Reasons for building a composting toilet
Traditional toilets use up to 5 gallons of water on a single flush whereas composting toilets need very little to no water.
Sewage treatment plants require significant infrastructure, operating expenditures, and energy to clean wastewater. Septic tanks are expensive and may not be possible depending on land conditions and environmental regulations.
A do-it-yourself composting toilet does not require any plumbing and is inexpensive to build. A composting toilet lets you dispose of your waste in the greenest possible way by turning it into compost that feeds your plants.
A composting toilet is not an outhouse (latrine, privy). When properly built and utilized, it gives off no foul odors since the composting process is carried out by bacteria under aerobic conditions enhanced by the addition of carbon material.
The Design
Your composting toilet should be comfortable, especially if you plan to use it day after day. There are 2 key elements to consider – toilet height and the seat. A taller toilet may be easier to sit on comfortably and may facilitate standing up more easily. However, a lower, squatting position may assist with emptying your bowels. Find the ideal height for you. The seat is up to you. There are numerous designs that can be purchased or you can build your own if you are so inclined.
Your toilet will consist of a 5 gallon (20 liter) bucket that sits inside a wooden box with a toilet seat on it. Next to the toilet, you will need a bin filled with sawdust or other material to be used to cover your waste (see below). Finally, you will need a compost heap or container for accumulating and composting your waste.
Building the toilet
To build a simple composting toilet with a plywood frame, you will need: plywood, four 2×4 wooden planks, screws, wood glue, drill, a five gallon (20 liter) bucket, toilet seat, optional urine separator with small container, and hinges and handles if using an alternative design.
Step 1 – Build the Box
Cut out six pieces of plywood for the top, bottom, and four sides of the box. (The dimensions depends on the size of your bucket plus the toilet seat, keeping your available space in mind.) Attach the pieces together using a pocket hole jig to make the box stronger and hide the screws, if desired. For now, keep the top section of your box loose so you can access the bucket.
Step 2 – Cut the Hole for the Seat
Cut a hole in the center of your top piece of plywood using your toilet seat as a template. Place the toilet seat on top of the plywood, open the lid, mark the inside circumference of the seat. Either make sure your waste bucket can be lifted thru the hole add a door to your box (see Alternative Design below). Start cutting by drilling a hole on the line to insert the jigsaw blade. Cut along the line to remove the center and then file off the rough edges finishing with the sandpaper for a smooth surface.
Step 3 – Add the Urine Separator (optional)
Screw the urine separator to the front of the underside of the plywood lid. The urine separator channels the urine into a separate container in your bucket to keep the solid waste drier. Although vault systems benefit from urine separators, hot compost heaps require liquid and may become too dry slowing down the processing time. In addition, urine adds nitrogen which balances the carbon of the cover material.
Step 4 – Install the Toilet Seat
Place the toilet seat on the top of the hole you cut and attach securely.
Step 5 – Add feet to your box
Although the box can set directly on the floor, adding feet will keep the box dry in the event the floor becomes wet. Place the 2×4 wooden planks at the four corners of the box bottom where it sets on the floor. Screw these in place. Check your box to make sure it is level and doesn’t wobble. If needed, add spacers between the feet and the box.
Step 6 – Insert the bucket(s)
Place your 5 gallon (2 liter) bucket inside the box. If you added a urine separator, you will need to install a long, narrow container at the front of your box to accumulate the urine.
Step 7 – Attach the lid
Place the panel that contains your toilet seat on top and either screw it in place or add hinges to one side to form a door for removing the bucket.
Alternative Design
If you don’t want to lift the bucket thru the seat hole, you could add hinges and a handle to one of the panels to make a door that can be used to easily remove the bucket(s). Or, if the box is small enough and not too heavy, you could eliminate the bottom panel and lift the box off to remove the waste bucket.
Cover Material
To eliminate odor, it is necessary to cover your waste with a material such as sawdust (ideally hardwood but softwood works). The sawdust should be a little damp to make it more effective at absorbing odors. Be sure to add enough sawdust to cover the waste. A 1-to-1 ratio of human waste to cover material is generally recommended.
The Compost Heap
Your compost heap is where you will empty your waste bucket. Site it accordingly.
For best results, your compost heap should be inside a 3-1/2 ft by 4 ft bin or similar container. The container can be a wooden box with a door, a large barrel with a lid, or some other type of containment. If you live in a cold environment, you may need to insulate your bin to retain sufficient heat. You will need at least two compost heaps – one for active use and one for curing. A third container for storing cover material is also useful.
Once you have your bins, place a thick bed of organic matter such as straw or shredded cardboard on the bottom of the first bin. Now you can begin to fill it. Each time you add waste to the bin, cover it with a layer of carbon material (branches, stems, dried leaves, pine needles, peels, bits of wood, bark dust or sawdust pellets, shredded brown paper bags, corn stalks, coffee filters, coffee grounds, conifer needles, straw, peat moss, wood ash, etc.).
There is no need to turn or disturb the underlying heap. Just add new waste to the center of the heap where it is the hottest. Dig a hole, add new waste, cover. If your heap starts to form a mound in the center, level it out and add more cover material if needed.
The ideal cover combination is something like a damp sawdust for soaking up liquids and adding nitrogen (the bulking agent) followed by a layer of straw or similar biodegradable matter capable of absorbing moisture while allowing rain through. The carbon-to-nitrogen ratio for composting human waste is about 30-to-13.
One of the main functions of the cover material is to block odor. Your heap should never smell. If it does, add more cover material.
Once you have filled your first bin or barrel, stop using it and let it rest to cure. Move to the second bin/barrel and start the process all over again.
Using your Finished Humanure Compost
Heat kills bacteria and parasites. Many people who plan to use their compost on food gardens, monitor the temperature of the heap to make certain it heats up sufficiently to destroy disease-causing organisms. Although heat may not guarantee that all bacteria and parasites are destroyed, time, exposure to sunlight, and competition with other microorganisms all contribute to produce compost that has the same level of pathogens as your normal garden soil.
If you are uncertain about using your humanure compost on your food gardens, you may want to only use it around your trees and ornamental plantings.
Resources
Humanure Handbook by Joseph Jenkins
Lifting the Lid by Peter Harper and Louise Halestrap
What’s the Scoop About Composting and Why Compost Now? by DBrower, published March 28, 2021.
