Thursday, July 26, 2012

Blue Plaines From the Past to the Future

AECOM Picture

The District of Columbia's sewage system, one of the oldest in the United States, began its story around 1810, when the first sewers and culverts were constructed to drain storm and ground water from the streets of Washington D.C. In 1815 the canal system was built and included the Washington Canal that ran down what is now Constitution Avenue. The canal system provided a convenient way to transport goods, provide access to water and also dispose of waste. Residents drew their water from a series of city owned springs. In the first half of the 1800’s streets in Washington began piping in spring or well water for residents' use, and sewage was discharged into the nearest body of water- often the canals.

The Washington Aqueduct bringing river water for potable use citywide was built in the 1859 when population growth required a dedicated clean drinking water supply. Lieutenant Montgomery C. Meigs, is credited with planning and building the Washington Aqueduct. The surge in population during the Civil War, quickly created a human waste problem in the city and there were epidemics of smallpox, typhoid and malaria, which took many thousands of lives during the war years. From 1871 to 1874, the city’s Board of Public Works built an estimated 80 miles of sewers to remove the human waste from the city. Although the amount of construction was impressive, sewer engineering and hydrology were in their infancy and much of the work was poorly planned, structurally unsound and hydraulically inadequate, but the vitrified clay and brick sewers remain part of the sewer system to this day. In the early 1880s when the Washington City Canal was covered over because it had become nothing more than a stagnant open sewer, the problem of open sewage was transferred to the marshes along the Potomac and Anacostia Rivers.

Up to this time, the sewer canals and pipes that served DC were a combined system that merely carried and discharged (without treatment) both sanitary sewage and stormwater into local creeks and rivers. In the 1890s, it was decided that the existing combined system should be retained (due to the costs of rebuilding the sewers as separate systems), but that any extensions of the system would be built using the more expensive system of separate lines to carry stormwater and sewage flows. In order to protect the health of city residents, who were drinking the river water provided by the Washington Aqueduct, all the sewage discharges would be extended away from the city to a point far enough down the Potomac River to prevent being taken up in the Aqueduct for drinking water. The discharge point selected was Blue Plains, the southernmost tip of the District of Columbia. Pumping raw sewage into rivers was state of the art in the 1800’s. Washington DC did not start treating the sewage waste until 1937 when the Blue Plaines sewage treatment plant was built.

Today, sitting on the southernmost tip of Washington DC, across the river from Alexandria is the Blue Plains Advanced Wastewater Treatment Plant. While there are larger sewer treatment plants, that remove the solids and bacteria, the modern day Blue Plains also has Tertiary Treatment to remove nitrogen and phosphorus making Blue Plains the largest advance treatment plant in the United States at 150 acres and with a rated annual average day capacity if 370 million gallons per day (mgd) and a peak wet weather capacity of 1,076 mgd. The system needs such a large storm rated capacity to accommodate the old central city section which accounts for one third the area of the District and still has the old combined sewer system that overflows with predictable regularity during large storms. The system has released excess storm flows averaging 54 million gallons per year to the Anacostia River. In addition, Blue Plains is under a consent order from the Environmental Protection Agency, EPA, to meet new effluent limits for total nitrogen released and better control of the system during storms.

Being an advanced waste water treatment plant is not as modern as it sounds. At Blue Plains and other sewer treatment plants primary treatment screens wastewater, and performs some rudimentary treatment to remove crude solids of human waste and skim off grease, oil and fat. Wastewater sits in settling tanks, which are designed to hold the wastewater for several hours. During that time, most of the heavy solids fall to the bottom of the tank, where they become a thick slurry known as primary sludge. The material that floats is also skimmed from the surface of the tanks. Secondary (or biological) treatment involves feeding oxygen to bio-organisms that break down any organic matter still in the wastewater.

Tertiary treatment further treats the effluent water to remove nitrogen, phosphorus, fine suspended particles and microbes, and to kill or slowdown disease-causing organisms and viruses. It is the tertiary treatment that makes Blue Plains an Advanced Wastewater Treatment Plant. At this time, Blue Plains cannot remove enough nitrogen (on average over the year including storm periods) from the waste stream to meet the EPA mandated limit for nitrogen of 4.689 million pounds of nitrogen per year. In addition, when it rains and the Blue Plains AWWT plant tries to provide complete sewage treatment to flows of 600-740 million gallons a day the performance of the clarification units deteriorates because of the large flow of water, turbulence and not enough time to settle the solids and scum. The deterioration in performance cascades from the primary treatment to the secondary and onto the advanced treatment. This results in a reduced treatment for the sewage that lasts not only during the rain, but can last for several weeks. Blue Plans is currently engaged in a $7.8 billion 20 year improvement program called the Clean Rivers Project that will meet the reduced total nitrogen released requirements of their operating permits and increase the control of the system during rain storms in addition sludge treatment will be improved and sewer piping improved in many areas.

The sludge is separated from the wastewater during the primary treatment is further screened and allowed to gravity thicken in a tank. Then the sludge is mixed with the solids collected from the secondary and denitrification units. The combined solids are pumped to tanks where they are heated to destroy pathogens and further reduce the volume of solids. With treatment sludge is transformed (at least in name) to biosolids. Blue Plaines biosolids are Class B; however, after the completion of the improvement project in 2014 utilizing thermal hydrolysis (heating to over 160 degrees under high pressure) followed by anaerobic digestors, the biosolids produced by the plant will be Class A and the methane captured will provide 20% of the power for Blue Plains. To ensure that biosolids applied to the land as fertilizer do not threaten public health, the EPA created the 40 CFR Part 503 Rule in 1989 that is still in effect today. It categorizes biosolids as Class A or B, depending on the level of fecal coliform and salmonella bacteria in the material and restricts the use based on classification. The biosolids are tested for fecal coliform and salmonella and composite sampling is done for metals and hydrocarbons; the presence of other emerging contaminants in the biosolids is not tracked.

The Clean Rivers project will maintain the peak flow rate of 1,076 mgd from the collection system to Blue Plains. Peak flow rates to Complete Treatment would be reduced to 555 mgd for the first 4 hours, 511 mgd for the next 24 hours and 450 mgd thereafter. A tunnel would be constructed between Poplar Point and Blue Plains, and flows exceeding the complete treatment capacity would be diverted to the tunnel and that tunnel connected to the other tunnel storage. The storage provided by the new tunnel would be an additional 31 million gallons for a total storage in the system of 157 million gallons spread over the Anacostia River tunnels system and the new Blue Plains Tunnel. This facility will allow flow from the collection system that exceeds the complete treatment capacity of the plant to overflow to the tunnel. Flow captured in the tunnels would be dewatered through a new enhanced clarification facility, ECF, with a capacity of 225 mgd. Operating provisions would include arrangements to dewater the tunnels during and following rain and to convey ECF effluent to a direct outfall (after disinfection) and/or through the complete treatment facilities depending on the capacity available at the time.

The Clean Rivers project will also include the construction of Enhanced nitrogen removal, ENR, facilities. The new ENR facilities will have the capacity to provide complete treatment for flow rates up 555 million gallons per day for the first 4 hours, 511 million gallons per day for the next 24 hours and at a rate of 450 mgd thereafter to meet the new total nitrogen effluent limit mandated by the EPA. When completed, the Blue Plains Advanced Waste Water Treatment Plant will be able to meet the nitrogen release standard under the operating permit, reduce the number of uncontrolled storm related releases of waste.

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