A 'shed on stilts', our faeces are dropped from a great height straight into huge portable plastic IBC tanks which can be shifted around underneath the superstructure and then transported by fork lift trucks, tractors etc. and used for growing specific fruit/vegetables.
Since building this structure, I have learned that most systems either use a very undersized tank for collection which needs emptying too often or no tank at all where the waste seeps into the ground and is accessible to all kinds of vermin and household pets. I myself believe that it is important to give proper attention to waste that we produce and process it in the best way possible.
Actually, this building is very much more than just a dog proof composting toilet as it also has a solar shower system, a garden tool store, dedicated rain water and grey water stores and even a communal kitchen area. The total floor area is 5.5m x 2.4m and the toilet itself is just a mere1.2m x 1.5m. For stability, the floor plan needs to be at least 2.4m x 2.4m or else it will be very susceptible to the wind and would look just plain stupid!
As far as design challenges go, the first question was: 'Would the hole in the IBC top be big enough for the waste?' I really wanted the waste properly contained in a plastic tank and really wanted to be able to screw the lid back on and maybe even collect the methane gas produced. The second question was 'How to brace the front of the stilts whilst allowing the IBCs to be able to be removed?' The structure would surely be a bit wobbly without such bracing. Third question: 'Would I need a pallet truck to move the IBCs on the concrete pad?' or 'Could I find a cheap second hand pallet truck?'. IBCs can easily be moved with pallet trucks, providing that the concrete is not too rough. Basically, an IBC is a 1,000 litre plastic tank in a steel cage on a pallet.
Part 1 of the project can be found here: https://www.instructables.com/id/The-Easy-Empty-Composting-Toilet-Project-Part-1-Gr/
Step 1: Design Features and Timber Profiles
- No nasty smells
- No flies
- No rats
- No flushing with water
- No splash-back
- Easy to empty and keep clean
- Environmentally friendly
- Waste is recycled
No calculations were made when designing this structure and all timber sizes are based on previous experience building similar structures.
Disclaimer: Check your local building codes and/or employ an architect/engineer to calculate loads and timber sizes.
The structure is 3.8m high, 2.4m wide and 5.6m long. It has 6 main upright posts of 100 x 100 mm which are anchored to the ground by welded steel post sockets set in concrete.
- Upright posts: 100 x 100 mm
- Floor joists: 100 x 50 mm spaced at 500 mm
- Floor: 18 mm plywood
- Roof rafters: 100 x 25 mm spaced at 500 mm
- Roof battens: 50 x 25 mm spaced at 400 mm
- Ridge board: 150 x 50 mm
- Wall plates: 150 x 50 mm
- Horizontal braces: 100 x 50 mm
- Diagonal braces: 100 x 50 mm
- Door frame (front): 100 x 50 mm
The critical factors in the design and construction are:
- Wind load
- Floor load (number of people standing on the floor at the same time)
- Roof load (roof must support weight of snow and people working on it)
- Corrosion (metal post holders must be protected with bitumen paint)
- Hygiene (the kitchen area must be separated from the toilet by at least one doorway)
- Ventilation (there must be windows or fans in the kitchen and toilet)
- Removable bracing on the front below the floor (not shown) to get access to IBC tanks.
- Removable steps for access to IBCs.
Step 2: Uprights and Floor
A couple of empty IBCs were positioned on the concrete slab to help support the upright posts and the floor joists were attached using 150 mm nails with temporary diagonal bracing.
Step 3: Diagonal Bracing
The floor joists are finished and some permanent diagonal bracing installed.
Step 4: Floor Boards
Floor boards are positioned and cut to size such that they butt together half and half over the floor joists. The edges of the floor MUST be supported or the floor will collapse.
Step 5: Toilet Construction
It was important for us to get this toilet working ASAP so the next stage was the tube and the frame for sitting on. The tube is constructed from a 12" plastic funnel and some flexible chimney pipe, which just happened to fit really well into the IBC top hole (ID 140 mm). After creating a rough surface and cutting off the funnel's spout, the two components were 'glued' together using copious quantities of glass fibre sheet and resin.
