Monday, December 10, 2012

The Dalecarlia Reservoir and Water Treatment Plant

From Army Corp of Engineers
I went up to the Dalecarlia Reservoir and Water Treatment Plant on MacArthur Boulevard for a tour and to speak with Thomas Jocobus, the General Manager. The Dalecarlia operations are the main location for the Washington Aqueduct. The Washington Aqueduct consists of the Dalecarlia Reservoir and Water Treatment Plant, the Georgetown Reservoir, and the McMillan Reservoir and Water Treatment Plant. The Washington Aqueduct draws water from the Potomac River and treats it to provide finished drinking water to the water distribution companies that buy water from them.

The Washington Aqueduct is a federally owned and operated by the Army Corp of Engineers and Mr. Jacobus like all employees of the Washington Aqueduct is a civilian employee of the Army Corp of Engineers. The Aqueduct was initially built with federal funds, but since 1927 the operating budget and capital budget have been paid for by the Aqueduct’s customers. Today, the operating budget is around $46 million that is supplied by the wholesale water rates charged for the water delivered. The Aqueduct produces an average of 155 million gallons of water per day and sells that water to the District of Columbia (about 75% of the finished water), Arlington County, Virginia (about 15%), and the City of Falls Church, Virginia (10%). In total about one million people a day use water supplied by the Aqueduct.

The maximum capacity of the Aqueduct is 320 million gallons of water per day much more than even the peak demand for drinking water and fire fighting for their customers. Though water use peaked at an average of 180 million of gallons a day about a decade ago, the system was expanded in the 1950’s anticipating serving Montgomery and Prince George counties, but the Washington Suburban Sanitary Commission (WSSC) instead built what is today the WSSC's principal water supply facility, the Potomac River Filtration Plant in western Montgomery County to supply their needs.

The Washington Aqueduct dates back to 1853 when congress appropriated $5,000 to develop the first portion of the system. The first portions of the system were the Dalecarlia Reservoir and Georgetown distribution reservoir. That portion of the system was designed to run on gravity, so that the system did not require pumps until much later when the system and the city expanded and demand for water required the expansion of the system. Even today the energy used is reduced because of the utilization of natural elevations in the design of the system. The Aqueduct first began delivering water in 1862. The Lydecker Tunnel and McMillian Reservoir and water treatment plant were added in 1905. The McMillian slow sand water treatment plant was the first treatment plant in the system and was built to address the increasing outbreaks of typhoid fever that were caused by contaminated drinking water. This was followed by a rapid sand filtration system at Dalecarlia to address the continued population growth after World War I.

Today the water for the Washington Aqueduct continues to be drawn from the Potomac River at the Great Falls and Little Falls intakes. This duel intake location about 10 miles apart allows for some degree of management of the water quality at intake if there should be a fuel spill or other water quality disturbance. On its way from the river intakes to the Dalecarlia reservoir, raw water passes through a series of screens designed to remove debris such as twigs and leaves and whatever trash finds its way into the Potomac River. Then copper sulfate and sodium permanganate are added as algaecides. All water drawn for the system enters the Dalecarlia Reservoir. While the water moves slowly through Dalecarlia Reservoir, much of the sand and silt settles to the bottom. This is called pre-sedimentation. After screening, the addition of the algaecides and pre-sedimentation the water is either pumped to the Dalecarlia or McMillan treatment plants.

The treatment plants filter and disinfect water from the Potomac River to meet safe drinking water standards. The treatment process is not identical at both plant, but it is very similar and includes sedimentation, filtration, fluoridation, pH adjustment, primary disinfection using sodium hypochlorite, secondary disinfection with chloramine through the addition of ammonia, and corrosion control with orthophosphate.
From the Army Corp of Engineers
The raw water from the Potomac River contains suspended solids, sediment, bacteria, and microorganisms that must be removed to produce finished drinking water. These are removed by the water treatment processes of the Washington Aqueduct after the initial screening and pre-sedimentation water treatment consists of:

Coagulation - A coagulant, aluminum sulfate (alum) and powdered activated carbon, is added to the water as it flows to sedimentation basins. Coagulants aid in the removal of suspended particles by causing them to consolidate and settle. Alum contains positively charged atoms called ions which attract the negatively charged particles suspended in water causing them to gather into clumps of particles heavy enough to settle. The activated carbon controls odor in the water.
Flocculation – The water is gently stirred with large paddles to distribute the coagulant; this causes particles to combine and grow large and heavy enough to settle. This process takes approximately 25 minutes. Cationic polymer and nonionic polymer are added.
Sedimentation – The water flows into quiet sedimentation basins where the flocculated particles settle to the bottom. After about four hours, approximately 85% of the suspended material settles out. Until recently, the sediment recovered was returned to the river, now the sediment residuals are collected from Dalecarlia, McMillan and Georgetown locations and then pumped to a central processing facility at Dalecarlia.  Residuals processing, including gravity thickening and dewatering, occur at the newly constructed Residual Management building. Following processing, trucks haul the residuals off-site to permitted land-disposal areas.
Filtration – Water at the top of the basins flows to large gravity filters, where the water flows down through filter media consisting of layers of small pieces of hard coal (anthracite), sand, and gravel placed in the bottom of deep, concrete-walled boxes. Filtered water passes through to a collecting system underneath. The filters are back washed every four days.
Disinfection – Chlorine in the form of sodium hypochlorite is added with precision equipment to kill pathogenic microscopic life such as bacteria or viruses. Ammonia is then added. The chlorine and ammonia combine to form chloramine compounds. This is the most recent significant change in the water treatment process and was required by changes in the Safe Drinking Water Act in the 1990's. The concentration of chloramines in the water is closely monitored from the time it is added at the treatment plants to points near the furthest reaches of the distribution systems.

