Criteria for the Dissemination of Biogas Plants for Agricultural Farm and Household Systems (GTZ, 1993, 25 p.) 2. Biogas technology 2.1. Digestion 2.2. The biogas plant 2.3. Supply of dung and gas production 2.4. The energy demand

Criteria for the Dissemination of Biogas Plants for Agricultural Farm and Household Systems (GTZ, 1993, 25 p.)

2. Biogas technology

2.1. Digestion

Biogas plants are an element of a waste recycling concept in which a part of the organic matter is converted into energy during a process of fermentation. In addition to the production of a high-grade source of energy, quite comparable to bottled gas, the biomass is processed into a valuable fertilizer referred to as slurry. A household biogas plant is normally a closed container in which animal and human excrement ferments under air-tight conditions. Theoretically, all organic materials can ferment or be digested. Only homogenous and liquid substrates can be considered for simple biogas plants: faeces and urine from cattle, pigs and possibly from poultry and the wastewater from toilets. When the plant is filled, the excrement has to be diluted with about the same quantity of water, if possible the urine occurring should be used. Waste and wastewater from food-processing industries are only suitable for simple plants if they are homogenous and in liquid form. During the digestion process, various kinds of bacterial cultures which harmonise with each other split large molecules typical for organic materials into smaller and more simple molecules in phases. The two main phases of this process are the acidic phase during which mainly CO2 is formed and the methanogenic phase. The quality of methanogenesis is centrally dependent on the carbon-hydrogen ratio of the substrate and on the bacterial communities existing in the substrate. Cattle dung is favourable for the beginning of methanogenesis. So-called starter material should normally be added to all other substrates to initiate the process. Starter material, which should comprise 10 - 20% of the total organic mass, can be cattle dung or slurry from a biogas plant which is already in operation.

Bacteria are living organisms which are dependent on certain environmental conditions. They can be poisoned by antibiotics, heavy metals, acids and detergent solutions which could, under some circumstances, lead to a complete breakdown of digestion.

At too low temperatures, bacteria reproduce and work so slowly that no production of gas worth mentioning occurs. In general, a biogas dissemination programme is only feasible where mean annual temperatures amount to around 20°C or where the average daily temperature is at least 18°C. Within a range of 20° - 28°C gas production increases over-proportionately. If the temperatures are low, methanogenesis will slow down. If the temperature of the biomass is below 15°C gas production will be so low that biogas plants are no longer worthwhile. Conditions for methanogenesis will be more favourable if the temperatures do not fluctuate too greatly since a different family of bacteria reproduces optimally at different temperatures.

The technical reaction to low temperatures is normally to construct a larger digester to increase specific gas production by longer retention times and so to meet the demand for energy as well as possible in colder seasons. Larger digesters prolong the retention times (HRT = hydraulic retention time). The temperature fluctuations between day and night are no great problem for plants built underground since the temperature of the earth below a depth of 1 m is practically constant. A large digester volume also creates a better buffer effect against toxins. However, larger digester volumes are reflected in higher costs. On the whole, methanogenesis is a very hardy process and adapts easily. In practice, apart from too low temperatures and unsuitable feedstock, there are hardly any biochemical problems which are relevant in deciding for or against a dissemination programme.

During methanogenesis many of the odorous materials connected with animal husbandry and the spreading of manure are degraded. Thus, e.g. fertilising with liquid slurry is also possible in the proximity of settlements due to this anaerobic treatment.

