Wastewater

DC Water operates the Blue Plains Advanced Wastewater Treatment Plant, the world's largest advanced wastewater treatment facility. At Blue Plains, DC Water provides wastewater treatment services to over 1.7 million people in our service area, including residents of the District of Columbia and significant portions of Montgomery and Prince George's Counties in Maryland and Fairfax and Loudoun Counties in Virginia. Wastewater treatment includes liquid process facilities that provide treatment for both sanitary wastewater flows and peak storm flows originating in the sanitary and combined sewer systems respectively, along with solids processing facilities that treat the residual solids removed by the liquid process facilities. Blue Plains is rated for an average flow of 370 million gallons per day (MGD), and is required by its National Pollutant Discharge Elimination System (NPDES) permit to treat a peak flow rate of 740 MGD through the complete treatment process for up to four hours, and continuous peak complete treatment flows of 511 MGD thereafter. The plant treats these flows to a level that meets one of the most stringent NPDES discharge permits in the United States. Additionally, up to 336 MGD storm water flow must receive partial treatment, resulting in a total plant capacity of 1,076 MGD.

Under DC Water's Biosolids Management Plan ("BMP", originally adopted by the Board in 1999), we evaluated a number of options for long-term biosolids processing and disposal, and identified full biosolids digestion as a common element of all long-term approaches. However, based on market conditions in 2006, the DC Water Board of Directors decided to defer implementation of the project, and to continue evaluation of alternatives. Since then, staff has evaluated a wide range of biosolids processing options. Four processing options were shortlisted, each of which involves anaerobic digestion, consistent with the 1999 BMP. This budget presents the recommended option for long term biosolids management. This option, which is described in more detail later in this document, includes construction of four Cambi thermal hydrolysis trains, four digesters, new dewatering equipment and a combined heat and power plant.

Overview of the Wastewater Treatment Process

Plant workers
Plant workers at Blue Plains.

The first wastewater treatment phase begins as debris and grit are removed by screens and grit chambers and trucked to a landfill. The sewage then flows into primary sedimentation tanks that separate more than half of the suspended solids from the liquid. The liquid flows to the secondary treatment process where oxygen is provided to allow bacteria to break down the organic matter. In the next stages of treatment, bacteria convert ammonia into other forms of nitrogen and then into harmless nitrogen gas. Residual solids are settled out in each biological process. The water is percolated down through dual-media effluent filters, removing most of the remaining suspended solids. The water is disinfected and then treated to remove residual chlorine and discharged into the Potomac River. The solids from primary sedimentation tanks go to gravity thickening process units where the dense sludge settles to the bottom and thickens. Biological solids from the secondary and nitrification processes are thickened separately using flotation thickeners. All thickened sludge is dewatered, lime is added to reduce pathogens, and the organic biosolids are beneficially reused through application to agricultural land in Maryland and Virginia. DC Water has performed an extensive analysis of alternatives to identify a cost-effective, long-term and sustainable biosolids management project for the Blue Plains Advanced Wastewater Treatment Plant that can produce a diverse Class A biosolids product, significantly reducing lime use and enhance land application.

Five liquid treatment processes (preliminary, primary, secondary, nitrification-denitrification, and filtration) comprise the liquid treatment processes at Blue Plains. The first phases of upgrades to four (of the five) liquid treatment processes are now in service. In tandem with the placing of these facilities in service, the process control system has also been implemented to enable monitoring and control of the upgraded equipment and systems, thus allowing DC Water to achieve greater process control and treatment efficiency and also yielding operating cost control. The current emphasis of the construction program for the liquid treatment processes is the upgrade of the nitrification-denitrification process and an upgrade to Raw Wastewater Pump Station 1. Construction is scheduled to begin in FY 2009 on Area Substation No. 6, Biological Sludge Thickening, and Rehabilitation of the Laboratory at Blue Plains. The design for additional nitrogen removal is scheduled to begin in FY 2009 and planning for improved treatment of excess flow and biosolids management will continue.

Long-term upgrade projects now under construction include:

  • Nitrification-Denitrification Facilities Upgrade — Upgrade to the nitrification-denitrification facilities to improve the process and to replace equipment that is at the end of its useful life.
  • Raw Wastewater Pump Station 1 — Upgrade to the Raw Wastewater Pump Station to replace equipment that is at the end of its useful life and improve reliability
  • Process Control Computer System — Will provide automated monitoring and control for the nitrification-denitrification process that will improve treatment, control and optimize chemical and power costs, and increase reliability of the facilities.

