In-depth understanding of the coagulation and sedimentation process of sewage treatment
The coagulation and sedimentation process is generally used in sewage treatment. It is often used in the pretreatment of the front stage and the deep treatment section after secondary treatment. The purpose is to remove suspended solids in the water and strengthen total phosphorus and other pollutants. removal effect. So what are the principles and influencing factors of coagulation, and what are its applications? Let us learn together.
01
Concrete concept
Coagulation refers to the process of aggregating colloidal particles and tiny suspended solids in water through some method (such as adding chemicals). It is a unit operation in water and wastewater treatment processes. Coagulation includes two processes: coagulation and flocculation.
Coagulation: The process of colloid losing stability is called coagulation;
Flocculation: The aggregation of destabilized colloids is called flocculation.
We collectively call agents that can coagulate and flocculate as coagulants.
According to the concept of coagulation dynamics, flocculation is divided into anisotropic flocculation and co-directional flocculation. The collision and aggregation of particles caused by Brownian motion is called anisotropic flocculation; the collision and aggregation of particles caused by fluid movement caused by hydraulic or mechanical stirring It is called co-directional flocculation, and the collision between particles is the primary condition for coagulation. The coagulation process involves: ① the properties of colloids in water; ② hydrolysis of coagulants in water; ③ interaction between colloids and coagulants.
02
Principle of coagulation
(1) Electric double layer compression mechanism
When an electrolyte is added to the solution to increase the ion concentration in the solution, the thickness of the diffusion layer will decrease. When two colloidal particles are close to each other, because the thickness of the diffusion layer is reduced and the ζ potential is reduced, their mutual repulsive force is reduced, and the colloidal particles can quickly agglomerate. This mechanism believes that the zeta potential can be reduced to 0. Therefore, the following two phenomena cannot be explained: ① If too much coagulant is added, the coagulation effect will decrease; ② Polymers or macromolecules with the same electrical signal as the colloidal particles also have good coagulation effects.
picture
(2) Adsorption electroneutralization mechanism
Adsorption and electric neutralization means that the surface of colloidal particles has a strong adsorption effect on parts with different charges. Because this adsorption effect neutralizes part of its charge and reduces the electrostatic repulsion, it is easy to get close to other particles and adsorb each other. This phenomenon occurs frequently in water treatment. It refers to the direct adsorption of polymer ions, polymer substances, colloidal particles, etc. with different charges on the surface of the colloidal core to reduce the zeta potential. Its characteristic is: when the dosage of the agent is too much, the ζ potential can reverse its sign.
(3) Adsorption and bridging effect
The adsorption bridging effect mainly refers to the mutual adsorption of polymer substances and colloidal particles, but the colloidal particles themselves are not in direct contact, causing the colloidal particles to aggregate into large flocs.
(4) Sediment trapping mechanism
When metal salts or metal oxides and hydroxides are used as coagulants, and the dosage is large enough to quickly form metal oxide or metal carbonate precipitates, the colloidal particles in the water can be trapped by these precipitates during their formation. catch. When the precipitate is positively charged, the precipitation rate can be accelerated due to the presence of cations in the solution. In addition, the colloidal particles in the water themselves can serve as the core for the formation of these metal hydroxide precipitates, so the coagulant is optimally dosed and removed. The concentration of the substance is inversely proportional, that is, the more colloidal particles, the less the dosage of metal coagulant.
