Research on the Application of Polyacrylamide as a Flocculant

Since the United States pioneered the development of polyacrylamide as a flocculant for water treatment in the 1950s, its production and application have developed rapidly. Since the 1960s, polyacrylamide has been widely used as a flocculant in water treatment in Europe and America. In China, since the 1960s, polyacrylamide has been widely used in drinking water plants in the Yangtze and Yellow River basins for the purification of highly turbid water, as well as for the treatment of urban sewage and industrial wastewater. Since the 1990s, some drinking water plants with severely polluted water sources have begun to use polyacrylamide as a coagulant aid in water treatment.

Polyacrylamide is the most widely used synthetic flocculant. Its long molecular chain and amide groups (---CONH2) can form hydrogen bonds with many substances through affinity and adsorption. This allows it to bridge the adsorbed particles, connecting several or even dozens of particles together to form flocs, accelerating particle settling and making it an ideal flocculant. Experiments have shown that adding alumina hydrate to a partially hydrolyzed polyacrylamide solution causes the polymerized anions to adsorb onto the alumina cations, resulting in a rapid increase in viscosity or gelation. This is similar to the general flocculation mechanism, where one molecule can simultaneously adsorb several particles, causing them to clump together and settle rapidly. The settling rate depends on the concentration of the flocculant and the concentration of suspended solids.

After years of research on water treatment applications by water purification experts, it is generally believed that the flocculation mechanism of polyacrylamide is:

(1) Due to its polar group—amide group—it easily adsorbs onto the surface of sediment particles through the action of its hydrogen bonds;

(2) Because it has very long molecular chains, the large number of long chains provides a huge adsorption surface area in water, resulting in good flocculation. The long chains can bridge between particles, forming large flocs and accelerating sedimentation. (3) With the help of polyacrylamide's flocculation-coagulant properties, double ionization compression may occur during the sludge coagulation process in water purification, reducing the stability of particle aggregation. Under the influence of molecular attraction, particles bind together, and the simple anions in the dispersed phase can be replaced by polymer anionic groups;

(4) Chemical interactions occur between the polymer and substances in the natural water composition, suspended matter in the water, or ions of hydrolytic coagulants added before it, possibly through complexation reactions;

(5) Due to the molecular chains being fixed on the surfaces of different particles, polymerization bridges are formed between the various solid phase particles.

Polyacrylamide is a chemically active polymer compound. The activity of the amide groups on the molecular side chains gives the polymer many valuable properties. Nonionic PAM flocculants, lacking ionic functional groups, exhibit the following characteristics compared to anionic PAM flocculants: their flocculation performance is less affected by fluctuations in water pH and salinity; under neutral or alkaline conditions, their flocculation effect (sedimentation velocity) is inferior to anionic flocculants, but they outperform anionic flocculants under acidic conditions; and their floc strength is stronger than that of anionic polymeric flocculants.

Anionic PAM flocculants typically have lower molecular weights than anionic or nonionic polymers, and their clarification performance is primarily achieved through charge neutralization. These flocculants mainly function to flocculate negatively charged colloids, providing turbidity removal and decolorization, making them suitable for water treatment with high organic colloid content.

Selection of Dissolution and Dosing Processes

Before use, polyacrylamide should be prepared into a 0.1-0.5% stock solution. Further dilution or hydraulic conveying can be used for dosing. The storage period for a 0.5% stock solution is seven days, and the storage period for a 0.05% dosing solution is three days. When preparing the solution, the following points should be noted:

(1) Dissolution temperature. Polyacrylamide requires a certain temperature to dissolve, which accelerates the dissolution process. However, excessively high temperatures can break the polymer chains, reducing its effectiveness. The optimal dissolution temperature is 50-60℃.

(2) Stirring conditions. Strong shearing should be avoided when dissolving polyacrylamide, as this can break the polymer chains and reduce its effectiveness. Low-speed impellers, such as anchor, frame, or multi-layer impellers, are recommended. The stirring speed should be around 60 rpm. High-speed centrifugal pumps should also be avoided during transport; piston pumps or diaphragm pumps are more suitable.

(3) Uniform dispersion of materials. The key to dissolving polyacrylamide is the uniform dispersion of the materials. Generally, the aqueous solution should be heated to 50-60℃ first. After starting the mixer, it is best to use a mechanical vibrating screen (10 mesh) for feeding to avoid the formation of large clumps or fish-eye-shaped insoluble particles, thus ensuring that the polyacrylamide is fully dissolved and achieves its best performance.

