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Application process and process of polyacrylamide in sewage treatment

    Once upon a time, cheap and abundant water resources were now reassessed as valuable resources due to limited availability. More urgently, the increasingly scarce water resources are facing unprecedented serious water pollution, which has become a major issue affecting human living environment and future development. The recycling of waste water (sealed systems) and the development of technologies for recovering useful substances from wastewater are becoming a strong demand in the era of water treatment and water use that save resources and save energy.
    The impurities in the wastewater can be classified into a soluble substance and a dispersion-suspended insoluble substance. The definition of dissolution and insolubilization is difficult to divide. However, in terms of water supply and drainage treatment, the substance in the filtrate after filtration is generally referred to as a dissolved substance, and the substance remaining on the filter material is referred to as a suspended substance, and is distinguished. In addition, the dispersed and suspended substances are also classified into colloids, suspended substances (usually written as SS) and coarse suspensions according to their size or specific gravity.
    Wastewater treatment is to separate harmful impurities and turbid substances from the wastewater system and further process them. Among them are physical treatment methods, chemical treatment methods and biological treatment methods.
The physical treatment method is a treatment method that does not use a chemical agent but uses only mechanical force and physical energy, and can be classified into a process of screening, sedimentation, flotation, and filtration. The chemical treatment method is a method for accelerating the separation or decomposition of a treatment substance by the action of a chemical agent when the physical treatment is difficult, or when the unit operation such as precipitation, flotation, filtration, or the like is further improved. The chemical treatment methods generally have the following classifications: pH adjustment, neutralization treatment; coagulation treatment; redox treatment; adsorption treatment; ion exchange treatment. The biological treatment method is mainly used for removing organic matter in water, and is a method for purifying BOD or COD components by utilizing the function of microorganisms, and is mainly used for drainage treatment and sludge treatment, and can be roughly classified into aerobic treatment and Anaerobic treatment. In practice, the above three methods are often applied in combination, including a combination of physicochemical methods (physical and chemical methods), biochemical methods (biological and chemical methods), and combinations of the three methods. The figure below shows the types and treatments of contaminants and the general combination of suitable agents.
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    In various types of wastewater, various pollutants are often mixed together. The nature of the wastewater depends on the type and content of the pollutants, and the following categories are classified according to the types of major pollutants.
Regarding their respective situations and cooperation, the treatment methods are as follows:
(a) Wastewater treatment of inorganic suspended solids
    In the wastewater such as civil engineering, steel and mining, it mainly contains inorganic suspended solids such as clays and fine ores. For this type of wastewater treatment, flocculation and sedimentation treatment is generally used.
(b) Organic wastewater treatment
    Waste water from pulp, food, fiber, dyes, petrochemicals, or machinery and automobile industries contains dissolved organic matter or emulsified oil in addition to suspended solids. At this time, the wastewater treatment is carried out by flocculation and sedimentation treatment, or a combination of activated sludge treatment and flocculation and sedimentation treatment.
(b) Waste water containing hazardous substances
    Hazardous substances are suitable for neutralization and redox treatment, or adsorption and ion exchange treatment, etc., depending on the type and nature. For the treatment of harmful substances, the effects on microorganisms should generally be considered. First, the harmful substances are removed, and then the other pollutants are treated by flocculation treatment and activated sludge treatment.
    Flocculation treatment is the most important method and means of wastewater treatment. The key to its closure is the choice of the type and type of flocculant. Our factory mainly produces three series of inorganic and organic polymer flocculants, papermaking auxiliaries and oilfield chemicals. In recent years, under the guidance of experts and professors from Tianjin University and the Chinese Academy of Environmental Sciences, we introduced the Japanese Kurita industrial plant. The theory and achievements of the research on water treatment chemicals in the club have developed YS and WS series organic and inorganic polymer flocculants and their application in wastewater treatment. Through various enterprises, wastewater treatment has reached the national emission standards.
    The flocculation process can be roughly divided into a tendency to be unstable caused by charge neutralization on the surface of the particles, and a tendency to flocculate due to bridging using unstable particles. However, sometimes the unstable particles are referred to as primary flocs (or coagulated flocs), and the coarsened flocs are referred to as secondary flocs.
The flocculating agents generally used in wastewater treatment are classified into inorganic flocculants used as destabilization and high-molecular flocculants used as flocculation. Further, an auxiliary agent is sometimes used in order to have good cohesiveness, flocculation sedimentation property, and floatability. The types and effects of these flocculants are as follows:
Types of flocculants and their effects
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    For the charge neutralization caused by inorganic flocculants such as polyaluminum chloride and the insolubilization of dissolved substances, and the bridging effect by polymer flocculants, we use the following model diagram to show the coagulation process.
