Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 233

One of the major areas of research concerning landfill is the use of bioreactors. A bioreactor is formed under specific land filling conditions. Bioreactor land filling is a process in which water and air are circulated into a specially-designed landfill, in order to cause accelerated biological decomposition of the waste material. The intention for this type of landfill operation is to maximize the generation of biogas, which is captured using a network of perforated pipes and burnt to generate electricity. Another desired outcome is the rapid stabilization of organic waste material (in order to minimize the length of time required to manage the landfill site or to make use of the decomposed material as compost).

By adding and recalculating liquids in bioreactors, decomposition is accelerated under anaerobic conditions, increasing the production of landfill gas by 2 to 10 times, approximately half of which is methane with at least 23 times the warming potential as carbon dioxide. The result is to shift substantial volumes of methane production, which otherwise would not occur for decades hence, to the present.

In the landfill, which acts as a bioreactor, anaerobic digestion occurs insofar as the naturally occurring processes of anaerobic degradation are harnessed and contained. Anaerobic digestion has a long history dating back to the 10^th century BC and involves four stages which are (i) hydrolysis, (ii) acidogenesis, (iii) acetogenesis, and (iv) methanogenesis. These stages result from the biological treatment of organic waste by two key bacterial groups – acetogens, and methanogens.

The hydrolysis stage is the chemical reaction where complex organic molecules are broken down into simple sugars, amino acids, and fatty acids with the addition of hydroxyl groups. The acidogenesis stage is the process where a further breakdown by acidogens into simpler molecules, volatile fatty acids occurs, producing ammonia, carbon dioxide, and hydrogen sulfide as by-products. The acetogenesis stage is the biochemical process where the simple molecules from acidogenesis are further digested by acetogens to produce carbon dioxide, hydrogen, and mainly acetic acid. The methanogenesis fourth stage is the biochemical process where methane, carbon dioxide, and water are produced by methanogens.

A simplified overall chemical reaction for the degradation of sugars produced by the hydrolysis of cellulose is:

Thus, the desirable product of the landfill bioreactor is methane. Other products may (depending upon the composition of the waste material in the landfill) include (i) a solid fibrous material, which is spread without further treatment, or after post composting (maturation), to provide organic matter for improvement of soil quality and fertility (improves soil structure and reduces summer irrigation demand) and (ii) a liquid fraction which contains nutrients and can be spread as a fertilizer and sprayed on crops. If the solid and liquid fractions are not separated, the slurry can be spread on the soil.

Effective gas generation in bioreactors under saturated conditions of rapid differential settlement without a low permeable cover is impossible. The result of this practice, which significantly increases early gas generation without effective gas capture, is a significant near-term increase of potent greenhouse gas emissions into the atmosphere.

The use of clay caps and advanced liner systems and bentonite is useful for the prevention of leachate and landfill gas release, but a consequence of this is that moisture, which is required for the biodegradative processes, is often excluded. The result of this is that much of the waste can remain intact entombed inside the landfill, potentially for longer than the lifetime of the barriers. One method to reduce this effect, by enhancing and accelerating waste stabilization, is to operate the landfill as a bioreactor. The bioreactor landfill attempts to control, monitor, and optimize the waste stabilization process rather than contain the wastes as prescribed by most regulations. This is carried out in an aerobic environment as opposed to the normal situation in landfill sites, where conditions within the wastes are commonly anaerobic.

The future of the bioreactor may well be found in its use with an anaerobic system i.e., a hybrid arrangement. For example, air could be added to the anaerobic processes after the degradation of waste has occurred, thus removing excess moisture from the landfill and fully composting the waste. The cycling of both conditions (aerobic and anaerobic) also offers the possibility of treating a greater range of chemicals such as the nitrification and denitrification of ammonia.

See also: Anaerobic Digestion, Landfill Gas.