A novel start-up strategy for mixotrophic denitrification biofilters by rhamnolipid and its performance on denitrification of low C/N wastewater.

A novel start-up strategy for sulfur-based mixotrophic denitrification biofilters (mDNBFs) by rhamnolipid was investigated for the first time. Rhamnolipid with gradient concentrations (0-120 mg/L) was added into five lab-scale mDNBFs. Results showed that rhamnolipid could promote biomass yield and nitrogen removal rate (NRR) by 71.7% and 68.7%, respectively, while its effect on EPS and adhesion force was concentration-dependent. The spatial distribution characteristics of microbial communities demonstrated the enrichment of main heterotrophic denitrifying bacteria outcompeted that of the autotrophs, with a more pronounced difference in high concentration rhamnolipid-treated mDNBFs. Furthermore, highest abundance of napA, narG, nirK and nosZ genes was observed in 80 mg/L rhamnolipid-treated mDNBF. Interfacial processes including solubilizing effect and hydration repulse and variations of organics were discussed to explicate the underlying mechanism. The study enlightened that an appropriate concentration (∼80 mg/L) of rhamnolipid may be a good solution for accelerating biofilm formation and enriching denitrifying bacteria to promote denitrification performance of mDNBFs treating low C/N wastewater.