Discussion on coagulation and sedimentation processes

Polyaluminum chloride (PAC) is a water purification material and an inorganic polymer coagulant; it is also referred to as polyalumina, with the English abbreviation PAC. It is an inorganic polymer water treatment agent with a high molecular weight and high charge, formed due to the bridging effect of hydroxide ions and the polymerization of multivalent anions. Morphologically, it can be further divided into solids and liquids. Solids are classified by color into brownish, beige, golden yellow, and white, while liquids can be colorless and transparent, slightly yellow, light yellow to yellowish-brown. Polyaluminum chloride in different colors also exhibits significant differences in terms of application and production techniques.

Chinese name: Poly Aluminium Chloride; English name: Poly Aluminium Chloride; Other names: PAC, polyalumina, polychlorinated aluminum; Chemical formula: Al2Cln(OH)6-n; CAS numbers: 1327-41-9; 101707-17-9; 11097-68-0; 114442-10-3; EINECS number: 215-477-2; Melting point: 190°C (253 kPa); Water solubility: Highly soluble in water; Density: Liquid ≥1.12; Applications: Water treatment; Hazard symbols: None; Hazard description: None

Concentration mixing methods: Based on the practice of using solid polyaluminum chloride (PAC) and diluting it into a liquid form, several common methods for determining the concentration of PAC are summarized and shared here, in the hope that they will be useful to everyone! The first step is to conduct a pilot test based on the conditions of the raw water in order to determine the optimal dosage before use. For the preparation of pilot-scale solutions, a weight ratio (W/W) is used, with 2–5% being a suitable concentration. For a 3% solution: Weigh 3 g of polyaluminum chloride PAC solid, place it in a 200 ml measuring cylinder, add about 50 ml of water, wait for it to dissolve, then add more water to reach the 100 ml mark and stir well. In the second step, when producing polyaluminum chloride PAC for use in production, it is sufficient to mix and dissolve it at a weight ratio of polyaluminum chloride PAC solid to clean water of 1:9 to 1:15. Solutions with an alumina content of less than 1% are prone to hydrolysis, which reduces their effectiveness; moreover, very high concentrations make it difficult to apply them evenly. Step three: Add the chemical at the optimal dosage determined through pilot testing. If there are few flocs in the sedimentation tank and a high level of residual turbidity, it indicates that the dosage is too low ； If the floc in the sedimentation tank is large and rises to the surface, along with high residual turbidity, it indicates that too much chemical has been added, and adjustments should be made accordingly.

Features of polyaluminum chloride: 1. Fast flocculent formation, good activity, and excellent filtration performance. 2. No alkaline additives are required; even if it deliquesces, its effectiveness remains unchanged. 3. It can adapt to a wide range of pH values, has strong adaptability, and is widely applicable. 4. The treated water contains less salt. 5. It can remove the contamination of water caused by heavy metals and radioactive substances. 6. It has a high concentration of active ingredients, making it easy to store and transport.

The flocculation effect of poly(chloro)aluminum is as follows: a. Strong electro-neutralization of colloidal substances in water. b. The excellent bridging adsorption effect of the hydrolysis products on suspended solids in water. c. Selective adsorption of soluble substances. Polyaluminum chloride is an inorganic polymer coagulant; it is an inorganic polymer water treatment agent with a high molecular weight and high charge, produced as a result of the bridging effect of hydroxide ions and the polymerization of polyvalent anions. Its characteristics are primarily determined by the working principle of pressure atomizers.

Process classification: Edit a – drum-type poly(chlorinated) aluminum. It has an average aluminum content and high levels of water-insoluble substances, and is mainly used in wastewater treatment. b. Plate-and-frame poly(chlorinated) aluminum – high aluminum content and low levels of water-insoluble substances; used for wastewater treatment and drinking water treatment. c. Spray-dried poly(chloro)aluminate: high aluminum content, low water-insoluble matter, and fast dissolution rate. Used for drinking water and water treatment requiring higher standards. Introduction to the combined use of polyaluminum chloride and polyacrylamide: First, we will analyze their relationship as a whole, and then introduce the steps for mixing them, the precautions to take, and related knowledge. Wastewater from detergent production is characterized by a complex composition, high levels of CODcr and LAS in the wastewater, which makes it difficult to biodegrade directly, as well as a low pH value. When the concentration of detergents in this wastewater reaches a certain level, it can affect processes such as aeration, sedimentation, and sludge digestion in wastewater treatment. In practical wastewater treatment, flocculants are often used to address the problem of the difficulty in biodegrading high-concentration LAS; therefore, studying the selection of flocculant types, their dosage, and the factors that influence them holds great practical significance in flocculation treatment.

