Prepared For: Small-Scale Appropriate
Energy Technology Grants Program
Region IX
333 Market Street, 7th Floor
San Francisco, CA 94105
Prepared By: M. L. Kroschel
The Farallones
Institute Rural Center
Coleman Valley Road
Occidental, CA 95465
February 1981
PROJECT SUMMARY
Solar Energy Utilization with Compost Toilets
A compost toilet is a waterless individual on-site human waste management device which uses biological processes to decompose wastes into a reusable humus-like end product. General problems with compost toilet operation have been 1) Vectors gaining access to the wastes. 2) liquid buildup (faster than evaporation). 3) odor problems from the toilet. 4) Health issues relating to the pathogenic nature of the humus end product (incomplete treatment). Manufacturers of commercial compost toilets have added electrical heat tapes, fans, and pumps to different models in an effort to alleviate some of these problems. Thus the compost toilet evolved from an energy saving device to an energy consumer in an effort to make it more trouble free and automatic so that it would be more generally acceptable to the public at large and health officials in particular.
The goal of the DOE study was to utilize solar energy in lieu of electricity to help alleviate the basic problems with compost toilets. Solar devices were first used to improve the performance of existing facilities and additional systems were designed from inception with solar heating and venting systems. Four independent studies were initiated and executed over the two years of the grant period.
1) The design and construction of a solar pasteurization unit for treatment of 55-gal drum waste collection units.
2) Retrofit of an existing Clivus Multrum toilet for direct passive solar heat gain.
3) Design and construction of an owner-built flow-through compost toilet with a solar hot air heating system for hill side installation.
4) Design and construction of an integrated bathroom facility that included a solar heated flow-through compost toilet with a solar chimney venting system.
The solar pasteurizer (a large solar oven) was able to heat a 55-gal. drum 3/4 full of privy wastes above 55 degrees C in one week of labor intensive operation (opening and closing an insulated lid, 10am and 2pm solar time). However, the management requirement seemed excessive and the investigations were discontinued.
A Clivus Multrum was installed in a greenhouse and painted black for direct solar heat gain. The vent stack was insulated and a wind turbine vent cap added to aid exhaust. Four thermo couple probes measured inlet air, compost, inside air, and exhaust temperatures. Compost temperatures were not significantly higher than non-solar controls. The Clivus began ponding after one year of use. Several recommendations are made in the body of the report.
An owner built flow-through toilet was build using a ferrocement stucco over insulated wood framed construction. A hillside installation allowed placement of the solar collector below the compost chamber inducing thermo convection of heated air up through the compost and out the vent stack. Constant overloading of the facility prevented a conclusive study of the unit's performance. Thermophilic temperature ranges were not attained by the compost. Further study under more controlled circumstances would produce more conclusive results.
The integral solar bathroom demonstrates water and energy conservation and alternative energy utilization specific to the bathroom. The toilet venting system did not perform satisfactorily as designed and odors were increased rather than alleviated. Thermophilic temperatures were not reached in the compost. Further study is recommended under controlled use rat after several design modifications have been made.
In general, the use of high temperature solar devices to enhance the biological process of composting within a privy vault was not successful. The compost toilets in the study demonstrated their vulnerability to shock over loading characteristic of public facilities and therefore demonstrated their unsuitability for such applications. However, their performance under more controlled use rates (at or below design maximum) merits further study.