Vacuum Evaporation of Liquid Fraction of Digestate

The aim of this work was to reduce the strain on a biogas plant by reducing the hydraulic pressure during winter using the technique of two-stage vacuum evaporation. In the first stage, the target was to concentrate as much as possible aqueous ammonium ions (NH4+) in condensate and neutralize it with sulfuric acid (H2SO4). Ammonium sulphate ((NH4)(2)SO4), or AS, resulting as a product from neutralization, is an inorganic salt that can be used as a fertilizer. Since the ammonium ion content in AS is low and the volume of solution is high, transport costs are considerably high. Therefore, a second stage was needed to reduce the volume of water and concentrate the salt to an acceptable limit, determined by the mass fraction of AS, i.e. 35 %. The parameters of evaporation (temperature, pressure, pH values) were changed in order to find the best conditions for evaporation, and determine whether AS can be successfully used as a fertilizer. Based on experimental results, the mass balances of vacuum evaporation were calculated. During the characterization of digestate sampled at different times, it was found that the physical and chemical characteristics fluctuate, particularly the ammonia content. This confirmed the high dependence of digestate characteristics on the conditions of anaerobic digestion. During the first stage of evaporation at 40 degrees C, the pH value of the digestate had not changed and remained at 7, meaning that all ammonia was in its volatile form and had converted to condensate during the process. The dependence of the evaporation rate on the concentration of the solute, i.e. ammonia, was established. The rate is higher at lower solute concentrations. After the neutralization of the first stage condensate, the AS content was 0.61 %. During the second stage of evaporation, the desired AS limit of 35.62 % in the concentrate was reached at 40 degrees C and pH 5. The AS content in the second stage condensate was 0.12 %. On the basis of laboratory experiments, it was found that the fraction of first stage condensate based on the input of digestate was 76 % and the fraction of first stage concentrate was 24 %. After the second stage, the fraction that passed into the second stage concentrate, i.e. AS solution, was 8 %, while the fraction in the second stage condensate, i.e. clean water, was 92 %.