(4) Avoid contact with iron. In the dissolving, mixing, and conveying dosing system, it is best to use materials such as plastic, enamel, aluminum, or stainless steel.

The choice of the dosing point for polyacrylamide has a significant impact on its performance. Some sources suggest that when treating high-turbidity water, polyacrylamide should be added first, and after thorough mixing, the coagulant should be added. Other sources suggest that adding polyacrylamide before adding the coagulant can lead to the protection of the sol. According to foreign sources, in seasons when the suspended solids content in water exceeds 50 mg/L, it is more suitable to add polyacrylamide and other anionic polymeric flocculants before the primary treatment structure. When the suspended solids content is low, it is more suitable to add them before the filter. To allow the suspended solids sufficient time to form fine flocs and prevent an actual decrease in particle number concentration (due to aggregation) and deterioration of the surface properties of the suspended solids, the time interval between adding coagulants and flocculants to the water is generally between 1 and 4 minutes. The lower the water temperature and turbidity, and the higher the water color, the longer the time interval between adding coagulants and flocculants should be. The water disinfection status should also be considered. If polyacrylamide is added before chlorination, the degree of water disinfection may be reduced, potentially worsening the process properties of cationic polymeric flocculants. This is because pre-chlorination may shield the cationic polymers from the disinfection process of biological organisms, and the polymers may be destroyed under the action of oxidants.

The stirring conditions during polyacrylamide addition significantly affect the flocculation effect. For optimal flocculation, it is best to add the polyacrylamide at the beginning of the reaction tank—when primary flocs are forming.

The dosage of polyacrylamide should be determined experimentally based on the specific water source quality and water purification process characteristics. Foreign literature indicates the following patterns in the flocculation process of polymeric flocculants:

(1) Optimal conditions are achieved when the polymeric flocculant dosage ensures coverage of the surface area of the dispersed phase particles;

(2) Oversaturation of the particle surface by polymer molecules leads to deterioration of flocculation because the free ends of the polymers can also adsorb onto the same surface, forming a curved shape, thus reducing the number of bridging bonds between adjacent particles;

(3) When vigorous stirring breaks down the polymer bonds, the flocculated particles disperse. If the polymeric flocculant dosage is less than the optimal dosage, the bridging bonds become weaker;

(4) There is a linear relationship between the optimal polymer dosage and the allowable adsorption area on the dispersed phase particle surface. A water plant in southern China believes that for its heavily polluted raw water with diverse pollutants and low pH and alkalinity, non-ionic polyacrylamide is relatively suitable for flocculation, anionic polyacrylamide is slightly less effective, and cationic polyacrylamide is less effective. Based on flocculation theory, it is inferred that during the dry season when pollution is severe, the raw water has low turbidity and fewer particles available for bridging. Adding liquid chlorine for disinfection before adding flocculants may have a destructive effect on the structure of cationic flocculants. In water bodies where coagulants and disinfectants have already been added, the pH is generally not higher than 7.0, and the water's charge tendency is not significant, so neither anionic nor cationic polyacrylamide shows any advantage.

Application of Acrylamide in Drinking Water Purification

Polyacrylamide has been used in a company's water supply production for many years, achieving significant results in improving flocculation, saving alum consumption, removing algae, reducing mutagenicity, improving water quality, and coping with sudden water quality incidents.

1. Improves flocculation effect, overcomes floc floating during the dry season, saves alum consumption, and reduces water purification costs. In low-temperature, low-turbidity water, organic matter constitutes a larger proportion of the turbidity components. Adding aluminum sulfate and polyaluminum sulfate alone results in loose, lightweight flocs that are difficult to settle. Adding 0.025-0.05 mg/L of polyacrylamide as a coagulant aid significantly increases the volume and specific gravity of the resulting flocs due to its large adsorption surface area and excellent bridging ability. This accelerates settling and rapidly improves the sedimentation capacity of the sedimentation tank, resulting in a substantial reduction in effluent turbidity. After adding polyacrylamide, the residual turbidity of the effluent is below 20 NTU after 5 minutes, which is 20-30 NTU lower than that of water samples without coagulant addition within the same timeframe. Water samples without coagulant addition, due to severe organic pollution in the raw water, have light and floating flocs, and the final turbidity cannot be reduced; even after standing for 1 hour, the residual turbidity does not decrease significantly. Adding polyacrylamide as a coagulant aid improves the flocculation effect, saves approximately 25% on alum consumption, increases water production by approximately 5-10%, and saves approximately 24% in costs.

