Screening of Heterotrophic Nitrification-Aerobic Denitrification Strains and Their Application in Nitrogen Removal from Soybean Product Wastewater

Abstract

Soybean product wastewater (SPW) is organic wastewater generated during the processing of soybean products. Untreated discharge of this wastewater would inflict severe damage on the receiving environment and pose significant risks to human health. However, to date, no studies have been reported on the application of Heterotrophic Nitrification-Aerobic Denitrification (HN-AD) bacterial strains for the treatment of SPW. To obtain HN-AD strains that can directly treat undiluted SPW, enrichment culture, gradient dilution-plate streaking method were used to screen, isolate, and purify epiphytic aggregates of cyanobacteria in Taihu Lake. Morphological analysis and 16S rDNA sequencing results showed that the isolated HN-AD strain was Gram-negative and belonged to the genus Enterobacter (Enterobacter CS-1). Gradient concentration acclimation results showed that the strain can grow normally in undiluted SPW. Single-factor optimization results showed that the optimal conditions for strain CS-1 to treat simulated wastewater were: using sodium succinate as carbon source, C/N=15, temperature 30°C, pH 6.50, shaking speed 180 rpm. Under these conditions, the removal rate of NH₄⁺-N in simulated wastewater reached 96.2% within 48 h, with a maximum removal rate of 2.95 mg L⁻¹ h⁻¹. Whole-genome scanning results showed that strain CS-1 mainly achieves nitrogen removal through two metabolic pathways: NH₄⁺-N is assimilated into glutamate via the GDH or GS-GOGAT process, while NO₃⁻-N is first dissimilated and reduced stepwise to generate NO₂⁻-N and NH₄⁺-N, which is then assimilated and utilized. Response surface methodology (RSM) was employed to analyze the influence of pH, temperature, and agitation rate on NH₄⁺-N removal from undiluted SPW by strain CS-1. The model identified optimal operational parameters as pH 6.85, a temperature of 32.1°C, and a shaking speed of 189 revolutions per minute, without the supplementation of an external carbon source. Under these optimized conditions, Enterobacter sp. CS-1 accomplished the concurrent removal of 90.1% NH₄⁺-N, 42.0% COD, 53.4% DTN, and 67.6% DTP from raw SPW within a 48-hour period. These results demonstrate the considerable practical potential of this strain for the treatment of SPW