High Hydraulic Loading Rates Favored Mainstream Partial Nitritation: Experimental Demonstration and Model-Based Analysis

Partial nitritation is required to provide nitrite for the anammox reaction in an autotrophic nitrogen removal process, which has been considered crucial to achieving energy-positive mainstream sewage treatment. In this study, three lab-scale reactors were operated to treat wastewaters with low ammonium concentrations at high hydraulic loading rates (nitrogen loading rates of 0.36 kg N/d/m³). Long-term experiments repeatedly demonstrated that a high hydraulic loading rate favored the startup of partial nitritation, as indicated by the nitrite buildup and effluent nitrite accumulation ratio maintained above 95% over 2 months. Despite many advantages of high loading rates, a major drawback resides in the process instability, i.e., unsustained nitrite-oxidizing bacteria (NOB) suppression. To elucidate the occurrence and disappearance of the partial nitritation, mathematical modeling was implemented. An integrated fixed-film activated sludge biofilm model was developed, calibrated, and validated based on the observed coexistence of granules and floccular sludge. Model-based analysis suggested that high hydraulic loading rates suppressed NOB likely via intensifying granule surface sloughing and restricting oxygen penetration. Based on the unraveled mechanisms, operational strategies were proposed and tested with mathematical models. The mechanisms illustrated in this work can guide the development of new operational strategies to facilitate mainstream partial nitritation and the anammox process.