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Point-of-Use / Point-of-Entry Treatment
Point-of-use (POU) treatment devices are placed on a single tap and used when treated water is only for drinking and cooking purposes. Point-of-entry (POE) treatment devices are used when the whole building is served with treated water.
Point-of-use (POU) Treatment
In accordance with the Safe Drinking Water Act (SDWA), POU devices (listed in the Arsenic Rule as SSCTs) can be used to remove arsenic from drinking water. The devices must be owned, controlled, and maintained by the public water utility or by an agency under contract with the water utility. The responsibility of operating and maintaining the devices cannot be passed to the customer.
POUs listed in the Arsenic Rule include:
POU devices are only appropriate for removing contaminants that pose only an ingestion risk, as is the case with arsenic. Since only a small fraction of the total water supplied to a given household is ultimately treated and consumed, only that small fraction needs to be treated in order to reduce the risk. In most cases, the POU unit is plumbed in at the kitchen sink (the device will have its own faucet) and treats only the water intended solely for consumption (i.e., one tap and the refrigerator ice maker and ice water dispenser).
The primary advantage of using POU treatment in a small system is a reduction in capital and treatment costs, relative to installing centralized treatment. On the downside, however, POU installations generally incur higher administrative and monitoring costs to make sure that all units are functioning properly. Studies have suggested that POU programs are an economically viable alternative to centralized treatment for systems serving roughly 50-500 people. Not all states allow the use of POU devices so systems interested in using POU treatment should first check with their state primacy agency.
Most POU devices do not address the issue of pre-oxidation. While RO may remove As(III) to acceptable standards, sorptive processes such as AA or iron based sorbents will probably not. In this case, water systems may need to conduct centralized chlorine treatment to convert As(III) to As(V). There is also a concern that even with centralized pre-oxidation, anoxic conditions could exist in the distribution system that allow As(V) to reduce back to As(III). Depending on the extent of reduction, this could be detrimental to a POU program.
If using POU, the water system should obtain approval from the State for a system specific monitoring plan that ensures public health protection. States may require water systems to use third-party performance certified treatment units, in addition to field testing and rigorous engineering design review.
While RO has been shown to be effective at removing arsenic (it is listed in the Arsenic Rule as a BAT and as a POU SSCT), it is only viable in terms of cost as a POU treatment technology.
Membrane separation technologies may be attractive arsenic treatment processes for small water systems. They can address numerous water quality problems while maintaining simplicity and ease of operation. RO is a pressure-driven membrane separation process capable of removing dissolved solutes from water by means of particle size, dielectric characteristics, and hydrophilicity/hydrophobicity. RO effectively removes constituents from water, including organic carbon, salts, dissolved minerals, and color. This treatment process is also relatively insensitive to pH.
In the RO process, a high-pressure force (100-150 psi) is used to force water through a membrane. Treated water is collected on the other side; contaminants and rejected water (up to 70% of influent water) are unable to pass through. RO membranes, can remove low molecular weight organic molecules and salts.
Source water parameters of concern for RO include:
Studies indicate reverse osmosis can treat water to below the MCL provided the arsenic exists as (or is converted to) As(V). However, some RO membranes are sensitive to chlorine. Increasing the pH increases As(III) removal but also, above pH 7, may increase scaling.
Point-of-entry (POE) Treatment
POE treatment systems are similar to centralized treatment systems. Treatment techniques used for POE could include IX, adsorptive media, RO or any other arsenic removal treatment.