(1) The Ideal System
(a) All the nutrient contained in the excreta is returned to the soil to be used again.
(b) No area of the environment, whether it be the earth, river or sea, should be contaminated in the disposal of the excreta.
(c) The system devised to transport the excreta to its area of use should have no risk of contamination to its users.
(d) The process between the production of the excreta and the use of it in the ground should be designed to require as little energy as possible.
(2) Water-borne versus Domestic Unit
(a) Sewage contains large quantities of water (necessary for the transport of the excreta) and is difficult to treat. Water does have a certain ability for self-purification but this requires oxidation and usually the volume of water is too small in proportion to supply the required quantity of oxygen. Consequently the receiving water becomes foul and normal fauna (fish) are destroyed. The receiving water also becomes contaminated with pathogenic bacteria, protozoa and with eggs and larvae of harmful liver flukes.
(b) In a water-borne situation (septic tank, sewage pond) contaminated material may remain viable for up to six months, whereas in an aerobic situation a large number of different species of bacteria deactivate the contaminated material quicker. (I then went on to talk about China's 4000-year history of disposing sewage to land and Nauru's dwindling supplies of phosphates).
(3) Anaerobic and Aerobic Decomposition.
(a) Anaerobic composting (as in a septic tank) takes place in the absence of oxygen. WHO (World Health Organisation) presents a compelling argument against handling excreta in an anaerobic manner. Although primarily aimed at the liquid manure tanks of the 'under-developed' tropical countries ... it lends respectability to the much-maligned aerobic composting system.
(b) Aerobic (with oxygen) decomposition, in an ideal system (where high temperature is generated, and with a large variety of bacterial and fungal populations competing), can neutralise pathogenic bacteria and worm eggs in a matter of days.
(4) Conditions most suitable for good aerobic composting
(a) The primary key is to establish the correct proportions of carbon to nitrogen. The desirable C:N ratio for excreta to break down is around 30:1. The raw excreta ratio is about 15:1 (difficult to pinpoint because of the high N content of urine). The C:N ratio is regulated with the addition of sawdust which has a C:N ratio as high as 500:1.
(b) Enzymes exist which aid the rate of composting. The Japanese use a complicated recipe of soybean powder and cereal enzymes. Small amounts of topsoil introduce beneficial bacteria.
(c) The addition a garden lime helps establish the optimal pH.
(d) Humus is the end product of good aerobic composting.
I went on to calculate the total weight of excreta produced daily in what was then known as Wanganui and pointed out that we were wasting 6.96kg of nitrogen, 2.16kg potassium and 1.08kg every day.
I proposed a system using interchangeable canisters and even included a sketch of council trucks, an "excreta spreader" and a rotary hoe and the resulting humus fertilising pine trees on the sand dunes (ah, such youthful idealism).
Over the years I have experimented with various aerobic composting toilet designs. The current inter-changeable canister system we use at our piece of land incorporates a couple of recycled plastic chemical drums with holes drilled in the bottom and buried in the ground.
After the toilet is used, a handful of 50:50 sawdust and pumice topsoil (for aeration), with a little lime, is thrown in. When the drum is full it is replaced with an empty one.
The full drum (with a lid on) warms in the sun and can be rolled around for further aeration. The system costs less than $100 and works just fine.
**When Fred Frederikse is not building, he is a self-directed student of geography and traveller. In his spare time he is co-chair of the Whanganui Musicians Club
