DIY Compost Toilet Installation – Part 1

When we decided to install a composting toilet in our shed, the easy option would have been just to get the luxurious Separett Villa 9010 (we have a 12 volt, solar-powered battery in there already), or even the functional Separett Weekend, but in an effort to understand what our customers might have to do, I opted for the low-cost Separett Privy 500 (the Privy 501 would be exactly the same from an installation point of view).

To Ventilate or Not?

The ventilation system I’ve chosen will be shown in the next part (or the part after), but I wanted to explore some of the options and reasons here before I started showing you the big build!

The first question was whether or not we needed additional ventilation. The Separett Weekend and the Villa 9000 / 9010 all come with a built-in fan and ventilation pipe. As you’ve probably read elsewhere on our website, the main function of the vent fan and pipe is to help dry the contents of the ‘solids’ bucket by creating an air-flow over the top (this creates a slightly lower air pressure in the bucket which will help pull the moisture out). This drying or moisture control, along with the fact that potentially smelly air is also extracted, means that these loos do not smell in normal everyday use and are therefore a pleasant experience for people to use.

The contrast to this is the ‘Loveable Loo’ that was pioneered in the USA by Joseph Jenkins – his approach is not to separate the ‘liquid’ from the ‘solids’, but to keep it simple and let everything go into one bucket. Importantly, you must use a lot of cover/soak material (he recommends sawdust from untreated wood) after each visit. Now this is all well and good and means the ‘loveable loo’ is extremely simple to make and use, but you do need a lot of cover material on hand at all times, and the physical bulk you create means that the bucket will need emptying every few days, and you’ll also need a large compost heap to put it all on.

The main issue of smells with compost toilets (of any type) is usually from mixing liquids and solids together, so separating is a step in the right direction (by separating, the volume of waste in the bucket is one fifth of the volume if everything is mixed together).

Initially I planned on not using any cover material (just using the fan, like the Separett Villa does, however, I also wanted to experiment with leaving the fan off for periods of time, so I would be using some woodshavings as cover my poo.

Unless your outhouse (or your intended location) is well ventilated and has a good draft through it, I would recommend some additional form of ventilation – the simple test is to poo into a bucket and leave it in your intended location for a few hours and see if you can smell anything – chances are you will, and that it won’t be too pleasant! Using extra cover material will certainly help, but the idea of the Swedish Separett models is to keep the bulk of the solids down, and hence the need to empty the bucket as little as possible. My shed is well insulated and doesn’t have any air flowing through it naturally…

It’s my intention to connect some standard drainpipe into the back of the toilet box, which will lead out through the back of the loo, through the shed wall and then go up a couple of metres. I’m planning on using black pipe, so my hope is that when the sun is out, the black pipe will warm quickly and create an updraft, negating the need for a 12 volt fan. We’ll see if it works without the fan, but if not, fitting one to my 12 volt battery etc should be straightforward enough.

[Update – the power of the sun on a small pipe has a negligible effect!]

Sizing Things Up

There are 3 things which will determine the size of the box that will become the toilet. First is the size of the bucket you will be using to collect the solids, Second is how you will access the bucket (ie will it have to come out from above, the side or behind of the loo) and Third, the route that the urine pipe will go.

In my situation, the loo box is going against a shed wall and the urine pipe, along with the vent pipe will come directly out of the back and through the shed wall. After thinking long and hard, I decided I’d run the urine pipe into the ground to drain in a soakaway. I had initially though about capturing urine to use as a garden fertiliser, but it can get a bit smelly if you don’t use it straight away, and if I need some, I can always wee into another container.

So, from underneath the base of the Separett Privy 500, there is a urine outlet which connects to the 2 metre flexible pipe. My intention was to have a portion of the lid of the box hinged so that when the bucket needed emptying, you would hinge up the lid and the pipe would be flexible enough to enable the bucket to come out. Once emptied, the bucket could be put back into place and the flexible urine pipe would be pushed to one side, around the bucket.

That all seems straightforward, so with the Privy 500, bucket and tape measure to hand, I worked out the absolute minimum width, depth and height the loo box could be. I wanted it as small as possible because firstly, my shed is quite small, and still needs to be a useable shed, and secondly, wood is expensive, so a smaller loo would cost less to make!

Initial drawings

I decided on the height by placing the Privy 500 over the bucket and moving it around until I got what I thought was the best place and measuring it. In my case, I decided on 43cm – you can go much higher of course, but you will need to consider a step or footrest for anyone with short legs! The width was easy as it’s based on the minimum you need for the Privy 500 or the bucket itself – in my case I chose the minimum internal dimension of 42cm (later discovered this was too narrow). The depth was the trickiest – I’d decided that I wanted the box lid hinged, but not right at the back – this was for strength as there would be a piece of wood across the box at the top, to the rear, screwed down and the hinges would be attached to this. The size of the resultant opening would need to be big enough for the bucket to be lifted out without fouling the urine pipes etc.

Outer panel dimensions sorted.


Having got the sizes sorted, I was really pleased to discover that some old shelves I had would be OK. The shelves were actually part of a very deep shelving system from IKEA and were surplus to requirements. The wood was solid (made up from smaller pieces glued together) and although it wasn’t too strong, used vertically, the weight would be coming down at the woods strongest point so should be fine. You could use plywood or other suitable panels. I wouldn’t recommend MDF and possibly not chipboard as they’ll deteriorate rapidly if there is any damp. I bought a length of 25mm square planed softwood to use as a bracing in the corners and to provide support under the seat hole as that’s the weakest area for the ex-IKEA shelves.


