lesson 9_ taste and odor

7/11/2014 Lesson 9: Taste and Odor

http://water.me.vccs.edu/courses/ENV110/Lesson9_print.htm 1/11

Lesson 9:

Taste and Odor

Objective

In this lesson we will answer the following questions:

How does taste and odor control fit into the water treatment process?

How does trihalomethane control fit into the water treatment process?What methods can be used to control taste, odor, and trihalomethanes?

Reading Assignment

Along with the online lesson, read Chapter 9: Taste and Odor Control, in your textbook Operation

of Water Treatment Plants Volume I .

Lecture

Taste and Odor Control Programs

Introduction

Taste and odor in drinking water are two of the most widespread causes of customer complaints.

Although there are no associated health effects, the extensive public relations difficulties resulting

from taste and odor make it important to treat these problems.

Treatment involves the implementation of a taste and odor control program, which should be foundat every treatment plant. Under some circumstances, this program may be as simple as routinely

monitoring for taste and odor problems and performing preventive maintenance on the system. In

other cases, treatment is more complex and can involve special equipment to treat the taste and

odor problems.

We will first consider prevention and testing, which are at the core of the taste and odor control

program. Later in this lesson, we will introduce various techniques which can be used for active

treatment of taste and odor problems. In any case, an understanding of the causes of taste and

odors in water will make treatment more effective.



Causes of Taste and Odor

Taste and odor can enter water in a variety of manners. Surface water sources can become

contaminated through algal blooms or through industrial wastes or domestic sewage introducing

taste- and odor-causing chemicals into the water. Groundwater supplies can be afflicted with

dissolved minerals, such as iron and manganese, which enter the water when it passes through

rocks underground. Tastes and odors can also enter either type of water in the raw watertransmission system and in the treatment plant due to algal growths, accumulated debris and sludge,

or disinfection byproducts. The distribution system can have many of the same causes of taste and

odor mentioned above, with the addition of problems resulting from cross-connections and low

flow zones.

The table below lists some of the chemicals which cause the most common taste and odor

problems in water.

Chemical cause Taste/odor Origin

Geosmin earthy or grassy odors Produced by actinomycetes, blue-green algae, and green algae.

2-Methylisoborneol(MIB)

musty odor Produced by actinomycetes and blue-green algae.

2t, 4c, 7c-decatrienal

fishy odor Produced by blue-green algae.

Chlorine bleach, chlorinous, ormedicinal taste and odor

Addition of chlorine as a disinfectant.

Chloramines swimming pool, bleach,or geranium odor

Addition of chlorine and ammonia as a disinfectant.

Aldehydes fruity odor Ozonation of water for disinfection.

Phenols andChlorophenols

pharmaceutical ormedicinal taste

Phenols usually originate in industrial waste. Chlorophenols areformed when phenols react with disinfecting chlorine.

Iron rusty or metallic taste Minerals in the ground.

Manganese rusty or metallic taste Minerals in the ground.

Hydrogen sulfide rotten egg odor Produced by anaerobic microorganisms in surface water or bysulfates in the ground.

Methane gas garlic taste Decomposition of organic matter.

Prevention



Algal bloom

The best way to treat taste and odor problems is prevention. Algal blooms in reservoirs can beprevented by using copper sulfate while algae growing on walls of the treatment plant basins can be

removed through shock treatment with chlorine. Since hydrogen sulfide is produced duringanaerobic conditions, preventing these conditions in the reservoir, the distribution system, and insludge in the sedimentation basin will prevent hydrogen sulfide formation. In the distribution system,

periodic flushing and maintaining an adequate chlorine residual will keep the pipes clean and odorfree.

Testing

An integral part of any taste and odor control program is testing the water for taste and odor

problems. The two methods used for these tests - the Threshold Odor Test and the Flavor ProfileAnalysis - are far more subjective than the methods used to test other water characteristics since

both the Threshold Odor Test and the Flavor Profile Analysis depend on human perception of thetaste and odor in the water. However, despite the difficulty of performing the tests objectively, they

still provide valuable information which can help the operator determine what is causing the taste or

odor problem, how concentrated the problematic chemical is, and how the problem should betreated.

The Threshold Odor Test is used to determine the amount of odor found in water. During theprocedure, the water being tested is diluted with odor-free water and is smelled. The dilutions

continue until no odor can be discerned. The last dilution at which odor is detected determines the

Threshold Odor Number (TON), which is a measure of the amount of odor in the water. If

several people independently perform the Threshold Odor Test, the averaged TON can berelatively accurate.



