City of Santa Rosa

Project Features and Performance Requirements

This project will modify a pump station located near Rohnert Park, used for commercial and agricultural irrigation, which is supplied with reclaimed wastewater from the City of Santa Rosa’s Laguna Treatment Plant.  Annual electrical energy usage will be reduced by 329 Megawatt-Hours (MWHr), a savings of 50% of the current energy use for the pump station.  Average annual power demand is estimated to be reduced by 36 KW for maximum demand, and 18 KW for peak period demand.

The total installed cost of the project is estimated at $292,000, with annual electricity savings of $49,200 at current PG&E rates.  This results in a simple payback of 5.9 years. The project will be eligible for a $26,300 incentive, and reducing the capital cost by the incentive amount results in a 5.4 year simple payback.

This upgrade project was initiated by the California Wastewater Process Optimization Program (CalPOP), administered by Quantum Energy Services and Technologies, Inc. (QuEST), in partnership with Sonoma County Energy Watch (SCEW).  This study takes the 2007 CalPOP audit a step further with additional data and analysis, adds a more detailed preliminary design to improve the savings estimate, and prepare for CEC loan application.

Facility Description

The Rohnert Park pump station is operated by the City of Santa Rosa, and provides approximately 1.25 million gallons per day (MGD) of reclaimed wastewater for mostly commercial, and some agricultural, irrigation.  It also can provide fire suppression if needed at the Sonoma State University campus in Rohnert Park.  Treated wastewater is supplied to the Rohnert Park pump station from the Meadowlane pump station next to the Laguna de Santa Rosa Subregional Wastewater Treatment Plant. 

To maintain irrigation pressure during periods of low demand, a bypass system returns water from the outlet header to the supply line.  During the 8-month irrigation season (beginning in March and ending in November), one of two 300 HP pumps operates all the time, with a 250 HP pump triggered very infrequently under unusually high irrigation demands.  A hydropneumatic tank regulates pressure and controls pipeline pressure surges during changes in demand while the VSD and bypass valve stabilize.  The air compressor for the pneumatic tank operates less than 30 minutes per day at approximately 4 KW, which is an insignificant load compared to the pumps.  If a 300 HP pump is not operating, the 20 HP jockey pump operates during the wet season to maintain pressure in the outlet header.  All the flow is bypassed unless irrigation demand and dropping header pressure triggers starting a 300 HP pump.  The only regularly scheduled operation of the 300 HP pumps during the wet season is to fill an irrigation storage reservoir.

To predict how changes in pump station equipment, controls, and operations will save energy, we developed a pump system energy model based on the 2007 SCADA data and an energy audit performed in the summer.  To validate the model, we compare its predicted energy use to the observed energy consumption as measured by the PG&E meter at the pump station.  The audit energy measurements were made in a using data logging equipment borrowed from the Pacific Energy Center. The audit was undertaken from July 20 to August 13, 2007.  To validate the pumping energy model developed, an energy balance was performed by selecting flow and pressure data during three summer PG&E billing periods, and applying the flow vs. KW model developed directly from the audit period.

The main source of energy waste is the need to maintain a 300 HP pump in operation around the clock, during the irrigation season, to maintain pressure on the reclaimed water distribution system even when there is no irrigation flow.  This consumes about 60 KW of power, with the pump running at the minimum speed. 

The recommended solution under zero irrigation demand conditions is to take advantage of the 10,200 gallon hydropneumatic tank that is installed at the pump station.  At zero irrigation demand, the air pressure in this tank can be used to maintain line pressure. As there will be some leakage in the irrigation distribution system, the tank water level drop will be monitored by the PLC.   We will add a variable speed drive to the existing jockey pump, and it will operate as needed to maintain water inventory in the tank.  When real irrigation demands are detected by the discharge flowmeter, the jockey pump will shut off and the 75 HP pump will come on.

The energy audit demonstrated there is considerable irrigation flow at 300-700 gpm, which corresponds to typical daytime pasture irrigation.  The existing 300 HP pumps are operating at ~40% irrigation efficiency during these flows; they are not designed to deliver water efficiently at this relatively low flow rate.  Furthermore, this irrigation often occurs during the day when summer peak electric rates are in effect, costing triple that of off-peak rates.  While less than 10% of the 2007 baseline energy use was on-peak summer, it accounted for 25% of the 2007 energy cost.  The recommended solution for typical irrigation demand conditions is to replace one of the 300 HP pumps with a 75 HP variable speed pump designed specifically for this flow range.  This pump will start as soon as appreciable flow is detected, and operate until flows reach 1000 gpm.  This will cover almost all the daytime irrigation, and be able to overlap the lower operating range of the large pump.