Water, usually surface water, contains many impurities in the form of dissolved and suspended particles. The suspended particles are different in size, composition, charge, shape, density etc. There are some methods to purify surface water from these types of impurities, like Coagulation and flocculation processes, filtration, disinfection etc. Both these methods are used for the separation of suspended solid particles from the water.
Coagulation is the process by which a colloid precipitates out of a solution. The precipitation is brought about by induced aggregation. For example, coagulation is the process by which a colloid precipitates out of a solution. The precipitation is brought about by induced aggregation.
Flocculation is a physical and chemical process which is used for the removal of the visible sediments and material from water which makes it a colloidal solution. It can be done through agitation or by adding flocculating agents. Colloidal solution is a heterogeneous mixture in which the collidal particles can't be seen by the eye and can pass through a filter paper. Swimming pool water and waste water contain visible sediments.
In the flocculation process, the polymers are used as flocculating agent for the formation of bridges between the flocs (clumps of bacteria and impurities which form a cluster). After slow addition of anionic flocculants or flocculating agents, these agents get adsorbed on the particles by reacting with positively charged suspension. It is very essential to gently mix the flocculating agent at a slow speed so that small flocs can easily agglomerate into large particles.
These large bind particles are known as clumps or agglomerates. They are formed by the aggregation of particles with the adsorption of the polymer chain segments on the particles. These agglomerated particles are fragile in nature. They can easily break into their particles by a sheer force, so mixing should be done gently. The flocculation reaction affects the size and physical nature (makes them less gelatinous) of flocs.
After flocculating the suspended particles into larger particles, they are generally separated from the fluid by a sedimentation technique as there is a density difference between suspended matter and the liquid. They can also be separated by other techniques like filtration, floatation and straining.
Filtration can be done in the absence of flocculating agents because the particles formed in the previous coagulation process are large enough to separate via filtration. The use of polymer as a flocculating agent should be done with great care because an overdose will make the setting process difficult. As they are lighter than water, a high dose of anionic polymer will increase the floating ability of flocculation.
Coagulation flocculation methods are important pretreatments in water purification plants. Both are used for waste water treatment. Both are done in a particular sequence.
- It is a combination of chemical and physical processes. In the waste water, these collides are suspended in the water. These particles are attracted by the electric charges on their surfaces and get stabilized.
- Due to this attraction force towards the surface, they don't have tendency to settle down or to make flocs.
- In the coagulation and flocculation processes, chemicals are mixed in water to increase the aggregation of the suspended solids into large particles so that they can be removed or settle down.
- Coagulation is the process for destabilization of colloids. This is done by neutralizing the forces. A cationic coagulant is added to the water which forms the attraction force among the suspended particles.
- The positive electric charge of the coagulant reduces the negative charge of the colloids. Thus the particles form larger particles or flocs.
- Quick mixing and sufficient amount of coagulant should be taken. As the overdose of coagulant can be the reason for charge reversal and affect the stability of the colloid complex.
- Thus, coagulation is a process of thickening of solid suspended colloidal particles while flocculation is the process of forming clumps of solids in a solution, which is called flocs.
- Flocculation is always done after the completion of coagulation because the previous process (coagulation) makes the solids thick enough for clumping in the flocculation process.
- So the main difference is that coagulation is a method of thickening the particles while flocculation is a process of formation of bigger clumps or flocculent.
- The coagulation flocculation process is completed in three steps. The first is flash mixing, then coagulation and the last is flocculation. After pre-chlorination and aeration (optional steps), the water it is ready for coagulation and flocculation process.
- Flash mixer- In the flash mixer, the water is quickly mixed after adding coagulant chemical. So the coagulant is uniformly distributed in the water. Flash mixing should be completed within a few seconds (approx a minute).
After flash mixing, the coagulant chemical is added. It helps in neutralization of the particles in the water. So the particles come closer and make large clumps.
