The effect of fermented superphosphate pretreatment and step-feed mode on biological denitrification of piggery wastewater

Highlights: • The COD/TN of piggery wastewater increased from 0.36 to 2.28 after fermented superphosphate (FSP) pretreatment. • In step-feed mode, the TN removal percentage in the FSP/Pretreatment-SBR was 57%. • The final effluent COD and NH₃-N and TP contents met the discharge standards. -- Abstract: Chemical pretreatment can reduce NH₃-N levels in piggery wastewater to a certain extent, but the lack of a carbon source for subsequent biological treatment leads to a low denitrification efficiency and poor total nitrogen (TN) removal percentage. Taking superphosphate (SP) pretreatment (SP/Pretreatment) as the control, this research studies the influence of fermented superphosphate (FSP; SP plus a carbon source) pretreatment (FSP/Pretreatment) on biological denitrification in a subsequent biological treatment step. Furthermore, the removal of pollutants under different influent modes is also evaluated. The experimental results show that with the addition of the SP pretreatment, the removal percentage of NH₃-N was 52%, and the chemical oxygen demand (COD)/TN ratio increased from 0.36 to 0.71. However, with the addition of the FSP pretreatment, the removal percentage of NH₃-N reached 64%, and the COD/TN ratio increased to 2.28. The combination of the FSP pretreatment and a subsequent sequencing batch reactor (SBR) step in the step-feed influent mode resulted in the best denitrification, with a TN removal percentage of 57%. This result was 51% higher than that of the SP/Pretreatment-SBR system, indicating that the addition of the FSP pretreatment improves the biological denitrification of piggery wastewater. After the full treatment process for piggery wastewater, the effluent COD was 57.33 mg·L⁻¹, the NH₃-N was 66.32 mg·L⁻¹, and the total phosphorus (TP) was 1.17 mg·L⁻¹, all of which meet the emission standards of the "Fouling Standards for Pollutants in the Livestock Breeding Industry" (GB 18596–2001) (400 mg·L⁻¹ COD, 80 mg·L⁻¹ NH₃-N, 8 mg·L⁻¹ TP).