Drinking Water Grade Polyaluminium Chloride (PAC) : Safe and Efficient Water Treatment Purifying Agent

Introduction

With the increasingly severe global water shortage and water pollution problems, safe and efficient drinking water treatment technologies have become the key to safeguarding public health. Polyaluminum Chloride (PAC for short), as a new type of inorganic high-molecular coagulant, has rapidly replaced traditional aluminum salts since its advent in the 1960s due to its excellent water purification performance and has become the mainstream agent in the field of water treatment. Especially in the field of drinking water treatment, drinking water grade PAC, with its characteristics of high efficiency, safety and wide adaptability, is widely used in urban waterworks, rural centralized water supply and various industrial pure water preparation systems.

China's "Hygienic Standard for Drinking Water" (GB5749-2022) has set higher requirements for water quality, which has further promoted the development of high-performance water treatment chemicals. Compared with traditional aluminum sulfate and aluminum trichloride, drinking water grade PAC has significant advantages such as good coagulation effect, low residual aluminum, wide pH adaptability range and less sludge. This article will systematically introduce the characteristics, production process, quality standards of drinking water grade PAC and its application technologies in various water treatments, providing a reference for the design and operation management of water treatment projects.

I. Overview of Drinking Water Grade PAC

Polyaluminium chloride is an inorganic macromolecular compound, and its chemical formula is [Al₂(OH)ₙCl₆ ₙ ₙ]ₘ, where n=1-5, m≤10. Drinking water grade PAC is a high-purity product specially developed for drinking water treatment. Compared with industrial-grade PAC, it has stricter restrictions on the content of harmful impurities such as heavy metals. From a chemical structure perspective, PAC is a hydrolyzation-polymerization product between AlCl₃ and Al(OH)₃, containing a large number of positively charged polynuclear hydroxyl complexes, such as [Al₆(OH)₁₅]³⁺, [Al₈(OH)₂₀]⁴⁺, etc. These highly charged polymers endow PAC with excellent electro-neutralization ability.

According to different production processes, PAC can be classified into three forms: drum-dried type, spray-dried type and liquid product. Drinking water grade PAC is mainly produced by spray drying process. This process, through high-temperature rapid drying, can effectively maintain the active components of PAC. The product is in the form of light yellow powder, with an alumina (Al₂O₃) content of over 30%, and the basicity is controlled between 40% and 60%. Liquid PAC is also applied in small and medium-sized water plants due to its convenience of use, but the transportation cost is relatively high.

China has strict quality standards for drinking water grade PAC. The "Polyaluminium Chloride for Drinking Water" (GB/T 22627-2014) stipulates: The Al₂O₃ content of drinking water grade PAC is ≥28%, the basicity degree is 40%-85%, the water-insoluble matter is ≤1.5%, the arsenic content is ≤0.0005%, the lead content is ≤0.002%, the cadmium content is ≤0.0005%, and the mercury content is ≤0.00001%. These indicators ensure the safety of PAC in drinking water treatment. Compared with international standards such as those of Japan and the European Union, China's standards are stricter in terms of heavy metal limits, especially for the control of the carcinogen arsenic.

Ii. The Mechanism of Drinking Water Grade PAC in Water Treatment

The purification mechanism of PAC in water treatment is complex and efficient, mainly including three aspects: electro-neutralization, adsorption bridging and net capture sweeping. The electro-neutralization effect is the core mechanism of PAC water purification. Colloidal particles in water usually carry negative charges, and the high-valent positively charged aggregates produced after the hydrolysis of PAC can effectively neutralize these negative charges, destroy the stability of the colloids, and cause them to destabilize and coagulate. Studies show that the higher the content of Alb (medium degree of polymerization component) in PAC, the stronger its electro-neutralization ability and the better the coagulation effect.

Adsorption bridging effect refers to the fact that the long-chain polymer structure of PAC can form "Bridges" among multiple colloidal particles, connecting these particles to form larger flocs. This effect is particularly important in the treatment of low-temperature and low-turbidity water. The net capture and sweeping effect occurs when the dosage of PAC is relatively high, and the formed aluminum hydroxide precipitate acts like a filter screen to capture fine particles in the water. In practical applications, these three effects often exist simultaneously, but the dominant mechanism varies with water quality conditions and the dosage of PAC.

Compared with traditional aluminum salts, PAC has significant advantages: Firstly, PAC is a pre-hydrolyzed product, and its effective components are formed at the factory and are not affected by factors such as water temperature and pH, ensuring a stable coagulation effect. Secondly, the polymeric structure of PAC endows it with a stronger charge neutralization ability and a wider pH adaptability range (pH5-9). Thirdly, the flocs produced by PAC are large and dense, with a fast sedimentation rate. The residual aluminum concentration in the treated water is low, usually controlled below 0.1mg/L, which is far lower than the 0.2mg/L limit stipulated by the national standard.

Iii. Application of Drinking Water Grade PAC in Drinking Water Treatment

In the application of waterworks, PAC, as the core coagulant, its dosing process directly affects the final effluent quality. The conventional treatment process usually adopts the "PAC coagulation - sedimentation - filtration - disinfection "flow. Among them, the PAC dosing point is generally set before the rapid mixing tank, and the rapid and uniform dispersion of the reagent is ensured through mechanical stirring or hydraulic mixing. The dosage should be determined through a beaker test based on the quality of the raw water, generally ranging from 5 to 30mg/L (calculated as Al₂O₃). For water with high turbidity, the "pre-chlorination - coagulation" process can be adopted; For low-temperature and low-turbidity water, it is recommended to use it in combination with a coagulant aid.

