The Fundamental of Biogas
The fundamentals are simple. In order to produce bio-gas (methane) with any efficiency, the following things are essential: no air should be present during the digestion; the temperature of the batch should be kept at a constant 95oF; and the ratio of carbon to nitrogen in the material should be about 30 to 1.
All this caters for methane bacteria. These ancient, energetic organisms break down waste material in the absence of air -- under water, for example. Methane bacteria were busy in swamps 500 million years ago and they are busy right now in sewer mains, septic tanks, marshes and ponds all over the world.
The rule of thumb is that 46 pigs equals one gallon of petrol per day. More precisely: the daily excreta from 46 pigs will yield methane with an energy content equal to one gallon of petrol.
For cattle, the conversion figure is 5.5 cows per gallon of petrol. (Which assumes that all the manure can be collected.) If you want to use chickens, you'll need 810 of them.
If you want to extract methane from wastes like cauliflower leaves, you can do it. You have to use the "single-batch" method. Pack everything into an air-tight vat, keep it at the right temperature, and draw off methane for as long as it comes. Again, this can be done on any scale, for oil-drum size upwards. The only snags are that the gas output is variable as the batch ages and you have to clean out the whole digester after each batch is exhausted.
It takes 5000 pounds per square inch to liquefy methane at ordinary temperatures.
If air gets into the digester -- or the first bit of gas produced isn't vented off so as to expel any air already there -- and anything should spark the mixture, the explosion could be memorable.
no compost heaps should be near the digester, at least not if the compost contains animal remains. Rotting meat gives off the gas phosphine. And if phosphine blends with methane and oxygen, the mixture bursts into flame like a sudden demon.
The greatest promise for methane production may not lie with processed animal manure or with ordinary vegetable wastes. The highest methane yields are from primitive algae. Elsewhere, schemes are afoot to unite algal ponds, methane production, and waste heat from power stations.
Here's the proposed cycle: power station waste-heat would warm pools of algae, producing tons of the wispy green material; the algae would be fed into digesters, generating methane; and the methane would be piped back into the engines of the power station. Almost a perpetual-motion machine. Except that the power station wouldn't run solely on methane -- just get a boost. And not to be forgotten, the sun contributes too, quietly and hugely, as it radiates energy into the fast-growing algae.
The hub of this system is the interplay between two of the oldest organisms on the planet: methane bacteria and algae cells. Something so fundamental and simple has appeal. After all, dead and fossilised ooze -- oil -- powers the world right now. Why not living ooze to help power whatever type of world lies ahead?