One of the proposed measures in the Baltic Sea Action Plan for reducing eutrophication, is to invest in biogas production and spreading of digestion residue based on farmyard manure.
Biogas is produced by anaerobic digestion - a microbial process, wherein organic material is decomposed incompletely under oxygen-free conditions. The products are biogas and a digestion residue. The digestion residue contains plant nutrients and may be used as fertilizer. In Sweden, most biogas plants use wastewater sludge as raw material. Other raw materials include solid waste at deponies, food industry waste, crops, garden waste and manure.
There are only a few farm-based small-scale plants producing biogas from manure. The interest in farm-based biogas is however increasing, and several plants are planned or under construction. This is partly a consequence of a governmental investment subsidy which was introduced in 2009.
How can biogas production from manure reduce eutrophication?
The main positive environmental effects of biogas production are no doubt related to the climate, since biogas is a renewable energy source. However, digestion of farm-yard manure is advantageous also when looking at eutrophication.
First, manure – whether digested or not – contains recycled nutrients. The use of manure as fertilizer thus leads to a lower net supply of nutrients to the ecosystem, as compared to when synthetic fertilizers are used.
Secondly, in comparison with undigested manure, digestion residue is also advantageous from the viewpoint of leaching. Digestion transforms the nutrients in manure into a form more readily available for plant uptake. Digestion residue also has a lower dry substance content than undigested. It is thus easier to spread with precision, and it becomes easier to adapt the amount of manure to the crop demand. An effective utilization of plant nutrients which gives a small residual nitrogen content in the soil after harvest, reduces the risk of nitrogen leaching the following autumn and winter.
This section contains the subcategories: Additives, Agitators, Biogas collecting and processing, Digesters, Heat recuperation and Roofs and coverings.
A cover for a digester, which comprises a water seal. The object is to prevent build-up of excessive pressure, and to separate moisture from the produced biogas.
A method of anaerobic digestion of organic matter. An additive comprising iron, cobalt and hydrochloric acid in specified amounts is supplied to the reactor. The additive increases the production of biogas without an excess trace metal consumption. The method may be used for co-digestion of green matter together with cow dung, sludge and/or dairy waste.
This patent deals with the problem of eliminating dead spots or no-flow volumes within a digester tank. Dead spots result in slow or no treatment and reduce the usable volume of the vessel. According to this patent, dead spots are difficult to avoid in conventional tanks, such as cylindrical tanks. The solution proposed here is a heart-shaped digester tank.
A little odd proposal for a digester placed directly on a cow.