Two-stage biofilter for effective NH₃ Removal from Waste Gases Containing High Concentrations of H₂S

A high H₂S concentration inhibits nitrification when H₂S and NH₃ are simultaneously treated in a single biofilter. To improve NH₃ removal from waste gases containing concentrated H₂S, a two-stage biofilter was designed to solve the problem. In this study, the first biofilter, inoculated with Thiobacillus thioparus, was intended mainly to remove H₂S and to reduce the effect of H₂S concentration on nitrification in the second biofilter, and the second biofilter, inoculated with Nitrosomonas europaea, was to remove NH₃. Extensive studies, which took into account the characteristics of gas removal, the engineering properties of the two biofilters, and biological parameters, were conducted in a 210-day operation. The results showed that an average 98% removal efficiency for H₂S and a 100% removal efficiency for NH₃ (empty bed retention time = 23–180 sec) were achieved after 70 days. The maximum degradation rate for NH₃ was measured as 2.35 g N day^-1 kg of dry granular activated carbon^-1. Inhibition of nitrification was not found in the biofilter. This two-stage biofilter also exhibited good adaptability to shock loading and shutdown periods. Analysis of metabolic product and observation of the bacterial community revealed no obvious acidification or alkalinity phenomena. In addition, a lower moisture content (40%) for microbial survival and low pressure drop (average 24.39 mm H₂O m^-1) for system operation demonstrated that the two-stage biofilter was energy saving and economic. Thus, the two-stage biofilter is a feasible system to enhance NH₃ removal in the concentrated coexistence of H₂S.