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biogas HANDBOOK Membrane Separation Technology A membrane is a filter with very fine pores, which can separate particles and solutes from most of the liquids on a molecular scale. The decision to use micro-, ultra-, or nanofiltration or soluble reverse osmosis depends on the size of the particles to be separated. The process is based on the difference of pressure between the two sides of the membrane, i.e. water, as well as minute particles, passing the membrane under pressure. Several conditioning steps are often connected, in successive series, in order to achieve the desired separation. For example, larger particles are removed from a decanter filtrate, through a first step of ultra-filtration and then the solubles are removed in a second step by reverse osmosis. Besides purified water, the membrane separation produces a nutrient rich concentrate, which can be sold either directly as liquid fertiliser, or further processed for volume reduction through evaporation. Evaporation Through evaporation, the liquid is further refined and separated into nutrients and purified water. Evaporation units require high energy consumption. In most cases, surplus heat from CHP-production is used in evaporation units, increasing the efficiency of energy utilisation and contributing to financing a part of the operational costs for the conditioning unit. Crucial for the choice of evaporation technology are the characteristics of the substrate to be evaporated. In the case of digestate, it is possible to use a closed-circulation evaporator, in which the heat transmission and the actual evaporation process run separately. This ensures a more stable process, especially if the substrate to be evaporated has a tendency to produce layers. 6.6.2 Necessaryconsiderations Conditioning technologies (especially the complete conditioning) require high energy consumption in order to create the pressure used in membrane technologies, or for the production of heat, used in evaporation processes. Up to 50% of the biogas produced electricity is necessary for the complete conditioning of the produced digestate, using membrane technology. Partial conditioning is less energy demanding, cheaper and, in regions where there is a surplus of phosphorus, it is the most economical conditioning technology. In all the cases, the conditioning technology is chosen according to the chemical and physical characteristics of digestate, herewith the tendency of the digestate to layer formation. If complete conditioning is aimed, it is important that most of the digestible dry matter is removed through complete separation of liquid and fibres, followed by ultra filtration (< 0,2 mm), so that the remaining liquid fraction has almost the quality of pure water. If the separated fractions do not reach the necessary level of purity, or if the chosen membranes and processes are not suitable for digestate, the expenditures for energy, labour, maintenance and cleaning of the system can increase considerably. 6.7 Digestatequalitymanagement 6.7.1 Digestatesampling,analyzingandproductdeclaration Recycling of digestate as fertiliser in agriculture should be done through integration in the fertilization plan of the farm. This implies accurate dosing which is possible because digestate is chemically analysed before leaving the biogas plant. Average samples of all loads 58PDF Image | biogas HANDBOOK
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