choose a holding tank and flush-water system for a marine toilet?
We have a 32-foot sailing yacht with a manual seawater toilet and usually cruise as a couple for three or four days. I want to fit a holding tank, but how big should it be, and would freshwater flushing be worth the extra plumbing?
Answer
Choose a holding tank by calculating how much sewage and flush water your crew will produce between pump-outs, then check that it fits and leaves the plumbing accessible. Choose seawater flushing to conserve drinking water, or a purpose-designed freshwater system to reduce seawater-related smell and scale; both need a properly vented tank.
Check the installation first
Trace the existing toilet inlet and discharge hoses, identify the toilet model and measure the proposed tank space, including the route through the locker opening. A tank that fits the compartment but cannot be installed, emptied or serviced is the wrong tank.
Check the height of the toilet and hose runs relative to the waterline, including when a sailing yacht heels. These measurements determine the anti-siphon arrangements. The toilet manufacturer's limits dictate pumping height and hose length.
Close the relevant seacocks before disconnecting toilet plumbing. Never connect a seawater toilet directly to the drinking-water supply.
A rigid polyethylene tank designed for sewage is the usual choice. Look for secure mounting points, accessible inspection and cleaning openings, and room above the fittings for hoses. Allow roughly 1 kg per litre of contents when assessing support: a full 80-litre tank adds about 80 kg before counting the tank itself.
Calculate holding capacity
Dometic's marine holding-tank range includes capacities from 42 to 137 litres, an indication of typical yacht installations. Crew numbers and flushing habits matter more than boat length.
Use this calculation:
People × toilet visits per day × litres entering the tank per visit × days between pump-outs.
Include both waste and flush water. For example, two people making five visits each daily, with 2 litres entering the tank per visit, produce 20 litres a day. Three days require 60 litres before allowing spare capacity; around 75 to 80 litres gives more margin. The 2-litre figure is an assumption, so check your toilet's consumption before ordering.
- Manual toilets: consumption varies considerably with pump strokes and discharge-hose length.
- Electric macerating toilets: check the specified consumption for each flush setting.
- Vacuum toilets: systems such as Dometic VacuFlush use little flush water, making freshwater supplies last longer and reducing the volume entering the holding tank.
Fit a level indicator, preferably with a high-level alarm. Leave spare capacity: an overfilled tank can send sewage into the vent and damage the system.
Choose your flush-water supply
Seawater flushing suits boats with limited freshwater capacity and an existing manual head. It saves drinking water, but stagnant inlet water can smell after the boat has been left unused. Mineral deposits can also restrict the discharge plumbing.