2. Improves water quality, removes color, organic matter, and algae, and reduces mutagenicity. Due to the coagulant effect of polyacrylamide, the turbidity of the effluent decreases significantly. The content of organic matter and algae is closely related to turbidity. Experts believe that when turbidity is reduced to 0.5 degrees, 80% of organic matter in the water can be removed. Therefore, adding polyacrylamide as a coagulant can effectively remove organic pollution and improve drinking water quality. Color removal rate increases by more than 10%, organic matter in water decreases by more than 46%, and algae removal rate increases by 16-26%. The mutagenicity of the effluent from the sedimentation tank decreases, changing from strongly positive to positive, proving that the reduction in effluent turbidity removes a considerable portion of organic matter, thus improving the mutagenicity of the effluent.

Safety of Polyacrylamide in Drinking Water Treatment

Polyacrylamide itself is basically non-toxic because after entering the human body, the vast majority is excreted in a short period of time, and very little is absorbed by the digestive tract. Most commercial products do not irritate the skin; only some hydrolysates may have residual alkali, which can cause irritation with repeated and prolonged contact. The U.S. Food and Drug Administration considers PAM and its hydrolysates to be low-toxicity or non-toxic. The toxicity of PAM comes from residual acrylamide monomers and toxic metals introduced during the production process. Acrylamide is a neurotoxin, damaging the nervous system and causing symptoms such as muscle weakness and motor incoordination after poisoning. Therefore, health departments in various countries have regulations on the residual acrylamide content in industrial polyacrylamide products, generally between 0.5% and 0.05%. When used for general water purification, the acrylamide content should be below 0.2%, and when used for direct drinking water treatment, it should be below 0.05%. The World Health Organization's 1985 standard for polyacrylamide states that when the residual AM content in PAM is controlled below 0.05% and the dosage is controlled, the concentration in the treated water will be below 0.25 ug/L, meeting the drinking water standards of most countries. Currently, major European and American countries generally stipulate that the residual AM content in PAM used for drinking water treatment and food should be below 0.05%, and the dosage of PAM should be controlled.

The situation is much more complex with certain cationic polyacrylamides. This is because the amino groups introduced into cationic polyacrylamides often have toxicities tens to hundreds of times higher than those of anionic and nonionic polyacrylamides, and their chronic toxicity is still under investigation.

For flocculation applications in drinking water treatment, food-grade products are appropriate. The "Implementation Manual for Water Supply and Drainage Standards and Specifications" clearly stipulates that the dosage of polyacrylamides used is 0.1 mg/L for infrequent use and <0.1 mg/L for frequent use. In coagulation aid applications in water treatment processes, the above standard values can be used as the maximum dosage. Purchasing high-quality, low-residue food-grade polyacrylamides can ensure the safety of drinking water. For example, a water plant using polyacrylamides as a coagulator had a maximum dosage of 0.09 mg/L, using food-grade polyacrylamides from the Guangzhou Polyacrylamide Engineering Center of the Ministry of Chemical Industry. No acrylamide monomers were found in the filter effluent or the treated tap water. Therefore, it is considered that as long as the quality and dosage of the product are well controlled, the use of polyacrylamides as a coagulator is safe for drinking water hygiene.

In the near term, the application of polyacrylamide-based organic products as water treatment agents in my country will gradually expand as the scope of water scarcity increases and the public's demands for drinking water rise. Applications include coagulation, flocculant aid, and filter aid. It is reported that adding 0.015-0.05 mg/L of polyacrylamide before filtration can increase the filtration cycle and improve production capacity by 10-16%. However, increasing the filtration cycle also increases head loss, requiring corresponding increases in backwash time and intensity. Nevertheless, adding polyacrylamide as a filter aid is economically worthwhile. Adding polyacrylamide can prevent algae from penetrating the filter bed and ensure the quality of filtered water in emergency situations. Some water supply plants in China have already adopted this filter aid technology with good results. The advantages of polyacrylamide-based organic products in water treatment processes—enhanced flocculation, improved filter capacity, improved water quality, increased water volume, and cost savings—are increasingly being accepted by water supply companies. The application of polyacrylamide in drinking water treatment and engineering water treatment is under continuous research and development, and its prospects are optimistic.