What is sedimentation of drinking water

The water that reaches our homes comes from rivers, streams, lakes, reservoirs and underground water reservoirs . This water naturally contains microorganisms (cyanobacteria, diatoms, dinoflagellates …), organic matter and inorganic particles, such as metals. The water we drink does not have the same characteristics as the water we find in nature. To avoid the health risks posed by non-potable water , it must undergo a purification process that consists in the elimination of microorganisms and particulate matter in suspension so that it is suitable for human consumption. Water purification It occurs in centers called Drinking Water Treatment Stations or ETAP, or in Water Purification Stations or EPA.

In the Water Treatment Stations, two main processes occur which consist, on the one hand, in the elimination of the particles that are in the water and, on the other hand, in the inactivation of the microorganisms. The water reaches the treatment stations through a series of pipes, aqueducts or built channels that connect the water intake areas with the stations. The water purification treatments that occur in these plants are:

• Coagulation: the water enters the tanks for “quick mixing”. A series of coagulating polymers and chlorine are added to create compounds that are then sedimented and removed. Chlorine is added to disinfect the water from pathogenic microorganisms. In these tanks the water moves at high speed to favor a greater union of the chemicals added with the water particles.
• Flocculation: then the water passes to the flocculating tanks. Here, the water moves more slowly to encourage the formation of flocs or clumps of the suspended solids in the water.
• Sedimentation: the water is then transferred to other tanks for the flocs to settle. For this it is necessary that the water moves very slowly.
• Filtration: the water moves towards filters for the elimination of small suspended particles that have not been eliminated before, such as remains of microorganisms, algae, smaller flocs, etc. Normally the filters are made of sand, gravel and activated carbon.
• Storage: finally the drinking water is stored in other tanks where chlorine and fluorides are also usually added to disinfect the water from possible microorganisms that have not been previously eliminated and also so that they do not proliferate while the water is stored in these tanks. From here, the water is distributed to the towns through a network of pipes.

You may also be interested in completing the information with this other AgroCorrn article on the different types of wastewater treatment .

Sedimentation is the process by which the solids that are in suspension in the water fall to the bottom of the container where the water is contained. Sedimentation is a natural process that occurs due to the effect of gravity. Although it happens in rivers and lakes, people have used this phenomenon to achieve purer and safer water .

Sedimentation is based on Stokes’ Law, according to which particles larger or heavier than water will have a higher sedimentation capacity. The viscosity of the liquid also influences, the lower the viscosity, the higher the sedimentation capacity and speed.

Suspended particles can be classified according to their diameter and state of suspension:

• Suspended particles up to 10 ^-4 cm.
• Colloids with particle sizes between 10 ^-4 and 10 ^-6 cm.
• Solutions with particles smaller than 10 ^-6 cm.

Based on this classification, there is a parallel way of classifying the methods or types of water sedimentation according to these types of particles:

• 1st case: Simple sedimentation. Particles up to 10 ^-4 cm are capable of sedimentation only by physical processes, such as gravity.
• 2nd case: The colloids must coagulate to form sedimentable flocs. You need to add chemicals.
• 3rd case: Soluble substances must become insoluble in order to form sedimentable flocs. As in the previous case, the addition of chemical products is necessary.

For the sedimentation of the particles to occur, it is necessary that the flow rate of the water is lower than the sedimentation rate of the suspended solids they contain. This concept of surface loading is essential when constructing sedimentation tanks.

Some examples of sedimentation speeds and times are, for a distance of 0.3 m, 38 seconds for sand grains with a speed of 88 millimeters per second (mm / s), 35 hours for bacteria conglomerates with a speed of 0 , 00154 mm / s and in the case of colloids the time could amount to 63 years and a speed of 0.000000154 mm / s. Naturally, the sedimentation rate also depends on other factors such as the water temperature and the diameter and specific gravity of the particles.

The sedimentation tanks can be circular or rectangular and have four distinct parts.

• Inlet zone: it is the zone through which the water enters the sedimentation tank. It is necessary to control the speed to avoid turbulence. Water usually enters with a velocity not exceeding 5 cm / s.
• Sedimentation zone: requires a uniform and slow flow of water to facilitate the deposition of the particles at the bottom, where the sludge zone is located.
• Sludge zone: sludge is the result of the deposition of solid particles contained in water. At the base there is a hatch through which the sludge is extracted. It is also necessary to remove them to avoid the obstruction of said hatch, but it must be done at a very low speed to avoid its re-suspension.
• Exit area: it is the area through which the water leaves this tank. In addition, it has structures that retain floating materials from the water.