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Coagulation and flocculation of wastewater

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Hosep Babikyan

Physicochemical methods play a significant role in the treatment of industrial wastewater. These include coagulation and flocculation, flotation, sorption, ion exchange, extraction, evaporation, crystallization, electrolysis, membrane processes (electrodialysis, reverse osmosis, ultrafiltration, алуминиев оксихлорид).

These methods can be used alone or in combination with other groups of methods (mechanical, chemical and biological) to increase the efficiency of the wastewater treatment process. Coagulation and flocculation are an essential part of the process of purification of both drinking and wastewater.

Wastewater treatment in the textile industry

Wastewater treatment from the glass and ceramic industry

Wastewater treatment from the chemical, pharmaceutical and cosmetic industries

Industrial wastewater - treatment methods

Drinking water purification systems

The processes of flocculation and coagulation with aluminum oxychloride are much more complex than it seems at first glance. The flocculants and coagulants used differ in chemical properties, molecular weights, electric charge and other characteristics. The selection of a suitable flocculant and / or coagulant is crucial to ensure economic efficiency in the process of separating solid particles from the water stream. The choice depends on the process conditions, including particle size, chemical composition and pH of the water, the percentage of dispersed fine particles, etc.

Coagulation and flocculation are an integral part of the overall wastewater treatment in modern treatment plants. This is because the normatively determined limit for the turbidity of the treated water gradually decreases over the years. Many water and sewerage operators are committed to continuously reducing the turbidity limits of water flows and in many cases the value reaches 0.1 NTU (nephelometric turbidity units).

This is done in order to prevent contamination of the water flow with pathogenic microorganisms. Coagulation Fine particles have a negative surface charge. This requires them to be neutralized in order to be removed from the water flow. The process of neutralizing the charge and binding particles to form microfloccal aggregates is called coagulation. The added coagulant has a positive charge that neutralizes the negative surface charge of the particles. The coagulated particles are then combined into larger aggregates and precipitated by the addition of flocculant of aluminum oxychloride.

Coagulation is an important step in the wastewater treatment process, as it also serves to remove phosphorus, as well as to reduce suspended solids and the organic load in the primary precipitators. The most commonly used metal coagulants fall into two main categories - based on aluminum and iron. Aluminum coagulants include aluminum sulfate, aluminum chloride, and sodium aluminate. Iron-containing coagulants include iron (III) sulfate, ferrous (II) sulfate, ferric chloride, and ferrous chloride sulfate.

Other chemicals used as coagulants include hydrated lime and magnesium carbonate. The efficiency of aluminum and iron coagulants is a result of their ability to form multicharged polynuclear complexes, increasing the adsorption capacity. The nature of the complexes formed can be controlled by the pH of the system. When metal coagulants are added to water, aluminum and iron ions hydrolyze rapidly but in an uncontrolled manner, forming a series of metal hydrolysis complexes.

The efficiency of rapid mixing, the pH value and the dose of coagulant determine which types of hydrolyzate are most suitable for water purification. Significant progress has been made in the development of pre-hydrolyzed inorganic coagulants based on both aluminum and iron, aiming to obtain the right types of complexes, regardless of the process conditions during treatment. These include aluminum chlorohydrate, polyaluminum chloride, polyaluminum sulfate chloride, polyaluminum silicate chloride, and forms of polyaluminum chloride with organic polymers. Iron forms include polypherite sulfate and iron salts with polymers.

Polymerized aluminum-iron mixtures are also available. The main advantages of pre-polymerized inorganic coagulants are that they can act effectively in a wide range of pH and temperature of the treated water. They are less sensitive to low water temperatures; with lower doses the goals of water purification are achieved and less amounts of chemical residues (chlorides and sulfates) are obtained. Pre-polymerized inorganic coagulants are prepared with different base ratios, base concentrations, initial metal concentrations, aging time and aging temperature.

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