For more information on the Blue Plains Advanced Wastewater Treatment Facility and our treatment processes, take a tour in our "Who We Are" section.

top

Liquid Processing Program

DC Water employee testing nitrification tanks
DC Water employee testing nitrification tanks

Projects in this program area encompass upgrading and rehabilitating facilities involved in handling flows from the sanitary and combined sewer systems. These flows progress sequentially through the plant processes to ultimate discharge of the treated effluent into the Potomac River. Liquid treatment systems include headworks facilities that screen and pump the wastewater flows, grit facilities that remove sand and grit particles, primary treatment facilities that remove solids by sedimentation, secondary treatment facilities that remove organic pollutants using a biological process, nitrification/denitrification facilities that remove nitrogen using a biological process, and effluent filtration, disinfection, and dechlorination facilities.

Specific major projects under this program that are substantially complete include:

  • Grit Chamber Facilities Upgrade — This project is for the construction of an automated, continuous grit removal system consisting of sixteen chambers in all. Impact on operations include the elimination of current manual cleaning of each grit tank and lowered maintenance costs of tanks and pumps due to reduced grit load into downstream processes. While all of the grit collection bridges and grit conveyance systems are in operation, a new heating system for Grit Chamber Building No.1, that meets the current low emissions regulations, will be constructed.
  • Influent Screen Facility — This project has provided for the installation of fine screens as a preliminary treatment step in the wastewater process. The fine screening removes rags and other debris from the wastewater and thereby improves treatment processes and provides protection for equipment.
  • Primary Treatment Facility — This project entailed replacing the clarifier mechanism in the primary sedimentation tanks. As a result, the primary treatment process has been removing additional suspended solids from the wastewater.
  • Secondary Treatment Facility — This project entailed replacing sludge and scum collection equipment and rebuilding deteriorated portions of the concrete sedimentation basins in the East and West Secondary Treatment Process.

Specific major projects under this program that are now underway include:

  • Raw Wastewater Pumping Station 1 Upgrade — This project will rehabilitate pumping equipment and appurtenances in one of the two stations that pump incoming wastewater into the plant. Construction began in 2007 and is scheduled to be completed in 2009.
  • Biological Nutrient Removal — Project TK is combined with Project TQ in a single construction contract to demonstrate and implement Biological Nitrogen Removal capability in order to meet the goals of the Chesapeake Bay Agreement; that is, to meet a total nitrogen discharge goal of 7.5 mg/l. Construction began in 2007 and will continue through 2011. This upgrade will provide for better flow distribution to the reactors and better process control within the reactors, and methanol feed control, and rehabilitation and upgrade of nitrification sedimentation basins. While this project alone will not enable DC Water to meet its new total nitrogen limit of 4.2 mg/l, it will continue to remove a significant portion of nitrogen from the wastewater, provide better process control and optimize methanol feed.
  • Nitrification/Denitrification Facilities Upgrade — This project includes major electrical rehabilitation of the entire facility, major HVAC and plumbing upgrade for all building and galleries, and architectural rehabilitation for the Nitrification Blower Building, Control Buildings, and Electrical Buildings. Benefits of this project include lower maintenance and energy costs due to improved efficiency. Design is currently underway.
  • Filtration and Disinfection Facilities Upgrade — Replacement of existing filter media and the addition of an air/water backwash system and improvements to pump operation will result in reduced power usage and treatment costs due to reduced backwash water usage. A portion of the work was designed and bid ahead of the rest of the project to expedite the full rehabilitation of the facility, which had experienced filter failures. The first contract, completed in 2007, restored all the filters to operability with new filter underdrains and media. A second contract, currently under construction, will provide a new air-water wash system and improve backwashing controls and instrumentation.

