Wastewater often contains fine suspended solids and colloidal pollutants that can not be removed by natural sedimentation method. For this kind of wastewater, chemical agents must be added to destroy the stable dispersion system of colloid and fine suspended solids in water, so that they can be aggregated into flocs with obvious sedimentation performance, and then separated by gravity method. This process includes two steps: coagulation and flocculation It is coagulation.
Among them, coagulation refers to the process of destabilizing colloid and super colloid and condensing into micro flocs, which includes the destabilization of colloid and the migration and aggregation of particles; while flocculation refers to the process of micro flocs forming larger flocs through adsorption, coiling and bridging, which only includes the migration and aggregation of particles.
The mechanism of flocculant is as follows
There are two kinds of chemicals: inorganic multivalent metal salts and organic polymer. The former is mainly composed of aluminum salt and aluminum salt, while the latter is mainly composed of Polyacrylamide and its deformation. Our common inorganic salts are polyaluminum chloride and ferrous sulfate, and the organic ones are polyacrylamide (PAM).
The coagulation mechanism of aluminum and iron salt coagulants is very complex. In short, it is the comprehensive result of their series of dissociation and hydrolysis products on the colloid and fine suspended solids in water, such as compressing electric double layer, electric neutralization, adsorption bridging and coiling net trapping.
Aluminum and iron salt coagulants play the following three roles in the hydrolysis process
The depolarization and condensation of Al3 + or Fe3 + with polynuclear complex ions with high oligomerization and high charge;
Bridging flocculation of complex ions with high degree of polymerization;
In the presence of hydroxides, the flocculation of the net catcher has been studied,
Sometimes the above three effects may exist at the same time, but under different conditions, one of them may be dominant.
Generally, in the early stage of reaction with low pH, high concentration of colloid and fine suspended solids and insufficient dosage, destabilization coagulation is the main form; when pH is high, pollutant concentration is low and dosage is sufficient, net catching is the main form; when pH and dosage are moderate, bridging and flocculation become the main form.
Polyaluminum chloride (PAC) is also called basic aluminum chloride or hydroxyl aluminum chloride. Through it or its hydrolysate, colloid in sewage or sludge can form precipitate quickly, which is easy to separate. The molecular formula of PAC is [Al2 (OH) nCl6-n] m, where n is any integer from 1 to 5, M is the degree of polymerization, that is, the number of chain links, and the value of M is not more than 10. The coagulation effect of PAC is closely related to the ratio (n value) of OH and Al in PAC, which is usually expressed by alkalization degree, B = [OH] / (3 [al]) × 100%. B requires 40-60%, the appropriate pH range is 5-9.
The hydrolysis process and mechanism of ferric salt coagulant are similar to that of aluminum salt.
Polyacrylamide (PAM), commonly known as flocculant or coagulant, belongs to coagulant. The average molecular weight of PAM ranges from thousands to tens of millions. There are several functional groups along the bond molecules, which can be mostly ionized in water. It can be divided into anionic polyacrylamide, cationic polyacrylamide and nonionic polyacrylamide according to the characteristics of its dissociative groups.
PAM appearance is white powder, soluble in water, almost insoluble in benzene, ether, esters, acetone and other general organic solvents, polyacrylamide aqueous solution is almost transparent viscous liquid, non-toxic, non corrosive, solid PAM has hygroscopicity, hygroscopicity increases with the increase of ionic degree, PAM thermal stability is good; heating to 100 ℃ stability is good, but above 150 ℃ It is easy to decompose and produce middle nitrogen, which is imidized between molecules and insoluble in water. The density is 1.302 mg / L (23 ℃). The glass transition temperature is 153 ℃, and PAM exhibits non Newtonian fluidity under stress.
Cationic PAM and anionic PAM are suitable for sewage or sludge with anionic and cationic charges respectively. The activated sludge produced by biochemical method has a negative charge and should be cationic. Anionic PAM has a good effect in treating wastewater or sludge with positive charge, such as iron and steel plant, electroplating plant, metallurgy, coal washing and dust removal. The non-ionic type has good effect on both cation and anion, but the unit price is very expensive, which increases the treatment cost.
