Enhanced biofilm formation and denitrification in slow sand filters for advanced nitrogen removal by powdery solid carbon sources addition

Slow sand filter (SSF) is reliable for its economical convenience but is limited by low NO3−-N removal effect and long start-up time. In this study, the feasibilities of powdery poly butylene succinate (PBS) and polycaprolactone (PCL) as solid carbon sources for enhanced nitrogen removal in SSFs were investigated. Results suggested that the PBS with 200 mesh (PBS200) was with better carbon release capacity than PCL with 200 mesh (PCL200). During the start-up phase, SSF added PBS200 (SSFPBS) was found to achieve stable after 8-day adaptation, and 94.2 % NO3−-N and 78.9 % TN would be removed. Though a relative longer start-up time was required in SSFPBS than SSF added PCL200 (SSFPCL), the cost and the risk of secondary pollution caused by excessive COD was lower in SSFPBS. Analyses to biomass and extracellular polymeric substances (EPS) revealed that the biofilm structure in SSFPBS was more stable, which was because of higher biodiversity and biomass, as well as higher proteins and lower CO, OCO in EPS. For the study of optimizing the operating performance of SSFPBS (quartz sand/PBS200, 24:1), the results suggested that 90 % NO3−-N could be removed from municipal secondary effluent under the flow rate of 0.07 m/h. Besides, 33 % COD was further removed by two stage filter system. This study demonstrated that powdery PBS-based SSF is a promising technique to eliminate nitrate‑nitrogen pollution.