Making and installing a urine-diverting compost toilet, Part 1

I’ve revived a series of posts from my old website from around 2013’ish because I knew from the feedback I got, that they were extremely popular and well received.

When I started a business (no longer operating) back in 2012 selling Separett waterless toilets, I knew that I had to be a user of them too – only then, could I say with any conviction, what they were like to use or manage.

In this series, I had used a Separett Privy urine-diverter, although the same principles apply to any make of urine-diverter – you just need to make sure the various parts line up.

So read on and learn how to build your own compost toilet!

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Having made the decision to install a composting toilet in my shed, the easy option would have been just to get the luxurious Separett Villa  (I have 12 volt, solar-powered battery in there already), but in an effort to understand what our customers might have to do, I opted to make one myself, using the low-cost Separett Privy 500 (the Privy 501 would be exactly the same from an installation point of view) urine-diverter as the starting point.

To ventilate or not to ventilate?

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

The first question was whether or not I needed additional ventilation. The Separett Villa (my main comparison product) comes with a built-in fan and ventilation pipe. The main function of the vent fan and pipe is to take away odours and excess moisture (poo is 80% moisture). By doing this, the toilet does not smell in everyday use and is therefore a pleasant experience for people.

However, the pipe and fan add a layer of complexity. You need electricity etc. The alternative is just to use a cover material, but then you potentially have issues with moisture build-up (and condensation).

The contrast to this is the ‘Loveable Loo’, 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(s) to put it all on and process it.

I decided 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.

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 is put back into place and the flexible urine pipe 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!

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 of 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 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.

Materials

So my sizes are sorted and I found I had some old, deep shelves that were perfect for repurposing. The shelves were actually part of a very deep shelving system from IKEA and were surplus to requirements. The wood is solid (made up from smaller pieces glued together) and although not especially strong, the weight is coming down at the strongest point so should be fine.

You could use plywood or other suitable panels. I wouldn’t recommend MDF or chipboard as they could 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.

Assembly

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 a strap clamp to keep it all tight and square as the glue set, but you might want to use metal corner braces.

The top is in 2 parts with the rearmost section screwed to the base to give extra stability and the forward section 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.

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.

Oh, the 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… Why didn’t I realise this earlier?

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. Grrr.

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!

The urine outlet part of the bowl (where the flexible pipe connects) could in theory sit over and/or in another, wider pipe – 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 out of the bowl 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!

Accepting failure and moving on…

I now accepted that prototype number 1 was simply not a workable solution and I needed a box that was a) wider and b) slightly deeper on the hinged part of the top 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…