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Drinking Water Treatment/Delivery System Overview

Water System Overview

The City of Modesto Water Operations Division supplies drinking water to residents in Modesto, Empire, Salida, Waterford, Hickman, Grayson, Del Rio, parts of Ceres and Turlock and county areas adjacent to the City system. About 250,000 residents receive their water from the city system and are billed according to a rate structure.

For many years, Modesto's water customers received all of their water from 140 wells. These wells pump water (groundwater) trapped in rock layers beneath the ground's surface. To continue to deliver clean, dependable drinking water to the customers, the City partnered with the Modesto Irrigation District (MID) in the early 90s to construct a surface water treatment plant. A third partner, the Del Este Water Company, was purchased by the City in 1995. The 30-acre plant, located at Modesto Reservoir, treats surface water from the Tuolumne River. This treated water is distributed to Modesto water customers at an average rate of 30 million gallons per day (mgd). Average daily water consumption has exceeded 74 million gallons, peak demand days have been as high as 133 million gallons, and peak hourly demands have reached 187 million gallons.

Water Treatment Process

  • Water from Modesto Reservoir flows by gravity into the water treatment plant where ozone is added for primary disinfection.
  • Liquid alum and polymers are mixed rapidly into the disinfected water to attract suspended particles in the water and cause them to mass together into a substance known as floc.
  • Sedimentation basins allow floc to settle to the bottom before the water goes to the filters.
  • The water moves through an anthracite coal and gravel filter where floc is removed and a final cleansing takes place.
  • Chlorine is added as a secondary disinfectant.
  • Lime and carbon dioxide are added to make the water less corrosive and more compatible with the existing groundwater supply.
  • Treated water is then moved into water storage reservoirs, pumped into the distribution system and delivered to area residents.

Water quality regulations require that all surface water be treated to meet strict drinking water standards for pollutants and pathogens. The plant's state-of-the-art treatment process ensures that the water supply meets or exceeds all state and federal drinking water standards now and in the future.