Partial nitritation of nitrogen rich refinery wastewater (sour water) with different IC/N molar ratios

For nitrogen rich streams, conventional biological treatment based on nitrification and denitrification usually lacks of efficiency and requires considerable amounts of an external carbon source to be supplied; on the other hand, physical-chemical processes are characterized by high operating costs. The possible application of partial nitritation SHARON (Single reactor for High activity Ammonium Removal Over Nitrite) coupled with autotrophic ANAMMOX (ANaerobic AMMonium OXidation) or heterotrophic denitrification via nitrite processes would represent a technical- and cost-effective technology: partial nitritation has been studied and commonly applied at full scale to treat anaerobic digester supernatant and landfill leachates, while only few studies focusing on the treatment of industrial wastewater containing toxic substances have been carried out so far. In this study, a SHARON reactor was used to treat synthetic and real ammonium rich refinery wastewater (sour water): since availability of inorganic carbon (IC) determines the amount of NH4+-N being converted into NO2--N by partial nitritation, different influent IC/N (as HCO3-/NH4+-N) molar ratios were tested and SHARON feasibility as the preliminary treatment in a double stage SHARON-ANAMMOX or SHARON-Denitrification via nitrite process was assessed. In order to retain only ammonium oxidizing bacteria (AOB) in the system, the reactor was run at controlled temperature and operated as a chemostat (no biomass recirculation) at low hydraulic and solids retention time. A synthetic medium containing NH4+-N (2,000 mg/L) was initially fed to promote biomass acclimation, then real sour water containing also organic substrate, cyanides, sulphides and phenols was supplied. In both synthetic and real wastewater, the IC/N molar ratio was progressively increased from 1 to 2. Effluent from the SHARON reactor fed with the synthetic medium (influent IC/N molar ratio of 1) was suitable for subsequent treatment by ANAMMOX; increasing the IC/N molar ratio up to 2 enhanced NH4+-N conversion into nitrite, producing a final effluent suitable for denitrification via nitrite. Such positive results were confirmed with real sour water, despite the presence of highly toxic substances: the progressive increase of influent IC/N molar ratio from 1 to 2 enhanced NH4+-N removal efficiency (up to 97.20.1%) and its conversion into nitrite, always with low nitrate production, indicating stable nitritation. Acute toxicity assessments carried out on biomass drawn from the SHARON reactor confirmed that acclimation to toxic substances contained in the real wastewater was successfully achieved. Moreover, removal of organic matter indicated the growth of heterotrophic biomass without any competition with autotrophic microorganisms. Results presented in this study proved that controlling influent IC/N molar ratio represents a key operating strategy to properly regulate SHARON performance: depending on the IC/N molar ratio, the SHARON reactor produced an effluent suitable for further treatment by either autotrophic ANAMMOX or heterotrophic denitrification via nitrit