Highlights: vacuum evaporation as an effective way of liquid digestate treatment in biogas plants, energy and mass balance models of three industrial evaporators suitable for digestate thickening, comparison of the evaporators in terms of their energy performance, Vacuum evaporation is an efficient method for reducing the volume of liquid digestate (LD) from biogas plants (BGP). Furthermore, thickening LD in BGP contributes to the efficient utilization of waste heat and also reduces fossil fuel consumption that is needed for transporting LD. However, the utilization of vacuum evaporation must be reasonable, and a comprehensive study should precede the integration of evaporation technology in a particular BGP. For this purpose, this study compares selected parameters of three types of industrial evaporators which may be suitable for LD thickening. Furthermore, this study provides a mathematical model that describes the mass and energy balances of the chosen evaporators and is able to evaluate their energy performance for a given set of input variables. It was concluded that the forced-circulation evaporator has the highest energy requirements and also requires a high cooling performance. This type of evaporator will be interesting for the plant owners only if the cost of power generation is extremely low. In terms of consumption of energy and cooling duty, the multi-stage flash evaporator is the most efficient and it also requires the least heat transfer area. The falling-film evaporator provides only slightly worse performance. viable without incentives since the capital cost of installing ORC is very high and the energy efficiency is usually lower than 20 % [6]. Despite this, many businesses are interested in utilizing the waste heat. If they are efficient in the way they utilize the heat, many governments will provide them with increased feed-in tariffs for electricity produced with a combination of heat and power generation. Moreover, surplus heat from cogeneration units must be cooled down, commonly in air coolers which may obviously consume more expensive electricity. The efficient utilization of heat relates to the BGP's increased efficiency as well as the plausibility of the whole concept. By fully recovering the waste heat, the environmental sustainability of biogas electricity production would improve significantly [7]. 1.2 Digestate and its treatment BGP owners have to face problems related to the management of large quantities of digestate. There are no accurate statistics available, but on average, 15 to 20 thousand m 3 of digestate per 1 MWel of installed capacity is quoted as being produced annually [8]. If the current BGP capacity in the EU 27, Switzerland, Croatia and Serbia amounts to 8.3 GWel [2], digestate production in these countries accounts for more than 120 million m 3. Digestate is a by-product of anaerobic digestion, and preserves minerals from the original materials (mainly nitrogen, potassium, phosphorus, sulphur, calcium and magnesium). It is for this reason that it mostly serve...
