The two main processes that OCWA—Central New York’s Water Authority—utilizes to treat the drinking water it delivers to its 500,000 consumers are filtration and disinfection. Today’s article focuses on the filtration process.
Customers of OCWA receive water that originates from one of three sources–Otisco Lake, Lake Ontario and/or Skaneateles Lake. OCWA operates two advanced water treatment facilities–one at Otisco Lake and the other at Lake Ontario, while the City of Syracuse is responsible for treating the water originating from Skaneateles Lake. The filtration process is different for each source of supply.
Otisco Lake (supplies customers mainly in southern and western Onondaga County)
OCWA has two water intake pipes located in Otisco Lake which deliver raw water, by gravity, approximately five (5) miles to OCWA’s Otisco Lake Water Treatment Plant (Otisco WTP) located in Marcellus. Water first enters the rapid mix tank where a coagulant (polyaluminum chloride) is added. After 30 seconds of mixing, the water enters “contact basins” where the calm conditions allow the coagulant to make small particles adhere together, forming larger particles. Some of these particles settle and are cleaned out later. The contact time in these basins also allows powdered activated carbon (a type of filtration media used only when needed) to adsorb organic taste and odor causing chemicals. After about one hour of contact time, the water passes through one of six multimedia filters where the coagulated particles are removed. These filters consist of granular activated carbon, silica-sand, and hi-density sand. The filters are cleaned periodically. The filtered water is stored in large tanks located at the treatment plant to provide adequate contact time for the disinfection process to work. Once the water leaves the tanks, orthophosphate is added to provide a coating for the pipes in the distribution system and in your home. This is done in order to prevent the leaching of lead and copper from your pipes into your water.
Lake Ontario (supplies customers in northern and eastern Onondaga County, as well as Oswego, Madison, Oneida, and Cayuga counties)
OCWA’s Lake Ontario water treatment system pumps water from Lake Ontario through an eight-foot diameter concrete intake it shares with the City of Oswego. Upon entering the “Raw Water” Pumping Station, lake water is treated with carbon dioxide to suppress pH thereby increasing the effectiveness of the coagulation process. The water is pumped approximately two miles to OCWA’s Ontario Water Treatment Plant (Ontario WTP). Water entering the plant is treated with sodium hypochlorite (disinfectant) and polyaluminum chloride (coagulant) and is flash mixed. The water then enters three contact basins where slow mixing allows small particles to accumulate and form larger, more readily filtered particles. After about two hours of contact time, the water flows into dual media filters consisting of granular activated carbon and filter sand whereby particulate contaminants are removed. After filtration three treatments are applied: fluoride to reduce tooth decay, sodium hypochlorite to disinfect, and sodium hydroxide for corrosion control.
Skaneateles Lake (supplies customers in Nedrow, Southwood and Jamesville areas)
The City of Syracuse does not filter the water that enters its intakes located in Skaneateles Lake. The City has been granted a waiver to provide its customers with unfiltered water subject to strict conditions set by the New York State Department of Health. These conditions include water quality monitoring, backup disinfection, and watershed protection. The City’s Water Plant is located in the Village of Skaneateles where the water is disinfected with chlorine and fluoride is added. Water then flows by gravity into the City’s storage reservoirs. Orthophosphate is added to the water (for lead and copper control) as it leaves these reservoirs and it is disinfected again by the addition of sodium hypochlorite. In 2013, an Ultraviolet Light Treatment Facility was put into operation at Westcott Reservoir. A UV Light Treatment Facility at Woodland Reservoir was completed in April 2014. Ultraviolet disinfection allows the City to strengthen protection against microbial contaminants, especially targeting cryptosporidium.