The Trouble with Composting Toilets

Dave Keenan, 29-Jul-98

[A version of this article appeared in ReNew #65, Oct-Dec 1998, p46 entitled "My toilet is alive!", followed by responses from two manufacturers: Environment Equipment (Rota-Loo) and DOWMUS Resource Recovery. For those of you who have already read the ReNew article, sections which did not appear in the ReNew article have been set in green type below.]

I want to live in as ecologically sustainable a manner as I can afford and I'm prepared to lose a little convenience if necessary, so 6 years ago I researched the theory of composting toilets and the models then available. They seemed the obvious way to go. I decided that Thomas Crapper's flush toilet was a fiendish invention.

So I bought and installed a 4-chamber Rotaloo. A Clivus wouldn't fit and these were the only ones I could find out about at the time. It was supposed to have a 200W heater in the bottom of it which would be turned on if the liquid level got too high, but I elected not to buy it. I wanted as passive a system as possible. I even decided to run the vent fan off a PV module (silly really). The instructions also said not to put fruit and vege scraps into it, but what is the point of having a composting toilet, if you can't put into it what you would put into a compost heap. Since I chose not to follow the manufacturer's recommendations, nothing I say here should be construed as a criticism of Rotaloos in particular.

I live in a sewered area on a 600m2 suburban lot in Brisbane so it was illegal for me to use a composting toilet, but of course I didn't ask permission and was never challenged about it. So it should be obvious that I didn't let any social barriers stop me either. My wife was totally behind it and my (now) 4 year old boy knew nothing else.

The next step was greywater treatment. When we built in under our house we had to install a flush toilet there but we had it drained separately. So now it goes into the sewer while our other wastewater is thoroughly treated on site and used to water the gardens via an automatic micro-irrigation system. Together with the composting toilet this cut our total water usage by about 40%.

With three adults using the composting toilet, as well as our fruit and vege scraps and much crumpled newspaper, we found that a chamber filled in about 6 months and then we would rotate to the next chamber. So after 2 years we needed to empty the first chamber. As promised, we found that the product had little smell and happily put it on our gardens. We also emptied our child's nappies down the chute and so two years later we learnt that nappy liners do not decompose at all! We had to pick them out of the compost to save them blowing around the yard and looking unsightly. It's amazing how little material is actually left after 2 years, it seemed as if it was mostly nappy liners!

We basked in the warm glow of having done the right thing for the environment, but one little problem continued to nag. In fact it nagged so much for so long that I have now decommissioned the composting toilet after about 4 years of use and installed a low flush conventional toilet instead. Through bitter experience I had come to understand that Thomas Crapper's invention was not at all fiendish, but in fact a brilliant solution to a serious problem. What problem? In two words "insect escape".

No matter what I tried over the years there were always times when one could not lift the lid without several flies lifting off and heading for the kitchen. The flies were of two main kinds, the tiny drosophila or fungal fruit fly and some larger wasp-like black flies. Incidentally drosophila can fit through the holes in ordinary insect mesh and even ordinary mesh gets clogged with dust too fast to use it on the vent outlet or inlet.

I tried more newspaper and I tried less newspaper. I tried urine separation with a half funnel siliconed to the front of the chute and polypipe going to a drum which I periodically diluted and poured around the fruit trees. I insulated my vent pipe where it went above the roof, by putting mineral wool and a larger PVC pipe around it. This was essential to stop evaporated water just condensing and running back into the tank in winter. I moved the vent connection from the top of the tank to the bottom, to be near the surface of the liquid that drains into the bottom of the outer tank. I changed to a 240V fan with more oomph. I arranged for air to be continuously drawn through the pedestal by blocking the intake on top of the tank and propping the seat up about 20mm with a piece of foam rubber taped under the front. I hoped that the continuous downdraft would prevent the insects from flying up, but they just walked up the sides instead where the airflow was least.

I then modified the pedestal in what I thought was a very clever way in order to maximise the airflow near the walls of the chute. Maybe some composting toilet manufacturer would like to use the idea. First the seat was returned to the original position so it sealed. The pedestal has its outlet hole somewhat smaller than the chute so the shit doesn't hit the sides of the chute on the way down. In this case the pedestal was a fibreglass shell. I completely removed the flange that joined the pedestal to the chute and simply centered the pedestal outlet inside the chute and sealed both the chute and the pedestal to the floor. I then cut a number of holes with a holesaw in the back of the pedestal (they could have been in the floor) so air could pass thru these into the cavity inside the pedestal and then down through the ring-shaped gap between the pedestal outlet and the chute. This also has the advantage of not being so draughty on the nether regions in winter. Alas it still wasn't enough. It seemed it would need a seriously turbo-charged fan to have any chance of stopping the little buggers. The main problem is that the chute is so much bigger than the vent.

I toyed briefly with the idea of painting the inside of the pedestal black and getting a black seat and lid, to eliminate any attraction to light, but paint would not wear well. In mad discussions with inventive friends we tried to come up with some automatic shutter system, but the complexity and the consequences of failure were always too messy to contemplate.