Classification of Polyacrylamide: Introduction to Polyacrylamide Products: Polyacrylamide (PAM) is a water-soluble polymer that is insoluble in most organic solvents. It possesses excellent flocculating properties and can reduce the frictional resistance between liquids. Based on their ionic characteristics, polyacrylamides can be divided into four types: non-ionic, anionic, cationic, and amphoteric. (Note: Polyacrylamide is different from acrylamide.) Chinese name: Polyacrylamide; English name: Poly acrylamide. Types: Cationic, Anionic, Non-ionic, Emulsion. Uses of cationic type: Sludge separation; Uses of anionic and non-ionic types: Adhesives, coagulants for high-turbidity water. Effective pH range: 7-14

Precautions for using cationic polyacrylamide: 1. Size of flocs: Too small flocs can affect the drainage rate, while too large flocs will retain more water, thereby reducing the compactness of the sludge cake. By selecting the molecular weight of polyacrylamide, the size of the flocs can be adjusted. 2. Sludge characteristics: The first point is to understand the origin, characteristics, composition, and proportion of sludge. Based on their properties, sludge can be divided into organic and inorganic sludge. Cationic polyacrylamide is used for treating organic sludge, while the corresponding anionic polyacrylamide flocculant is used for inorganic sludge. Cationic polyacrylamide is used in highly alkaline conditions, whereas anionic polyacrylamide is not suitable in highly acidic conditions. When the solid content is high, more polyacrylamide is generally required. 3. Floccule strength: The floccules should remain stable and not break under shear stress. Increasing the molecular weight of polyacrylamide or selecting an appropriate molecular structure helps to improve floccule stability. 4. Ionization degree of polyacrylamide: For dehydrated sludge, flocculants with different ionization degrees can be tested on a small scale to select the most suitable polyacrylamide. This approach ensures the best flocculation results while minimizing the amount of chemical used and thus saving costs. 5. Dissolution of polyacrylamide: Good dissolution is necessary to fully exert its flocculation effect. Sometimes, there is a need to accelerate the dissolution rate; in such cases, considering increasing the concentration of the polyacrylamide solution might be an option. Can polyacrylamide and polyaluminum chloride be used together? In fact, when treating wastewater, for some types of wastewater, using just one type of flocculant is not sufficient; both must be used in combination. Using an inorganic flocculant like PAC along with polyacrylamide as a composite flocculant yields better results. However, it is important to pay attention to the order in which these chemicals are added, as using them in the wrong order will result in ineffective treatment. The correct order for adding polyacrylamide along with other flocculants is crucial: when using a composite flocculant, one must pay attention to the sequence in which it is added as well as the time intervals between additions! The combined use of PAC and PAM involves first using PAC to neutralize the charges and destabilize the colloids, thereby forming fine flocs; thereafter, increasing the size of these flocs facilitates their complete precipitation. Since the reaction time for polyaluminum chloride PAC is very short, vigorous mixing is required after it is added. The action time for PAM is longer, and when mixing, it is important to start with strong mixing and then reduce the intensity – strong mixing is needed to ensure uniform distribution, while reducing the intensity helps prevent the destruction of the flocs! Polyacrylamide is a flocculant, while polyaluminum chloride is a coagulant. Generally, the coagulant is added first followed by polyacrylamide, but for safety reasons, it is still recommended to determine the order of addition based on experimental results! The dosing point, dosage amount, dosing time, and mixing intensity need to be determined through experimentation. It is crucial not to use these two types of chemicals together, as this will affect their effectiveness and increase the cost of use.

Methods for determining when polyacrylamide flocculants have lost their effectiveness: It is common to encounter many sewage treatment plants, especially in southern regions, where the humid climate causes polyacrylamide to absorb moisture and clump together due to prolonged storage or inadequate sealing of its packaging. Regarding this problem of clumping, many people wonder whether the polyacrylamide has lost its effectiveness and can still be used. In fact, as long as it can be dissolved and the resulting solution still has viscosity, it has not lost its effectiveness; however, it is very difficult to dissolve polyacrylamide that has clumped together, which also represents a waste of resources. The shelf life of different types of polyacrylamide varies significantly, and this is related to their structure. Relatively speaking, anionic polyacrylamide has a longer shelf life, while for cationic polyacrylamide, we generally specify a shelf life of 1 year. Any period beyond this limit is considered to be past the expiration date. There is a risk of failure; the failure of polyacrylamide can be determined from two aspects: one is a decrease in viscosity, and the other is a deterioration in flocculation efficiency.