Types of wastewater treatment

First, the wastewater is collected by a series of collector tubes that make it reach the Water Treatment Stations (WWTP) where it will be subjected to different treatments to purify it. In these stations they usually stay an average of 24-48 hours before being returned to their receiving channel, either a river, a reservoir or even the sea. Once they enter the WWTP, they are subjected to:

• Pretreatment , which consists of the removal of the largest solids such as sands and oils.
• Primary treatment .
• Secondary treatment , in case you want to further purify the water
• Finally, a tertiary treatment is carried out when the water is discharged to protected areas, since due to the cost it has, it is not normally done. These treatments are explained below.

Primary treatment consists of a set of physical-chemical processes that are applied to reduce the content of suspended particles in the water . These suspended solids can be settling or floating. The former are capable of reaching the bottom after a short period of time, while the latter are made up of very small particles (less than 10 microns) integrated in the water so they are not able to float or settle and to remove them requires other techniques.

Some of the wastewater treatment methods are the following:

• Sedimentation: process by which the particles fall to the bottom thanks to the action of gravity. Up to 40% of the solids contained in the water can be removed. This process occurs in tanks called decanters.
• Flotation: consists of the removal of foams, fats and oils since, due to their low density, they are located in the surface layer of water. Low density particles can also be removed, for which air bubbles are injected facilitating their ascent. With flotation, up to 75% of the suspended particles could be removed. This occurs in other tanks called dissolved air floats.
• Neutralization: consists of normalizing the pH, that is, adjusting it to a value in the range of 6-8.5, which is typically the value of water. In the case of acidic wastewater (low pH) such as those containing heavy metals, alkaline substances (high pH) are added to raise the pH of the water. On the contrary, in alkaline wastewater, CO2 is usually introduced so that the pH of the water falls to normal values.
• Other processes: to achieve greater purification of wastewater, other techniques such as the use of septic tanks, lagoons, green filters or other chemical processes (ion exchange, oxidation, reduction, etc.) can be applied.

Secondary treatment consists of a set of biological processes that seek to eliminate the organic matter in the wastewater. These biological processes consist of the work carried out by some bacteria and microorganisms and which is based on the transformation of organic matter into cellular biomass, energy, gases and water. This treatment is 90% effective.

Several processes can be distinguished, aerobic and anaerobic:

• The aerobic processes are performed in the presence of oxygen so it is necessary to introduce in the tank where wastewater are. In this stage, part of the degradation of organic matter occurs, from which water and CO2 are released, as well as the elimination of nitrogenous products. Ammonium, derived from very toxic nitrogen, is transformed into nitrate in a reaction called nitrification. However, nitrate, although it is no longer toxic, is an assimilable form of nitrogen and, therefore, could cause a proliferation of algae and the enrichment of nutrients in the waters in the receiving environment (eutrophication), so through denitrification This turns into nitrogen and is released into the atmosphere.
• On the contrary, anaerobic processes are carried out in the absence of oxygen. In this process fermentative reactions occur in which organic matter is transformed into energy, methane and carbon dioxide.

Here are some wastewater treatment methods :

• Active sludge: it is an aerobic process that consists of adding flocs or lumps of organic matter with microorganisms to the wastewater and constantly infiltrating oxygen for reactions to take place.
• Bacterial beds: aerobic process. These are supports where the microorganisms are found and the residual water is poured in small quantities to maintain aerobic conditions.
• F ilters green: is crops irrigated with wastewater as these have the ability to absorb compounds.
• Anaerobic digestion: it is an anaerobic process that takes place in completely closed tanks. Bacteria that produce acid and methane are mainly used when they degrade organic matter.
• Others: biodisks, biocylinders, electrocoagulation, electrooxidation, membrane biological reactor, etc.

Tertiary treatment consists mainly in the elimination of pathogens , especially fecal bacteria and nutrients. This treatment is optional and is usually done when the water is going to be reused, for example, in gardens or other public spaces so that they do not pose a danger to human health, or in the event that the receiving channels are in spaces protected or with a high quality in its waters. The most common wastewater treatment processes are the following:

• Ultraviolet radiation: In order to apply, the waters must be very clear and without much dissolved particulate material so that the light can reach everywhere. Ultraviolet radiation prevents the reproduction of microorganisms and prevents them from developing their infectious capacity. It is capable of eliminating around 99% of microorganisms.
• Ion exchange: Technique used to remove salts in low concentrations and for this, resins are used that are capable of temporarily retaining ions.
• Reverse osmosis: It consists of the elimination of salts when the water passes from a more concentrated solution to a more dilute one.
• Filtration: It consists of the elimination of organic particles that have not been able to be extracted in the previous treatments. Sand and gravel are used for this.
• Chlorination: It consists of the elimination of microorganisms through the application of chlorinated products. In addition, they contribute to the elimination of ammonia and prevent the oxidation of inorganic elements.

This is a small part of the processes commonly used in treatment plants, but new techniques are currently being investigated to make water treatment cheaper and more complete. In addition, everything explained in this article focuses on the water line of the treatment plants, in parallel there is a sludge line in which the solid waste that is extracted from the wastewater is treated and purified.

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