GeoInsighter Summer/Fall 2003 Newsletter

A Primer On Small Water Treatment Methods
Point-of-Entry and Point-of-Use Systems – Part Two

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In a previous article, we provided general information regarding how and why Point-of-Entry (POE) and Point-of-Use (POU) water treatment systems could be used. This article describes general treatment methods used to improve water quality. In most cases, POE and POU technologies are similar in principles of operation to full-scale systems that might be applied at a municipal water treatment facility or as part of a contaminated site ground water remediation system. In fact, the definition between community and individual treatment systems can become blurred; for instance, when an individual POE system is used to address impurities in a community supply well because it is more economical to operate than a larger system.

While selection of systems may sometimes require a bench-scale or pilot-scale test, most technologies are sufficiently well known that an appropriate system can be relatively easily selected for a particular set of ground water quality issues and performance criteria. Typical POE and POU systems are based upon the treatment technologies that follow.

• Filtration - Unwanted impurities in the form of particulates are removed from the water by trapping particles (such as sediment, colloids, and pathogens) larger than a specified size on a special medium. Media used for this purpose include synthetic materials (such as woven or non-woven cartridges or bag), carbon, and special sand or granular media. The selection of filter media will depend up the size and type of particulate to be trapped and flow rate. Some filter media can be back-washed to be rejuvenated; others are disposed after being clogged with particulate. Many other types of treatment systems include filtration as the first step in the process to maximize the efficiency of subsequent steps. Reverse Osmosis is a type of filtration where a pressure gradient is created on one side of a special membrane that functions at the molecular level to trap impurities but allows water to pass through the membrane for use.

• Chemical Treatment – For certain impurities in water, chemicals can be added to physically change or sequester the impurity in some way such that it is rendered safe, or can then be removed easily by another method. The addition of the chemical causes a reaction with or transformation of the impurity and is done in a manner that ensures complete mixing of raw water with a treatment chemical specific to the impurity. Examples of this technology include the introduction of oxygen to react with organic constituents, zinc/phosphate blends to control corrosion or scale formation, lime to control pH, and chlorine to kill certain bacteria.

• Physical Removal – Many different processes involve removal of constituents from the water, where a physical action takes advantage of constituent solubility, vapor pressure, or affinity to another material to transfer impurities from water to another medium. These technologies include: distillation, where water is converted to steam and the steam condensed to recover only water (not effective for water-soluble organics); air stripping to volatilize organic constituents into the air; absorption of contaminants on granular activated carbon; and ion exchange, where a replaceable resin, mineral salt bed, or activated sand bed (such as potassium permanganate) is used to react with and sequester minerals dissolved in the water.

• Destruction – A few technologies destroy certain impurities such that they are rendered harmless in the resulting treated water. These technologies include ultraviolet light, electro-hydraulic cavitation, and boiling of water.

Depending upon the characteristics of the water to be treated, many technologies require pre-treatment to maximize system efficiency. Accordingly, often two or three treatment technologies are used in series to address different impurities. Examples of common treatment trains include:

• removal of radon that will otherwise accumulate in activated carbon (causing radioactivity concerns for disposal) using aeration prior to being routed to the carbon;

• removal of iron and manganese that can be precipitated by an aeration process (fouling the aerator) using ion exchange to prevent precipitate formation during aeration; and

• removing particulates with pre-filters to increase the effectiveness reverse osmosis and remove turbidity that will interfere with light transmission in ultraviolet oxidation or disinfection.

Another important consideration in selection of a treatment system is the scope and frequency of confirmatory water quality testing to evaluate system performance. Confirmatory testing is normally carried out on a regular schedule specified by the system manufacturer or considering the nature of the impurity being treated. In general, the greater the risk associated with ingestion of untreated or inadequately treated water, the more frequent the testing schedule should be to assure protectiveness. Confirmatory testing can also guide maintenance activities, such as changing filters, rejuvenating or changing out reactive media when spent, repairing damaged parts, and cleaning scale and microbial growth off internal system components so they operate efficiently.

The length of time of system operation is dependent upon the quality of the water being treated. If impurities in water are natural, treatment system operation will be indefinite and will likely require replacement when its normal operating life ends. Other unnatural impurities, such as gasoline in ground water from a storage tank release, may require treatment for a long time (e.g., 5 to 20 years) because of their persistence in the subsurface. When a system is installed for a short-term condition that is expected to improve over time (such as a small fuel oil release to ground water), system operation may be discontinued after only a few years.

Michael C. Penney, P.E., L.S.P.
mcpenney@geoinc.com

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