Lesson 4:

Coagulation and Flocculation

Objective

In this lesson we will answer the following questions:

How do coagulation and flocculation fit into the water treatment process?

Which chemical principles influence coagulation and flocculation?Which chemicals are used in coagulation?

What factors influence coagulation and flocculation?

Reading Assignment

Along with the online lesson, read Chapter 4: Coagulation and Flocculation, in your textbook Operation ofWater Treatment Plants Volume I .

Lecture

Overview of the Process

Location in the Treatment Plant

After the source water has been screened and has passed through the optional steps of pre-chlorination andaeration, it is ready for coagulation and flocculation.

In theory and at the chemical level, coagulation and flocculation is a three step process, consisting of flash

mixing, coagulation, and flocculation. However, in practice in the treatment plant, there are only two steps inthe coagulation/flocculation process - the water first flows into the flash mix chamber, and then enters the

flocculation basin.

In this lesson, we will primarily be concerned with the theory behind coagulation/flocculation. In laterlessons, we will consider the practice in more detail.

Purpose

The primary purpose of the coagulation/flocculation process is the removal of turbidity from the water. Turbidity is a cloudy appearance of water caused by small particles suspended therein. Water with little or

no turbidity will be clear.

Turbidity is not only an aesthetic problem in water. Water with a high turbidity can be very difficult orimpossible to properly disinfect. As a result, the maximum allowable level of turbidity in water is 0.5 NTU,while the recommended level is about 0.1 NTU. (NTU, or TU, stands for nephelometric turbidity units, ameasurement of the turbidity of water.)

In addition to removing turbidity from the water, coagulation and flocculation is beneficial in other ways. Theprocess removes many bacteria which are suspended in the water and can be used to remove color from the

water.

Turbidity and color are much more common in surface water than in groundwater. As surface water flows

over the ground to streams, through streams, and then through rivers, the water picks up a large quantity ofparticles. As a result, while aeration is more commonly required for groundwater, treatment involvingcoagulation and flocculation is typical of surface water.

Three Steps

As I mentioned above, the chemistry of coagulation/flocculation consists of three processes - flash mix,coagulation, and flocculation. Each of these processes is briefly explained below.

In the flash mixer, coagulant chemicals are added to the water and the water is mixed quickly andviolently. The purpose of this step is to evenly distribute the chemicals through the water. Flash mixing

typically lasts a minute or less. If the water is mixed for less than thirty seconds, then the chemicals will notbe properly mixed into the water. However, if the water is mixed for more than sixty seconds, then themixer blades will shear the newly forming floc back into small particles.

After flash mixing, coagulation occurs. During coagulation, the coagulant chemicals neutralize the electricalcharges of the fine particles in the water, allowing the particles to come closer together and form largeclumps. You may already be familiar with the process of coagulation from cooking. You can see

coagulation occurring when preparing gelatin (jello) or when cooking an egg white.

The final step is flocculation. During flocculation, a process of gentle mixing brings the fine particles formed

by coagulation into contact with each other. Flocculation typically lasts for about thirty to forty-fiveminutes. The flocculation basin often has a number of compartments with decreasing mixing speeds as thewater advances through the basin. This compartmentalized chamber allows increasingly large floc to form

without being broken apart by the mixing blades.

Floc

The end product of a well-regulated coagulation/flocculation process is water in which the majority of the

turbidity has been collected into floc, clumps of bacteria and particulate impurities that have come together

and formed a cluster. The floc will then settle out in the sedimentation basin, with remaining floc beingremoved in the filter.

The best floc size is 0.1 to 3 mm. Larger floc does not settle as well and is more subject to breakup in theflocculation basin. Smaller floc also may not settle.

Chemistry

Introduction

Why do we need such a complex process to remove particles from water? Some particles would settle out

of the water on their own, given enough time. But other particles would resist settling for days or months dueto small particle size and to electrical charges between the particles.

We will consider the chemical processes which prevent and aid settling below. But first, we will list the threetypes of objects which can be found in water.

Particles in Water

There are three types of objects which can be found in water. In order from smallest to largest, theseobjects are chemicals in solution, colloidal solids, and suspended solids. Coagulation/flocculation will

remove colloidal and suspended solids from water.

Chemicals in solution have been completely dissolved in the water. They are electrically charged and can

interact with the water, so they are completely stable and will never settle out of the water. Chemicals insolution are not visible, either using the naked eye or using a microscope, and are less than 1 Mu in size. (A

Mu, or millimicron, is equal to 0.000000039 inches.) An example of a chemical in solution is sugar in

water.