In addition, some pathogens and weed seeds are killed. The biological oxygen demand (BOD) is reduced by up to 80% which is important for wastewater released into open waters. The pollution of surface and groundwater can possibly be substantially reduced by anaerobic treatment. However, anaerobic treatment cannot be considered to completely purify the substrate. For this, other aerobic treatment phases or special processes would be necessary to e.g. eliminate phosphates, halogens or heavy metals. Biogas plants do not withdraw any plant nutrients from the substrate so that the fertiliser value of the original substrate is retained in total. During digestion, part of the total nitrogen is mineralised and can thus be more rapidly taken up by many plants. In a number of applications, slurry from biogas plants is even superior to fresh dung especially when the slurry is spread directly on fields with a permanently high nitrogen demand (e.g. fodder grasses) or when using slurry compost to improve the structure of the soil. When the slurry is dried a large proportion of the nitrogen is lost. For this reason, dry slurry is more suitable as fertiliser for roots and tubers than to generally improve the soil. How the farmers ultimately make use of the slurry primarily depends on the extent of transport and labour involved in spreading it and on their traditional methods of fertilising.

2.2. The biogas plant

Each biogas plant consists of the digester and the gasholder. The size of the digester depends on the amount of substrate occurring daily at given temperatures. The size of the gasholder depends on the daily gas production and mainly on the gas consumption times. For reasons of economy and construction, it is not feasible to store the gas over several days. For household plants, the digester volume (VD) is about five times as great as the volume of the gasholder (VG). The gas is fed directly from the gasholder to the burner through pipes. For reasons of costs and gas pressure, it is an advantage when the point at which the substrate occurs and where the biogas plant normally stands is not too far from the place where the gas is used. Saving biogas in pressurised containers or in bottles in liquid form is not economical.

There are three types of plant which would basically and technically be suitable for households. These are the floating-drum plant, the plant with a plastic balloon-type gasholder and the fixed-dome plant. Only the fixed-dome plant has proven successful for dissemination programmes for household plants as it is hard-wearing and requires little maintenance.

The technology of fixed-dome plants today - in contrast to earlier years - is no longer a great problem. The type of plant which has asserted itself in German development cooperation is the one developed by CAMARTEC in Tanzania with the so-called "strong ring" and the gas-tight dome plastered with the aid of cement agents. When this has been erected technically perfectly its service life is practically unlimited. Periods of 15 - 20 years are most frequently assumed for economic efficiency calculations. However, specific training of masons is necessary if this model is to be built to operate correctly. In the fixed-dome plant, the gas is stored in the upper area of the digester. The gas then pushes part of the substrate into a compensation chamber. This then flows back into the digester as soon as gas is taken out. The compensation chamber is consequently the same size as the inner gasholder. The gas pressure then falls or rises depending on the quantity of gas stored. The gas pressure occurs due to the deference in level between the level of liquid in the digester and that in the compensation chamber. To avoid gas pressure which is too high, the maximum size of a fixed-dome plant with a digester-gasholder ratio (VD:VG) of 5:1 at about 50 m³ VD. During construction, a household fixed-dome plant requires an area of about 7 by 4 metres. After construction has been finished, the plant will only take up a space of about 5 m².

For the floating-drum plant, which was previously preferred, the demands on quality in production can be met more easily. The corrosion on the steel gas dome which can hardly be avoided and the higher costs mean that this model is less suitable for dissemination. Even Indian entrepreneurs specialised in the construction of floating-drum plants are now beginning to build fixed-dome plants. Experiments on replacing the steel by ferrocement, fibre glass or other materials currently have to be seen as unfruitful. Floating-drum plants today are only considered for digester volumes over 50 m³. The steel drum then floats in a water bath.

The plant with plastic foil is the most low-cost in its production but, due to its short service life, incurs high operation costs. Biogas dissemination programmes still using these plants need a secure service structure. In individual cases however, the foil plant has proven itself for larger plants (VD > 100 m³ (e.g. Ferkessedougou/lvory Coast).

