I built a 1000L small scale digester about 10 years ago which incorporated chamber heating via solar hot water circulation as well as a biogas fired heat-exchanger as a back-up for cold nights. The system could readily achieve blood temperature and maintain this with minimal management and a small amount of electricity to drive the circulation pump intermittantly. A solar powered pump would have been sufficient. I was not so interested in the biogas component at the time as the unit was readily able to produce 2 cubic meters of up to 90% methane per day and I had no other use for this gas other than to flare it off. I used the gas volume as an indicator of digestation reactivity and a determinant for retention time. My interest was in the spent digestate, particularly the colloidal nutrient contained within it. The bio-gas was a bonus, the fertilizer was beyond compare. I split the digestate into colloidal liquid and pelletized the remainder. The colloids were sprayed as a foliar feeder and the pellets fed the soil and plant roots. Carbon content of the soil was raised considerably. Plant growth was exceptional and the fertilizer would enable the growth of vegetation on long term bare ground (at least 20 years barren) which would not previously support any growth at all. The full-scale plant (38,000L) was reproduced in India and achieved 22% better output results than I managed with the prototype. They called it the BioBowser. Peter Allison.
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