Nitrification with High Nitrite Accumulation for the Treatment of “Old” Landfill Leachates

“Old” leachates are generally difficult of access to conventional biological and physicochemical nitrogen removal processes due to high-strength ammonium, recalcitrant carbon source, and strong buffering capability. The new processes based on partial nitrification of ammonium to nitrite coupling with anaerobic ammonium oxidation (ANAMMOX) are considered a promising option for treating “old” leachates and stable nitrite accumulation is the prerequisite step. Many studies have been conducted on the nitrite-type nitrification process, but few relative investigations were reported on the treatment of “old” leachates in the literature. In this research, temperature, hydraulic retention time (HRT), and dissolved oxygen (DO) were selected as operational variables to study the possibility of stable nitrite-type nitrification from “aged” leachates without pH adjustment. The experimental results showed that satisfactory nitritation (nitrite-type nitrification) performance could be easily achieved for the treatment of “old” leachates under ALR (ammonium load rate) varying from 0.0690 to 0.2843 g NH₄⁺-N g VSS⁻¹ day⁻¹ and the NO₂-N/NO_x-N(%) of effluent was in the range of 87–95%. It is concluded that the alternate inhibitions on nitrite oxidizers between free ammonia (FA) and free nitrous acid (FNA) in an operating cycle can be mainly responsible for satisfactory nitrite accumulation in the research. Additionally, this research confirms the feasibility of providing the appropriate nitrite/ammonium mixture for the following ANAMMOX reactor.