Washington Aqueduct
Sodium Hypochlorite & Caustic Soda Project
System Improvements of the Dalecarlia
WTP
and the McMillan WTP for Disinfection and pH Control
Washington Aqueduct, a
division of the U.S. Army Corps of Engineers (USACE), Baltimore District,
operates the Dalecarlia and McMillan Water Treatment Plants (WTPs) in
Washington, D.C., serving potable water to over one million persons
in the District of Columbia and northern Virginia. The treatment process
removes solid particles from the Potomac River supply water, treats
and disinfects the water, and distributes the finished water to the
metropolitan service area. Washington Aqueduct has decided to modify
two components of the treatment process – disinfection and control
of pH – at both the Dalecarlia WTP and the McMillan WTP to enhance
the reliability of the production of safe drinking water and to reduce
operational risk. Washington Aqueduct prepared an Environmental
Assessment (EA) on the proposed system modifications, and determined
that there were no significant impacts associated with the proposed action.
The preferred alternative, as described below, was selected for implementation
in a Finding of No Significant Impact document, which can be downloaded
below.
Disinfection
Bulk liquid chlorine, created by compressing
pure chlorine gas, has been used throughout the history of disinfection
at the Dalecarlia WTP and the McMillan WTP. Due to the hazardous nature
of the liquid chlorine, engineering and management controls are employed
to minimize risks associated with its handling and use. As an alternative
to using liquid chlorine, chlorine as aqueous sodium hypochlorite, an
inherently safer form, is commercially available and frequently used
in the water treatment industry. In the Draft EA, Washington Aqueduct
had considered converting the disinfection process at the Dalecarlia
WTP and the McMillan WTP from using bulk liquid chlorine to using aqueous
sodium hypochlorite for disinfection in order to eliminate the inherent
risks associated with storing and handling liquid chlorine.
pH Control
In 2004, in the interest of managing corrosion
observed in parts of the District of Columbia water distribution system,
the United States Environmental Protection Agency approved a Washington
Aqueduct plan to take steps to modify the water treatment process. The
initial step taken was to introduce a chemical corrosion inhibitor.
In addition, the acceptable range for pH in finished water was modified.
So in the Draft EA, in order to comply with the new corrosion control
requirements for drinking water in the District of Columbia, Washington
Aqueduct considered using caustic soda for pH control as a supplementary
or replacement process for lime, which is currently used at both the
Dalecarlia WTP and the McMillan WTP. During the development of the EA,
it was determined that sulfuric acid will also be needed periodically
to control pH at the McMillan WTP.
Preferred Alternative
The preferred alternative, which is also the
environmentally preferred alternative, identified in the EA includes
the following features:
- Design, construction and operation of bulk sodium hypochlorite
storage and feed systems at both the Dalecarlia WTP and the McMillan
WTP, with consideration for facilitating the possible installation
of on-site sodium hypochlorite generation equipment in the future.
- Continued study and future consideration of on-site sodium
hypochlorite generation systems for the Dalecarlia WTP and the McMillan
WTP.
- Design, construction, and operation of a caustic soda
storage and feed system in order to trim pH following pH adjustment
with lime at the Dalecarlia WTP.
- Design, construction, and operation of caustic soda and
sulfuric acid storage and feed systems for the control of pH at the
McMillan WTP.
- Construction of a new structure adjacent to an existing
storage building at the Dalecarlia WTP.
- No new structures at the McMillan WTP.
The construction phase of the project is underway and scheduled to be complete in 2010. The preferred alternative allows Washington
Aqueduct to eliminate the use of liquid chlorine at both the Dalecarlia
WTP and McMillan WTP, to achieve the corrosion control requirements
for pH, and to further investigate the potential option of generating
aqueous sodium hypochlorite on-site at the two facilities.
The following measures will also be implemented
as part of the selected alternative:
- Chemical offloading areas will be designed to control
and minimize the potential for offsite observance of noise.
- Washington Aqueduct will study and consider further the
operational uncertainties associated with installing on-site sodium
hypochlorite generation equipment. Consideration of installing on-site
sodium hypochlorite generation equipment would be described in additional
National Environmental Policy Act documentation, if it is determined
to be technically feasible.
- Washington Aqueduct will revise existing emergency response
planning documentation to incorporate necessary spill prevention and
response planning for the new bulk chemicals that will be used.
- Deliveries will typically occur during off-peak traffic
hours.
The final EA can be downloaded from the link below,
or can be viewed at the Palisades and Mt. Pleasant Branches of the District
of Columbia Public Library, or at the Little Falls Branch of the Montgomery
County Public Library.
For further information, please contact the
Washington Aqueduct NEPA Coordinator at the address shown, at 202-764-2771
or at washingtonaqueduct@usace.army.mil
U.S. Army Corps of Engineers
Baltimore District, Washington Aqueduct
5900 MacArthur Boulevard NW
Washington, D.C. 20016-2514
Attn: NEPA Coordinator
FINAL ENVIRONMENTAL ASSESSMENT
APPENDICES
FINDING OF NO SIGNIFICANT
IMPACT
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