Chemically enhanced primary sedimentation and acidogenesis of organics in sludge for enhanced nitrogen removal in wastewater treatment

Abstract

Ferric iron-based chemically enhanced primary sedimentation (CEPS) can remove over 70% of organics from municipal wastewater into sludge, and acidogenic fermentation converts 27.9% of solid organic substances in the CEPS sludge into soluble ones, mostly in the form of volatile fatty acids. In this study, the CEPS sludge supernatant after fermentation (CEPS-FL) was studied as an organic carbon source for biological denitrification, in comparison with the common external carbon sources of methanol (CH3OH), sodium acetate (NaAc), and sodium propionate (NaPr). The specific denitrification rates of NaAc, NaPr, and CEPS-FL were 701.3, 449.8, and 410.3 g N·kg VSS−1·d−1, respectively, obviously higher than the value of CH3OH (237.2 g N·kg VSS−1·d−1). During heterotrophic denitrification, lower N2O yields were observed in VFAs-based denitrification reactors (0.53% of NaAc, 1.11% of NaPr, and 1.14% of CEPS-FL) than in the reactor fed with CH3OH (3.42%). The bacterial community of the denitrification sludge was significantly affected by the type of carbon sources. The sludge fed with CEPS-FL had a greater alpha diversity than that fed with pure organic carbons, with the dominance of Zoogloea (14.7%), Alicycliphilus (7.6%), and Thauera (6.2%). Based on the mass balance of CEPS-fermentation-denitrification combination, the ammonium-nitrogen in the CEPS-FL would not affect the overall nitrogen removal performance. In comparison with the municipal wastewater treatment supplemented with CH3OH or NaAc as the external carbon source for improved nitrogen removal, the operating cost of this combined system can be considerably lower. Overall, a small modification in primary sedimentation by CEPS in combination with the sludge fermentation would not only decrease the organic load on the subsequent biological treatment and significantly improve the nitrogen removal performance, but also provide an effective, chemical-saving, and energy-efficient way for wastewater treatment.