DIY Compost Toilet Installation – Part 2

So in Part One, we ended the blog post with the realisation that a few mistakes had been made! No problem, I always knew this was a prototype and that learning by doing (and making the mistakes) would help me understand the whole process better, but there’s no getting away from the fact that it can be frustrating at times!

To briefly re-cap, the two areas I had problems were:

  1. The width of the box was too narrow and,
  2. Because of the way I wanted the top to hinge, the flexible urine pipe wasn’t going to work, so I was looking at another method of coupling things together.

Fixing the width turned out to be straightforward due to way I had made the box, and I managed to re-use the four pieces of wood I had previously used for the sides. I got the extra width I wanted because the sides were previously attached to the back of the front section – attaching them to the side gained me 3cm in interior width. I needed a new top piece as the box was now slightly wider, so this was duly measured and cut. Sorted!

Now onto the urine pipe. In Part One, I explained that I had found some plastic plumbing pipes and joints (domestic sink waste pipe). I concocted an arrangement of a larger 90 degree elbow (40mm pipe diameter), going into a reducer (down to 32mm diameter), some straight pipe, another corner piece and finally a straight coupler, into which the flexible pipe would go, sealed with copious silicone sealant.

Where the supplied flexible pipe fits onto the base of toilet bowl, there are two protrusions – these needed to come off…


In my naivety, I had not realised that the two types of fittings (push fit and solvent weld) were not interchangeable due to the plastic pipes having slightly different outer diameters (the inners are the same), so there was no way my solvent weld corners would fit with the pipe I purchased.

You know you get to a point and wonder if it’s all worth it? Well I got to that point and seriously considered just taking a complete unit like the Separett ’Weekend’ out of stock and just fitting that.

I eventually decided that would be cheating and I would not let these setbacks defeat me…


So back to the DIY store and with the correct pipes in tow, I managed to complete the pipe work. It was attached to a piece of wooden batten that enabled me to get it the right distance from the front, and hence perfectly line up with the urine outlet from the Separett Privy toilet bowl. Some cable ties and other bits of strapping held it all in place. The important thing is to make sure the urine pipe is always flowing downwards so nothing can settle – hopefully, no nasty urine smells will come back up as nothing should be ’sitting’ in the pipe and going stale.

If you do get any urine-type smells coming back up (either due to urine ‘sitting’ in the pipe, or simply crystallising in the pipe), we sell Eco Urinal Tablets in packs of 5 or 10 which you place in the urine bowl – these contain natural enzymes which stop the urine crystallising and are scented too.

If you look at the pipe I’ve used, or indeed any of the pipes that come with Separett toilets, you might think that are very large bore. I don’t know exactly the reasoning, but I’d guess that a large bore pipe will have no chance of getting air locks and the like, so it ensures smooth and free-flowing drainage at all times. [Update – the reasons for having larger bore pipes are 1) to help prevent scaling and build up of urine crystals, which can eventually block the pipe, and 2) ensures the urine flows freely even when the outside temperature is below freezing.]

Putting it in place

I’m now at the point where I’m ready to install the toilet in my shed and decide on the final position, bearing in mind where the urine outlet pipe and vent pipe will go.

I used a hole cutter attachment on my drill to cut an appropriate hole for the flexible pipe (32mm) and ran it through the shed wall and to the outside. I then rammed the end of the flexible pipe into the straight connector pipe using loads of silicone sealant (I really didn’t want a leak!).


A small soak pit was dug in front of the shed and I ran the pipe into the pit and put in some gravel etc to complete that part.

Ventilation – the final part

The final part is the ventilation, and I have to say, this is the part that can make or break the experience that someone will have using your toilet! Nobody wants to sit on a toilet or be in a room with strong smells of shhh… you-know-what (shall we just call it ‘poo’?), whether they are your own or someone else’s!

If the chamber that holds the poo bucket is well ventilated (i.e. has a good draft blowing through it) and/or is open to the elements (needs to be screened ideally to stop flies etc), then maybe you can get away without any mechanical ventilation. I’ve used ‘long-drop’ compost toilets in the past, and they are generally OK, but every now and them, you will get a whiff – some people are OK with this (part of the ‘experience’), whereas others are not. As mine would be a demonstration unit for family and friends, I wanted it as smell-free as possible.

