By Matthew Wirth
Removing arsenic is relatively simple, but the projects can be complex. The prime purpose of this article is to share one dealer’s experience in helping their customer so that others can learn from the process and to help everyone streamline the placement of arsenic systems by paying attention to details.
The Immanuel Lutheran Church rests on a hilltop in rural Minnesota, a few miles northwest of Hutchinson. The site holds a 45-student school, a parsonage and a church with meeting facilities. According to US EPA and the Minnesota Department of Health (MN DOH) maximum contaminant level (MCL) requirement, Immanuel had too much arsenic in its water. (The current MCL in Minnesota is <10 ppb). For a small congregation, this was a huge challenge – a challenge that included months of discovery, multiple contractors and contact with several state agents from MN DOH and vendor engineers.
At first glance, the job appeared to be a pretty simple problem. The water chemistry showed that the arsenic was just over the MCL and the well produced only 15 gallons per minute (56.78 liters per minute). Since the church was considered a public, non-transient water supply, the treatment required approval from the MN DOH. After months of extensive investigation and multiple quotes, the church contacted a local water treatment company to correct their problem.
When the water specialists first visited the site, they realized that there was more to this project than providing water treatment. The distribution lines from the wellhead ran in three different directions – all buried underground – with no central connection location. Providing central treatment for all three facilities’ lines required directional boring into the basement of the parsonage and extensive backhoe work to convert the multiple distributions to a single feed line. (This is Minnesota where water lines lay six to eight feet underground to prevent freezing.)
Initially, the State of Minnesota suggested that an easy solution to the arsenic problem at Immanuel Lutheran was oxidation and iron filtration. In arsenic removal, iron is the number one ally. It is a chemical reality that arsenic (V) binds to iron creating ferric arsenate. The industry norm for removing arsenic by coagulation precipitates arsenic and iron to create ferric arsenate and then mechanically strains this particulate from the water.
When the church sent in quarterly water samples for testing, the water panel included the normal constituents (i.e., iron, manganese, pH, arsenic). Unfortunately, those reports did not provide all the information needed to design the proper arsenic treatment system. Test results indicated the following:
- Total Arsenic: 14 ppb As * Manganese: 0.159 ppm
- Total hardness: 29 gpg pH: 7.41
- TDS: 495 ppm Iron 3.89 ppm or 3,890 ppb
- Alkalinity: 360 ppm
*The application design converts converted all arsenic, to arsenic V, to maximize the bed life of the adsorptive media. Projected bed volumes increase significantly for most adsorptive technologies if the arsenic is converted to its pentavalent state.
The rule of thumb for the ratio of arsenic to iron is 1:20. In other words, for every ppb of arsenic, 20 ppb of iron is required. At Immanuel, the arsenic was 14 ppb and the iron was 3,890 ppb (3.89 ppm). At first glance, there is a 1:277 ratio of arsenic to iron. Removing the iron should easily reduce the arsenic to below the MCL.
In arsenic adsorption, the arsenic (V) binds to iron sites on the media. The issue is ensuring that there is a proper ratio of iron to arsenic and maintaining that ratio to effectively reduce the arsenic below US EPA and individual state standards. One common omission in calculating the proper ratio is building in a factor for competing constituents such as phosphate (as PO4-3). Looking at The Periodic Table, one will find phosphorus located directly above arsenic. Arsenic and phosphorus are in the same chemical family and will compete for the available adsorption sites on the available iron. Iron cannot tell the difference between the two.
New to arsenic treatment, the selected water treatment company sought assistance from two additional companies that specialize in arsenic issues for guidance to ensure that they covered all the bases. Utilizing an outside testing lab and one company’s onsite laboratories, it was recommended to reconfirm the iron, arsenic, pH and manganese, as well as testing for phosphate and silica. Through this testing, they discovered a phosphate level of 1.20 ppm in Immanuel’s water. Since phosphate and arsenic are similar, the ratio of 1:20 remained in place.
The presence of 1,200 ppb (1.20 ppm) of (PO4) phosphate required a new calculation:
- The phosphate and arsenic together were 1,214 ppb.
