Removal of coliform pollution using a gravel-packed contact bed: reduction effect and microbial community analysis

A gravel contact oxidation system is an ecological engineering packed-bed reactor that uses gravel as a biofilm carrier to remove waterborne contaminants. While widely studied for pollution control, little is known about its microbial communities. In this study, we evaluated the system's effectiveness in reducing coliform bacteria, including antibiotic-resistant and hemolytic strains, as indicators of pollution and public health risk in Dongda Creek, Taiwan. Results showed a significant reduction in coliform levels, from 10⁵-10⁶ CFU/100 mL to 10² CFU/100 mL, along with notable improvements in water quality parameters like turbidity, total dissolved solids, and oxidation-reduction potential. Antibiotic-resistant coliforms were less prevalent in treated water, demonstrating the system's efficacy in control of bacterial flora that poses a risk to public health. Third-generation sequencing revealed reduced species richness in treated wastewater, while biofilm samples showed greater bacterial diversity. Gravel's surface area supported biofilm formation, promoting microbial communities capable of degrading organic pollutants. The activity of biofilm microbial community also decreased turbidity and dissolved solids, while shifting bacterial composition in the wastewater. These findings underscore the system's potential as a sustainable, effective model for ecological wastewater treatment.