Composting Toilets ‐ Design 1, The Weekender, Circulating Air.





Aerating the Compost

The compost heap or mass, needs to be kept raised from the bottom plastic lining and away from the sides of the compost bin to increase air flow, allowing the microbes to work more efficiently. This also makes it easier for liquids to drain.

The easiest way I could think of achieving this, was to use a series of plastic trays or baskets. I found a variety of shapes and sizes of seed raising trays at my local gardening supply store. Seed raising trays are usually shallow and have holes and/or slots in the base for drainage. Some have holes on the sides as well. They are cheap to purchase at only a few dollars each.

Make sure you get ones made of sturdy material so they will support the weight of a moist compost heap. They can be stacked, cut to different sizes and joined together through the holes by twisting plastic coated wire.

Plastic seed raising tray (left) and sturdier bread tray (right)-

plastic seed raising tray   plastic seed raising tray

Some ideas and instructions follow. This is quite hard to explain in writing, so check the diagram below and arrange the baskets so you achieve what I've tried to show.
  • cover the urine drain with a small seed raising tray. If you have trouble finding a suitably sized tray, just cut a section from a larger one with a hacksaw or craft knife. Make sure liquids flow freely down to the urine drain. This tray needs to have fairly small holes to prevent anything other than liquids passing through and potentially clogging the urine drain. I would suggest turning the tray upside-down so the top lip of the tray is resting on the plastic lining of the compost bin, providing there are still places the liquid can escape. The main point here is to make sure liquid can get to the urine drain at the lowest point as we don't want liquids to gather or "pool".
  • the next tray or basket will support the tropical fish tank heater - more on this later. First, use a hacksaw or craft knife to remove one wall of the tray, making sure not to leave any sharp edges or the liner may be punctured!. Your pretty 4-walled tray will now have 3 walls! The wall is removed so compost material (ok, poo etc) can move easily from the sloped tray you can see in the diagram, onto this tray. We're trying to encourage the compost material to end up on this tray, as the heater will be resting here. Rest the tray on top of the urine drain cover, sloping slightly downward to the middle of the composting toilet bin. The end with the missing wall will have its base/bottom touching the bin liner
  • the steeply sloped tray is next. The tray you choose needs to have small holes in the bottom and if possible on the sides. If it doesn't have any holes in the side, you can easily drill a few holes. This tray will catch the poo, toilet paper and material you add to the mix when it first enters the bin. Take note of the slope of this tray in the diagram. The slope is needed to assist the compost pile move down to the fish tank heater tray. The end nearest the fish tank tray will have its wall removed. If you get the angle right, the material will accumulate on the slope and begin to breakdown. As more material is added it will slide down to the other tray. You can use smaller baskets underneath to prop the sloped tray at the correct angle. Same as before - make sure the trays have holes on the bottom so liquid can seep away. To make the tray area large, I had to join two trays together by removing one wall from each (like we did above) then slide one inside the other so there is some overlap and wire them together. You need to cover as much of the circumference of the bin interior as possible, so you may have to shape the corners to achieve this.
  • when you're happy with the position of the trays and/or baskets, use some plastic coated wire or plastic wire ties and secure any loose trays together.
  • Not essential but if you have room, add another layer of trays over the first layer , keeping about 2" between their bottom surfaces to allow air to penetrate.
  • If you want to get even more technical about things, consider using a different style of tray above the first layer by using one with larger holes and gaps . An alternative to using a second layer of trays is to use a layer of plastic garden mesh or plastic trellis with 1/2" gaps. In effect, you're really creating two composting areas - larger material will compost on the first level, then work its way through to the second level as it breaks down. Of course, this will assist with aeration.


Plastic garden mesh works quite well when placed on top of the first layer of trays. It's easy to cut to the shape you need.

plastic garden mesh

Exhaust system - using a fan

To help with air flow, we can force air out of the composting heap by using a fan. There are two main advantages to moving air out of the compost bin-
  1. it helps remove moisture from the compost i.e. prevents the compost being "stodge"
  2. it adds oxygen to the system so the microbes work efficiently


When talking about the bin to choose for composting, an air intake vent was mentioned. Air will enter the composting bin via the gauze-covered vent. There may also be some air entering from around the toilet seat. Air flowing in, is helped by the draft created from the exhaust fan which extracts air. In other words, the air will be forced through the bin. We're not talking gale-force though! You'd be better to think of it as a small draft. The draft will be enough to achieve our two goals above.

I used two fans for my design, housed in a chamber in the middle of the exhaust pipe. I chose to use one AC mains powered fan and one 12v DC fan, powered directly from a small solar panel located on the roof of the shed.

Exhaust Pipe and Fan Chamber I used 1.5" (40mm) diameter PVC pipe for the exhaust. You need to be aware that the size of pipe you use is important as it affects the drying process - too large a diameter pipe will mean there is too much air entering the system which will cause the compost to dry more quickly. A pipe with small diameter will mean there is not enough air flowing through. In general, use the same diameter pipe for the exhaust as the hole of your air intake vent. The sizing can be tricky though, as it also depends on the climate you live in, the seasonal variations in temperature, the size of fan and number of hours the fan is going, the number of "deposits" the loo gets etc etc.

