The 'Easy Empty' Composting Toilet Project: Part 2 - Superstructure




Introduction: The 'Easy Empty' Composting Toilet Project: Part 2 - Superstructure

About: Ugly pirate roaming the seas in search of Treasure.

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:

Step 1: Design Features and Timber Profiles

Operational features:

  • 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.



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    28 Discussions

    It would be interesting to see what the survive-ability and benifits of worms might be????

    4 replies

    Yes, I'm trying to think of ways to accelerate the decomposition process but first I think the waste needs to dry out quite a bit so I'm thinking maybe some sort of fan and using cardboard for it's wicking ability?

    I doubt they can handle 140deg but I know they can handle at least 110deg in some parts of Idaho and Arizona. In the Seattle area they can handle at least 40inchs of rain a year, but in Idaho they come out of the ground and may drown if irrigation submerges them for more than a few hours.

    In Seattle I had students build a 4'x4' compost box and the worms could consume about 4" of waste per week - so they are VERY efficient. I saw a commercial plant that had worms process about 6" per week in 10x40ft bins 4

    I found this on ...

    "Another problem with fresh manure, or any composting material, is that it goes through an initial hot composting
    stage. In other words the composting process generates heat. Large quantities of fresh composting materials can
    actually heat up a worm bin enough where worms will try to escape or worse die."

    "We find that pre-composting for a about a week works well for manure. We also pre-compost many other foods we
    intend on using for worm food. One exception is material we add to our compost tumbler where it is allowed to
    compost for weeks, or months on end. This composted material is a key part of our preferred bedding mixture."

    Worms love cellulose (wood & paper) and table scraps, so a handful of sawdust or or a few newspaper pages every day or two may be all that's needed (toilet paper may even be enough). They naturally climb to the soils top so if they can stand manure, they will always be at the surface of the pile. Off hand I would try gravity feed of the liquids to a tank below the solids. As any motor home owner will tell you, yeast will turn manure and toilet paper to sludge in 2-3 days; so I am thinking that you could let the yeast eat the liquid and figure out the optimum way to feed the worms the solids.

    There must be campers that have figured out how to make waste pathogen safe - so that has probably been solved and an Internet search might find it. I remember some Mother Earth articles but don't remember what was said.

    Our way to enjoy this adventure is a bit different but at least we don't need to go outside for it. It means emptying the buckets every day and having an unusual morning walk, but it's working anyway.. Great project, looks cosy!

    1 reply

    Thanks! It's certainly a great feeling to be recycling this valuable commodity. The shed is quite cosy although a bit draughty.


    2 years ago

    Very interesting project! We also try to reuse human waste, but I heard you shouldn't use human waste, since it wont destroy viruses and diseases, it helps them to propagate, I would like to learn about it more...

    10 replies

    Read Joseph C. Jenkins "The Humanure Handbook" for the lowdown on eliminating pathogens in 'humanure'. He would cringe with this system as there is no significant heat created in an IBC full of feces to kill harmful pathogens. I don't think anyone would call this composting. Note that the author warns of using end product on any food that might come in contact with it. What does that tell you about safety of it? I wouldn't want to use it anywhere near anything I was producing for food.

    From memory, I think the temperature required is 158 degrees Fahrenheit, but as you mentioned, should be checked.

    I've got one full IBC here already but, during use, it was severely abused with far too much urine, many large sacks of sawdust, grass cuttings and even an old carpet! For a whole year, it bubbled away steadily and now it's starting to go dry and is a good habitat for plants. I think the best way to dry it out is to get something really big and fast growing established in it like a butternut squash or two which will suck out loads of moisture and look quite attractive. I don't know if I'm brave enough to eat the fruit or not.

    I think that the next IBC will take a year to fill and that's with no sawdust or urine etc - just 99% poo. A bit of cardboard might be useful for it's wicking capabilities. It will start to dry out with natural ventilation and then I reckon one year in storage and then grow plants in it for another 6 months.

