Go Green! A Complete Guide to Eco-friendly Marine Composting Toilets

Okay, we need to talk about toilets. Not pretty I know, but for liveaboards, they’re essential for a comfortable onboard experience.

Whether you sail, cruise or barge, toilets are a big deal when it comes to boats. Having toilet facilities is excellent, especially when you’re in the middle of nowhere. It makes boating feel more homely, with all the creature comforts.

But which is the best toilet for your boat?

In the main, there are four types of toilets used on vessels: Cassette, pump-out, incinerating, and composting toilets.

I wanted to focus on composting toilets because of their “Green” credentials.

What Are the Advantages of Composting Toilets?

Ease of Use

Composting toilets are simple really!

They usually have two chambers: Liquid and solids. The solids are separated because if they come into contact with the liquid, decomposition cannot happen. The solids have to be kept dry. The fluids are emptied traditionally, via a disposal point.

However, because urine contains potassium, phosphorous and nitrogen (three of the essential elements to plant growth) just like peeing in the bushes, you can dispose of the liquids by the bank side (as long as it’s away from popular spots).

Good for the Environment

Because there are no chemicals used to break down the waste products, there’s nothing in the toilet to harm the environment. And the composting process that occurs will take all the bad stuff and turn it into plant food. Now that’s got to be a good thing!

Emptied Less Often

Because the solids in the toilet undergo an “Active” composting process, they break down into mulch, much like natural compost soils for the garden.

Composting toilets also have a fan, which aerates the chamber where the waste gets stored. The fan has two functions; it speeds up the drying process, and in turn, the composting process, and it helps to stop any odors from escaping back into the boat.

To improve the process, try adding a bulking agent to the tank. Sawdust, ground coffee, and cocoa shell are the most widely used, and all help to kick-start the composting process, as well as giving a pleasant smell to the toilet.

What Are the Disadvantages of a Composting Toilet?

They Are Seriously Expensive

If you compare them to a cassette toilet, for example; a standard cassette toilet will set you back between $70 and $200. A decent composting toilet will cost somewhere in the region of $1000 to $2000. Ouch! That’s quite a difference.

Why spend all those dollars on a composting toilet? Well, think about the ongoing task of emptying the cassette, for one thing. Cassette toilets may be cheap, but they’re not very convenient, especially on longer journeys. They tend to smell, and the chamber where the waste collects isn’t significant.

And pump-out toilets are massively expensive, compared to composting toilets. And they’re complicated and have many moving parts that can malfunction.

How Does a Composting Toilet Work?

Time to get a bit gritty!

Separating the Waste

As already mentioned, composting toilets work by separating the solids from the liquid. In essence, there are two chambers to collect the waste product; one at the front for urine, and the other at the rear for feces. Male users are encouraged to sit rather than stand so the urine can collect in the chamber at the front.

The waste tank at the rear for solids, is the larger of the two, for obvious reasons, and is accessed via a flap, to be opened before use. Most composting toilets encourage the use of a bulking agent; something like sawdust, or cocoa shells. They help to extract any moisture from the feces and start the composting process, as well as help minimize any smells.

A Word of Caution

The solids chamber should avoid being exposed to moisture, as it will encourage bacteria to grow, causing a reversal of the composting process, and bad smells. So always remember the liquids tank fills up faster than the solids tank, and if left unattended, could back-fill into the feces tank.

No Smells

The composting toilet is fitted with a small fan, much like the type you find in a desktop computer, to keep the feces aerated (again, to actively compost the hard stuff). This also reduces the risk of bad smells. Anecdotally, some users of composting toilets state there are no smells at all!

It’s possible to get a composting toilet fitted with a heater that keeps the temperature at a steady 55 celsius, making the drying and composting much quicker. Toilets equipped with a heater generally use evaporation to remove the liquids from the holding tank.

I’ve been asked many times whether a composting toilet is any larger than a standard pump-out toilet, and depending on the design, I don’t see much difference. They can look a little space-age, however, more akin to something you’ll find on the space station.

The Environment Will Love You

So, they don’t use water to flush, which is better for your finite water supply, they don’t smell, which is better for your boat, they don’t take up any more room than a cassette or pump out, and best of all, they don’t damage the environment.

Everyone’s a winner!

So let’s look at some examples of composting toilets:

The Best Marine Composting Toilets on the Market

Sun-Mar Compact Self-Contained Composting Toilet, Model# Compact
Sun-Mar Excel Non-Electric Self-Contained Composting Toilet, Model# Excel-NE
Nature's Head Self Contained Composting Toilet with Close Quarters Spider Handle Design
Sun-Mar Compact Self-Contained Composting Toilet, Model# Compact
Sun-Mar Excel Non-Electric Self-Contained Composting Toilet, Model# Excel-NE
Nature's Head Self Contained Composting Toilet with Close Quarters Spider Handle Design
Sun-Mar Compact Self-Contained Composting Toilet, Model# Compact
Sun-Mar Compact Self-Contained Composting Toilet, Model# Compact
Check Price
Sun-Mar Excel Non-Electric Self-Contained Composting Toilet, Model# Excel-NE
Sun-Mar Excel Non-Electric Self-Contained Composting Toilet, Model# Excel-NE
Check Price
Nature's Head Self Contained Composting Toilet with Close Quarters Spider Handle Design
Nature's Head Self Contained Composting Toilet with Close Quarters Spider Handle Design
Check Price

Sun-Mar Self-Contained Composting Toilet

Coming in at just under a cool $2000, the Sun-Mar is by far the most expensive option.

