Letter to the Editor

Najm's paper, “The National Cost of Compliance with a Drinking Water MCL for Hexavalent Chromium” published in the September 2025 edition of AWWA Water Science, offers readers information needed to assess the costs of introducing a nationwide regulatory limit on the presence of hexavalent chromium (Cr(VI)) in drinking water supplies. Transparency in such analyses is essential for policymakers, utilities, and the public. However, the paper contains important inaccuracies and omissions that result in the costs being significantly overstated. A credible cost analysis should begin with a comprehensive overview of all available treatment methods. If a method is excluded, the rationale for doing so should be explicitly stated. Najm's paper does not meet this standard. Specifically, it excludes consideration of a major technological innovation in Cr(VI) treatment based on the in-situ generation of stannous, which has been widely demonstrated at scale in the field since 2018. In 2023, the technology was designated a Best Available Technology (BAT) by the State of California (State of California 20231). Najm, who cites this same State of California 2023 report, does not reference this technology despite the fact that it has been reported to have lifetime costs that are an order of magnitude lower than the traditional technologies presented in the paper. Accurate characterization of the treatment systems for Cr(VI) that could be employed to achieve regulatory compliance, and the related key operating parameters should also be considered when estimating the cost of regulatory compliance. Najm's paper does not meet this standard on at least two counts. First, the treatment system he characterizes as reduction, coagulation, and filtration (RCF) is, in fact, reduction, coagulation, oxidation and filtration (RCOF). Oxidation is a critical part of the Cr(VI) treatment process required to convert excess ferrous reagent into insoluble ferrous hydroxide. The operating costs of this and the associated capital costs of an oxidation tank and necessary ancillary equipment are not made explicit in Najm's paper. Second, the addition of excess ferrous reagent generates sludge, which is defined as a hazardous material by Najm; however, the paper provides no explanation of how sludge volumes or disposal costs are calculated. In addition, Najm's model does not include the capital and operating costs of online, real-time monitoring in the RCOF system specification necessary to ensure compliance with regulatory standards. Furthermore, without providing a rationale, Najm has applied a 30-year amortization rate to the capital costs in his calculation of lifetime costs. Industry practice typically applies a 20-year amortization rate (California State 20242). This difference means that Najim's lifetime cost estimates for the traditional water treatments he cites are significantly understated. To derive a true total system cost, we must reduce the years for amortizing costs and then mark up the adjusted number by 250% (a percent taken from Najm's paper) to reflect construction costs and the professional fees of consulting engineers, etc. The exclusion of in-situ stannous-based RCF, a Best Available Technology, from the paper has important cost implications for municipalities as they consider treatment technology, public funding requirements, and the allocation of that funding to communities impacted by the new regulatory limit. in-situ stannous RCF is a far simpler treatment system than RCOF, resulting in significantly lower equipment costs. The fundamental reason for this is the higher reductive power of stannous compared to ferrous. This results in a far faster conversion of Cr(VI) to Cr(III) at stoichiometric dosing rates. Subsequently, stannous hydroxide is a far more effective coagulant than ferrous, especially in the presence of silica, which is present at elevated levels in many California water sources. Both characteristics of in-situ generated stannous-based RCF contribute to lower capital and operating costs and, therefore, far lower lifetime costs than ferrous-based RCOF. A costing of in-situ stannous-based RCF would also exclude the capital and operating costs of building and equipment required for safe chlorine dosing, storage, and handling equipment as these are not required. In addition, the RCF system produces far less sludge that is nonhazardous and can be disposed of in a general landfill or beneficially reused. These capabilities of stannous-based RCF significantly reduce both equipment and operating costs. Finally, Najm's RCOF process model does not permit a calculation of equipment costs based on treating a by-pass stream of water to < 1 parts per billion (ppb)—a level of treatment that is possible with in-situ stannous generation at a minimal marginal operating cost—and then blending this treated stream with the untreated water to produce a finished product flow with Cr(VI) below the regulatory limit. By reducing the flow to be treated with stannous has a significant impact on equipment costs. The core of the in-situ stannous RCF treatment system is a flexible reagent dosing module capable of treating to < 1 ppb a broad range of flows of Cr(VI) impacted water sources (100–4000 gal per minute (gpm)). This standardized system, combined with the high selectivity of the stannous reagent toward Cr(VI), dramatically simplifies the design and operation of this treatment technology compared to the RCOF system. The RCOF system requires a bespoke design for each deployment site to accommodate differences in the water matrix composition, which impacts the entire reduction, coagulation-oxidation, and filtration process. By contrast, the RCF treatment system is delivered to the site preassembled, pretested, and containerized in a manner that has a small footprint. The standardized packaged system significantly reduces the percentage markup for construction and professional services typically added to water treatment technologies to derive a total system cost. If the paper had included an unbiased review of the in-situ stannous RCF treatment system, there would also be a mention of the value in terms of operating costs. The absence of chlorine use and the relative simplicity of the RCF system and its high degree of automation will require considerably less labor for supervision and maintenance than RCOF, which has multiple points of failure that risk noncompliance. In summary, the exclusion by Najm of any analysis of the cost of the in-situ stannous-based RCF, which has significantly lower lifetime costs than traditional treatment technologies, has resulted in a significant overstatement of the lifetime costs of treating Cr(VI) in California and, by extrapolation, the whole of the USA. The paper risks compromising the setting of regulatory standards because the costs of achieving regulatory compliance appear significantly overstated because the costing of in-situ stannous RCF treatment system has been excluded from his analysis. At the same time, the costs of RCOF are significantly understated. A rigorous analysis of the lifetime costs of removing Cr(VI) with a BAT designated RCF process based on in-situ generated stannous reagent would better inform policymakers, regulators, and utilities in setting an appropriate nationwide regulatory limit for Cr(VI) as to the costs that this would impose on communities, those who must fund the cost of bringing water systems into compliance, policy makers, and utilities' choice of Cr(VI) treatment technology to meet that regulation. It would also better inform those in California who are currently having to decide which treatment system to adopt in order to become compliant with the recently introduced MCL for Cr(VI). Vladimir Dozortsev: formal analysis, investigation, methodology, writing – original draft, writing – review and editing. The author declares no conflicts of interest. The author has nothing to report.