Taste and Odor Treatment
The choice of an active treatment method for taste
odor problems depends on the cause of the problem. In addition,
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,
ion exchange units, air
stripping, performing chemical or mechanical oxidation, or performing
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.
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.
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 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
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.
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
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.
use large pores, such as the one shown below, to increase their surface
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
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.
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