Abstract
Anaerobic digestion of high-nitrogen wastes is inhibited by the presence of ammonia. We investigated a method to
alleviate this inhibition by reducing free ammonia through pH control in the digester. The pH was reduced to a level lower
than usual by adding acid. The results showed that the pH 6.8 could be controlled, without compromising methane
production. Furthermore, low pH control enables operating the system under high nitrogen concentrations, up to a total
ammonia nitrogen of 10,000 mg L−1, while keeping a free ammonia at 100 mg L−1 or less. This method can contribute to
producing liquid fertilizers with high nitrogen concentration derived from highly nitrogenous wastes, and can be used as
substitutes for both biomass energy and chemical fertilizers.
