Coagulation and Flocculation

ObjectiveIn 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 AssignmentAlong with the online lesson, read Chapter 4: Coagulation and Flocculation, in your textbookOperation of Water Treatment Plants Volume I.

LectureOverview of the ProcessLocation in the Treatment PlantAfter the source water has been screened and has passed through the optional steps of pre-chlorination and aeration, 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 in the 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 later lessons, we will consider the practice in more detail.PurposeThe primary purpose of the coagulation/flocculation process is the removal of turbidity from the water.Turbidityis 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 or impossible 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, orTU, stands for nephelometric turbidity units, a measurement of the turbidity of water.)In addition to removing turbidity from the water, coagulation and flocculation is beneficial in other ways. The process 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 of particles. As a result, while aeration is more commonly required for groundwater, treatment involving coagulation and flocculation is typical of surface water.Three StepsAs 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 theflash mixer, coagulant chemicals are added to the water and the water is mixed quickly and violently. 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 not be properly mixed into the water. However, if the water is mixed for more than sixty seconds, then the mixer blades will shear the newly forming floc back into small particles.After flash mixing, coagulation occurs. Duringcoagulation, the coagulant chemicals neutralize the electrical charges of the fine particles in the water, allowing the particles to come closer together and form large clumps. 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 isflocculation. 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-five minutes. The flocculation basin often has a number of compartments with decreasing mixing speeds as the water advances through the basin. This compartmentalized chamber allows increasingly large floc to form without being broken apart by the mixing blades.FlocThe end product of a well-regulated coagulation/flocculation process is water in which the majority of the turbidity has been collected intofloc, 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 being removed 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 the flocculation basin. Smaller floc also may not settle.Chemistry

IntroductionWhy 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 due to 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 three types of objects which can be found in water.Particles in WaterThere are three types of objects which can be found in water. In order from smallest to largest, these objects 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 in solution are not visible, either using the naked eye or using a microscope, and are less than 1 Mu in size. (AMu, ormillimicron, is equal to 0.000000039 inches.) An example of a chemical in solution is sugar in water.Colloidal solids, also known asnonsettleable 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 several years 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, orsettleable,solidswill 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 of suspended solids include sand and heavy silts.Electrical ChargesThe chemistry of coagulation and flocculation is primarily based on electricity.Electricityis 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 positively charged 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, they stay 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 thezeta potential, a measurement of the magnitude of electrical charge surrounding the 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 negativelycharged particles due to electricity.The combination of positive and negative charge results in aneutral, 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 forcesrefer 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 drift toward each other and join together into a group. When enough particles have joined together, they become floc and will settle out of the water.Particles and coagulants jointogether into floc.Coagulant ChemicalsTypes of CoagulantsCoagulant chemicals come in two main types - primary coagulants and coagulant aids.Primary coagulantsneutralize the electrical charges of particles in the water which causes the particles to clump together.Coagulant aidsadd density to slow-settling flocs and add toughness to the flocs so that they will not break up during the mixing and settling processes.Primary coagulants are always used in the coagulation/flocculation process. Coagulant aids, in contrast, are not always required and are generally used to reduce flocculation time.Chemically, coagulant chemicals are either metallic salts (such as alum) or polymers.Polymersare man-made organic compounds made up of a long chain of smaller molecules. Polymers can be eithercationic(positively charged),anionic(negatively charged), ornonionic(neutrally charged.) The table below shows many of the common coagulant chemicals and lists whether they are used as primary coagulants or as coagulant aids.Different sources of waterneed different coagulants, but the most commonly used are alum and ferric sulfate.

Chemical NameChemical FormulaPrimary CoagulantCoagulant Aid

Aluminum sulfate (Alum)Al2(SO4)3 14 H2OX

Ferrous sulfateFeSO4 7 H2OX

Ferric sulfateFe2(SO4)3 9 H2OX

Ferric chlorideFeCl3 6 H2OX

Cationic polymerVariousXX

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

Calcium oxide (Quicklime)CaOX*X

Sodium aluminateNa2Al2O4X*X

BentoniteClayX

Calcium carbonateCaCO3X

Sodium silicateNa2SiO3X

Anionic polymerVariousX

Nonionic polymerVariousX

*Used as a primary coagulant only in water softening processes.AlumThere 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 liquid form with a concentration of 8.3%, or in dry form with a concentration of 17%. When alum is added to water, it reacts with the water and results in positively charged ions.Coagulant AidsNearly all coagulant aids are very expensive, so care must be taken to use the proper amount of these chemicals. 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 flocculation and sedimentation basin. A couple of coagulant aids will be considered below.Limeis a coagulant aid used to increase the alkalinity of the water. The increase in alkalinity results in an increase inions(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.Bentoniteis a type of clay used as a weighting agent in water high in color and low in turbidity and mineral content. 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

