Chapter ten: Integrated approach to the anaerobic digestion process
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Methodology to assess integrated systems
Existing integrated systems
Biomass is understood to mean all land and water plants, their wastes or by-products, farmyard wastes (including manures) and the wastes and by-products resulting from the transformation of these plants or of what they produce. This transformation is usually accomplished by the technological processes of the agro- food industries. The production of biomass is primarily derived from the process of photosynthesis - the capture and conversion of sunlight by plants. The energy produced in this fashion is about 10 times the present world energy needs, and about 20 times the food needs, as expressed in terms of energy (Pellizzi 1981). This is so, although the yield harnessed is very low - about 0.1% of the global sunlight radiation reaching earth. Energy transformation processes of biomass can reach much higher degrees (5 -10%) in some known processes, but not in Nature.
The utilization of biomass for energy conversion poses some difficulties. The actual availability and geographic distribution, the harvesting, transportation, pretreatment and storage problems bring the operation cost to a higher level than most rural people can afford. It is based essentially on the harvesting of the biomass needed for food use only, neglecting the potential utilization of the by-products of these processes. It is time to realize the potential of the agricultural and agro- industrial byproduct, and use them in an integrated resource recovery system. A more integrated approach has to dominate the outlook of energy and food as one plan. Biomass can be used in many ways to generate energy available for human use. Fig 10.1 presents the overall picture of fixing energy from the sun, by utilizing biomass produced by photosynthesis, including organic wastes, for the production of biogas. It must be understood that although biogas is presented here as the only energy generating system, it is not the only way of extracting the energy back from biomass (burning, pyrolysis are some of many examples). Except from direct burning, biogas is the main means of to generating energy from biomass that can bring to rural areas the opportunity to develop industry, in addition to its use for cooking, lighting and heating.
The most efficient use of biogas systems in Developing Countries is incorporation the harvesting of the sun's energy into food and fuel cycles, in an integrated resource recovery scheme.
In some known cases the "Utilization of Agricultural Wastes", in which the integrative approach was applied and used, to obtain an economic operating system, gave an answer that was specifically tailored to differently structured farms, using all materials existing on the farms (Maramba 1978; Rousseau et al. 1979; Hu Bing-hong 1982; Marchaim 1983; Gunnerson and Stuckey 1983).
Possible integrated systems
The variety of feeds, biogas techniques, and end uses of biogas and slurry results in a large number of possible integrated systems. The interactions between fuel, food and fertilizers in such a system are complex (Figure 10.1). If fuel supply is the primary desired output from such a system, then the nutrients present in the slurry could either be recycled back to the fields to grow more crops and provide residue for feedstock, or be used to grow feedstock directly (Gunnerson and Stuckey 1983). This is a good example how, although all components of the material is used, the output is not optimal and not the most economic one.
Fig 10.1: Schematic representation of the integrated approach to the utilization of biomass (From Marchaim et al. 1983).
The end use of the biogan also has implications in terms of fuel efficiency. If it is used solely to satisfy cooking and lighting requirements, it will not result in any feedback into fuel production. However, if the Gas is used in a dual fuel engine, the power generated can be used to irrigate fields, resulting in increased agricultural residues available for food and for digestion. In addition, the waste heat produced by the engine can be used as heat for household purposes, or to heat the digester, which would allow a smaller digester to produce the same amount of gas, reducing capital investment and produce a stable quantity of biogas all the year long. The net result could be an energy loop, leading to increased amounts of energy available from a given amount of land. The relative fractions of gas used for cooking and power generation influence the amplitude of this loop, and optimization techniques would maximize the gains from these "feedback" loops.
However, energy must not be looked on as the main product of biogas processes. Food production is influenced by the presence of nutrients within the slurry. The most common method of using this slurry in integrated systems is to recycle it to the fields as a fertilizer/soil conditioner. The method of handling the slurry can influence its efficacy as a fertilizer, and hence the quantity of biomass, food and residuals produced. Examination of the most profitable process to be used on an integrated farm can not be generalized, but must be tailored to the specific farm. There are, for instant, other methods of utilizing the slurry which can increase the amount of food, including re-feeding to animals and growing algae or fish. Also, the end use of biogas is important in this context since utilization of all the gas for pumping irrigation water and powering tractors, as opposed to cooking and lighting, would increase the quantity of food produced from a given amount of land. Yet this would ignore the problems of deforestation and fossil fuels.
Methodology to assess integrated systems
In evaluating an integrated system, it is important to define the boundaries of the system being studied. In many rural situations in Developing Countries, ecosystems can be defined which are relatively closed; i.e. there are little input or output of fuel and feed outside the system. However, in many rural-urban areas the systems are quite "open," with fuel and food input balanced by monetary output. The primary focus of development in recent years in most Developing Countries has been on satisfying the basic needs of the rural poor. The methodology is slightly simpler for closed than for open systems, but this discussion will deal with both systems, with an emphasis on developing new concepts and new ideas for rural areas, in order to develop these sector in particular.
In a typical small village in a rural area, the system boundaries can be said to include the village and all the agricultural land which supports it. All ecological and economic aspects must be considered. Fuel or food entering the system is an input, in addition to solar radiation (which is rarely used by people in villages in Developing Countries), while those leaving the system are the output. In assessing the potential of the integrated system to improve the quality of life within such an ecosystem, the economic evaluation of the food and energy sources, their flows and transformations and the ecological aspects must be considered in a comprehensive manner. The modelling of ecosystems, based on material and on energy flows, and energy conversion efficiencies was pioneered by Odum (1971). Reddy and Subramanian's (1979) rural development approach included:
(a) elucidation of current rural energy consumption patterns;
(b) translation of these patterns into a set of energy needs arranged according to priority;
(c) consideration of feasible technological options for satisfying these energy needs with the available resources;
(d) selection of the "best" option for satisfying each category of need;
(e) integration of the selected options into a system.
