How is Water Treated?

How is Water Treated?

Baltimore operates three treatment plants that safely produce up to 405 million gallons per day of drinking water. All three treatment plants treat the raw water using the same basic steps: pre-chlorination, coagulation, flocculation, sedimentation, filtration, fluoridation, post-chlorination, and corrosion control treatment. These steps are needed in order to produce water that will meet federal and state drinking water standards, that is free of pathogens, and will be suitable for public consumption.

Prechlorination

Chlorine is added to the raw as it enters each plant. Chlorine kills microorganisms such as bacteria, protozoa, and viruses that may cause illness in humans. Chlorine also prevents the growth of algae at the treatment plant that may interfere with treatment of water and cause taste and odor problems. Enough chlorine is added to maintain a residual of 1 part per million or residual chlorine in water after filtration. A chlorine residual is needed to prevent re-growth of bacteria in the piping network otherwise known as the distribution system.

Coagulation/Flocculation

Alum (aluminum sulfate) is added to the water to destabilize natural fine particulate matter suspended in water. This process in known as coagulation. These particles enter water through land erosion, airborne contaminants, decay of natural organic matter such as plants and animal wastes, and discharges resulting from human activities. The impurities found in water suspensions consist of charged colloids (colloid - a very small solid suspended in water which varies in size, density and electric charge) ranging in size from 5 nm to 1 um and particulates greater than 0.5 mm. Most of these particles will not settle by gravity due to their charge and because they are lighter than water (their density is less than water). The addition of alum causes these particles and colloids to clump together to form heavier particles which will settle in water.

Alum is added quickly to the water at the plants using mechanical mixers. This process is known as Rapid Mix. After passing through the Rapid Mix process, the water travels through flocculation basins where it is gently stirred by slow moving paddles. Gentle mixing brings small particles into contact so that they will collide, stick together, and grow to a size that will readily settle. These large particles are referred to as floc. Formation of proper floc is necessary for particles to be removed during sedimentation and filtration.