AWWA WQTC60669

This paper discusses an effort by United Water New York (UWNY) to design/build the first high rate dissolved air flotation (DAF) system in the United States at its Lake DeForest Water Treatment Plant (LDF WTP). The reasons for the upgrade included: increase plant capacity from 10 mgd to 20 mgd; improve algae removal; capability for pH adjustment prior to coagulant addition (CO₂ was selected); improve filtered performance (lower turbidities and longer filter run times); and, lower disinfection byproducts (DBPs). LDF is the largest reservoir in a chain of reservoirs that comprise the Hackensack River Watershed. It is a eutrophic lake that receives runoff from suburban fertilized lawns, septic systems and inputs from streams and smaller lakes that are also eutrophic. Typical water quality ranges in LDF: Algae - up to 5,000 counts; TOC - 4 to 6 mg/L; color - up to 50+ units; and, low turbidity - 3 to 8 ntu. In addition to these treatment challenges, UWNY also needed to maximize microbial removal/inactivation and minimize DBPs. Major improvements included: CO₂ feed system for pre-coagulation pH adjustment; 48-inch in-line static mixer for improved chemical mixing; DAF Pretreatment - hydraulic flocculation; high-rate DAF system; supersaturated air addition; solids separation/DAF Float; residuals wasting/DAF effluent; AquaDAF recycle stream; and, 6,000 sf pre-engineered building designed to match existing structures. The total cost for these improvements was $9 million. LDF overall performance is discussed along with chemical optimization, copper sulfate treatment event, aluminum breakthrough, and high filter turbidities. Includes table, figures.