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biogas HANDBOOK than 15 % and dry digestion, when the DM content of the substrate is above this value, usually between 20-40 %. These definitions and their limit values have some regional variations or they can be differentiated by legislation and support schemes, like e.g. in Germany. Wet digestion involves feedstock like manure and sewage sludge, while dry digestion is applied to biogas production from solid animal manure, with high straw content, household waste and solid municipal biowaste, green cuttings and grass from landscape maintenance or energy crops (fresh or ensiled). Both dry and wet digesters are described in the next subchapters, with emphasis on wet digestion systems. From the point of view of feedstock input and output, there are two basic digester types: batch and continuous. 7.6.1 Batch-typedigesters The specific operation of batch digesters is that they are loaded with a portion (batch) of fresh feedstock, which is allowed to digest and then is completely removed. The digester is fed with a new portion and the process is repeated. Batch-type digesters are the simplest to build and are usually used for dry digestion. An example of batch digesters are the so-called “garage type” digesters (Figure 7.18) made of concrete, for the treatment of source separated biowaste from households, grass cuttings, solid manure and energy crops. Treatment capacity ranges from 2 000 to 50 000 tonnes per year. The feedstock is inoculated with digestate and fed in the digester. Continuous inoculation with bacterial biomass occurs through recirculation of percolation liquid, which is sprayed over the substrate in the digester. Unlike wet digestion, dry digestion needs no stirring or mixing of the AD substrate during digestion. The temperature of the process and of percolation liquid are regulated by a built-in floor heating system, inside the digester, and by a heat exchanger, which acts as a reservoir for percolation liquid. Compared to other systems, batch digestion has the advantage of low operation costs and costs of the mechanical technology behind it and the disadvantage of high process energy consumption and maintenance costs. Figure 7.18 Garage-type batch digester, loaded by a loader (BEKON 2004) 75PDF Image | biogas HANDBOOK
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