Anaerobic Digestion

Digestion Process

AD is also the name of the process of biological decomposition that occurs outside the presence of oxygen due to the activity of microorganisms. There are four stages to the process, as complex organic polymers are gradually broken down into their basic constituents; they include hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Though this process occurs naturally, it is actually rather sensitive. It is ideal for anaerobic digestion to occur within landfills in order to increase decomposition and ultimately create more capacity, but there need to be sufficient conditions to promote microbe colonies, including the right pH, a digestible substrate, and an adequate temperature. Increasingly, landfills are tapped for their biogas, which also contains methane, albeit in smaller proportions than in naturally occurring biogas. However, landfills are not designed to provide easy access to the waste they contain. In this way, they resemble the simplest form of anaerobic digester, a batch system. In these contexts, waste material is sealed off and breaks down undisturbed. Other digesters are designed to promote more control over the process, although this is difficult to achieve.

Steel anaerobic digestion towers in Germany. The Clement plant, built in 1976, processes over 400 million cubic meters of purified wastewater per year. Heavy rain and floods risk the discharge of untreated wastewater into the Rhine, as occurred in 1981 and 1995.

Digesters can be designed in a variety of ways, depending on how the process is maintained, at what temperature it occurs, and the kind of feedstock it is meant for. When digesters are developed independently from landfills—typically contained within steel, cylindrical vessels—they can be harnessed continually and predictably. The benefit of continual mixing is better gas production in addition to the ability to add material to alter the pH to a more optimal level; for example, adding basic compounds to lower acidity. Similar advantages come from heating digesters, so that the microorganisms that power them are the more efficient thermophilic variety, rather than mesophilic. Overall, the digestion process must sustain a relative chemical balance between stages of fermentation so that none of the microbes overwhelm the others. One common problem involves the overproduction of volatile fatty acids, or VFAs, which are the product of the acidogenesis stage and can disrupt the pH necessary to maintain later stages. One way of dealing with such complications is through a multistage digester, which allows for a more sustained, controlled process by separating the four stages into particular tanks, each corresponding to the microbial populations involved in the process.

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