Missing Tribe Inc Earthen Digester

Organic reactions in an Earthen Digester

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.

These five stages play an important role in the proportions of the leachate constituents that the Digester produces. Leachate and the factors that affect its quality are described in more detail below.

	MISSING TRIBE INC Solid Waste and Recycling Consultants
	An Earthen Digester is a specially designed landfill that takes only yard waste and uses natural processes to convert organic matter into methane. The methane can be cleaned and sold to natural gas distributors, or burned on site to produce electricity or heat for industrial purposes. Once the methane gas is depleted from the site, the process is stabilized, and the entire Digester site can be harvested as clean composted soil, free of herbicide residue as with conventional composting.
	Organic reactions in an Earthen Digester 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. These five stages play an important role in the proportions of the leachate constituents that the Digester produces. Leachate and the factors that affect its quality are described in more detail below.
	Leachate Unlike typical landfill leachate, Digester leachate will only contain materials from the decomposition of plant life. Therefore, metals and other hazardous matter will not contaminate the leaching process. At the start, the leachate produced is typically generated under the aerobic, oxygen-rich conditions, and this produces a complex solution with near neutral pH of 7.5. This process generally only lasts a few days or weeks and is relatively unimportant in terms of the quality of the leachate. However the heat produced during aerobic decomposition can sometimes rise to as high as 80ºC - 90ºC, and if this heat is retained it can enhance the later stages of leachate production. As waste decomposition progresses, the oxygen within the waste gradually becomes diminished, and the conditions become anaerobic. This initially creates high concentrations of soluble degradable organic compounds and an acidic pH. After several months or years, the onset of methanogenic conditions changes the pH of the leachate, making it more neutral or even slightly alkaline. As biodegradation nears completion, aerobic conditions may return, and the leachate will eventually become neutral to the environment.