The ANAMMOX process as the second step for the treatment of ammonium rich refinery wastewater with high Corg/N ratio

The combination of partial nitritation and anammox (anaerobic ammonium oxidation) has been mainly applied to the treatment of wastewaters with high ammonium concentration and low content of biodegradable organic carbon. So far, only few studies have focused on the application of partial nitritation-anammox process to the treatment of ammonium-rich wastewaters characterized also by a high organic carbon to nitrogen ratio (Corg/N), as well as by the presence of toxic substances: in this study, an anammox reactor was started-up and fed with the effluent from a partial nitritation reactor treating IGCC (Integrated Gasification Combined Cycle) wastewater, in order to evaluate its feasibility as an alternative to the currently applied chemical-physical-biological treatment. A sequencing batch reactor was inoculated with granular anammox biomass and run at controlled temperature (35±0.5 °C) and pH (7.7±0.3). The synthetic influent containing NH4-N (up to 250 mg/L) and NO2-N (up to 330 mg/L) was progressively replaced by the IGCC wastewater, which had been pre-treated in the lab-scale partial nitritation reactor. When the reactor was fed with the synthetic medium at the target nitrogen loading rate (NLR, 0.350 gN/L·d), the observed NH4-N removal efficiency was 93±5%, and no nitrite was detected in the effluent. Good overall process performance was maintained as increasing amounts (up to 65%) of the effluent from the partial nitritation system were fed to the anammox reactor: NH4-N and NO2-N removal efficiencies were 98.9±1.0% and 96.6±2.1%, respectively, and nitrite specific removal rate peaked at 0.28 gNO2-N/gVSS∙d. On day 154, a nitrogen shock load was applied to evaluate anammox stability during start-up: despite system sensitivity to the sudden increase of nitrogen load, process performance was recovered and the percentage of IGCC wastewater in the influent could be raised to 100% with fairly good NH4-N and NO2-N removal efficiencies (85.7±5.8% and 88.2±2.3%, respectively). Anammox granules were compact (diameter, 636±20 m) and dense (86.5±3.4 gTSS/Lgran), with good settling properties. The total organic carbon (TOC) removal efficiency was low: since most of TOC (around 80±8%) had been removed in the preliminary partial nitritation step (results not shown), it can be assumed that the residual TOC entering the anammox reactor was slowly biodegradable, therefore heterotrophic denitrifiers did not compete with anammox biomass for nitrite. The results indicate that anammox start-up can be successfully achieved and the process can be applied in combination with a preliminary partial nitritation step for the treatment of ammonium-rich IGCC wastewater, thus providing useful information also for the treatment of similar wastewaters with high Corg/N ratio and containing toxic substances.