The Washington Aqueduct Searching for the Best Possible Drinking Water

The Washington Aqueduct is a federally owned and operated by the Army Corp of Engineers. The 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. Thomas Jocobus, a civilian employee of the Army Corp of Engineers is the General Manager and the Aqueduct produces an average of 155 million gallons of water per day which it sell to the District of Columbia (about 75%), Arlington County, Virginia (about 15%), and the City of Falls Church, Virginia (10%).

The Washington Aqueduct is a very conservative organization reflecting its structure, management, and response to events of the past. Water systems consist of three systems: The first is the water treatment systems that draw water from the source and treats it to meet the US EPA Safe Drinking Water Act standards (SDWA). The second is the water delivery system that moves the water from the finished water storage through the water mains and throughout the community so that there will be on demand water and adequate water for firefighting. The third is the plumbing systems in homes and building that deliver water to your sink, toilet or shower.

Somewhere in the past the Washington Aqueduct had begun to view their mission as providing finished water that met all EPA SDWA requirements. After all, (unlike most water utilities that are regulated by state regulators) EPA Region 3 was their direct regulator and they were attentive and responsive to the regulatory requirement and deeply concerned about the quality and cost of the finished water they sold.  All the costs of the Aqueduct operations are directly passed on to wholesale customers. In the 1990’s something happened to remind the Aqueduct that really they are in the public health (and fire safety) business and that their operations could not be viewed separately from the water delivery and plumbing systems that ultimately brought water to the households and businesses that bought water  from the distribution companies. What happened was chloramine.

from DC Water

In 1994 amendments to the Clean Water Act SDWA resulted in changing from chlorine to chloramine for disinfection. The EPA issued regulations concerning disinfectionby-products formed when chlorine (used for a hundred years) reacts with organic matter in drinking water; the EPA considered these byproducts to be a potential health threat. Chloramines do not produce disinfection byproducts. The treatment process for the Washington Aqueduct was changed to add ammonia after primary disinfection to react with the remaining chlorine to prevent the formation of disinfection byproducts (haloacetic acids and trihalomethanes). When planning the chloramine project as advised by  EPA guidance manual the Aqueduct considered the possibility that the disinfectant change would increase the level of nitrification in the distribution system. Such an increase could cause a lowering of pH in the distribution system, increasing the possibility of corrosion. The Aqueduct put a plan in place to minimize the potential for nitrification and monitored for nitrification for six months following conversion. Unfortunately, corrosion was not caused by nitrification.  Shortly after the change, increasing pipe failures and levels of lead began appearing in the homes of Washington DC residents. It turns out chloramine-treated water picks up lead from pipes and solder and does not release it, resulting in elevated levels and deterioration of the pipes. Extreme lead concentration began appearing in Washington DC homes and water delivery pipe and plumbing systems began to fail at an accelerated rate.

  

Dr. Marc Edwards a MacArthur Prize winning professor of engineering at Virginia Tech ultimately  identified the cause and solution of the increasing incidence of leaks in copper water pipes. The lead problem was addressed by the Washington Aqueduct adding additional treatment steps to add orthophosphate and tightly control the pH of the water.  Orthophosphate controls 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, but only works in a narrow pH range. Calcium hydroxide (lime) is also added to adjust the pH of the water to ensure optimal performance of the orthophosphate. In addition, DC WASA spent $97 million to replace a portion of 15,000 pipes and 2,000 full pipe replacements.

The change in the water treatment process prevents the finished water from dissolving lead in the water mains, solder joints, and fixtures. The changes in the operation of the Washington Aqueduct were more profound. The Aqueduct had to rethink the management of their relationship with their customers, how had they made a change in the water treatment without testing the impact on all the other water systems? That is unlikely to ever happen again.  The Washington Aqueduct's customers (District of Columbia Water and Sewer Authority, Arlington County, and the City of Falls Church) entered into a memorandum of understanding with the U.S. Army Corps of Engineers that restructured the business relationship and created the Wholesale Customer Board and Technical Committee to oversee and approve Washington Aqueduct's operating and capital budgets and long term strategy.

The Washington Aqueduct’s official mission became to provide the “best possible” drinking water while: Minimizing any negative effects on the environment, being fiscally responsible to consumer who bear all the operating and capital costs through water rates, and anticipating and avoiding unintended negative consequences. The Aqueduct has been working with their Wholesale Customer Board to navigate the challenges that face drinking water utilities in our modern world; microbial water quality issues, trace chemicals and emerging contaminants in the source water, and water quality issues resulting from treatment and distribution.

Chemicals are everywhere in our modern world. The technology used for chemical analysis has advanced to the point that it is possible to detect and quantify nearly any compound known to human kind down to less than a nanogram per liter or parts per trillion (1/1,000,000,000,000). This enhanced analytical ability has allowed scientists to discover that trace levels of pharmaceuticals, potential endocrine disrupting compounds (EDC) and other emerging contaminants exist in surface water, have appeared in some groundwater and may persist in the water through conventional and some advanced treatment trains to appear in our finished drinking water. The Washington Aqueduct and its partners on the Wholesale Customer Board and technical working group expert panel from consulting, academia, EPA and other Water Utilities are working to develop a framework to make decisions and chart courses of action when faced with these challenges. The first and continuing challenge is to keep evolving the understanding of what is the “best possible” drinking water and remember to consider all consequences before taking any action. The Washington Aqueduct of today is unlikely to be the first water treatment operation to make any change, but is unlikely to be surprised by unanticipated consequences of any changes in treatment that are made in the future. 

The Dalecarlia Reservoir and Water Treatment Plant

from Army Corps of Engineers via Wikimedia

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.