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PORTLAND'S HYPOCHLORITE CONVERSION – SAFE DISINFECTION IS NO ACCIDENT

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Project Background

Prior to implementation of this project, the City of Portland's Columbia Boulevard Wastewater Treatment Plant used gaseous chlorine to provide disinfection of final plant effluent and reuse water, as well as to supply process control needs. This chlorine was delivered and stored in 20,000-gallon rail tank cars in an enclosed Chlorine Containment Building. During maintenance of the gaseous chlorine system in 2000, the City installed a small temporary sodium hypochlorite disinfection system and was pleased with its operation. This experience, along with increasing plant loadings, staff levels, training needs, risk management program administrative costs, and safety concerns associated with the use of chlorine gas, prompted the City to initiate a predesign study in 2001 to evaluate long-term disinfection options, and proceed with design and construction of the selected improvements commencing in 2003.

Due to the stringent disinfection requirements in the City's National Pollutant Discharge Elimination System (NPDES) permit, it was essential that the disinfection system operate continuously and safely under all circumstances. In addition, the new system needed to be highly flexible, provide redundancy, and meet demand for flows ranging from 20 mgd at startup, to 450 mgd under ultimate peak flow conditions. Construction of the project began in March 2005, and was completed in two phases in July 2006.

Project Elements

The project included:



Removal of the existing gas chlorine system while maintaining disinfection at all times.


Bulk sodium hypochlorite storage in the Hypochlorite Receiving Building (previously the Chlorine Containment Building).


Pumped transfer of hypochlorite from the bulk storage tanks to intermediate loop storage tanks.


Two remote intermediate loop storage tanks to supply one of two hypochlorite loops with continuously recirculating hypochlorite.


Two redundant piping loops from the loop storage tanks to various disinfection and process application points throughout the plant.


New peristaltic metering pumps at four distribution points, including wet weather and dry weather disinfection points.


Conclusions

This project has met the City's objectives of significantly improving safety for plant staff and the surrounding community by eliminating the potential for release of gaseous chlorine. It also has provided redundancy to minimize the potential for disruption of disinfection, and will meet ultimate disinfection chemical demands for the City of Portland's Columbia Boulevard Wastewater Treatment Facility with no further addition of chlorination system infrastructure.

The result of this project has been a sodium hypochlorite disinfection system that has proven to be very flexible, reliable, and well accepted by plant staff. The City's project manager commented: “If people knew how easy this was and what a great system they could have, no one would still be using gas!” The system has increased plant staff and community safety, allowed maintenance and repairs to be conducted with no negative impact to plant performance or permit compliance, and it will provide for the long-term needs of the facility.
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Document Type: Research Article

Publication date: 2007-01-01

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