Meeting arsenic MCLs in northern California wine country
Wine producers in the northern California wine country, including Sonoma, Napa, Lake and Mendocino counties, encounter high levels of arsenic in groundwater extracted for use in wine processing and irrigation. These producers must lower arsenic levels to newer drinking water standards set by the U.S. Environmental Protection Agency (EPA) and California’s Department of Health Services. Arsenic removal is also indicated to ensure product safety and to maintain customer confidence in wine products.
Ongoing water quality compliance testing has confirmed the presence of arsenic in water systems in this region. Arsenic, which has been implicated as a cancer risk, is a metallic contaminant of greatest concern in the western U.S. Experts believe that water in some geologic formations acquires arsenic by flowing through bedrock and dissolving iron or manganese oxides and trace amounts of arsenic under anoxic conditions. The arsenic in this area appears as two species: arsenic V and arsenic III. The water also demonstrates an elevated pH and high concentrations of competing contaminants such as silica.
For many years, federal and California drinking water standards for arsenic were 0.05 mg/L. Following reassessments, the EPA established a federal Maximum Contaminant Level (MCL) of 0.01 mg/L that the agency required states to adopt by 2006.
Graver Technologies LLC, in cooperation with its water treatment partners, has installed more than a dozen state-of-the-art treatment systems that use MetSorb HMRG adsorbent media to remove arsenic in the Northern California wine country. They set a goal of meeting these new state and federal standards and further lowering the levels of both arsenic species to the maximum extent feasible. Treatment options for removing arsenic identified by the EPA include adsorption, lime softening, ion exchange, reverse osmosis and coagulation/filtration.
Adsorption technologies have been extensively evaluated and are increasingly recognized as the most economically and operationally feasible treatment processes for small water systems such as those in the four wine-producing counties.
Adsorption technologies present a number of benefits, including: lower capital costs (equipment and installation, and small footprint); reduced operational and maintenance activities (less operational oversight, less mechanical/electrical sophistication); and reduced waste generation (minimal backwash, no waste sludge generation).
When evaluating treatment options for arsenic, minimal waste generation and waste handling significantly influence the selection criteria. Uniquely associated with nano-titanium oxide adsorbents, negligible leaching of arsenic from exhausted media supports non-hazardous disposal classification, even under the rigorous Waste Extraction Test required under California disposal regulations.
To define the treatment effectiveness of an adsorbent technology in source water containing arsenic, a number of small water systems in northern California measured arsenic levels prior to and after onsite treatment, presented in Table 1 (page 13).
A typical treatment design consisted of two 36-by-72-in. fiberglass reinforced plastic (FRP) vessels plumbed in series charged with 25 cu ft of adsorbent media. The adsorbent media chosen for the testing program was the patented nano-titanium oxide (MetSorb brand from Graver Technologies) based on results realized in previous full-scale arsenic treatment installations. The treatment system was designed to accommodate 100% of the 60 gal per minute well pump capacity. A prefiltration step, using traditional 25-micron bag filters, was installed to keep sand or dirt from the well from accumulating on the adsorbent media.
The two FRP vessels containing the MetSorb adsorbent media were plumbed in a series (lead/lag) configuration. This design allows the lead vessel tank to act as the “worker” tank for arsenic adsorption while the lag vessel acts as the “guard” column, providing adsorption capacity as the lead column reaches exhaustion. A flowmeter and flow totalizer is typically installed after the treatment system to capture the total quantity of water treated. In this specific situation, the lead adsorption vessel was capable of treating a total of 7 million gal of groundwater prior to exhaustion.
These systems have been extremely effective in removing arsenic and co-occurring metallic contaminants of concern such as manganese, silica, vanadium, selenium and uranium. The data concluded that the nano-titanium oxide adsorbent media effectively removed both arsenic species to below the safe drinking water MCL. The adsorbent was very effective despite high pH levels in the water and high concentrations of competing contaminants such as silica. The adsorbent media did not demonstrate the release of arsenic often association with traditional iron-based adsorbent media. A graph of post-treatment data is presented in Figure 1.
The continued application of adsorbent technologies is an efficient and cost-effective treatment solution to remove arsenic from groundwater. It is extremely useful for small water systems where financial and human resources are limited and cost-effective, easy-to-operate treatment solutions are needed. It is recommended that a characterization of the saturated media be conducted for proper disposal consideration under the Federal Resource Conservation and Recovery as well as state and local disposal restrictions.
Author’s Note: Additional information provided by Gene Broderic, NorthCoast Water Treatment, Santa Rosa, Calif. Broderic can be reached at [email protected]