The boring part is then marking out, measuring, checking, checking again, and then cutting all the wood up to the right sizes. Having done that, I assembled the four sides of the box by glueing and screwing the pieces together, along with the supports in the corners. I used an strap clamp to keep it all tight and square as the glue set, but you might want to use metal corner braces.

The top, as mentioned before, would be in 2 parts with the rearmost part screwed to the base to give extra stability and the forward part hinged and cut out to take the Privy 500. I made the overall piece around 1 cm longer than the base so there would be an overhang at the front – this makes it easier to grip and lift.

Compost loo carcass Showing wooden strip reinforcement Corner reinforcement

Before you cut the hole for the separating part, check and check again it’s position – I wanted the urine pipe to be as close to the front of the box as possible, but at the same time, not so close that the pipe would foul the box. Having worked it out, I made the cut, added some reinforcing strips and screwed the Privy 500 into place.

Privy 500 screwed into place Privy 500 seat in place Privy 500 seat and cover in place

Excitement and Disappointment

At this point, I must admit I’m getting quite excited! I have a completed toilet box with a lovely urine-separating seat/bowl and it looks like we’re on the home straight.

I put the bucket inside and realise that I’ve made things very tight for this bucket. In fact, the bucket needs to be turned so that the protrusions where the handle is are front to back. OK, that’s not a problem – I can live with that (note to self – next model needs to be slightly wider). Next issue is a non-issue really, just an observation that the box is deeper (front to back) than it needs to be – there’s a lot of wasted space behind the bucket.

Next problem sort of combines with the issue of the box being too narrow – the urine pipe. The flexible urine pipe needs to lay to the edge of the box and run by the side of the bucket – due to my mistake with the width, the only way the pipe will fit past the bucket is if it goes right at the bottom (the bucket tapers in slightly towards the base), but I need to ensure there is a gradual and continual downward run for the pipe, or the urine will collect in places and you’re likely to get some smells coming back up the pipe. I was thinking I could use some clips or something to secure the pipe to the side of the box and maintain the slope, but then the next issue hit me – I need to have enough pipe at the front so that the seat can be hinged upwards to put in/take out the bucket. That’s fine in principle, but in practice, that leaves a lot of pipe, which is hard to deal with when the lid is down and you want to make sure you don’t have a dip where stuff can collect.


I’m now beginning to get frustrated as I can’t see a way of dealing with the urine pipe issue, whilst maintaining good access to the bucket. I suppose the lid could lift up rather than hinge, or if the bucket was accessed from the rear or side, then the urine pipe would be less of an issue, but the way I want it is proving an issue.

Taking the Piss

I eventually accepted the fact that the flexible pipe is not going to work with a hinged lid and with the dimensions of the box I have (a taller box would make it much less of an issue), so I set about thinking of alternatives. I went to a DIY store and spent some time looking at all the plumbing bits and eventually thought of a way that might work!

Alternative urine catcher – click to enlarge

The urine out part of the bowl (where the flexible pipe connects) could, in theory sit over and/or in another, wider pipe which would act a bit like a funnel. The rest of the urine pipework could connect to this ‘funnel’. I eventually found domestic ‘waste’ pipework in plastic – a 40mm 90° bend piece was wide enough for the urine exit outlet from the separator to sit in (with the flexible pipe supports taken off) – it gives enough play to allow for some movement. That goes into a 40mm to 32mm reducer and from there, rigid plastic pipes go around the corner and eventually connect to the original Separett flexible urine pipe (using copious amounts of silicon).

Obviously, this pipework/funnel needs to be accurately placed and secured to prevent liquid malfunctions, and in my case would need a small block of wood between it and the box to position it correctly. The additional problem here was that this now effectively narrowed the gap into which you placed/removed the solids bucket. I realised that although it was possible to use it, putting in an empty bucket is not the same as trying to carefully remove a bucket full of …. well, you get the idea!


I now accepted that prototype number 1 was simply not a workable solution and I needed a box that was wider and where the hinged part of the lid was deeper to enable easier access to the bucket.  Fortunately, I managed to salvage most of the wood and the only waste was the top part of the box.

In part 2, I’ll go through the final dimensions and assembly, fitting the urine funnel and pipes into place and so on…

Read Part 2


Posted on 7 Comments

7 thoughts on “DIY Compost Toilet Installation – Part 1

  1. […] in Part One, we ended the blog post with the realisation that a few mistakes had been made! No problem, I […]

  2. […] bedoeling is. We hebben beslist om dat niet te doen en volgden het principe dat de leverancier zelf op zijn blog had getoond. Bij het composttoilet zit standaard een opzetstuk als WC-bril + deksel in polystyreen. […]

  3. Hello Martin,
    A while ago I bought the Separett Privy 500 in your store.
    I wrote a review about it, we are really happy to have bought and installed it. We are using it for now as a temporary toilet while we are building our house 🙂
    The blog is in Dutch, but here you find the link:
    Kind regards, Isabel

    1. Thank you Isabel. I have looked at your website using Google Translate to read it in English – your toilet looks brilliant!

  4. Great story on how to build this toilet. In the USA we build small campers into cargo trailers converted to campers. Thanks for sharing & great you tube video.

    1. Hi Jerry. Thanks for the comments – your cargo trailer campers look great!

  5. […] you buy the seat, DO NOT use the minimum requirements. This is our fix and another one can be seen here. The minimum requirements will make you a cute little unit, but you will not be able to remove the […]

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