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biogas HANDBOOK 6.2.3 Sanitation The AD process is able to inactivate viruses, bacteria and parasites in the treated feedstock substrates, an effect which is usually called sanitation. The sanitation efficiency of AD depends on the actual retention time of the feedstock inside the digester, the process temperature, the stirring technique and digester type. The best sanitation is obtained at thermophilic temperatures (50-55°C) in e.g. an elongated plug flow reactor, with the appropriate retention time. In this digester type no mixing of digestate with fresh feedstock occurs, allowing up to 99% of all pathogens to be destroyed. In order to ensure veterinary safe recycling of digestate as fertiliser, European legislation requires specific sanitation measures in the case of feedstock types of animal origin. Depending on the type of feedstock pre-sanitation by pasteurisation or by pressure sterilisation is required before supplying the substrate to digester. More details about sanitation can be found in Chapter 7.2. 6.2.4 Destructionofweedseeds A considerable reduction of germination capacity of weed seeds occurs throughout the AD process. This way, biogas production contributes to ecological weed reduction. Experience shows that loss of germination capacity can occur for the majority of weed seeds within 10- 16 days HRT, with some differences to be noticed between different types of plant seeds. Like in the case of sanitation, the effect increases with increased retention time and temperature. 6.2.5 Avoidanceofplantburns Application of raw slurry as fertiliser can cause burning of plant leaves, which is the effect of low-density fatty acids, such as acetic acid. When fertilising with digestate, plant burns are avoided, as most fatty acids have been broken down by the AD process. Digestate flows more easily off the plants vegetable parts compared to raw slurry, which reduces the time of direct contact between digestate and the aerial parts of the plants, reducing the risk of leaf damage. 6.2.6 Fertiliserimprovement Through the AD process, most organically bound nutrients, in particular nitrogen, are mineralised and become easily available to the plants. Figure 6.2 shows nitrogen utilisation from digested slurry, applied to winter wheat and spring barley, compared to nitrogen utilisation from untreated slurry. Because of the increased availability of nitrogen, digestate can be integrated in the fertilisation plant of the farm, as it is possible to calculate its fertiliser effects in the same way as for mineral fertilisers. Digestate has lower C/N ratio, compared to raw manure. Lower C/N ratio means that digestate has a better short term N-fertilisation effect. When the value of the C/N ratio is too high, micro-organisms take hold in the soil, as they successfully compete with the plant roots for the available nitrogen. 52PDF Image | biogas HANDBOOK
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