Anaerobic Digestion of High-Nitrogen Tannery By-products in a Multiphase Process for Biogas Production

Industrial by-products and wastes can be very challenging substrates for anaerobic digestion, since their composition in terms of nutrient content can be strongly unbalanced. Tannery by-products, for instance, are biomasses of animal origin with high nitrogen content since their composition is dominated by proteins, peptides and amino acids. The use of this biomass as main substrate to feed anaerobic digestion is limited because of the high concentrations of ammonia formed during fermentation. Ammonia is originated from the biological degradation of proteinaceous materials in anaerobic environment. Free ammonia permeation into bacterial cells can cause intracellular pH change, inhibition of specific enzymatic activities and increased energy requirement for cell maintenance. Therefore, high ammonia amount cause severe inhibition or failures of anaerobic digestion, with important consequences for both process stability and productivity. The aim of this study was the development of a multiphase anaerobic digestion process for tannery byproducts, characterized by a C/N ratio below 5, in order to convert them into biogas. The process needed an acclimation phase of anaerobic microorganisms due to high concentrations of ammonia (NH4+/NH3), above 9,000 mg/L, and the low C/N ratio. Implication for process stability, microbial activity and for the achievement of the target performances are discussed.