Orange County's Groundwater Replenishment System as a scalable model for water security

In this June issue, you will also find an opinion article by Mehul Patel, Executive Director of Operations at the Orange County Water District, who shares a first-hand account of the system’s inception, evolution, and technical leadership. This article aims to complement his perspective by focusing on GWRS as a replicable model, exploring the factors that have enabled its success and potential applicability in other contexts.

A circular solution in Orange County

The Groundwater Replenishment System, operational since 2008, is the world’s largest advanced water purification facility for indirect potable reuse. Jointly managed by the Orange County Water District (OCWD) and the Orange County Sanitation District (OC San), the facility applies a rigorously engineered multi-barrier treatment train. OC San provides approximately 185 million gallons per day of secondary-treated effluent, produced through conventional primary and secondary wastewater treatment, including bar screening, grit removal, trickling filters, activated sludge, clarification, and disinfection. A source control program curtails industrial pollutants and emerging contaminants before they enter the wastewater stream.

The GWRS purifies wastewater to near-distilled quality using microfiltration, reverse osmosis, and ultraviolet advanced oxidation

The tertiary treatment process at GWRS begins with microfiltration (MF), which utilises 0.2-micron hollow-fibre membranes to remove suspended solids, protozoa, bacteria, and some viruses. The filtered water then undergoes reverse osmosis (RO), where high-pressure pumps drive it through semi-permeable membranes that exclude dissolved salts, organic compounds, pharmaceuticals, and viruses. The final stage, ultraviolet (UV) advanced oxidation, combines high-intensity UV light with hydrogen peroxide to degrade trace organic contaminants, including NDMA and other micropollutants.

The result is a water quality profile approaching distilled water, necessitating post-treatment stabilisation before recharge. Once treated, the water is distributed strategically to support regional groundwater sustainability. Approximately 30 MGD are injected into coastal wells in Fountain Valley and Huntington Beach, forming a seawater intrusion barrier. Another 90 to 100 MGD are conveyed to percolation basins in Anaheim, where the water filters through sand and gravel into the deep aquifers of the Orange County Groundwater Basin, replenishing the local drinking water supply. Additionally, up to 10 MGD can be directed to mid-basin injection wells located in Santa Ana to further stabilise groundwater levels. Following its final expansion in 2023, GWRS now operates at a capacity of 130 MGD—producing up to 44 billion gallons annually—making it a cornerstone of Orange County’s resilient and diversified potable water supply.

Operational excellence and integration

Operational resilience and treatment performance are foundational to GWRS’s credibility. The system achieved 98.9% uptime in 2023, delivering over 112,000 acre-feet of purified water. It is embedded in a diversified water portfolio that also includes flows from the Santa Ana River, stormwater capture, and imported supplies from the Colorado River and State Water Project. Approximately 85% of the water demand in north and central Orange County is met through the groundwater basin, with imported water bridging the balance.

What distinguishes GWRS is not just its engineering sophistication but its seamless integration into Orange County’s long-term water management strategy. This integration ensures adaptive operational control, hydraulic stability in the basin, and independence from increasingly variable imported supplies. A jointly staffed steering committee governs the project, reflecting the institutional collaboration between the water supply and sanitation sectors. California’s progressive regulatory framework for indirect potable reuse, combined with OCWD’s proactive compliance and outreach, has established a precedent-setting governance model.

Beyond scarcity: why reuse matters everywhere

The GWRS combines engineering sophistication with seamless integration into Orange County’s long-term water management strategy

Although born of water scarcity concerns, the rationale for reuse transcends arid climates. GWRS also addresses effluent management, energy efficiency, and greenhouse gas mitigation—providing a decentralised, climate-resilient source that aligns with integrated resource planning principles. For utilities in coastal or high-density regions, reducing ocean discharge volumes and diversifying risk exposure are increasingly compelling drivers.

