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)2SO4), 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 °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 °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 %. This work is licensed under a Creative Commons Attribution 4.0 International License.

Cilj ovog rada bio je rasterećenje postrojenja za proizvodnju bioplina smanjenjem hidrauličkog tlaka tijekom zime primjenom dvostupanjskog vakuumskog uparavanja. U prvom stupnju cilj je što više povećati koncentraciju amonijevih iona u vodenoj otopini (NH4+) i neutralizirati ih sumpornom kiselinom (H2SO4). Amonijev sulfat ((NH4)2SO4), skraćeno AS, koji nastaje kao proizvod neutralizacije, anorganska je sol koja se može upotrijebiti kao gnojivo. S obzirom na to da je koncentracija amonijevih iona u AS-u mala, a volumen otopine velik, troškovi transporta su znatni. Radi toga je potrebno uvesti drugi stupanj uparavanja kako bi se smanjio volumen vode, a udjel AS-a povećao do prihvatljive vrijednosti 35 mas. %. Parametri procesa uparavanja (temperatura, tlak i pH) podešavani su kako bi se pronašli najbolji uvjeti i utvrdilo može li se AS uspješno upotrijebiti kao gnojivo. Na temelju eksperimentalnih rezultata izvedene su bilance tvari procesa. Tijekom karakterizacije uzoraka digestata prikupljenih tijekom vremena provođenja eksperimenta utvrđeno je da se njegova fizikalna i kemijske svojstva kolebaju, a posebno udjel amonijaka. Time je potvrđen velik utjecaj svojstava digestata na uvjete anaerobne digestije. Tijekom prvog stupnja uparavanja pri 40 °C, pH vrijednost digestata iznosila je 7 i nije se mijenjala, što znači da je sav amonijak prisutan u hlapljivom obliku i tijekom procesa prelazi u kondenzat. Utvrđena je ovisnost brzine isparavanja o koncentraciji otopljene tvari, odnosno amonijaka. Brzina isparavanja je viša pri nižim koncentracijama. Poslije neutralizacije u prvom stupnju udjel AS-a bio je 0,61 %. Tijekom drugog stupnja, ciljani udjel AS-a postignut je pri 40 °C i pH = 5, a iznosio je 35,62 %. Udjel AS-a u kondenzatu drugog stupnja bio je 0,12 %. Na temelju eksperimenta utvrđeno je da je udjel kondenzata temeljen na ulaznoj količini digestata 76 %, a koncentrata 26 %. Nakon drugog stupnja udjel koncentrata (otopina AS) temeljen na količini koncentrata prvog stupnja koji u njega ulazi iznosi 8 %, a udjel kondenzata (čista voda) 92 %. Ovo djelo je dano na korištenje pod licencom Creative Commons Imenovanje 4.0 međunarodna.