Colloidal solids, also known as nonsettleable solids, do not dissolve in water although they are

electrically charged. Still, the particles are so small that they will not settle out of the water even after severalyears and they cannot be removed by filtration alone. Colloidal solids range between 1 and 500 Mu in size

and can be seen only with a high-powered microscope. Examples include bacteria, fine clays, and silts.

Colloidal solids often cause colored water, such as the "tea color" of swamp water.

Finally, suspended, or settleable, solids will settle out of water over time, though this may be so slow that

it is impractical to merely allow the particles to settle out in a water treatment plant. The particles are more

than 1,000 Mu in size and can be seen with a microscope or, sometimes, with the naked eye. Examples ofsuspended solids include sand and heavy silts.

Electrical Charges

The chemistry of coagulation and flocculation is primarily based on electricity. Electricity is the behavior of

negative and positively charged particles due to their attraction and repulsion. Like charges (two negatively

charged particles or two positively charged particles) repel each other while opposite charges (a positivelycharged particle and a negatively charged particle) attract.

Negatively charged particles repel each other due to electricity.

Most particles dissolved in water have a negative charge, so they tend to repel each other. As a result, theystay dispersed and dissolved or colloidal in the water, as shown above.

The purpose of most coagulant chemicals is to neutralize the negative charges on the turbidity particles to

prevent those particles from repelling each other. The amount of coagulant which should be added to the

water will depend on the zeta potential, a measurement of the magnitude of electrical charge surroundingthe colloidal particles. You can think of the zeta potential as the amount of repulsive force which keeps the

particles in the water. If the zeta potential is large, then more coagulants will be needed.

Coagulants tend to be positively charged. Due to their positive charge, they are attracted to the negative

particles in the water, as shown below.

Positively charged coagulants attract to negatively

charged particles due to electricity .

The combination of positive and negative charge results in a neutral, or lack, of charge. As a result, the

particles no longer repel each other.

The next force which will affect the particles is known as van der Waal's forces. Van der Waal's forces

refer to the tendency of particles in nature to attract each other weakly if they have no charge.

Neutrally charged particles attract due to van der Waal's forces.

Once the particles in water are not repelling each other, van der Waal's forces make the particles drifttoward each other and join together into a group. When enough particles have joined together, they becomefloc and will settle out of the water.

Particles and coagulants join

together into floc.

Coagulant Chemicals

Types of Coagulants

Coagulant chemicals come in two main types - primary coagulants and coagulant aids. Primary coagulants

neutralize the electrical charges of particles in the water which causes the particles to clump together. Coagulant aids add density to slow-settling flocs and add toughness to the flocs so that they will not breakup during the mixing and settling processes.

Primary coagulants are always used in the coagulation/flocculation process. Coagulant aids, in contrast, arenot always required and are generally used to reduce flocculation time.

Chemically, coagulant chemicals are either metallic salts (such as alum) or polymers. Polymers are man-made organic compounds made up of a long chain of smaller molecules. Polymers can be either cationic

(positively charged), anionic (negatively charged), or nonionic (neutrally charged.) The table below showsmany of the common coagulant chemicals and lists whether they are used as primary coagulants or ascoagulant aids.

Different sources of water need different coagulants, but the most commonly used are alum and ferric sulfate.

Chemical Name Chemical Formula Primary Coagulant Coagulant Aid

Aluminum sulfate (Alum) Al2(SO4)3 · 14 H2O X

Ferrous sulfate FeSO4 · 7 H2O X

Ferric sulfate Fe2(SO4)3 · 9 H2O X

Ferric chloride FeCl3 · 6 H2O X

Cationic polymer Various X X

Calcium hydroxide (Lime) Ca(OH)2 X* X

Calcium oxide (Quicklime) CaO X* X

Sodium aluminate Na2Al2O4 X* X

Bentonite Clay X

Calcium carbonate CaCO3 X

Sodium silicate Na2SiO3 X

Anionic polymer Various X

Nonionic polymer Various X

*Used as a primary coagulant only in water softening processes.

Alum

There are a variety of primary coagulants which can be used in a water treatment plant. One of the earliest,

and still the most extensively used, is aluminum sulfate, also known as alum. Alum can be bought in liquidform with a concentration of 8.3%, or in dry form with a concentration of 17%. When alum is added towater, it reacts with the water and results in positively charged ions.

Coagulant Aids

Nearly all coagulant aids are very expensive, so care must be taken to use the proper amount of thesechemicals. In many cases, coagulant aids are not required during the normal operation of the treatment plant,

but are used during emergency treatment of water which has not been adequately treated in the flocculationand sedimentation basin. A couple of coagulant aids will be considered below.

