New Regulations Turn California Wastewater Into Drinking Water

California’s new Direct Potable Reuse regulations, which took effect on October 1, 2024, let water agencies purify wastewater and deliver it as drinking water without first routing it through a natural buffer like an aquifer or reservoir. The State Water Resources Control Board adopted the rules on December 19, 2023, making California the first state with a comprehensive regulatory framework for turning sewage directly into tap water.¹ The framework pairs a fixed sequence of advanced treatment technologies with some of the strictest pathogen-removal and monitoring standards in the country.

What the Rules Actually Allow

The regulations define a Direct Potable Reuse project as one that introduces recycled water either directly into a public water system or into a raw water supply immediately upstream of a drinking water treatment plant.² That is a meaningful change from what California has permitted for years.

Under the older indirect potable reuse model, purified wastewater is injected into groundwater or added to a surface reservoir, where it sits for weeks or months before being pulled back out and treated again. That holding period, called an environmental buffer, gives operators extra time to catch problems. Orange County’s Groundwater Replenishment System has used this approach since 2008, purifying 130 million gallons a day and supplying roughly 39 percent of the water demand for about one million people.³

Direct Potable Reuse removes that buffer. To compensate, the regulations load the safeguards into the treatment process itself through required technologies, continuous monitoring, and tighter operational controls. Adoption by water agencies is voluntary; nothing in the rules forces any utility to build a facility.

The Required Treatment Sequence

The regulations don’t just set a performance standard. They specify the order of treatment steps. The core treatment train must run ozone combined with biologically activated carbon first, then reverse osmosis, then advanced oxidation.⁴ Each step handles a different class of contaminant, and earlier steps protect the equipment that follows.

Ozone breaks down organic compounds and kills microorganisms. The water then passes through biologically activated carbon, where microbes consume the broken-down organics. This reduces the load on the reverse osmosis membranes downstream.
Reverse osmosis forces water through membranes fine enough to filter out dissolved salts, pharmaceuticals, PFAS, and most remaining contaminants at the molecular level. The regulations note that reverse osmosis can remove up to 99 percent of long-chain PFAS molecules.
Advanced oxidation, typically UV light combined with hydrogen peroxide, destroys trace organic chemicals that slipped through reverse osmosis, including smaller molecules the membranes cannot catch.

The ozone and biologically activated carbon step can be skipped only in narrow circumstances, such as when the wastewater contribution to a blended supply falls below 10 percent.⁴ For most projects, all four processes will be required.

Pathogen Removal Standards

The pathogen reduction numbers are the most aggressive of any potable reuse rule in the United States. A Direct Potable Reuse system must achieve:

20-log reduction for enteric viruses
14-log reduction for Giardia cysts
15-log reduction for Cryptosporidium oocysts

Log reduction is an exponential scale. A 20-log reduction for viruses means the treatment process must remove 99.99999999999999999999 percent of any viruses in the source water. The numbers were set by adding the reduction already expected for conventional drinking water treatment to extra reduction values that account for the missing environmental buffer.⁵

Hitting those targets requires at least four separate treatment processes for each pathogen category, drawing on at least three different mechanisms: physical membrane separation, chemical inactivation, and UV disinfection. No single process can be credited with more than 6-log of reduction, which prevents over-reliance on any one barrier.⁶ If monitoring shows pathogen reduction dropping below acceptable levels, the system has to automatically divert water away from the drinking supply.

PFAS, Pharmaceuticals, and Other Emerging Chemicals

The regulations also require operators to actively look for chemicals that don’t yet have formal drinking water limits. PFAS, pharmaceuticals, and personal care products all fall into this category. Each facility must review, every year, information from industrial sources in its service area, local monitoring data, and published lists of chemicals with known or suspected health effects. From that review, the operator selects specific chemicals to add to its monitoring list.

The approach is deliberately adaptive. Rather than lock in a fixed list that would age poorly, the framework requires surveillance that evolves with the science. The ozone and biologically activated carbon step and the advanced oxidation step together provide broad-spectrum removal of many unregulated organics, and reverse osmosis physically blocks most dissolved contaminants regardless of whether they appear on anyone’s list.⁴

Who Is Accountable, and Who Is Watching

Every Direct Potable Reuse project must designate a single public water system as the Direct Potable Reuse Responsible Agency, or DiPRRA.² That agency carries full responsibility across the whole project, from the sewer collection system to the point where water enters the distribution network. The single-agency structure is designed to prevent finger-pointing between a wastewater utility and a drinking water utility.

