Composting toilets

A composting toilet treats human waste without flushing it away. Instead of the waterlogged conditions in a septic tank, it keeps waste moist but unsaturated and well aired, so aerobic bacteria and fungi break it down, much like a garden compost pile. Toilet paper and a carbon additive such as sawdust go in with it, and some units take kitchen scraps too.

Types

• Continuous composters have a single chamber. Waste goes in at the top and finished material comes out at the bottom.
• Batch composters use two or more chambers. One fills while the others sit and cure, so fresh waste never mixes with finished material. Alternating double bins and 55-gallon drums are common in site-built versions.

Many designs divert urine into a separate container, or add a waterless urinal, to keep the chamber from getting too wet. Too much liquid turns the process anaerobic, which causes odours.

Some units are small appliances that sit on the bathroom floor. Others are large tanks that need a basement or crawlspace under the toilet.

Where they fit

They suit seasonal cabins, remote homes, places where water is scarce, and sites with poor percolation, a high water table, shallow soil or rough terrain, all places where a drainfield is hard to build. Cutting the toilet out of the wastewater stream can also let an overloaded or undersized septic system cope.

Trade-offs

For: no flush water, much less wastewater to treat on site, low power use, and nutrients kept out of streams and lakes.

Against: most units still need a power source for a fan. Small units can be overwhelmed by peak use, like a full house of guests. Poor upkeep leads to odours and health risks. Too much liquid in the chamber stalls the process unless it drains properly. Most importantly, you still need somewhere for greywater from sinks, showers and laundry to go.

The finished material

The end product is a soil-like humus. Rules on what you can do with it are set by the state and county: it may have to be buried or removed by a licensed septage hauler. Material that isn’t fully treated can carry pathogens, so it shouldn’t go on food crops without checking local rules.

For a Leafhouse project

Ask the county health department early whether it accepts a composting toilet as the home’s only toilet, and what it requires for greywater. The answer can change the size, and cost, of the septic system you need.