Feasibility of coupling anaerobic digestion and struvite precipitation in the same reactor: Evaluation of different magnesium sources

• Different magnesium sources were evaluated to reduce ammonia inhibition.
• A magnesium based stabilizing agent was used to facilitate struvite precipitation.
• The stabilizing agent was the unique non-harmful magnesium source.
• Anaerobic digestion and struvite precipitation were coupled in the same reactor.

The feasibility of coupling anaerobic digestion and struvite precipitation in the same reactor was evaluated to enhance manure anaerobic digestion methane yields through ammonia inhibition mitigation. Five different magnesium sources were tested as a struvite (ammonia sequestration agent) precursor, i.e. MgCl2, Mg(OH)2, two industrial by-products rich in MgO but with different reactivity, and a stabilizing agent. The latter was formulated in advance with the low reactivity industrial by-product and phosphoric acid. The effect of each magnesium source on anaerobic digestion as well as its struvite precipitation capacity was evaluated through biomethane potential tests. Results indicated that all magnesium sources were able to reduce ammonia concentration to different extents. However, the stabilizing agent was the unique magnesium source that did not inhibit the anaerobic digestion process. The avoidance of adding a phosphate source directly into the digester medium and the high newberyite content were the advantages of the stabilizing agent over the other magnesium sources. Finally, a series of experiments indicated that if anaerobic digestion and struvite precipitation are combined in a single reactor, stabilizing agent addition should be carried out through several small additions rather than few large additions.