floc. 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 the plant. Coagulation will be affected by changes in the water's pH, salt content, alkalinity, turbidity, and temperature.
Full Answer
Why is coagulation and flocculation used in water treatment?
first process in drinking water treatment, coagulation, is to destabilize the particles and allow them the potential to collide and stick together. There are four main mechanisms for coagulation: double layer compression, adsorption and charge
What are the factors that affect coagulation?
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 Calculations
What causes flocs to form in flocculation?
Many water purification methods require pretreatments such as coagulation and flocculation. A coagulant is added to the source water in traditional coagulation-flocculation-sedimentation to produce an attraction between the suspended particles. Slowly stirring the fluid causes the particles to clump together forming “flocs.”. The water is then pumped into a silent …
What happens to the particles in water during coagulation?
Apr 03, 2018 · It’s what our blood does when it combines with oxygen, thereby making a scab or a blood clot. It’s the same principle with wastewater treatment. In coagulation treatment, a harmless chemical such as alum causes all of the particles to give off a positive charge and thus clump together, making them easier to filter.
What are the factors affecting coagulation and flocculation?
The factors that influence coagulation–flocculation are, among others, temperature, pH, effluent quality, dosage and coagulant type (Nnaji 2012; Jin 2005; Ma et al. 2001). The suspended particles vary considerably in source, composition charge, particle size, shape, and density.Feb 1, 2015
What affects coagulation in water treatment?
Factors. Coagulation is affected by the type of coagulant used, its dose and mass; pH and initial turbidity of the water that is being treated; and properties of the pollutants present. The effectiveness of the coagulation process is also affected by pretreatments like oxidation.
How can we improve the coagulation and flocculation process for improved water treatment?
Water Treatment 101: 6 Tips for Improving Coagulation, Flocculation and ClarificationUse the correct polymer or coagulating chemicals. ... Keep injection point close. ... Slow mixing is good mixing. ... Velocity gradient is important. ... Keep an eye on temperature. ... Balance the velocity.May 15, 2014
What causes flocculation in water treatment?
Flocculation is a water treatment process where solids form larger clusters, or flocs, to be removed from water. This process can happen spontaneously, or with the help of chemical agents. It is a common method of stormwater treatment, wastewater treatment, and in the purification of drinking water.Jul 31, 2019
What does coagulation and flocculation remove?
Coagulation and flocculation remove a large number of organic compounds as well as suspended particles, including inorganic precipitates. The process removes particles and dissolved matter; however, the water may still contain pathogens, including viruses and bacteria.
What factors affect coagulation?
The following are coagulation factors and their common names:Factor I - fibrinogen.Factor II - prothrombin.Factor III - tissue thromboplastin (tissue factor)Factor IV - ionized calcium ( Ca++ )Factor V - labile factor or proaccelerin.Factor VI - unassigned.Factor VII - stable factor or proconvertin.More items...
What is flocculation and how does it improve filtration in a water treatment plant?
Flocculation is gentle stirring or agitation to encourage the particles thus formed to agglomerate into masses large enough to settle or be filtered from solution.May 24, 2019
How does pH affect coagulation and flocculation?
pH EFFECTS pH that is too low may not allow the coagu- lation process to proceed, while high pH can cause a coagulated particle to redisperse. The size of the coagulated particles is also affected by pH, which, in turn, determines the density of the flocculated slime and its tendency and rate of settling out.
What makes a good flocculant?
Polymers are useful as flocculants because they are robust molecules and sometimes carry charges. Because they are so large, small particles can get trapped in the curves of the polymer causing them to accumulate a mass heavy enough to prevent their retention in solution.
What is coagulation in water treatment?
Coagulation is the chemical water treatment process used to remove solids from water, by manipulating electrostatic charges of particles suspended in water. This process introduces small, highly charged molecules into water to destabilize the charges on particles, colloids, or oily materials in suspension.Dec 20, 2019
Why is coagulation and flocculation necessary?
Coagulation and flocculation are used to separate the suspended solids portion from the water. Suspended particles vary in source, charge, particle size, shape, and density. Correct application of coagulation and flocculation depends upon these factors.
What is flocculation coagulation?
