Trelino Composting Toilet Review — A Boater’s Take on a New Marine Composting Toilet

Disclaimer: Trelino provided us with an Evo M composting toilet to try. What follows is our honest take on this toilet for use on our sailboat. 

If you’ve ever engaged me in a conversation about toilets (and really, why haven’t you?), you probably know my feelings about our boat’s human litter box composting head. We’ve lived full-time, 365 days a year, with an Airhead marine composting head for five years, so we might be close to experts on the topic by now.

But we’re both ready for something new, so we couldn’t say no when Trelino offered to let us review their new, simple, clean, and small composting toilet in our boat. Trelino makes three different separating toilets and markets them to campers or van lifers. Here’s our take on how this new breed of composting toilet might work for boaters, how it compares to our old Airhead, and how we like the change. 

A Few Notes About Marine Composting Toilets

There are so many misconceptions about marine composting toilets; they swirl around the internet like 💩 flushing down a properly plumbed toilet. I will never understand how this “technology” has earned so many fanboys and fangirls.

First, there is no such thing as a composting toilet in a boat (or RV, for that matter). To put it plainly, these toilets only separate the liquids from the solids and then do their best to dry out the solids to keep the smell down. The terms “separating toilet” or “desiccating toilet” are much more appropriate, but those didn’t catch on. 

Legally, marine composting toilets are just Type III MSDs (Marine Sanitation Devices). That’s the same designation any holding tank, including your trusty bucket, would have. The Coast Guard does not approve or disapprove of these things; they just happen to meet the same standards that a bucket does. They cannot be dumped overboard — even the liquids! — unless you are three miles or more offshore (actually, you need to go a lot farther than that in many cases).

Overall, here are some of the pros and cons of using a composting toilet on a boat.

Pros of compositing toilets on boats

• No plumbing is required. Typical marine toilets have long runs of expensive hose that must be replaced every 5–10 years. 
• No through-hulls are required. Typical marine toilets have two underwater through-hulls: one for flush water and one for offshore discharging. Fewer holes in the bottom of your boat is always a good thing.
• Separating liquids from solids means less smell and easier disposal.
• It can’t clog because there are no hoses, joker valves, or plumbing.
• No holding tank is necessary, which might clear up space in your boat for something else (but not in our boat, unfortunately). 

Cons of marine composting toilets

• Puts guests and some users off. It’s not the same as a flush toilet; your waste doesn’t magically disappear. 
• Complex instructions for guests; it’s new and a bit intimidating.
• Not a great option for full-time liveaboards (the solids don’t have enough time to dry out properly).
• Must keep a stash of composting media onboard. Coconut coir is the most common but must be purchased online.
• Still smells, especially the liquids, which are typically emptied in marina restrooms. 
• Attracts bugs. Usually, tiny fungus flies are the biggest problem, but the vent also draws in regular house flies. This can be managed with insecticide use.
• Emptying the urine tank is disgusting; it’s easy to slosh and spill and smelly.
• There is no good way to dispose of solids. Most bag and throw them in a dumpster, which is pretty gross. What if you don’t have access to a dumpster?

Trelino vs. Airhead Composting Toilet

When we purchased our boat in 2019, we were certain a fancy Airhead toilet was the way to go. After several years of struggle and frustration, we were living with it — but we hated it. By 2024, we had decided to make a change, but we weren’t certain it was worth the cost. The Airhead was on its way out, and the company refused to sell me parts, so I started hunting for another option. We wanted to reinstall a standard marine head, but we decided to try a newer design of composting toilet first. The Trelino caught our eye because of its simplicity and smaller size. 

The Trelino toilet is designed from the ground up to work for many off-the-grid living situations. Many Airheads are also installed in these situations, but the toilet is much larger, more complex, and requires much more space. 

The Trelino is similar to a head you may see in some older boats called the C-Head. C-Head went out of business, but they made a similar product: a simple composter without the solids churn handle. Instead of stirring the solids, you use the layering method. After each deposit, you cover it with a scoopful of composting medium.

