Quels matériaux sont utilisés dans le traitement des eaux usées?

Le traitement des eaux usées est un processus complexe qui vise à éliminer les contaminants des eaux usées, making it safe to discharge back into the environment or suitable for reuse. A variety of materials play crucial roles in this process, each contributing to different stages of treatment. Ici, we explore the key materials used in wastewater treatment.

• Polymeric Aluminium Chloride (PAC): Also known as basic aluminium chloride or hydroxy aluminium chloride, PAC is a widely used coagulant. Its formula is (AL₂(OH)ₙCl₆₋ₙ)ₘ, where n ranges from 1 - 5 and m represents the degree of polymerization. PAC functions by neutralizing the negative charges on suspended particles in wastewater. This neutralization causes the particles to aggregate, forming larger, more easily - settleable flocs. The effectiveness of PAC depends on the ratio of OH to AL (expressed as the alkalinity degree, B=(OH)/(3(AL))X100%, with an optimal range of 40 - 60%). It is effective across a pH range of 5 - 9. Par exemple, in treating industrial wastewater with high turbidity, PAC can quickly form large flocs, reducing the turbidity significantly.

• Ferric Salts: Similar to aluminium - based coagulants, ferric salts such as ferric chloride (FeCl₃) and ferric sulfate (Fe₂(SO₄)₃) work by hydrolyzing in water to form positively - charged ions. These ions neutralize the negative charges on colloidal particles in wastewater, promoting coagulation. Ferric salts are often used in wastewater treatment plants due to their relatively low cost and high efficiency in removing suspended solids and certain heavy metals. They can also be effective in treating wastewater with a high organic matter content, as they can help in the precipitation of organic - bound pollutants.

• Polyacrylamide (PAM): PAM is a flocculant available in anionic, cationic, et non - ionic forms. With an average molecular weight ranging from thousands to tens of millions, PAM has functional groups along its chain - like molecules. Cationic PAM is suitable for treating wastewater with negatively - charged particles, such as those in biological wastewater treatment plants where the activated sludge has a negative charge. Anionic PAM, d'autre part, is effective for wastewater with positively - charged particles, like in some metal - processing industries. Non - ionic PAM can be used in situations where the charge of the particles is less clear or when a more universal flocculating effect is desired. PAM enhances the flocculation process by bridging the gaps between small flocs, forming larger and heavier aggregates that can settle or be filtered more easily.

• Acids: Sulfuric acid (H₂SO₄) is commonly used to lower the pH of alkaline wastewater. In industrial settings where wastewater from processes like metal plating or chemical manufacturing has a high pH due to the use of alkaline cleaners or reagents, sulfuric acid is added in a controlled manner. The acid reacts with the alkaline substances in the wastewater, neutralizing them and bringing the pH to a more suitable range for further treatment. Hydrochloric acid (HCl) is another option, especially in cases where the presence of sulfate ions in the treated water may cause issues, such as in some food - processing or pharmaceutical industries.

• Bases: Calcium hydroxide (Ca(OH)₂), also known as slaked lime, and sodium hydroxide (NaOH), or caustic soda, are used to raise the pH of acidic wastewater. Par exemple, in mining operations where acidic mine drainage is a problem, calcium hydroxide is added. The hydroxide ions from the base react with the hydrogen ions in the acidic wastewater, neutralizing the acidity. Sodium hydroxide is often preferred in situations where a more rapid pH adjustment is required or when the wastewater contains substances that could react unfavorably with calcium ions.

• Hydrogen Peroxide (H₂O₂): Hydrogen peroxide is a powerful oxidizing agent used in wastewater treatment to break down organic pollutants and reduce the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of wastewater. In industries such as textile manufacturing, where wastewater contains dyes and other organic compounds, hydrogen peroxide can be added to oxidize these pollutants into simpler, less harmful substances. It can also be used in combination with other catalysts, comme le fer (II) ions in the Fenton's reagent process, to enhance the oxidation efficiency.

• Sodium Hypochlorite (NaClO): Commonly used as a disinfectant in wastewater treatment, sodium hypochlorite releases chlorine, which kills pathogenic microorganisms in the treated wastewater. In municipal wastewater treatment plants, sodium hypochlorite is added at the final stage of treatment to ensure that the water is safe to discharge into water bodies. It is also effective in controlling the growth of algae and bacteria in recycled water systems.

