Sulfur-pyrite-limestone biological filter for simultaneous nitrogen and phosphorus removal from wastewater treatment plant effluent: Interaction mechanisms of autotrophic and heterotrophic denitrification

To efficiently remove nitrogen(N) and phosphorus(P) from wastewater treatment plant effluent (WWTPE) with insufficient carbon, sulfur-pyrite-limestone autotrophic denitrification (SPLAD) reactor and gravel heterotrophic denitrification (GHD) reactor were constructed. Results showed that SPLAD was successfully started after 8 days and was faster than GHD. Under optimal hydraulic retention time (30 min) and backwash cycle (2 d), SPLAD achieved higher removal rates for TP, TN, NO3--N, NH4+-N at 70.49 %, 90.35 %, 92.82 %, 79.66 %, respectively, surpassing those of GHD. The N removal primarily occurred through nitrification and denitrification with a proportion of 2.42-2.51 between heterotrophic and autotrophic denitrification, while P removal predominantly relied on the combination with Fe2+/Fe3+ generated by FeS2 autotrophic denitrification. Microbial community analysis revealed that Thiobacillus (17.86 %), Thioalbus (7.30 %) and Geothrix (0.81 %), belonging to autotrophic denitrification bacteria using S-0/FeS2 as electron donors to reduce NO3-N, were existed exclusively in SPLAD. Besides, the denitrification genes, such as nitrate reduction (narG, narZ, nxrA, and narH, narY, nxrB) and nitrite reduction (nirB and nirD), were higher in PSLAD than that in GHD. High abundance of sulfur oxidation genes (soxA, soxX, soxB, soxY and soxZ) and iron oxidation gene (ccoN) for autotrophic denitrification were also found in SPLAD. This study highlights the practical engineering application potential of the sulfur/pyrite/limestone involved mixotrophic denitrification process due to its simplicity, cost-effectiveness, and efficient N and P removal.