The arrangement of final effluent storage for the biogas plant is critical to the effective operation of the plant. If a fibre separator is fitted after the digester, then a buffer store is important. This should be small enough to be stirred by the separator pump or an auxilliary pump and sized to enable the separator to run without reference to the loading cycle of the digester.
The final effluent tank is sized to meet the needs of the agricultural community or further processing. In an agricultural digester it is possible to store slurry for 9 months a year with little nitrogen loss. The residual methane from such stores has a significant global warming effect and should be captured. This may increase gas yields by 5 - 15 percent. The relative size of the digester to the final effluent tank is an important consideration in the design of a site. A typical digester treating 10 tonnes a day of high solids wastes has a retention time of up to 20 days - a 200 cubic meter digester. However, the storage of final effluent for 9 months would require a final effluent tank of 2700 cubic meters and this could well be the most expensive single item in the system. Thus a detailed analysis of the flow dynamics of the system over the year can produce massive capital savings.
Composting stores are required for solids. This may require drying floors and dry sheds to cope with a year's dry fibre. If longer retention times are forcast, it is probably more practical to burn the dry compost and accept a 10% total system loss in Nitrogen over complete recycling.
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