One of the most appealing aspects of the Trelino is its size. It comes in three sizes (small, medium, and large), each with different liquids and solids capacities. But it allows the user to pick the one that best fits their space. The small is so compact that it could be used as a porta-potty in any car, SUV, or small boat.

This is a great benefit for boaters because the Airhead is too tall. If you can mount it on a flat surface, it’s as tall as you could possibly live with. But on many boats, you’ll have to put it on a platform that raises it even higher. This makes the Airhead comically tall, like a precarious perch. The shorter Trelino works much better in our boat, but its base is still large for the platform. Also of note, the Airhead is available with a notch cut out of the bottom rear that allows it to sit closer to the boat’s hull shape, but the Trelino is perfectly square. No matter what toilet you choose, it always comes down to the measurements of your space and where you can mount it.

Big Problems with our Airhead Composting Toilet

What did we decide to ditch our Airhead? Our first issue was bugs. Even though the toilet is completely sealed with gaskets and bug screens, tiny flies get in every summer. We’ve found a few bug sprays and insecticides that work, but it’s a pain. Prevention is so much better than the cure, however. Once there are flies, you must rigorously clean the toilet. 

And that’s the next big problem with the Airhead — it became impossible to clean. First, the inside of the toilet is so complex, with so many nooks and crannies, that you can’t reach everything. The best we could do was spray it out with a high-power hose and spray in a lot of bleach. 

But the entire toilet and bowl were hard to keep clean. It’s made of porous rotomolded plastic. After five years, it was too stained to be cleanable, even with scraping, sanding, and polishing. The company recommends cleaning with gentle cleaners, but we never found anything that worked well on the white plastic. Then, the more brutal you become to remove mineral buildup and staining, the more porous the material becomes for future staining. 

In the end, we found that it had just reached the end of its life. I asked the company for a replacement bowl, but they refused, stating that the bowl was such a large part of the total toilet that they could not sell it on its own. That’s when we knew it was time to replace it with something else.

Here are a few other problems we had over the years with the Airhead. Read on to see if the Trelino is any better.

• Too tall in our bathroom, so sitting on it at sea always seemed dodgy. 
• The separating bowl was a bit shallow, so when sailing, it was not unusual for some urine to slosh into the solids bin. 
• The urine tank was impossible to clean, and it had a tiny lid that was prone to leaking while being carried to the bathroom. 
• Cleaning the solids tank was a huge hassle, too — the churner always got in the way, and it was impossible to just cleanly dump it into a trash bag. I usually had to resort to using gloved hands. 🤮  It was a job I absolutely dreaded, but at least it was only needed about once a month. 

Trelino Composting Toilet Review — Simpler Design, Lower Cost, Smaller Size

The Trelino is a different type of composting toilet. Instead of having a crank handle to churn the solids, you layer a little bit of soil media after each use. The solids bin is just a small bucket that you line with a trash bag. When it’s time to empty it, you just tie the trash bag closed and move on. While you have to empty it more often than the churn system, it’s so much cleaner and easier that it’s no big deal. Plus, you’re dealing with a small five-pound bag instead of a huge 20 or 30-pound bag of 💩, which is much easier to deal with. 

Another big benefit of the Trelino is how easy it is to clean. It’s made of harder ABS plastic that wipes clean. And it doesn’t have the intricate nooks and crannies that were so hard to clean on the Airhead. Plus, the urine tank has a large lid, so it’s actually possible to clean the container properly. The lid fits snuggly, but it’s still easy to cross the treads if you aren’t careful. I’ve had a few leaks while carrying it to the bathroom, so it’s not foolproof — you need to be careful.

The separating bowl of the Trelino is much deeper than the Airhead. After a few months of testing, it has had no spillage problems and works flawlessly, even when the boat is healed at an angle while sailing.

Overall, I think the Trelino is a better value than the Airhead. It’s roughly half the cost owing to its relative simplicity. That’s still $475 for a plastic box with some buckets inside, though. 

