Sodium Hypochlorite & Caustic Soda Project
System Improvements of the Dalecarlia WTP
and the McMillan WTP for Disinfection and pH Control
Washington Aqueduct is moving forward in modifying 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 late 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.
EPA APPROVAL LETTER
FINAL ENVIRONMENTAL ASSESSMENT
APPENDICES
FINDING OF NO SIGNIFICANT IMPACT
DECISION RATIONAL FOR APPROVAL OF WASHINGTON AQUEDUCT TREATMENT
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