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.