Valorization of nitrogen and sulfur from digestate and biogas: A closed-loop approach to sulfur-rich microbial protein synthesis

An integrated route for the conversion of anaerobic digestion outputs: biogas (CH4/H2S) and digestate (NH4+), into microbial protein (MP), is proposed. The MP production from H2S-rich biogas and digestate nitrogen through a mixed culture of methane oxidizing- (MOB) and sulfur oxidizing-bacteria (SOB) was performed in two hollow fiber membrane bioreactors. The use of digestate and biogas for the supply of N and S, key nutrients for the microbial fermentation process, was compared to the supply of mineral N and S, and its effects on biomass concentration and yield, gas and nutrient consumption, as well as on the amino acids profile and the microbial community composition of the harvested biomass were investigated. Ammonium nitrogen was provided through direct stripping from the liquid digestate using the recirculating biogas stream from the fermenter. The ammonia stripped from the digestate, with nitrogen stripping rates of up to 109.6 mg NH4+-N/L·d obtained with at 45 °C and 4 vvm (gas:digestate), met the nitrogen requirements of the biogas-to-MP conversion process. Sulfur was supplied through the H2S present in the biogas, that was oxidized into sulfate by SOB. Biomass growth occurred even at high sulfide concentrations in the biogas (260.5 mg/L of volatile suspended solids (VSS) at 6000 ppmv of equivalent H2S), with a high protein content (up to 65.4 %) of the biomass, while relying on the sulfate produced by H2S oxidation as sulfur source. Increasing H2S concentrations in the biogas resulted in an amino acids profile rich in sulfur-amino acids (140.4 mg/g VSS at 6000 ppmv).