What are the Various Wastewater Treatment Processes?

The different stages employed in a wastewater treatment plant is dependent on the source of wastewater, the characteristics of waste and the facility of the treatment plant. Hence, there are different types of wastewater treatment plant like industrial , agricultural , sewage treatment plant etc. Each plant employ different order of treatment stages and method. Even if the treatment methods vary, any wastewater treatment facility conduct a set of typical processes like coagulation, filtration, flocculation etc.


The article explains the different processes through which the wastewater passes through as a part of treatment.

1. Coagulation

Coagulation is defined as the process of removing bulk suspended solids and other contaminants in the wastewater by adding various chemicals. The process is conducted in a reaction tank. At first, the mixing reactor starts to add chemicals to the wastewater and assort the mix. The method makes the finer particles to combine with the chemical to form heavier particle, which later on settles due to heavy weight. Aluminium -based coagulants like alum and polyaluminium chloride are the most commonly used for coagulation.

2. Flocculation


The flocculation process comes after coagulation. The water after coagulation, enters the flocculation chamber where it is subjected to slight stirring along with some charged molecules. These charged molecules are generally long-chain polymers that attracts all the colloidal particles present in the wastewater and pull them together. These finally convert colloidal particles into settleable particles.

3. Sedimentation


The sedimentation process is performed in a sedimentation tank. It is a large circular device into which the flocculated material and water flow enters by undergoing a circular motion. A slow settling process happens, where the solids settle down at the bottom of the clarifier and then into a sludge blanket. At the same time water rises up and overflows at the perimeter of clarifier.

The solids settled are raked to the centre of the clarifier. The sludge is mixed, pumped out or undergoes further dewatering.


4. Filtration

After sedimentation, the water is allowed to pass through gravity sand filters. The arrangement consists of finely crushed silica sand placed at a depth of two to four feet width which is properly packed at the bottom of the filter. As the feed particles pass through this, the particles trap within the sand.

The filter used for filtration varies with the type of industry. A packed bed pressure multimedia filter is used for small industrial treatment systems. Sand filter is the most conventional filtration system.

Another choice is the use of ultra-clarification (UF). The process can replace the whole process or it is used after clarification.

5. Disinfection


The bacteria present in the water after the filtration process is removed by disinfection. Chlorination is the most commonly used disinfection process.

6. Lime Softening


If the water obtained after the final treatment possess hardness or sulphates in undesirable amounts, then lime softening process is undertaken. The method raises the pH of the water and make the hardness and metal to precipitate. The method used can be either cold, warm or hot lime processes based on the efficiency required.

7. Iron Exchange Softening


Iron exchange softening is the process of removing hardness by means of a softening resin which undergoes a strong acid cation exchange process. This is a post treatment employed to remove hardness of water.

8. Distribution

Industries reuse the wastewater by pumping the treated water to a holding tank and used based on demand. It is given to distribution system of water towers and various collection services if it used for municipal purposes.

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