As low-water fixtures and plumbing gain more popularity among homeowners and renovators, a simple and easy-to-make composting toilet may yet be the greenest solution of all.
The average person throws away 9 times their body weight in waste every year. What is worse, nearly 30% of that waste is food scraps and yard waste that could be used to create compost.
Compost is organic material that you can add to the soil to help plants grow. People have used it alongside manure to enrich the soil with nutrients that plants use, but also to improve moisture retention in regions affected by drought.
It may come surprising that human waste can also be successfully composted by using an easy-to-make composting toilet that everyone can build.
A composting toilet is a defecating system that collects human waste and turns it into compost. It’s a great way of turning refuse into a resource.
It’s not an outhouse, latrine, or privy. If you build it right, it gives off no foul odor.
Since it does not require any plumbing, you can build it almost anywhere, and since there are no sewer drains, it doesn't add to pollution.
A composting toilet combines human feces with dry materials such as sawdust, crushed leaves, or wood ash. You dump this mixture onto your composting heap, and straw and high heat will do all the rest.
This way, you can safely use composted “humanure” in your garden, even for fruit trees and vegetables. 
In self-contained composting toilets, the compost system is enclosed within the toilet, beneath the bowl.
This type of composting toilet is best suited for small homes, boats, and recreational vehicles (RVs). It's also commonly used in seasonal dwellings like cabins and cottages.
If you have the time to do it yourself, however, you can easily add one to your full-time residence.
If you live in a large home, using a central or remote composting toilet could be a better option.
This version takes both pee and poop to a central composting system that is at another location (and not beneath the bowl). In many cases, this location is in the basement or even outside the house.
If you have a large remote system, you can channel several of these in your home.
On the outside, these central composting toilets look like traditional toilets with a bowl.
A composting toilet works by separating the liquid from solid waste, mixes the solid with sawdust or similar absorbent materials, and then turns it to compost.  Whether you choose to get one type of DIY compost toilet the other, they both follow a similar concept and consist of these parts:
The composting chamber needs to be lined with carbon materials like dried peat moss or coconut fiber. This is to keep proper carbon/nitrogen balance for bacteria to do their work.
In a simpler variant, just throw in a trowel of sawdust each time you use the toilet.
Urine drains into a removable container, while solid waste falls through a manual flush door into the composting chamber. If the bowl gets dirty, use a spray bottle with a mix of water and vinegar to flush the remains and keep the business clean.
Now you need to stir the compost up. Turn the spider handle that is attached to the agitator bar to mix the feces with the existing compost and carbon material. If you’re using a DIY toilet with a simple plastic bucket, just throw another layer of sawdust.
Once the toilet is full, remove the urine container and empty it in a safe manner. For the composted part, you need to follow your state or local guidelines to dispose of compost. The best place to start is your state’s department of health website.
We began to see that the effects of properly cured and aged composts could be more far-reaching than just sanitizing the soil safely. As we identified one beneficial strain of soil microbe after another, it was apparent that some of them could induce systemic resistance to diseases in the plants themselves.
Harry Hoitink, Professor Emeritus in the Department of Plant Pathology, The Ohio State University
If there is any leftover compost matter clinging to the sides of the composting chamber, rinse the container using a garden hose with a high-pressure attachment. After you’ve cleaned it, tip the water onto your outdoor compost heap. Dry the bucket with a rag and spray the interior with vinegar.
Remember not to use chemical cleaners as their residue may kill the good bacteria that break down the waste into compost.
Angie's List  has a terrific graphic below that clearly illustrates how the whole thing works.
You'll be able find many composting toilets DIY plans online, but if you want to build the simplest and the most effective variant with a plywood frame and a 5-gallon bucket, you're going to need:
Your DIY composting toilet needs to be comfortable as this is where you’ll be releasing your humanure day after days. If it’s comfortable, it will make the experience more enjoyable. When it comes to toilet comfort there are two key things:
Height — Naturally the taller the toilet, the easier for you to sit and stand up. However, the doctors say that the more you squat, the better you may empty your bowels. Find the ideal height by trying to sit and stand from objects of different height to get the ideal measure.
Toilet seat — This is entirely up to you, just make sure it’s durable and comfortable.
To build the frame of your composting toilet, you need to decide on the dimensions, by considering the available space. Remember that it needs to include the size of the toilet seat.
Cut out six pieces of plywood for the top, bottom, and four sides of the box. Attach the pieces together using a pocket hole jig to make the box stronger and to hide all those screws from plain sight.
You need to keep the top section of your plywood box loose so you can access the bucket. On the other hand, you’ll need to cut a hole in the center for the waste to fall inside. You can use the toilet seat as a template:
You can freely cut the circle a bit wider to give yourself more wiggle room. Start cutting by drilling a hole somewhere on the line so you can insert the jigsaw blade. Unless you’re a seasoned DIY carpenter, your hole is not going to be perfect, so use a file to soften the rough edges and finish them with sandpaper.
Once the box is done you may choose to stain or paint it to give it more character.
Attached to the top, the urine separator channels the urine into a separate container in your DIY composting toilet. This helps compost toilets neutralize the smell, but also makes it easier to dispose of urine and keep the solid waste bucked drier.
Screw the urine separator to the underside of the plywood lid, at the front.
Place the toilet seat assembly on top of the hole you’ve just cut. Mark the drill holes with a pencil. Use a strong screwdriver to drill small holes for the toilet seat hardware, making sure the holes are tight enough so the screws don’t get loose with use.
While the whole plywood box can rest on the floor by itself, it’s always better to give it legs. Bathrooms are wet places, and sooner or later there’s going to be water on the floor, and plywood and water don’t play nice.