Other Liquid Processing Program projects but not scheduled to start until later, include:

  • Dual Purpose Sedimentation Basin Rehabilitation — Replacement of sludge collection equipment, sludge and scum pumps, and support process equipment with design starting in 2011.
  • Filtration/Disinfection Facility Phase II — Design is scheduled to begin in 2009 on an upgrade to major electrical equipment serving the Fiiltration/Disinfection Facility.
  • Primary Treatment Facilities Phase II — Design is scheduled to begin in 2015 for structural repairs to the primary sedimentation tanks.
  • Grit Chamber Facilities Phase II — Design is scheduled to begin in 2015 for upgrades to the grit chamber building structures and facilities. These upgrades include structural, architectural and building system renovation of office and storage spaces in each building.

top

Plantwide Facilities Program

Workers at Blue Plains adjust machinery
DC Water employees adjust plant machinery

This program provides for upgrading, rehabilitating, or installing support systems and facilities that are required for both the liquid processing and solids processing programs. Systems include a Process Control System (PCS) for monitoring and control of all processes and facilities, upgrades to city and plant water systems, chemical systems, electrical power and distribution systems upgrade, telephone service, and data highway infrastructure for process, safety, security and information needs. Facilities comprise chemical receiving, storage, transmission and feed systems for chemicals used throughout the liquid and solids processes, including metal salts, polymers, sodium hypochlorite, and sodium bisulfite. Support facilities projects include the rehabilitation of the Central Operations Facility and the Central Maintenance Facility.

Specific major projects under this program that are substantially complete include:

  • Process Control and Computer System — This system allows for automation of a significant number of plant processes at Blue Plains, and better management of processes that are currently manually monitored. Operating savings are anticipated from lowered chemical usage and electricity consumption, by minimizing peak demand, as well as lower staffing levels. This project is critical to achieving the goals presented in the Blue Plains Internal Improvement Plan. The new system is being implemented in three phases. Phase I which began with the screens, grit chambers, primary and secondary treatment facilities, and dewatering processes, is substantially complete. Phase II will include nitrification, filtration, and disinfection facilities, and Phase III will add the solids processing facilities. Construction on the project began in August 2002 and will continue through 2010. The new system is being constructed in conjunction with the major upgrade projects and will be placed in service in tandem with the upgrade-projects becoming operational.
  • Central Operations Facility Renovations — This project provided for the renovation of the Central Operations Facility and will improve the functionality and appearance of the building. Project includes replacement of existing building windows, HVAC upgrades and renovation of the DETS and Procurement Offices. Construction is underway and is expected to be completed in 2009.

Specific major projects under this program include:

  • Plantwide Fine Bubble Aeration System — This project involves replacing the coarse bubble diffusers in the secondary treatment aeration system with fine bubble diffusers. The conversion will provide the capability to transfer more oxygen to the process while saving overall energy consumption.
  • Electrical Power System Switchgear — This project involves replacing or upgrading aging electrical switchgear and transformers throughout Blue Plains. The project includes electrical systems that are not being upgraded as part of other facility upgrade projects.

top

Solids Processing Program

Twilight over the Blue Plains gravity seperation tanks
Twilight at Blue Plains

Biosolids processing involves reductions in volume along with treatment to meet applicable federal, state and local requirements for the ultimate disposal method. Treatment is provided by a system of processing facilities that include gravity thickening of primary sludge, floatation thickening of the biological waste sludges produced by the secondary and nitrification/denitrification processes, dewatering by centrifuge and lime stabilization. Dewatered-stabilized biosolids are conveyed to the Dewatered Sludge Loading Facility, from which the biosolids are loaded into tractor-trailers and hauled offsite for beneficial reuse. Examples of beneficial reuse are land application, silviculture, and land reclamation. Solids processing facilities are required to produce a biosolids product that can be reused or disposed of in an economical and environmentally acceptable manner.

We are continuing implementation of our Biosolids Management Plan (BMP), originally adopted by the Board in 1999. This plan, which included input from our neighbors, environmental groups, and other stakeholders, evaluated a number of options for long-term biosolids processing and disposal, and identified full biosolids digestion as a common element of all long-term approaches and recommended continuing land application as long as financially advantageous. However, an unacceptably high bid for construction of the Egg-Shaped Digester project led to a decision by the DC Water Board of Directors to defer the project. The decision by the DC Water Board of Directors to defer the project until market conditions improve was based on an independently conducted economic analysis and an internal cost-benefit evaluation. As requested by the Board, DC Water has continued to monitor the construction market, and regulatory initiatives related to land application of biosolids and evolving wastewater treatment technologies. Since then, we have evaluated a wide range of biosolids processing options, narrowed the options to four processing options. Each of the options involves anaerobic digestion, consistent with the 1999 BMP. Various types of digestion vessels have been investigated that would be less expensive to construct than the egg shaped digesters.