Source control sits upstream of the treatment plant. Each project must set up a source control committee, expand local limits on what industries can discharge into the sewer, and conduct upstream monitoring to catch contamination before it reaches the facility. The cleaner the wastewater entering the system, the easier it is to treat safely.

Staffing requirements are strict. Either the chief operator or a shift operator must be physically on-site 24 hours a day, seven days a week. An agency can later apply for an exception if it can show that automated monitoring provides equivalent oversight, but continuous human presence is the default. The chief operator must hold an advanced water treatment certification.

Transparency requirements run before and after a facility opens. Before launch, the water agency has to hold at least one public meeting and make project information readily available. Once operating, the DiPRRA must include detailed project information in its annual consumer confidence report: identifying municipal wastewater as a source, describing the treatment train, listing all chemicals detected in finished water during the previous year along with average and range values, summarizing any violations, and explaining how the project addresses climate change impacts.⁷ The report must also link to the facility’s full annual report.

Federal rules still apply underneath all of this. All drinking water in the United States must meet Safe Drinking Water Act requirements, including the EPA’s chemical and microbial contaminant standards under 40 C.F.R. § 141.⁸ California’s Direct Potable Reuse standards exceed those federal minimums in most categories, but the federal floor remains the backstop.

Which Agencies Are Building Facilities

Since October 1, 2024, water agencies have been able to submit permit applications to the Division of Drinking Water. Several large systems are already in planning or construction:

Pure Water Southern California, run by the Metropolitan Water District, is the largest planned project. At full scale it would purify 150 million gallons a day, with operations scheduled to begin in 2032.⁹
Pure Water San Diego is being built to produce 30 million gallons of purified water daily in its first phase, with a goal of supplying one-third of the city’s water by 2035.⁹
The East County Advanced Water Purification project, led by Padre Dam Municipal Water District in eastern San Diego County, is scheduled to come online in 2025 and is expected to be among the first operational advanced water purification facilities in the state.⁹

Southern California is leading the early adoption because the region imports most of its water from hundreds of miles away, which makes locally produced recycled water a practical alternative. Given the engineering, environmental review, and construction involved, most facilities built under the new rules are still years away from serving customers.

Is the Water Safe to Drink

Research consistently finds that advanced-purified recycled water is comparable to conventional drinking water, and in some respects cleaner. A peer-reviewed study of microbial composition found that finished water from potable reuse systems “overlapped substantially” with conventional drinking water, regardless of the specific treatment technologies used.¹⁰ Reverse osmosis strips out dissolved minerals and salts that conventional surface water treatment leaves behind, producing water that is essentially ultrapure before being remineralized for distribution.

The practical track record is also relevant. Orange County has recycled wastewater into drinking water through its indirect system since 2008, producing billions of gallons without a reported health incident.³ Direct Potable Reuse uses the same core purification technologies with additional regulatory safeguards layered on top. Many existing drinking water sources already contain treated wastewater from upstream discharges, just without the advanced purification step; the new framework replaces unplanned reuse with an engineered system designed to make that water safe.

1
California State Water Resources Control Board. Regulating Direct Potable Reuse in California
2
State Water Resources Control Board. California Code of Regulations Title 22 – Direct Potable Reuse
3
OCWD. GWRS
4
Legal Information Institute. California Code of Regulations Title 22 64669.50 – Chemical Control
5
U.S. Environmental Protection Agency. Summary of California’s Water Reuse Guideline or Regulation for Direct Potable Water Reuse
6
Legal Information Institute. California Code of Regulations Title 22 64669.45 – Pathogen Control
7
Legal Information Institute. California Code of Regulations Title 22 64669.130 – Consumer Confidence Report
8
U.S. Environmental Protection Agency. Summary of Texas’ Water Reuse Guideline or Regulation for Potable Water Reuse
9
Metropolitan Water District. Pure Water Southern California
10
PMC. What is the Difference between Conventional Drinking Water