IntroductionIn 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 the plant. Coagulation will be affected by changes in the water's pH, alkalinity, temperature, time, velocity and zeta potential.The effectiveness of a coagulant is generallypHdependent. Water with a color will coagulate better at low pH (4.4-6) with alum.Alkalinityis needed to provide anions, such as (OH) for forming insoluble compounds to precipitate them out. 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 thetemperature, the faster the reaction, and the more effective is the coagulation. Winter temperature will slow down the reaction rate, which can be helped by an extended detention time. Mostly, it is naturally provided due to lower water demand in winter.Timeis an important factor as well. Proper mixing and detention times are very important to coagulation.The highervelocitycauses the shearing or breaking of floc particles, and lower velocity will let them settle in the flocculation basins. Velocity around 1 ft/sec in the flocculation basins should be maintained.Zeta potentialis 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 reducing zeta potential charge to almost 0.CoagulantThe proper type and concentration of coagulant are essential to the coagulation process. The coagulant choice will depend on the conditions at the plant. The concentration of coagulant also depends on the water conditions, 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. Agravimetric feederfeeds dry chemicals into the water by weight. Ametering pumpfeeds 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 coagulant Using the wrong concentration of coagulant. This may result from setting the wrong feed rate on the gravimetric feeder or metering pump or from a malfunction of the equipment.Common Coagulation and Flocculation Problems

ReviewCoagulation/flocculation is a process used to remove turbidity, color, and some bacteria from water. In the flash 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 is gently stirred for 30 to 45 minutes to give the chemicals time to act and to promote floc formation. The floc then 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/flocculation process is affected by pH, salts, alkalinity, turbidity, temperature, mixing, and coagulant chemicals.

ReferencesAlabama 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.

AssignmentsRead 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 wide and 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 alum chemical 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. CompleteAssignment 4on Coagulation and Flocculation. You may do the Assignment online to get credit or print it out and send it to the instructor.(50 points)Laboratory ProceduresObjectiveIn this lesson we will learn the following: What laboratory procedures need to be done in the water plant.

Reading AssignmentAlong with the online lesson, read Chapter 11: Laboratory Procedures, in your textbookOperation of Water Treatment Plants Volume I.

LectureA laboratory is a place for precise work to determine appropriate treatment of raw water and the quality of the finished water. It must be kept organized, well maintained, and clean. All instruments must be kept clean and routinely calibrated with proper records. A number of lab tests are needed daily, quarterly, semiannually, annually, and at other specified intervals to monitor the water quality before, during, and after the treatment. A test is not better than the sample, and the sample is not better than the manner in which it is collected.SamplingValid testing starts with an adequate and representative sampling. A sample is either a grab or a composite. Agrab sample, as the name indicates, is a specific volume collected at one site at one time. These samples indicate the quality of water at that time and at that site. Grab samples are taken for bacteriological and disinfection residual tests. Acomposite sampleis a mixture of a number of portions taken at the specific intervals. This reduces the number of tests. Each portion can be proportionate to the flow or volume. For each test operators should follow the prescribed sampling size, collecting, and preserving procedure given in the Standard Methods for the Examination of Water and Waste Water. Testing must be done as soon as possible and not later than the specified holding time.TestsVarious regularly performed common tests by the operating staff are for tastes and odor, turbidity, jar test, pH, alkalinity, hardness, disinfection residual, coliform bacteria, and the heterotrophic plate count. All other tests are run either by highly trained chemists and microbiologists of the lab or by certified contract laboratories.Tastes and OdorsTesting for taste and odor is important because of aesthetic value. The majority of water quality complaints are of this type. Most of the organic and some inorganic chemicals cause tastes and odors. These chemicals come from the decaying organic matter, runoffs, industrial wastes, and municipal sewage discharges. Geosmin and methyl-isobarneol (MIB) are the serious odor-causing chemicals; they are produced by bacteria, particularly actinomycetes, while decomposing dead organic matter at the bottom of the water bodies. Even a very low concentration of these chemicals can cause earthy-musty odors. The odors are common in spring and fall due to the turn over of the lakes and reservoirs. In the groundwater, the tastes and odors can be due to iron, manganese, and hydrogen sulfide (H2S).These are general classes of odors: Aromatic (spicy) Balsamic (flowery) Chemical Disagreeable Earthy Musty Grassy VegetableThese are called thereference odorin the water samples.TurbidityTurbidity is the murkiness in the water caused by colloidal (1 to 100 nanometer particles) and other suspended particles, such as clay, sand, silt, organic matter of plant and animal origin, planktons, and other microscopic organisms. Turbidity particles can be waterborne pathogens or particles harboring them. The lower the turbidity, the less is the amount of the particulate matter. It means there is less probability of the presence of waterborne pathogens, and the water is safer. Therefore, turbidity is one of the primary standards for the drinking water. The finished water turbidity is tested at least every four hours.

