Compost toilets: an option for human waste disposal at remote sites

Abstract

Compost toilets are used for sewage treatment at remote sites, however there are often problems with anaerobic conditions creating smell. The composting process is well understood, thanks to large scale composting, but the applicability of the research to small scale compost toilets is uncertain.

The heat flows and evaporative performance of a Soltran Compost Toilet at Routebum Falls Hut on the Routebum Track in Mount Aspiring National Park, New Zealand, were assessed. Up to 8 kg of liquid were evaporated from the evaporator during a hot day, but 2 kg of this were condensed in the compost room. This condensation reduced net evaporation, but improved the heat transfer performance of the toilet. The heat released by the condensing water exceeded the heat transferred by hot air. Overall the amount of heat transferred was small and what was transferred was quickly lost from storage by evaporation.

Separation of urine and faeces at source has been identified as the factor that could most improve composting performance in an existing toilet. Auxiliary heating improves compost performance in existing toilets, by a combination of increased rate of composting and increased evaporation of urine from the compost. Separation of urine is likely to achieve good composting without the need for auxiliary heating.

The high air flows, needed in compost toilets to ensure odours do not reach the users' nostrils, result in evaporative cooling. This ensures the compost, in existing toilet designs, will remain close to ambient temperatures; any surplus heat (biological or auxiliary) will be quickly removed by evaporation. In addition, only the recent (less than one month) additions to the compost will contribute heat. The remaining mass (one to two years of compost) contributes no heat. The conduction losses through the walls of the large container necessary to hold this mass, eliminate the possibility of maintaining high temperatures in the pile without extremely good insulation.

However composting will initiate at temperatures above 4°C, although the rate of composting is affected by temperature. A formula relating compost temperature, daily usage, surface area of compost, and oxygen penetration depth is proposed as a means of identifying the overload point within a compost toilet.

This research concludes that successful compost toilets can take one of two paths:

a. high temperature (high speed) composting - to achieve this will require a small, sophisticated composting chamber.

b. ambient temperature composting - a vault/pit composter with separation of urine. Because composting occurs at ambient temperatures, the rate of composting will vary as temperature varies. Design considerations for these toilets will need to take ambient temperature into account by sizing the surface area of the receiving chamber accordingly.

Existing compost toilet designs do not get hot, so do not fit into category (a) and have a restricted surface area, so do not fit into category (b). In many ways they are more like an ambient temperature compost toilet, but are susceptible to overloading because of the restricted surface area and addition of urine.

Composting systems occur on a continuum from the ambient temperature, adequate airflow system (double vault compost toilets) which can be likened to a 'forest floor ecosystem', to the high rate 'bacterial' dominated composting system. The design considerations for the two are totally different and one cannot be easily transferred to the other. High use sites in colder areas will need a design based on the high rate system.