Are you gagging at the smell in the heads? Nigel Calder explains the best way to eliminate that sea toilet smell from your heads and holding tank
Sea toilet smell? Get a fresh heads
If you close the boat up with a toilet system that has not been thoroughly flushed, and leave it for a while, especially in a warm climate, I can pretty much guarantee it will not smell too good when you come back! The odour-generator is generally a pocket of standing effluent somewhere in the system which is permeating through one of the hoses, although it might also be tiny leaks around Y-valves (commonly used for an overboard discharge) and hose fittings, or odours escaping through poorly-seated vented loops (necessary in many systems to prevent back-siphoning of seawater into the toilet and boat).
Even some heavy-duty hoses are minutely porous and in time will give off foul odours. To test, rub a clean rag up and down the hose jacket, then sniff the rag. For a more thorough test, wrap a hot, damp cloth around the low point of a discharge hose, leave to cool, then remove and sniff. Special impermeable hoses are required in toilet systems, generally labelled ‘Sanitation Hose’. There are two choices: some variant of PVC, or heavy-wall rubber sanitation hose. Often the PVC hose is considerably cheaper (less than half the price), but more prone to problems.
PVC versus rubber
To increase its resistance to moisture absorption, PVC hose for sanitation purposes is specially formulated (compounded) from a high-density material with a greater-than-normal wall thickness. Ultimately, however, almost any flexible PVC hose that is permanently filled with effluent will absorb enough moisture to begin to smell. The reason for this is that to make the hose flexible it must contain a plasticiser, which creates a larger molecular structure that in turn is minutely permeable. Rigid PVC pipe (used for household effluent) does not have a plasticiser and so does not suffer from the same problem; as a result it makes an excellent (and extremely cost-effective) choice where it can be used. However, it is more difficult to run than hose and tends to develop leaks at the joints as a result of the constant flexing that takes place when a boat is at sea. If used, it may be best to include flexible connections – lengths of hose at either end of the rigid piping.
PVC sanitation hose is relatively stiff. It is essential to make it fit closely to any hose barbs. If the barb is undersized, the clamping pressure needed to seal the hose to the barb will cut into the hose, causing it to develop microscopic cracks at the edge of the clamp (resulting in hard-to-trace leaks). On the other hand, if the barb is too big and the hose has to be stretched over it, the hose will harden and once again develop microscopic cracks in the clamping area. Rubber sanitation hose is more tolerant of poor hose barb fits than PVC and it also has greater flexibility. Its resistance to permeation is directly related to wall thickness – with rubber hose, thickness rules!
Regardless of quality, ultimately any hose that is filled with standing effluent will start to smell, so when installing either PVC or rubber sanitation hoses, do all you can to avoid low spots that retain effluent. If you cannot, flush the head sufficiently after each use to clear all the effluent out of the line. Better yet, substitute rigid PVC pipe for those hose runs that may end up with standing effluent. Alcohol-based antifreeze, petrochemicals and most toilet bowl deodorizers should also never be used, as all contain chemicals that will destroy the moisture-absorption resistance of the hose (PVC hose will be damaged faster than rubber hose).
Redesign the system
This still doesn’t deal fully with low spots, such as Y-valves and vented loops, that gather standing effluent and other potential sources of odours. I have a simple solution – get rid of them! I have always plumbed the discharge from my toilet directly into the top of our holding tank with no Y-valve, vented loop or anything else in between. When we flush the toilet, we pump enough water to totally clear this line of effluent. The holding tank has the usual overboard vent, which becomes a substitute for any vented loop that would otherwise be needed on the discharge line. I also have the normal deck pump-out fitting going into the holding tank.
The holding tank is situated up under the side deck such that its bottom is above the boat’s waterline. I have a drain from the base of the holding tank that goes directly overboard. This has a valve at the tank end and a seacock at the hull end. For overboard discharge mode, I open the two valves. As the effluent gets pumped into the top of the holding tank, it simply falls to the bottom and drains out. Boat motion constantly back-flushes the drain line with clean seawater. For holding tank mode, I close both valves. There are now no hoses, pipes, valves or vented loops with effluent in them – only the tank, which is vented overboard. There is no route for odours into the boat.
If so desired, the holding tank odours themselves can actually be eliminated. It is simply anaerobic bacteria forming in an oxygen-starved environment that create the smells. If you provide sufficient airflow by installing one or more larger holding tank vents than normal, or else by pushing air through the tank with a small fan (there are some purpose-built holding tanks that do this), you will eliminate most of these odours. Personally, I don’t bother.
I do, however, include one other feature in our systems. I make the discharge hose from the tank to the seacock direct (no bends) and install the through-hull for the seacock below the waterline, but not so low that I can’t reach it from the dinghy with my deck wash hose. If the discharge line should become bunged up then it can be easily back-flushed. The drain will not get blocked in normal use, but if someone, for example, uses large amounts of non-fibrous toilet paper when in holding tank mode, or puts something else in the toilet that should not be there, things might settle out and prevent draining the next time the overboard discharge valves are opened. In practice, if there is any boat motion this will likely back-flush the drain pipe and clear the system.
We have had this toilet and tank arrangement on our boats for the past eight years. It is a simple and effective solution to an age-old problem. We leave our boat closed up for months at a time and she smells just as fresh when we get back as she did when we left.