Anaerobic digestion 456 Subject

Environmental pollution is the subject of many political and social discussions in the Netherlands. This is mainly caused by the concentration of the animal husbandry farms close to the villages and cities. There are technical treatments of air and slurry to decrease the odour of it. Anaerobic digestion is one of these. This technology is known from biogasproduction. Most countries in the western world have studied this possibility for renewable energy. A lot of technical problems are solved but on most places it is not possible to produce biogas from farm waste to a competitive price. What is the feasibility for biogas production if there is a need for reduction of the odour emission on the farm ... 

Lactose is readily fermented by lactic acid bacteria, especially Lactococcus spp. and Lactobacillus spp., to lactic acid, and by some species of yeast, e.g. Kluyveromyces spp., to ethanol (Figure 2.27). Lactic acid may be used as a food acidulant, as a component in the manufacture of plastics, or converted to ammonium lactate as a source of nitrogen for animal nutrition. It can be converted to propionic acid, which has many food applications, by Propionibacterium spp. Potable ethanol is being produced commercially from lactose in whey or UF permeate. The ethanol may also be used for industrial purposes or as a fuel but is probably not cost-competitive with ethanol produced by fermentation of sucrose or chemically. The ethanol may also be oxidized to acetic acid. The mother liquor remaining from the production of lactic acid or ethanol may be subjected to anaerobic digestion with the production of methane (CH4) for use as a fuel several such plants are in commercial use. 

If nitrogen and phosphate removal are required, combinations of chemical and biological action are required. Secondary effluent is treated with lime to decrease the pH and then subjected to anaerobic digestion to convert up to 90% of the combined nitrogen compounds present to ammonia (Eq. 5.26). The ammonia formed in solution is air-stripped from the effluent... 

Most commonly, sewage sludge is subjected to anaerobic digestion in a digester designed to allow bacterial action to occur in the absence of air. This reduces the mass and volume of sludge and ideally results in the formation of a stabilized humus. Disease agents are also destroyed in the process. 

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