Water purification is the heart of water treatment, the process of purifying water involves the removal of unpleasant chemicals, biological pollutants, and suspended particles from water. The contaminants required levels and the quality of treated water will be based on water utilization purposes.
The removal of suspended solids in water is one of the important parts of the purification process in many water applications such as desalination, wastewater, and direct cooling systems. An efficient solids/liquid separation is a key factor for effective water management.
As water contains different types of suspended solids, suitable water treatment techniques must be selected according to the quality of the treated water, these techniques can be mechanical, chemical, and biological.
Typically, settling and filtration are the most common techniques to remove the suspended solids (SS) from water, however, the particle size of SS varies and thus the settling time required will differ as well.
Figure 1: Settling time required for different particle sizes considering a specific gravity of 2.65
Inadequate settling time is a challenge for a treatment plant to separate suspended solids from water. To overcome this limitation of the plant design, settling time should be expedited, Chemical treatment is applied to carry out mechanical treatment, such as settling, flotation, and filtration, more efficiently.
Solutions are classified based on the particle size inside of them into three different types, true solutions, colloidal solutions, and suspensions.
Figure 2: Solutions classification based on particle size
Colloidal particles are small insoluble particles with a dimension scale between nanometers to micrometers. These fine particles normally carry a negative charge on their surface, this charge creates repulsion forces which made the particles stabilized and suspended in the solution.
So, colloidal particles are fine in size, have low density, and are suspended in the solution due to the repulsion forces, making their tendency low to agglomerate and settle, However, liquid density is also one of the factors that also influence the tendency of fine particles to settle.
Coagulation is the destabilization of colloidal particles brought about by the addition of a chemical reagent called a coagulant. The destabilization is essential to the neutralization of the electrical charge present on the surface of the particle thus facilitating the aggregation of the colloids.
The coagulation mechanism includes two steps, the first step is a chemical reaction between the coagulant and the colloidal particles, it corresponds to lowering the repulsion effect between the negatively charged colloidal particles by adding the positive ions of the coagulant. The second step is a physical separation of the formed aggregates, where the micro flocs will settle down because it becomes heavier and dense.
Figure 3: Coagulation Mechanism
Coagulants are classified into three groups:
- Mineral coagulants are based mainly on mineral salts such as ferric chloride and ferric sulfate, this type of coagulant is the most used commonly due to its low cost and the wide range of applications that are suitable for.
- Organic coagulants are based on a long polymer chain that has positively charged functional groups, these coagulants have many advantages:
- It has a lower required dosing rate.
- It formed less sludge.
- It does not add metals to the treated water.
- It is not affecting the pH of water.
- Blend coagulants are a mixture of both mineral and organic coagulants, the main reason is to bring a solution for the treated water that has the advantage of a reasonable price and low dosage.
The choice of coagulant chemical depends upon the different parameters:
- The process design of the treatment plant and the used equipment.
- Operational conditions include injection point, mixing time, and contact time.
- Quality of the treated water.
- Required water quality such as Turbidity, TSS, and COD.
- The cost of the treatment.
Coagulant selection is done using different lab techniques, the most common one is jar testing, in this method, coagulation will be simulated at a laboratory scale typically at two sets of testing. First, different coagulants will be used at the same dosage to select the most proper coagulant, then the best-performed product will be used at different dosages for to define the best-required dosage to achieve certain water quality.
Figure 4: Jar Testing
After selecting the coagulant at the laboratory scale, a field test is done at the plant to verify the results, and quality of treated water and optimize the coagulant dosing rate based on real conditions.
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