Lime is a coagulant aid used to increase the alkalinity of the water. The increase in alkalinity results in anincrease in ions (electrically charged particles) in the water, some of which are positively charged. These

positively charged particles attract the colloidal particles in the water, forming floc.

Bentonite is a type of clay used as a weighting agent in water high in color and low in turbidity and mineralcontent. This type of water usually would not form floc large enough to settle out of the water. The

bentonite joins with the small floc, making the floc heavier and thus making it settle more quickly.

Factors Influencing Coagulation

Introduction

In a well-run water treatment plant, adjustments are often necessary in order to maximize the

coagulation/flocculation process. These adjustments are a reaction to changes in the raw water entering theplant. Coagulation will be affected by changes in the water's pH, alkalinity, temperature, time, velocity andzeta potential.

The effectiveness of a coagulant is generally pH dependent. Water with a color will coagulate better at lowpH (4.4-6) with alum.

Alkalinity is needed to provide anions, such as (OH) for forming insoluble compounds to precipitate themout. It could be naturally present in the water or needed to be added as hydroxides, carbonates, or

bicarbonates. Generally 1 part alum uses 0.5 parts alkalinity for proper coagulation.

The higher the temperature, the faster the reaction, and the more effective is the coagulation. Wintertemperature will slow down the reaction rate, which can be helped by an extended detention time. Mostly, itis naturally provided due to lower water demand in winter.

Time is an important factor as well. Proper mixing and detention times are very important to coagulation.

The higher velocity causes the shearing or breaking of floc particles, and lower velocity will let them settle inthe flocculation basins. Velocity around 1 ft/sec in the flocculation basins should be maintained.

Zeta potential is the charge at the boundary of the colloidal turbidity particle and the surrounding water.The higher the charge the more is the repulsion between the turbidity particles, less the coagulation, and vice

versa. Higher zeta potential requires the higher coagulant dose. An effective coagulation is aimed at reducingzeta potential charge to almost 0.

Coagulant

The proper type and concentration of coagulant are essential to the coagulation process. The coagulantchoice will depend on the conditions at the plant. The concentration of coagulant also depends on the waterconditions, and a jar test can be used to determine the correct concentration to use at any given time.

Coagulants are usually fed into the water using a gravimetric feeder or a metering pump. A gravimetricfeeder feeds dry chemicals into the water by weight. A metering pump feeds a wet solution (a liquid) into

the water by pumping a volume of solution with each stroke or rotation.

Improper coagulation related to coagulant may result from:

Using old chemicals

Using the wrong coagulantUsing the wrong concentration of coagulant. This may result from setting the wrong feed rate on thegravimetric feeder or metering pump or from a malfunction of the equipment.

Common Coagulation and Flocculation Problems

Review

Coagulation/flocculation is a process used to remove turbidity, color, and some bacteria from water. In theflash mix chamber, chemicals are added to the water and mixed violently for less than a minute. These

coagulants consist of primary coagulants and/or coagulant aids. Then, in the flocculation basin, the water isgently stirred for 30 to 45 minutes to give the chemicals time to act and to promote floc formation. The flocthen settles out in the sedimentation basin.

Coagulation removes colloids and suspended solids from the water. These particles have a negative charge,

so the positively charged coagulant chemicals neutralize them during coagulation. Then, during flocculation,the particles are drawn together by van der Waal's forces, forming floc. The coagulation/flocculationprocess is affected by pH, salts, alkalinity, turbidity, temperature, mixing, and coagulant chemicals.

References

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

Belmont Water Treatment Association. 1997. Coagulation.

Nearly all of the section "Factors Influencing Coagulation" is drawn directly from this site.

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

Assignments

Read about detention times on page 110 of your text and answer the following question: (25 points)

1. A water treatment plant treats a flow of 1.7 MGD. The flocculation basin is 6 feet deep, 17 feet wideand 40 feet long. Calculate the detention time in minutes.

Read about chemical feeder settings on page 117 of your text and answer the following question:

(25 points)

2. The optimum liquid alum dose from the jar tests is 10 mg/L. Determine the setting on the liquid alumchemical feeder in gallons per day when the flow is 2.8 MGD. The liquid alum delivered to the plant

contains 4.83 pounds of alum per gallon of liquid solution.

3. Work the following crossword puzzle that comes from definitions in your textbook. You may eitherprint 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. (50 points)

Labs

Read the following jar test lab procedure. You may have test questions come from this lab.

Quiz

Answer the questions in the Lesson 4 quiz . When you have gotten all the answers correct, print the pageand either mail or fax it to the instructor. You may also take the quiz online and submit your grade directlyinto the database for grading purposes.