The tube was then positioned in a sheet of 18 mm plywood on top of a wooden frame.
Now it was time to start testing the system ........ Thankfully, it worked just fine!
Step 6: Cubicle
Not wanting to distress the neighbours, we built a cubicle for privacy.
Step 7: Wall Plate, Rafters and Ridge
The rafters are joined together with a horizontal brace and some pre-drilled metal plates on the floor and then positioned over the wall plates using clamps. Notice that there is a temporary piece of timber slotted in where the ridge timber is going to sit. If this stage is done carefully, the ridge timber itself is really very easy to install and it just slots in place with no effort at all.
Step 8: Battens Are Nailed Onto the Rafters
2" x 1" battens are nailed onto the rafters at 400mm spacing. The nails should be galvanised and thin enough so as not to split the wood and nailed in 2 per joint at opposing diagonals or else the wind will pull them off.
Step 9: Roof Box Profile Sheets Screwed On
Self drilling roofing screws with washers are used to fasten the box profile steel roofing sheets onto the battens.
Lastly, a ridge profile sheet is added along the top of the ridge.
Step 10: Steps
The steps were made by welding 40 x 40 x 3 box section. Firstly, the sides are laid out on the floor as in the photo and then put together with the horizontal sections.
There are all kinds of regulations concerning steps and it's important that they are uniform so that people do not trip up and fall off. Some steps need railings. These steps are slightly steeper than 'normal' and each one is 250mm high and 250mm wide.
Step 11: Doorway
The whole front of the structure is going to be clad with waney edged timber, but before we do this we must create the doorway and supports for the timber.
Step 12: Cladding the Front
This is the really fun part. Waney edged larch is nailed to the front, making sure that the nails go into the support timbers and not just into thin air the other side!
Starting at the bottom, the cladding is cut extra long and trimmed down afterwards to size, making sure to mark off where the doorway is with a marker pen.
Lastly, a chainsaw is used to very carefully cut out the door itself.
Step 13: Finishing the Doorway
The door is clamped back in place tight against on the hinges and the top frame, with the clamps on the inside (not seen). Next, the hinges are screwed on and the door is opened to check that the cladding does not interfere with a full open swing or the hinges will get broken. I used the chainsaw to remove small slithers of cladding that were stopping the door from fully opening.
Step 14: Painting and Other Finishing Jobs
The metal posts will eventually rust if they are not protected against moisture effectively. Here, I have used a very liberal quantity of heavy bitumen paint to keep the metal work protected.
Step 15: Using the Toilet and the Compost
The main thing to be aware of with this toilet is that it does not accept urine. Past experience has shown that urine adds far too much moisture which then takes literally years to dry out properly in the IBC. Some systems have separate chambers and clever urine traps and diverters, but I did not want to have any of this as, sooner or later, it's inevitably going to get clogged up and need cleaning - YUK - no thank you! The bulk of the urine needs to go elsewhere, in a separate toilet going into a grey water collection tank, which, by the way, really does smell bad!
If the toilet for faeces is used properly, it does not smell at all and with this design there does not seem to be any flies or rats ...... yet!
When the IBC is full, it is transported away from the structure with a tractor or fork lift and stored outside in the sun with the lid unscrewed, but not allowing rain to get in. The moisture level inside starts to reduce and the bacteria start to make compost from the top downwards. Eventually, the whole IBC will be full of nice dryish fluffy compost.
Now, the top of the IBC needs to be cut off with a 9" grinder and the compost shovelled out. It can be used as a general soil improver, but not for potatoes or any other root crop. Rhubarb or squashes would be ideal as they love very rich soil. Tomatoes are another good one. Some people put the compost around fruit trees which is ok as long as the fruit does not fall off the tree onto the compost and then get eaten by a human.
The photos above show some very 'fresh' compost which looks pretty nasty and some 'mature' compost with plants growing out of it.
Step 16: Have a Poo Party!
Inauguration ceremony with the neighbours and cutting of ribbon to the entrance. The compost toilet was given an overall score of 9/10, which I was very pleased with.