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Safety of biogas slurry, and how time effective is biogas vs rocket stove?

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I have been reading up about building or making our own biogas plant.  It is presented as a steady stream of great fertiliser, but then I have read conflicting reports about its safety for humans and soil life.  Some say it is better than manure, but I also read the below from this website.  http://journeytoforever.org/biofuel_biogas.html

Has anyone researched this aspect?  We would not put human manure in it, and we could put animal or just greenwaste in our biogas generator.  This would reduce the microbes going back to humans, but what about the toxicity to soil life?

We have also read about the (lack of) efficiency of biogas on this website and have wondered if it is the best way to go.  We currently have the house and water heated with solar water panels and wood.  our current usage is about 10kW of power a day, most of which is cooking.  We plan to get solar and already have the inverter and batteries (we have this to run a food trailer at the markets) so really only need the PV panels and charger.  We thought it best to get rid of our cooking needs first, and then add the solar panels to cover the rest.  We are looking at building a covered picnic area with a rocket stove/oven/thermal mass against the house, but thought we would also have biogas for the quick "five minute" boil of the kettle, when you don't want to light the fire.  But is this reallty time effective, or would the time spent tending a 1000L IBC tank generator be more than that of lighting a stick fire to make a cup of coffee?  We already plan to have stored water for heat in the house, so it would be an easy thing to put the IBC tank in side a wooden tank and keep it at 28C or more.

We also have sufficient grass clippings with toxic leaves and weed scraps to keep a biogas generator feed on a regular basis.  (we are in a temperate climate with frost but no snow and grass growing slowly through winter).  

We also have heaps of manure that we do collect in the field but it seems to me that the best use of that would be to grow compost worms to feed back to the chickens, as it feed value for the biogas seems not worth the hassle of turning into a slurry and feeding in.


Apart from the gas, digesters also produce waste sludge and supernatant, the spent liquid of the original slurry. The waste is rich in plant nutrients, and it's often touted as a great organic fertiliser. Many of the resources listed below claim that.

However, the people who design biogas digestion systems are nearly always engineers, not biologists, and they tend to think chemicals are chemicals, as indeed they are, and these particular chemicals are indeed both nutrients and organic. But that doesn't make it a fertiliser — in fact it kills earthworms and wrecks the soil micro-life, which is the basis of soil fertility. This is what biologists say about it:

"Placement of E. foetida [manure worms] into sludge freshly removed from an anaerobic digester or in freshly-passed human excreta results in 100% mortality within a few hours." — From "Physicochemical Requirements in the Environment of the Earthworm Eisenia Foetida", by David L. Kaplan, Roy Hartenstein, Edward F. Neuhauser and Michael R. Malecki, Soil Biology and Soil Biochemistry, Vol. 12, pp 347-52, Pergamon Press, 1980.
Biogas digestion is an anaerobic process (no oxygen involved), unlike composting, which is aerobic (with oxygen). Compost gets hot, up to 60 deg C or more (140 deg F), biogas digesters don't get hot. Anaerobic digestion produces volatile fatty acids and volatile organic acids, both of which are phyto-toxins — plant poisons. Not what you want in your soil.

Add the sludge and supernatant to your compost pile. Biogas digestion makes the best sense when it's coupled with hot aerobic composting, then nothing will be wasted. Also, the heat in an active compost pile can be harnessed to produce a hot water supply, which means you can use the compost to keep the digester at working temperature during cold weather.

Another myth is that the digestion process kills off the pathogens in the manure (cow, pig, poultry and human manure are common feedstock for biogas digesters). Hot composting kills off pathogens reliably, when it's done right, but biogas digestion doesn't kill the pathogens in the manure.

"Indian biogas plants have short detention times. These are unlikely to destroy intestinal parasites, which are widely prevalent in rural areas of India. As a result, if the biogas sludge were used as a fertilizer, it would likely increase the spread of intestinal diseases." — From "Community Biogas Plants Supply Rural Energy and Water", UNDP

"Work I did and studied in the 1970s with 'dungas' production (anaerobic digestion) in South Africa and India showed that the drawback was the high concentration of pathogens in the resultant slurry. ... It's the pathogen problem that caused me to abandon this line of research as a viable source of alternative energy production and return to aerobic composting." — Walker Bennett, Organic Gardening Discussion List, 15 Oct 1999.
This isn't a problem as long as you're aware of it. Adding the sludge and supernatant to the compost pile will kill all pathogens.

If you're not in a position to make compost, flush the sludge and waste liquid down the toilet — let your municipal wastewater treatment facility handle it for you, that's what they do all the time.

If you keep poultry, there's a way to make better use of the sludge, though it doesn't need very much of it. See High-protein poultry feed from thin air

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