In the latest development in a long-running regulatory process that began more than a decade ago, the State of California has released its proposed regulations for the direct potable reuse (DPR) of municipal wastewater. Once finalized, the DPR rule is expected to help California water agencies extend and increase the resilience of their existing supplies by creating a source that is essentially drought-proof. 

In 2010, California lawmakers passed legislation directing the state’s Department of Public Health to investigate the feasibility of developing regulations for DPR, a task that was later transferred to the California State Water Resources Control Board (SWRCB). A 2017 law directed the SWRCB to adopt criteria for DPR by December 31, 2023.

After making available an advance copy of the rulemaking on July 11, the SWRCB published the official version of the notice of proposed rulemaking on July 21. Publication of the notice of proposed rulemaking began a 45-day comment period ending on September 8. 

Recycled wastewater produced by means of DPR is expected to play a major role in California’s ambitious plans to expand its available water resources. In August 2022, California Governor Gavin Newsom released his water supply strategy, which calls for reusing at least 800,000 acre-ft of water annually by 2030 and 1.8 million acre-ft/yr by 2040. As of 2022, California was recycling approximately 728,000 acre-ft/yr, according to the strategy.

Joint plans and source control

Through its proposed rulemaking, the SWRCB intends to amend Title 22 of the California Code of Regulations, Division 4 (Environmental Health), Chapter 17 (Surface Water Treatment) by adding Article 10 (DPR). 

The proposed rule covers DPR projects involving raw water augmentation, or the use of recycled water to augment a source of supply for a drinking water treatment facility, as well as treated water augmentation, or the introduction of recycled water directly into a drinking water distribution system.

The proposal spells out the requirements for what it terms a “direct potable reuse responsible agency” (DiPRRA), or the public water system that intends to operate a DPR system. Among its provisions, the proposed rule notes the elements of the “joint plan” that a DiPRRA must implement with the agency or agencies supplying the municipal wastewater for a DPR project. “The joint plan, and the components therein, are necessary to coordinate all aspects of a DPR project to ensure that a DPR project successfully produces water that meets all drinking water standards and is protective of public health,” according to the initial statement of reasons that the SWRCB released in conjunction with the proposal.

The proposed rule includes a significant emphasis on source control. Within this arena, a DiPRRA must develop an “early warning program” to inform it of a “potential occurrence” that could interfere with or reduce the reliability of the operation of a treatment process at a water treatment plant that is part of a DPR system or “result in an increase in contaminant levels in the advanced treated water,” according to the rule.

This early warning system must include “online monitoring instrumentation that measures indicator compound(s) or surrogate parameter(s) and that indicates an increase in chemical contamination that may adversely impact the operations of the DPR project treatment or cause contamination of the advanced treated water,” the rule notes. 

“Online monitoring is necessary to track changes in wastewater quality that may occur due to an unpermitted discharge, such as might result from an accidental release of a chemical or from an illicit disposal of hazardous materials,” according to the initial statement of reasons. 

Stringent pathogen controls

In the proposed rule, the SWRCB imposes exacting requirements for pathogen control. Treatment systems must have a minimum design log reduction value (LRV) of 20 for enteric virus, 14 for Giardia lamblia cyst, and 15 for Cryptosporidium oocyst. These LRVs include 4 logs of redundancy to account for an occasional, brief lapse in treatment. However, the LRVs of 20 for enteric virus, 14 for Giardia lamblia cyst, and 15 for Cryptosporidium oocyst must be achieved 90 percent of the time.

Treatment trains must employ at least four separate treatment processes for each pathogen involving at least three different treatment mechanisms. “A treatment process may be credited with no more than 6 log reduction, and at least four processes must each provide at least 1.0 log reduction for each reference pathogen,” according to the initial statement of reasons. 

Three particular mechanisms must be included: membrane physical separation, chemical disinfection, and disinfection involving ultraviolet light. Each mechanism may be validated for no less than 1.0 log reduction.

Up to 2 logs of reduction credits may be received by means of continuous blending of DPR project water with certain groundwater or surface water sources, continuous mixing of DPR project water in a reservoir under certain conditions, and recharge or storage of DPR project water into a groundwater basin under certain conditions.

By comparison, Colorado and Texas — the first two U.S. states to document their procedures for approving DPR — have less stringent log reduction requirements. In its DPR rule that took effect this past January, the Colorado Department of Public Health and Environment requires that projects meet what it calls a log removal target (LRT) of 12 for enteric virus, 10 for Giardia, and 10 for Cryptosporidium. Colorado also provides a process for determining an alternate minimum LRT, which may not be less than 8 for virus, 6 for Giardia, and 5.5 for Cryptosporidium removal.

In its regulatory guidance manual for DPR that it released last November, the Texas Commission on Environmental Quality set minimum LRV treatment levels of 8 for virus, 6 for Giardia, and 5.5 for Cryptosporidium.

Chemical removal requirements

To account for the potential presence of chemicals of emerging concern in municipal wastewater, California’s proposed DPR rule requires that a treatment train “consist of no less than three separate treatment processes, using no less than three diverse treatment mechanisms, for chemical reduction,” according to the proposal. The proposed rule stipulates that a DPR project shall include the following treatment processes in the following order: an ozonation process,  biologically activated carbon (BAC), a reverse-osmosis (RO) membrane process, and an advanced oxidation process (AOP).

