Rhamnolipid-assisted solid-phase denitrification process for enhanced nitrate removal of recirculating mariculture system water

Solid-phase denitrification (SPD) is a promising approach for treating recirculating mariculture systems (RMS) water, but its relatively slow nitrate removal rate presents a challenge. This study, for the first time, investigates the enhanced nitrate removal in the SPD process by rhamnolipid (RL) addition. The enhancement by RL addition (20 and 40 mg L-1) was significant and largely persisted even after RL dosing was stopped. Specifically, the nitrate removal in RL-assisted SPD reactors was enhanced by 1.7-2.6 and 1.9-2.1 times for influent NO3--N concentrations of 100 and 200 mg L-1 , respectively. The enhancement was attributed to the improved biofilm formation, as indicated by the increased contents of biomass and extracellular polymeric substances. Moreover, the abundances of denitrification and biofilm formation functional bacteria (Pseudomonas and Denitromonas) also increased. RL addition increased abundances of functional genes involved in carbon degradation and nitrate removal. The denitrification and assimilatory nitrate reduction pathways achieved nitrate removal together. Therefore, RL-assisted SPD presents a novel strategy for treating nitrate-rich RMS water.