Authors: Taj Dufour, Engineering Manager/Chief Engineer, Soquel Creek Water District; Mike Waite, Technical Director, Ionex SG; Chad Seidel, Vice President, Corona Environmental Consulting, LLC; Craig Gorman, Water Process Engineer, Corona Environmental Consulting, LLC
On July 1, 2014, California became the first State in the nation to mandate an MCL of 10 ppb for hexavalent chromium (chromium-6). The MCL of 10 ppb as chromium-6 requires quarterly compliance monitoring at individual water sources or entry points to the distribution system after treatment. Compliance with the MCL is to be determined by a running annual average of quarterly samples. Water utilities must begin compliance sampling by the end of 2014.
Starting in 2012, Soquel Creek Water District (District) in Santa Cruz County and Corona Environmental Consulting, LLC, in collaboration with the Water Research Foundation, began implementing a small-scale bench and field pilot study to find an optimum treatment for chromium-6. Approximately one-third of the District’s water supply is impacted by naturally occurring chromium-6. The District already treats for iron and manganese, as well as arsenic, in other well supplies.
The imperatives were to find a reliable, proven technology and minimize capital and O&M costs. Criteria included: a system that’s easy to deploy, utilizes a standard, approved ion exchange resin, with components that are NSF 61 certified, and a demonstrated capability to remove chromium-6. Also, a technology that produced the smallest possible waste volume for disposal was considered especially critical.
For the full-scale demonstration study, the District selected a strong-base anion exchange technology developed by UK-based technology company Ionex SG, LLC. While the treatment method is new to the District, the system, known as the IX-C™, was chosen for its modular design, as well as its cutting-edge brine treatment process that showed promise as an efficient method to handle the waste generated. It is expected that such advantages would improve the treatment economics. Ionex SG, LLC mobilized quickly with a full-service plan to meet the District’s demanding demonstration specifications.
The District will shortly deploy the system as a temporary full-scale (1,000 gpm) facility until a permanent facility can be evaluated under the California Environmental Quality Act (CEQA), as well as design, permits and construction — a process that is expected to take approximately two years.
Installation & Progress
The 1,000 gpm demonstration system was designed, built and connected over a six month period. This streamlined installation was enabled by the system’s modular, plug-and-play configuration that requires just a simple inlet/outlet connection, along with a power connection.
The installation process included construction of a half-mile of raw water main and on-site utility modifications. Although the District’s chromium-6 impacted wells are geographically dispersed, construction of a centralized strong-base anion exchange treatment facility rated at 2,000 gpm is planned to treat any two of three wells in the vicinity. Approximately one mile of raw waterline is required to convey untreated water to the centralized treatment facility. A fourth chromium-6 impacted well located in a separate pressure zone will be placed on standby status with the State Water Resources Control Board’s (SWRCB) Division of Drinking Water (DDW), and an intertie between the two pressure zones is planned to supplement the lost production.
Outcome and Results
Through bench and pilot studies conducted in collaboration with Ionex SG, LLC and Corona Environmental Consulting, LLC, the District demonstrated that the strong base anion exchange treatment process is by far the most suitable technology to use given its water quality. Not only did the pilot testing capably reduce chromium-6 levels and produce water that exceeds the requirements of the DDW, but the equipment also provided confirmation for the chromium treatment capacity of the plant, and demonstrated the feasibility of brine reuse and waste volume minimization and treatment.
In particular, the technology demonstrated these advantages during the pilot study:
First, the study showed that it was feasible to remove the chromium-6 from the water to treatment objective of 8 ppb with greater than 99.99% water efficiency.
Second, the small system footprint exceeded expectations. This is important because available vacant land within the District is limited. Other possible treatment processes require larger footprints to implement, or large volumes of additional chemicals. The modular configuration also allows for flexibility and quick deployment. The 1,000 gpm system, including associated brine treatment tanks occupies just under 800 sq. ft. of space.
Over the next six months, the District will continue to work with Ionex SG, LLC to optimize the brine treatment technology – important activities that will inform the design of a full-scale permanent treatment facility.
Finally, in part due to the successful pilot, the District’s Board decided to set a treatment goal of 2 ppb where treatment is implemented, and directed implementation of one full-scale 2,000 gpm treatment facility at the District’s Bonita Well Site.