GWRS also excels in stakeholder engagement. From its inception, OCWD invested in building public trust through transparency and education. Over 60,000 individuals—including water professionals, regulators, and students—have toured the facility. Initiatives such as classroom outreach, media engagement, and even distributing bottled GWRS water have helped normalise the concept of potable reuse. Surveys and feedback indicate a consistent uptick in community support, converting initial scepticism into advocacy.

Exporting the model: opportunities and requirements

Replicating GWRS demands specific enabling conditions. A consistent supply of secondary effluent is essential, as is access to a hydrogeologically suitable aquifer for indirect potable reuse. Regulatory clarity is paramount: jurisdictions must define water quality objectives, environmental buffers, and pathogen/chemical log removal requirements. Equally important is the institutional capacity to coordinate between utilities, manage stakeholder relationships, and ensure sustainable finance mechanisms.

The GWRS treatment train has also proven highly effective in managing emerging contaminants. PFAS compounds, including PFOA and PFOS, are routinely monitored. The multi-barrier system—particularly RO and UV/AOP—achieves non-detect or trace concentrations, far below California’s health advisory levels. This positions GWRS as not only a supply solution but a public health safeguard against contaminants of concern.

Financing the vision

The financial structure behind GWRS is as significant as its engineering. From inception to completion of its final expansion, the project has required over $900 million in capital investment. The system first came online in 2008 with a capacity of 70 MGD at a construction cost of approximately $481 million. It underwent two major expansions: in 2015 to 100 MGD (at a cost of $142 million), and in 2023 to its current capacity of 130 MGD (at a cost of $284 million).

This level of investment was made feasible through a combination of local, state, and federal funding mechanisms. OCWD and its partners leveraged grants, low-interest loans, and operational subsidies to keep long-term costs manageable. These included Clean Water State Revolving Fund loans, federal support through the Bureau of Reclamation’s Title XVI program and the EPA’s WIFIA financing, and support from California’s various water bond propositions.

For international water professionals, the key takeaway is the importance of diversified funding streams and early stakeholder alignment. While the precise sources and terms of financing will differ by country, the GWRS model shows that a complex, high-performance reuse project can be made economically viable through layered, long-term financial planning. The project's evolution also underscores the importance of building financial resilience into phased expansion, enabling utilities to scale infrastructure responsively as demand and regulatory conditions evolve.

Global recognition and influence

Over 60,000 people have toured GWRS, part of a long-term effort to build trust and normalise the concept of potable water reuse

The international relevance of Orange County’s efforts is underscored by the global recognition OCWD has received. In 2014, OCWD was awarded the prestigious Lee Kuan Yew Water Prize by Singapore’s Public Utilities Board (PUB), in acknowledgement of its pioneering leadership in groundwater recharge and potable reuse. This recognition not only honoured the success of GWRS, but also highlighted OCWD’s earlier innovation—Water Factory 21—which was among the first in the world to produce potable-quality water from treated wastewater using reverse osmosis.

The influence of OCWD’s work extended directly to Singapore’s own water reuse strategy. Inspired by OCWD’s technologies and public engagement practices, Singapore developed the NEWater initiative, a cornerstone of its water sustainability efforts. OCWD’s approach—particularly its use of advanced treatment barriers, long-term monitoring, and emphasis on building public trust—helped shape international benchmarks for potable reuse.

The GWRS model demonstrates how local solutions can catalyse global change, offering transferable insights for cities confronting water security issues. Its success reinforces the value of integrating technology, governance, and public communication in the development of reuse systems.

A blueprint for the future

The GWRS exemplifies the transition from linear to circular water systems: potable reuse is not only feasible but scalable and sustainable

As regulatory frameworks evolve to accommodate direct potable reuse and climate pressures intensify, GWRS stands as a proven reference model. It has inspired analogous projects in Texas, Arizona, Singapore, and Australia. Utilities globally can look to GWRS for both technical validation and strategic guidance.

In essence, GWRS exemplifies the transition from linear to circular water systems. Its value lies not just in its infrastructure, but in the institutional, regulatory, and social architecture that supports it. For water professionals tasked with securing future supplies, the lessons from Orange County are clear: potable reuse is not only feasible but scalable, sustainable, and essential.