Hexavalent chromium [Cr(VI)] in drinking water is a topic of substantial debate in the State of California and the U.S. However, knowledge regarding the potential cost implications for Cr(VI) treatment has been limited to research with only a handful of drinking water utilities. In order to help fill this knowledge gap, the Water Research Foundation funded project #4450, “Impact of Water Quality on Hexavalent Chromium Removal Efficiency and Cost.” Building on past and ongoing research on Cr(VI) treatment, the specific objectives of this project are to:
(1) identify the impact of water quality conditions on the removal efficiency of Cr(VI) and total chromium with three leading treatment technologies and
(2) develop defensible capital and annual operation and maintenance (O&M) cost estimates for implementing Cr(VI) treatment systems of various sizes that can comply with potential drinking water maximum contaminant levels (MCLs) ranging from 1 to 20 ug/L or parts per billion.
As part of this project, for which a final report is anticipated in 2013, Water Quality & Treatment Solutions, Inc. (WQTS) developed a cost estimation tool to help drinking water systems estimate a range of potential costs to remove Cr(VI) from their water based on system-specific information about the impacted well, water quality, and residuals handling. This tool estimates potential cost ranges for three Cr(VI) technologies that have emerged as leading approaches with respect to feasibility and cost:
- reduction with ferrous iron/coagulation/filtration (RCF),
- weak base anion exchange (WBA), and
- strong base anion exchange (SBA)
Reduction/Coagulation/Filtration. The RCF process is similar to typical coagulation/filtration processes used for groundwater treatment, although a reductant (ferrous sulfate or ferrous chloride) is used to convert Cr(VI) to Cr(III) and provide for iron/chromium floc formation that can be removed by filtration. In addition, oxidation of residual ferrous is also necessary prior to filtration. This can be achieved with aeration or with a low dose of chlorine.
Weak Base Anion Exchange. WBA resin has emerged as a leading technology for Cr(VI) removal due to the high capacities and use of WBA as a single-pass approach. Operational attention required for a WBA process is significantly less than that for an RCF process. pH adjustment is a necessary condition for WBA resins. Spent WBA resins may be challenging for disposal depending on the constituents that accumulate on the resin during operation (e.g., uranium). Overall, WBA offers the simplest approach for small systems with constraints on operational complexity, brine disposal, and space available, but it can require high chemical usage and potentially cost more than RCF or SBA.
Strong Base Anion Exchange. SBA resin is operated with periodic regeneration using a high concentration of sodium chloride, NaCl. Significant full-scale experience around the country exists for SBA for nitrate and arsenic treatment. During regeneration, SBA yields a brine waste that can require treatment to precipitate the hazardous chromium component from the brine before it can be disposed. SBA can be an attractive approach if brine disposal is feasible and removal of other constituents such as nitrate, perchlorate, and arsenic is desirable.
IMPORTANT NOTE: This cost tool was developed by Water Quality & Treatment Solutions, Inc. (WQTS), and is made publicly available with financial support from the Water Research Foundation (WaterRF). This tool is provided solely for informational purposes. The calculations are based upon literature, empirical experience, and research, but are not a substitute to site-specific data collection and economic analysis by a licensed design professional. WaterRF and WQTS provide no guarantee of the accuracy of these calculations and assume no responsibility for their use.