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      The cationic polymer flocculant is used alone, and the charge neutralization and flocculation are simultaneously performed, and flocculation treatment is performed. Most of the methods are mainly used to remove large suspended solids such as gravel, pulp and fiber fragments, as well as for sludge concentration and dewatering. Further, in order to prevent dissolution of a soluble organic substance such as lignin or a dye, a low molecular weight cationic polymer flocculant may be used. In addition, in the ore flotation, surfactants and various xanthates are readily available as auxiliaries.

     First, the nature and use of inorganic flocculants
     As the inorganic flocculating agent, a divalent or higher metal salt is generally used. The neutralization power of the monovalent sodium and potassium salts is weak, and the insoluble hydroxide is not produced. Therefore, the effect of one suicidality is also weak. However, as the ion valence increases, the charge neutralization force becomes strong, and an insoluble hydroxide is formed, which has the property of capturing dirt (primary flocculation), so that the effect is also improved. We refer to the cohesiveness of the dirt caused by the neutralization of this valence and charge as the condensation price, and the relationship is in accordance with Schultz-Hard's law (ie, the law of valence).
     The types and characteristics of inorganic flocculants generally used in wastewater treatment are as follows:

Types and characteristics of the product flocculant W S series

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When using these inorganic flocculants, the following should be noted.
    (a) PH adjustment
    The inorganic flocculants listed in the above table have their respective pH ranges. The suitable pH value is usually the pH range of insoluble hydroxides. For example, when using bauxite and PAC, the optimum pH is 5-7.5. Within this range, it is most likely to generate flocs once.
    (b) flocculation and agitation
In order to increase the number of times of contact between the flocculant and the suspension, and sufficiently carry out the charge neutralization reaction, it is necessary to enhance the intensity and time of the stirring.
(c) Adding order
Generally, after the inorganic flocculant is added, the pH is adjusted to fully react, and then the polymer flocculant is added. In addition, inorganic flocculants and pH adjusters are sometimes added, but when the charge neutralization is paid more than the flocculation caused by the hydroxide, it is preferable to add the pH adjuster after the inorganic flocculant is added. .
Second, the type and nature of polymer flocculants
    In order to give full play to the good effect of polymer flocculants in water (bridge action), they should have the following properties:
    (a) should be water soluble
    In general, in the case of suspended particles in water, in order to uniformly act, it is necessary to uniformly disperse and dissolve in water.
    (b) should have an active reactive group
    By adsorbing the active group, the suspended particles are attached to the primary floc, or the reacted particles are aggregated (flocculated), and the types of the active groups are nonionic, anionic, and cationic.
    (c) should have a wide distribution in the water
    In order to concentrate as many particles as possible and to form coarse flocs, it is required that the high-molecular flocculant itself has a wide distribution in water. The factors that increase the distribution are mainly to increase the molecular weight and to adjust the content of a certain active group that expands the action of the molecule.
The YS series is an organic polymer with this property. It is used according to the different nature and type of wastewater. It is not only used for flocculation of wastewater, but also widely used for sludge concentration, dehydration and recycling. The general properties and characteristics of YS series polymer flocculants are as follows:

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Note: VSS is the volatile suspended solids of sludge, SS is the total suspended solids content of sludge, and VSS/SS is the proportion of organic matter in SS.
At present, the production of domestic sludge dehydrating agent (cationic polymer) has just started, and the variety specifications are few. Our factory introduces French production technology, and the main raw materials are also imported from France. Now we produce a variety of dehydrating agents. The types and methods of use of dehydrating agents are briefly described.
(1) The nature of the dehydrating agent
The main component of the sludge, the microbial floc, has a negative charge, and in order to cause it to agglomerate, a cationic dehydrating agent must be used. The centrifugal dewatering machine and the belt filter press machine form a coarse and non-breakable floc, the molecular weight of 100,000 to 10 million, or even more than 20 million molecular weight, currently mainly using amino methacrylate and amino acrylate. Polymer and Mannich denature of polyacrylamide.
The reason is summarized as the following two points.
1 High molecular weight products are readily available (and there are also more than 20 million molecular weights).
    2 The former is a copolymer with acrylamide, and the latter can obtain products of different cationic degrees by adjusting the cationic denaturation conditions.