The pipe leads vertically from a hole in the lid of the bin, to the chamber about 18" (450mm) away. The chamber is made from PVC flange pipes. These should be the correct diameter for the pipe you use and allow the pipe to expand to a larger diameter so the fans fit, then reduce back down again to the standard PVC pipe. Think of the chamber as being a bit like two plastic funnels with their widest ends cupped together. The join is sealed with duct tape for easy access at a later date. From the chamber, the pipe continues through the back wall of the shed to the outside and covered with a make-shift cowl.

The "inside-the-bin" end of the exhaust pipe is covered in gauze as a precaution to prevent any dry debris being sucked up the exhaust into the fan chamber - I don't think this step is essential but better to do this before the system is in use! The "outside" end of the exhaust pipe is covered with gauze mesh to prevent flies entering the system. I also bent a piece of scrap corrugated iron to act as a "cowl" to prevent wind and rain from outside entering the exhaust the wrong way.

Fans I recycled a 12V cooling fan from an old computer to use as one of the fans. Alternatively, this could be purchased at your local electronics supply store or computer hardware shop. I wired this directly to a 12V 10W solar panel, easily available to purchase online or at the store. These are often used for car battery trickle-feed charging. As my fan is rated only about 3 or 4 watts (I can't remember exactly) it only draws what it needs from the panel, obviously only during daylight. This means the fan only operates for about 6 hours in the middle of winter, more in summer. I've found that this is fine as I don't really use the property during winter and the whole composting process is slowed during cooler periods anyway. I like the solar panel option, as it means I don't have to leave the mains power on when I'm away from the property and I know the composting bin is getting at least 6 hours of good air flow.

12v computer cooling fancomputer cooling fan

A further enhancement I might make if needed (to be honest I don't think it's needed) would be to connect a 12V deep-cycle battery. The solar panel would charge the battery during the day and the fan would have a power source after dark.

The other fan is mains powered. I purchased this from an electronics store for a few dollars. This is also mounted in the exhaust chamber and is only there as a backup fan for when the old computer cooling fan wears out. It's only used when I'm at the property i.e. when mains power is turned on. Eventually the whole property will be solar powered - yet another project!

The steps-
  • place the lid on the composting bin. Cut a 2' (600mm) length of pipe, place it on the lid towards the back of the bin and trace around its diameter. Cut this hole as neatly as possible in the lid so the pipe can feed into it
  • fit a PVC flange on one end of the pvc pipe and glue into position using PVC cement.
  • Place one of the fans into the chamber and mark where the cables need to exit through the side of the chamber. You can cut the plastic connector from the end to expose the cables. Make sure you position the fan the correct way up so the air blows in the direction you need it to i.e. out of the compost bin and through the exhaust pipe to the outside. You might have to temporarily connect the solar panel jumper leads to the cables on the fan to check the air flow direction (connect red from solar panel jumper to red on fan, and black-to-black). Drill two small holes the diameter of the cables in the side of the flange and feed the cables through the holes. Don't worry if you don't have the correct size drill bit, just remember to seal the holes around the cables with some RTV silicone sealer or similar - you need to stop extra air getting in and prevent flies entering. NB - if using a computer cooling fan with 3 cables (red, black and yellow) you can cut the yellow cable off at the fan as this is used in computers as a RPM sensor which we do not need - we only need the red and black cables.
  • If you've been lucky and found a suitably shaped PVC flange and fan, the corner holes on the fan will line up at the correct spot in the flange and you can screw the fan down into the flange using narrow self-tapping screws or a very small gauge bolt, washer and nut.   In my case, as picture below, I cut a piece of plastic from a recycled plastic container and screwed it into the pvc plastic flange to use as a mount for the fan. I used some RTV silicone sealer to help glue and secure it in position to reduce any vibration. You can't see from the photo, but there is a screw in each corner to fix the fan securely onto the plastic. This forms one half ( the bottom half) of the exhaust pipe system.

    computer cooling fan inside composting toilet exhaust
  • the above procedure is repeated to make the top half of the exhaust pipe. Remember to orientate this fan correctly so the air flows in the correct direction when the bottom half and top half are assembled.
  • I made a small triangular shelf from plywood to act as a support for the chamber. This has a hole cut the correct size to allow the chamber to sit on it but not fall through it. Not shown in the picture, is a piece of plywood that acts as a cover for the whole exhaust system. It goes vertically from the plywood base behind the toilet seat, up the height of the exhaust. You can see the exhaust system and chamber in the following picture. Note the black duct tape around the two sections of pvc flange pipe-

    pvc exhaust fan system
  • place the bottom section of pipe into the hole you cut in the lid of the composting bin. Invert the top section of pipe so the "cup-ends" from both sections meet (think of this as joining the wide opening of two plastic funnels together and taping them in position). Seal this join with duct tape to keep the sections together and keep the flies out. Using duct tape will allow easy future access if you need to replace a fan
  • continue with the pvc pipe so it leads to the outside world. Take it through the roof of the shed or like I did, out the side wall of the shed
  • make sure you cover the "air-flow exit end" with more of the fine gauze to prevent flies entering
  • create a "cowl" for the end of the exhaust pipe protruding from the shed, to prevent rain entering and strong wind blowing down the pipe
  • connect the cables for the 12V DC fan to the cables attached to the solar panel, using an electrical crimp or connecting block. You can get these cheaply from an electrical supply store. Cover the connecting block or crimps with electrical insulating tape or plastic covers. If you chose to install a secondary fan, connect this one remembering to do so safely!


Exhaust and fan for composting toilet