Performance and mechanism of synchronous nitrate and phosphorus removal in constructed pyrite-based mixotrophic denitrification system from secondary effluent.

ABSTRACT

The performance and process of the constructed pyrite-based mixotrophic denitrification (POMD) system using pyrite and residual organic matters as the co-electron donors were investigated for simultaneous removal of N and P from secondary effluent. After the batch experiments, 61.80 ± 3.26% of phosphate and 99.99 ± 0.01% of nitrate were removed, and the obtained nitrate removal rate constant can reach 2.09 days-1 in POMD system, which was significantly superior to that reported (0.95 day-1) in pyrite-based autotrophic denitrification (PAD) system. PO43--P removal was mainly achieved via chemical precipitation as FePO4 with iron, and it was irrelevant with the initial nitrate and ammonium concentrations. High-throughput 16S rRNA gene sequencing analysis showed the coexistence of heterotrophic and autotrophic denitrifiers in the mixotrophic environment. The denitrification process could be divided into two stages according to the carbon balance and calculation of sulfate accumulation (a) nitrate was mainly reduced heterotrophically during 12-36 h and (b) nitrate was reduced autotrophically after 36 h. The calculated proportion of heterotrophic denitrification was 58.17 ± 3.78%, which was promoted by a higher ammonium concentration. These findings are likely to be useful in understanding the mixotrophic denitrification process and developing a cost-effective technology to simultaneously remove N and P from secondary effluent. Graphical abstract.