I love anaerobic biodigestion as a topic. To an eco-geek like me this is like turning lead into gold - biological alchemy at its best. Why? Because we need to integrate better with nature, making our waste into a useful resource to the ecosystem is what it's all about.
Anaerobic biodigestion is the process of microorganisms breaking down organic material in an oxygen depleted environment. Biodigestion is commonly used for treating municipal sewage and agricultural effluent, but nearly any organic material can be digested including grass clippings, paper, and food waste. The process has four stages: - Hydrolysis
- Acidogenesis
- Acetogenesis
- Methanogenesis
(wikipedia: Biodigester) "The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide." Bio-digestion also produces trace amounts of hydrogen sulphide (H2S), a toxic gas. Methane - The last stage produces methane (CH4) or "biogas," which differs from natural gas in that the carbon is not fossil. Burning biogas does not increase the amount of C02 in the atmosphere becuase the carbon contained in the organic wastes came from the current atmosphere not a fossil one - Biogas is a "living" fuel, not a "fossil" fuel. Carbon dioxide - again the carbon dioxide that is a direct product of biodigestion is carbon neutral. However, the HDPC design uses the CO2 to encourage algae and plant growth. Hydrogen sulphide - is a broad spectrum poison and was used as a nerve gas in WWI by the British. The HDPC design captures the H2S and recommends it be used to clean water laced with heavy metals from heap leaching. Filtrate - the water produced from biodigestion is highly enriched nutrient soup perfectly suited for hydroponic agriculture. Digestate - the solid material left over from bio-diegestion is suitable fodder for composting. Energy - the bacteria responsible for bio-digestion operate in moderate (mesophilic - 20-45 oC) to high (thermophilic 50-70 oC) temperatures. The advantages of high temperature include faster gas production and greater sterilization of undesirable bacteria. However, the higher temperatures require energy input, so systems should be carefully designed to ensure energy output is greater than input. Cost - a municipal bio-digestion system capable of handling 20,000 tonne/year of biogenous waste from Kompogas costs $5.27 million (4 million Euro). |