- At the ratio of 1:20, the iron requirement for coagulation/filtration was 24,280 ppb of iron.
- Expressed in common values, the iron requirement would equal 24.28 ppm for this technology to work.
- The water only had 3.89 ppm of iron.
- To maintain the correct ratio would require ferric chloride injection and subsequent removal of over 20 ppm of iron through filtration.
- Injection of ferric chloride for a public system required the supervision of a licensed operator, which was a budget buster for the church – not to mention the challenge of filtering that much iron.
Workable solutions through thoughtful planning
Had the water dealer not taken the extra step to have the water tested for all the necessary constituents, they would have likely designed the system to remediate the arsenic by way of iron removal. Had the state approved this widely known technology, the system would likely fail. .
The company went into this project knowing that it was a learning process for them, the customer and the MN DOH. Small-scale (see Flow Schematic) projects like this one are common, but far from simple. These projects are complex and require knowledge, practice and extreme attention to details. In the end the final project included:
- Directional boring into the basement of the parsonage to connect into a central treatment location
- Backhoe work to connect all the distribution lines to create a single treated source
- Installation of three 10-inch diameters filtration units plumbed in parallel with an 18” D x 65” T aeration system
- A twin alternating 10”D x 54”T water softener
- A lead/lag 12”D x 52”T arsenic adsorptive system
Figure 1. Flow schematic
Conclusion
In the end, the project utilized proportional air make-up and retention to convert the ferrous iron to ferric for filtration. In addition, in this application, the oxidation process converted As3 to As5. A percentage of the As5 combined with the available ferric iron to create ferric arsenate, which came out of the water as part of the filtration process. The twin alternating water softeners provided continuous soft water to the facilities and helped to keep the non-backwashing arsenic adsorptive system clear of debris and iron residuals. The arsenic adsorption system used a hybrid iron media in a lead/lag configuration to ensure little or no slippage through the treatment train.
Looking at this job, one would question, “Why do it? It sounds like a time and money waster.” It is not being ignoble; wanting to make profit as a business. On the other side, part of being in business is learning and expanding one’s experience. When looking at projects involving level-one contaminants, the business side looks to make spreadsheet decisions and dehumanize the issue. The ethical side looks at the human component and those affected by drinking unsafe water. Arsenic may not outwardly harm everyone but it does greatly harm someone. It’s not just about the statistics. If one does their due diligence up front, they can be profitable and rewarding at the same time. Having proper water chemistry reports, flow rate information and a knowledgeable system provider makes for a smooth installation. Keeping state and local agents informed about the process and the application requirements eliminate issues on final inspection. Pay attention to details, play by the rules, read the instructions, and everyone comes out a winner.
About the authors
Matthew Wirth works for Layne Christensen in Commercial Sales in the Water Technologies POE/POU Division, responsible for the region west of the Mississippi River. He is a 31-year professional of the water industry and an active trainor for several national organizations. Wirth has extensive experience in light C&I, PEO and POU problem water applications. He is a graduate of Concordia University in St. Paul, MN with a B.A. Degree in organizational management and communications. Wirth received his engineering training at the South Dakota School of Mines and Technology in Rapid City, SD. He can be reached at [email protected] or cell phone, (319) 333-4174.
About the companies
Layne Water Technologies (www.laynewater.com) Layne owns the LayneRT adsorptive technology and offers it in multiple residential and commercial configurations through a network of approved professionals. They can be contacted at (800) 216-5505. Finken Water (www.finkens.com) Companies (est. 1961) offer commercial water treatment installation, services and systems, in addition to carrying a complete line of residential equipment. The company’s home office is in St. Cloud, MN and they provide service to a 100-mile radius area with their satellite offices. They can be reached at (877) 346-5367. Hellenbrand, Inc. (www.hellenbrand.com) provides proprietary demand air aeration systems for oxidation and iron control with multimedia filtration and patented air make-up in the Iron Curtain product line. In addition, they offer multiple tank configurations for softening/IEx in the ProMate and H Series product offerings. Contact number is (608) 849-3050.