I think the Trelino is the perfect replacement for any small boat with a porta-potty. You could easily camp comfortably on your boat with this thing; the small versions could fit anywhere. The smell factor and ease of emptying and cleaning it are far superior to a porta-potty.  And the Trelino is small enough to keep one onboard for emergencies, like when you can’t get a pump out or your main head is busted. 

Pros of the Trelino Composting Toilet

• Compact sizes work nearly anywhere, including for temporary installations
• Three styles (Evo, Origin, and Timber) are available, and all come in three sizes (S, M, and L) {The difference between the models is the material they are made from: Evo from ABS plastic, Origin from PE plastic with a wood lid, and Timber from wood.}
• Ideal replacement for a boat porta-potty
• You could carry it as a spare should the proper marine head break or you can’t get a pump-out
• Easy to keep clean thanks to hard ABS plastic (Evo model)
• Easy to modify to suit your boat — more on that below
• More frequent emptying means less chance for bug problems (and less mess if it does happen)
• The unique liquid tank lid has a clever membrane closure to prevent sloshing
• The deeper separating bowl means no spills while underway

Cons of the Trelino

• It does not come with a vent fan, so it can smell if you don’t use enough media
• Requires more frequent emptying, especially the small or medium sizes
• Must purchase special black bags to use (available online) plus composting media (coconut coir)

Modifying the Trelino Toilet to Work on a Boat

No installation or setup is necessary to get a Trelino up and running, which is one of its best points. On a boat, however, you really need to screw it down to the floor. The ABS plastic is easy to drill through, and we used four substantial wood screws to hold it onto the platform we had built for our Airhead.

I think there are two modifications every boater should make if they are going to use a separating toilet long-term: add a vent fan and plumb it to a larger holding tank. 

Trelino sells a venting fan kit as an add-on. We used the old fan and plumbing from our Airhead. All we had to do was cut out a hole in the Trelino toilet box that fit the tube. The vent fan is important because it ensures that all stray smells go outside your boat, and it helps keep the solids as dry as possible. It’s not 100% necessary with the Trelino, but it was a priority for us.

Plumbing the liquid tank is a bigger project and greatly depends on your holding tank’s location. Luckily, ours is in the bilge immediately below the toilet. With a few hoses and adapters, gravity can drain the toilet directly to the tank.

The simple design of the Trelino makes it pretty easy to modify to suit your needs. I decided to remove the urine jug and rig a simple plumbing system to let the liquid drain into the holding tank. For the funnel, I found that a 1-liter water bottle fit perfectly (I actually used a plastic white vinegar bottle, which was a little heftier than most water bottles). With a brass hose fitting (3/4″ female garden hose to 1/2″ female pipe thread, 5200 to make sure it never leaks) attached to the bottom, the bottle then attaches to a 1/2″ hose barb elbow that drains into the holding tank.

Why bother plumbing the tank? Carrying the urine tank to the bathroom at the marina only works if you’re at a marina. We prefer to go months between marina stays. By plumbing our head’s liquid tank to our holding tank, we can go over a month between pumpouts, or we can pump the tank overboard if we’re offshore. Pumpouts are a major pain if you need one every 3 or 5 days, but if you need one every 30 or 40 days, it’s not a big deal at all.

Is a Trelino — Or Any Composting Head — Right for Your Crew?

So far, both Lucy and I are convinced that the Trelino is way better than the Airhead. It’s not perfect, but it’s a whole lot better than the old toilet and it looks better in the bathroom. To learn more about Trelino, check out the company’s website. The company ships quickly and has a nice selection of parts available for future replacement and upgrades. You can see a comparison of the sizes and features of the different models here.

One of the best sources for information I’ve found is Peggie Hall’s book Get Rid of Boat Odors: A Boat Owner’s Guide to Marine Sanitation Systems. It covers conventional heads and composting heads, with problems and solutions for each. If you’re struggling with a problematic marine toilet and hoping that a composter is the magic bullet solution, I highly recommend reading this book first. Chances are, by implementing the solutions Hall has outlined, you’ll can get your existing system working better — and with fewer problems — than dealing with a composting toilet.