Design Of Coagulation And Flocculation Tank Pdf


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design of coagulation and flocculation tank pdf

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This article provides an overview of the processes and looks at the latest thinking. Material for this article was largely taken from reference 1.

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Suspended matter in raw water supplies is removed by various methods to provide a water suitable for domestic purposes and most industrial requirements. The suspended matter can consist of large solids, settable by gravity alone without any external aids, and nonsettleable material, often colloidal in nature. Removal is generally accomplished by coagulation, flocculation, and sedimentation. The combination of these three processes is referred to as conventional clarification.

Coagulation is the process of destabilization by charge neutralization. Once neutralized, particles no longer repel each other and can be brought together. Coagulation is necessary for the removal of the colloidal-sized suspended matter. Flocculation is the process of bringing together the destabilized, or "coagulated," particles to form a larger agglomeration, or "floc.

Sedimentation refers to the physical removal from suspension, or settling, that occurs once the particles have been coagulated and flocculated. Sedimentation or subsidence alone, without prior coagulation, results in the removal of only relatively coarse suspended solids. Steps of Clarification. Finely divided particles suspended in surface water repel each other because most of the surfaces are negatively charged. The following steps in clarification are necessary for particle agglomeration:.

Coagulation can be accomplished through the addition of inorganic salts of aluminum or iron. These inorganic salts neutralize the charge on the particles causing raw water turbidity, and also hydrolyze to form insoluble precipitates, which entrap particles.

Coagulation can also be effected by the addition of water-soluble organic polymers with numerous ionized sites for particle charge neutralization. Therefore, coagulation involves neutralizing charged particles to destabilize suspended solids. In most clarification processes, a flocculation step then follows. Flocculation starts when neutralized or entrapped particles begin to collide and fuse to form larger particles. This process can occur naturally or can be enhanced by the addition of polymeric flocculant aids.

Inorganic Coagulants. Table lists a number of common inorganic coagulants. Typical iron and aluminum coagulants are acid salts that lower the pH of the treated water by hydrolysis. Depending on initial raw water alkalinity and pH, an alkali such as lime or caustic must be added to counteract the pH depression of the primary coagulant.

Iron and aluminum hydrolysis products play a significant role in the coagulation process, especially in cases where low-turbidity influent waters benefit from the presence of additional collision surface areas.

Variation in pH affects particle surface charge and floc precipitation during coagulation. Iron and aluminum hydroxide flocs are best precipitated at pH levels that minimize the coagulant solubility.

However, the best clarification performance may not always coincide with the optimum pH for hydroxide floc formation. Also, the iron and aluminum hydroxide flocs increase volume requirements for the disposal of settled sludge. With aluminum sulfate, optimum coagulation efficiency and minimum floc solubility normally occur at pH 6. Iron coagulants can be used successfully over the much broader pH range of 5.

If ferrous compounds are used, oxidation to ferric iron is needed for complete precipitation. This may require either chlorine addition or pH adjustment. The chemical reactions between the water's alkalinity natural or supplemented and aluminum or iron result in the formation of the hydroxide coagulant as in the following:. The term polyelectrolytes refers to all water-soluble organic polymers used for clarification, whether they function as coagulants or flocculants.

Polymeric primary coagulants are cationic materials with relatively low molecular weights under , The cationic charge density available positively charged sites is very high. Polymeric flocculants or coagulant aids may be anionic, cationic, or nonionic. Their molecular weights may be as high as 50,, Table describes some typical organic polyelectrolytes.

For any given particle there is an ideal molecular weight and an ideal charge density for optimum coagulation. There is also an optimum charge density and molecular weight for the most efficient flocculant. Because suspensions are normally nonuniform, specific testing is necessary to find the coagulants and flocculants with the broadest range of performance. They exhibit strong cationic ionization and typically have molecular weights of less than , When used as primary coagulants, they adsorb on particle surfaces, reducing the repelling negative charges.

These polymers may also bridge, to some extent, from one particle to another but are not particularly effective flocculants. The use of polyelectrolytes permits water clarification without the precipitation of additional hydroxide solids formed by inorganic coagulants. The pH of the treated water is unaffected. The efficiency of primary coagulant poly-electrolytes depends greatly on the nature of the turbidity particles to be coagulated, the amount of turbidity present, and the mixing or reaction energies available during coagulation.

With lower influent turbidities, more turbulence or mixing is required to achieve maximum charge neutralization. Raw waters of less than 10 NTU Nephelometric Turbidity Units usually cannot be clarified with a cationic polymer alone. Best results are obtained by a combination of an inorganic salt and cationic polymer. In-line clarification should be considered for raw waters with low turbidities.

Generally, waters containing 10 to 60 NTU are most effectively treated with an inorganic coagulant and cationic polymer. In most cases, a significant portion of the inorganic coagulant demand can be met with the cationic polyelectrolyte. With turbidity greater than 60 NTU, a polymeric primary coagulant alone is normally sufficient. In low-turbidity waters where it is desirable to avoid using an inorganic coagulant, artificial turbidity can be added to build floc.

Bentonite clay is used to increase surface area for adsorption and entrapment of finely divided turbidity. A polymeric coagulant is then added to complete the coagulation process. Coagulant Aids Flocculants. In certain instances, an excess of primary coagulant whether inorganic, polymeric, or a combination of both may be fed to promote large floc size and to increase settling rate.

However, in some waters, even high doses of primary coagulant will not produce the desired effluent clarity. A polymeric coagulant aid added after the primary coagulant may, by developing a larger floc at low treatment levels, reduce the amount of primary coagulant required. Generally, very high-molecular-weight, anionic polyacrylamides are the most effective coagulant aids.

