AWWA WQTC58935

A forced draft aeration (FDA) process has been in operation for approximately 18 months at the Keller Water Treatment Plant, Pinellas County Utilities (PCU), Florida. During this time, the system has effectively removed sulfides, a major contaminant of concern because of long-standing impacts associated with water disinfection, corrosion, turbidity and taste and odor in the Pinellas County Water System (PCWS). However, increased turbidity levels in the FDA effluent were encountered within the first few months of operation prompting further investigation. Turbidity has consistently exceeded 1 NTU with values exceeding 1.5 NTU within 3 months of operation. This represents a substantial increase from the 0.2 NTU level commonly observed in the WTP raw water stream. Visual inspection confirmed significant solids accumulation within the packing media and microbial analysis performed for the County confirmed the presence of Thiothrix. This organism, whose numbers greatly increase in the FDA process, is at least partly responsible for the turbidity problem. The Floridan Aquifer appears to be the source of Thiothrix (Brigmon et al. 1994). Note that while Thiothrix is not reported to pose a human health threat, the slime like substances (EPS, or extrapolysaccharide) excreted by the bacterial biofilm could provide a protective matrix for a wide variety of microorganisms to live and multiply, a condition which raises several fundamental concerns: adverse water quality impacts caused by sloughing of biofilm material into the WTP process stream; degradation of FDA treatment performance; and, mechanical failure of the FDA media support structure. The information presented in this paper looks at low-cost filtration methods for removing solids sloughed from the FDA media into the process water. Based on organism size, cartridge filters and backwashable strainers were identified as sieving processes potentially capable of mitigating turbidity following FDA. This study characterized the nature of solids accumulated/sloughed and evaluated turbidity removal via either cartridge or strainer filtration techniques. Includes 2 references, tables, figures.