After seeing the first two steps of effluent treatment, let's talk now about the final step. If you did not have the opportunity to learn about these previous methods, access the following texts:
- Types of effluent treatments;
- Secondary effluent treatments.
Tertiary effluent treatments consist of tphysicochemical or biological techniques for the removal of specific pollutants that have not been removed by the other more common processes. Some of these specific pollutants can be organic matter, non-biodegradable compounds, nutrients, heavy metals, among others.
These tertiary treatments can include several steps that will depend on the type of effluent pollution and the degree of purification that is desired. In addition, the different processes that can be applied in tertiary treatments can be classified into two main types:
* Phase transfer technologies: the pollutant is simply passed to another state of aggregation, that is, it passes from the aqueous phase to another phase, which can be conveyed to the atmosphere or be transformed into solid waste. The latter occurs, for example, with the activated carbon adsorption method that will be explained later.
* Destructive Technologies: the pollutant is actually transformed, that is, it ceases to exist as such. This is achieved by the oxidation of organic matter, which leads to chemical species that are increasingly oxidized until their complete mineralization occurs. Chemical oxidation is a type of treatment that will also be explained further below.
See now the main examples of tertiary effluent treatments:
* Microfiltration: is a separation process using membranes with pores in the micrometer scale (1 µm = 10-6 m) in which the force that promotes the separation of the liquid part of the polluting solids is the pressure through the membrane and its pores.
* Precipitation and coagulation: Coagulant chemicals that form flakes when added to the suspended matter are added to the water. For example, adding lime to iron-containing drains produces flakes that sink to the bottom of the container.
* Adsorption (activated carbon): Pollutants are adsorbed on the surface of the coal: they are transferred. Adsorption can take place in two ways: chemical or physical. Chemical adsorption or chemisorption occurs through chemical bonds, mainly covalent bonds. On the other hand, physical adsorption or physisorption occurs through intermolecular interactions of the Van der Waals type, such as the induced dipole force and the permanent dipole force.
* Ion exchange: uses certain polymers with sites that can retain ions. Thus, the polluting ions that are in the water, which are retained in the polymeric resin, can be exchanged for other ions with the same charge. For example, if this ion exchange resin is cationic, it might have the H ions+, which are exchanged for salt cations or even heavy metals that are in the effluent. If the ion exchange resin is anionic, it may have OH ions- which are exchanged for anions present in the effluent. So the H ions+and oh- that are in the water coming out of the resin react to form more water.
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* Reverse osmosis: By applying pressure, pure water from the effluent is forced through a semi-permeable membrane of polymeric organic material that ions cannot pass through. This method is used to desalinate water, for example. See how this is done in the text Reverse Osmosis in the desalination of sea water.
reverse osmosis illustration
* Ultrafiltration: It is a selective fractionation process that uses pressures above 145 psi (10 bar).
* Electrodialysis: a series of semipermeable membranes are placed vertically and alternately inside an electrical cell. Through this membrane, only small cations or anions can pass through. In this way, an electric current is applied that causes the water to decompose into its ions. These, in turn, migrate to the corresponding poles, that is, the cations migrate to the cathode and the anions to the anode. Thus, in alternate zones, the liquid is more concentrated and, in others, it is less concentrated in ions. The concentrated part of ions is discarded and the purified water is discarded in the environment.
* Chlorination: Chlorine (chlorine gas or sodium hypochlorite) is added to water for two main actions, which are (1) destroy or nullify the activity of pathogenic microorganisms, algae and bacteria, and (2) act as an oxidizer of organic and inorganic compounds present in water. In addition to leading to disinfection, the addition of "chlorine" can also lead to odor control, BOD (Biochemical Oxygen Demand) removal, fly proliferation control, cyanide and phenols destruction, in addition to nitrogen removal.
* Ozonation: Ozone (O3) is used because it acts as a potent oxidizing agent, in addition to being easily absorbed by water. It is mainly used to oxidize non-biodegradable organic compounds.
ozone molecule
* PAOs (Advanced Oxidation Processes): In addition to ozone, chemical oxidation can also be carried out using hydrogen peroxide or another conventional oxidizer. To accelerate these processes, extremely oxidizing and poorly selective radicals that can be obtained are used. through different combinations between ultraviolet radiation, oxygen peroxide, ozone and photocatalysts.
By Jennifer Fogaça
Graduated in Chemistry
Would you like to reference this text in a school or academic work? Look:
FOGAÇA, Jennifer Rocha Vargas. "Tertiary wastewater treatments"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/tratamentos-terciarios-efluentes.htm. Accessed on June 28, 2021.
Chemistry
Chemical Oxygen Demand, amount of dissolved oxygen, acidic medium, degradation of organic matter, Biochemical Demand of Oxygen, oxidation of biodegradable organic matter, biological attacks, chemical oxidant, potassium bichromate, permangan
Chemistry
Water pollution, physical aspects of water, chemical aspects of water, biological aspects of water, industrial waste, heavy metals, drinking water, organic matter, water turbidity, sewage.
