Chapter two: Introduction and overview
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Anaerobic treatment is the use of biological processes, in the absence of oxygen, for the breakdown of organic matter and the stabilization of these materials, by conversion to methane and carbon dioxide gases and a nearly stable residue. As early as the 18th Century the anaerobic process of decomposing organic matter was known, and in the middle of the 19th Century, it became clear that anaerobic bacteria are involved in the decomposition process. But it is only a century since anaerobic digestion was reported to be a useful method for the treatment of sewage and offensive material. Since that time, the applications of anaerobic digestion have grown steadily, in both its microbiological and chemical aspects. The environmental aspect and the need for renewable energy are receiving interest and considerable financial support in both Developed and Developing Countries, expanding research and application work in these directions, and many systems using anaerobic digestion have been erected in many countries.
Anaerobic digestion provides some exciting possibilities and solutions to such global concerns as alternative energy production, handling human, animal, municipal and industrial wastes safely, controlling environmental pollution, and expanding food supplies.
Most technical data available on biogas plants relate primarily to two digester designs, the floating cover and fixed dome models. Promising new techniques such as bag, dry fermentation, plug flow, filter, and anaerobic baffled reactors should be explored to establish a firmer technical base on which to make decisions regarding the viability of biogas technology. Along with this increase in interest, several newer processes have developed, that offer promise for more economical treatment, and for stabilizing other than sewage materials - agricultural and industrial wastes, solid, organic municipal residues, etc. - and generating not only an alternative energy source, but also materials that are useful as fodder substitutes and substrates for the mushroom and greenhouse industries, in addition to their traditional use as organic fertilizers. Other benefits of anaerobic digestion include reduction of odours, reduction or elimination of pathogenic bacteria (depending upon the temperature of the treatment) and the use of the environmentally acceptable slurry.
The technology of anaerobic digestion has not yet realized its full potential for energy production. In most industrialized countries, biogas programs (except for sewage treatments) are often hindered by operational difficulties, high costs of plants and as yet low energy prices. In most Developing Countries, expansion of biogas programs have been hindered because of the need for better economic initiatives, organized supervision and initial financial help, while in other Developing Countries, on the other hand, slow development has been observed, and a lack of urgency, because of readily available and inexpensive non-commercial fuels, such as firewood.
Biogas technology is also potentially useful in the recycling of nutrients back to the soil. Burning non-commercial fuel sources, such as dung and agricultural residues, in countries where they are used as fuel instead of as fertilizer, leads to a severe ecological imbalance, since the nutrients, nitrogen, phosphorus, potassium and micro-nutrients, are essentially lost from the ecosystem. Biogas production from organic materials not only produces energy, but preserves the nutrients, which can, in some cases, be recycled back to the land in the form of a slurry. The organic digested material also acts as a soil conditioner by contributing humus. Fertilizing and conditioning soil can be achieved by simply using the raw manure directly back to the land without fermenting it, but anaerobic digestion produces a better material. Chinese workers report that digested biomass increases agricultural productivity by as much as 30% over farmyard manure, on an equivalent basis (van Buren 1979). This is due in part to the biochemical processes occurring during digestion, which cause the nitrogen in the digested slurry to be more accessible for plant utilization, and to the fact that less nitrogen is lost during digestion than in storage or comporting. The stability of the digested slurry and its low BOD and COD are also of great importance. This aspect of biogas technology may, in fact, be more important than the gas produced (Gosling 1980; Marchaim 1983).
In the area of public health and pollution control, biogas technology can solve another major problem: that of the disposal of sanitation wastes. Digestion of these wastes can reduce the parasitic and pathogenic bacterial counts by over 90% (Feacham et al. 1983; McGarry and Stainforth 1978; van Buren 1979; Klinger and Marchaim 1987), breaking the vicious circle of reinfection via drinking water, which in many rural areas is untreated. Industrial waste treatment, using anaerobic digestion, is also possible.
Many planners and engineers have expressed an interest in obtaining information on anaerobic digestion and biogas technology. Application to FAO in Rome of the fundamentals of design and operation of digesters to enhance their technical and economic viability, were the main reason for this review. An additional review, which will describe more technical aspects, will be published, in order to explain the complexity of this interdisciplinary technology, which requires a broad overview of the whole program for optimal selection of size and style of the digestion system.
The present review attempts to present only very basic information on the engineering aspect, while giving some more detailed description of the biochemistry and microbiology, and emphasizing the economic and socio-cultural aspects of biogas programs and the uses of the products of the anaerobic digestion process, especially as they may be applied in Developing Countries. References cited can provide studies of given areas of interest, in greater depth. The chapter on biogas products and their uses gives an idea of the potential applications of biogas and digested slurry technologies.
This review is also intended to assist engineers and government officials/funding agencies to meet present and future challenges, and make decisions on the promotion of anaerobic digestion as an alternative source of energy, for soil conservation and enrichment, as fodder for fish and animals, for pollution reduction and other ecological benefits, such as pathogen reduction in human and animal wastes.