It uses a heater to evaporate the fluids from the chamber, as opposed to a separate solids and liquids tank. The heater keeps a steady 55 celsius, ensuring there’s no bacterial growth in the holding tank which could cause bad smells, reversing the composting process. The liquid soaks through the bulking agent and feces material to a tray on the floor of the toilet (where the heater is located) and evaporates.

Sun-Mar recommends using a bulking agent in the chamber, like cocoa shells, or sawdust. The unit comes with a venting kit to remove any odors, and they claim this composting toilet is bug-free, meaning no flies can access the goods inside.

A word of warning: Some users have claimed the urine tray at the bottom can be difficult to access and clean. They also claim not having a separator tank for the urine means the toilet can be a little on the smelly side, meaning the fan needs to be on 24/7.

Also, because the heater takes time to evaporate the liquid, if the toilet gets some serious use, like on special occasions when you have guests, the urine tray can become waterlogged and overflow onto the floor. And nobody wants that!

Sun-Mar Excel Non-Electric Self-Contained Composting Toilet

If you want to step it up a notch, Sun-Mar also offers a larger “Excel” version.

This is ideal for entire liveaboard families.

Its odorless ability is achieved with a 4″ vent located in the top back of the unit.

If that ventilation is not enough, there is also an optional 12-volt fan available.

Natures Head Self-Contained Composting Toilet

Natures Head is one of North America’s leading manufacturers of composting toilets. Their units are robust in feel and design, with quality materials. But remember, you’re paying a lot more for that privilege. This toilet retails at just under $1000. But the best bit, Natures Head is wholly American made, so you’ll be supporting a home-grown business. Bonus!

The principles of use are the same as the other composting toilets. The Natures Head has a fan that aerates the waste, and a “Spider” handle on the side to mix the feces with the bulking agent in the chamber. This helps speed the decomposition of the solids.

Again, male users are going to have to sit to pee, as the collection tank sits at the front of the unit.

Word of warning: If you have a strong flow (time to get personal), be careful splashback doesn’t occur, causing the liquid to cross-contaminate the solids, as some reviewers have mentioned. Could this be a design fault with the Natures Head toilet? Time will tell.

Also, the accessories and spares are expensive. It’s a good idea to have a spare urine collection chamber, but that could set you back over $40.

And make sure you choose which side of the pond you buy on carefully: The same Natures Head composting toilet in the UK could cost over £1600 (that’s over $2000 at the current exchange rate).

Kildwick Composting Toilet

Check Price on Kildwick


Kildwick is a UK based manufacturer, building composting toilets that are seemingly traditional in their appearance, but appearances can be deceptive. Designed around a capital D shape, Kildwiick toilets retail at around $890 (£695), and are available in an array of colors from traditional white to glittery purple.

Kildwick toilets have a 10-liter fluid tank and a 23-liter solids tank. They come with variable speed fan options, as well as opportunities for fitting a larger external fluid tank, and they work on the same separation principles as most compost toilets.

Many boaters in North America may not be familiar with the name, but Kildwick toilets are gaining a following in the UK.


Check Price on Simploo


Designed by a boater living on the River Thames, this composting toilet is cheap compared to the rest featured in this article. Simploo is a small concern, aimed at the affordable end of the composting toilet market. The Simploo works on the same principle as the others, separating the liquids from the solids, actively composting via a fan and the addition of bulking agents.

It retails at less than $400 dollars. It is small and compact and takes up the same space as a cassette toilet or pump-out.

Word of warning: Because of the low cost, the production values aren’t as good as the high-end manufacturers, but the Simploo gets a thumbs up in reviews from satisfied customers. And let’s face it, you can’t beat a good review for honesty value!


If you travel further than the lakes and rivers, or you’re a liveaboard boater looking for a way to escape the endless emptying of a cassette toilet, then composting toilets could be the answer. Imagine only emptying the solids chamber every three to four weeks as opposed to every two to three days.

I accept that the composting toilet in its current form is far from perfect. But as an eco-friendly option, and compared to the alternatives, it’s hard to beat. The technology is sure to improve, but without support now, eco-based opportunities won’t challenge mainstream thinking.

And remember, the environment needs your support too!

Mark Weir lives on a canal boat in the heart of England, with his wife, Julie, and his grumpy dog, Eric. Mark likes to travel the waterways in his wide-beam barge, filming his exploits as he travels. Julie paints the wildlife, and Eric likes to bark, mostly.