AWWA ACE65313

Granular filter media used in chemical clarification treatment practicing "sweep floc" coagulation will acquire metal surface coatings on the filter media over extended periods of time. In essence, the metal hydroxide floc carryover from the settling tanks and the aging of these hydroxide floc on the filter media over years of exposure have resulted in a durable metal-oxide coating on filter media that can be conducive for enhanced NOM and arsenic adsorption over the entire filter run cycle. The objective of this overall study was to quantify natural organic matter (NOM) and arsenic removal using naturally coated sands from conventional rapid rate filters. The major research goals of this paper were to evaluate dissolved organic carbon (DOC) and ultraviolet (UV) adsorbance (254 nm) as a function of metal surface coating (i.e. aluminum-based vs. iron-based), pH of regenerating filter media backwash (BW) solution, pH of the influent NOM challenge solution, and challenge water quality. Preliminary results indicate that DOC and UV absorbance removals were significantly enhanced with increasing BW regeneration pH (up to pH 10-12) and lower challenge solution pH. Increasing the pH to 13 resulted in a significant loss of the aluminum coating and a corresponding decrease in DOC removal. The influent challenge solution pH had an important impact on DOC removal efficiency. The BW regeneration was maintained at pH 11 during this phase while the influent pH was adjusted to 6, 5, and 4. DOC and UV absorbance removals were dramatically increased (>80%) with decreasing pH. At run termination at the lower pHs, DOC removals were at least as comparable if not exceeding removals noted using commercially available iron-oxide coated sands. Includes 7 references.