Simultaneous nitrate and phosphorus removal in novel steel slag biofilters: Optimization and mechanism study

The simultaneous nitrate (NO₃^--N) and phosphorus (P) removal systems are considered to be an effective wastewater treatment technology. However, so far, there are few studies on system optimization to improve NO₃^--N and P removal. In this study, nine simultaneous NO₃^--N and P removal biofilters (SNPBs) were constructed to treat simulated wastewater. In order to optimize the NO₃^--N and P removal, different material loading positions were set: (1) red soil, steel slag, and rice straw (RSR), (2) steel slag, red soil, and rice straw (SRR), and (3) red soil, rice straw, and steel slag (RRS). Results showed that the above three treatments had mean removal efficiencies of 58%-91% for NO₃^--N and 55%-81% for TP, with the best N and P removal occurring in the SRR. The TN mass balance indicated that microbial removal was responsible for 78.2% of the influent TN in the SRR biofilter. The key microorganisms were Enterobacter, Klebsiella, Pseudomonas, Diaphorobacter, and unclassified_f_Enterobacteriaceae, which accounted for 61.9% of the total microorganisms. The main P-removal mechanism was the formation of Al-P, Fe-P, and Ca-P in red soil or steel slag layer. In addition, the decrease of SRR effluent pH from 11.86 in 1-7 days to 7.75 in 8-50 days indicated that red soil and rice straw had a synergistic effect on water pH reduction. These results suggest that a reasonable combination of steel slag with red soil and rice straw not only simultaneously removes NO₃^--N and P but also additionally solves the problem of high pH caused by steel slag.