The main problem is usually too much liquid from urine. If the contents are too wet, you will not get compost. You will get a sodden, stinking mass of toxic sewage. Odor problems abound. In theory the liquid should be evaporated by the built in heater and/or fan. But in practice, the volume of liquid may simply be too great for this to be accomplished. Emptying a toilet full of un-composted material is hard to even think about.
To try and solve this problem, many non-urine seperating composting toilets have overflow drains to handle excess liquid. These drains MUST be clear and working properly. The overflow drain must be led somewhere below the toilet itself so gravity can cause it to flow away. That may be difficult, especially if you are already at ground level or don’t want to cut holes in your floor. Also, you need a sump, container or pit where this excess urine can be stored. If you have a properly working overflow drain, you should have no problems. The overflow will be urine mixed with feces, so it cannot go into the gray water system. It cannot be drained into a pit, and it cannot be used as fertilizer. In other words, it’s a problem.
A better mousetrap?
A superior solution in most ways is a urine diverting composting toilet. These have only recently become more widely known. You use the toilet normally, but most of the urine is diverted automatically into a separate container. This means the solid contents remain only slightly moist – perfect for composting. Separating the urine solves the excess liquid problem common to most conventional composting toilets. The urine has not been contaminated with feces, so in many areas it is legal to send it into the gray water system. Typically, this will be a tube running from the toilet to the drain below the bathroom sink. Or you can store the urine in a tank, dilute it with water, and use it as fertilizer. It can also simply be led into a small drain pit beside the dwelling.
According to the group SOIL (sustainable organic integrated livelihoods) – a non-profit that builds composting toilets in Haiti – urine diverting toilets are the way to go. They have experimented with many different methods. From their website:
“Urine Diversion toilets have the significant advantage of reducing the amount of waste that needs to be transported therefore reducing gasoline consumption and costs. Because the excreta remains dry, the toilets are also less odorous than other toilet models and very pleasant to use.”
Another advantage of the urine diverting composting toilet is that no heater is necessary. Some composting toilets made by Envirolet, Biolet and Sun-Mar use heaters of up to 500 watts. That could be a significant cost, with our soaring energy rates. And what is the point of installing a “green” toilet if you use large amounts of electricity to run it? The heaters may eventually fail, costing more money. All manufacturers do have non-electric or 12 volt models, which use little electricity. However, you must be extremely careful to have a working overflow drain in those non-electric toilets.
Urine diverting composting toilets need only a small, inexpensive, easy-to-replace fan that uses very little electricity. Or they can be hooked up to a solar vent and you can forget about the fan and power use entirely.
Solid human waste is approximately 85% water by volume. This means that a urine separating composting toilet will have enormous capacity for its size. The solid material dries out and shrinks dramatically (just like a compost pile in you backyard).
And finally…urine separating composting toilets do not have any problems whatsoever with odor – unlike some older models of composting toilets.
Still not perfect
However, even with a urine separating composting toilet you still need a way to dispose of the urine. Fortunately, this is fairly easy. Urine contains very little or no bacteria and does not pose a health risk.
The easiest solution is to run the urine drain into the gray water system. If you don’t want to go that route, you can build a simple “French drain” to solve the problem. This is a small pit, about 2′ wide and 2′ deep, filled with small rocks. You cover the rocks with landscape fabric and place soil and grass seed on the top.
Run a tube from the toilet, through the wall or floor and directly into the French drain adjacent to the building. This way you will never have to think about the urine again, as it is getting disposed of constantly.
You can also store the urine in a plastic tank, and dilute it with water 10:1. It is then exceptionally good fertilizer.
Many people install the smaller urine diverting composting toilets on boats and in RVs. In this case, the urine can be dumped in any conventional toilet or outhouse as needed.
Some urine diverting composting toilets, such as the Nature’s Head, are smaller, self contained units suitable for 2 people full time or weekend use for up to 4 people. The larger Separett toilets have more capacity and are suitable for a family and general household use. Urine diverting toilet seats are available for the handy do-it-yourselfer that wants to build their own urine separating composting toilet system.