Coagulation is the destabilization of colloidal particles brought about by the addition of a chemical reagent called as coagulant. Flocculation is the agglomeration of destabilized particles into microfloc and after into bulky floccules which can be settled called floc.
What is the best coagulant for organics removal?
Organics removal and enhanced coagulation are effective with traditional coagulants like aluminum sulfate, ferric chloride and ferric sulfate, as well as formulations like polyaluminum chloride (PACl) and acid alum. Acid alum formulations are aluminum sulfate with 1 to 15-percent free sulfuric acid.
What are the variables in coagulation?
The efficiency of the coagulation-flocculation process is dependent on many variables. For a particular water these may include: 1 Type of coagulant used 2 Coagulant dosage 3 Final pH 4 Coagulant feed concentration 5 Type and dosage of chemical additives other than primary coagulant (e.g. polymers) 6 Sequence of chemical addition and time lag between dosing points 7 Intensity and duration of mixing at rapid mix stage 8 Type of rapid mix device 9 Velocity gradients applied during flocculation stage 10 Flocculator retention time 11 Type of stirring device used 12 Flocculator geometry.
How does orthokinetic flocculation work?
Orthokinetic flocculation arises from induced velocity gradients in the liquid. It is here that primary particles are induced to approach close enough together, make contact and progressively form larger agglomerates, or flocs. The principal parameter governing the rate of orthokinetic flocculation is the velocity gradient applied. The degree or extent of flocculation is governed by both applied velocity gradients and time of flocculation. These two parameters influence the rate and extent of particle aggregation and the rate and extent of breakup of these aggregates.
What happens when you add coagulants to water?
When metal coagulants are added to water the metal ions (Al and Fe) hydrolyze rapidly but in a somewhat uncontrolled manner, forming a series of metal hydrolysis species. The efficiency of rapid mixing, the pH, and the coagulant dosage determine which hydrolysis species is effective for treatment.
What is the rapid mixing stage?
The rapid mixing stage is possibly the most important component of coagulation-flocculation processes, since it is here that destabilization reactions occur and where primary floc particles are formed, whose characteristics markedly influence subsequent flocculation kinetics. In general it is likely that the metal coagulant hydrolysis products that are formed within the time range 0.01 to 1.0 seconds are the most important for effective destabilization. In many instances, traditional 30 to 60 second retention times during rapid mixing are unnecessary and flocculation efficiency may not improve beyond rapid mix times of approximately 5 seconds or less. Indeed, beyond a certain optimum rapid mix time, a detrimental effect on flocculation efficiency may result.
What are some examples of coagulation operations?
Coagulation operations can be useful in some cases for the removal of inorganics. Examples of successful applications are copper and mercury reductions from wastewaterplant effluents. Two applications discussed in more detail below are arsenic and fluoride removals in potable water treatment:
What is the sequence of chemical addition?
Traditionally, the sequence of chemical addition for coagulation operations is to first add chemicals for pH correction, then add the metal coagulant, then add the flocculant aid. Not all these chemicals are necessarily added, but the sequence logic is often as described. However, there are instances when other sequences are more effective, including inverting the sequence of metal coagulant and polymer addition, and the sequence of metal coagulant addition and pH adjustment. The best sequence for a particular application can be determined by jar test experiments.
How do coagulants feed into water?
Coagulants are usually fed into the water using a gravimetric feeder or a metering pump. A gravimetric feeder 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.
What is a coagulant aid?
Coagulant aids add 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.
Why is it important to mix and detention times?
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 in the flocculation basins.
What are the two main types of coagulants?
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.
What is the most commonly used coagulant in water treatment plants?
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 liquid form with a concentration of 8.3%, or in dry form with a concentration of 17%.
What is lime used for?
Lime is a coagulant aid used to increase the alkalinity of the water. The increase in alkalinity results in an increase 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.
What is the charge at the boundary of the colloidal turbidity particle and the surrounding water?
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.
What is a flocculant?
Flocculants are lightweight, medium weight and heavy polymers that cause the destabilized clumps of particles to agglomerate and drop out of the solution, removing them from the filtered water. The weight used depends on the type of particle.
What percentage of giardia is removed?
