Water Treatment

Diagram showing the process of treating water


The process of treating the water supply is as follows.

Raw Water

Raw Water is drawn by gravity from the primary source, the Catskill Aqueduct, through a siphon connection constructed through the top of the aqueduct. There is a standby connection to the Croton Aqueduct in the event of a Catskill Aqueduct shutdown.

Rapid Mixers

The Rapid Mixers provide about one minute of high intensity dispersion of chemicals added to the raw water. The chemicals that can be added are alum, polyaluminum chloride, chlorine, caustic soda, polymer and potassium permanganate.


Flocculators provide about 30 minutes of controlled mixing of the water in three stages with coagulants that make particles stick together into large masses called floc particles, such as:

  • Algae
  • Bacteria
  • Clays
  • Minerals
  • Silts
  • Viruses

Dissolved Air Flotation (DAF)

Dissolved Air Flotation (DAF) removes the floc particles and clarifies the water. The DAF involves dissolving compressed air into a recycled water stream, and then releasing the air as microscopic bubbles that float the floc particles to the surface, where it is skimmed off. The clarified water is removed from the bottom of the DAF tanks. 

The detention time in the DAF units is about 25 minutes. The DAF process is used because it adapts better than other processes to the seasonal changes that occur in the quality of the raw water (variations from low to high turbidity, or amounts of particles suspended in the water) coming from the aqueducts New Castle uses.


Ozone, the strongest oxidizing agent among commonly used disinfectants, is the primary disinfecting agent. It is generated on site from the air and is injected into the clarified water in the ozone contact chambers, which provide about six minutes of effective detention time. 

By applying ozone after clarification, but before filtration, the ozone demand is minimized while any inorganic and organic oxidized material can be removed by filters. The ozone oxidizes any organic matter in the water, killing bacteria, viruses, algae, and microorganisms like giardia and crytosporidia.


Filtration is the final step in the physical removal of particulates and floc that have not been removed in the clarifiers. Filters, made up of 24 inches of anthracite coal on top of 12 inches of sand, strain the water and remove any remaining particles and floc. 

The filters are cleaned by backwashing with air and water, depending upon the turbidity of the raw water, filters usually operate for 72 hours or more between backwashes. Well over 1,000 gallons of sludge is removed each day from the entire treatment system and pumped to a drying lagoon.

Final Chemical Treatment

Final Chemical Treatment of the filtered water with a low dose of chlorine is done to maintain a residual of about 0.5 parts per million of free chlorine throughout the system. This assures the bacteriological quality of the water is maintained and prevents regrowth of any dormant bacteria in the distribution system piping. Caustic soda is added to raise the pH of the treated water from slightly acidic (less than 7.0) to slightly basic (more than 7.0). 

This optimizes the effectiveness of the orthophosphate corrosion inhibitor, which is also added to the water. The corrosion inhibitor reduces leaching of lead and copper from water pipes, particularly the individual service lines to residential buildings and plumbing within buildings. Fluoride is also added at this point.

Treated Water Pumps

The Treated Water Pumps lift water from elevation 335 feet in the filter control chamber of the treatment plant to a pressure level at the pumping station equivalent to an elevation of about 740 feet. The purified water is pumped out of the treatment plant into the distribution system. The 7.5 million gallon per day capacity treatment and pumping station sends the water through the recently relined pipe network on its way to remote pumping stations, storage towers and individual residences. 

The station includes four single-stage, 300 horsepower, centrifugal pumps with variable speed electric motors. The pumps are automatically controlled based on the water level in the system's primary storage tank.

Quality Control

Quality control for New Castle's water is assured by:

  • Computerized monitoring and feedback systems
  • Continuous water quality testing at all stages of treatment
  • Multiple backup systems: 
    • Croton Aqueduct connection
    • Emergency generator
    • Standby ozone generators
    • Etc.