Everything you need to know about boat toilet systems

In short, it stinks, badly. Marine algae tends to be high in sulphur. When it gets into a boat’s toilet and the algae dies, the sulphur is released as a strong rotten egg smell. It’s why if there is a marine toilet – in boating language it’s known as a head – on a boat, it will be flushed using freshwater rather than salt.

Marine sanitation is a difficult topic. First of all, it’s rarely seen, usually tucked away behind bulkheads and under bunks, with the mystery of how they operate difficult to decipher.

But not anymore. We’re about to take a deep dive into marine sanitation so that if you’re thinking of installing one or upgrading your existing system, you’ll know where to start.

Before we start, bear in mind that under Australia’s maritime laws, a bucket of untreated human waste can’t be tipped overboard unless you are more than 12 nautical miles offshore. Even offshore, if you are anywhere inside a marine park, it’s also a no-go zone for dumping. In other words, you’re going to have to keep it onboard.

How then do you handle toilet waste on a boat? Let’s look at some of the options available to us.

The simplest form of toilet – other than a bucket – is either a chemical toilet that stores the waste in a holding tank filled with a cocktail of fluids to stop it from decomposing or a cassette toilet that contains waste in a removable container.

These toilets are extremely portable, meaning they can be carried to a waste dump and emptied – although you do tend to get up rather close and personal to your work.

Chemical and cassette toilets tend to have very limited storage, so if it is covering the needs for a family it will only last a matter of days before needing to be emptied. 

If you don’t need the portability, a better toilet is a more permanent solution that acts all the world like a conventional household one.

The idea behind a fixed toilet is that it is connected to a holding tank that stores the waste until you can get back to shore and pump it out to a sewage point.

The toilet can add waste to the holding tank by either using gravity or via a manual or automatic flushing system.

One of the main considerations with a flushing marine toilet is water use. We’ve already determined that we’re not using salt water to flush because of the smell it can create.

That means you’ll be using fresh water to flush. It’s important, then, with limited freshwater resources to ensure that whatever the marine toilet, it conserves as much water as possible.

Don’t forget, too, that the toilet will be competing for a scarce resource that will be used for everything from washing saltwater off the kids after a swim to making ice cubes for the cocktails you plan to enjoy while watching the sun go down at the end of the day.

The other big consideration is how much volume you’re likely to need to hold all the human waste and water flowing into the holding tank. There’s no hard and fast rule, but expect at least a litre per person for each day in ‘black’ waste and another litre per person a day in ‘grey’ waste such as shower or dishwashing water.

If space is an issue, you’re going to need a gravity toilet.

The simplest forms of a gravity toilet are the portable chemical toilets that see duty on everything from boats to campervans and caravans.

The next step up from this is a flushing gravity toilet. These look more like a conventional toilet and use freshwater to flush, but the holding tank is built into the pedestal below it. These are simple to plumb in and take up much less space than a fully plumbed head with a separate holding tank.

This brings us to the next level, the conventional-look plumbed toilet. 

These toilets look very similar to what we have at home and feed into a holding tank that will need to be pumped out regularly to ensure they do not overflow or back up.

Even these differ, but mainly in the way they flush. A manual flush toilet – many of which will use ‘raw’ water, the polite name for seawater – will need you to pump it to empty the bowl, similar to a portable toilet at a music festival. 

Electric flush heads use a pump to flush, usually passing the contents through a macerator that breaks everything up into small pieces, while vacuum flush heads suck everything out of the bowl.

Boats can also use composting toilets that allow the blackwater tank’s contents to break down naturally while not releasing odours and incinerator toilets that convert waste into ash. This last type of toilet places a huge electrical demand on a boat’s batteries, so may be impractical.

Always use sanitation-grade pipes to contain smells. If your marine sanitation system does start to smell, it’s probably time to replace the pipes that service it. Gas molecules are much smaller than water, so while an old pipe may not leak fluids, it can easily leak smells. You can tell if a hose needs replacing by looking at the condition of it where it connects with a fitting. If it is hard rather than pliable, it’s overdue.

All hoses on a boat that drain outside the hull need to have a seacock, the marine version of a tap, installed between the system and the skin fittings. This is important to stop seawater coming back up the pipes while the boat is underway.

It’s also important for a marine system to have a loop in the pipes that sits above the waterline to ensure that no water from outside can flow backwards into the hull, mainly as a backup for the one-way valve that’s built into the head.

This is usually done using a gooseneck, a U-shaped fitting that’s well above the waterline outside the hull. If you own a sailing boat, the gooseneck will need to be even higher to account for the times when the boat will be heeled over. A handy tip is to mount the gooseneck against a bulkhead in the centre of the boat’s beam.

As a rule, a gooseneck should not be more than 1.8 metres high

The goosenecks will also usually feature something called an anti-siphon valve that bleeds air into the system and helps to prevent the head’s contents from being sucked back through the system and into the boat’s hull.

Don’t use see-through pipes. They may be handy when trying to trace a blockage, but because they’re not rated for use in sanitation, over time they will start releasing some of that sulphurous stink that we talked about earlier – gas molecules are much smaller than water molecules, so while a plastic pipe won't leak water, it may leak gas. This is a frustration for those owners who don’t realise it’s the pipes, and not the system, that’s generating the off smells.

It’s one thing to have a head on a boat, but it is an entirely different thing to store its fruits onboard.

This means you’re going to have to have a holding tank. There are two different types: a blackwater tank for the toilet, and a greywater tank for holding used shower and dishwashing water.

If you’re not venturing far offshore, you’re most likely going to be pumping out both these tanks at a marina-based pump-out station.

It pays to know how to do this correctly, as some systems may need the seacocks open to flush out properly.

Don’t use household cleaners to tidy up your toilet. Instead, use commercially available septic system cleaners.