It is the last step. In the flocculation process, the particles formed by coagulation are gently mixed together with flocculent. This is done in a flocculation basin with the use of mixing blades. Thus this process makes the flocs. These are then allowed to settle down in the sedimentation basin and remaining is removed in the filter.
The water treatment process is completed in some continuous steps. These steps are described below.
- Coagulation and flocculation
Coagulation and flocculation
The purpose of coagulation is to make solid, suspended colloidal particles thick. This is done by adding sufficient amount of a cationic coagulant. It helps in neutralization of the suspended particles. In flocculation, a flocculent is added to form clumps of impurities. This is also called flocs. These flocs are heavy
enough to settle down. The flocculation is done after coagulation process because it’s easy for thick particles to make clumps of heavy particles.
This method is for the separation of solids or suspended solid particles which aggregate in the form of flocs. After the flocculation, water enters in the sedimentation basin. A deep basin is used so that more heavy flocs can settle down. The clumps or flocs gather at the bottom of the basin due to the gravitational force.
After removing the flocs, filtration is done for the removal of the remaining small particles such as microorganism, clay and slit etc. Different types of filters are used like rapid sand, slow sand filters etc.
This is the final step in the water treatment process. It is a process in which the amounts of pathogenic organisms, effect of organic compounds, suspended solids and contaminants are controlled. The disinfection efficacy is evaluated by using indicator organisms’ especially “Total coliform” because the presence of a specific pathogenic organism is very difficult to measure. Mostly fecal coliform is used in waste water treatment.
There are various chemicals and methods for disinfection of water. The most common systems are ozone, ultra-violet irradiation, chlorine, chlorine dioxide, and potassium permanganate. Thus the water comes after filtration, is allowed to go through the process of disinfection for its purification.
An iron (III) hydroxide sol can be made to aggregate by addition of an ionic solution. A positively charged particle of iron (III) hydroxide gathers a layer of anions around it. The thickness of this layer is determined by the charge on the anions. The greater the magnitude of the negative charge, the more compact the layer of charge. For e.g., phosphate ions gather more closely to the positively charged iron (III) particle than chloride ions.
Layers of ions surrounding a charged particle of iron (III) hydroxide.
- A: Fe(OH)3 surrounded by Cl- ions
- B: Fe(OH)3 surrounded by PO43- ions
If the ion layer is gathered close to the colloidal particles, the overall charge is effectively neutralized and two colloidal particles can approach close enough to aggregate and precipitate out.
The coagulation of colloids by an electrolyte takes place only when the electrolyte has a certain minimum concentration. The minimum concentration of electrolyte in milli moles that is added to one liter of the colloidal sol to bring about complete coagulation is called the flocculation value of the electrolyte for the solution.
Different electrolytes have different coagulation values. Smaller the coagulation value of the electrolyte, larger is its coagulating power. According to Hardy and Schulze, coagulation of colloids by electrolytes is governed by two factors, namely
- Ions carrying a charge opposite to that of the colloidal particles are effective in bringing about coagulation.
- Coagulation power of an electrolyte is directly proportional to the valency of its ions.
These two observations made by Hardy and Schulze are called the Hardy Schulze rule. On the basis of this rule, it is possible to predict that for colloidal sols consisting of negatively charged particles such as (AS2S3), Al3+ ions are more effective than Ba2+ or Na+ ions. For positively charged colloidal particles, PO43- is much more effective than Cl- to bring about coagulation.
Colloidal sols can be coagulated by mutual precipitation, electrophoresis, repeated dialysis and heating.
Some examples of coagulation which occur in nature are
- Curdling of Milk - Milk is a colloidal suspension in which the particles are prevented from aggregating because they have electric charges of the same sign. When milk sugar (lactose) ferments, ions responsible for curdling are formed.
- When river water meets the concentrated ionic solution of an ocean, coagulation of the colloidal suspension of the soil in river water occurs.