Arsenic in Drinking Water
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Ion exchange (IX) is a physical-chemical process in which ions are swapped between a solution phase and solid resin phase. If As(III) is present, it must be oxidized to As(V) in order for IX to be effective.
The solid resin is typically an elastic three-dimensional hydrocarbon network containing a large number of ionizable groups electrostatically bound to the resin. These groups are exchanged for ions of similar charge in solution that have a stronger exchange affinity (i.e., selectivity) for the resin.
In drinking water treatment, this technology is commonly used for softening and nitrate removal. Arsenic removal is accomplished by continuously passing water under pressure through one or more columns packed with strong-base anion (SBA) exchange resin. These resins are insensitive to pH in the range 6.5 to 9.0.
The exchange affinity of various ions is a function of their net surface charge. Therefore, the efficiency of the anion exchange (IX) process for As(V) removal depends strongly on the concentration of other anions, most notably sulfate and nitrates. These sulfate and nitrates and other anions compete for sites on the exchange resin according to the following selectivity sequence.
SO4-2 > HAsO4-2 > NO-3 , CO3-2 > NO-2 > Cl-
High levels of total dissolved solids (TDS) can adversely affect the performance of an IX system. In general, the IX process is not an economically viable treatment technology if source water contains over 500 mg/L of TDS or over 50 mg/L of sulfate (SO4-2). Although these relationships will not be exactly the same for all water sources, it does provide a general indication of the impact of TDS and sulfate on IX treatment.
Hydraulic considerations associated with IX include empty bed contact time (EBCT) and headloss. The recommended EBCT range is 1.5-3 minutes. EBCTs as low as 1.5 minutes have been shown to work in some installations. The presence of suspended solids in the feed water could gradually plug the media, thereby increasing headloss and necessitating more frequent backwashing. Therefore, pre-filtration is recommended if the source water turbidity exceeds 0.3 NTU.
One of the primary concerns related to IX treatment is the phenomenon known as chromatographic peaking, which can cause As(V) and nitrate levels in the treatment effluent to exceed those in the influent stream. This can occur if sulfate are present in the raw water and the bed is operated past exhaustion. Because sulfate is preferentially exchanged, incoming sulfate anions may displace previously sorbed As(V) and nitrate. In most groundwaters, sulfate are present in concentrations that are orders of magnitude greater than As(V). Therefore, the level of sulfate is one of the most critical factors to consider for determining the number of bed volumes that can be treated.
A useful technique for avoiding chromatographic peaking is to perform careful monitoring of the effluent stream during startup. Then, based on the analysis, determine a setpoint for the total volume treated before breakthrough occurs. This volumetric setpoint would then be used to trigger the regeneration cycle. Regular monitoring of the column effluent should be continued to ensure that loss of capacity in the media does not lead to premature breakthrough. Frequently, the volumetric setpoint is based on the breakthrough of nitrate. The kinetics of breakthrough are rapid; therefore a margin of safety should be provided or a guard column should be used in series with the IX column.
Another concern is resin fouling. Resin fouling occurs when mica or mineral-scale coat the resin or when ions bond the active sites and are not removable by the standard regeneration methods. This can have a significant effect on the resins capacity as the media becomes older. Replacement of the media or reconditioning may be needed after a number of years.
Resin can be regenerated on-site using a four-step process: (1) backwash; (2) regeneration with brine; (3) slow water rinse; and, (4) fast water rinse.
IX will produce liquid residual consisting of the backwash water, brine solution, and rinse water. The spent regenerant may contain high levels of arsenic and may need to be treated before disposal. In accordance with RCRA, systems are required to determine whether the waste is hazardous using knowledge of the waste generation process, analytical testing, or a combination of both. One analytical method, the Toxicity Characteristic Leaching Procedure (TCLP) (EPA Method 1311), is designed to determine the mobility of both organic and inorganic analytes present in liquid, solid, and multiphasic wastes. The TCLP predicts if hazardous components of a waste are likely to leach out and become a threat to public health or the environment. Liquid waste streams that contain more than 5.0 mg/L of arsenic are hazardous waste based on toxicity characteristics listed in RCRA.
Indirect discharge may be an option for IX wastes since wastes that pass through a sewer system to a publicly owned treatment works (POTW) are exempt from RCRA regulation. However, the critical factor dictating the feasibility of indirect discharge, will be technically based local limits (TBLL) for arsenic and TDS. Water systems that elect to use brine recycle will further concentrate the dissolved arsenic and solids, making it even more unlikely that the stream will meet local TBLLs. Check with your POTW and State for more information.
Based on previous studies, spent IX resin does not exceed any regulatory toxicity concentrations, enabling it to be disposed of in a municipal solid waste landfill.