Effects and mechanism of carbon and nitrogen removal of wastewater using Fe2O3-TiO2/UV/O3+PSAF treatment on a swine farm

Wastewater in livestock and poultry farming is the top priority to prevent and control the non-point source pollution in modern agriculture, due mainly to its high concentrations of nitrogen, suspended solids, and organic matter. In this research, a combined treatment was proposed to assemble Fe2O3-TiO2 photocatalytic ozonation and polyaluminum ferric silicate coagulant (Fe2O3-TiO2/UV/O3+PSAF), in order to achieve the deep decarbonization and denitrification of swine farm wastewater with low-carbon and high-ammonia-nitrogen (NH3-N) content. Response Surface Method (RSM) was employed to optimize the operating conditions of the combined process: the dosage of Fe2O3-TiO2 was 1.06 g/L, the flow rate of O3 was 3.02 g/h, the aeration time was 90.75 min, and the dosage of aluminum ferropolysilicate (PSAF) was 833.29 mg/L. The experimental value (COD: 88.79%; NH3-N: 82.24%) and the model predicted value (COD: 89.46%; NH3-N: 82.70%) were deviated by 0.75% and 0.56%, lower than 5%, indicating the excellent performance of the model. The interaction of composite factors demonstrated that the COD removal rate increased firstly, and then decreased with the increase of Fe2O3-TiO2 dosage, O3 flow rate and PSAF dosage, and finally peaked at the maximum in the range of 1.04-1.12 g/L Fe2O3-TiO2 dosage, 2.90-3.10 g/h O3 flow rate and 800-900 mg/L PSAF dosages. The NH3-N removal rate increased first and then decreased, with the increase of O3 flow rate and aeration time, and finally reached the maximum in the range of 2.90-3.10 g/h O3 flow rate and 84-96 min aeration time. Furthermore, three-dimensional fluorescence spectroscopy (3DEEMs) and ultraviolet-visible absorption difference spectroscopy (UV-Vis) were utilized to analyze the variation characteristics of fluorescence and molecular structures of Dissolved Organic Matter (DOM) in wastewater. The features and mechanism of carbon and nitrogen removal in the combined treatment were elucidated using the morphological changes of Dissolved Organic Carbon (DOC) and the Total Dissolved Nitrogen (TDN). A remarkable effect was achieved under the optimal operating conditions, particularly on the treatment of soluble microbial metabolites and humus-like substances in swine farm wastewater. Specifically, the removal rates of DOC and TDN were 77.7% and 82.6%, and the synergy factors were 1.11 and 1.50, respectively. Anyway, the Fe2O3-TiO2/UV/O3 unit behaved substantially reduced humus-like substances. Humus-like substances were transformed into small molecules through synergistic oxidation by O3,•OH and h+, where NH3-N was oxidized to NO3--N and N2 (N2 conversion rate was 39.71%), indicating the improved polarity of organic matter and the number of oxygen-containing functional groups. As such, the conversion and mineralization of nitrogen was promoted to create great water inlet conditions for PSAF coagulation. The removal of organic matter was facilitated with high oxygen and carbon ratio, as well as inorganic nitrate nitrogen using the bidentate complexation and electrical neutralization, where the surface of polynuclear hydroxyl polymer was used to adsorb, and finally to deposit in the bottom sludge in the PSAF coagulation system. This finding can provide a novel idea for the advanced treatment of low-carbon and high-ammonia-nitrogen organic wastewater, particularly for the control of agricultural non-point source pollution and ecological treatment of the water environment.