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San Diego County Water Authority approves expansion of Twin Oaks water treament plant



The San Diego County Water Au­thority board approved change orders to the SDCWA’s contract with NEW­est Construction Company, Inc., for the Twin Oaks Valley Water Treatment Plant Expanded Service Area project.

The September 24 CWA board vote accepted three change orders totaling $315,145 which had been authorized by CWA staff under the authority of CWA general manager Maureen Sta­pleton while also authorizing Stapleton to execute an additional change order for $93,392. The $400,537 change or­der amount increases the total contract amount to $4,090,537.

A change order for $282,599 in­cludes the replacement of the existing pump inlet piping and valves which were smaller than shown on the re­cord drawings. The change order for $42,046 was executed after the pres­ence of slurry backfill beyond the lim­its shown on the record drawings led to the decision to relocate the Hauck Mesa Valve Vault and to excavate and back­fill the original location. An adminis­trative modification credit of $9,500 corresponds to reduced CWA field of­fice requirements. The new $93,392 change order allows for modifications required to the heating ventilation and air conditioning (HVAC) system.

The Carlsbad seawater desalina­tion plant is expected to be in service this fall and will relieve demand on the CWA’s Twin Oaks Valley Water Treatment Plant, which will allow for a redistribution of water from Twin Oaks through the Valley Center Pipe­line and reduce the amount of treated water needed from the Metropolitan Water District of Southern California (MWD).

Treated water from the Twin Oaks Valley Water Treatment Plant is initial­ly distributed along the CWA’s Second Aqueduct which includes Pipelines 3, 4, and 5. The Valley Center Pipeline runs from the Gopher Canyon area to Valley Center and connects the Second Aqueduct to Pipelines 1 and 2 on the First Aqueduct. Pipelines 1, 2, and 4 currently convey treated water to CWA turnouts while Pipelines 3 and 5 carry untreated water through the San Diego Aqueduct system.

The Valley Center Pipeline cur­rently carries 20 cubic feet per second (cfs) while the CWA takes 60 cfs from the MWD delivery point to provide 80 cfs of treated water along the First Aqueduct south of the Valley Center Pipeline.

The Valley Center Pump Station itself was completed in 2001. A review of the station’s mechanical, structural, electrical, and security systems determined that the pump station could be expanded to in­crease the pumping capacity to 41 cfs, which would satisfy day-to-day and emergency treated water needs projected through 2035.

The retrofit includes replacing the existing pumps, motors, electri­cal system, computer control system, and certain valves and modifying the building roof to comply with current building code requirements.

Conveying additional water from the Twin Oaks plant would cost less than importing treated water from MWD, and the expected return on investment period is five to eight years.

The Valley Center Pump Sta­tion expansion will also increase the CWA’s chances of meeting water de­mands in an emergency event. In Au­gust 2014 the CWA board authorized a $3,682,000 design-build contract with NEWest Construction Company, Inc., for the capacity expansion.

In August 2014 the CWA board autho­rized a $3,682,000 design-build contract with NEWest Construction Company, Inc., for the capacity expansion.

The inlet piping was believed to be 24 inches in diameter, but some segments had a smaller diameter. That piping will be replaced and will be 26 inches in diam­eter to meet current standards.

As work was being performed, ad­ditional concrete slurry surrounding the Hauck Mesa Valve Vault was encoun­tered. Relocation was determined to be preferable to removing the slurry backfill, so the vault will be moved ten feet to the west.

NEWest or a subcontractor had been required to retrofit the pump station’s ex­isting HVAC system based on the maxi­mum design indoor temperature of 104 degrees for electronic equipment and an average outdoor air high temperature of 91 degrees in the Escondido area. Since the Valley Center Pump Station is in Val­ley Center, the more appropriate average outdoor air high temperature was 100 de­grees. The HVAC modifications will in­clude procuring and installing additional air conditioning units and ducting, high- temperature alarms on the variable fre­quency drives and active harmonic filter equipment, and seismic bracing for the roof-mounted HVAC equipment.

The capacity expansion has a January 2016 completion estimate.



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