Yeast-enhanced activated sludge for improved nitrogen removal in wastewater treatment: Focus on dissolved organic...

Dissolved organic nitrogen (DON) in effluent of wastewater treatment plants (WWTP), particularly hydrophilic DON, is usually more effective than dissolved inorganic nitrogen (DIN) in stimulating phytoplankton growth and increases the risk of eutrophication in receiving waterbodies. Proteins, amino acids, and nucleic acids, which are the main sources of DON in the effluent, are produced during the hydrolysis of extracellular polymeric substances (EPS) in activated sludge. Herein, a yeast strain Candida tropicalis O2, which was highly efficient in degrading DON in EPS was screened. Within 48-h batch experiments, the DON removal rates of the extracted hydrophilic and hydrophobic EPS reached 68.26% and 59.27%, respectively. During the continuous 35-day operation of sequencing batch bioreactor (SBR) fed with synthetic wastewater, the yeast-enhanced activated sludge (AS-Y) reactor demonstrated a marked improvement in removing various pollutants compared to the traditional activated sludge (AS) reactor. Specifically, DON removal increased by 1.53 mg/L (24.75%), hydrophilic DON by 1.24 mg/L (27.13%), hydrophobic DON by 0.28 mg/L (12.08%), and COD removal by 4.04 mg/L (6.48%). Although the DIN removal decreased by 0.38 mg/L (3.86%), it did not attenuate the overall TN removal from the system, and an additional TN reduction of 1.15 mg/L (7.13%) was achieved. Metagenomic analysis showed that adding strain O2 slightly inhibited the DIN metabolism, and the relative abundances of napB, nirK/S, norB/C, and nosZ involved in denitrification somewhat decreased. Kyoto Encyclopedia of Genes and Genomes and Carbohydrate-Active Enzymes annotations revealed that adding strain O2 promoted amino acid and carbohydrate metabolism. The increased relative abundance of Candida indicated that strain O2 was able to colonize the sludge in AS-Y reactor, which was conducive to synergistic interactions with other microorganisms. This study provided a novel method for in situ improving nitrogen removal in WWTP and reducing the eutrophication risk of the effluent to receiving waterbodies.