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Hypochlorite Conversion in Portland, ME – Compliance, Wet Weather Disinfection, Improved Automation, and Success

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Abstract:

The Portland Water District (PWD) owns and operates the 19.8 mgd East End Wastewater Treatment Facility (EEWWTF) in Portland, Maine. While providing effective treatment of secondary flows of up to 36.8 mgd, the facility also provides primary treatment and high rate disinfection/dechlorination of up to an additional 43.2 mgd during wet weather events. Faced with obsolete gaseous chlorination and dechlorination systems, compliance issues, safety concerns and regulatory program requirements related to gaseous chemical handling, PWD decided to replace the disinfection and dechlorination systems.

In response to fecal coliform and effluent residual violations, PWD had already replaced an ORP based control system with a new programmable logic controller (PLC) based residual trim controlled system. Prior computation fluid dynamics modeling of the chlorine contact tanks indicated that the tanks were performing reasonably well. While some effluent violations persisted, most of those were attributed to the aged equipment, the lack of qualified service personnel to assist in maintaining the equipment, inadequate levels of redundancy, and capacity limitations of the existing system. The hypochlorite conversion project focused on addressing all of these issues and improving the performance and operability of the system.

The new system includes properly sized pumps and storage facilities. The pumping system is comprised of peristaltic pumps designed to deliver the wide range of flows required for both dry and wet weather and to avoid issues such as off-gassing of the product. Each pump was equipped with a flow meter to confirm pump function and output. Failure of a pump will generate an alarm, start a second pump, and notify the operator of a successful transition. The system includes residual trim control of chlorine dosing and feed-forward control of the dechlorination process. Chemical use is totalized in SCADA for record keeping and reporting.

The project embraced a new approach to project delivery. In addition to design and construction administration services, the engineer, Wright-Pierce, took on the responsibility of control logic programming of the new system. Once the contractor completed the physical installation, the engineer's team had 3 weeks to install program logic, test and verify program function, and complete no less than 7 days of continuous operation of the system while using water in place of the actual chemical. This was possible with the existing systems still operating. The plant SCADA system was used to track actual chemical use and to confirm that the new system was performing adequately. This approach minimized violations and resulted in a turnkey acceptance by PWD.

With the chlorination system fully upgraded and performing, PWD has focused on enhancing wet weather control. Through use of SCADA, PWD has modified flow regulation and the plant's transition to and from a wet weather mode. Through the proper application of equipment, automation, and functionality, PWD has dramatically decreased the number of violations associated with its disinfection system. Wet weather chlorination events that once seemed chaotic and difficult are now nearly routine.

Document Type: Research Article

DOI: https://doi.org/10.2175/193864707787932568

Publication date: 2007-01-01

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