Flavor Profile Analysis

While the Threshold Odor Test is used to determine the concentration of odor-causing problems

in water, the Flavor Profile Analysis can be used to determine which tastes and odors arepresent in water. This test uses a panel of trained judges who taste the water and list which tastes

they can detect. Since the tastes present are described carefully, the Flavor Profile Analysis can be

helpful in determining which chemicals are at the root of the problem.

These tests can be performed to find the source of a particular problem or as part of routine

monitoring. To find the source of a problem, the water should be tested at various locations, from

the source water to the customer's tap. In contrast, routine monitoring can be less intensive but

requires good record-keeping. Past records can help the operator predict seasonal variations intaste and odor problems so that he can prevent problems before they reach the customer. Records

of past treatment methods can make it much easier to determine which treatment methods will be

effective during current outbreaks.

Trihalomethanes

While not a taste and odor problem, we will discuss trihalomethanes in this lesson because they can

sometimes be treated with the same methods used for taste and odor control. As you will

remember, trihalomethanes are by-products of chlorination and include several chemicals such aschloroform, bromodichloromethane, dibromochloromethane, and bromoform. Since these

substances are suspected to cause cancer in humans, their concentrations in drinking water are

carefully controlled.

Trihalomethanes are formed when organic compounds in the water come in contact with chlorine.

The organic matter is primarily humic substances, the organic part of the soil which results from

the decay of plant matter. Humic substances are more likely to be found in surface water than ingroundwater since they can enter water from algae, leaves, bark, wood, or soil.

Treatment methods include prevention of trihalomethane formation, removal of trihalomethanes

from treated water, or removal of trihalomethane precursors (humic substances) before chlorine isadded to the water. The first treatment method - prevention - involves using a disinfectant other

than chlorine to treat water. If the equipment for other types of treatment are not already available

at the treatment plant, then using an alternative disinfectant is usually the most economical option for

trihalomethane control.



The methods used to remove trihalomethanes or trihalomethane precursors will be discussed in the

rest of this lesson since the same methods can be used for taste and odor control. If treatmentinvolves removing only trihalomethanes themselves, it should be realized that more trihalomethanes

can form when the chlorine residual reacts with organics in the distribution system. Therefore, it is

most effective to remove the precursors before the trihalomethanes have a chance to form.

Active Treatment Methods

Introduction

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 mayinvolve 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 alsoone 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 removetrihalomethane precursors in many cases if prechlorination is not used at the treatment plant.

Ion Exchange

Ion exchange units 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 beexpensive 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 waterusing 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 methodcalled 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 activatedcarbon (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 wellagainst 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. As you will remember from the filtration

lesson, 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 verylarge 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.

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 nutshells 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. We will discuss activated carbon in more depth in a later

section.

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 aswell as to remove arsenic and selenium. Synthetic resins are more relevant to this lesson since they

can remove trihalomethanes from water. However, synthetic resins are very costly and their use is

still in the developmental stages.

Types of Activated Carbon

Powdered Activated Carbon

Powdered activated carbon, or PAC, is a form of activated carbon with a very small particle

size. Treatment involves adding PAC to water, allowing the PAC to interact with contaminants in

the water, then removing the PAC by sedimentation or filtration.

The feed location of PAC can be at any point prior to filtration. The most common locations are inthe flash mixer or flocculator since these pieces of equipment will mix the PAC into the water very

well. However, some plants feed PAC just before filtration so that the PAC will form a layer on

top of the filter and ensure that all water comes in contact with the activated carbon. Adding PAC



just before filtration can cause problems, though, since the small PAC particles can pass through thefilters and cause dirty water complaints from customers or can cake filters and result in reduced

filter runs.

Regardless of the feed location, PAC can be added to water using either a dry feeder or as a

slurry. Dry feeders are most often used in small plants when PAC is fed at intervals in response to

periodic taste and odor problems. In contrast, slurries (mixtures of PAC with water) are used in

larger plants or when PAC is fed continuously. Since it is difficult to make the PAC mix with water,the mixer should have an overhead spray system.

The effectiveness of PAC in adsorbing tastes and odors depends on adequate mixing, contact time,

dosage, and on the cause and concentration of the taste/odor problem. Mixing and contact time

are determined by the location at which the PAC is added in the treatment process, so adjustments

made by the operator will usually involve only dosage adjustments. The dosage usually ranges from

1 to 15 mg/L but must be much higher, in the range of 100 mg/L or more, when the PAC is beingused to remove trihalomethanes or trihalomethane precursors. The operator chooses an

appropriate dosage using jar tests and the results from odor and taste tests.