• Sodium Bisulfite (NaHSO₃) and Ferrous Sulfate (FeSO₄): These are reducing agents used in wastewater treatment to convert certain pollutants to less toxic forms. Par exemple, in wastewater containing hexavalent chromium, a highly toxic form of chromium often found in electroplating wastewater, sodium bisulfite or ferrous sulfate can be added. These reducing agents convert hexavalent chromium to the less toxic trivalent chromium, which can then be removed from the wastewater through precipitation or other methods.

• Aerobic Bacteria: In processes like the activated sludge process, aerobic bacteria are the workhorses of wastewater treatment. These bacteria, such as those from the genera Pseudomonas, Bacillus, and Nitrosomonas, require oxygen to survive. They consume organic pollutants in wastewater, breaking them down into carbon dioxide, eau, and biomass. Par exemple, in a municipal wastewater treatment plant, aerobic bacteria in the aeration tank consume the organic matter present in the sewage. The oxygen supply in the aeration tank allows these bacteria to thrive and efficiently decompose substances like carbohydrates, protéines, and fats, reducing the BOD of the wastewater.

• Anaerobic Bacteria: Anaerobic bacteria are used in processes such as anaerobic digestion, which is often applied to treat high - strength organic wastewater, like that from food - processing plants or livestock farms. Bacteria such as Methanobacterium and Clostridium species work in the absence of oxygen. They break down organic matter through a series of complex metabolic reactions, producing biogas (a mixture of methane and carbon dioxide) and a nutrient - rich digestate. The biogas can be used as a renewable energy source, while the digestate can be used as a fertilizer. Anaerobic digestion not only reduces the organic load in wastewater but also provides a sustainable solution for energy generation and waste management.

• Microalgae: Microalgae are increasingly being used in wastewater treatment, especially for nutrient removal. Algae such as Chlorella and Scenedesmus species can absorb nutrients like nitrogen and phosphorus from wastewater. These nutrients are essential for the growth of microalgae, and in the process of uptake, they effectively remove these pollutants from the water. After treatment, the microalgae biomass can be harvested and used for various purposes, such as producing biofuels, animal feed, or fertilizers. This makes microalgae - based wastewater treatment a sustainable and resource - recovering solution.

• Trickling Filter Media: In trickling filter systems, media such as plastic, céramique, or gravel are used to support the growth of biofilms. These media provide a large surface area for microorganisms to attach and form biofilms. As wastewater trickles over the media, the microorganisms in the biofilm consume the organic pollutants and nutrients in the water. Plastic media, comme haut - densité en polyéthylène (HDPE) or polypropylene, are popular due to their light weight, high porosity, et durabilité. They can be designed in various shapes, such as honeycomb or corrugated structures, to maximize the surface area available for biofilm growth.

• Rotating Biological Contactor (RBC) Disks: RBCs consist of a series of circular disks made of plastic or fiberglass. These disks are partially submerged in wastewater and rotate slowly. As they rotate, a biofilm forms on the surface of the disks. The biofilm contains a diverse community of microorganisms that remove pollutants from the wastewater as it comes into contact with the disks. The rotation of the disks ensures that the biofilm is alternately exposed to the wastewater and the air, providing the necessary oxygen for aerobic microorganisms to function effectively.

• Powdered Activated Carbon (PAC): PAC is made from materials such as wood, coconut shells, or coal. It has a highly porous structure, providing a large surface area for adsorption. PAC is often used to remove organic compounds, taste - et - odor - causing substances, and certain heavy metals from wastewater. In situations where there is a sudden contamination of wastewater, such as from an industrial spill, PAC can be added to quickly adsorb the pollutants. Par exemple, in treating wastewater from the pharmaceutical industry, PAC can remove residual drugs and other organic contaminants that may be present.

• Granular Activated Carbon (GAC): GAC is typically used in fixed - bed adsorption systems. It has a larger particle size compared to PAC, which allows for easier handling and longer - Utilisation du terme. GAC is effective in removing a wide range of organic and inorganic pollutants, including chlorine, pesticides, and some heavy metals. In water treatment plants that produce drinking water from surface water sources, GAC filters are often used to polish the water, removing any remaining contaminants and improving the taste and odor of the treated water.

1. What is the difference between coagulants and flocculants in wastewater treatment?

2. How do microorganisms contribute to wastewater treatment?

3. When should activated carbon be used in wastewater treatment?