Now you may be thinking that it's only the Rotaloo that has this problem, and then only when not used strictly in accordance with the manufacturer's recommendations. A 4 chamber Rotaloo without the heater may be a worst case, but even the best, which I believe are the Dowmus range, have this problem. I mentioned the problem to a friend who has a Dowmus and he said "No, I don't have that problem". The Dowmus has a light trap which is intended to attract the flies away from the pedestal. When the opportunity came to visit his place I went straight to his toilet, lifted the lid and said, "What's this then" as a large yellow fly sat there looking at us for a few seconds before taking off inside his house. Now this may have been the only fly that ever escaped from his system but it does seem unlikely. I would guess that most people are not very observant when it comes to looking down a toilet.

Don't get me wrong, it's fine to have all kinds of worms, flies, spiders, cockroaches, a whole mini-ecosystem in your composter but you don't really want them coming out of the pedestal and into your house. Even if I was to be convinced that there was little health danger from flies coming out of the toilet and landing on food (e.g. drosophila go straight for the fruit bowl) how would I convince my guests that it was ok. Guests rarely had a problem with the composting toilet per se, although I did move the light so it didn't shine straight down the chute, and some wanted a tape-recorded flushing sound to really feel that the act was complete. But at one stage I was spraying low-toxic personal insect repellant down the chute just before the guests arrived, and hoping for the best.

Manufacturers will say that if the toilet is working properly, the flies won't breed. I don't doubt this, but the fact is that a composting toilet is a very delicate biological system (mainly because it's such a small one), and it will get out of balance at times no matter how hard you try. There's temperature, oxygen, water content, carbon nitrogen ratio, ammonia etc.

The system we have for greywater treatment involves a septic tank followed by a large area (10m2) aerobic sand filter followed by planted ponds. The thing is, if this system had been made twice as big (20 m2), it would not have cost much more and it could have handled the toilet waste as well. This is what it was originally designed for. The sand filter is practically invisible since it is built by lining and filling a 1 metre deep hole in the ground and it can be finished back to original ground level and, turf (or strawberries, or anything shallow rooted) can be grown on top of it. The ponds are optional and can be replaced by a simple pump chamber. For a similar system, designed by a civil engineer with a postgrad diploma in environmental engineering, try this Google search or these excellent articles by David Venhuizen.

Since Thomas Crapper's day we have learnt to make flush toilets that use very little water, and if treated wastewater is used in applications where it replaces fresh water (it could even be returned to the cistern) then it is not a net user of water at all. The S-bend solves the insect escape problem beautifully, acting as a liquid shutter.

You then have the problem of what to do with your kitchen scraps. Well you could flush them, or get one of those horrible in-sink grinders. But we solved that little problem by getting chooks.

Another problem with composters is that much of the useful plant nutrient goes up the chimney. My plants just love my treated greywater. And of course this is water I don't have to buy from the city council.

Another problem with composting toilets is that, despite the advertising, there can be a smell from the vent pipe, and despite having it well above my roof peak there were some wind conditions that brought it down to ground level. Not very nice for us or our neighbours. Again, this would probably not be noticeable if the system was "working properly".

One aesthetic advantage that is sometimes cited for composting toilets is that there is no smell in the toilet room at all. This is true. With a flush toilet the stuff sits there stinking the place out until you're finished, but with a composter even your farts are immediately whisked away by the action of the vent fan.

It should be noted that there may be some health-diagnostic value in briefly smelling, and looking at, your own shit. Humans probably did this for hundreds of thousands of years before the invention of any kind of toilet (or any kind of health).

However the same aesthetics might be achieved for a flush toilet with the addition of a small fan and a T-junction coming off the pipe that goes between the cistern and pedestal, and going out through the wall and up. The fan draws air from the bowl, back through the path that the water takes when you flush. The fan would be outside, above the cistern height, so it doesn't get hit by the flush water. Although it would use very little power, even running continuously, it could be controlled by a switch that is operated by pressure on the seat. A witty friend suggested using a "strain gauge" and another said that what it really needs is a "motion detector".

So, from my experience, I cannot recommend composting toilets to anyone, unless they have a serious water shortage, and they live in a non-urban area, and they locate it outside their insect-screened house envelope (on a verandah would be fine). Since greywater treatment is required anyway, why not save money and use the same system (only larger) for treating toilet waste as well. Although Dowmus, and possibly others, offer a flush "composting toilet" that you can put your greywater through, I remain to be convinced that they can achieve anywhere near the efficiency and resilience of a 20 square metre aerobic sand filter.

I recently learned that Thomas Crapper, the man with the memorable name, was not the inventor of the flush toilet. He did however install the first "royal flush" for Queen Victoria. You can read about it yourself on the web at

Footnotes [added after ReNew publication]:
It is clear, since replacing the composing toilet with a low flush, that almost all of the 40% saving in water usage comes from greywater reuse and not the composting toilet.

Here is a dry toilet that may overcome some of the problems mentioned above: No endorsement is implied.

Carol Steinfeld tells me that many of the issues above are addressed in her 1999 book The Composting Toilet System Book: No endorsement is implied.

In 2012 Angela Bivens kindly wrote "I'm not sure if you meant it to be, but it was hilarious and I found myself laughing throughout the article. Since it was written a little more than 14 years ago, you may have tried some of the newer composting toilets like the foam flush composting toilet, which I am interested in because it would clean the bowl - a topic which, irritatingly never comes up by those marketing their toilets." Angela also suggested the following article re insects and composting toilets No endorsement is implied.

Thanks Angela, I was chuckling to myself the whole time I wrote it. The subject matter somehow lends itself. We have stuck with the low flush conventional, plus greywater treatment and reuse, but your suggestions may help others. Thanks.