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 similar to 90% by the I IAR was achieved. Meanwhile, the lowest COD/NH4+-N ratio of similar to 2.0 in the HAR effluent ensured the lowest observed NO3--N/NH4+-N ratio of similar to 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. (C) 2018 water Environment Federation