Anaerobic Digestion
Anaerobic digestion is the breaking-down of organic matter (biomass that comes from plants or plant wastes) through bacteria in absence of oxygen. In the industry, an anaerobic digester is the system that harnesses the process of anaerobic decomposition to create products/by-products. This technology allows us to treat biodegradable material, produce biogas in order to power machinery, produce heat for homes, and manufacture soil conditioners to improve plant growth.
The digesters have existed for a long time, especially for processing sewage and animal feces. With the increasing awareness of the need to reduce wastes and the decreasing space available dispose of it, digesters have become very important as it reduces a lot of the biodegradable items that would have gone in landfills or garbage incinerators.
Many things can act as the fuel for the digestion. Any organic matter that is unwanted can be disposed of in this fashion. Specific crops grown for the sake of anaerobic digestion, called energy crops, also are one of the major contributors to increase biogas production. Before any digestion occurs, unwanted items, like inorganic and toxic substances, are first removed from the material by sorting or screening. After, they are cut up to maximize surface area and boost the process rate. The fuel is dumped into a digester where the bacteria are given the optimum environment to do their work. There may be devices in the digesters that rotate the contents and remove excess.
There are two different temperatures that allow anaerobic digestion. The mesophilic way works the best around 37-41ºC or room temperature (20-25ºC). The thermophile method is most efficient at 50-52ºC to up to 70ºC.
Digestion can occur in a wet or dry environment, dry being that the mixture is 30% or more solid and wet being that it is less than 15% solid.
The length of time necessary to decompose the organics varies according to the temperature, amount and type of feed material. With mesophilic digestion, the process may go for 15 to 30 days in order to go to completion. The latter process is quicker as it takes about two week, but is the more expensive choice, considering the heat that is necessary for the bacteria to work effectively. Thus, the first process is preferred and more commonly found in use.
There are four major steps to anaerobic digestion. The first step is hydrolysis, where the complex materials are broken down into their basic building blocks of monosaccharides, amino acids, and fatty acids. Then acidogenesis (acid fermentation) occurs, where these things are broken into smaller and simpler ones. By-products (ammonia, carbon dioxide, and hydrogen sulphide) are made in this process. Acetogenesis follows. At this stage, the simple molecules are processed even more to create carbon dioxide, hydrogen, and, largely, acetic acid. The last step is methanogenesis (methane fermentation), where, as its name suggests, the production of methane, water and carbon dioxide by methane-forming bacteria result.
There are several types of reactors, two types of digester operations being more commonly found: batch and continuous. Batch is simply placing the material in the container and leaving it there until the process is complete. However, this method is unpleasant as odours are emitted when emptying. Continuous, which is more often used, is adding the feed constantly while removing the finished products simultaneously. This method is better because there is not the long wait for the bacteria to repopulate.
Three main products come from this process: biogas, acidogenic digestate, and methanogenic digestate.
1. Biogas is made of a mixture of methane and carbon dioxide, with some hydrogen and traces of hydrogen sulphate. The production of the gas is not even at all times, with the peak amount being made in the middle of the reaction. At the beginning there is very little gas because the bacteria culture has not multiplied enough; by the end, there is very little gas, the reason being that only the hard-to-digests are left. Normally, this gas escapes into the air, but it can be captured from landfills, farms, and even household sewage. Some methane also exists underground and even deeper in ocean sediments. Biogas can be used to generate electricity and warm digesters/buildings. It can also be used to substitute propane, butane, natural gas, or fuel oil in certain adapted machines.Some factors that influence the quality of the biogas produced are:
- the quality and characteristics of the substance that’s being digested
- temperature
- toxic substances in the substance
- pH
- rate of digestion due to how much substance is added to the digester at one time
2. The second by-product resembles domestically decomposed products. It is made mostly of lignin and chitin, with some minerals, and possibly plastic, mixed in. this material can be used as compost or to manufacture low-quality building materials such as fibreboard.
3.The last by-product is a liquid that can be used as a fertilizer, depending the quality of the feed material.
It is often with a good amount of nutrients that plants need, like nitrates and phosphates. The sludge is often separated by filtration.
Characteristics of biogas include:
- explosive nature like natural gas or liquid propane, especially when mixed with oxygen
- may need to be cleaned because hydrogen sulphide is very corrosive and poisonous
- it should be used immediately or otherwise stored
- lower heat value than most fuels
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