The U.S. EPA surface water treatment rule requires 99.9 percent (3-log) Giardia removal or inactivation, and at least 99 percent (2-log) removal of Cryptosporidium. The combination of coagulation and flocculation is particularly useful at exceeding these guidelines.
Why is flocculation so popular?
Coagulation and flocculation processes have become more and more popular due to the increasingly stringent filtration requirements for industrial and municipal water treatment and wastewater treatment facilities levied by the U.S. Environmental Protection Agency (EPA.)
What is coagulation in water treatment?
Coagulation water treatment is the first step in chemical wastewater treatment. Instead of passing over particles that would otherwise slip through the filter and fall too slowly to be trapped as sediment, coagulation clumps them together so they are more easily removed. Most of us know coagulation from anatomy class.
When was coagulation water used?
Yet coagulation water treatment is far from being a new process. In fact, it was in use by the Egyptians as early as 2,000 B.C. Later the Romans used the coagulation process in water treatment, as did the English in the 18th century.
Is alum a good coagulant?
It’s the same principle with wastewater treatment. In coagulation treatment, a harmless chemical such as alum causes all of the particles to give off a positive charge and thus clump together, making them easier to filter. Coagulation is especially useful in removing the chemical phosphorus from water. Yet coagulation water treatment is far ...
How to remove turbidity from 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.
How does flocculation work?
Flocculation follows coagulation in the conventional water treatment process. Flocculation causes the agglomeration or collection of small particles into larger, more readily settleable floc particles. It is usually a tapered process with two or more units in series. The first unit has a higher mix speed and the mixing speed decreases in subsequent basins to promote the growth of floc particles. The total detention time in the flocculation basin is normally 30 to 60 minutes. Through experience, we see that effective mixing reduces the required amount of chemicals and greatly improves the sedimentation process, which results in longer filter runs and higher quality finished water. The goal of flocculation is to form a uniform, feather-like material similar to snowflakes - a dense, clingy floc that traps the fine, suspended, and colloidal particles and carries them down rapidly in the settling basin. To increase the speed of floc formation and the strength and weight of the floc, polymers are often added.
What is coagulation reaction?
cationic, anionic, and nonionic) and molecular weight. Coagulation is a reaction caused by adding salts of iron or aluminum to the water.
Why is coagulation important in water treatment?
It is, however, an important primary step in the water treatment process, because coagulation removes many of the particles, such as dissolved organic carbon, that make water difficult to disinfect. Because coagulation removes some of the dissolved substances, less chlorine must be added to disinfect the water.
What is the purpose of coagulation?
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.
What happens when water flows over the ground?
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.
How long does it take for a water mixer to shear?
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.
What is the purpose of flocculation and clarification?
The general purpose of this sequence is to remove solids and turbidity from the water to produce clear product water and meet turbidity standards, expressed as NTUs (nephelometric turbidity units). Coagulation, flocculation and clarification can ...
How to remove bacteria and color from water?
Coagulation, flocculation and clarification can also remove bacteria and color from the water. First, coagulating chemicals are mixed with water in a rapid-mix step for even distribution. The chemicals cause particles in the water to clump together. In the flocculation step, the water and chemicals are gently mixed, ...
What is clarified water?
The clarified water passes on to downstream processes such as filtration and disinfection. Like most water treatment processes, coagulation, flocculation and clarification sound simple but are subject to many variables that affect the efficiency and success of the operation.
Is slow mixing good for mixing?
“The further away the mixing is from the injection point, the less (effective) mixing you’re going to see,” Zurcher says. 3. Slow mixing is good mixing.
What is the purpose of coagulation and flocculation?
The primary purpose of the coagulation and flocculation is to destabilize the charged colloidal particles in water and make them to settle so as to remove turbidity from the water. In addition to removing turbidity from the water, coagulation and flocculation process removes many bacteria which are suspended ...
What is the process of adding coagulant to water?
Coagulation is a unit process of addition of coagulant chemicals to water and rapid mixing so as to neutralize the electrical charges of the colloidal particles in the water, and allow them to come closer and form fine clumps or micro flocs. No related topics.
What are the main phenomena that control the behaviour of colloids?
The principal phenomena that control the behaviour of the colloids are zeta potential (electrostatic force), Vander Walls forces and Brownian motion.