(5) DLVO theory
The interaction between colloidal particles is determined by repulsive energy and attractive energy, which are generated by electrostatic repulsion and van der Waals attraction respectively. Repulsive potential energy: ER—1/d2 Attractive potential energy: EA—1/d6 (some think it is 1/d2 or 1/d3). From this, the relationship between the interaction potential energy of colloidal particles and distance can be drawn. When the colloid distance x
03
Commonly used coagulants
(1) PAC
Polyaluminum chloride (PAC): It can achieve good flocculation effect on various wastewaters, can quickly form large alum flowers, has good sedimentation performance, has a wide suitable pH range (pH between 5-9), and is easy to treat. The pH and alkalinity of the water then dropped less. When the water temperature is low, it can still be maintained that the product is an inorganic polymer compound, which is a product between AlCI3 and Al(OH)3. It is polymerized through hydroxyl bridge and has varying numbers of hydroxyl groups in the molecule. Its general chemical formula is: [AI2(OH)NCI6-N]m. The color is yellow or light yellow, dark brown or dark gray resinous solid. The solid color is yellow or light yellow, dark brown or dark gray resinous solid. The color is yellow or light yellow. The alumina content in PAC is about 25%, and the alkalization degree is 65-85%. It has strong bridging adsorption performance, and physical and chemical processes such as chemistry, condensation, adsorption and precipitation occur during the hydrolysis process. It is suitable for purifying domestic water and high-purity water, and is used to remove COD, BOD and decolorize industrial sewage. It has a sustained and stable flocculation effect, and its alkalization degree is higher than other aluminum salts and iron salts, so the chemical liquid has less corrosive effect on the equipment.
(2)PAM
Polyacrylamide (Polyscrylae), referred to as PAM, is divided into cationic, anionic and non-ionic types. The molecular weight ranges from 4 million to 20 million. The product appearance is white powder, easily soluble in water, and easy to decompose when the temperature exceeds 120°C. The polyacrylamide molecule has a positive group (-CONH2), which can adsorb and bridge suspended particles dispersed in the solution, and has a strong flocculation effect, so it is widely used in the field of water treatment. PAM is used for sewage treatment. It has the characteristics of high removal efficiency of organic matter in water, low dosage, fast sedimentation and low cost. It is a product that cannot be replaced by other flocculants.
Polyacrylamide is divided into: anionic, cationic and nonionic.
In urban and industrial wastewater treatment, anionic type is mainly used to treat neutral suspensions mainly composed of inorganic solids, and improve the removal effect of suspended solids, BOD and phosphate in wastewater. For example, by adding 0.25 mg/L hydrolyzed polyacrylamide to the primary wastewater sedimentation tank, the removal rates of suspended solids and BOD can be increased to 66%-23% respectively; adding 0.3 mg/L anions to the secondary wastewater treatment sedimentation tank The removal rates of flocculant, suspended solids and BOD can be increased to 87% and 91% respectively.
The cationic type is mainly used for sludge dehydration. It should be a product with a molecular weight corresponding to the properties of the sludge. It has the characteristics of producing large flocs, non-stick filter cloth, low dosage, and high dehydration efficiency. The moisture content of the mud cake is below 80%. It is also used in industries where the wastewater contains a lot of negative charges, such as breweries, beverage factories, MSG factories, sugar factories and other organic wastewater treatment.
The non-ionic type is mainly used for the treatment of acidic wastewater. At this time, PAM plays an adsorption role, causing the suspended particles to flocculate and settle, achieving the purpose of purifying water. The non-ionic type is non-toxic, especially when used in conjunction with inorganic flocculants. The processing effect is better.
(3) Coagulant aid
Coagulation may or may not be involved.
①Acid-base: adjust the pH of water, such as lime, sulfuric acid, etc.;
② Increase the particle size and firmness of alum flowers: such as activated silicic acid, polymer flocculant polyacrylamide, etc.;
③Oxidants: Destroy substances that interfere with coagulation, such as organic matter, Cl2, O3, etc.
04
Factors affecting coagulation
The factors affecting the coagulation effect are relatively complex, mainly including:
(1) Water quality characteristics, including water temperature, water quality, pollutant properties and concentrations, etc.;
(2) Type and quantity of coagulant added;
(3) The flocculation equipment used and its related hydraulic parameters.
(1) Water temperature
When the water temperature is low, flocs usually form slowly, the flocculated particles are small and loose, and the coagulation effect is poor. The reasons are:
①Desorption heat of inorganic brine;
② As the temperature decreases, the viscosity increases and Brownian motion weakens;
③When the water temperature is low, the hydration of the colloidal particles is enhanced, hindering aggregation;
④The water temperature is related to the pH of the water.
Therefore, generally speaking, a temperature of 20~30℃ is more conducive to coagulation.
