The WA water treatment process is considered “Conventional” treatment. “Conventional” water treatment refers to systems providing coagulation/flocculation/ sedimentation followed by filtration, and finally disinfection. Many conventional water treatment plants incorporate additional steps (such as pre-sedimentation.) The basic steps in conventional treatment can be accomplished using a variety of methods.
Following is a summary of the Washington Aqueduct water treatment process. Chemicals are indicated in italics and the conventional treatment processes are indicated in bold.
Source Water – The Washington Aqueduct draws raw water from the Potomac River from two locations, a gravity intake at Great Falls, Maryland and at Little Falls pumping station.
Receiving and Pre-Sedimentation – Raw water enters the Dalecarlia reservoir. Here pre-sedimentation occurs. The reservoir also provides for storage to permit dampening of flow variations in and out as water demands vary daily.
Sodium permanganate is added during summer months at the downstream end of the Dalecarlia Reservoir to control algae and thus reduce taste and odor issues.
Powder Activated Carbon can also added, as needed, after the Dalecarlia Reservoir to respond to and treat taste and odor issues. The carbon absorbs tastes and odor and is then removed in the following treatment processes.
Aluminum sulfate (alum) is added to water entering flocculation/sedimentation basins. Alum, the coagulant, initiates the coagulation process. It works by charging the particles in the water causing them to be attracted to one another and then combine to create larger, heavier particles for improved settling.
A flocculation aid polymer is also added at this point to aid in the process.
Chlorine can also be added at this point as needed for oxidation and enhanced particle removal. Chlorine is only added here as needed because most organic materials have not yet been removed and the reaction between chlorine and organic materials produces disinfection by-products.
Flocculation is the process of gently “stirring” the water, causing suspended particles to aggregate into clumps or masses (coagulation) that then settle in the sedimentation process.
Sedimentation takes place in large basins. Coagulated particles settle by gravity to the bottom of the basin over time. The residuals that accumulate in the bottom of the sedimentation basins must be removed and disposed of. This removal can be done continuously or in batches. Currently the Washington Aqueduct removes the residuals in batches (approximately 4 times per year from the sedimentation basins at Dalecarlia and twice a year from the Georgetown reservoir, the sedimentation basins for the McMillan plant) and dispose of them by returning them to the Potomac River under an NPDES permit. WA is in the process of constructing facilities that will allow continuous removal of the residuals and transfer of the residuals to a processing facility where they will be dewatered and sent by truck for off-site land disposal or reuse.
Chlorine can, again, be added after the coagulation/flocculation/sedimentation process as needed.
In the Filtration process settled water is moved by gravity through a filter media consisting of anthracite coal and sand. Suspended solids are removed by straining (physically capturing particles larger than the pores through which the water is flowing) and by adsorption (particles “sticking” to the surface of the filter media by the same forces as those at work in the coagulation process.) A filter aid polymer is added prior to the filters to aid in the adsorption of particles in the filters.
After filtration the water looks clean and clear but harmful microscopic organisms, not removed in the previous processes, remain. Chlorine is added to disinfect the water.
Disinfection takes place in clear water reservoirs, also known as “clearwells". The “clearwells” are large, underground, concrete basins that provide holding time for the chlorine to inactivate organisms. The amount of disinfection provided is a function of the concentration of the disinfectant (i.e. milligrams of chlorine per liter of water) and time the disinfectant is in contact with the water before use.
Prior to finished water leaving the treatment plant to the distribution system other chemicals are added.
Ammonia is added to convert the free chlorine to chloramine. Chloramine is a weaker disinfectant than chlorine but by combining the free chlorine with ammonia to produce chloramine a less reactive compound is created. This results in nearly eliminating the further production of undesirable disinfection by-products.
Lime is added to adjust the water to the proper pH for optimum corrosion control and distribution. The pH of filtered must be raised to meet the current WA pH target of 7.7.
Phosphoric acid is added to inhibit corrosion.
Fluoride is added, as required by EPA, to reduce tooth decay.
The Washington Aqueduct produces safe, high quality potable water for approximately one million people who live and work in the District of Columbia and Northern Virginia. Water is not only needed for domestic use but also for fire fighting, industrial needs and many other uses. The Washington Aqueduct operated, without fail, 24 hours per day, 7 days per week.
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