AWWA WQTC55036

The Second Taxing District Water Department (STWD) of Norwalk, Connecticut owns and operates a 10-MGD water filtration plant located in Wilton, CT. The filtration plant, last upgraded in 1968, is a conventional plant with pre-aeration, rapid mix, single-stage flocculation, sedimentation, and shallow-bed automatic backwash (ABW) sand filters. The STWD filtration plant produces water that meets all current standards, but like many water utilities, may not meet all expected facets of future regulations. This paper presents the results of the feasibility study that was conducted to determine the processes most suitable for upgrade of the STWD filtration plant. The STWD has a limited site area, so a major constraint of the project was that any upgrade must fit within the existing plant footprint. The primary water quality goal of the study was to improve the removal of natural organic matter to thereby reduce disinfection byproduct formation. Improved removal of aesthetic characteristics such as color and taste and odor was also desired. A long list of upgrade alternatives was screened down to a short-list of the most suitable processes for the STWD plant. Eight alternative treatment trains composed of a variety of unit processes including, plate/tube settling, high rate clarification, ozonation, GAC filtration, and membrane ultrafiltration were developed. Conceptual layouts were prepared for the alternatives and compared on the basis of advantages and disadvantages, capital cost, operations and maintenance cost and present worth. On a process and layout feasibility basis, the most favorable process trains were ballasted settling/ozone/granular activated carbon (GAC) filtration, dissolved air flotation/ozone/GAC filtration, ultrafiltration with immersed vacuum-driven membranes, and ultrafiltration with pressurized membranes. Preliminary cost estimates for the four alternatives were comparable. Two-season parallel piloting of the four process trains was recommended to determine the best process for the STWD plant. Includes tables, figures.