Tertiary denitrification by sulfur/limestone packed biofilter

Autotrophic denitrification and the simultaneous NH₄ ⁺-N and total phosphate (TP) removal performance of elemental sulfur/lime (S/L) biofilter were investigated for the tertiary treatment of secondary effluent. Removed NO₃ ^--N loading rate increased with the influent NO₃ ^--N loading rate and complete denitrification was obtained at loading rates <0.298 kg/(m³·d). Half-order constants were 2.359, 3.314, 3.437, and 4.288 mgNO₃ ^--N^1/2/L^1/2·h, respectively, for Empty Bed Residence Time (EBRT) of 2, 1, 0.75, and 0.5 h. Moreover, NH₄ ⁺-N and TP removal performance was studied for the first time, and results indicated that NH₄ ⁺-N and TP could be simultaneously removed in the S/L biofilter for tertiary denitrification. Approximately 0.081 mg/L NH₄ ⁺-N was removed per mass of nitrate denitrified under condition where no nitrite was accumulated. About 0.103 mg/L TP could be removed under different EBRTs and pH was a limiting factor for phosphorus removal performance. NH₄ ⁺-N and TP removal characteristics were related to NO₃ ^--N removal along the biofilter depth. Moreover, backwashing was required when hydraulic head loss reached ∼50 cm with about 30 g NO₃ ^--N removed. Nutrients removal performance could recover within 3-4 h. In general, sulfur/limestone packed biofilter is an economical and effective approach for tertiary denitrification. Results achieved in this study can improve the understanding of sulfur-based autotrophic denitrification for engineering application of nitrate removal from municipal wastewater treatment plant secondary effluent.