Encyclopedia of Renewable Energy. James G. Speight
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Whereas the production of volatile fatty acids (VFAs) is increased when the process pH is in excess of 5, the production of ethanol (C2H5OH) is characterized by a pH lower than 5 with reaction process coming to a halt at a pH less than 4.
In a balanced bacterial process approximately 50% of the monomers (glucose, xylose, amino acids) and long-chain fatty acids (LCFA) are broken down to acetic acid (CH3COOH). Twenty percent is converted to carbon dioxide (CO2) and hydrogen (H2), while the remaining 30% is broken down into short-chain volatile fatty acids (VFAs). Fatty acids are monocarboxylic acids that are found in fats and have fewer than six carbon atoms whereas long-chain fatty acids. If there is an imbalance in the digester process, the relative level of volatile fatty acids will increase with the risk of accumulation, since the bacteria that degrade the volatile fatty acids have a slow growth rate and cause an imbalance between the various phases of the process. A steady degradation of the volatile chain fatty acids is therefore crucial and often a limiting factor for the biogas process.
Hydrolysis of simple fats results in 1 mol glycerol and 3 mol long-chain fatty acids and, therefore, high proportions of fat in the digester feedstock will result in large amounts of long-chain fatty acids, while large amounts of protein, which contain nitrogen in amino groups (-NH2), will produce large amounts of ammonium/ammonia (NH4+/NH3). In both cases this can lead to inhibition of the subsequent decomposition phase, particularly if the composition of the biomass feedstock varies.
See also: Acetogenesis, Acidogenesis, Acidogenic Digestate, Anaerobic Digestion, Methanogenesis.
Acidogenic Digestate
Anaerobic digestion produces two main products: (i) digestate and (ii) biogas.
The digestate is the material remaining after the anaerobic digestion of a biodegradable feedstock which is then used as a source of renewable energy through the participation of a variety of chemical reactions (Table A-4).
Acidogenic digestate is fibrous and comprises structural plant material which includes lignin and cellulose. It is the acidogenic digestate that possesses the high moisture retention properties and the raw digestate usually also contains minerals and the remains of the micro-organisms (mainly bacteria) which were active during the digestion process. On the other hand, methanogenic digestate is a liquid or sludge (sometimes referred to as a liquor) which is often high in nutrients such as ammonium derivatives and phosphate derivatives.
Table A-4 Common reactions in anaerobic digestion process.
Key: acetate CH3CO2− , propionate C3H9CO2− , butyrate C4H9CO2−
There are four stages to the digestion process, each of which has different characteristics: (i) hydrolysis, which is a chemical reaction where particulate matter (solid matter, such as carbohydrate derivatives) is solubilized and high molecular weight chemicals are converted into lower molecular weight products, (ii) acidogenesis, which is a biological reaction in which lower molecular weight products are converted into volatile fatty acid derivatives, (iii) acetogenesis, which is a biological reaction in which the volatile fatty acid derivatives are converted into acetic acid where volatile fatty acids are converted into acetic acid, carbon dioxide, and hydrogen, and (iv) methanogenesis, which is a biological reaction in which acetate derivatives are converted into methane and carbon dioxide, while hydrogen is consumed. Thus, using carbohydrate derivatives as the example of the digester feedstock:
(i) Hydrolysis
(ii) Acidogenesis
(iii) Acetogenesis
(iv) Methanogenesis
The acidogenic digestate is a stable organic material comprised largely of lignin and chitin, but also of a variety of mineral components in a matrix of dead bacterial cells and some plastic may be present. This resembles domestic compost and can be used as compost or to make low-grade building products such as fiberboard. Another byproduct is a liquid (methanogenic digestate) that is rich in nutrients and can be an excellent fertilizer dependent on the properties of the digester feedstock (Table A-4).
If the feedstock sent to the digester includes low levels of toxic heavy metals (i.e., metals with relatively high density, atomic weight, or atomic number) or synthetic organic chemicals such as pesticides or polychlorobiphenyls, the effect of digestion is to concentrate such materials in the product (i.e., the digester liquor). In such cases further treatment will be required in order to dispose of this liquid properly. In extreme cases, the disposal costs and the environmental risks posed by such materials can offset any environmental gains provided by the use of biogas. This is a significant risk when treating sewage from industrialized catchments.
See also: Anaerobic Digestion, Digester, Digestion.
Acid Rain
Acid rain, or acid deposition, is a broad term that includes any form of precipitation with acidic components, such as sulfuric acid or nitric acid that fall to the ground from the atmosphere in wet or dry forms. This can include rain, snow, fog, hail or even dust that is acidic. Acid rain is another environmental problem that affects much of the eastern United States, damaging crops, forests, wildlife populations, and causing respiratory and other illnesses in humans.
Acid rain is formed when oxides of sulfur and oxides of and nitrogen react with water vapor and other chemicals in the presence of sunlight to form various acidic compounds in the atmosphere. Acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents. The sulfur dioxide and nitrogen oxides react with water, oxygen and other chemicals to form sulfuric and nitric acids.
These then mix with water and other materials before falling to the ground:
These acids can form particulate matter by reaction with, for example, ammonia in the air or with metals particulates.
While a portion of the sulfur dioxide and oxides of nitrogen that cause acid rain is from natural sources such as volcanoes, another part is due to the combustion of carbonaceous fuels. Wind currents can carry these oxides over long distances and across borders making acid rain a global problem and not just for those who live close to these sources.
Other pollutants that are emitted as a result of the combustion process are hydrocarbon derivatives (including unburned fuel and hydrocarbon products of the process) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which the nitric oxide is converted to nitrogen dioxide (NO2) – a brown gas and at sufficiently high levels can contribute to urban haze. However, nitrogen dioxide can absorb sunlight and break apart to produce oxygen atoms that combine with the oxygen in the air to produce ozone (O3), a powerful oxidizing agent, and a toxic gas.
A final comment related to acid rain since the formation of the rain is usually is also attributed to the combustion of fossil fuels and fossil fuel products. Caution is advised when making such a grandiose statement. Any renewable energy source that contains nitrogen and/or sulfur will contribute to the formation of acid rain. In fact, when air is used as the oxidant in a combustor, part of the nitrogen in the air can be converted to nitrogen oxides and thence to acid rain. In all cases, as the fossil fuel users have learned, the gas from combusters such