Coagulation of water refers to preliminary physico-chemical methods for its purification. The essence of the process is the enlargement and deposition of mechanical impurities or emulsified substances. This technology is used in modern wastewater treatment and water treatment plants.
Physical fundamentals
Coagulation of water, or in another way its clarification, is a process in which small particles in suspension are combined into larger conglomerates. Carrying out this procedure allows you to remove fine impurities from the liquid during its further sedimentation, filtration or flotation.
In order for the particles to “stick together”, it is necessary to overcome the forces of mutual repulsion between them, which ensure the stability of the colloidal solution. Most often, impurities have a weak negative charge. Therefore, to purify water by coagulation, substances are introduced that have an opposite charge. As a result, particles of suspensions become electrically neutral, lose mutual repulsive forces and begin to stick together, and then precipitate.
Materials used
As coagulants, 2 types of chemical reagents are used: inorganic and organic. Of the first group of substances, the most common are salts of aluminum and iron, and mixtures thereof; salts of titanium, magnesium and zinc. The second group includes polyelectrolytes (melamine formaldehyde, epichlorohydrindimethylamine, polychlorodiallyldimethylammonium).
In industrial conditions, wastewater coagulation is most often done using aluminum and iron salts:
- aluminum chloride AlCl 3 ∙ 6H 2 O;
- iron chloride FeCl 3 ∙ 6H 2 O;
- aluminum sulfate Al 2 (SO 4 ) 3 · 18H 2 O;
- iron sulfate FeSO 4 · 7H 2 O;
- sodium aluminate NaAl (OH) 4 and others.
Coagulants form flakes having a large specific surface area, which ensures their good adsorption capacity. The choice of the optimal type of substance and its dose is made in laboratory conditions, taking into account the properties of the liquid of the object being cleaned. To clarify natural waters, the concentration of coagulants is usually in the range of 25-80 mg / l.
Almost all of these reagents belong to the 3rd or 4th hazard class. Therefore, the areas on which they are used must be in isolated rooms or detached buildings.
Appointment
The coagulation process is used both in water treatment systems and for the treatment of industrial and domestic wastewater. This technology helps to reduce the amount of harmful impurities:
- iron and manganese - up to 80%;
- synthetic surfactants - 30-100%;
- lead, chromium - by 30%;
- petroleum products - by 10-90%;
- copper and nickel - by 50%;
- organic pollution - by 50-65%;
- radioactive substances - by 70-90% (except hard-to-remove iodine, barium and strontium; their concentration can be reduced by only a third);
- pesticides - by 10-90%.
Water purification by coagulation with subsequent sedimentation allows reducing the content of bacteria and viruses in it by 1-2 orders of magnitude, and the concentration of simple microorganisms by 2-3 orders of magnitude. The technology is effective against the following pathogens:
- coxsackie virus;
- enteroviruses;
- hepatitis A virus;
- E. coli and its bacteriophages;
- Giardia cysts.
Key factors
The speed and effectiveness of water coagulation depends on several conditions:
- The degree of dispersion and the concentration of impurities. Increased turbidity requires the introduction of higher doses of coagulant.
- The acidity of the medium. Purification of liquids saturated with humic and sulfic acids, better occurs at lower values of the hydrogen index. With normal clarification of water, the process is more active at elevated pH. To increase alkalinity add lime, soda, caustic soda.
- Ionic composition. At a low concentration of the electrolyte mixture, the coagulation efficiency of water decreases.
- The presence of organic compounds.
- Temperature. With its decrease, the rate of chemical reactions decreases. The optimal mode is heating to 30-40 ° C.
Technological process
There are 2 main coagulation methods used in wastewater treatment plants:
- In free volume. For this, mixers and flocculation chambers are used.
- Contact lightening. First, a coagulant is added to the water, and then it is passed through a layer of granular materials.
The latter method of coagulation of water is most widespread due to the following advantages:
- High speed cleaning.
- Smaller doses of coagulating substances.
- The absence of a strong influence of the temperature factor.
- No need to alkalize the fluid.
The technological process of wastewater treatment by coagulation includes 3 main stages:
- Dosing of the reagent and its mixing with water. Coagulants are introduced into the liquid in the form of 10-17% solutions or suspensions. Mixing in containers is carried out mechanically or by aeration with compressed air.
- Flocculation in special chambers (contact, thin-layer, ejection or recirculation).
- Precipitation in sedimentation tanks.
Sewage sedimentation is more efficient with a two-stage method, when at first it is carried out without coagulants, and then after treatment with chemical reagents.
Traditional mixer designs
The introduction of a solution of coagulants in the treated water is carried out using various types of mixers:
- Tubular. Inside the pressure pipeline, static elements are installed in the form of cones, diaphragms, screws. The reagent is fed through a venturi.
- Hydraulic: cloisonne, perforated, vortex, washer. Mixing occurs due to the creation of a turbulent flow of water passing along the partitions through the holes, a layer of suspended coagulating sediment, or an insert in the form of a washer (diaphragm) with a hole.
- Mechanical (lobed and propeller).
Combination with flotation
Wastewater treatment by coagulation is associated with difficulties in the regulation of the process due to the constant change in the quality of the liquid. To stabilize this phenomenon, flotation is used - the separation of suspended particles in the form of foam. Together with coagulants, flocculants are introduced into the purified water. They reduce the wettability of suspensions and improve the adhesion of the latter with air bubbles. Gas saturation is carried out in flotation plants.
This technique is widely used for coagulation of water contaminated with products of the following industries:
- oil refining industry;
- artificial fiber production;
- pulp and paper, leather and chemical industries;
- engineering;
- food production.
3 types of flocculants are used:
- natural origin (starch, hydrolytic feed yeast, oilcake);
- synthetic (polyacrylamide, VA-2, VA-3);
- inorganic (sodium silicate, silicon dioxide).
These substances can reduce the required dose of coagulants, reduce the cleaning time, increase the rate of deposition of flakes. The addition of polyacrylamide even in very small quantities (0.5-2.0 mg / kg) significantly complicates the deposited flakes, which increases the rate of water rise in vertical clarifiers.
Process intensification methods
Improving the process of coagulation of water is carried out in several directions:
- Change in processing mode (fractional, separate, intermittent coagulation).
- Regulation of the acidity of water.
- The use of mineral opacifiers, particles of which play the role of additional centers for the formation of conglomerates, sorption materials (clay, clinoptilolite, saponite).
- Combined processing. Combination of coagulation with the magnetization of water, the application of an electric field, exposure to ultrasound.
- The use of a mixture of ferric chloride and aluminum sulfate.
- The use of mechanical mixing, which allows to reduce the dose of coagulants by 30-50% and improve the quality of cleaning.
- The introduction of oxidizing agents (chlorine and ozone).