(2) pH and alkalinity
The degree of influence of pH on coagulation effect is related to the type of coagulant. The optimal pH range during coagulation is closely related to the quality of the incoming water and removal objects. When a metal salt coagulant is added, its hydrolysis will generate H+, but the alkalinity in the water has a buffering effect. When the alkalinity is not enough, lime needs to be added. The lime dosage is estimated according to the following formula: [CaO]=3[a]-[x] + [δ], where
[CaO]: pure lime CaO dosage, mmol/L;
[a]: Coagulant dosage, mmol/L;
[x]: Raw water alkalinity, measured in mmol/L, CaO;
[δ]: Generally 0.25~0.5mmol/L (CaO).
The dosage of lime is generally determined through experiments.
(3) Suspended solids concentration
The concentration of pollutants is low, the probability of collision between particles decreases, and the coagulation effect is poor. The countermeasures that can be taken are: ①Add polymer coagulant; ②Add clay; ③Filter directly after adding coagulant. If the content of suspended solids in raw water is too high, in order to reduce the amount of coagulant, polymer coagulants are usually added.
(4) Organic pollutants
Since the dissolved organic matter molecules in the sewage are adsorbed on the surface of the colloidal particles, they protect the colloidal particles, hinder the collision between the colloidal particles, and hinder the destabilizing agglomeration between the coagulant and the colloidal particles. Therefore, in the presence of organic matter, the colloidal particles It is more difficult to destabilize than when there is no organic matter, and the amount of coagulant needs to be increased. Potassium permanganate, ozone, chlorine, etc. can be added as pre-oxidants, but it is necessary to consider whether toxic by-products are produced.
(5) Coagulant type and dosage
The hydrolysis characteristics of different types of coagulants and the quality of the water used are not exactly the same, so the appropriate type of coagulant should be optimized and selected based on the quality of the incoming water. For inorganic salt coagulants, it is required to form a form that can effectively compress the electric double layer or produce strong electrical neutralization. For organic polymer flocculants, it is required to have an appropriate amount of functional groups and polymeric structure as well as a larger molecular weight.
Generally speaking, the coagulation effect increases as the amount of coagulant increases. However, when the amount of coagulant reaches a certain value, the coagulation effect reaches its peak. If the amount of coagulant is increased further, re-stabilization will occur, and the coagulation will become unstable. On the contrary, the effect is reduced. Considering the cost issue, the optimal coagulant dosage in actual production usually takes into account that the water quality after purification reaches the national standard and the coagulant dosage is lower.
(6) Adding method
There are two ways of adding coagulant: dry dosing and wet dosing. Because the solid coagulant and liquid coagulant or even liquid coagulants of different concentrations have different coagulation effects that can compress the electric double layer into having the ability to neutralize electricity. If other coagulants are added in addition to coagulants, the order in which the various agents are added will also have a great impact on the coagulation effect. The appropriate dosing method and combination must be determined through simulation experiments and production practice. order.
(7) Hydraulic conditions
After adding coagulant, the coagulation process can be divided into two stages: rapid mixing and flocculation reaction. However, in the actual sewage treatment process, the two stages are continuous and inseparable, and continuity is also required in terms of hydraulic conditions. During the flocculation reaction stage, the stirring intensity and water flow speed should gradually decrease as the flocs increase to prevent the aggregated flocs from being broken and affecting the coagulation and sedimentation effect. At the same time, since the flocculation reaction is a slow process in which the flocs gradually grow, if the flocs need to grow to a large enough particle size to be removed by sedimentation after the coagulation reaction, a certain flocculation time needs to be ensured. If the gas is used after the coagulation reaction, Floatation or direct filtration process, the reaction time can be greatly shortened.
05
Application of coagulation process (excerpt)
(1) Advection sedimentation tank
In order to ensure that coagulation achieves better treatment effects, the following points need to be noted when using the advection sedimentation tank process:
① During coagulation and sedimentation, the suspended solids content in the effluent generally does not exceed 20 mg/L.