“Ozone will break down organic molecules into smaller organic molecules, some of which will be biodegraded by the biological filter organisms in the BAC,” according to the initial statement of reasons. “The ozone/BAC process will reduce the amount of low molecular weight chemicals like acetone and formaldehyde that are not removed by the downstream RO and AOP treatment. The ozone/BAC treatment must precede the RO treatment in the treatment train so that there is enough organic material to support the biological activity of the ozone/BAC. Next in the treatment train is RO, which will remove all sizable molecules. Then AOP degrades most of the remaining low molecular weight material.”

The ozone/BAC process must be designed to provide at least 1.0 log reduction of each of the following indicators: the organic compounds formaldehyde and acetone and the medications carbamazepine and sulfamethoxazole. The BAC must be designed with an empty bed contact time of at least 15 minutes. In certain cases, a continuous blending process may be substituted for the initial ozonation and BAC processes. 

For the RO process, each membrane must achieve a “minimum rejection of sodium chloride of no less than 99.0 percent and an average (nominal) rejection of sodium chloride of no less than 99.2 percent,” according to the proposed rule. Finally, the AOP must be designed to provide no less than 0.5 log reduction of the solvent 1,4-dioxane.

The proposed DPR rule also includes provisions pertaining to operator certification, water safety plans, monitoring, laboratory analysis, engineering reports, operation plans, pathogen and chemical control monitoring and response, compliance reporting, cross-connection control, corrosion control and stabilization, public notification, and consumer confidence reports.

‘Careful and thorough’ approach

Although long in development, the proposed rule will help to ensure the protection of public health as it pertains to drinking water produced by means of DPR, said Darrin Polhemus, the deputy director for the SWRCB’s Division of Drinking Water. 

“This moment has been some time in the making because we have been careful and thorough to produce regulations that ensure, down to a chemical level, that water treated to these standards will be pure and wholesome,” Polhemus said in the SWRCB’s July 11 news release announcing the availability of the advance copy of the proposed rule. “In fact, the extensive treatment requirements we’ve proposed mean that direct potable reuse processes in California will produce water of higher quality and lower risk than many traditional drinking water sources.”

“We’ve seen real enthusiasm and interest from major urban water agencies, who are the ones that will take direct potable reuse forward, adding it to their water supply portfolios to increase resiliency,” added Polhemus. “California has been a leader for years in water recycling, and this last step – going directly from treatment to usage as drinking water – builds on that experience and the expertise of scientists and engineers who have worked with recycled water for many years.” 

Overly conservative requirements

Although supportive of the overall thrust of the proposed DPR rule, the Metropolitan Water District of Southern California (MWD) finds that some of its requirements exhibit a “bit of over-conservatism,” says Heather Collins, the assistant group manager in water system operations for the district, which supplies water to 26 agencies in Southern California. 

For its part, the MWD is partnering with the Los Angeles County Sanitation Districts to develop the Pure Water Southern California project, a planned 150 mgd water reuse effort that is expected to employ DPR as well as indirect potable reuse. Design work on the massive project began earlier this year.

As an example of the conservative nature of the proposed DPR rule, the requirements for pathogen removal are “very stringent,” Collins says. In part, this results from the “extremely conservative” risk models used by the SWRCB to develop the requirements, she notes. 

In effect, the strict requirements will necessitate a “lot of monitoring, a lot of redundancy, a lot of additional treatment, perhaps, to meet those different levels of log removal, depending on how your project is laid out,” Collins says. “It very much translates into increased capital and increased operational costs for agencies, which then could impact the rate payers ultimately.”

While the MWD is “glad” that the SWRCB is doing its best to protect public health by means of the proposed regulation, Collins says, “we also want it to match with current science and data available, to reach that right level of treatment requirement so it can be effectively and efficiently implemented and not be cost-prohibitive.”

With respect to the proposal’s requirements pertaining to treatment train order, the MWD would like to have more “flexibility” to accommodate future technological advances and the development of additional performance data, Collins says. Instead, the proposed rule includes “very rigid” requirements related to pathogen control and chemical removal, she notes.  Inclusion of an “alternative clause” would afford the necessary flexibility to enable utilities to test and validate new treatment processes that potentially could meet public health protections more efficiently or cost-effectively, Collins says. 

That said, the MWD appreciates the fact that the SWRCB agreed to include “more defined pathogen control credits” for projects that blend recycled water with groundwater or surface water, Collins says.

At the same time, the proposed regulation dictates certain treatment process design criteria to an unusual degree, Collins says. “This is the first time we've seen regulations really dive into design criteria.” However, the SWRCB provided “some greater flexibility in this version for biological activated carbon design,” she notes. “They did allow a default of 15-minute empty bed contact time, rather than a 5-minute requirement.”

Similarly, the SWRCB agreed to require monitoring for total organic carbon once every 15 minutes, rather than once every 5 minutes, Collins says.

Completion date uncertain

On July 20, the SWRCB announced that it intends to hold a public hearing on the draft regulations on September 7 in Sacramento. Precisely when the final version of the DPR rule will be released and take effect remains to be seen. 

“The statute directs the Water Board to ‘adopt’ the regulations by December 31, 2023,” says Blair Robertson, a public information officer for the SWRCB. “They are not effective until approved by the Office of Administrative Law, which may happen several months after adoption.”