Enhanced Organic Micropollutants Removal through Biofilm Thickness Management in Drinking Water and Water Reuse Applications

Abstract

The objective of this research is to investigate if targeted substrate removal is possible through biofilm thickness management in drinking water and water reuse system. The primary hypothesis is, management of thick and thin biofilms will allow for increased contaminant removal for all contaminants in a competing system by eliminating the effects of diffusion through a biofilm. Prior to this study, biodegradation of three organic micropollutants (acetaminophen, salicylic acid, and ibuprofen) were examined by the bacteria from backwash seed collected from full-scale biofiltration plant at different seasons. These compounds were chosen due to their predominant occurrence in water bodies, their consideration as micropollutants, and their different water partitioning behavior. The degradation was observed in the presence of oxygen. Complete removal of salicylic acid is observed in 30 to 60 hours depending on different environmental conditions, while acetaminophen requires more than 200 hours to achieve greater than 90% removal. Ibuprofen exhibited poor removal efficiencies in the 40% to 50% range after 220 hours depending on different seasons. Followed by the biodegradation experiments, experiments on three different biofilm thickness (25 µm, 100 µm and 400 µm) were conducted at an initial concentration of around 5 mg/L. Per the biofilm thickness experiments, SA degradation was faster in the thin biofilm scenario as compared to the thick scenario. The thinnest biofilms achieved 100% salicylic acid removal within 23-28 hours, whereas the thickest one required 32-37 hours depending on the biofilm source; however, the system contained dual limitation constraints for both oxygen and substrate concentrations. Unlike SA, both ACT and IB degraded faster by the thickest biofilms than the thinnest one. For ACT, depending on the biofilm sources, complete degradation required 87-107 hours and 109-121 hours by the thickest and thinnest biofilms, respectively. Moreover, the thickest and thinnest biofilms completely degraded IB at 130-140 hours and 145-152 hours, respectively. The differences in these removal rates were more prominent at the end of the reaction period, strengthening the hypothesis that the substrate removal was diffusion limited and not biomass limited. Hence, the limiting bulk substrate concentration is driving the reaction rates. These batch experiments provided the first clues on the general potential for micropollutant degradation under different biofilm thickness. Moreover, focusing on the management of thick and thin biofilms to potentially facilitate increased contaminant removal in competing systems allows for the isolation and investigation of diffusion-controlled degradation. Ongoing research is investigating the impact of degradation rates and biofilm thickness on overall pollutant removal in pilot-scale biofiltration columns to ultimately increase removal efficiencies. The research will also investigate if rapid and slow microbial growth are capable through SRT management of two filtration media: ceralite (expanded clay) and granular activated carbon (GAC).