Click here to enlarge imageIon exchange columns can achieve 95%+ removal of arsenic but may be too complex for single wellhead operations.
The use of coagulants followed by either pressure filtration or membrane filters also are viable processes. They involve the addition of ferric chloride to the raw water followed by a short mixing step to bind the arsenic to the ferrihydrite precipitates that form. These precipitates are then removed by low-pressure granular filters or low-pressure membranes (frequently microfiltration units, although ultrafiltration units also have been used.) This process can be water efficient and generates only small quantities of ferrihydrite residuals for disposal. However, equipment costs are expected to be higher than sorption technologies, especially for smaller systems.
Specialty Sorbents
A tremendous amount of research and development of specialty sorbents for arsenic removal has been conducted within the past five years. A number of iron- and aluminum-based adsorbents have been developed in the past few years to overcome the shortcomings of the traditional treatment techniques.
These "designer adsorbents" have several distinct advantages over the older technologies. First, they have been formulated to preferentially adsorb arsenic ions as opposed to a range of constituents such as sulfates and nitrates that could compete for the adsorption sites (and prematurely exhaust the media). Second, some are meant to be disposable media and simply discarded into a landfill as opposed to regenerated on-site by the utility. A fresh column of media can then be placed into service.
Treatment Cost Estimates
Regardless of the technology chosen, the cost of arsenic removal is significant - both in initial capital costs as well as ongoing operational and maintenance (O&M) costs. For example, the projected capital costs of a 1-mgd (700 gpm) well can range from $0.75 to $1.50 per gallon, depending upon the process and raw water quality. Land acquisition, if needed, may further increase this cost. Annual O&M costs are forecast to be $100,000-$180,000 for a 1-mgd facility. For a 0.1 mgd (70 gpm) facility, capital costs will be approximately $75,000-$150,000 with annual O&M costs in the $15,000-$30,000 range.
About the Author:
T. David (Tim) Chinn is a Vice President and National Director for Potable Water with HDR, a firm providing architecture, engineering, consulting and project development services. HDR's Water Program is ranked as one of the Top 10 in the nation by Engineering News Record (ENR.) Chinn is intimately familiar with the Arsenic Rule, having chaired the American Water Work Association's (AWWA) Arsenic Task Force since its inception in 1992. He is nationally recognized in water issues and has testified before numerous Congressional and US EPA Panels concerning the regulation of water supplies.