A method for determining the cationicity of a dehydrating agent is usually a colloid titration method. According to the degree of cationicity, it is divided into high, medium and low cations.
(2) Relationship between sludge properties and dehydrating agent
The value of VSS/SS in the sludge property project has a great influence on the type and effect of the dehydrating agent. That is, VSS/SS is big:
1 A dehydrating agent with a high degree of cationicity is required.
2 The amount of dehydrating agent should be increased.
When the sludge is mixed and produced by sludge, VSS/SS is not considered, and the Mannich denature of polyacrylamide can be used.
When the SS concentration is high, the high molecular weight dehydrating agent is effective, and when the SS concentration is low, the lower molecular weight dehydrating agent is effective. The sludge with a large amount of fiber/SS has a low moisture content of the filter cake, but it has nothing to do with the dehydrating agent used.
When a flocculating sludge (polyaluminum hydroxide-containing) is mixed, the dehydrating agent is applied to the low-cationic side, and it is effective to use a Mannich-type dehydrating agent.
The pH of the sludge is independent of the cationicity and molecular weight of the dehydrating agent. However, it should be noted that when the pH is high, the functional group is a dehydrating agent having a structure of a tertiary amine or a tertiary amine, and the effect is lowered. The amino methacrylate and the amino acrylate dehydrating agent usually have a quaternary ammonium functional group.
(3) Relationship between type of dehydrator and dehydrating agent
    Because the centrifugal dehydrator performs solid-liquid separation at a high centrifugal force of 2000-3000 G, a high molecular weight dehydrating agent may form a strong floc compared to a belt press. In the case of a belt press, when the molecular weight is too high, the viscosity of the dehydrating agent remains on the floc, so that the peeling property of the filter cake on the filter cloth is poor.
If observed with a cation, the same VSS/SS belt press requires a high degree of cationicity. In addition, the centrifugal dewatering machine is used in a large amount for the amount of the dehydrating agent to be dosed.
(4) Effective use of dehydrating agent
The following items can be listed: 1 should understand the quality of the treated sludge and its changes, 2 should select the dehydrating agent suitable for the nature of the sludge and the type of dehydrator, 3 should pay attention to the injection point of the dehydrating agent, the reaction conditions, dosing The amount, 4 should be dissolved and stored by careful dehydrating agent.
(a) Select an experiment
Take the centrifugal dehydrator as an example:
1 CST (capillary suction time)
The rate at which the filtrate of the flocculated sludge rises along the capillary is measured under strong agitation.
2 Gravity dewatering and sedimentation experiments
Under strong agitation, the water repellency of the condensed sludge and the sedimentation of the floc were measured.
    3 pressing experiment
The cake obtained by gravity dehydration described above was subjected to press dehydration, and the moisture of the filter cake was measured.
For the belt pressure type dewatering machine, the gravity dewatering property of the condensed sludge, and the press dehydration of the filter cake, and the water content and the peelability of the filter cloth were measured. However, it is sometimes evaluated by a pressure filtration test.
    (b) Precautions for the actual device
1 centrifugal dewatering machine
When the properties such as the concentration of the sludge change, the centrifugal dehydrator can be processed relatively stably. When the dosage of the dehydrating agent exceeds a certain ratio, although the recovery of SS is stable (sometimes with high dosage, the recovery is also bad), the water content of the filter cake and the treatment cost (dosing amount) and stable treatment should be considered. Therefore, it is best to maintain an appropriate dosage. In addition, according to changes in operating conditions, the following results can be obtained, but the conditions of the corresponding targets need to be set.
Operating conditions and effects of centrifugal dewatering machine
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2 belt pressure type dewatering machine
The belt press is characterized by a small amount of dehydrating agent and a low moisture content of the filter cake. Because the optimal dosage range is narrow, it is necessary to carefully carry out the dosing management. In addition, depending on the operating conditions, the dewatering effect will change, as shown in the following table:
Operating conditions and effects of belt press dewatering machine
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(c) Dissolved water quality of dehydrating agent and preservation of solution
Some water quality items that seriously affect the dehydrating agent in the quality of dissolved water include M alkalinity (100 ppm or more), phosphate ion, and the like. As a dehydrating agent, the tendency is that the lower the cation, the more susceptible it is to the influence. Recently, in sewage treatment plants, there is a case where treated water is used as dissolved water, but this requires consideration of shortening the storage time of the solution.
