Exploitation of Solar Energy for Ammonium Sulfate Recovery from Anaerobic Digestate of Different Origin

Digestate represents the semi-liquid byproduct of the anaerobic digestion process. It was estimated that 56 Mtonnes of digestate are annually produced only in Europe. Digestate composition depends on the initial total solids (TS) mixture fed to the digester and by its origin (manure, food wastes, agricultural residues, wastewater sludge). Typical values for the different components include a stabilized organic matter content between 20 and 50 g/L, a good content of nitrogen compounds (2-8 g/kg) and of phosphorous compounds (up to 3 g/kg). In particular, ammonia presence in digestate is interesting for ammonium sulfate production, a fertilizer which favors plants' growth. Traditional ammonium sulfate synthesis is conducted by stripping followed by sulfuric acid absorption, a process which requires high energy and chemicals consumptions. This work investigates the exploitation of solar energy to evaporate the liquid phase of digestate in a lab-scale greenhouse. Digestate vapors, rich in ammonia, are sent in a Drechsler trap, filled with 38% w/w sulfuric acid solution, through three solar air fans. The process has been tested on digestates differing for TS content and origin. It emerged that the process is favored by digestates with high ammonia content, as those originated from manure, and a TS content of about 10% w/w, which allowed to obtain an ammonium sulfate concentration of about 2 M in the final solution.