Potable Water Reuse Report

One challenge with Approach 1, is that some emerging chemicals are so new that they lack (1) analytical methods that are needed to detect them, and/or (2) toxicity information. In a recent research study to develop health thresholds, Jessica Steigerwald found that nearly 75% of the compounds of interest did not have sufficient toxicity information to evaluate their health risk. The reason for this is due to the complexity of developing toxicity data, which typically requires years of studies often using animal- and/or cell-based experiments. Snyder highlighted this challenge: “The regulatory paradigm with mammalian toxicology studies can’t keep up with the growing list of chemicals."

Recently, the US EPA announced it would eliminate mammalian-based toxicity testing in lieu of “new approach methods” using non-animal testing including: (1) methods using cultured cells, tissues, or microorganisms, (2) assays testing a chemical’s reactivity with biological compounds like proteins, and (3) computer-based methods (including artificial intelligence) to predict the potential harmful effects of chemicals. Opponents worry that the information developed through the new methods alone (i.e., in the absence of animal testing) may not be as transferable to human health. Proponents argue, however, that it will provide more rapid and cost-effective information and allow more efficient evaluation of chemical toxicity.

Approach 2: Identify Indicators and Surrogates

The second approach refines the list of chemicals developed in Approach 1. The goal of Approach 2 is to identify a set of indicators and surrogates; control of the indicators and surrogates would assure protection against the full list of chemicals in Approach 1. An indicator is a chemical that has greater resistance to treatment and whose removal can demonstrate the effectiveness of treatment against a wider suite of compounds. A surrogate is a parameter that can be measured continuously (e.g., total organic carbon or conductivity) to assess the effectiveness of treatment in real-time. The main benefit of Approach 2 is its potential to reduce the number of compounds to monitor compared to Approach 1

Using indicators for chemical control is akin to reference pathogens for pathogen control. In Issue 2.1, we showed that treatment and monitoring are developed for a smaller number of reference pathogens, whose reduction ensures protection against the broader diversity of pathogens. Approach 2 applies this same logic to chemicals.

Several groups in the US have undertaken efforts to determine appropriate surrogate or indicator compounds for the universe of chemical contaminants. For example, the California State Water Board has assembled a panel of experts (including Drewes and Snyder) to develop monitoring strategies for emerging chemicals in recycled water. Drewes summarized their plan: “We need to evaluate which chemicals have the properties to go through treatment. These are the key indicator groups we should use to assess treatment.” Rather than measuring an ever-expanding list of new compounds, this approach focuses on compounds that provide confidence that the treatment train protects against the diversity of chemicals potentially present in wastewater (Figure 2).

This approach is being carried into many potable reuse regulations in the US. For example, most potable reuse regulations require treatment trains that demonstrate removal of 1,4-dioxane, a low molecular weight, recalcitrant compound that can pass through many treatment barriers. In this case, 1,4-dioxane serves as an indicator for a wider suite of contaminants and provides confidence in the effectiveness of treatment. Many regulations also require monitoring of total organic carbon as a surrogate for removal of trace organics.

Approach 2 can also be used to evaluate compounds that lack occurrence and/or toxicity data. In this case, a compound’s physical-chemical characteristics (e.g., chemical structure, biodegradability, polarity, molecular size, and volatility) are compared to those of existing indicators and surrogates to understand if the barriers that are already in place will also control this new compound. Dickenson said, “Regardless of a compound’s toxicity, I always come back to treatment. What are the barriers that would be able to control that compound? If we have a treatment barrier in place that removes it, then I feel reassured that we can control it.”

How do the approaches work together?

Using PFAS as an example: if we only use Approach 1, we may have a list of hundreds of PFAS compounds to monitor. Using Approach 2, we can refine this list by evaluating the characteristics of the PFAS compounds based on treatability and identify a smaller number of conservative indicators. Controlling for the indicators would ensure that the broader class of PFAS chemicals would also be controlled even if they are not directly monitored. In this way, Approach 1 is used to develop an initial list of relevant compounds and Approach 2 is used to refine the list to a smaller number of indicator and surrogate compounds that can be reasonably measured.

Keeping the list of chemicals up to date

The experts agree that these lists should be continually updated. Drewes emphasized, “Risk assessment is not a one-time effort. It's something you do on regular cycle.” Fortunately, many institutions have implemented on-going programs to identify unregulated chemical threats for potential regulation. For example, every 5 years the EPA develops a list of unregulated contaminants that may require future regulation (Contaminant Candidate List, or CCL). The EPA also implements a monitoring program to support regulatory decisions on unregulated contaminants (Unregulated Contaminant Monitoring Rule, or UCMR). Steigerwald also noted the European Union’s REACH program as a key reference for assessing chemical risk. Beyond the EPA, REACH, and other national institutions, Dickenson added, “We also rely on academics to identify new compounds and predict their toxicity.”

Monitoring Approaches

Current monitoring approaches rely mainly on "targeted" analyses that look for specific, known compounds in water samples using techniques like mass spectrometry. But not only do we have to know the compound exists, we must have a method and standards to measure and quantify it. Targeted analyses do not address how we can control compounds that we don’t know exist or aren’t looking for.

Non-targeted analysis (NTA) refers to a suite of techniques that can identify the presence of chemical compounds in water samples without prior knowledge of the compounds. High-resolution mass spectrometry is one form of NTA that scans water samples to detect and characterize chemical structures that are present. This information can be used to identify compounds, including unexpected or previously unidentified ones.

Bioassays are another type of NTA that evaluate whether a water sample contains chemicals that impart a biological effect on living cells. Bioassays can consider not only the effect of individual chemicals but also mixtures of chemicals that may have synergistic or antagonistic effects. Snyder—a longstanding proponent of NTA—emphasized the need for a multi-pronged monitoring approach that includes NTA in combination with other targeted strategies to understand the broad world of chemical threats.

2) Controlling Chemical Threats

The approach to controlling chemicals needs to be multi-faceted to account for an ever-evolving chemical landscape that includes both knowns and unknowns. Rather than emphasize a single strategy (e.g., treatment alone), most approaches rely on multiple complementary strategies. In addition to targeted and non-targeted monitoring, these strategies include source control, multi-barrier treatment, and blending/dilution (Figure 3). Indirect potable reuse schemes also benefit from the environmental buffer. Thus, no single strategy is expected to be infallible, instead, a collective set of strategies is relied on to reduce chemical threats to negligible levels.