Nonionic or cationic types have proven successful in some clarifier systems. Essentially, the polymer bridges the small floc particles and causes them to agglomerate rapidly into larger, more cohesive flocs that settle quickly.

The higher-molecular-weight polymers bridge suspended solids most effectively. Coagulant aids have proven quite successful in precipitation softening and clarification to achieve improved settling rates of precipitates and finished water clarity. Color Reduction. Frequently, the objective of clarification is the re-duction of color. Swamps and wetlands introduce color into surface waters, particularly after heavy rainfalls.

Color-causing materials can cause various problems, such as objectionable taste, increased microbiological content, fouling of anion exchange resins, and interference with coagulation and stabilization of silt, soluble iron, and manganese. Most organic color in surface waters is colloidal and negatively charged.

Chemically, color-producing compounds are classified as humic and fulvic acids. Color can be removed by chlorination and coagulation with aluminum or iron salts or organic polyelectrolytes.

Chlorine oxidizes color compounds, while the inorganic coagulants can physically remove many types of organic color by neutralization of surface charges. The use of chlorine to oxidize organic color bodies may be limited due to the production of chlorinated organic by-products, such as trihalomethanes.

Additional color removal is achieved by chemical interaction with aluminum or iron hydrolysis products. Highly charged cationic organic polyelectrolytes can also be used to coagulate some types of color particles. Coagulation for color reduction is normally carried out at pH 4. Optimum pH for turbidity removal is usually much higher than that for color reduction. The presence of sulfate ions can interfere with coagulation for color reduction, whereas calcium and magnesium ions can improve the process and broaden the pH range in which color may be reduced effectively.

Conventional Clarification Equipment. Originally, conventional clarification units consisted of large, rectangular, concrete basins divided into two or three sections. Each stage of the clarification process occurred in a single section of the basin. Water movement was horizontal with plug flow through these systems.

Because the design is suited to large-capacity basins, horizontal flow units are still used in some large industrial plants and for clarifying municipal water.

The retention time is normally long up to hr , and is chiefly devoted to settling. Rapid mix is typically designed for min and slow mix for min. This design affords great flexibility in establishing proper chemical addition points.

Also, such units are relatively insensitive to sudden changes in water throughput. The long retention also allows sufficient reaction time to make necessary adjustments in chemical and polymer feed if raw water conditions suddenly change.

However, for all but very large treated water demands, horizontal units require high construction costs and more land space per unit of water capacity. Compact and relatively economical, upflow clarifiers provide coagulation, flocculation, and sedimentation in a single usually circular steel or concrete tank. These clarifiers are termed "upflow" because the water flows up toward the effluent launders as the suspended solids settle.

They are characterized by increased solids contact through internal sludge recirculation. This is a key feature in maintaining a high-clarity effluent and a major difference from horizontal clarifiers. Because retention time in an upflow unit is approximately hr, upflow basins can be much smaller in size than horizontal basins of equal throughput capacity. A rise rate of 0.

design of coagulation and flocculation tank pdf

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Enhancing the coagulation process is currently a popular research topic. In this review article, the latest developments in enhanced coagulation are summarized. In addition, the mechanisms of enhanced coagulation and the effect of process parameters on processing efficiency are discussed from the perspective of ballast-enhanced coagulation, preoxidation, ultrasound, and composite coagulants. Finally, improvements and new directions for enhanced coagulation are proposed. Material from this article can be used in other publications provided that the correct acknowledgement is given with the reproduced material and it is not used for commercial purposes. Information about reproducing material from RSC articles with different licences is available on our Permission Requests page. Fetching data from CrossRef.

Many water treatment plants use a combination of coagulation, sedimentation, filtration and disinfection to provide clean, safe drinking water to the public. Worldwide, a combination of coagulation, sedimentation and filtration is the most widely applied water treatment technology, and has been used since the early 20th century. For more information about the disinfection portion of the treatment process, see the Chlorination fact sheet. The coagulation process involves adding iron or aluminum salts, such as aluminum sulphate, ferric sulphate, ferric chloride or polymers, to the water. These chemicals are called coagulants, and have a positive charge. The positive charge of the coagulant neutralizes the negative charge of dissolved and suspended particles in the water. When this reaction occurs, the particles bind together, or coagulate this process is sometimes also called flocculation.


Once floc has reached it optimum size and strength, water is ready for sedimentation. Design contact times for flocculation range from 15 or 20 minutes to an hour.


Suspended matter in raw water supplies is removed by various methods to provide a water suitable for domestic purposes and most industrial requirements. The suspended matter can consist of large solids, settable by gravity alone without any external aids, and nonsettleable material, often colloidal in nature. Removal is generally accomplished by coagulation, flocculation, and sedimentation. The combination of these three processes is referred to as conventional clarification. Coagulation is the process of destabilization by charge neutralization.

However, water currents also tend to distribute the floc unevenly throughout the tank; as a result, it does not settle out at an even rate. The gentle mixing is usually done mechanically although hydraulic mixing is sometimes required. Coagulation and flocculation are an essential part of drinking water treatment as well as wastewater treatment. When coagulants are added, the stabilised colloidal system of water is disturbed and the coagulant forms primary flocculation particles.

Coagulation is one of the preliminary steps in treating wastewater to make it safe for drinking. When chemicals are used, key design practices on the storage tank can help ensure safety, longevity, and operational efficiency. Every water source contains small floating particles in the water, which must be removed to make the water potable. These floating particles, called colloids, are too light to settle by gravity. They are made up of both non-organics such as clay or silt and organics such as algae or bacteria.

The coagulation and flocculation process requires a great amount of attention to detail along the way.

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LIST OF TABLES. Table 1: Summary of Routine Coagulation-Flocculation-​Clarification Process Actions. 40 Other shapes and designs of tank utilizing the.


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