The reaction conditions and dosing requirements of flocculant are as follows
Function of flocculation tank:
The function of the flocculation tank is: after the coagulant is added into the raw water, it is fully mixed with the water body, most of the colloidal impurities in the water lose stability, and the destabilized colloidal particles collide and agglomerate with each other in the flocculation tank, and finally form flocs that can be removed by precipitation.
The reaction conditions were as follows
The process of floc growth is the process of contact and collision of small particles. The flocculation effect depends on the following two factors
The ability of polymer complex formed by hydrolysis of coagulant to form adsorption bridge is determined by the property of coagulant;
The collision probability of small particles and how to control them for reasonable and effective collision.
The discipline of Water Treatment engineering believes that in order to increase the collision probability, it is necessary to increase the velocity gradient, and to increase the velocity gradient, it is necessary to increase the energy consumption of the water body, that is, to increase the flow rate of the flocculation tank
If the flocs grow too fast, their strength will be weakened. In the process of flow, the strong shear will make the adsorption bridging be cut off, and it is difficult for the cut adsorption bridging to be continuous again. Therefore, the flocculation process is also a speed limited process. With the growth of flocs, the water flow speed should be reduced continuously, so that the formed flocs are not easy to be broken;
If some flocs grow up too fast, the specific surface area of flocs in water will decrease sharply, some small particles with imperfect reaction will lose reaction conditions, and the collision probability between these small particles and large particles will decrease sharply, so it is difficult to grow up again. These particles can not be intercepted by the sedimentation tank, but also by the filter.
Dosing requirements:
In the early stage of coagulant adding reaction, it is necessary to increase the chance of contact between coagulant and sewage as much as possible, and increase the stirring or flow rate. Depending on the collision between the water flow and the folded plate and the multiple turning of the water flow between the folded plates, the collision rate of the particles in the water increases and the flocs condense. In the later stage of the reaction, in order to reduce the velocity gradient, better flocculation and sedimentation effect can be obtained.
Preparation method of flocculant (taking PAM as an example)
Preparation method:
The use form of PAM is 0.1% - 0.2% aqueous solution, which is prepared with tap water. During the configuration, it must be noted that PAM must be evenly and dispersedly dropped in the continuously stirred water, and it must be ensured that when entering the water, it is scattered and individual particles, and does not form clusters. Otherwise, once large particle clusters are formed, it is difficult to continue to dissolve and form large particle clusters of water drug. The preparation should be fully stirred to make it dissolve. The prepared solution is easy to hydrolyze and should be used up on the same day.
Dosage:
PAM should be effectively mixed in sewage or sludge, and the mixing time is generally 10-30 seconds, generally not more than 2 minutes. The specific usage of PAM has a lot to do with the concentration, property and treatment equipment of colloid and suspended solids in sewage or sludge. The usage of PAM in Sewage Treatment is generally within 3-10ppm, that is, adding 3-10g per ton of water, the usage of PAM in sludge treatment should be more, and the best dosage should be obtained through a large number of experiments. According to the optimal dosage concentration (the concentration of polyacrylamide to be added to ppm1), the influent flow rate (T / h) and the configured concentration of polyacrylamide solution (the concentration of polyacrylamide prepared by ppm2), the displayed value (LPM) on the flowmeter of dosing pump can be calculated, that is, the influent flow rate (T / h) / 60 × the concentration of polyacrylamide to be added to ppm1 / the concentration of polyacrylamide prepared by ppm2. For example, if the water inflow is 100 t / h, the optimal dosage is ppm1 = 10 ppm, and the preparation concentration is 2 ‰ (3 kg drug is dissolved in 1.5 T water), then the dosing flow indication should be adjusted on the scale of 100 / 60 * 10 / 2 = 8.3 LPM.
Note: ppm is one part per million; in the unit of flowmeter value of dosing pump, LPM is L / min; GPM is Gal / min.
matters needing attention:
The preparation of liquid medicine should be carried out in strict accordance with the operating procedures and instructions.
The sundries shall not be mixed into the liquid medicine to avoid blocking the dosing pump and flowmeter.
According to the influent quantity and the notice of dosing, the dosing quantity should be adjusted in time to achieve the best treatment effect.