    There's probably all kinds of ways of mechanically accelerating the drying process - an aquarium bubbler or two with some long tubes might be useful?

    Once the beneficial organisms and plants have done their job I'd consider blasting it with heat. By this time the compost will be fluffy so the heat could be in the form of hot exhaust fumes from burning wood piped into old steel oil drums or something custom made. I have steel tank building skills and capabilities so could knock up a 1,000 litre steel drum fairly easily. Stick that over a wood fire and rotate it slowly like a hog roast, monitoring the internal temperature, YUM! (Just ideas. In the UK the word 'could' does not mean 'will')

    So, I'm thinking 18 months to dry out naturally which could be reduced to 6 months with mechanical assistance. I'd then want to be sure that 100% of my compost was safe and probably not rely on internal naturally generated heat from doing the job. Unless the IBC was well insulated, the heat distribution would be fairly poor. In any case, I think some DIY temperature probes would be a good idea and they're cheap and easy to make - we have the technology.

    It's really great to bounce some ideas around, which is why I love this website!

    "Just ideas. In the UK the word 'could' does not mean 'will".... I can totally relate to that! good luck... will be following your ideas/progress.

    Hello Blender, thanks for your comment! I would be very wary of any of the 'popular' literature to do with composting and pathogens and you'd proberly need a PHD in compostology to really understand it. Just because a book is popular does not mean it is correct.

    As hsteinbe mentions in the comment below, human waste is routinely pumped into the soil in fields in the form of fairly nasty liquid sludge. Whether this sludge is heat treated or not, I do not know. Whether it is good for the soil structure I would assume not.

    It might well be the case that heat IS produced in the IBCs as, for one, it is quite a large volume to surface area so heat could easily be generated when the moisture level gets near the optimum level. I guess I should bury a load of temperature probes in it to see what is happening?

    Joseph Jenkins does have some credentials, including a Master of Science in sustainable systems. Regarding hsteinbe's comment, to my knowledge any septic systems contents first goes through a municipal wastewater treatment facility before it is turned out as sludge, unless it's done illegally, or perhaps some municipalities have very lax laws. I'd be curious about the heat generated in your system too as anaerobic composting tends to be cooler and will not kill weed seed or other plant pathogens, not to mention human pathogens. Thanks for your efforts.

    I think that there is probably an easy way to pasteurise my IBC contents to make it 100% safe to use. At the moment it is safely contained within the plastic container and is drying out nicely with a few plants growing in it. When the composting process has finished and it has all turned nice and fluffy I could shovel it into some kind of wood fired pasteurising chamber with an array of temperature probes inserted. Another exciting Instructable possibility!

    As I understand it, and I need to re-read the specifics on temperatures, during a thermal aerobic composting the temps can get too high and will kill beneficial microbial life as well as pathogenic. Therefore the temps need to be limited to somewhere around 140-150 deg F. if I recall correctly. Too high you kill everything and too low doesn't rid pathogens. I suppose this would apply to an anaerobic compost also.... just guessing. What kind of timeframe are you speaking of before yours gets all nice and fluffy? I can imagine that the contents of an IBC just drying out slowly from the top down would take a considerably long time to reach that stage.

    It starts off as anaerobic and then goes aerobic from the top down as the moisture level decreases and then actually starts to make compost. Takes a while to dry out.

    Can you pee in it if you add sawdust or ash? It seems very inconvenient to pee and poop in different places. Plus, sometimes you can't help but do both.

    1 reply

    We tried this in version 1 of the IBC system, but alas, it was a miserable failure as huge quantities of sawdust would be needed to actually dry the urine out. In my friend's 'open' system, he can throw whatever he likes at it and the soil seems to absorb the moisture, but then animals can also burrow in through the soil ..... errrrr ..... Not ideal!

    We now have a fine/penalty system in place for anybody that pees in the toilet, which seems to work fairly well.