Gunnerson and Stuckey (1983) described the model in figures based on data gathered by Ravindranath et al. (1980) on rural energy consumption patterns. This evaluation leads the authors to the selection of a limited set of energy paths, subjected to the following constraints: time dependence of the energy utilizing task; self-reliance; environmental soundness; power requirements of certain tasks; and the availability of the technology. Reddy and Subramanian (1979) evolved an energy scheme, based on this concept, for a community scale biogas unit. In the systems described above, there is a limited role for the optimization of all the techniques involved. In most cases the households have their own system next to the pigsty (and in better cases next to the latrine, too), and use the energy for cooking and lighting their houses, and the slurry as a fertilizer for the fields.
The comprehensive approach involves emphasizing other considerations, in addition to energy, such as nutrient recycling, public health or the environment, and especially the output of the development on the advancement of the rural community by industrializing the village, bringing light and power. The integrated approach can lead to a system that includes a community system, that gathers all wastes from the village; and by maintaining a central system by experienced technicians and using more sophisticated equipment in order to produce electricity, small agro-industrial plants can be developed in the farm which will attract the new generation. The production of light and power can accelerate the greenhouse and mushroom industries on the village level, and hence use the digested slurry in a much more economic manner, as a substitute for peat- moss. Marchaim, during his visit to China (1990), discussed the question of integrating the separated biogas systems in a certain village in order to generate electricity and power. Some Provincial Rural Energy Officers did not accept the idea, since they estimated that the farmers would not agree to operate in an integrated manner. Other farmers and other officials found the idea very attractive. The success of such a system must be evaluated on a demonstration system in a village.
Existing integrated systems
In recent years, a number of integrated systems have been established in developing countries (Chan 1973; Alviar et al. 1980; Solly 1980; Marchaim et al. 1981; Meta Systems 1981). Probably the best known of these are Maya Farms in the Philippines and Xinbu Village in China.
Maya Farms, which covered 36 hectares and contained 25,000 pigs, 70 cattle and 10,000 ducks, designed and implemented three integrated farming systems, varying in size from a small family farm model to a large commercial feedlot venture (Judan 1981). The family farm is based on 1.2 ha of land with 1.0 ha used for crops (rice or corn) and the rest devoted to a cattle shed, fishpond, biogas works, accommodation and a pigsty containing four sows. The biogas, produced from the swine waste and manure from two water buffalo, is more than enough to supply the family's energy requirements for cooking, and also powers a refrigerator and gas mantle lamp. Solids in the slurry are reefed to the pigs, constituting 10% of their feed, while the liquid slurry is used to raise fish in a 200 m3 pond, and to fertilize all the cropland throughout the year. This is a very comprehensive and complicated farm to handle for a family.
The medium scale system is based on 12 ha of land and a 48 sow piggery. The gas is sufficient to pump water for the farmhouse and livestock and to irrigate the 12 ha of cropland. The large system was designed for 500 sow units and no agricultural land, approximating an intensive animal feedlot. The gas produced is used for pumping water, lighting the pigpens and operating a feed mill; however, in this case there is a gas surplus amounting to roughly 40% of the output. Various uses for this gas have been suggested. Payback periods varied from 18 to 39 months (for the family farm system).
More efficient systems are possible if all the energy and food within the system, as well as the digested slurry, are fully integrated. With more "open" systems, such as intensive animal feedlots, the prime parameter to consider may be financial returns. This was found as the main constraint in Developing Countries.
Hu Bing-hong (1982) described the Xinbu Brigade in China, which started to install biogas units in 1976, and where 80% of the families use biogas. These units supply some 50% of the families' fuel requirements and, in addition, 17 families use solar roof panels which, with biogas, supply 70 - 80% of their energy needs. The biogas is used for cooking and generating electricity for lighting, and the waste heat from the engine is used to dry silkworm cocoons. Solar dryers are also used to carry out the latter task. The slurry is used to feed fish ponds and fertilize the fields growing mulberry, sugar cane and Napier grass. In addition, some of the slurry is used to grow mushrooms. In the six years the scheme has been in operation, the output from the Brigade (in Yuan) has risen by 150% (Hu Bing-hong, 1982), and the general sanitary conditions of the village have improved considerably.
The integrative approach to the subject of biomass production and utilization raises the demand to reconsider the introduction of biomass from sources which were neglected and abandoned (plant residues) in the past, and even cultivation of some energy crops, to increase the amount of substrate for the process, thus improving the feasibility of an integrated system. Anaerobic biodegradability factors should be considered as well as the quality of the digested slurry produced for further utilizations on a specific locality. The potential benefit of using biomass from different sources i" the possibility of controlling the chemical composition of the digester feed especially the C\N ratio. The fibrous materials (lignocellulosic complex, hemicellulose etc.), although having low biogas production under anaerobic condition, are important in the digested slurry, especially when the material is used as a soil conditioner.
The integrated approach has to evaluate energy as a means to produce more food for the world and to consider food as metabolic energy. It is not surprising that in Nature these two aspects are connected, and this lead. the way to a similar consideration by rural planners. The combination of producing food and energy simultaneously, using the by-products of one as substrate, or complementary to the other, is the right path to be take.
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