Small Systems Treatment Technologies
- Types of Treatment Technologies
- Point-of-Use and Point-of-Entry Treatment
- Arsenic Removal
Most public water systems (PWSs) treat drinking water to make it safe and appealing. The use of a specific treatment technology depends on source water quality and operator skill. Treatment technologies remove:
- chemical, and
- biological contaminants.
More than one treatment technology may be applied, depending on the type of contamination. EPA is sensitive to cost restrictions, which tend to affect small systems that serve less than 10,000 people.
For small systems, the challenge is to determine which treatment technology is appropriate considering:
- maintenance, and
Treatment technologies fall into three broad types:
- Particulate and turbidity removal
- Simple filtration
- Advanced filtration
- Reverse osmosis
- Chemical contaminant removal
- Ion exchange
- Others (aeration, softening, electrodialysis, reverse osmosis)
- Biological contaminant removal
- Ultraviolet light
- Other disinfection technologies (ammonium compounds, nonoxidizing biocides [i.e., formaldehyde], heat, and peracetic acid)
The Safe Drinking Water Act addresses small system drinking water concerns. It also requires EPA to assess small systems treatment technologies. The two classes of small systems treatment technologies are:
- Compliance technologies: comply with the Maximum Contaminant Level (MCL) or satisfy a treatment technique requirement. Possible compliance technologies include packaged or modular systems and point-of-use (POU) or point-of-entry (POE) treatment units. Design and Cost Considerations for small systems.
- Variance technologies: are specified for only those PWSs with a system size and source water quality that have no listed compliance technologies. While variance technologies may not achieve compliance with the MCL or treatment technique requirement, they must achieve the maximum reduction that is affordable and protects public health.
In many cases, small drinking water systems use POU/POE treatment units, which may be the best solution for providing safe drinking water to:
- individual homes,
- apartment buildings, and
- even small towns.
These consumers may not have:
- the financial resources,
- technical ability, or
- physical space to own and operate custom-built treatment plants.
POU/POE units can be used for treating raw water, and are excellent for treating finished water that may have degraded during distribution or storage. The units also ensure that vulnerable consumers, such as the very young, the very old, or the immune compromised, receive safe drinking water.
POU/POE units are a practical treatment method. In many cases, they are essentially the same as central treatment. The regulations require that POU/POE units be owned, controlled, and maintained by the PWS or by a person under contract with the PWS. This ensures proper operation, maintenance, and compliance with the maximum contaminant levels or treatment technique. The units must have automated warning device to notify when there are operational problems.
POU/POE treatment is used for inorganics, synthetic organic chemicals, and radionuclides, but not for volatile organic compounds (VOCs) or micro-organisms. POU/POE treatment reduces levels of:
- organic contaminants,
- microorganisms including cysts, and
- many other contaminants.
Taste, odor, or color may also be improved with POU/POE treatment.
The MCL for arsenic in drinking water is 10 micrograms per liter. The following are Small System Compliance Technologies (SSCTs):
- Ion exchange
- Activated alumina (centralized and POU)
- Reverse osmosis (centralized and POU)
- Electrodialysis reversal
- Coagulation/filtration, enhanced coagulation/filtration, and coagulation-assisted
- Coagulation-assisted microfiltration
- Lime softening and enhanced lime softening
Small systems are not required to use these technologies, but they may be included in an arsenic mitigation strategy. The arsenic mitigation strategy may include:
- switching to a source of water with a lower arsenic concentration,
- locating and installing a new source,
- purchasing water from a nearby system,
- using chlorine disinfection optimized to provide concurrent arsenic (III) oxidation, or
- using membrane separation, which addresses numerous water quality problems while maintaining simplicity and ease of operation.
Point-of-Use Treatment for Arsenic
POU devices were approved as SSCTs for meeting the revised arsenic MCL. The SDWA requires that the devices 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.
Because a very small fraction of the total water supplied to a given household is ultimately consumed, POU devices are particularly attractive for removing contaminants that pose only an ingestion risk, as is the case with arsenic. In most cases, the POU unit is plumbed in at the kitchen sink (the device will have its own faucet). As such, the kitchen tap would be the only source from which water should be used for consumption.
The advantage of using a POU treatment in a small system is reduced capital and treatment costs. Studies have suggested that POU programs are an economically doable alternative for systems serving roughly 50–500 people. On the downside, these programs generally incur higher administrative and monitoring costs. Another downside is that the media or membranes used in POU treatment devices may be susceptible to microbial contamination. Higher levels of bacteria have been found in the finished water produced by some POU treatment devices, particularly those that incorporate an activated carbon element, than in the corresponding untreated water.
No illnesses have been reported as a result of using a POU device. Additional monitoring and post-treatment disinfection may be required to ensure public safety. This increases overall costs. The criteria for selecting an appropriate POU treatment device are:
- arsenic removal performance,
- third party certification to NSF/ANSI standards,
- appropriate mechanical warning devices, and
- ease of serviceability.