Start-up of a full-scale two-stage partial nitritation/anammox (PN/A) process treating reject water from high solid anaerobic sludge digestion (HSAD).

High solid anaerobic digestion (HSAD) achieves the benefits of high volumetric loading rates and lower reject water production, which, however, results in much more concentrated reject water with a remarkable increase in organics and nitrogen compared with that from conventional AD with low solid content. The high concentrations of ammonium (2000-3500 mg/L) and COD (3000-4000 mg/L) were reported to exert inhibition on anammox bacteria (AnAOB), posing challenges to the application of the partial nitritation/anammox (PN/A). To date, no cases of PN/A process start-up for sludge HSAD reject water were reported. This study demonstrated the start-up process of a 480 m/d PN/A project without anammox sludge inoculation and treating HSAD reject water from a centralized dewatered sludge treatment plant. The project did not construct new infrastructures but utilized previously constructed tanks to upgrade the process from existing short-cut nitrification-denitrification to a two-stage PN/A process. Although no external anammox sludge inoculation was performed to save seeding sludge cost, the start-up was successfully achieved in about 9 months (273 days) based on a three-step method of "AnAOB enrichment - sludge acclimation - capacity doubling". During start-up, the relative abundance of AnAOB (Candidatus_) increased from near zero to 12.0%. After start-up, the total inorganic nitrogen (TIN) removal load reached 0.74 kgN/(m•d), with a total nitrogen removal efficiency of over 90%. Compared to the traditional nitrification-denitrification process, the PN/A process remarkably reduces the addition of organic chemicals and aeration energy consumption, saving approximately 4.2 million yuan (RMB) in operational costs annually. In summary, this research provides a full-scale reference for the start-up of the PN/A process treating sludge HSAD reject water.