CASE STUDY: TREATMENT OF SECONDARY EFFLUENT FOR USE AS COOLING TOWER MAKEUP AT A COMBINED-CYCLE NATURAL-GAS POWERGENERATING FACILITY
FPL Energy's Rhode Island State Energy Partners LP (RISEP) has constructed a 540-megawatt combined-cycle, natural-gas-fired power plant in the town of Johnston, Rhode Island. During the development phases of the project, it was determined that an adequate potable and natural water
supply was not available to supply makeup water to the power plant's cooling towers. As a result, RISEP established a working partnership with the neighboring community of Cranston, Rhode Island to use secondary effluent from the city's wastewater control facility (WCF), normally
discharged to the Pawtuxet River.
The use of treated effluent in the cooling towers led the Rhode Island Department of Environmental Management (RIDEM) Office of Air Resources to issue stringent permit limits. The permit required the cooling tower supply makeup water to have a total suspended
solids concentration (TSS) of 5 mg/l or less. In addition, the cooling tower makeup water was required to have a total residual chlorine (TRC) concentration of not less than 1 mg/l at the power plant (12 miles away from the facility). To meet these requirements, a filtration and chemical
feed system were proposed to filter particulate matter from the effluent and to provide enhanced disinfection using sodium hypochlorite. These processes were incorporated into a tertiary treatment facility located at the Cranston WCF. RISEP's aggressive schedule required that permitting,
design, construction, and start-up of the tertiary facility be completed in approximately 18 months. Design-build project implementation was selected as the best approach to meet the schedule requirements. M&E designed a tertiary treatment facility at the WCF and 12 miles of 18-inch ductile
iron supply pipeline and 12-inch return pipeline for the cooling tower makeup and blow down and power plant wastewater discharge. The blow down is conveyed back to the WCF's headworks for treatment. The facility is designed to supply the power plant's cooling towers with an average
flow of 2.5 mgd and peak flow of 4.3 mgd using three 450-horsepower pumps.
This paper provides a discussion of the project including an overview of the regulatory and technical requirements that drove the overall design basis for the supplemental treatment facility, high-pressure pipeline
design considerations, features of the instrumentation and control system, and start-up aspects of the project.
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