How IRWD optimizes recycled water & codigestion to meet energy goals
Location: Orange County, California
Size: 447,000 population served; 120,000 drinking water connections
Equipment: screens, grit chambers, activated sludge, membrane bioreactor, centrifuge dewatering, acid digesters, methane digesters
Irvine Ranch Water District (IRWD) has received a gold-level status from The Climate Registry for demonstrating leadership in meeting rigorous voluntary greenhouse gas reporting criteria. The recognition followed nine years of verified greenhouse gas inventories reported through 2019 to The Climate Registry. Additionally, IRWD received Water-Energy Leader Gold status for participating in the Water-Energy Nexus Registry, which tracks and reduces California water system carbon emissions.
“We’re proud of the distinction,” said Paul Weghorst, IRWD executive director of water policy. “We signed on to reporting our greenhouse gas emissions around 2012 when we first put together our Energy and Greenhouse Gas Master Plan. Our distinct values center around environmental stewardship, transparency, accountability, and collaboration.”
District at a Glance
IRWD is a not-for-profit local public agency serving residents and businesses in a 181-square-mile section of central Orange County, California. Its customer base consists of 447,000 people at 120,000 service connections and swells to 600,000 during the day when businesses are open.
IRWD provides customers with drinking water, sewage collection, sewage treatment, recycled water and urban runoff treatment. All of that requires energy, which led IRWD to commit to meeting the environmental challenges of the water-energy nexus.
“There’s embedded energy in all water,” Weghorst notes. “It takes energy to produce, pump, convey and distribute water.”
To determine where the agency can derive energy efficiencies, IRWD maintains an Embedded Energy Study. Using GIS overlays, the study shows the locations of higher energy intensity uses of water, “giving us the ability to focus on working with customers in those areas to reduce water use and do what we can to control pumping costs,” Weghorst said.
Customers at higher elevations – who require extra pumping – are charged based on actual cost of service, with energy factored in as part of IRWD’s commitment to reducing energy use and environmental impacts in a cost-effective manner as laid out in the District’s Energy and Greenhouse Gas Master Plan. IRWD was established in 1961 as a drinking water provider, adding sewage collection and treatment in 1963, and a recycled water system later that decade.
“Our recycled water program started in 1967 when the Michelson plant opened and began delivering about 2 million gallons per day of tertiary-treated recycled water to agricultural users,” Weghorst said, adding recycling operations expanded with community growth.
Recycled Water Operations
The Michelson Water Recycling Plant (MWRP) in Irvine is IRWD’s primary recycled water source. Tertiary (three-stage) sewage treatment creates high-quality recycled water used for landscape and agricultural irrigation, toilet flushing and industrial and commercial needs.
Water flowing through sewers to the MWRP is transformed into recycled water through a multi-step treatment process beginning in the headworks, which screens out debris and uses grit chambers to remove sand, rocks, grit and other inorganic debris. Sewage then flows via gravity into the primary clarifiers. Two treatment processes occur in parallel: a conventional activated sludge (CAS) process and an advanced mechanical membrane bioreactor (MBR) process.
Both processes use beneficial microorganisms to remove nitrogen and phosphorus from the water. There’s a final disinfection in the chlorine contact tank. The MBR process concludes with the use of ultra-violet light to deactivate remaining pathogens.
Solids and grease removed in the water recycling process are sent through an underground pipeline to IRWD’s $200 million Biosolids and Energy Recovery Facility that began operating in 2020. Solids are converted into pellets for soil amendment. Three methane digesters recover biogas and a scrubbing system purifies the methane, running it through microturbines.
IRWD plans to install enough microturbines to produce up to 1.6 megawatts of power. The microturbines currently in place power one-third of the recycling plant’s energy requirements. Sludge is thickened through centrifuges, going through two digester phases: acid and methane. Additional leachate recovered from the process is sent to the Michelson plant for recycled water processing.
A fats, oils and grease (FOG) receiving station addresses an upcoming program in which FOG will be collected from restaurants and other businesses for codigestion. The biosolids project is projected to save IRWD and its customers $160 million by 2030.
After meeting quality standards at the Michelson plant, the recycled water exits through a purple pipe and is delivered back to the community. To avoid confusion with pipes carrying drinking water, IRWD pioneered purple pipe in the early 1980s to certify a standard color for pipes carrying recycled water to distinguish them from potable water systems. It has now become the universal standard for recycled water.
IRWD recycles about 9 billion gallons of water annually. MWRP has the capacity to treat up to 28 million gallons per day (mgd) and averages 25 mgd. Combined with water from IRWD’s smaller Los Alisos Recycling Plant in Lake Forest, the recycled-water operation provides 28% of the water supplied to customers. Recycled water is delivered to more than 6,000 metered connections through 572 miles of purple pipeline.
“In a place like California, where droughts are common and water is precious, recycled water is important because it is drought-proof and saves us from having to buy expensive imported water from northern California or out of state,” Weghorst said.
IRWD also is engaged in renewable energy projects, using solar power to help meet district needs. This includes a 1 MW solar power array at the District’s Baker Water Treatment Plant in Lake Forest; a 100 kW array at the District’s headquarters, providing 20% of the building’s energy needs; and a 250 kW system at the District’s Zone 3 reservoir.
IRWD has one of the nation’s largest networks of high-efficiency lithium batteries to store energy and provide onsite power to district operations. They provide power stored during off-peak hours for operations. The energy also can be used by Southern California Edison as a demand response resource for energy grid resilience.