Place each 2x4 wood at the four corners of the plywood box and screw together. Place the whole thing on the floor to see if it stands firmly. If any of the legs are shaking, check the screw to find any additional spacings.
To complete your composting toilet DIY project you need to find buckets of the right size. One for number 1s and the other for number 2s.
An ideal container for urine is long and narrow. If you have trouble finding something like that, a 3.5-liter plastic pot would be enough for a day.
As for the solid waste bucket, you can use 5-gallon (about 20 liters) plastic buckets. Be sure you have at least three buckets in rotation — two can be full at all times and one being used.
For the last step, add a few inches of absorbent materials into the bucket. You may use dried leaves, wood ash, or sawdust (hardwood sawdust is the best). The sawdust soaks up the excess moisture but also neutralizes the bad odors.
Here's a video that shows all the steps I've outlined above. Now the steps here may be in a different order, but they all add up to a similar final result. Check it out below.
Septic tanks are expensive, and in certain layout and land conditions, the use of a septic tank would not be possible at all. Not to mention environmental regulations that may prohibit the use of septic tanks where you live. A DIY composting toilet, on the other hand, lets you deal with your stuff on your property in the greenest possible way — by turning it into food for plants.
Even in this time and age, there are regions that struggle with water scarcity, and not all of them are in developing countries.  When compared to traditional toilets that waste up to 5 gallons of water on a single flush, for a DIY composting toilet described here, you need very little to no water.
Even when sewage makes it to the treatment plant, it takes a huge amount of money and energy to clean all that wastewater, and the process must be repeated over and over as the water is cycled back through people's toilets. I think that you should really make people understand how stupid it is to poop into clean water.
Cecilia Lalander, Environmental Engineering Researcher at the Swedish University of Agricultural Sciences in Uppsala
If you’re looking into compost toilet DIY tutorials, chances are that you plan to use one where plumbing is not available, such as an outdoor cabin. A composting toilet is also a great option for an old building, where plumbing is either inefficient or non-existing. From an environmental point of view, having a composting unit makes much less impact than you having to tear out the old and running new pipes through the building.
While you can go and buy a ready-made composting toilet, this is all about being self-sustainable and making do with materials you have at hand, with minimum waste. The 5-gallon bucket version with a plywood frame and urine separator will set you back about $10. You won’t be creating any waste and won’t have to pay for someone to install it.
When compared to an outhouse, a DIY composting toilet has a significant advantage — you don’t have to deal with foul odors in your property. Helped with the carbon material, the whole composting process is carried out by bacteria under aerobic conditions. The final product is eco-friendly odorless compost which you can use in your garden.
Yes, you can make your own composting toilet. A composting toilet is easy to build with a few basic hand and battery tools. All you need are a few sheets of thick plywood, a toilet seat assembly, screws, and a bucket with the absorbent material. You can even create a simpler unit by using a medical toilet seat.
Yes, you can pee in a composting toilet. In that case, you’ll have to use more absorbent material, like sawdust, to neutralize the odor, as urine and feces also smell much worse together. Sawdust is rich in carbon and you need more of it to balance the extra nitrogen content, so composting can take place.
No, composting toilets don’t smell bad. If you follow the instructions and make sure there is enough absorbent material in the bucket you won’t have to deal with bad smell. If you want to be extra sure, you can install a vent with a small 12V computer fan that activates every time you open the lid.
Yes, you can use human feces for compost. Keep in mind that it’s much easier to compost feces from a toilet with a urine separator. While some municipalities recommend using this type of compost only for flower beds, if you use a hot compost heap, you can use it on your vegetable patch safely as well.
You can get rid of composting toilet waste by emptying it onto your outdoor compost bin. If you used plenty of damp sawdust or peat moss, the bucket won’t smell bad, and you can rinse it with a garden hose, dry it, and spray it with vinegar. No chemical cleaners needed.
Yes, you normally use toilet paper with a composting toilet. Toilet paper won’t decompose as quickly as feces so it will be visible long after the matter has broken down. You can use any type of paper, but keep in mind that marine and RV paper will compost the quickest. You should never throw diapers, wipes, and tampons in a compost bin.
You can dump a composting toilet on your composting heap or apply it directly to your garden. Between November and April you shouldn’t apply compost to your plants, so dump the content of your bucket into a composting bin or heap. In warmer months, empty the toilet directly at the base of shrubs and flower beds.
No, you don’t need electricity for a composting toilet. The best thing about compost toilets is that they can be used off-grid, with zero power, emissions, and pollution. If you use one in your home and want to eliminate any chance of bad odor, you can use a small 12V computer fan for venting.
Yes, earthworms can eat human waste. They live off any organic matter that is composting. When given human excrement, they will digest it and convert the remaining material into their excrement, called worm castings. Worm castings are among the best balanced natural plant foods there is.
You can make a compost toilet not smell by making sure there’s always enough absorbent material in the bucket. For this, you can use hardwood sawdust, dried peat moss, dry leaves, or coconut coir. You can also install a small computer fan to vent any lingering odors outside.
A DIY composting toilet is a great alternative to a chemical toilet or outhouse. It’s odorless, eco-friendly, and can fit almost anywhere you need a restroom.
Above all, these toilets are among the most sustainable options today. They need very little or no water, so they are great if you want to minimize your water consumption. They produce no pollution, as there’s no septic tank that you need to empty, and you don’t add to municipal sewage.
This type of toilet is also easy to build: if you can handle standard house tools, then you should start building a functional composting toilet today.