For a ‘small system,’ consider cartridge filters

Oct. 13, 2010

They offer simplicity of maintenance, operation.

For some point-of-use and point-of-entry (POU/POE) water treatment dealers, the world of small systems may offer some challenges. Knowing the basics can help.

The US Environmental Protection Agency (EPA) defines "small community water systems" as systems serving 10,000 people or fewer. Approximately 85 percent of all US public water systems (PWS) are classified as small community water systems; these systems service approximately 26 million people.

The EPA has defined three types of public water systems. They are:

  • The community water system (CWS), which supplies water to the same population year-round
  • The non-transient non-community water system (NTNCWS), which regularly supplies water to at least 25 of the same people at least six months per year, but not year-round (some examples are schools, factories and hospitals that have their own water systems).
  • The transient non-community water system (TNCWS), which provides water in such places as gas stations, restaurants or campgrounds where people do not remain for extended periods of time.

Small community water systems are further categorized into three groups according to the number of people they serve: 25-500 people, 501 to 3,300 people or 3,301 to 10,000 people. The majority of the small community water systems rely on groundwater as their primary water source.

Treatment options outlined
The EPA has compiled a list of potential disinfection and filtration treatment options for small systems. Disinfection treatment options include: chlorine, ozone, chloramines, chlorine dioxide, ultraviolet (UV), and on-site oxidant generation.

Filtration treatment options include: conventional filtration treatment or direct filtration, slow sand filter, and diatomaceous earth filters.

Alternative filtration includes: Membrane filtration (such as reverse osmosis [RO], nanofiltration, ultrafiltration and microfiltration), and bag and cartridge filtration.

Depending on water source and quality, small community water systems often are challenged in meeting Safe Drinking Water Act (SDWA) requirements. Many of the accepted treatment technologies used by large systems are too expensive and require advanced operator training. Operators with advanced levels of training may be difficult to locate in some of the remote locations where these small community water systems are located.

Technology pros and cons
Conventional filtration can include chemical coagulation, rapid mixing and flocculation, followed by floc removal via sedimentation and then filtration. Conventional filtration has the advantage of being able to treat a wide range of water qualities.

It also has several disadvantages for small drinking water systems. Depending on the influent water quality and the demand requirements, the equipment footprint for a conventional filtration system can be large.

Additional disadvantages include extensive chemical pretreatment costs and that an operator with advanced training is required.

Slow sand filtration has the advantages of low maintenance and monitoring requirements. However, slow sand filtration does require a "ripening period" after cleaning, which may lead to wastewater production. Depending on demand requirements, slow sand filtration can have extensive land requirements due to its low-flow operation (0.03 to 0.10 gallons per minute per square foot of filter bed area).

Diatomaceous earth (DE) filtration can be an inexpensive filtration option for water supplies with low bacterial count and low turbidity. Initial water quality can be a determinant in the viability of DE filtration as an option for a small community water system.

A caveat regarding DE filtration: Coagulants and filter aids may be required for virus protection, which may then require an operator with advanced experience.

Membrane filtration systems can be a viable option for small systems because they remove bacteria, microorganisms, particulate material and natural organic materials for taste and odor. Membrane filtration generally requires limited space depending on the pre-filtration requirements. The maintenance requirements for membranes include periodic chemical cleaning to remove contaminant buildup.

Disadvantages of RO membranes for small community water systems are the issues of disposal of concentrates and high levels of wasted reject water (up to 50 percent of feed).

Cartridge filtration can be an economical solution for small community water systems. Operating requirements for cartridge filtration are low, because they demand minimal energy consumption, produce no wastewater and require a minimal footprint area.

Installation and maintenance of a cartridge filtration system does not require advanced operator training. If water quality, usage requirements or government regulations change, the systems can be easily modified by upgrading replacement cartridges or installing additional levels to the filtration chain.

Advancements in the filtration media available for cartridge filtration have vastly improved their performance potential. These new media technologies, such as nanotechnology, are producing cartridge filters that are capable of virus and bacterial removal, while maintaining low pressure drop and offering high dirt-holding capacity. Such filters also will work well in the pre-filtration train prior to membrane filtration.

Depending on raw water turbidity, roughing filters or a series of filter cartridges may be required to remove larger suspended solids and prevent the rapid fouling of the smaller-micron polishing cartridge filters. Highly turbid water can lead to cartridge fouling, requiring frequent replacement and increasing operating budgets.

Growth in ‘largest of the small’
POU/POE treatment devices may be an option for PWSs when central treatment is not affordable for meeting maximum contaminant level (MCL) requirements.

The SDWA prohibits the EPA from listing POU treatment units as an affordable technology for a microbial contaminant, radon or for most VOCs, including trihalomethanes, since these devices do not provide adequate protection against inhalation or contact exposure to these contaminants at untreated taps. The statute does not exclude the use of POE devices as an option for removal of these contaminants.

In view of this, here are some key points to consider:

  • Over the next several years small community water systems using "surface water or groundwater under the direct influence of surface water (GWUDI)" will be required to phase into the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR). This will require more stringent monitoring and control of Cryptosporidium levels in drinking water, as well as maintaining low disinfection byproduct and turbidity levels.
  • Historically the very small water systems (those serving 25-500 people) have experienced little growth in service population — around 2.5 percent in median connections in a five-year period. However, the category of 3,301-10,000 connections experienced a 10 percent increase in the number of connections over the same period.

Cyndi Benson, a mechanical engineer, is a process and product development engineering consultant for Harmsco Filtration Products, headquartered in North Palm Beach, FL. She can be reached by telephone at (561) 848-9628, ext. 159, and via e-mail at: [email protected].

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