On March 7th Fairfax Water and the Arlington Department of Environmental Services began flushing their water distribution systems. Each spring Fairfax Water and Arlington DES flushes its water mains by opening fire hydrants and allowing them to flow freely for a short period of time. In addition, the Washington Aqueduct and Fairfax Water temporary change how the water is disinfected.
For most of the year, chloramines, also known as combined chlorine, is added to the water as the primary disinfectant. During the spring the water treatment plants switch back to chlorine in an uncombined state, commonly referred to as free chlorine. This free chlorine reacts with sediments suspended during flushing and kills bacteria that may be in the bio-film that forms on the pipe walls. Many water chemistry experts believe this short exposure to a different type of disinfectant maintains a low microbial growth in the bio-film and improves the quality and safety of the water.
This change in disinfection is an annual program to clean the water distribution pipes and maintain high water quality throughout the year. The U.S. Army Corps of Engineers Washington Aqueduct provides water to the District of Columbia, Arlington County, and other areas in Virginia. Fairfax Water provides water to Fairfax county and parts of both Loudoun and Prince William County. Both Fairfax Water and the Aqueduct switch from chloramine to chlorine during this period that runs from March 7th to May 2nd at the Aqueduct and into June for Fairfax Water.
You may notice a slight chlorine taste and smell in your drinking water during this time, this is not harmful and the water remains safe to drink. If you are a coffee and tea lover like me, use filtered water or leave an open container of water in the refrigerator for a couple of hours to allow the smell to dissipate. Water customers who normally take special precautions to remove chloramine from tap water, such as dialysis centers, medical facilities and aquarium owners, should continue to take the same precautions during the temporary switch to chlorine. Most methods for removing chloramine from tap water are effective in removing chlorine. The annual chlorination is important step to remove residue from the water distribution system.
Flushing the water system entails sending a rapid flow of chlorinated water through the water mains. As part of the flushing program, fire hydrants are checked and operated in a coordinated pattern to help ensure their operation and adequate flushing of the system. The flushing removes sediments made up of minerals which have accumulated over time in the pipes as well as bacteria on the bio-film. An annual flushing program helps to keep fresh and clear water throughout the distribution system. Removing the residue ensures that when the water arrives in your home, it is the same high quality as when it left the water treatment plant.
Drinking water in Fairfax comes from either the Potomac River or Occoquan Reservoir. The Washington Aqueduct draws its raw water from the Potomac. This water needs to be treated to remove impurities and disinfected to kill disease causing germs such as Salmonella, Campylobacter, and norovirus. Giant screens on the water intake pipes prevent trash, debris and fish, but the water is only screened and not yet drinkable. In the water treatment plant potassium permanganate (KMnO4) is added to the water to control taste and odors, remove color, prevent biological growth within the water treatment plant, and remove iron and manganese which are naturally occurring predominantly nuisance contaminants.
Then water is pumped into a series of water chambers where the pH is adjusted by adding either caustic soda or sulfuric acid and a coagulant to remove small particles of dirt suspended in the water. The water moves through a series of mixing chambers with progressively slower mixing to allow the particles to coagulate into larger and larger particles until dirt floc is formed. Then the water is held in sedimentation basins and the floc is allowed to settle to the bottom of basins by gravity where they are removed.
The next step in the water treatment process in Fairfax is the infusing of the water with ozone gas and the first of two disinfection steps. This step is not used in every water treatment plant. Ozone is highly effective in eliminating the Cryptosporidium bacteria and other naturally occurring microorganisms present in water. Unlike ultraviolet and chlorine disinfection systems (which are still used in many locations), there is no re-growth of microbes after ozonation. Ozonation also reduces the formation of trihalomethanes (chlorine breakdown products) because of the reduction of organic materials in the water before chlorination.
The final steps in the water treatment process is the second disinfection, fluoridation and the addition of a ammonium hydroxide to adjust the pH slightly to prevent corrosion of piping and fixtures of the plumbing systems in customer homes to prevent the leaching of lead from older plumbing systems and old connector lines into water. For most of the year Fairfax Water and the Washington Aqueduct use chloramine as the final disinfection step in water treatment. However, during the spring of every year they use chlorine to disinfect and flush the delivery network. Free chlorine is better suited to remove residue that may have collected in the pipes and a coordinated opening of fire hydrants serves to flush the system.