Turbidity is measured as the amount of scattered light by the suspended particles in the sample. Turbidity of the finished water should be equal to or less than 0.3 nephalometric turbidity unit (NTU) in 95 percent of the samples/month.Jar TestingJar testing is a useful tool to determine the practical optimum dose of a chemical under the simulated plant conditions. It uses a range of increasing dose of a particular chemical in a series of six jars with a stirring and illumination mechanism.

Most of the problems in the source water, particulary in the surface water, quality are due to seasonal variations or other unusual circumstances, such as drought, heavy rains, unexpected discharge of raw sewage, or runoffs from farm land. These problems can be solved by this test, which is important for coagulation, softening, sedimentation, removal of synthetic organics, and for tastes and odor control. It makes the water treatment more effective, easy, and economical.pHpH, hydronium ion index, is the measurement of acidity (H+). Acidity in water is usually due to carbon dioxide (CO2) from rain water, mineral acids, chlorine, and heavy metal salts, such as alum. pH is an important parameter in the water utility. It is used to determine the condition of water for proper coagulation, softening, and stabilization.AlkalinityAlkalinity of water is its capacity to neutralize acidity. Carbonates, bicarbonates, and hydroxides are the most common forms of alkalinity in natural waters. These chemicals are mostly compounds of calcium and magnesium coming from mineral deposits such as limestone and dolomite. Industrial discharges can also cause alkalinity. Bicarbonate alkalinity is present between pH 4.3 and 8.3. Carbonate and bicarbonate alkalinity is present between pH 8.3 and 9.4, and carbonates and hydroxides are present between pH 9.4 and 14. Alkalinity does not exist below pH 4.3. Alkalinity test is important to determine proper coagulation and the stability of water.Disinfection ResidualChlorine is one of the most effective disinfectants and is quite commonly used for water disinfection. Chlorine, combined with ammonia, forms chloramines, which are called combined residual chlorine. Total residual chlorine is the sum of the free residual chlorine and combined residual chlorine.Coliform Bacteria TestsBacteriological quality of water is important to determine the degree of disinfection and possible presence of waterborne pathogens. Bacteria, being small, are present almost everywhere, such as in air, water, and on lab equipment. Therefore, all equipment and handling is done in a sterile environment to ensure the accuracty of data.Media are the food for the bacteria to culture them in the laboratory. Different bacteria have different food requirements; therefore, each medium will allow certain types of bacteria to grow. Media are either liquid, known as broths, or semisolid (gelatinous), which are called agars.To ensure the absence of waterborne pathogens, the water is tested for coliform bacteria. Coliform bacteria are present in human wastes and in soil contaminated with human wastes. These bacteria in human wastes are known asfecal coliformbacteria. Those in the soil are callednonfecal coliforms. Both fecal and nonfecal coliforms are called the total coliform group. This group is used as an indicator of the presence of human wastes in water and the possible presence of waterborne pathogens. The two most common techniques for testing coliform bacteria are the membrane filter technique and the multiple tube fermentation/most probable number technique.Heterotrophic or Standard Plate Count (HPC)This test gives the total count of almost all types of bacteria in the water sample that can grow on a general medium called the standard plate count agar or nutrient agar. A count less than 500 colonies/mL of the sample means that the water is properly disinfected, and vice versa. Furthermore, a count of higher than 500 colonies/mL interferes with the growth of total coliform bacteria.

ReviewA number of lab tests are needed daily, quarterly, semiannually, annually, and at other specified intervals to monitor the water quality before, during, and after the treatment. A sample is either a grab or a composite. Agrab sample, as the name indicates, is a specific volume collected at one site at one time. These samples indicate the quality of water at that time and at that site. Grab samples are taken for bacteriological and disinfection residual tests. Acomposite sampleis a mixture of a number of portions taken at the specific intervals. This reduces the number of tests. Each portion can be proportionate to the flow or volume.Various regularly performed common tests by the operating staff are for tastes and odor, turbidity, jar test, pH, alkalinity, hardness, disinfection residual, coliform bacteria, and the heterotrophic plate count. All other tests are run either by highly trained chemists and microbiologists of the lab or by certified contract laboratories.