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Tour of DC Water’s Blue Plains Advanced Wastewater Treatment Plant

The chapter’s December 3 tour of the Blue Plains Advanced Wastewater Treatment Plant was combined with the Association of Energy Engineers since IIE was well below its quota of participants. Blue Plains is the largest advanced wastewater treatment facility of its type in the world, categorized as “advanced” because of its nitrification, denitrification and filtration processes.

Left to right, Don Wulfinghoff, P.E., Bob Charlton, Richard Leshuk, P.E., tour guide Nick Passarelli, and Joe Scheibeler shown visiting the Blue Plains Central Control Room.

Although last on our tour, the main control room (photo above) is the facility’s nerve center, where all flows and processes are monitored for volume and chemical content, facilitating centralized decision-making. Three additional Area Control Centers can operate the plant in parallel, if necessary.

Our tour stopped to view all the processing steps. 1) Wastewater collected by the District of Columbia sewer system and from the Maryland and Virginia suburbs is delivered to the Blue Plains Advanced Wastewater Treatment Plant by gravity flow. 2) Nine huge 4,160-volt motors totaling 6,740 HP pump nine divided wastewater flows through primary screening to remove debris that mixed into sewerage such as leaves, cigarette butts, small pieces of garbage, plastic, etc. 3) 16 divided flow paths proceed through 16 aerated grit chambers (rectangular tanks) powered by 32 grit pumps where heavier solids such as sand sink and get removed by 16 wedge-like grit classifiers which move slowly from one end to the other of their respective tanks, plowing the solids on the bottom to be collected, de-hydrated and sent to land fills. 4) Flows further divide though 36 cylindrical primary sedimentation tanks to remove settleable organic particles – primary sludge – from the wastewater, while fats, oils, and greases are skimmed off the surface as scum. (When operated in enhanced primary removal mode, a mixture of metal salts and polymer is added to increase solids removal.) 5) Flows proceed to six separate aeration reactors and slowly pass through four rectangular tanks each containing microbes in mixed liquor for secondary treatment, a biological process in which air purification provides oxygen for micro-organisms that feed on the waste products and reproduce, so that waste sludge can be removed while the micro-organism content is maintained. 6) Ammonia, converted from nitrogen in organic material in wastewater during secondary treatment, is converted to nitrates in the nitrification process. Flows proceed to 12 separate 30-foot deep nitrification/denitrification reactors. Denitrification requiring the absence of dissolved oxygen and the addition of methanol to replace the source of carbon removed in secondary treatment, forces the microbes to consume oxygen in nitrates for respiration, converting the nitrates into harmless nitrogen gas released into the air. This process takes place in the same tank as nitrification, but the nitrification section is aerated and the denitrification section is un-aerated. The mixed liquor is sent to one of 28 sedimentation tanks to separate solids, sludge and scum from the waste water. 7) Wholly within the Blue Plains plant, the world’s largest wastewater filtration system, provides multimedia filtration. Before exiting into the Potomac River, the treated water is disinfected by sodium hypochlorite-based chlorination, filtered through anthracite and sand and then residual chlorine is completely removed with sodium bisulfite. The exiting water is claimed to be indistinguishable from tap water. One of the stops on our tour was at a building used to periodically test the effluent water for ph level and purity.

The solids treatment processes at Blue Plains are composed of thickening and dewatering processes for sludge. These processes include screen and degritting processes, gravity thickeners, dissolved air flotation thickeners, sludge blending centrifuge dewatering. All sludge is lime stabilized. Sludge waste is chemically converted to bio-solids used in fertilizer and land-fills. Eight anaerobic digesters are being built to begin operating in 2011 to substantially reduce pathogens, odor and the volume of biosolids.

The major plant operating cost is for utilities ($19 million/yrear) including electric power used primarily for enormous pumping equipment to maintain flows through the plant averaging 370 million gallons per day. Biosolids disposal is $17 million/year, chemicals $14 million/year, labor $8 million/year, and contracting costs $5 million/year. Automation has drastically reduced operating personnel requirements while increasing maintenance staff.

About one third of DC’s sewer system has combined sewer overflow (CSO), which means that sewerage and storm water use the same pipes. When storms cause the CSO flow to exceed Blue Plains’ 1.1 billion gallons per day capacity, the untreated overflow goes directly into the local river system, causing water pollution. The District of Columbia Water and Sewer Authority (DCWASA) has embarked on a $2.6 billion 15-year Clean Rivers Project to remedy the overflow problem which has already achieved a 40% overflow reduction and by 2018 will provide overflow storage tunnels to reduce the overflow by 1.5 billion gallons per year or 62% of the current 2.4 billion gallon annual overflow. DCWASA continues to achieve the goals of the Chesapeake Bay Agreement and is well-within its EPA permit limits for each of its effluent and biosolids parameters. DSWASA is proud of the progress made since taking over management of the Blue Plains facility in 1996 and implementing a $1.4 billion capital improvement program which protects the Potomac and Chesapeake Bay through its nitrogen and phosphorus removal process and biosolids end-use programs. Whereas DCWASA is investing in costly improvements at the limits of technology to further reduce nitrogen effluent from its current 5.3 mg/L average to meet the more stringent requirement of 4.0 by 2014, it is rumored that gross sources of Bay pollution from other sources in the region get lesser attention and are not under compulsion to reduce more damaging discharges.

A virtual tour of Blue Plains is available on line at http://www.dcwater.com/about/model_flash.cfm.

(Submitted by Joe Scheibeler, Chapter Secretary)

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