The Dalecarlia operation has an EPA certified laboratory to perform the tens of thousands of analysis required each year under the Safe Drinking Water Act. One of the coolest features of the tour was seeing the surveillance of the water treatment train. In one of the laboratories is a trough sink with 15 spigots continually running. Each spigot continually draws water from one step in the treatment process. If a problem arises water at every point can be checked to make adjustments.

Fluoride, in the form of hydrofluorosilicic acid, is added to the finished water to reduce tooth decay; this is especially beneficial for children. Orthophosphate is added to control corrosion in pipes, service lines, and household plumbing throughout the distribution system. It works by building up a thin film of insoluble material in lead, copper, and iron pipes and fixtures. This thin film acts a barrier to prevent leaching of metals into the water. Calcium hydroxide (lime) is also added to adjust the pH of the water to ensure optimal performance of the orthophosphate.

After the water has completed its path through the treatment process, it is referred to as finished water and meets all requirements under the Safe Drinking Water Act. Because the Washington Aqueduct is in Washington D.C. it is directly regulated by the US EPA Region 3 office. The Washington DC regulators do not have primacy. Unlike most large urban water systems the Aqueduct does not have state regulators to answer to and its customers are the three water distribution systems DC Water, Arlington and Falls Church section of Fairfax County. Falls Church has come to an agreement to turn over their distribution system to Fairfax Water who will continue to buy water for that portion of the system from the Aqueduct. The Washington Aqueduct does not currently engage in any advanced water treatment, but is studying the options.  


  1. Hi Elizabeth. Could you tell us a bit more about what happens to the residuals from the filtration portion of the treatment chain? How much do the plants produce and how is the land disposal regulated? What happens to the residuals after they are applied? What crops (if any) are permitted to be grown on the sites? What controls prevent the sites from returning the residuals to the watersheds?
    Thanks. Hope you are doing well!

  2. Fluoride accumulates in the bone and in other calcified tissues over a lifetime. It is still not known what the true half-life of fluoride is in the human bone, but an estimate of 20 years has been made (NRC, 2006, p 92). This means that some of the fluoride absorbed by infants will be retained for a lifetime in their bones. Early symptoms of fluoride poisoning of the bones are identical to arthritis. Lifelong accumulation of fluoride in bones can also make them brittle and more prone to fracture.

    f) Fluoride is known to have toxic properties at low doses (Barbier et al, 2010).

    g) Children in fluoridated countries are being over-exposed to fluoride as demonstrated by the very high prevalence of dental fluorosis. According to the CDC (2010) 41% of American children aged 12-15 have some form of dental fluorosis. Black and Mexican American children have significantly higher rates (CDC, 2005, Table 23).

    h) A 500-page review by the National Research Council in 2006 revealed that several subsets of the population (including bottle-fed babies) are exceeding the EPA's safe reference dose (0.06 mg / kilogram bodyweight/day) when drinking fluoridated water at 1 ppm (NRC, 2006, p85). The NRC panel also indicated that fluoride causes many health problems at levels close to the exposure levels in fluoridated communities (NRC, 2006).

    i) An un-refuted study conducted at Harvard University shows that fluoride may cause osteosarcoma (a frequently fatal bone cancer) in young men when boys are exposed to fluoridated water in their 6th, 7th and 8th years (Bassin et al., 2006). Despite promises by Bassin’s thesis advisor (Chester Douglass) a subsequent study by Kim et al. (2011) did not refute Bassin’s key finding of the age-window of vulnerability.

    j) There are many animal and human studies, which indicate that fluoride is a neurotoxin and 37 studies that show an association between fairly modest exposure to fluoride and lowered IQ in children. Twenty-seven of these studies were reviewed by a team from Harvard University (Choi et al., 2012). In an article in Lancet Neurology, Grandjean and Landrigan (2014) have since classified fluoride as a developmental neurotoxicant. All these papers can be accessed at

    m) Dental caries is a disease, according to the ADA, CDC's OHD, and the American Association of Pediatric Dentistry, and others. Fluoridation is designed to treat a disease but has never been approved by the FDA. The FDA has never performed any trial to ascertain the safety of fluoride. FDA classifies fluoride as an “unapproved drug.”

    n) The effectiveness of swallowing fluoride to reduce tooth decay has never been demonstrated by a randomized control trial (RCT) the gold standard of epidemiology (McDonagh et al., 2000).

    o) The evidence that fluoridation or swallowing fluoride reduces tooth decay is very weak (Brunelle and Carlos, 1990 and Warren et al., 2009).

    p) The vast majority of countries neither fluoridate their water nor their salt. But, according to WHO figures, tooth decay in 12-year olds is coming down as fast –if not faster – in non-fluoridated countries as fluoridated ones ( ).

    q) Most dental authorities now agree that the predominant benefit of fluoride is TOPICAL not SYSTEMIC (CDC, 1999, 2001)– i.e. it works on the outside of the tooth not from inside the body, thus there is no need to swallow fluoride to achieve its claimed benefit and no justification for forcing it on people who do not want it.

    r) Many countries (e.g. Scotland) have been able to reduce tooth decay in low-income families using cost-effective programs without forcing fluoride on people via the water supply (BBC Scotland, 2013).

    Paul Connett, PhD
    Director of the Fluoride Action Network
    Co-author, The Case Against Fluoride (Chelsea Green, 2010)