The storage time of the solution is, in principle, controlled within one day. However, the longest period of use of low cationity should be controlled within two days, and the longest period of time with high cationity should be controlled within seven days.
    ZS series of high-molecular flocculants are powdery (or granular) and liquid, and the concentration of these flocculants is different. In general, the powder is more than 90%, and the liquid is about 5-10%. Due to the high transportation cost of each component of the liquid flocculant, special transportation methods, storage methods, transport distance, etc. should be considered. Production, material system. The powdered flocculant can be transported and stored by a general conveying method, and can be circulated regardless of the amount used. Therefore, powdered flocculants account for the majority of wastewater treatment.
When using a powdery ZS series polymer flocculant, it is usually powdered by an automatic powder feeder or a simple disperser to disperse and dissolve it in a certain amount of water, and then inject wastewater. The general procedure is as follows:
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When using powdered ZS series polymer flocculant, the following should be noted:
    (a) Points of attention when dispersing and dissolving
When the powdery polymer flocculant is dissolved, if it is simply added to water, it forms a lump (so-called "dough") and does not dissolve even after a long time of stirring. Not only in vain, but also often cause the clogging accident of the dosing pump. Therefore, the dispersing device in which the dissolved water and the powder are in contact with the dispersed film state is generally used. In the automatic powder feeder, the disperser is generally installed, but when manually dispersed, Use the simple disperser as shown below:
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Polyacrylamide high-molecular flocculant usually requires 15-90 minutes of dissolution and stirring time due to different varieties and particle sizes. While stirring vigorously for a long time, it is dissolved by pump circulation, sometimes causing molecular breakage and deterioration, and thus causing poor effect. The following table is an example of a drop in effect.
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Nowadays, there is a powder adding system on the market for special processing of powder. Since the time required for dissolution is extremely short, after dispersing, it can be directly injected into the wastewater without dissolving and stirring. The system does not need to dissolve the stirring tank.
    (c) water quality related to dissolved water
    The water-soluble organic polymer is generally affected by the quality of the dissolved water, so that the diffusion of the molecule is suppressed, and the dissociation of the ionic group is suppressed, resulting in a decrease in activity. When the ZS series polymer flocculant is used, it is also adversely affected by the nature of the dissolved water, which tends to cause a decrease in the viscosity of the solution and a poor effect. Substances that give such adverse effects are mainly polyvalent metal salts such as calcium and iron salts, and oxidizing agents. The concentration at which it affects varies depending on the type and physical properties of the polymer flocculant.
    Therefore, in the case where the polymer flocculant is in a state of being largely dispersed in water, it is preferable to use pure water and soft water. When industrial water and treated water are used as dissolved water, water quality analysis and measurement must be performed in advance. Solution properties and confirmation effects, etc.
    (d) Adding order and flocculation stirring
    The order of addition is basically to add a polymer flocculant after adding an inorganic flocculant and adjusting the pH.
    The flocculating agent should firstly disperse the high-molecular flocculant in the wastewater, and then stir vigorously to adsorb the reaction of the particles and the primary floc, and the stirring time should be short. Thereafter, slow and weak agitation was carried out to grow the flocs and become coarse. If the vigorous stirring and the weak agitation are carried out, the floc not only does not grow, but the reverse side is destroyed. Therefore, the order of addition and the order of stirring are also important factors for improving the effect.
   Fourth, the coagulation test method (field experiment)
   (a) Flask test
The flask test is to obtain the type of the best flocculant and the required dosage, to predict the volume of sludge (sludge volume) due to the treatment of water quality and agglomeration, and all data related to flocculation treatment, for example: When judging the effect of the flocculation and sedimentation treatment, the data in the following table is usually collected. At this time, the operation procedure and test conditions such as the number of revolutions of the stirring blade and the stirring time, and the processing method and purpose of the production apparatus are optimally designed.
Flask experiment data chart
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Note: The dosage is in ppm, the sedimentation velocity m/h, the sludge volume %V, and the formation time s.
(b) Column flotation test
     The column flotation test is mainly used to judge the effect of flocculation flotation. The data shown in the following table is usually collected. In addition, the operation procedure is the same as the above-mentioned flask test, and is designed in combination with the processing mode and purpose of the production apparatus.
Column flotation test data chart
Column flotation experiment data chart
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Note: The dosage is ppm, the floating speed is m/h, and the volume ratio is %V.
     (c) When the effect is judged by specific data such as sediment compaction and filterability of the floc, a test tool such as a settling tube or a filter may be used depending on the purpose.