Vacuum evaporation treatment of digestate with use of heat from anaerobic digestion

Within manure management strategies, anaerobic digestion followed by vacuum evaporation of digestate represents an interesting solution for both the reduction of nitrogen and phoshorous surpluses in soils and to avoid odour and gas emissions connected with the operations of treatment and storage of effluents. In reality, both anaerobic digestion and evaporation processes take place in a confined reactor, collecting gaseous emission as biogas and condensate, respectively. The aim of vacuum evaporation is i) to reduce the volume of the slurry to be spread on the field, thus reducing transport and distribution costs, and ii) to produce a condensate that can be discharged, to reduce the storage volume only to the concentrated fraction. Previous anaerobic digestion (AD) provides the heat necessary for the evaporation process, without it wasting in the atmosphere, as usually happens for the amount exceeding the needs of the digestor. With the goal to verify concentration efficiency, energy consumption and characteristics of concentrate and condensate, tests were performed using a one-stage semi-continuous pilot plant fed with the liquid fraction of a cattle slurry and maize silage digestate, without acidification. This practice is used to prevent ammonia volatilisation, but requires significant quantities of acid, thus increasing operational costs and causing problems for the on-farm storage of this product. A 12% TS concentrate was obtained, representing the 40-50% of the effluent. The condensate, because of its ammonia content (2.7 g/L on average) cannot be discharged. A solution for the recovery of ammonia from condensate is the filtration on reverse osmosis (RO) membranes, with previous acidification. Tests are on-going with a RO pilot plant to verify whether discharge limits can be matched. With a heat requirement of 0.87 kWh/kg of condensate, heat is the limiting factor of the process when the objective is to treat the entire quantity of digestate effluent from the biogas plant, especially in winter when the heat demand from AD plant increases.