Effects of external recirculation on a two-stage mainstream anaerobic-anammox treatment system

Nitritation-anammox treatment can be a potentially energy- and resource-efficient technology for treating mainstream wastewater. However, the issue of nitrate residue from anammox treatment remains to be addressed. Herein, external recirculation of the anammox effluent to a hybrid anaerobic reactor (HAR), which was also to provide a continuous flow with low COD/N for the nitritation-anammox reactor, was employed to decrease the residue compounds. The recirculation ratio of 50% was observed to be the optimal to achieve the best overall performance with potential savings in energy demand. Specifically, in the operation scenario of R = 50%, the highest COD removal of ~90% by the HAR was achieved. Meanwhile, the lowest COD/NH₄ ⁺ -N ratio of ~2.0 in the HAR effluent ensured the lowest observed NO₃ ^- -N/NH₄ ⁺ -N ratio of ~14% in the nitritation-anammox reactor. These results have demonstrated the feasibility of applying external recirculation for nitrate residue removal via denitrification in the anaerobic pretreatment stage. PRACTITIONER POINTS: Nitritation-anammox treatment is an attractive method for mainstream wastewater treatment. Nitrate residue from anammox processes contributes to total nitrogen in the final effluent. Recirculation of anammox effluent to an anaerobic reactor can decrease nitrate residue. A recirculation ratio of 50% results in a low COD/NH₄ ⁺ ratio of 2 that benefits the subsequent anammox.