My original plan was to have a standard 68mm black plastic drainpipe coming out the back of the toilet box (at the height of the top of the solids bucket), run it through a 90 degree bend and take the pipe up, and above the eaves of the shed. My hope was that air movement over the pipe outside, plus the act of the sun heating the pipe during the day would create an updraft which would take away any smells and help dry the contents of the bucket.

The more I thought about this, the more I thought it wouldn’t be effective in a shed. Final confirmation came when I was testing the toilet out before I fitted the vent – I was so eager to give it a go, I made a ‘solid’ deposit in the bucket. As you’d expect, the shed had a certain ‘aroma’. I put the lid on the bucket and still found that it was hard to shift the smell.

In the ‘Humanure Handbook’, Joseph Jenkins composting toilet is based on no ventilation, but instead uses a cover material (usually sawdust) after each visit. I’ve not used this myself, so can’t comment on its effectiveness (perhaps you could let me know if you have, and what it was like?). According to the manufacturer, all the Separett models that come with a fan, don’t need or use any cover material at all. In a future blog post, I’ll expand on this and the times when a little cover material might be beneficial.

Plan B was to fit a small 12 volt computer fan to the toilet box, in front of the vent pipe exit, to create a forced airflow through the vent and out. Fortunately in my shed, I already have a solar panel and 12 volt deep-cycle battery to power the lights etc, so I ran an appropriate wire from my battery to the toilet box and bought an 80mm fan from Maplins (electronics accessory shop) and fitted it. They cost around £6.


I thought an 80mm fan would be fine sitting over a 70mm hole, but when powered up, I was getting a lot of buffeting from the fan where parts of the blades where forcing air against the wood around the hole. I needed a smaller fan! Back to Maplins and I purchased a 60mm fan and made a wooden mounting plate for it to fit on.

It worked – no buffeting! The only problem was noise. I was quite surprised at how loud the fan was. I know from fitting other Separett models that come with built-in fans, that they are virtually silent, so was it something I had done wrong or was it the fan itself? I’ll come back to the noise issue later…

In use – does it smell?

Anyway, the toilet is now fitted and complete! The urine pipe runs out without leaks, the vent pipe and fan seem to be working so time to test it fully! I used the standard Separett Compostable Bag Liner and put about an inch of ‘coir’ (waste product from coconut production) in the bottom to act as a soak. After some performance anxiety, I managed to make both liquid and solid deposits – so far so good, but what about the smell?


After leaving it overnight, I ventured to the shed the next day and opened the door to find… no smells! Seriously, absolutely no smells whatsoever! Not even a faint whiff, nothing. Keen to see whether this was a one off, I continued to use the composting toilet exclusively and still nothing smells. Even visitors comment on the lack of smell, and some are actually keen to use a composting loo!

Overjoyed with the success, my final hurdle was to address the noise of the fan. Some research online and I found that it’s possible to buy ‘silent’ 12 volt computer fans, although they are a lot more expensive than the Maplin ones I bought earlier.

In part 3, I’ll cover the fitting of the final fan and report back on a slightly longer term experiences of using the loo, along with the different types of ‘soak’ and ‘cover’ materials.

Read Part 3


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7 thoughts on “DIY Compost Toilet Installation – Part 2

  1. Very helpful, probably make my own mistakes and trials but interesting blueprint, thanks

  2. Will try without fan, using coir. havent got cheap shed for it yet.

  3. Try to reduce all pipe bends. You’ll see.

  4. Hi IW – I assume you mean on the vent pipe? I agree that the best performance is from the shortest pipe with the fewest (if any bends) – in my situation, I have to have the pipe do a 90 degree bend as it exits the shed. I did think about just venting straight out, but if are gardening near the shed, you’ll be well aware of the smell, so I decided up, up and away. Now I’ve had it all fitted a couple of weeks, the noise is a non-issue with my new fan.

  5. […] DIY Compost Toilet Installation – Part 2 October 22, 2012 […]

  6. So what were the final dimensions of the box? Trying to construct my own and it would be helpful.

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