Waste holding tanks are now becoming a must have item on any boat. With the advent of tighter environmental legislation in Europe and the Americas, holding tanks are in some countries compulsory.

GENERAL LAYOUTS

Plumbing layouts for holding tanks can vary greatly depending on where the tank is going and how it is going to be used. The position of the tank wants to be as close to the toilet as possible to avoid long pipe runs and will be either above or below the waterline. For more information and comprehensive plumbing layout diagrams please visit here. It is possible to fit a system that involves a carry off tank that would comply but is probably not very desirable from an emptying point of view!

MATERIALS

Holding tanks are best manufactured in polyethylene or polypropylene. Waste is highly corrosive and will “rot out” an aluminium tank in no time at all with stainless steel holding tanks failing at the welds eventually. Some stainless steel tanks will however last a considerable amount of time depending on the quality of the steel and the manufacturer. The fittings attached to the tank are probably better off being plastic as well, as brass fittings will corrode in a very short time.

TANK DESIGN

Tanks ideally should have a sloping bottom or a funnel design so as the waste can collect at one end. The outlet or dip pipe will need to be at the deepest part of the tank and ideally the inlet wants to be near the outlet so the contents do not have to migrate through the length of the tank. A popular question often asked is “how big should the tank be?” The industry rule of thumb is 2 gallons per person per day using a manual toilet. An electric toilet will produce far less and a figure of ½ a gallon per person would be quite realistic.

TANK FITTINGS

The fittings for the vent, inlet and outlet(s) want to be on the top of the tank if possible in case there are any leaks from the pipe or fittings. Most holding tanks have 1 ½” inlet, an 1 ½” outlet and an 1 ½” vent. On smaller tanks a 1″ vent is acceptable. It is important to get a good flow of fresh air into the tank so as it does not turn sceptic and start to smell.

TANK POSITION

A holding tank should be positioned as close to the toilet as possible to avoid any long pipe runs. If the tank has to be situated some distance from the toilet then any dips and sags in the pipe should be avoided to prevent waste sitting in the pipes. If waste is left in contact with the pipe for a long period of time then the chances of odours permeating the pipe will be increased. Holding tanks are quite often mounted up high under the side deck actually in the heads compartment or can be found under a forward berth next to the heads compartment bulkhead. The choice of position on a large boat is not normally a problem due to the increased space available.
1. Fuel tanks
2. Water tanks
3. Grey water tanks
4. Waste tanks