Microbial community structure and dynamics in a mixotrophic nitrogen removal process using recycled spent caustic under different loading conditions

► Heterotrophic denitrification which uses organics increases operating cost of WWTP. ► We applied MSC to nitrogen removal process instead of organic carbon. ► MSC application proved to be efficient for nitrogen removal of sewage with a low C/N. ► The NLR of 0.15kgN/m3d is recommended to satisfy water quality regulations. ► MSC caused the increase of genus Thiobacillus known as an autotrophic denitrifier. A laboratory-scale Bardenpho process was established to investigate the proper nitrogen loading rate (NLR) when modified spent caustic (MSC) is applied as electron donor and alkalinity source for denitrification. MSC injection induced autotrophic nitrogen removal with sulfur as electron donor and heterotrophic denitrification. The nitrogen removal rate (NRR) did not increase proportionally to NLR. Based on the total nitrogen concentration in the effluent observed in the trials with MSC, the NLR in the influent should not exceed 0.15kgN/m3d in order to satisfy water quality regulations. Microbial communities in the anoxic reactors were characterized by pyrosequencing of 16S rRNA gene sequences amplified by the polymerase chain reaction of DNA extracted from sludge samples. Microbial diversity was lower as MSC dosage was increased, and the injection of MSC caused an increase in SOB belonging to the genus Thiobacillus which is responsible for denitrification using sulfur.