The San Joaquin Valley of California is approximately 220 miles long and varies in width to a maximum distance of approximately 80 miles. The occurrence of groundwater over the entire extent of the valley is abundant. Recharge for the varied depths of aquifers is from precipitation in the form of rain and the snow pack from the western Sierra Nevada Mountain Range. High capacity wells constructed in the valley range in depth from 200 to 2,000 feet. For the most part the water quality is highly mineralized due to interaction with geological conditions as it travels westward for great distance and depth. Intermingled with the various unconsolidated geological deposits are layers of organic material.
In many areas there is a presence of arsenic, an undesirable element, for which the maximum contaminant level (MCL) was recently lowered by the United States Environmental Protection Agency (EPA) from 50 ug/l to 10 ug/l affecting large and small communities across the country.
The City of Hanford, California, and nearby industries, as well other municipalities, are dependent on groundwater for a major portion of their water supply. In l984 a study was performed by the University of Houston under sponsorship of the EPA to address arsenic removal from drinking water. The resulting report stated that the wells in the Hanford area all contain varying degrees of arsenic. The study also revealed that arsenic was present at depths of 1,500 feet and construction to greater depth would not be without risk of arsenic contamination. The cost of constructing wells to such depths as well as the associated risks of capacity and quality were strong arguments in favor or treatment of existing wells.
Filtronics entered into discussions with a company located in the Hanford area. The company had reviewed several reports concerning the various methods of treating water for arsenic removal. The methods discussed were activated alumina adsorption, reverse osmosis, electro-dialysis, ion exchange, oxidation and filtration. After review of the reports company management concluded that the process of oxidation, coagulation, flocculation and filtration would offer the most practical and economical solution. Filtronics was selected because it had pioneered an arsenic removal system in California with its Electromedia I filtration process. The Electromedia I process is capable of achieving up to 70% reduction of arsenic. However, company management established a goal of 100% removal or to non detectable levels. To achieve this goal, it was recognized that additional pretreatment would be required.
Discussions culminated in a pilot test program in which the goal was set at 100% removal of arsenic.
The pilot plant consisted of the following processes and equipment:
- Rapid mix
- Electromedia I Process
- Analytical - Hach DR 2000
The arsenic level found in the raw water was analyzed to be at 58 ug/l. All tests of raw and filtered water were performed on site with the Hach DR 2000 Spectrophotometer and split samples were sent to a laboratory.
As previously stated, arsenic removal may be achieved through reverse osmosis; ion exchange; activated alumina; and oxidation, adsorption, utilizing iron and aluminum salts, and filtration, utilizing Filtronics Electromedia I adsorptive media filtration process.
The most cost effective process was determined to be Filtronics oxidation, adsorption and filtration method.
THE Electromedia Process
The raw water is oxidized utilizing chlorine as the oxidant and ferric chloride and polymeric aluminum silicate sulfate (PASS) for adsorption. The chlorine and ferric chloride are introduced in front of a one-minute detention and mixing vessel. The PASS sulfur dioxide are introduced at the exit of the first reaction vessel and in front of the second one-minute detention and mixing vessel. The flow from the second reaction vessel is directed to Filtronics Model GWT-1 gravity water treatment system consisting of:
- Long chain organic polymer addition to aid coagulation and flocculation
- Rapid mix, 1000 sec -¹ velocity gradient using counter current mixing with a 20-second retention time to maintain consistent mixing and thorough dispersion of the polymer
- Flocculation, low energy, variable speed turbine, 50 second velocity gradient, 20-minute retention in a baffled chamber to produce a slow, random circulation, maximizing the flocculation process
- Clarification with Filtronics exclusive SuperSlant parallel plate clarification at very low velocities (600 gallons per day per square foot)
The product of the GWT treatment system is pumped through Filtronics Electromedia I iron and manganese adsorptive media. Recent testing demonstrates that Electromedia I adsorptive characteristics extend to a wide range of heavy metals, including arsenic.
Raw water arsenic levels of 52 to 56 ug/l were consistently reduced to 4 ug/l or less.
The tests were conducted over a three-week period. The final results of testing yielded the following:
Most groundwater found in the Hanford area has varying degrees of obnoxious smelling hydrogen sulfide. An ancillary effect of the Filtronics treatment process is the removal of hydrogen sulfide and some forms of organic’s from the water.
Electromedia I has demonstrated its ability to remove arsenic concentrations to levels approaching 50% removal. With the addition of ferric chloride, removal can be increased to 70%. With the addition of a gravity water treatment system, consisting of rapid mix, flocculation and clarification, arsenic levels can be reduced to almost non-detect.