2.8 Biogas

Biogas is a mixture of colorless, flammable gases produced by the anaerobic fermentation of organic waste materials. Biogas is a mixture of methane, carbon dioxide, small amounts of carbon monoxide, hydrogen, nitrogen, oxygen, hydrogen-suIphide, and hydrocarbon gas. The actual percentage of each gas varies with raw materials, the ratio of input materials, temperature, and fermentation stages. Typically the composition of biogas is as follows (Fernando and Dangaggo 1986).

Methane-54 -70%

Carbon monoxide-0.1%

Carbon dioxide-27-45%

Oxygen-0.1%

Nitrogen-0.5-3%

Hydrogen sulphide- Trace

Hydrogen-1-10%

Methane is the major combustible component of biogas. Others usually in small quantities are carbon dioxide, hydrogen, and hydrogen sulphide.

Biogases are obtained by the fermentation of organic materials such as animal, human, agricultural and industrial wastes. These include animal feces, municipal sludge and garbage, abattoir waste, paper waste, and waterweeds. The rate of conversion of the organic waste to the end products at an appropriate temperature depends on the complexity of the waste. In the treatment of complex wastes such as sewage and slaughterhouse waste, generally slower loading rates should be used due to the much slower conversion rate of the biodegradable suspended compounds than of the soluble compounds such as young plant materials. It is thus expected that the complexity of the waste is a significant factor that affects the rate of anaerobic digestion of any type of waste.

2.8.1 Biogas Production

A study carried out by Almoustapha et al., 2009, revealed that it is possible to produce biogas from a mixture of water hyacinth and fresh rumen residue. Chanakya et al., 1993; Kivaisi and Mitla, 1998; Kumar, 2005 as well as Kunatsa et al., 2013 among other authors confirmed the possibility of producing biogas from water hyacinth. Almoustapha et al., 2009, highlighted that biogas outflow is related to seasonal variations in temperature. For the same retention time, the total volume of biogas obtained during the warm season is 1.8 times greater than that obtained during the cool season. According to Nijaguna (2002), aquatic plants generate high-quality biogas but their salinity of 35-50 parts per 1000 is a serious problem. Water hyacinth generates biogas that has greater methane content and more soil nutrients than digested dung. Nijaguna highlights that water hyacinth produces 20.3 liters of biogas per kg of dry matter. On the other hand, Dr. Jagadeesh, 1990, noted that a hectare of water hyacinth can produce enough biogas each day to generate between 90 and 180 cubic meters of methane, equivalent to 3.44 to 6.88GJ of energy production. Despite the serious drawbacks, water hyacinth invasions in lakes can be harnessed for environmental benefit and renewable energy production. Water hyacinth has a very high cellulose content making them a potential renewable energy source. While controlling water hyacinth populations has proven to be beyond the capabilities of local government, using these plants for energy production provides an alternative approach to dealing with this invasive species. Water hyacinth can be used to produce biogas, an energy source that already has been embraced the world over. An investigation into the possibility of bio-converting water hyacinth to yield biogas adds value and solves the problem of water hyacinth management as well as gives a solution to the energy and power shortages since people would no longer rely on the expensive LPG or grid electricity. Biogas will lead to reduced use of fuel wood and diesel generators hence an innovative technology to the reduction of greenhouse gas emissions. Besides energy production, other valuable products, such as high-quality bio-fertilizer are obtained from the anaerobic digestion of water hyacinth and this will minimize the use of expensive mineral fertilizers. The option of biogas production as a way of energy exploration using water hyacinth may not only sustain the energy availability but also improve environmental sustainability by improving the social, economic, and physical well-being of the environment.

2.8.2 Process

The anaerobic digestion process involves a high number of microorganisms, which convert the feedstock to methane and carbon dioxide-rich biogas through a series of biochemical reactions that can be described by four steps, viz. hydrolysis, acidogenesis, acetogenesis, and methanogenesis. These microorganisms include hydrolytic bacteria, acid-forming bacteria (acidogens), acetic acid-forming bacteria (acetogens), and methanogenic bacteria (methanogens).

Hydrolysis In the first step, hydrolysis, the complex molecules of carbohydrates, proteins, and lipids are split into simple components (sugars, fatty acids, and amino acids) with the help of extra-cellular enzymes secreted by microorganisms which are mostly obligate anaerobes. A complex consortium of microorganisms participates in the hydrolysis and fermentation of organic material. It is the first step is inhibited by lignocellulose-containing materials, which are degraded only very slowly or incompletely (Rilling, 2005).

Acidogenesis It is the second phase; the monomers produced in the hydrolysis phase are further degraded by fermentative bacteria into short-chain organic acids, with one to five carbons (valeric acid, butyric acid, propionic acid, acetic acid, and formic acid), alcohols, hydrogen, ammonia, and carbon dioxide. In a stable process, with a low partial pressure of hydrogen, the main products formed by the fermentative bacteria are acetate, carbon dioxide, and hydrogen. Acidogenesis products are volatile fatty acids (VFA), alcohols, aldehydes, hydrogen, and carbon dioxide. Decomposers are fermentative bacteria or anaerobic oxidizers. When the partial pressure of hydrogen is high, more intermediates such as volatile fatty acids and alcohols are formed.

Acetogenesis In the third step, acetogenesis, the products of the acidification are converted into acetic acids, hydrogen, and carbon dioxide by acetogenic bacteria. Acetogenic bacteria such as Syntrobacter wolini and Syntrophomonas wolfei convert volatile fatty acids (e.g. propionic acid and butyric acid) and alcohol into acetate, hydrogen, and carbon dioxide, which are used in methanogenesis. The acetogenesis is regarded as thermodynamically unfavorable unless the hydrogen partial pressure is kept below 10-3 atm, by an efficient hydrogen removal pathway of hydrogen-consuming organisms such as hydrogenotrophic methanogens and/or homoacetogens. The first three steps of anaerobic digestion are often grouped together as acid fermentation. In acid fermentation, no organic material is removed from the liquid phase: it is transformed into a form suitable as a substrate for the subsequent process of methanogenesis.

Methanogenesis The last step in anaerobic digestion is methanogenesis. The methanogenic microorganisms work under strictly anaerobic conditions. The methanogens, which belong to the group archaea, differ from the other organisms in the anaerobic reactor, which are bacteria. Archaea are more sensitive than bacteria with regards to environmental stresses in the reactor, such as pH, or toxic compounds such as heavy metals or different toxic organic materials. The methanogens mainly use acetate, carbon dioxide, and hydrogen, but also methylamines, alcohols, and formate for the production of methane. About 70% of the methane production arises from the acetate, and about 30% of the methane arises from hydrogen and carbon dioxide. The methanogens have the longest generation times (2-25 days) of the microorganisms in the reactor, which makes this step the most time-limiting step for easily hydrolyzed materials.