Arsenic in Drinking Water
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Non Treatment Options
Where possible, systems should consider non-treatment options. Not only are these options often less costly, but they also avoid the creation of residuals (waste products). Non-treatment options include: (1) using alternative sources that meet the MCL; (2) seasonal use of high arsenic sources while maintaining a running annual arsenic concentration that meets the MCL at each entry point to the distribution system; and, (3) blending high and low-arsenic sources to meet the MCL.
Perhaps the simplest non treatment option is abandonment of the water source with a high concentration of arsenic and procurement of a new source that meets the arsenic MCL. This option is most realistic for utilities with multiple water sources where at least one source can be relied upon to produce water with arsenic below the MCL. There may however, be constraints to abandoning one source, such as inadequate capacity of other sources or water rights.
Many small utilities only maintain a single water source and do not have the flexibility of simply switching production to another location. In this particular case, the utility can locate or install a new low arsenic source, or purchase water from or negotiate consolidation with a nearby system. Installation of a new source could be more costly than treatment.
Another non treatment option is to switch sources with high arsenic concentrations from full-time production to seasonal or peaking use only. Systems with only one source will need to find another primary source of supply while systems with one or more source can switch the problematic source to a back-up or seasonal source. When this option is used, systems must conduct monitoring (in accordance with the primacy agency's requirements) at the entry point to ensure the running annual average (RAA) of arsenic is at or below the MCL. State requirements may preclude this option. Check with your State for additional information.
Blending is also an option if the system has low arsenic sources which can be operated in conjunction with the high arsenic sources. In order for blending applications to be practical, the wells with low arsenic levels must be continuously reliable and the high and low arsenic sources must be brought together for mixing in a common header. Generally, the target arsenic concentration of the final blended stream is set below the MCL by a safe margin (e.g., 10-20% below the MCL). States may have established their own safety margin. Check with your State for more information.
Sidestream treatment, a combination of non-treatment and treatment options, involves splitting flow, treating one stream, and blending it with the untreated stream prior to distribution. Blending and sidestream treatment can be used independently or in combination. Sidestream treatment is feasible when a water source exceeds the revised MCL by a relatively small margin. This approach is viable because most arsenic treatment processes (operated under optimal conditions) can achieve at least 80% arsenic removal and, in many cases, this high level of performance is not needed to meet the MCL. Typical treatment efficiencies and water losses for processes operated under normal conditions should be taken into account when considering sidestream treatment.