Effect of potassium on nitrate removal from groundwater in agricultural waste-based heterotrophic denitrification system

• The threshold of K + concentration for nitrate reduction was 229.78 ± 25.80 mg-K/L. • 1.15–1.88 fold denitrification rate was improved under different K + concentration. • The evolution pathway and utilization sequence were revealed. • Pseudomonas and Thiobacillus are the unique species in 229.78 ± 25.80 mg-K/L. Heterotrophic denitrification based on solid carbon sources has been widely investigated for nitrogen removal in recent years. In this study, the response of the heterotrophic denitrification process under different K + concentrations was clarified. Additionally, the denitrification enhancement mechanism was revealed and resource utilization of agricultural waste was achieved. A series of batch tests were conducted to study the effect of different K + concentrations on the denitrification performance, dissolved organic matter (DOM) dissolution and microbial community structure. Results demonstrate that the threshold of K + concentration for the NO 3 − -N and NO 2 − -N reduction rates were 229.78 ± 25.80 and 159.10 ± 24.60 mg-K/L, respectively. Excitation-emission matrix (EEM) analysis identified the main DOM components associated with tyrosine-like, tryptophan-like and humic-like substances, as well as illustrated the evolutionary behavior and utilization of DOM. High throughput 16S rRNA gene sequencing indicates that a K + concentration of 229.78 ± 25.80 mg-K/L exhibited the highest diversity of functional species associated with fermentation and denitrification. The genera Pseudomonas and Thiobacillus were the unique denitrifiers at this K + concentration. The correlation of K + concentration, DOM dissolution of different regions and microorganism structure were analyzed using correlation matrix and PCA, and the appropriate K + concentration of different functional microorganisms survival was optimized by this analysis method.