Addition of GAC caps and ozonation to conventional filters for improved organics and disinfection by-product reduction

Abstract

Disinfection by-product (DBP) control is a concern for many water utilities, especially as increasingly stringent regulatory limits are implemented. It has been previously determined that GAC improves removal of organic matter and may support biomass related to “biofiltration” to provide prolonged organics control. The objective of this pilot study was to explore the advantages of shallow GAC beds, or GAC caps, added to existing filters with or without prior ozonation in comparison to anthracite/sand filters in terms of DBP formation potential (FP) reduction and organics removal. Seven different filter configurations were examined to quantify the impacts associated with each treatment variable. DBPs of interest were haloacetic acids (HAAs) and trihalomethanes (THMs), the formation potential and removal of which, along with dissolved organic carbon (DOC) were monitored. Filters were also monitored for biomass development and biodegradation potential via adenosine triphosphate (ATP) and enzyme activity, respectively. The results of this pilot study confirmed significant increases in the removal of DBP FP alone (7%) and in combination with GAC caps (11%) inferring synergistic organics removal mechanisms. Increasing ATP concentrations were observed on GAC when operated to promote biological acclimation (e.g. non-chlorinated backwash). Monitoring biomass characteristics was important to distinguish the adsorptive or biological ‘mode’ of the filters. The GAC caps provided short-term improvements with respect to specific organics removal, and demonstrated biological indicators for potential long-term treatment improvements when compared to filters which did not incorporate GAC. GAC caps may be a practical solution for facilities looking to improve performance, specifically in regard to DBP removal, by taking advantage of the higher biological density and removal capacity of GAC compared to anthracite, at a greatly reduced cost.