Coagulation of water refers to the preliminary physical and chemical methods of its purification. The essence of the process lies in the enlargement and precipitation of mechanical impurities or emulsified substances. This technology is used in modern wastewater and water treatment plants.
Physical foundations
Coagulation of water, or in other words its clarification, is a process in which small particles in suspension are combined into larger conglomerates. Carrying out this procedure allows you to remove finely dispersed impurities from the liquid during its further settling, filtering 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 with opposite charges are introduced. As a result, particles of suspensions become electrically neutral, lose their mutual repulsion forces and begin to stick together, and then fall out.in the sediment.
Materials used
2 types of chemical reagents are used as coagulants: inorganic and organic. Of the first group of substances, the most common are s alts of aluminum, iron, and mixtures thereof; titanium, magnesium and zinc s alts. The second group includes polyelectrolytes (melamine-formaldehyde, epichlorohydrindimethylamine, polychlorodiallyldimethyl-ammonium).
In industrial conditions, wastewater is most often coagulated with aluminum and iron s alts:
- Aluminum chloride AlCl3∙6H2O;
- ferric chloride FeCl3∙6H2O;
- Al sulfate 2O;
- iron sulfate FeSO4 7H2O;
- sodium aluminate NaAl(OH)4 and others.
Coagulants form flakes with 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 treatment object. For clarification of natural waters, the concentration of coagulants is usually in the range of 25-80 mg/l.
Practically all of these reagents belong to the 3rd or 4th hazard class. Therefore, the areas where they are used must be in isolated rooms or detached buildings.
Destination
The coagulation process is used both in water treatment systems and for cleaning industrial andhousehold waste water. This technology helps to reduce the amount of harmful impurities:
- iron and manganese - up to 80%;
- synthetic surfactants - by 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 for hard-to-remove iodine, barium and strontium; their concentration can only be reduced by a third);
- pesticides - by 10-90%.
Water purification by coagulation followed by sedimentation allows to reduce the content of bacteria and viruses in it by 1-2 orders of magnitude, and the concentration of the simplest microorganisms - by 2-3 orders of magnitude. The technology is effective against the following pathogens:
- Coxsackievirus;
- enteroviruses;
- hepatitis A virus;
- E. coli and its bacteriophages;
- giardia cysts.
Key Factors
The speed and efficiency of water coagulation depend on several conditions:
- Degree of fineness and concentration of impurities. Increased turbidity requires higher doses of coagulant.
- Acidity of the environment. Purification of liquids saturated with humic and fulvic acids occurs better at lower pH values. With normal water clarification, the process is more active at elevated pH. To increase alkalinity add lime, soda, caustic soda.
- Ionic composition. At low concentrationmixture of electrolytes, the efficiency of water coagulation is reduced.
- Presence of organic compounds.
- Temperature. With its decrease, the rate of chemical reactions decreases. The optimal mode is heating up to 30-40 ° С.
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. A coagulant is first added to the water, and then it is passed through a layer of granular materials.
The last method of water coagulation is the most widely used due to the following advantages:
- High cleaning speed.
- Smaller doses of coagulants.
- No strong influence of the temperature factor.
- No need to alkalize fluid.
The technological process of wastewater treatment by coagulation includes 3 main stages:
- Reagent dosing and 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.
- Floc formation in special chambers (contact, thin-layer, ejection or recirculation).
- Settling in settling tanks.
Wastewater sedimentation is more efficient with a two-stage method, when it is first carried out without coagulants, and then after treatment with chemicalreagents.
Traditional faucet designs
The introduction of the coagulant solution into the treated water is carried out using various types of mixers:
- Tubular. Static elements in the form of cones, diaphragms, screws are installed inside the pressure pipeline. Reagent is supplied 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 holes, a layer of suspended coagulating sediment or an insert in the form of a washer (diaphragm) with a hole.
- Mechanical (blade and propeller).
Combination with flotation
Wastewater treatment by coagulation is difficult to control 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 adhesion of the latter with air bubbles. Gas saturation is carried out at flotation plants.
This technique is widely used for coagulation of water contaminated with products of the following industries:
- refining industry;
- artificial fiber production;
- pulp and paper, leather and chemical industries;
- mechanical engineering;
- productionfood.
3 types of flocculants are used:
- of natural origin (starch, hydrolyzed fodder yeast, bagasse);
- synthetic (polyacrylamide, VA-2, VA-3);
- inorganic (sodium silicate, silicon dioxide).
These substances make it possible to reduce the required dose of coagulants, shorten the cleaning time, and increase the rate of flake settling. The addition of polyacrylamide even in very small amounts (0.5-2.0 mg/kg) significantly weighs down the settling flakes, which increases the rate of water rise in vertical type clarifiers.
Methods of process intensification
Improvement of the water coagulation process is carried out in several directions:
- Change processing mode (fractional, separate, intermittent coagulation).
- Regulation of water acidity.
- Use of mineral opacifiers, whose particles play the role of additional centers for the formation of conglomerates, sorption materials (clay, clinoptilolite, saponite).
- Combined processing. Combination of coagulation with magnetization of water, application of an electric field, exposure to ultrasound.
- Using a mixture of ferric chloride and aluminum sulfate.
- The use of mechanical agitation, which reduces the dose of coagulants by 30-50% and improves the quality of cleaning.
- Introduction of oxidizers (chlorine and ozone).