If PAC is fed as a slurry, then the actual concentration of PAC in each slurry tank will be different

and an accurate dosage can be difficult to determine. In that case, an imhoff cone, such as the one

shown above, can be used to determine the concentration of PAC in the slurry. Slurry is placed in

the cone and is allowed to sit while PAC settles to the bottom of the cone. The amount of PAC

can be measured using the gradations on the side of the imhoff cone.

Granular Activated Carbon



Granular activated carbon, also known as GAC, has a larger particle size than PAC with an

associated greater surface area. Like PAC, GAC can remove trihalomethane precursors as well as

taste and odor compounds.

GAC Contactor

GAC is used as a filter medium, either as a layer in a rapid-sand filter or in a separate filter knownas a contactor. When contactors are used, the contactor is placed downstream of the filter so that

turbidity won't clog the contactor.

Like filters, contactors must be designed to provide adequate contact time of water with the filter

medium. This is done by calculating the empty bed contact time, or EBCT, which is calculated

similarly to detention time, as the volume of the filter divided by the flow rate. The calculation is

called "empty bed contact time" because the volume taken up by the GAC in the contactor is not

taken into account. Empty bed contact time should be about ten minutes.

During operation of a GAC filter or contactor, a variety of factors must be monitored. If the GAC

is part of a filter designed to remove particulate matter as well as to adsorb tastes and odors, then

the effluent turbidity should be monitored. Similarly, the taste and odor contaminants in the effluent

should be monitored to determine whether the GAC is operating properly. The operator should

make regular checks for bacteria since microorganisms often grow on GAC filters and result in

clogging problems. Finally, head loss must be monitored as it would be for any other filter todetermine when the unit needs to be backwashed. Washing a GAC filter involves backwashing

with a 50% bed expansion and surface washing.

Although GAC filters can be operated like a rapid sand filter in most ways, backwashing and

surface washing are not the only cleaning required for the units. The entire surface of the GAC will

eventually become covered with contaminants, just as a softener's resin will become covered with

magnesium and calcium ions. A GAC filter can typically operate for months or years beforereaching this state, depending on the contaminant levels in the influent water. Once the GAC has



reached its adsorption capacity, it must be regenerated using the same heating process used to

activate the carbon. In many plants, GAC is simply replaced rather than investing in the equipment

required for regeneration.

Choosing a Type of Activated Carbon

GAC and PAC each have advantages and disadvantages. In general, PAC is used more often due

to the low initial cost and to the flexibility of dosage which allows the PAC concentration to be

adjusted to deal with changing contaminant levels. However, PAC has a high operating cost if used

continuously, cannot be regenerated, produces large quantities of sludge, and can break throughfilters to cause dirty water complaints by the customers. In addition, the dust resulting from the

small particles of PAC make handling difficult, as does the flammability of the particles.

GAC becomes a more economical choice in larger systems or where taste and odor must be

controlled continuously. Disadvantages of GAC include a high initial cost to buy the filter or

contactor, and the tendency of GAC filters to grow bacteria.

Review

Tastes and odors can be caused by algae or bacteria, industrial waste, dissolved minerals, or

disinfection reactions. Control programs are necessary at every treatment plant and may include

prevention, monitoring, and active treatment. Many of the active treatment methods can also be

used to treat trihalomethanes. Active treatment methods for trihalomethane, taste, and odor control

include optimizing existing plant processes, ion exchange, air stripping, oxidation, and adsorption.

Activated carbon is a commonly used adsorption method. Two types of activated carbon used in

water treatment are PAC and GAC.

References

Alabama Department of Environmental Management. 1989. Water Works Operator Manual.

Hou, J., and B. Clancy. 1997. Taste and Odor Problems in Water Treatment. Virginia Tech:

Blacksburg.

Kerri, K.D. 2002. Water Treatment Plant Operation. California State University: Sacramento.

http://www.cee.vt.edu/program_areas/environmental/teach/wtprimer/taste2/frontpg3.html



Letterman, R.D., ed. 1999. Water Quality and Treatment: A Handbook of Community Water

Supplies. McGraw-Hill, Inc.: New York.

Assignments

Work the following crossword puzzle that comes from definitions in your textbook. You may either

print the puzzle out, complete it and mail or fax back to the instructor or you may send an email

with the correct answers numbered accordingly

Quiz

Answer the questions in the Lesson 9 quiz . When you have gotten all the answers correct, print

the page and either mail or fax it to the instructor. You may also take the quiz online and submit

your grade directly into the database for grading purposes.

http://water.me.vccs.edu/courses/ENV110/crosswords/lesson9.pdf

http://water.me.vccs.edu/courses/ENV110/quiz9.htm

lesson 9_ taste and odor

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