Comparison between ferrihydrite adsorption and full-scale advanced drinking water treatment processes for controlling bacterial regrowth potential.

Bacterial regrowth in drinking water systems is a threat to public health. In this study, ferrihydrite (Fh) adsorption was compared with advanced drinking water treatment processes (ADWTP) during one and a half years of sampling to test the reduction in assimilable organic carbon (AOC) and bacterial regrowth potential (BRP). Dissolved organic matter (DOM) was removed by Fh through ligand exchange and electrostatic interaction. The DOM removal was higher below pH 6 due to protonation of Fh surfaces. The ADWTP exhibited higher removal rates of DOM than Fh and lower phosphate removal rates than Fh. Excitation-emission matrix (EEM) and size exclusion chromatography (SEC) revealed that Fh removed aromatic DOM larger than 1000 Da, while the biological activated carbon (BAC) of ADWTP could remove DOM smaller than 1000 Da. These differences of organic compositions resulted in the lowest AOC of BAC treated water, and the lowest BRP of Fh-treated water, indicating that it was the most biostable water. Phosphate addition experiments illustrated that phosphorus was the primary rate limiting nutrient, indicating that the higher phosphate removal of Fh made it possible to produce waters with lower BRP than ADWTP. Therefore, BRP is considered to be a better indicator of bacterial regrowth than AOC when phosphorus is a rate-limiting nutrient, as is the case with the Fh treatment.