2.3. Supply of dung and gas production

Biogas, consisting of 60% methane and 40% carbon dioxide, is a high-grade source of energy. However, the calorific value is lower in comparison to bottled gas. One m³ of biogas only replaces about 0.4 kg bottled gas. One kilogram of cattle dung produces about 40 litres of biogas and one kilogram of pig dung about 60 litres of biogas per day. To produce 1 m³ biogas, 25 kg cattle dung or 17 kg pig dung are necessary. The amount of dung occurring is approx. proportional to the live weight of the animals. Random measurements of the amount of dung occurring per animal are essential to define the gas potential on the farm since the biomass produced per animal varies greatly from region to region. A zebu cow in Orissa/India provides e.g. about 5 kg dung per day; a Javanese zebu cow crossed with a Friesian for high milk output, in contrast, produces 15 kg per day. The amount of dung must be known to allow the energy potential occurring daily in the housing to be estimated. Where animals are put out to pasture during the day and only housed at night, experience shows that only about half of the total amount of dung occurs in the housing and thus is available for the biogas plant. Livestock systems, where animals are only out to pasture, cannot be considered for biogas plants as the collecting of the dung dropped is too labour-intensive. This would also mean depriving the pasture of the dung.

A biogas programme can only be considered for pig farming if the animals are kept in sties with concrete floors. It is more difficult to determine the quantity of dung than in cow sheds. The changing numbers also make rapid estimation of the substrate quantities more difficult. However, the number of brood sows often provides the basis for an adequate estimation.. In this respect however it is important to know whether piglets or pigs for slaughter are being reared. Pig breeders often normally know what quantities of dung occur daily or annually. As a rough idea, it can be assumed that each pig with a live weight of over 50 kg produces about 2.5 kg dung. The piglets are then neglected when counting, or they are counted in proportion to their weight. Established values in literature which mostly comes from industrialised nations only rarely apply to developing countries.

2.4. The energy demand

Although simple biogas plants embody multi-functional agricultural technology, the benefit of energy is almost always primary in the farmer's decision to buy. The demand for fuel in the household - and with this the need for biogas - varies extremely from region to region. Whilst gas from household plants is used for cooking, lighting and cooling, larger plants could be interesting as agricultural inputs. Where there are larger quantities of gas (over 15 m³ per day), motors, incubators, heaters or generators can be operated by the gas. The size of the biogas plant will essentially be linked to the amount of substrate occurring. The gas consumption is then adapted to the gas quantity available. The recurring consumption patterns which are feasible and, primarily, which are realistic is a matter which will have to be investigated.

There are no overall and generally valid values for gas consumption since the consumption patterns and quantities depend on other energy supplies and on the possibilities and preferences for the utilisation of the gas. Lighting is particularly attractive for farming families in regions where there is no electrification. 0.5 m³ gas/per day should be assumed per biogas lamp.

The household demand for energy is greatly influenced by eating and cooking habits. Gas demand for cooking is lower in regions where e.g. preserved vegetables are eaten with white bread or millet soaked in milk than in areas where rice or beans are part of daily nourishment. The supply of energy from biogas plants reaches the limits of its capacity when cooking is carried out only once or twice a week and then as a supply for several days. This method is common e.g. in Central Africa for beans. The biogas plant would then have to be designed for this peak consumption pattern. A disproportionally large gasholder combined with a larger digester would correspond to this peak demand. For reasons of construction, this would lead to high costs particularly for the fixed-dome plant.

Although the women become quickly accustomed to using the biogas stove and can adapt to the use of this new source of energy without any trouble, substitution of conventional sources of energy by biogas also has its limits. In this respect, the following questions have to be answered: can the biogas plant, due to cooking habits, only meet a relatively low share of the energy demand? Does the source of energy which is otherwise used (firewood, charcoal) serve as a source of heating for the inhabitants during winter months? Does the smoke from the open fire conserve foodstuffs or control insects? Does the kitchen also serve as a smoking chamber?

The possible and probable substitution of firewood by biogas should thus be investigated particularly in programmes with specifically ecological objectives where the saving of firewood is a central question.

To define the demand for gas, the previous energy consumption can be taken as a rough value. Here, the profiles of utilisation are to be compiled and these converted into biogas equivalents using standard figures and more exact measurements. The standard figures which are based on experience can be taken from the following list (cf. also the chapter on "Data collection":

Gas for cooking in an Indian or West African household:

1.3 to 2.5 m³/day (depending on family size and eating habits which also depend on the prosperity of the family).