Thermal thickening of nitrified liquid phase of digestate for production of concentrated complex fertiliser and high-quality technological water

Large production of liquid phase of digestate (LPD) characterised by low dry matter content increases the costs of LPD storage and transportation. Additionally, significant nitrogen losses result from LPD handling. The operation of biogas plants also requires large amounts of technological water, a scarce resource. The method of LPD nitrification and subsequent thermal thickening as a promising alternative treatment able to overcome those problems was investigated. The properties of thermally thickened nitrified LPD and of the distillate derived from vacuum evaporation of nitrified LPD were examined. The results show that over 99.9% of nitrogen was accumulated in thickened nitrified LPD. Total nitrogen (TN) concentration in thickened LPD ranged from 12 to 18 g/L depending on the distillate/thickened LPD volume ratio applied. The thickened LPD was also richer in other nutrients, which indicates its possible use as a concentrated complex fertiliser. TN concentration and conductivity in the distillate did not exceed 6 mg/L and 0.03 mS/m respectively, suggesting that the distillate may be useful as alternative source of technological water. It was concluded that the system consisting of nitrification and subsequent thermal thickening of nitrified LPD yields final products exhibiting significantly superior properties compared to thermal thickening of raw LPD, without requiring any additional chemical agents. The quality of these products is comparable to those obtained through advanced membrane processes.