Adsorption treatment systems add a cleansing substance directly to the water supply or via a mixing basin. Adsorbents combine chemical and physical processes to remove organic contaminants and the compounds that impart color, taste, and odor to water.
The most commonly-used adsorbent is activated carbon—a substance which is quite similar to common charcoal. Activated carbon, however, is treated by heat and oxidation so that it becomes extremely porous and able to readily adsorb, or capture, the impurities found in water.
Activated carbon also attracts not only known contaminants, but also naturally dissolved organic matter (much of which is harmless). Therefore, monitoring is needed to ensure that carbon doses are high enough to adsorb all contaminants.
There are two different forms of activated carbon in common use, granular activated carbon (GAC) and powdered activated carbon (PAC). Physically, the two differ as their names suggest—by particle size and diameter.
Powdered activated carbon is an inexpensive treatment option (capital cost) that can typically be added to an existing treatment system’s infrastructure. This flexibility makes PAC an attractive option for short-term treatment responses to poor water conditions. It is particularly useful to treat taste and color deficiencies.
PAC works quickly and efficiently but it is limited to lower removals than GAC and becomes expensive if it must be used on a continuous basis. When the process is complete the powdered carbon must be removed, usually by filtration.
Overall, activated carbon is better than ion exchange for removing organic substances.
Granular activated carbon (GAC) consists of particles about a millimeter in size—ten to 100 times the size of the powdered form. It is typically arranged in a bed or column through which source water is slowly passed or percolated. Sometimes several adsorption columns are linked together in a single system.
Like powdered activated carbon, granular activated carbon also attracts not only known contaminants, but also mostly harmless, naturally dissolved organic matter. Therefore, careful monitoring is needed to ensure that enough carbon remains active to adsorb all contaminants. Particulates may also clog systems and compromise their effectiveness. GAC systems have a higher capital cost but are capable of accomplishing higher levels of removal, and their operating costs (mostly the cost of replacing spent GAC) are lower if removal is required on a continuous basis.
These systems may also serve as biological water filters without compromising effectiveness if beneficial microbes are allowed to grow within the system.
Electrically charged atoms or molecules are known as ions. The ion exchange treatment process uses special resins to remove charged, inorganic contaminants like arsenic, chromium, nitrate, calcium, radium, uranium, and excess fluoride from water.
When source water is passed through a series of resin beads, it exchanges its charged contaminants for the harmless charged ions stored on the resin surface. Ion exchange resins then store the attracted contaminants. Because of this accumulation process, resins must be periodically cleaned with a solution that recharges their supply of harmless, interchangeable ions.
Ion exchange resin comes in two forms: cation resins, which exchange cations like calcium, magnesium, and radium, and anion resins, used to remove anions like nitrate, arsenate, arsenite, or chromate. Both are usually regenerated with a salt solution (sodium chloride). In the case of cation resins, the sodium ion displaces the cation from the exchange site; and in the case of anion resins, the chloride ion displaces the anion from the exchange site. As a rule cation resins are more resistant to fouling than are anion resins. Resins can be designed to show a preference for specific ions, so that the process can be easily adapted to a wide range of different contaminants.
This treatment process works best with particle-free water, because particulates can accumulate on the resin and limit its effectiveness.
Ion exchange is a common water treatment system that can be scaled to fit any size treatment facility. It may also be adapted to treat water at the point-of-use and point-of-entry levels.
Activated alumina (a form of aluminum oxide) is typically housed in canisters through which source water is passed for treatment. A series of such canisters can be linked together to match the water volume requirements of any particular system.
As alumina absorbs contaminants, it loses its capacity to treat water. Therefore, treated water quality must be carefully monitored to ensure that cartridges are replaced before they lose their treatment effectiveness. Also the capacity of the alumina is strongly influenced by the pH of the water. Lower pH is better. Many systems use acid pretreatment to address this need.
Source water quality is an important consideration for activated alumina systems. The treatment agent will attract not just contaminants, but many other negatively charged ions found in source water. This can limit the alumina’s ability to attract and remove the targeted contaminants.
Activated alumina technology can be expensive, and many of its costs are associated with disposal of the contaminated water that is created when alumina is purged of contaminants and “reset” for future use. Large-scale activated alumina systems also require a high level of operational and maintenance expertise, and consequently are relatively rare.
Small-scale systems are more common and can be tailored to accommodate any specific water volume requirements.