Composting toilets

Composting toilets

Composting toilets are part and parcel of the Sustainable Building movement, the practice of incorporating renewable and green friendly features into every aspect of construction.

Composting toilets in Surrey has proved popular with children and gardeners alike since being opened on allotments in Kingston in 2011. The ‘donations’ provided by the toilet users are used to fertilise the allotments.

composting toilets

Composting toilets installed on an Allotment in surrey


‘Humanure’ v ‘Biosolids’

It’s barely 100 years since water based excrement disposal systems (modern toilets) became commonplace. People have used their own manure as fertiliser for millennia. Urine is a liquid nitrogen fertilizer in itself and human solids are rich in nutrients.

‘Humanure’ is made from unadulterated human excrement and is free from the industrial chemicals present in ‘Biosolid’ ‘sludge’. ‘Sludge’ contains everything that goes into the sewers, including toxic chemicals and heavy metals.

But by not using sludge in gardening and agriculture we divert the bulk of our human excrement away from the land. Leaving aside the chemical fertilisers we use to replace it, it is not at all clear what  long term results will emerge from introducing large volumes of biosolids directly into rivers, the water table and seas. On the other hand however having mixed it with the industrial waste which goes through the sewers we pollute the ‘Biosolid’ product itself. So using it on the land might also have risks.

This difficulty has arisen because the sewers mix everything together. The only resolution to the dilemma is to cycle all human excrement through composting toilets.

Composting Toilets

Composting toilets use managed aerobic processes to break down the waste. What little water is required in the process is used for cleaning the installation. Composting toilets operate without mains drainage, a water supply, or any treatment facility. In fact the less water they use the less they smell. Typically, compost toilets are used in roadside facilities and in National Parks.

Late 19th Century Composting Toilet or ‘Earth Closet’

composting toilets

A very early composting toilet before the invention of sewers

Once deposited in the unit the excrement is mixed with sawdust, coir,peat moss or any other suitable available medium capable of supporting aerobic processing, absorbing liquids and containing smells. The waste decomposes faster than it would in the anaerobic processes used in wet sewage treatment systems such as septic tanks. The final waste product is ‘Humanure’.


In blunt terms, basic composting toilets is a box used as a toilet with a bucket beside it containing wood shavings, peat moss etc.  Users are invited to place some of the material into the box after using the bucket. There are no pipes involved.

Basic Model

The installation can however be made to look more like a normal toilet by dispensing the composting medium into the compost toilet in a more sophisticated way or by having the chamber separate and below the main toilet bowl. In countries such as Sweden where environmental values are highly cherished composting toilets closely resemble traditional water based systems.

Modern Composting Toilets

composting toilets

Modern composting toilets are now available

Regardless of design and sophistication however the principles are the same. Aerobic compost toilets use bacteria which live at high temperatures to break down the waste, reducing its volume and removing potential poisons. To avoid smells the compost toilet should be operated with as little moisture compatible with the system operating at a reasonable speed. The dryer the system the slower it will compost.

Whatever the system however a product equal in bulk to about 10% of the input volume should eventually emerge. The time taken could take from a few months (if sufficient heat and biological matter is introduced) to several years if cold passive decomposition is relied upon. Dependent on regulations the product can be used as fertilizer.

In the UK unless here is some specific commercial application there are no special regulations associated with installing or using a composting toilets

Levels of sophistication

“Self-contained” composting toilets start and sometimes complete the composting process in the unit itself. Some units have fans to aerate the matter, and sometimes have heaters to keep the matter dry or to maintain optimum composting temperatures.

Basic composting toilets

composting toilets

composting toilets a basic model

however might be a receptacle with a lid and funnels attached to the bowl to divert urine out of the solids. The urine flows into containers and the solid waste is released from the bowl down to containers located as far below the toilet seat as possible, possibly in another room and into a composting facility which allows the material to develop for up to a year. The composting ecosystem can be populated with worms, beetles and microorganisms to accelerate breakdown.

An important ingredient is oxygen. The system needs as much air to circulate around it as possible. This can be achieved by drawing in air from the room and out through a vent. In a simpler system the compost can be stirred manually. But a variety of more sophisticated systems are available on the market some including heaters, mechanical aeration, carousels to keep batches separate, and some pre seeded with worms beetles and bacteria.

“Remote,” “central,” or “under floor” composting toilets collect the waste in a manner which more closely resembles a conventional toilet.  By various means a waterless flush can propel the matter into a chamber. “Vacuum-flushes” can propel the matter horizontally or even upward with little water and into the composter. “Desiccating toilets” dry the waste and kill pathogens, although some studies suggest that rehydration can occur later bringing the germs back to life.

Remote Unit

composting toilets

composting toilets a remote underfloor unit

Any readers considering installing composting toilets should examine the various products on the markets. They vary widely in sophistication and operation and the determining factor is the circumstances of the user and the location. KCS would be happy to answer any specific enquiries you would like to raise. You may however consider building your own we offer suggested design below

Urine-diverting Compost Toilet

A:B:C Building Levels

 1: Composting medium chamber.      2:Ventilation pipe to outside.      3:Compost Toilet sea.      4:Urinal bowl.      5:Collection and dehydration of urine.

Given the enormous breadth of this subject we may return to composting toilets from time to time with more specific articles on different types and would like to hear from you if you have particular designs you would like us to feature in more detail