Denitrification performance, biofilm formation and microbial diversity during startup of slow sand filter using powdery polycaprolactone as solid carbon source

Abstract

To solve the problem of unsatisfactory nitrogen removal effect and long start-up in slow sand filter (SSF), the powdery polycaprolactone (PCL) with different particle sizes (40 and 200 mesh) were added to the lab-scale SSFs. This study compared the two solid carbon sources release capacity and enhanced denitrification performance in each SSF, and revealed the inner mechanism by analyzing the physical and chemical characteristics of the biofilm. The results represented that PCL200 could release organic carbon up to 273.9 mg/L, which was much higher than PCL40 (121.2 mg/L). Since the higher organic carbon release and specific surface area, the PCL200 was conducive to the rapid start-up of the SSF and achieving a desired removal efficiency of NO3--N (> 95%) and TN (> 80%) at the early stage of operation. In addition, large amounts of biomass and extracellular polymeric substances (EPS) were detected in the PCL200 system. Spectroscopic characterization further revealed EPS from two SSFs expressed similar fluorescence peak locations but different intensities. Specifically, the aromatic and tryptophan protein-like substances were the main fractions in EPS, and more functional bonds (C-(C, H), C˭O, O-C-O) conducive to microbial flocculation were observed when adding PCL200. Furthermore, the high-throughput sequencing analysis showed the denitrifying bacteria almost accounted for 62.12% in the reactor with PCL200, and microorganisms in two SSFs were significant difference in amino acid and carbohydrate transportation and metabolism. Findings from this study would be valuable for enhancement of the denitrification and stability of SSF treating nitrogen-contaminated water bodies.