|【Practical Application of Digested Liquid and Methane Fermentation Treatment of Livestock Wastes】|
This system combining anaerobic digestion and biogas generation is drawing attention in that not only does the said system treat livestock manure as wastes, but it produces energy (methane gas) by utilizing carbon-neutral biomass. That is, the system is being developed as an effective treatment method from the point of view of a countermeasure against global warming, as well.
Currently in the world, this system is advancing in Germany, with a generated power output exceeding 250MW. Stems of onion, wheat, corn, root residue of root vegetables, waste vegetable oil and food residues, as well as livestock wastes, are used as source materials. In Denmark, power generation by biomass accounts for 3% of the country’s total power output, which is an important position for their energy policy. The use of mixed materials including food waste and sewer sludge increases its efficiency.
In Europe, most digested liquid is effectively used as liquid fertilizer. Its odor reduction and thinner viscosity are evaluated.
Anaerobic digestion (methane fermentation) of livestock wastes generates digested liquid. When the digested liquid is treated with water, it needs immense expenditure and energy costs, so the CO2 reduction effect is low. However, assuming all digested liquid can be diverted into liquid fertilizer, CO2 reduction effect increases significantly. On the other hand, most Japanese livestock farmers, different from those of European countries and the US, do not have fields to spray liquid fertilizer onto. Thus, in Japan, it is practical to spray the digested liquid of livestock wastes as liquid fertilizer onto paddy fields. However, paddy rice farming requires a relatively sophisticated fertilization plan. Additionally, the spraying of digested liquid onto paddy fields is not seen in any other country in the world.
There are several problems to overcome spraying liquid fertilizer onto paddy fields. Paddy fields are surrounded by dikes, unlike fields. So spraying liquid fertilizer onto the paddy fields using a spray vehicle is impossible. Measurements to reduce the expenses of liquid fertilizer transportation and consistent spraying onto agricultural fields are required.
In addition, there are problems concerning both production efficiency and environmental safety in using organic fertilizer, including liquid fertilizer from digested liquids. Organic fertilizer contains a lower concentration of nitrogen than chemical fertilizer and its availability is easily affected by environmental conditions. Therefore, it is difficult to comprehend the results of liquid fertilization. Establishment of a fertilization plan based on form changes of nitrogen is essential. Generally, since it contains a large amount of slow-acting nitrogen, excessive application can easily occur and nitrogen contamination of the environment is easily caused.
Organic fertilizer contains heavy metals, saline matter, pathogenic microorganisms and pesticides, which are of biological origin, so it is likely to contaminate crops and soil. Moreover, in recent stock farming, the breeding cycle is shortened to enhance its productivity and high-density feeding is conducted in addition to the use of pharmaceuticals, hormonal agents and high nutrition feed. For these reasons, with respect to environmental safety from spraying digested liquid onto agricultural land, an investigation on the influence of not only nutrient enrichment, trace substances and drug-tolerant pathogenic microorganisms on the environment and crops, but also of volatile organic compounds on air quality is required.