Current research, experimental and functional programs throughout the world, are rapidly adding to our knowledge of anaerobic digestion, and should provide increasingly efficient and useful designs to improve the quality of life everywhere. The AD meetings - The International Symposiums on Anaerobic Digestion that are held every 2 - 3 years - are a very good occasion to get up-to-date information on latest developments. The Proceedings published after each of these meetings can be very valuable to researchers, officials and economic analysts.
In order to draw conclusions about the feasibility of the anaerobic digestion process, it can be examined in one of two ways: a strictly financial approach, involving analysis of monetary benefits such as sale or re-use of products (methane, carbon dioxide and slurry, with all its applications) and the costs of constructing and maintaining facilities; or as a social assessment of input and output, including such intangibles as improvements in public health, reduced deforestation and reduced reliance on imported fossil fuels, in a social cost benefit analysis. There is no agreed methodology for quantifying these social benefits, so rigorous economic comparisons between biogas and other renewable, as well as conventional, energy sources are difficult, and must be done according to local conditions.
In assessing the economic viability of biogas programs, it is useful to distinguish between four main areas of application:
1) individual household units;
2) community plants;
3) large scale commercial animal rearing operations, and
4) municipal/industrial projects.
In each of these cases, the economic feasibility of individual facilities depends largely on whether output in the forms of gas (for cooking, lighting, electricity and power) and slurry (for use as fertilizer/soil conditioner, fishpond or animal feed) can substitute for the costly fuels, fertilizers or feeds which were previously purchased. For example, a plant has a good chance of being economically viable when the farmers or communities previously paid substantial percentages of their incomes for fuels (e.g. gas, kerosene, coal) and/or fertilizers (e.g., nitrates or urea) or peat-moss as soil conditioner for greenhouses. The economics may also be attractive in farming and industry, where there is considerable cost involved in disposing of manure or effluent. In these cases, the output can be sold or used to reduce energy costs, repaying the original capital investment. In those cases when the community is charged for treatment of the wastes, or if fines are imposed, the process is of great financial importance. If output/products do not generate income or reduce cash outflow, then the economic viability of a biogas plant decreases; for example, when cooking fuels such as wood or dung can be collected at no cost, or where the cost of commercial fuel is so low that the market for biogas is limited.
If the broader criteria are used to evaluate anaerobic digestion, especially in countries where officials and natural strategy people are more aware of ecology, where the effects of global warming over the long term (the Greenhouse Effect) is considered, then determination of viability requires knowledge of real resource or opportunity costs of input and output. When such output as improved public health, greater rural self-sufficiency, reduced deforestation and reduced dependence on imported fossil fuels can be incorporated, the global economic analysis usually results in more positive conclusions than a purely monetary analysis.
Technical, social and economic factors, government support, institutional arrangements, and the general level of commercial activity in the construction of biogas plants and related equipment are highly interrelated. All influence the development of biogas programs. Focusing attention on any one aspect will not bring about successful results. A large variation exists in the number of digesters installed in Developing Countries throughout the world, depending on the extent of government interest and support. China, India and South Korea, have installed large numbers of units, ranging from some seven million plants in China to approximately 30,000 in South Korea. Other Developing Countries have fewer than 1,000 - usually less than 200. The relative poverty of most rural and urban people in Developing Countries, and their concomitant lack of capital, are especially powerful economic considerations. On the one hand, program growth will be slow if facilities require a relatively large number of people to cooperate and alter their behaviours simultaneously, but on the other hand this can improve tremendously the economics of the plant and the benefits to the community.
Some of the large scale projects erected in rural area proved to be economic viable much more than the household small scale systems, which lack maintenance and efficient exploitation of the plant's products. Some very interesting activities in communal biogas operation have already been working for a number of years in Italy (De Poli 1990). Commercial and private sector interest in anaerobic digestion is steadily increasing, in conjunction with government tax policies, subsidies which alter prices of competing fossil fuels and fertilizers, and pollution control laws, all of which affect the growth of biogas programs. Institutional program structure and government policies are the primary administrative and driving forces behind biogas implementation. In many developing countries the infrastructure to disseminate information on biogas to technical personnel, policy makers and potential users does not exist. Both qualitative and quantitative assessments of ongoing activities are needed to improve technology and adapt its use to each specific country. Generally, program coordination does not exist, except in China, between R & D projects and implementing agencies. Biogas programs which have expanded rapidly have had strong government support, including subsidized capital and tax incentives.
To summarize, biogas technology is receiving increased attention from officials in Developing Countries, due to its potential to bring an economically viable solution to the following problems:
a. Dependence on imported sources of energy;
b. Deforestation, which leads to soil erosion and therefore to a drop in agricultural productivity;
c. Providing inexpensive fertilizers to increase food production;
d. The disposal of sanitary wastes, which cause severe public health problems;
e. The disposal of industrial wastes, which cause water pollution.
With the growing significance of this process, it is appropriate to mention some the historical developments which have occurred during the last 100 years of anaerobic digestion. In many cases, this may help to clarify the state-of-the-art at the end of the 20th Century.
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