How does a horrible material turn into nice safe compost? It seems the transformation occurs by magic. Why do we want composting to happen in the toilet? There are two reasons; firstly to heat the compost pile up to 70°C to kill the pathogens and secondly, to reduce the amount, and change the structure of the waste.

The bacterial population in the compost pile does the composting. In compost, aerobic bacteria are the most important decomposers. They are very abundant; there may be millions in a gram of soil or decaying organic matter. You would need 25,000 of them laid end to end on a ruler to make an inch. They are the most nutritionally diverse of all organisms and can eat nearly anything. In order for them to perform, we must create the correct environmental conditions for them to thrive. The factors affecting composting are:

• Moisture
• Air / Oxygen
• Carbon Nitrogen (C:N) ratio
• Temperature

Moisture – the compost pile needs to have the correct level of moisture. Too dry and the bacteria will perish, too wet and the conditions will turn anaerobic as there is no air spaces or air flow in the compost pile. A moisture content of 50-60% by weight is about right. In practical terms this means that the compost should look wet. If you see dry parts it’s probably too dry.

Air – Aerobic breakdown of the waste by bacteria is much faster than anaerobic breakdown and it also generates heat required for thermophilic composting. Aerobic breakdown of organic matter produces no objectionable odour (explanation here). If odours are noticeable the process is not entirely aerobic and so it just needs more air. Turning the compost at intervals, either using the unique Sun Mar BioDrum or by using bark chips to create air spaces in the Biolan compost pile is important.

C:N Ratio – Bacteria utilize carbon as a source of energy (to keep on eating) and nitrogen to build protein in their bodies (so they can grow and reproduce). Organisms that decompose organic matter use carbon as a source of energy and nitrogen for building cell structure. They need more carbon than nitrogen; a C:N ratio of 25:1 is good. Too little nitrogen and decomposition slows when the nitrogen is used up and some organisms die. Nitrogen is required for cell growth. If the population of bacteria reduces the pile does not get to the high temperatures required to kill the pathogens (thermophilic composting) that are in the waste. At ambient temperatures it can take up to 18 months for the pathogens to perish. This is why true composting toilets (internal composting toilets) do not divert the urine. Urine is the source of the nitrogen.

Temperature – If the compost pile has sufficient air, moisture and C:N it will generate it’s own heat through aerobic microbial activity. Initially, mesophilic organisms, which live at temperatures of 10°C to 45°C, colonise the waste. When the temperature exceeds about 45°C thermophilic organisms, which grow and thrive in the temperature range 45°C to 70°C, develop and replace the mesophilic bacteria. Decomposition at thermophilic temperatures (45-70°C) takes place more rapidly than at mesophilic temperatures (10-45°C) and, hence, a shorter time is required for decomposition. The high temperatures will destroy pathogenic bacteria and protozoa (microscopic one-celled animals), which are detrimental to health. Compost piles under aerobic conditions attain a temperature of 50°C to 70°C in one to five days. This temperature can also be maintained for a few days before further aeration is required.

If the toilet is in an uninsulated building in winter in cold climates the outside temperature can overcome the heat generated by aerobic microbial activity and the temperature of the compost pile will cool and composting will slow. Some units have heating elements (Sun Mar Electric Range) and others have insulated bodies and feature air flow adjusters (Biolan Eco Hot Composting Toilet) to keep the temperature up to support that thermophilic bacteria.

So now we have looked at the factors required for the bacteria to compost the waste. But where does the waste go? How does composting result in less material than we started with? To explain this, imagine your favourite large tree. Imagine walking up to this tree and feeling how solid the tree trunk is and wondering; Where did all this hard wood (carbon) come from? Many people that I have asked this question to answer by saying “it comes out of the ground” which is instinctively what you might expect. But it is the incorrect answer. The tons of carbon that make the wood come from the air. Through the process of photosynthesis the tree “fixes” carbon in the presence of sunlight and releases Oxygen. It breathes in CO2 and exhales (for want of a better term) Oxygen. The reverse is happening with composting. Bacteria, through the process of respiration, break the waste down by releasing carbon and using oxygen, just like we do. So much of the solid material is turned to harmless CO2 gas. Something similar is happening when wood is burned. Fire turns the wood to a smaller amount of ash with CO2 being released. Composting does the same thing, only less dramatic and at a slower rate.

So when we go through the logic of the composting process and understanding the Carbon cycle we can understand how a composting toilet can consume so much waste and produce only a small amount of harmless broken down compost compost.

Patrick Boylan (BSc) for Toilet Revolution