MadSci Network: Microbiology

Re: gasses emitted from a compost pile

Date: Wed Jul 28 17:10:29 1999
Posted By: Andrew D. Brabban, Faculty, Biology, The Evergreen State College
Area of science: Microbiology
ID: 932945979.Mi

Above ground organic material is broken down (aerobically) into carbon dioxide and water by bacteria - just like you do with sugars in your body. The main thing released from your compost heap is therefore carbon dioxide (CO2). When you bury organic material, be it paper, waste food etc., in a landfill which is highly analogous to your description, there is one huge change in the life present in the system.

Many people consider oxygen as being a necessary item for life - it is not. Life has existed on this planet for 3.8 billion years, only om the last 1-2 billion has it existed with oxygen. So us oxygen users are the new kids on the block; life evolved without oxygen.

To most of these organisms oxygen is toxic, highly toxic, leading to instant death. But these organisms are dependent as is all life on water. As long as water is present life will thrive in a compost heap above ground or below (landfill). So what happens under ground in landfills. Well as you may recall from news programs houses built on landfills have a tendency to explode due to methane gas buildup. Why ?. Well this is the big difference. When oxygen is present organisms oxidize organic material to carbon dioxide

Glucose + oxygen --> carbon dioxide + water

Without oxygen life uses other methods. The first thing is that some organisms ferment the organic material to small compounds such as acetic acid, lactic acid, ethanol, propanal, propionic acid etc., as their way of degrading the carbohydrates. These small organic compounds are then utilized by methanogens which turn them into methane (CH4) and carbon dioxide. So the total process is the conversion of carbohydrates to methane and carbon dioxide (together termed biogas).

Glucose --> carbon dioxide + methane

Hence, you can see why houses built on waste sites can blow up. Other gases produced include hydrogen sulfide (H2S) and ammonia (NH4+ from the break-down of proteins. These gasses produce the scent of rotten eggs and window cleaner, respectively.

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