②The number of pools or divisions is generally not less than 2;
③The average horizontal flow velocity in the pool is generally 110~25mm/s;
④The precipitation time should be based on the raw water quality and water quality requirements after precipitation, generally 1.0~3.0h;
⑤The coagulant generally passes through a static tubular mixer, so that the added coagulant and coagulant aid are instantly mixed in the pipeline. The mixing rate can reach 90%~95%, and the mixer should be installed as close as possible to the sedimentation tank.
(2) Air flotation sedimentation tank
The coagulation air flotation sedimentation process has the characteristics of small footprint and high suspended matter removal efficiency. It is widely used in the field of industrial sewage treatment, especially in food, printing and dyeing, tanning, papermaking and other industries; flocculating agents are generally dosed accurately using metering pumps. To ensure the effect of air flotation treatment, the design of the air flotation sedimentation tank: the tank depth is generally 1.5-2. 5m, and the protection height is preferably 0.5m. The ratio of pool length to pool width is not less than 3.0, the ratio of pool depth to pool width is greater than 0.3, contact chamber: upward flow rate 10-20mm/s, residence time >60s. Separation chamber: The surface load is usually 5-10m³/(m²·h) and the residence time is 30-40min. The flow rate of the water collecting pipe is less than 0.5m/s, and the slag scraping speed is less than 5m/min.
(3) High-efficiency sedimentation tank
The working principle of an efficient sedimentation tank is based on the following five aspects:
①The original concept of integrated flocculation reaction tank;
② Slow transmission from the push-flow reaction tank to the sedimentation tank;
③External sludge recirculation system;
④ Inclined tube precipitation mechanism;
⑤ Use synthetic flocculant + polymer coagulant.
Daily process control should be adjusted based on the following points:
① The coagulant is added through a metering pump. The concentration of the coagulant polyaluminum chloride solution remains unchanged, but its dosage changes in direct proportion to the inlet water flow of the sewage plant;
② The amount of return sludge is controlled based on the inlet water flow, inlet suspended solids (SS) and return sludge concentration. When the inlet water flow and inlet water turbidity fluctuate, the amount of return sludge needs to be appropriately increased (or reduced). In order to keep the flocculation effect in the flocculation zone in a more ideal state;
③The mud discharge cycle is controlled by the mud level height set in the sedimentation and clarification area. When the mud level reaches the design height, the mud discharge pump is started to discharge mud (when the water inlet conditions are stable, a timer can be used to control mud discharge). The remaining sludge is discharged intermittently.
(4) Magnetic coagulation reaction cell
1. The magnetic flocculation precipitation process includes:
① Magnetic flocculation reaction: The precipitation process of magnetic flocculation is generally similar to the ordinary flocculation process. The difference is that after adding high specific gravity magnetic powder, the colloidal particles and the magnetic powder particles aggregate with each other. The specific gravity of the flocculation produced by flocculation is much higher than that of ordinary flocculation. Flocculated floc. As a result, the settling speed is greatly increased.
②Solid-liquid separation: The high-speed solid-liquid separation process relies on the specific gravity of the magnetic flocculation itself to enable it to form a static settling speed of more than 40m/h, thereby quickly separating pollutants from the water body.
③ Magnetic powder recovery: The magnetic flocculation is crushed by a high-speed shear and then enters the magnetic separator. On the one hand, the magnetic powder is recovered into the reaction tank for recycling. On the other hand, the separated sludge is discharged into the sludge storage tank for subsequent sludge dehydration treatment.
2. Design parameters:
The magnetic flocculation reaction tank is generally set up with 4 cells, and the total residence time of the reaction tank is controlled between 10-14min; the surface load of the magnetic flocculation sedimentation tank is acceptable: 20-40m³/(m²·h). The residence time should be between 10-20 minutes. The magnetic flocculation sedimentation tank is equipped with a sludge return system, and the return ratio is controlled at about 10%, with the main purpose of achieving magnetic powder recycling. The residual sludge discharge is about 0.5% -2% of the designed daily treatment capacity, and the residual sludge system can operate intermittently.
Source: Water Treatment Research Institute