The presently favored option includes construction of four Cambi thermal hydrolysis trains, four digesters, new dewatering equipment and a combined heat and power plant. The option produces power from digester gas to meet over one third of DC Water's electric demand at Blue Plains and the digestion process destroys nearly one half of the biosolids which will result in lower reuse costs. There is the possibility that DC Water can market a substantial portion of the biosolids product, further reducing land application reuse costs.

DC Water's award-winning Biosolids Management Program has been recognized by the U.S. Environmental Protection Agency for its outstanding operations, technological advances, and promotion of the beneficial uses of municipal wastewater biosolids.

Specific major projects under this program that are substantially complete include:

  • Gravity Thickeners — This project has rehabilitated gravity thickeners 1-6.
  • Additional Dewatering Facilities — This project provides new centrifuges to expand the dewatering capacity and efficiency of solids processing at Blue Plains.

Major projects underway in this program include:

  • Biological Sludge Thickening Facilities (formerly Centrifuge Thickener Facility) — This project will upgrade the existing dissolved air floatation thickening units and provide mechanical thickening equipment. Improvements are expected to reduce sludge processing and chemical costs through improved efficiency. The construction contract for this project was bid in FY 2008.
  • Solids Processing Building / DSLF — This project involves repairs to chemical systems and provides for miscellaneous improvements to the Solids Processing Building and Dewatered Sludge Loading Facility. This project replaces aged equipment to ensure integrity and reliability of the systems and facilities which results in improved performance of chemical feed systems and other solids processing operations, and improved biosolids quality.
  • Area Substation No. 6 — This project involves installation of a new Area Substation No. 6 (ASS-6) with three feeds to replace aged Area Substation No. 4 (ASS-4). ASS-4 would become a vault for other electrical equipment.

Other Solids Processing Program projects included in the CIP but not scheduled to start until later, include:

  • Gravity Thickening Facility Upgrade — This project will demolish Thickener Units 5 and 6, and provide a major upgrade to Thickener Units 7-10, including new collector mechanisms, thickened sludge pumps, and scum pumps. Project would also repair cracks in gallery roof in vicinity of Thickener Units 7-10.

top

Blue Plains Enhanced Nitrogen Removal Program

This program provides for new facilities and upgrades to existing facilities needed at Blue Plains to meet the total nitrogen discharge limit that has been included in DC Water's NPDES permit. Projects included in the Blue Plains Enhanced Nitrogen Removal Program were identified through a strategic planning process that resulted in development of DC Water's proposed Total Nitrogen/Wet Weather (TN/WW) Plan, which addresses the requirements of the Clean Rivers Project as well as the Chesapeake Bay Tributary Strategies for reducing nitrogen discharged into the Chesapeake Bay. The recommended alternative in the plan removes additional nitrogen from the wastewater prior to discharge and improves the quality of discharge to the Potomac and Anacostia Rivers during wet weather events. Significant work on refining and resequencing the alignment, facilities and schedule has been completed. Review of the estimates and inflationary risks is ongoing.

  • Enhanced Clarification Facilities — The principal components of this project are grit removal and screening for influent wastewater followed by an enhanced clarification facility. The new facilities will treat excess flow during wet weather events resulting in improved water quality of the excess flow discharge.
  • Nitrogen Removal Facilities — This project includes a new or expanded facility to remove additional nitrogen from the wastewater prior to discharge to the Potomac River as well as any improvements to upstream processes that may be required to ensure the reliability of the new or expanded system.
  • Centrate Treatment Facilities — This project provides for the treatment of recycle streams from the sludge dewatering process. Digestion of sludge, which results in a greatly reduced volume of sludge, also results in a high concentration of ammonia in the centrate from the dewatering process. This high concentration of ammonia has the potential to overload the existing and new nitrogen removal processes. DC Water is currently participating in research to determine the most cost-effective and reliable methods to provide separate treatment of the centrate recycle stream. Specifics of this project are dependent on the findings and recommendations of the updated Biosolids Management Plan.
  • Wet Weather Peak Mitigation (aka Blue Plains Tunnel — The principal components of this project are a 23 foot diameter tunnel from Main and O Streets to Blue Plains and a tunnel dewatering pump station at Blue Plains. The impact of this project will be to reduce peak flow rates through Blue Plains without reducing the total volume of wet weather flow that receives treatment. It is important to note that the proposed project will not increase combined sewer overflows beyond those anticipated in the Clean Rivers Project.

top

What We Do