Stable long-term operation and high nitrite accumulation of an endogenous partial-denitrification (EPD) granular sludge system under mainstream conditions at low temperature.

In this study, the robustness of an endogenous partial-denitrification (EPD) system was demonstrated during a 105 days operation under mainstream conditions at low temperature. The results showed a high nitrate-to-nitrite transformation ratio (NTR) of 88.7%, a suitable NO-N/NH-N ratio of 1.09, and no residual NO-N in the effluent, as reactor temperature was decreased from 25 °C to 13.5 °C. The stable morphology of the EPD granular sludge with high extracellular polymeric substance content (273.6-308.1 mg/g VSS) contributed to the steady performance of the EPD system. Bath tests revealed that the maximum NTRs (95.6% and 96.3%) were slightly higher at 10 °C and 15 °C than those (90.4% and 92.0%) at 20 °C and 30 °C, resulting from the lower poly-β-hydroxyvalerate degradation rate or the negligible denitrifying dephosphatation activity at low temperatures. Thus, the EPD system has the potential to provide stable nitrite for mainstream anammox at low temperature.