A draft California Public Health Goal could lead to a requirement for California water agencies to reduce hexavent chromium to the ultra-low level of 0.02 parts per billion.
The naturally occurring suspected carcinogen is present in as much as a third of the state’s groundwater supply. The federal government currently only regulates total chromium levels at 100 parts per billion. The draft goal, released in December 2010, was reduced from a previously suggested 0.06 parts per billion “to account for increased sensitivity associated with early-in-life exposures,” according to a state study. The California Dept. of Public Health could set drinking-water maximum contaminant level standards as close to the draft standard as is “economically and technically feasible” by 2013.
James Borchardt, vice president and water technology expert for the Southern California office of MWH Americas Inc., said, “Agencies would have to spend millions on capital costs and even more on operating costs to reach these levels.”
Analytical procedures would require improvement because “0.02 parts per billion is way below current detection limits,” said Leighton Fong, a civil engineer at Glendale Water and Power.
GWP currently is building two demonstration plants. The Weak-Base Anion system plant pushes 425 gal per minute of water through a pair of lead/lag pressure vessels in series with upstream acid addition. WBA is easier to implement on individual wellheads but becomes less effective the longer it runs and results in a toxic sludge.
The other system, Reduction Coagulation Filtration, adds excess ferrous iron to 100 gpm of water to break down chromium 5 to chromium 3, which is then precipitated along with the resulting ferric iron. Filtration removes floc after aeration and coagulation. Adding microfiltration and reverse-osmosis treatment could attain even lower levels, he says.
The RCF system is better suited to large-scale treatment plants at which agencies have real estate to devote to settling basins and multiple treatment stages, Fong says.