What is the zeta potential of a coagulant?
The amount of coagulant to be added to the water will depend on the zeta potential, a measurement of the magnitude of electrical charge surrounding the colloidal particles. The zeta potential is the amount of repulsive force or electric charge, which keeps the particles in the water.
What are the three types of suspended particles in water?
In the order from smallest to largest, these particles are chemicals in solution, suspended colloidal solids, and suspended discrete solids.
What happens when enough particles join together?
When enough particles have joined together, they become floc and will settle out of the water. The chemistry of coagulation and flocculation is primarily based on electricity. Electricity is the behavior of negatively and positively charged particles due to their attraction and repulsion.
What are some examples of solids that can cause water to be colored?
Examples include bacteria, fine clays, and silts. Colloidal solids often cause colored water, such as the “tea color” of swamp water. Suspended settle able 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.
Coagulation and Flocculation in Water and Wastewater Treatment
The Coagulants
- The commonly used metal coagulants fall into two general categories: those based on aluminum and those based on iron. The aluminum coagulants include aluminum sulfate, aluminum chloride and sodium aluminate. The iron coagulants include ferric sulfate, ferrous sulfate, ferric chloride and ferric chloride sulfate. Other chemicals used as coagulants include hydrated lime and magn…
Removal of Natural Organic Matter
- Natural organic material (NOM) is usually associated with humic substances arising from the aqueous extraction of living woody substances, the solution of degradation products in decaying wood and the solution of soil organic matter. These substances are objectionable for a number of reasons: they tend to impart color to waters; they act as a vehicle for transporting toxic substan…
Pathogen Removal
- The U.S. EPA surface water treatment rule requires 99.9-percent (3-log) Giardia removal or inactivation, and at least 99-percent (2-log) removal of Cryptosporidium. Adequately designed and operated water treatment plants, with coagulation, flocculation, sedimentation and filtration are assigned a 2.5-log removal credit for Giardia, leaving only 0.5-log inactivation to be achieved by …
Removal of Inorganics
- Coagulation operations can be useful in some cases for the removal of inorganics. Examples of successful applications are copper and mercury reductions from wastewaterplant effluents. Two applications discussed in more detail below are arsenic and fluoride removals in potable water treatment:
Wastewater Treatment
- Physical-chemical treatment of wastewater was widely practiced until the late 19th century, until the advent of the trickling filter for biological treatment. The early 1970s saw a partial revival of interest that has continued to the present day, particularly for treatment plants that are overloaded during peak flow events. The addition of coagulant chemicals to primary clarifiers, or to other de…
Factors Affecting Coagulation Operations
- Temperature
Temperature significantly affects coagulation operations, particularly for low turbidity waters, by shifting the optimum pH. This can be mitigated by operating at an optimum pOH as given by: pH + pOH = pKW; where pKW = 0.01706xT + 4470.99/T – 6.0875 and T = temperature in °K = 273.15 … - Sequence of chemical addition
Traditionally, the sequence of chemical addition for coagulation operations is to first add chemicals for pH correction, then add the metal coagulant, then add the flocculant aid. Not all these chemicals are necessarily added, but the sequence logic is often as described. However, t…
Rapid Mixing
- The rapid mixing stage is possibly the most important component of coagulation-flocculation processes, since it is here that destabilization reactions occur and where primary floc particles are formed, whose characteristics markedly influence subsequent flocculation kinetics. In general it is likely that the metal coagulant hydrolysis products that are formed within the tim…
Flocculation
- Orthokinetic flocculation arises from induced velocity gradients in the liquid. It is here that primary particles are induced to approach close enough together, make contact and progressively form larger agglomerates, or flocs. The principal parameter governing the rate of orthokinetic flocculation is the velocity gradient applied. The degree or extent of flocculation is g…
Testing and Control
- The efficiency of the coagulation-flocculation process is dependent on many variables. For a particular water these may include: 1. Type of coagulant used 2. Coagulant dosage 3. Final pH 4. Coagulant feed concentration 5. Type and dosage of chemical additives other than primary coagulant (e.g. polymers) 6. Sequence of chemical addition and time lag between dosing points …