PAC shows unique advantages in dealing with sudden water pollution. When abnormal algae, organic matter or heavy metal pollution appears in the raw water, these pollutants can be effectively removed by adjusting the dosage of PAC and pH value. Studies show that under optimized conditions, PAC can achieve a removal rate of over 90% for heavy metals such as lead and cadmium, and a removal rate of more than 95% for algal cells. When dealing with seasonal algal outbreaks, a certain water plant increased the dosage of PAC from 15mg/L to 25mg/L, while controlling the pH within the range of 6.5-7.0. As a result, the number of algae in the effluent was successfully reduced from 5,000 per mL to less than 50 per mL.

In rural safe drinking water projects, the application of PAC is more flexible and diverse. For centralized water supply systems, automatic dosing equipment can be adopted. For decentralized water supply points, simple dosing devices and small-packaged products have been developed. In the rural drinking water renovation project of a certain province, PAC was promoted and used, raising the water supply qualification rate from 60% to over 90%, effectively solving the long-standing problem of high fluoride and high arsenic water quality in rural areas. It is worth noting that in rural applications, special attention should be paid to the simplicity of operation and cost control. Liquid PAC or pre-prepared PAC solutions are more favored.

Iv. Application of Drinking Water Grade PAC in Urban Water Supply and Industrial Water Supply

In addition to the conventional production of tap water, the urban water supply system also includes the reuse of reclaimed water and the maintenance of water quality in the pipeline network. PAC is widely applied in these fields. In the treatment of reclaimed water reuse, PAC can effectively remove colloidal organic matter and phosphorus in water. A certain city's reclaimed water plant adopted the PAC coagulation + membrane filtration process, reducing the COD of the effluent to below 10mg/L and the total phosphorus to less than 0.3mg/L, fully meeting the standards for landscape water use. For the "yellow water" problem in the pipeline network, by adjusting the dosage of PAC in the water plant to optimize the coagulation effect, the deposition of iron and manganese in the pipeline network can be significantly reduced. After a certain city implemented the optimized dosage of PAC, the complaints about water quality at the user end decreased by 70%.

In the field of industrial water supply, different industries have varying requirements for water quality, and the application of PAC also shows diverse characteristics. In the boiler feed water treatment of the power industry, PAC, as a pretreatment agent, can effectively reduce the load of the subsequent ion exchange system. In the preparation of ultrapure water in the electronics industry, high-purity PAC is used to remove trace metals from water. The food and beverage industry pays more attention to the safety of PAC and its impact on taste. A certain brewery has adopted a specially designed low-residue PAC to replace the traditional coagulant, significantly improving the quality of brewing water and making the product taste purer.

The reuse of industrial wastewater is another important application scenario of PAC. In the advanced treatment of wastewater in industries such as steel, dyeing and printing, and papermaking, PAC can simultaneously remove COD, color and heavy metals, achieving wastewater reuse. A certain paper mill adopted the PAC coagulation + ozone process, increasing the wastewater reuse rate from 40% to 75% and saving 2 million tons of fresh water annually. Unlike municipal water supply, the dosage of PAC in industrial applications is usually higher, and sometimes it needs to be used in combination with organic polymer flocculants to achieve the best treatment effect.

V. Development Trends and Prospects of Drinking Water Grade PAC

With the advancement of water treatment technology and the increasing environmental protection requirements, drinking water grade PAC is developing towards high performance, functionality and greenness. New types of composite PAC products are constantly emerging, such as silicon-containing PAC (PACS) and iron-containing PAC (PAFC), etc. These products have better treatment effects for specific water qualities. The introduction of nanotechnology has also given rise to nanoscale PAC, whose larger specific surface area and higher activity have significantly enhanced the coagulation efficiency. The arsenic removal capacity of the nano-PAC developed by a certain research team is more than three times that of the traditional PAC.

Green production is an inevitable trend in the PAC industry. The waste residue and acidic wastewater generated in the traditional PAC production process are being effectively controlled through process improvement. The new spray drying technology reduces energy consumption by 30% and the recovery rate of by-products exceeds 95%. In the future, the technology for producing high-quality PAC from industrial waste acids, aluminum dross and other waste materials will be further improved to achieve resource recycling. The biological method of PAC is also in the laboratory research stage and is expected to further reduce the environmental impact of the production process.

Intelligent applications will change the traditional way PAC is used. The intelligent dosing system based on big data and artificial intelligence can analyze water quality changes in real time and predict the optimal dosage of PAC. A certain smart water plant project has achieved advanced control of PAC dosing through a machine learning model, reducing chemical consumption by 20% while ensuring the stability of effluent quality. With the development of Internet of Things (iot) technology, remote monitoring and optimization of PAC dosing will become possible, which is particularly suitable for application scenarios in decentralized water supply points and rural water plants.

Conclusion

Drinking water grade polyaluminium chloride, as a core chemical agent in modern water treatment, plays an irreplaceable role in ensuring drinking water safety, improving urban water supply quality and supporting sustainable industrial development due to its high efficiency, safety and strong adaptability. With technological advancement and the improvement of environmental protection standards, PAC products and their application technologies will continue to innovate, providing more comprehensive solutions to address global water resource challenges. The water treatment industry should fully recognize the scientific value of PAC, continuously optimize its production and application technologies, and at the same time strengthen the research and development of new coagulants to provide mankind with safer and cleaner drinking water.