Taste and Odor Treatment

The choice of an active treatment method for taste and odor problems depends on the cause of the problem.  In addition, some methods can be used to solve other problems, such as trihalomethane formation, which should be factored into the choice of a treatment method.  Active treatment may involve optimizing plant processes, using ion exchange units, air stripping, performing chemical or mechanical oxidation, or performing adsorption.



Optimizing Plant Processes


Chlorine smells are one of the most common problems reported by water customers and are also one of the simplest odor problems to treat.  Chlorine smells can be dealt with by simply optimizing the chlorine dosage. 

If the tastes and odors are associated with color and turbidity or with floating algae, then optimizing the coagulation/flocculation, sedimentation, and filtration processes may take care of the problem.  This is often the simplest and most economical treatment method for taste and odor problems since the equipment is already in place.  In addition, these typical plant processes can remove trihalomethane precursors in many cases if prechlorination is not used at the treatment plant. 



Ion Exchange


Ion exchange units are most often used for softening.  They are not usually used for taste and odor removal but can be used to remove trihalomethane precursors if anion exchange resins are used.  However, this process can be expensive and creates the problem of waste disposal. 



Air Stripping


Volatile compounds can sometimes be removed from water using aerators which strip the compounds from the water.  This technique is usually more helpful at controlling odors than tastes, and is very effective at removing hydrogen sulfide.  Trihalomethanes can be removed from water using aeration if the aeration follows chlorination.  However, in that type of setup, the operator must be aware that passing air through treated water can add contamination back into the water. 



Oxidation


Oxidation is another frequently used method to remove tastes, odors, and trihalomethane precursors.  Oxidation can be either mechanical (using an aerator) or chemical (by adding chlorine, potassium permanganate, ozone, or chlorine dioxide.)  Use of aerators is usually only effective at removing tastes associated with iron and manganese.  In other cases, chemicals must be used. 

Chlorine is the most widespread chemical used for oxidation of tastes and odors since chlorine is already in use in many treatment plants as a disinfectant.  When using chlorine to oxidize taste and odor problems, the dosage of chlorine must be greater than that used for disinfection, a method called superchlorination.  After superchlorination has removed the taste and odor problems, the excess chlorine must be removed from the water, which can be achieved using granular activated carbon (which will be discussed later.)  Chlorination can deal with fishy, grassy, or flowery odors and with iron and hydrogen sulfide.  However, chlorination can make some problems worse, especially those caused by phenols.  And, of course, chlorination will increase the trihalomethane concentration. 

Other chemicals used for oxidation include potassium permanganate, chlorine dioxide, and ozone. Potassium permanganate is used to treat organic contaminants while chlorine dioxide does well against phenolic and algal tastes.  Ozone is a very strong oxidant which will treat more problems than chlorine and lacks the objectionable by-products.  All of these methods can also be used to remove or modify trihalomethane precursors, but with variable efficiency. 



Adsorption


The final treatment method we will discuss is adsorption.  Adsorption occurs when Van der Waal's forces pull contaminants out of the water to stick them onto the surface of some other material.  This material, known as the adsorbent, has a very large surface area to allow the removal of large amounts of contaminants.  Adsorbents use large pores, such as the one shown below, to increase their surface area. 

Large surface area


Several different materials can be used as adsorbents in water treatment.  The most widespread of these materials is activated carbon which is formed when carbon from wood, coal, peat, or nut shells is exposed to heat in the absence of oxygen.  Activated carbon has been used medicinally since 1500 BC in Egypt and is now used in over a quarter of water treatment plants across the U.S.  The popularity of activated carbon stems from its lack of specificity which allows it to treat many different taste and odor problems. 

The other two types of adsorbents are activated alumina and synthetic resins, both of which are typically used as filter media.  Activated alumina is used to remove excess fluoride from water as well as to remove arsenic and selenium.  Synthetic resins can remove trihalomethanes from water, but are very costly and their use is still in the developmental stages.