Making Drinking Water Safe Where Arsenic is Abundant

Arsenic, a naturally occurring chemical found in the earth's crust, can be dangerous to humans when released into drinking water supplies as rocks, minerals, and soils erode. This poisonous, metallic element is found in abundance in the state of Michigan. In fact, the state is one of only seven throughout the United States that hosted three or more demonstration sites during the United States Environmental Protection Agency's National Arsenic Treatment Demonstration Project, Phase 2.

In June 2003, The City of Brown City, Michigan, located about 60 miles north of Detroit, was awarded the first of the US Environmental Protection Agency's (EPA) treatment systems as part of the EPA Arsenic Removal Demonstration Program. As they would at a number of other demonstration sites throughout the country, the EPA and its team of consultants on the Demonstration Program panel selected Severn Trent Services' SORB 33® arsenic adsorption system for use at the site.

Brown City's water system serves a population of 1,300 with water from two wells, Number 3 and 4. Wells Number 1 and 2 were capped when 3 and 4 came on line in the late 1960s. Prior to the demonstration study, both wells provided water on an alternating basis. But during the demonstration, Well Number 3 was reserved for emergency backup and Well Number 4 became the sole source of water. The well has a capacity of 640 gallons per minute (gpm), and the water is delivered to a storage tank at a pressure of 60 psig. The well water has an arsenic (As) level of 19-25 micrograms per liter (μg/L), most in the form of reduced As(III), and also contains a modest level of iron (Fe), typical of Michigan well waters. The pump building, which also houses a motor control center and chlorination unit, required a significant increase in floor space to accommodate the arsenic removal system.

Severn Trent Services proposed to install two of its automated SORB 33® APU 300 package units, each with a capacity to treat 320 gpm or half of the well pump's total flow. The 16'L x 6'W x 9'H skid-mounted units each consist of two 5 1/4 Ø FRP adsorbers and 152 ft³ of Bayoxide® E33 iron oxide media which were installed as modules after the building was completed. Each unit includes 10 automatic valves, five for each adsorber, which are controlled by individual PLCs. The 4 adsorbers operate in parallel flow configuration. Piping for the system is schedule 80 PVC with PVC butterfly valves.

After the pump building was expanded to house the additional equipment, the two APU systems were installed and began operation in April 2004, and their performance has been monitored since then by EPA's contractor.

The SORB 33® system was designed to treat Brown City's unchlorinated water directly from the well pump, although an injection point was installed upstream of the adsorbers during construction. Spent backwash water is discharged through a swale to a nearby county storm water drain. Backwash is initiated by the operator, although the system does have features for automatic backwash on a timed schedule or on high differential pressure across the adsorber vessel. Because the well operates only four to six hours per day, backwash is initiated manually, and the operator ensures the backwash procedure is completed before the pump shuts down.

The system performed well, removing the arsenic and iron in the water. However; after several months of operation, reduced arsenic, As(III), began to break through because the adsorbed reduced iron, Fe (II) lowered the media's ability to oxidize and adsorb As(III). To remedy the breakthrough, the chlorination injection point was moved upstream of the SORB 33 system. As a result, the effectiveness of the Bayoxide E33 media improved, reducing arsenic to <2 μg/L in the treated water, and performance has been consistent since prechlorination was implemented. Manganese is also being removed in the process. Backwashing has been increased to once every two weeks.

With the successful removal of arsenic at Well Number 4, the City has decided to build a second arsenic removal facility and has selected the SORB 33 system. Construction of the new facility, which is being financed by the City, is scheduled to begin in June 2007 and will be operational by the end of the year.