Facility Description
The City of Santa Rosa’s Laguna Wastewater Facility is a tertiary level activated sludge facility with biological nutrient removal with an average daily dry weather flow of 17.5 million gallons per day. All the effluent used for irrigation passes through the Meadowlane pumping and storage complex, located adjacent to the City’s Laguna wastewater treatment facility. The Meadowlane complex has two main functions: 1) to store water in its holding ponds, particularly during wet weather; and 2) to transport water to either remote storage ponds or irrigation booster pump stations.
The Meadowlane pumping complex is supplied with power from the adjacent Laguna treatment plant. Therefore, there is no separate utility metering for the Meadowlane system. Nor does the Meadowlane complex have its own continuous power monitoring. Furthermore, about half of the power for the Laguna plant is supplied from co-generation, using biogas and utility natural gas. For the purpose of the energy cost baseline and savings, any load reductions from efficiency improvements at Meadowlane are assumed to directly reduce the power purchased from the utility grid, rather than reduce co-generation.
Efficiency Improvements
The recommended Meadowlane improvements will save electricity by closely matching the hydraulic energy output of the pumps to the varying system demand in the irrigation trunk lines, thereby reducing energy waste. This will be done by replacing two of the fixed speed pumps with variable speed pumps. The new pumps will be speed controlled and sequenced automatically. In combination with the existing fixed speed pumps, a wide range of flow demands will be met at a constant discharge pressure by varying the combination of pumps in operation and the speed of the new pumps. This should all but eliminate the need for flow bypass and floating discharge pressure caused by the present inability to modulate the hydraulic output of the pumps.
Furthermore, to improve the decision making ability of the owner to refurbish or replace the pumps when efficiency losses due to wear become excessive, electrical power and discharge flow measurements will be added to the Meadowlane complex. This will allow a “real time” calculation and logging of the energy efficiency of the pumping system, which can be tracked over time. The current poor state of efficiency of the EB pumps is not atypical of the water industry, since large pump repair is expensive and disruptive, and is usually undertaken only when vibration becomes excessive or a pump fails. If no measurements exist on which to base a policy of replacement for poor efficiency, the same replacement practices will likely continue into the future.
