Beat the Pong - marine toilets 101

Marine sanitation expert John Wilson.
Marine sanitation expert John Wilson.

Most of us know the problem: a highly unpleasant smell that seems to come from somewhere in our toilet system but which is almost impossible to trace.

Although it might smell that way, it is most often not a problem with cleanliness and no amount of scrubbing or use of "de-odourisers" makes any difference.

The problem, according to marine sanitation expert John Wilson, is often caused not by clogged toilets or incorrectly installed holding tanks but rather by the almost-always overlooked link between the two: the humble hose pipe.

"There are a lot of different things that cause an unpleasant odour to hang around marine toilets and on board sewage systems," says Wilson.

"However, there are two causes that are far more common than any other. They are the use of the wrong hose and hoses that have not been installed correctly."

Ensuring the correct hose is installed is vital, says Wilson, a marine engineer at Westhaven-based General Marine Services Ltd.

"Always use proper sanitation hose," he says. "It needs to be flexible, kink- and odour-resistant. It also needs to be able to handle human waste that is around 30 times more concentrated that normal residential sewage."

Wilson says there are basically three types of hose used in a standard marine toilet installation. These are the inlet hose (bringing fresh water into the toilet); the waste line hose (taking sewage from the toilet to either an overboard discharge fitting or a holding tank or both); and the holding tank vent hose (which releases both the air from the tank as it is displaced by each flush and the pressure that builds up as a result of sewage oxidation in the tank).

While proper sanitation hose must always be used for the second and third of these, there are three different grades from which to choose. As with most things, the better the performance, the higher the initial cost. However, as Wilson points out, it is hard to put a price on a toilet system that operates without bad odours or other problems for many years.

Standard sanitation hose will do the job and is found on many boats. It is a PVC hose with what is known as a hard vinyl helix construction. It is reasonably long-lasting but is often not completely odour-free.

Extra Odour Resistant Sanitation Hose is a clear step up. It has the same construction as standard sanitation hose but also has an anti-bacterial formulation and has been proven to be many times more odour resistant.

The top of the line option is Premium Rubber Sanitation Hose. It has a thicker wall than the others and features two ply reinforcement and a wire helix. Not surprisingly, both laboratory testing and more than a decade of trouble free use in the marine environment has proven this to be the most odour and trouble free of the three.

"However, choosing the correct hose is only the half the story," stresses Wilson. "Incorrectly installed sanitation hoses will cause problems even with the best hose grades."

This is largely because the primary aim when installing a sanitation system is to ensure that there are no places where the raw sewage can collect and fester. This means keeping hose lengths as short and straight as possible; ensuring that the outlet hose is self draining into the holding tank or seacock; and that there are no loops, low spots or dips.

It is also important to ensure that any overboard discharge lines are connected to dip tubes at the top of the tank and there is no (or minimal) use of Y valves.

Wilson also advises using rigid PVC pipe anywhere where there is even a slight possibility sewage may be left in the system and supporting this every 1.8 metres.

"Also use as few connecting fittings as possible," he advises. "Use sweeps instead of elbows and make sure seacocks are both installed and easily-accessible on all toilet intake and overboard discharge lines."

Extra care clearly also needs to be taken whenever a toilet is installed below the waterline.

Wilson says a siphon break (a bronze or plastic vented loop with a one-way valve at its highest point) must be installed on both the intake and discharge lines.

"This must always be looped above the maximum heeled waterline," says Wilson. "This allows air to be drawn into the line and therefore prevents a water leak from siphoning into the toilet and potentially sinking the boat.

"Installing a vent hose from the discharge hose's one way valve is also a good way to reduce to possible odour permeation."

Wilson says following these simple guidelines will ensure most marine toilet systems remain both odour- and trouble-free.

"Owners installing their own toilet systems simply need to ensure they use the correct type of sanitation hose and install it correctly. Those having their toilet system professionally installed should be okay but it does pay to check that the correct hose is being used and that all the outlet hoses are correctly installed.

"If in any doubt, get an experienced marine engineer or sanitation expert to check over the system before it is used.

"This could save a lot of trouble - and a lot of bad smells - later on."

For more information contact John Wilson, General Marine Services Ltd on 09 309 6317.

Boating NZ