Cambridge, Mass., facility uses DAF system to comply with strict drinking water standards
Fresh Pond is a vital part of the drinking water supply system for the residents of the City of Cambridge, Mass. Fed by the 2.5-billion-gal Hobbs Brook Reservoir and the 255-million-gal Stoney Brook Reservoir via gravity, Fresh Pond provides a raw water source for the 24-mgd Cambridge Water Treatment Plant (WTP).
To ensure treated water complied with the most stringent drinking water standards—including the U.S. Environmental Protection Agency Stage 2 Disinfectants and Disinfection Byproducts Rule (State 2 DBPR)—the Cambridge WTP decided to implement a robust multi-barrier treatment solution. A crucial step in the treatment process is the dissolved air flotation (DAF) and clarification system. The DAF system is a removal method for total organic carbon (TOC)—it prevents the formation of regulated disinfection byproducts (DPBs) such as trichloromethanes (TTHMs) and haloacetic acids (HAA5s).
The utility installed a Leopold Clari-DAF clarification system with two 12-mgd trains consisting of 12 concrete flocculators, six concrete DAF basins, two full recycle systems, and monitoring and controls. The Clari-DAF system is followed by ozone for primary disinfection, granular activated carbon gravity filters operated as biological filters and then chlorine to provide a secondary disinfectant.
The removal of particles in the water is a chemical and physical process. Aluminum hydroxide, a chemical coagulant, is rapidly mixed into the water to create particle collision—a process known as flocculation—to create clumps of particles. By using pressurized nozzles, the Leopold DAF system creates millions of micro-bubbles, approximately 30 to 60 microns in diameter, to float the clumps of particles to the surface of the water to be removed using mechanical and hydraulic skimming. The clarified water is drawn from the Clari-DAF system tank through underflow collectors located at the bottom of the basin.
The goal of this process is to comply with the Stage 2 DBPRs requiring a 35% TOC removal when TOC concentrations are below 4 mg/L and a 45% TOC removal when TOC concentrations are above 4 mg/L.
With the source water having an average TOC concentration of 3.73, the Clari-DAF system achieved average TOC removals of 45% in train 1 and 46% in train 2. After one year of operating the Leopold Clari-DAF system, the treatment plant’s TOC removal has consistently exceeded the Stage 2 DBPR requirements.
In addition, the high TOC removal lead to TTHM’s and HAA5’s average concentrations of 8.7 μg/L and 6.5 μg/L, respectively—significantly below the Stage 2 DBPR limit of 80 μg/L TTHM and 60 μg/L HAA5.