Lescure Engineers, Inc. working with John Rosenblum PhD, prepared engineering and construction documents for replacement of the effluent pump station located at the Graton Community Services District Wastewater Treatment Plant. Applications and supporting analyses were submitted to secure California Energy Commission (CEC) and PG&E financing for the project.
project summary
This existing effluent pump station was originally designed to transfer wastewater from Graton to the Forestville treatment plant for tertiary treatment. Instead, it is being used to supply irrigators with treated wastewater, and also for winter discharge to Atascadero Creek. This change in use from the design intent necessitates continuous by-pass flows (pumping water in a circle) so that flow through the pumps remains within the manufacturerˇ¦s lowest flow requirements. The continuous by-pass flow means that energy demand has been far higher than necessary. In addition, since the pumps operate continuously at conditions they were not designed for, pump wear has accelerated, and this pump station has become unreliable and expensive to maintain.
An extensive energy audit was performed, sponsored by the PG&E Savings by Design energy efficiency program. Dr. John Rosenblum guided the audit and analyzed the results. As expected, the pump station was consuming far more energy than necessary. As much of 80% of the energy consumed was being wasted.
Given the conditions stated above, Graton Community Services District determined that the best course of action is to replace the existing pump station with an energy efficient, new pump station, designed specifically to meet the Districtˇ¦s current and projected needs for effluent discharge and irrigation. Given the complexity of controls for an energy efficient irrigation pump station, Lescure Engineers set out to find experienced vendors who can supply integrated package pump stations designed to specification. The vendors were asked to provide budget estimates based on the 20 year life cycle costs including energy and maintenance, in addition to capital costs.
Lescure Engineers and Dr. Rosenblum estimated the demand (pressure, flow, solids removal) in order to specify a new effluent pump station. They evaluated the need for reliability using multiple pumps and developed a set of design criteria. They reviewed the system for winter discharge to Atascadero Creek and recommend an energy efficient strategy that will allow the operators to maintain flexibility in discharge flow, allowing for the normal range of creek levels.
Information and cost estimates were gathered from multiple vendors to determine the probable cost of a new pump station. Dr. Rosenblum assisted Lescure Engineers in developing a conceptual design of the wetwell, mounting pad, piping, and other work that will not be included in the vendor supplied pump station. The goal was to have a solid conceptual design and cost estimate meeting the standards for a CEC / PG&E financing application. Lescure Engineers developed the cost estimate from the conceptual design, as required to apply for funding.
A new effluent pump station will be constructed at a new location, with new pumps, variable speed drives (VSDs) for the pumps, and new controls and instrumentation. The main elements of the upgrade are:
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A 15 hp pump with a VSD to cover all discharge flows up to 960 gpm. Very infrequent flood-stage discharges up to 1,025 gpm can be provided with the irrigation pumps.
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A fixed-speed 7.5 hp pump will provide irrigation flows up to 60-70 gpm (depending on the system curve), and maintain pressure in the reclaimed wastewater trunkline when there is no irrigation demand.
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Two 50 hp pumps with VSDs will provide irrigation flows up to 650 gpm, and transfer flows to Forestville at 750 gpm (1 MGD). These pumps will also support the fire-protection system at the wastewater plant.
Upgrading the pump station will allow integration of energy efficiency into the design, and will save Graton CSD a large fraction of their existing energy bills. The recommendations developed through Savings by Design reduces energy use and costs by an average of 90% compared to the 2006 baseline. Peak power demands will be reduced by an average of 47 kw (74%).
Life-cycle cost analysis was performed for the project. The replacement project will cost $285,500 but will generate energy cost savings resulting in a payback period of approximately 9.8 years. Payback periods of 31 years are typical for standard pump stations. This payback period comparison demonstrates that making a engineering decisions based on capital costs alone can be very short-sighted.
