Hydrolysis & aerobic degradation
The initial stage of organic decomposition occurs during the emplacement of the yard waste in the Digester and for the period of time after, when oxygen is available within the waste. There chemical processes are initiated and facilitated by the presence of aerobic micro-biota which metabolize a fraction of the organic waste to produce simpler hydrocarbons, water, carbon dioxide and, as this is an exothermic reaction, heat (the heat generated can raise the temperature of the waste to up to 70ºC - 90ºC, however compacted waste achieves lower temperatures due to the reduced availability of oxygen). In these reactions, water and carbon dioxide are produced in the greatest concentrations. The carbon dioxide can dissolve in the water, forming a leachate that is rich in carbonic acid, which, in turn, lowers the pH of the surroundings. This stage generally lasts for a matter of days or weeks, depending on the amount of oxygen that is available within the waste.
Hydrolysis and fermentation
The removal of oxygen in Stage 1 facilitates a change in conditions from aerobic (oxygen present) to anaerobic (oxygen absent). Thus the majority of micro-biota found within the waste change to anaerobic species. Carbohydrates are hydrolyzed (a chemical process in which a molecule is split into two parts by the addition of a water molecule) to sugars, which are then further decomposed to form carbon dioxide, hydrogen, ammonia and organic acids. Proteins decompose via deaminisation (the removal of an amino - NH2 group) to form ammonia, carboxylic acids and carbon dioxide. The leachate that is produced at this stage contains ammoniacal nitrogen in high concentration. Acetic acid is the main organic acid formed but propionic, butyric, lactic and formic acids and acid derivative products are also produced, and their formation is affected by the composition of the initial waste material. The temperatures in the Digester drop to between 30ºC and 50ºC. Gas composition may rise to levels of up to 80% carbon dioxide and 20% hydrogen.
Acetogenesis
In this stage, anaerobic conditions are still present and the organic acids that were formed in the hydrolysis and fermentation stage are now converted, via specific microorganisms to acetic acid, acetic acid derivatives, carbon dioxide and hydrogen. Other microorganisms convert carbohydrates directly to acetic acid in the presence of carbon dioxide and nitrogen. Hydrogen and carbon dioxide levels begin to diminish towards the end of this stage, with the lower hydrogen concentrations promoting the methane-generating microorganisms (methanogens), which subsequently generate methane and carbon dioxide from the organic acids and their derivatives generated in the earlier stages.
Methanogenesis
This stage encompasses the main processes that lead to the production of Digester gas. At this point, the chemical processes involved are comparatively slow and can take many years to complete. Oxygen-depleted, anaerobic conditions still remain as in the previous two stages. Low levels of hydrogen are required to promote the methanogenic organisms, which generate carbon dioxide and methane from the organic acids and their derivatives such as acetates and formates formed in the earlier stages. Methane generation may also occur from the direct conversion of hydrogen and carbon dioxide (via microorganisms) in to methane and water: Hydrogen concentrations, produced during Stages 2 and 3, therefore fall to low levels during this fourth stage.
Oxidation
Oxidation processes mark the final stage of the reactions involved in the biodegradation of waste. As the acids are used up in the production of landfill gas (as seen in Stage 4), new aerobic microorganisms slowly replace the anaerobic forms and re-introduce oxygen to the region. Microorganisms that convert methane to carbon dioxide and water may also become established.
At this stage, air can be added to the anaerobic processes after the degradation of waste has occurred, thus removing excess moisture from the Digester and fully composting the waste. The remaining product will be safe and clean composted soil.
Spokane Waste to
Energy - Advancing Intelligent Solid Waste Management Solutions
Missing Tribe Inc
CH2MHill.pdf
Final
Field Summary
Solid Waste Management Plan
Osprey Communications
Puget Sound Electric - Power
Purchase Agreement
Menke directs sweetheart deal to
old cronies
Mayor Verner declares dictatorial
power over Regional System
HDR report: Spokane's WTE is falling
apart
Spokane's plan to buy County support
with YOUR taxes
Menke controlled audit of Regional
System
Spokane - Wheelabrator O&M
Agreement (1989)
