Biodigesters

Biodigesters convert manure into biogas and nutrient rich fertilizer, through an anaerobic reaction.[1]  The biogas can be used in cooking stoves as an alternative to fossil fuels. Biodigesters are generally simple to construct and prevents methane from entering the environment by turning animal manure into fuel for cook stoves or generators. The residual sludge can be used as fertilizer due to its high nutrient content.   The raw sludge may still contain residual pathogens though, so it should be used on non-edible plants and trees or composted to render it benign. This is brilliant in terms of stacking functions—we take a pollution problem (excess methane) and turn it into fuel for cooking and a fertilizer. 

Biodigesters can save people money they would have spent buying cooking fuel, just by directing their waste into this system instead of flushing it down the toilet or simply composting it.  Biodigesters are especially useful for farmers with lots of livestock, which generates tons of waste, creating a major pollution problem.  If every pig farmer in the world was processing their waste this way we could power cook stoves for millions of people in the developing world virtually for free and build soil on farms at the same time.

There are three main types: floating drum digesters, fixed dome digesters, and bag digesters.  Floating dome digesters originated in India and consist of a digester tank, an influent chamber, an effluent chamber, and a floating dome that rises and falls depending on the amount of gas in the chamber.  The gas pressure depends on the weight of the cover.[2]  Fixed dome digesters originated in China, and are the most expensive to make, but are the most durable.  The sides are made of brick, stone or poured concrete with a hemispherical top and bottom to withstand the high pressure of the gas-slurry mixture.  Bag digesters have caught on in many parts of the developing world because they are relatively inexpensive and easy to build.  Unfortunately the bags are plastic and are prone to punctures, making them less durable than the fixed dome option.