Digester Beneficial Use Program
About the DGBU Program
The San Bernardino Municipal Water Department (SBMWD) owns, operates, and maintains a regional Water Reclamation Plant (SBWRP). The SBWRP generates digester gas (DG) as a byproduct of the wastewater treatment process that the facility had beneficially used since 2005 to fuel internal combustion engines driving process and power generation equipment. Driven by regulatory changes, and expanded to efficiently achieve other resiliency, environmental, operational, and cost benefits, SBMWD initiated the Digester Gas Beneficial Use (DGBU) Program. This included multiple projects:
- Fuel cell (through a Power Purchase Agreement);
- Ultra-low emissions duty flare;
- Low emissions backup flare;
- Electrified turbo blower system with Ammonium-Based Aeration Control (ABAC);
- Arrowhead Lift Station (ALS) Reliability Projects;
- 170,000 cf DG storage system;
- Electric utility primary metering project;
- ALS electrical upgrade; and
- Mega Duct Bank project, to minimize the energy consumption and emissions while maximizing the beneficial use of DG.
Ultra-Low Emissions Duty Flare and Low Emissions Backup Flare
The fuel cell system is intended to utilize all of the Digester Gas (DG) that the Department produces that is not first used to operate the boilers. In the event that the fuel cell is out of service, the Department must be capable of fully combusting all DG produced in compliance with the regulations of the South Coast Air Quality Management District (SCAQMD). The Department installed two new flares under the DGBU Program:
- Ultra-Low Emissions Duty Flare (meeting 0.025 lbs NOx/MMBTU)
- Low Emissions Backup Flare (meeting 0.06 lbs NOx/MMBTU)
Electric Turbo Blowers
Previously, the Department utilized four (4) centralized rotary lobe blowers that fed 16,500 SCFM (each) to provide aeration air to diffusers within the various aeration basins of the SBWRP. Two of these four blowers were driven by DG-fueled ICE. There was insufficient redundancy for the remaining two blowers to provide sufficient air to operate the full SBWRP. As part of the DGBU Program, the Department added five (5) ultra-efficient electric turbo blowers in a decentralized approach to feed air primarily to the largest of the aeration basins with the ability to temporarily backfeed air to the other basins when the rotary lobe blowers required maintenance.
Ammonium-Based Aeration Controllers (ABAC)
The Blower Electrification Project also incorporated development and installation of an automated Dissolved Oxygen (DO) control system with custom cascading control to more efficiently control the level of DO in the aeration zones of the activated sludge treatment process. This resulted in greater control over the nitrification and denitrification processes as well as energy savings through reduction of overfeeding of unnecessary air provided by the blowers. This improved the quality of secondary effluent by lowering typical Total Inorganic Nitrogen (TIN) levels from 9 mg/L to 5 mg/L.
Digester Gas Storage
Fuel cell systems typically require a steady, equalized flow of DG to the fuel cells in order to achieve maximum performance. Changes in gas feed rate and pressure significantly deteriorate the operational ability of the fuel cell system. In addition to equalizing flow for the fuel cell entity, the Department desired to provide greater ability to store DG in order to provide operational flexibility for staff. With nearly a half-day’s production of DG in storage ability, Department Staff can perform maintenance and temporary shutdowns of connected components.
Arrowhead Lift Station Reliability Projects
The Arrowhead Lift Station (ALS) is one of three (3) large influent lift stations for the SBWRP. It is a critical lift station that must remain in operation 24-hours per-day, 365-days-per-year and transmits, on average, a flow of 6 million gallons per day (MGD). This lift station previously had four (4) pumps driven by ICE and one electric motor-driven pump. Two of the four pumps were fueled by NG and two were fueled by DG. The NG-fueled ICE are primarily used as backup equipment when the DG ICE were undergoing maintenance or repair. They were also used during high storm-flow events, which at times in the past have required all four ICE to run simultaneously. With the cessation of DG to ICE, one ICE was converted to an electric motor and the other was converted to a propane fuel source. This provides the Department with greater redundancy on multiple energy sources.
Arrowhead Lift Station Electrical Supply Project
Installation of the ALS Pump #5 motor equipment in 2014 and 2015 resulted in usage of the last remaining capacity on the 480-volt power supply. No additional electrical capacity was available through which to power any electrification of Pump #2. The existing switchgear had sufficient capacity on the 4,160-volt supply but lacked sufficient space for additional switches. This project included an extension of duct bank and junction boxes from the new Mega-Duct-Bank noted below.
SCE Primary Metering Project
Prior to the DGBU Program, the SBWRP had two electrical service feeds from SCE: a 4,160-volt service and a 480-volt service. The Department anticipated that, following the cessation of flow from one of its Satellite Collections Systems, it would be able to utilize all power produced by the fuel cell system. However, that load was distributed amongst the two separate utility service feeds. The fuel cell system required connection to only one electrical service. Neither existing service alone was anticipated to have sufficient load to utilize all produced power. Connection to either of the existing services as is would have required the fuel cell system to be de-rated, yet the Department would be required to purchase the power produced at the system’s full capacity (Deemed Generated Energy). Therefore, the SBWRP’s electric load was consolidated into one system by tying both services into one primary SCE electric meter. This also included a switchgear expansion component with four new switch sections.
Mega-Duct Bank Project
This project included construction of a plant-wide electrical duct bank (underground electrical raceway) to serve both current and future electrical facilities. Given the inclusion of the ALS Electrical Supply Project, this also included installation of an electrical substation, including a step-down transformer and 480-volt panel board to serve the Pump #2 Electrification Project.