GAC to BAC: Does it make chloraminated drinking water safer?

Area of Science:

• Environmental Science
• Water Treatment Technology
• Analytical Chemistry

Background:

• Biological activated carbon (BAC) is a key polishing step in drinking water treatment, primarily for taste, odor, and organic carbon removal.
• Previous research on BAC focused on regulated disinfection by-products (DBPs), often with pre-ozonation, but less is known about its efficacy with chloramine and a wider range of DBPs.
• Chloramine, while producing fewer regulated DBPs, can increase concentrations of unregulated nitrogenous and iodinated DBPs.

Purpose of the Study:

• To evaluate the removal efficiency of 71 DBPs from ten classes by BAC filters in two full-scale drinking water plants using pre-chloramination and chloramine.
• To assess the impact of BAC filter age on DBP removal and formation.
• To analyze the changes in total organic halogens (TOX) and calculated genotoxicity and cytotoxicity associated with BAC treatment.

Main Methods:

• Analysis of 71 DBPs across ten classes (e.g., haloacetonitriles, nitrosamines, haloacetic acids) and speciated total organic halogens (TOCl, TOBr, TOI).
• Sampling and analysis were conducted across six BAC filters of varying ages (operational times) in two full-scale drinking water treatment plants.
• Calculated genotoxicity and cytotoxicity were assessed based on the identified DBP concentrations.

Main Results:

• Most pre-formed DBPs were effectively removed by BAC, but some, including specific nitromethanes, nitrosamines, and haloacetic acids, were poorly removed or increased.
• Certain DBPs, such as dibromoacetaldehyde and bromochloroacetamide, were newly formed after BAC treatment.
• Total organic halogen removal was variable, and while calculated genotoxicity decreased, calculated cytotoxicity increased in some filters, indicating DBP concentration alone is insufficient for risk assessment.

Conclusions:

• BAC treatment demonstrates variable efficacy in removing a broad spectrum of DBPs formed during pre-chloramination and chloramination processes.
• The formation of certain DBPs and potential increases in toxicity highlight the complexity of BAC's role in DBP control.
• Risk assessments for drinking water treatment using BAC should consider a wider range of DBPs and their potential formation/removal dynamics, not solely relying on concentration levels.