Compost ToiletsComposting toilets (also called biological, dry or waterless toilets) are systems that treat human excrement through biological processes, turning it into organic compost material that can be used to fertilise the soil. They are small-scale, complete sewage processing systems not connected to the mains sewage system. The Chinese have been using composting toilets for hundreds of years, but it is only since the 1960s that they have become popular in the rest of the world.


Types of Composting Toilets

There are hundreds of different composting toilets, ranging from simple DIY designs to advanced high-tech commercial models. They can be classified as:

  • Self-contained

    – where the toilet and composting container are one unit.

  • Remote

    – where the toilet is located separately from the composting site.

  • Batch

    – where waste is collected and composted in two or more sealed containers, mounted on a rotating carousel. When one container is full it is replaced with an empty one.

  • Continual process

    – where waste is composted slowly in a single container, and compost is harvested from the bottom on an ongoing basis.

All composting toilets decompose waste by creating the aerobic conditions for bacteria, fungi, worms and other macro and micro-organisms to thrive. The objective is to destroy harmful pathogens, reduce the risk to human health and environment, and transform the waste nutrients into fertile soil. They typically break down waste to 10% of the original volume. Most toilet composting systems are low-temperature, known as mouldering toilets, but others are hot (thermophiliac). The waste material must be heated high enough that pathogens are destroyed, or left for long enough that pathogens break down naturally.

Compost that is too wet becomes anaerobic and produces unpleasant odours. Because of this some composting toilets separate the urine from the faeces. The collected urine goes through a process of nitrification, resulting in an odourless, bacteria-free liquid that can be used as a fertiliser or leached safely into the ground. Other models collect urine and faeces together, and either evaporate the liquid off completely or require the addition of carbon material such as sawdust, leaf mould, straw or grass clippings to soak up the liquid.

Commercial models often have advanced features, such as electric fan ventilation systems, oxygen injections, or mechanical mixing and heaters to facilitate the fast decomposition of human excrement (good for cold climates). However, there are also low-tech models that use passive ventilation systems to prevent odours, passive solar design to heat the compost, and hand-turning to increase aeration.

Bucket systems are the simplest and cheapest type of composting toilet, with a bucket placed under the toilet seat, the contents of which are kept covered at all times with clean organic carbon material to prevent odours, absorb urine and deter flies. When the bucket is full it is deposited onto an outdoor compost pile.

Treebogs are another simple type of composting toilet, with an outdoor chamber open to the air, and willows and nettles planted around it. The plants convert the nutrient waste to biomass, which can be usefully harvested, and consequently treebogs never need emptying.

Clivus Multrum is one of the more popular commercial models, and utilises a two-level container connected to the toilet with a variable chute. The unit has an inclined base where solid waste slowly decomposes (over two years) and slides down to the lower level. Excess liquid is drained to the lowest part of the container, and collected or evaporated.

Sustainable Advantage of Composting Toilets

All composting toilets require some form of management to ensure that they remain clean, hygienic and odour free. And all need to have compost material removed at regular intervals. But these are minor inconveniences compared to the advantages of composting toilets. They protect groundwater, surface water and soil from sewage pollution, prevent the accumulation of hazardous pathogenic waste, and solve the problem of disposing sewage sludge to landfill. They save huge quantities of water in a world where water is an increasingly scarce resource, and require very little infrastructure. They are low-impact, low-maintenance and can also adapt to any situation, even in places where it is difficult or inappropriate to establish a mains toilet system such as hard rocky soils, high water tables, near springs or in an environmentally sensitive area.

Composting toilets are an excellent example of sustainable design. They provide a safe and effective way to reduce resources and prevent pollution, whilst saving money and energy for the household and the community. At the same time they produce a valuable end-product that can be used to fertilise the soil. Once these remarkable benefits are understood they are likely to become even more popular.