Clear and clean pool water is an important factor in retaining swimmers. Photo: Compass Pool.
I myself am also a swimming enthusiast, and so are the children in my family. Unfortunately, during one visit to a nearby swimming pool, we had such a bad experience that we never dared to return. The reason was not only the large amount of suspended particles in the water, but also the presence of floating impurities that made me feel extremely uncomfortable.
Poor water quality is one of the key factors that causes swimming pools to lose customers or see a gradual decline in visitors over time. If you are an investor and your pool is facing a similar situation, you should start by improving water treatment.
When operating a swimming pool, the use of specialized pool chemicals to clean the water and prevent algae growth is essential. Each type of chemical has a different function and is designed to dissolve in large volumes of water to achieve high efficiency. Among them, commonly used coagulants such as PAC or aluminum sulfate directly affect water clarity, operating costs, and customer safety.
What is aluminum sulfate?
Aluminum sulfate (Al₂(SO₄)₃) is an inorganic compound widely used in water treatment and many other industries. When dissolved in water, it releases aluminum ions that can form flocs and bind tiny dirt particles together into larger masses, making them easier to settle and filter out.
In domestic water and wastewater treatment, aluminum sulfate works through hydrolysis. When it comes into contact with water, especially water with neutral to slightly alkaline pH, aluminum ions form aluminum hydroxide flocs. These flocs act like magnets, attracting suspended particles, organic impurities, and even bacteria, aggregating them into larger flocs. As a result, the water becomes clearer after sedimentation or filtration. This is why aluminum sulfate is commonly used in water treatment plants, although the dosage is always carefully controlled to avoid excessive pH reduction.
What is PAC?
PAC, short for Poly Aluminium Chloride, is a coagulant widely used in clean water treatment, domestic wastewater, and industrial wastewater treatment. When PAC is diluted and added to water, positively charged aluminum ions quickly combine with impurities to form larger and heavier flocs. These flocs are tightly bound, settle easily, and can be easily separated from water, helping the water become clear quickly while significantly reducing color and improving odor.
One major advantage of PAC is its ease of use. It dissolves quickly in water, produces less sludge than traditional alum, and has a wide working pH range, making it suitable for both industrial and domestic systems. In water supply plants, PAC has almost become the standard choice because it clarifies water quickly, has a short flocculation time, and causes less equipment corrosion. In wastewater treatment, PAC is favored for its ability to remove color, turbidity, TSS, COD, and difficult-to-settle impurities.
Comparison of PAC and aluminum sulfate in swimming pool water treatment
Image: Google Images.
Although both are aluminum-based coagulants used in water treatment with the function of aggregating impurities, effectively removing suspended solids, supporting filtration, and reducing turbidity, each has very different chemical characteristics and operational performance.
Coagulation efficiency
PAC hydrolyzes very quickly in water and releases high-charge-density hydroxy-aluminum clusters. These multi-nuclear clusters can almost immediately neutralize the surface charge of colloidal particles, while forming large, dense, and heavy flocs. Thanks to this compact structure, sedimentation occurs rapidly, making the water clearer and more stable in a short time.
In contrast, alum mainly relies on Al³⁺ ions to neutralize charges, so the coagulation process is slower and less efficient. The flocs formed are often loose, light, and easily re-dispersed if the flow is strong or mixing conditions are not suitable, resulting in a significantly slower settling rate.
Under the same treatment conditions, PAC shows superior coagulation and sedimentation performance compared to alum, both in floc formation speed and in the quality of treated water.
Water quality range
PAC can operate effectively over a wide pH range from 4 to 11, allowing it to adapt easily to various water conditions. Even when the water has high turbidity, contains a large amount of organic matter, or is affected by low temperatures, PAC maintains stable coagulation performance. This makes PAC a flexible choice for swimming pools where water quality frequently changes with the seasons or is strongly influenced by external environmental factors. Therefore, under complex water conditions or low temperatures, PAC demonstrates greater stability and adaptability than alum, ensuring higher treatment efficiency and less dependence on environmental fluctuations, while also reducing the cost of pH adjustment chemicals such as NaOH, soda, and HCl.
Meanwhile, alum only achieves optimal performance within a narrow pH range of 5.5 to 7.5 and works best in neutral or slightly alkaline water. When the water deviates from this range, especially at higher or lower pH levels, coagulation efficiency decreases significantly. In addition, alum is strongly affected by temperature; in cold water, the reaction rate slows down, leading to a noticeable decline in treatment performance.
PAC swimming pool chemical can operate effectively over a very wide pH range. Photo: The Spruce.
Dosage and cost
Overall, although alum appears cheaper at the initial stage, PAC delivers lower actual costs thanks to its high efficiency, lower dosage, and reduced post treatment expenses. Specifically, PAC has a high degree of polymerization and strong coagulation efficiency, so the typical required dosage is only about 1 to 5 g per cubic meter of water, around 30 to 50 percent lower than alum. During reactions, PAC also generates less sludge, reducing the load on filtration systems and saving on cleaning and sludge treatment costs. Although the unit purchase price of PAC may be slightly higher, when total operating costs are considered, from chemical consumption to post treatment expenses, PAC usually offers more optimal overall spending.
In contrast, alum has a lower initial price, which easily creates a perception of cost savings. However, to achieve equivalent treatment performance, alum requires a significantly higher dosage, typically 15 to 30 g per cubic meter of water. In addition, alum coagulation produces more sulfate ions and a larger volume of sludge, increasing operating and sludge treatment costs. As a result, the total actual cost of using alum is often much higher than its initial purchase price.
Post treatment water quality and filter residue
PAC forms large, dense, and well bonded flocs, making filtration more efficient and resulting in highly clear treated water. Due to its stable polymer structure, PAC leaves much lower residual aluminum levels compared to traditional coagulants, allowing the water to more easily meet safety standards, including drinking water standards. This makes PAC a safe and reliable choice for systems that require strict output water quality.
Meanwhile, alum produces looser flocs and leaves higher residual aluminum levels. If dosage is not tightly controlled or water conditions are not optimal, residual aluminum can adhere and accumulate, causing secondary contamination and directly affecting the safety of water supply systems. Therefore, despite its lower price, alum requires more rigorous monitoring processes to ensure water quality standards are met.
Specialized swimming pool chemicals help keep the water clean and clear. Photo: Godopools.
Safe for swimmers
PAC is widely used in domestic water treatment thanks to its high stability and effective flocculation ability without causing strong fluctuations in water parameters. As a result, the treated water quality remains stable, safe, and has minimal impact on users.
In contrast, aluminum sulfate tends to lower water pH quite quickly if the dosage is not properly controlled, leading to an overly acidic water environment. This pH change not only affects treatment efficiency but also increases the risk of skin and eye irritation for swimmers, especially in pools with high usage frequency or unstable operating systems.
Conclusion
In summary, both PAC and aluminum sulfate are common coagulants used in water treatment, but they are not the same. From the comparison above, it can be seen that PAC offers more advantages. These include forming dense flocs that settle quickly, effectively removing turbidity, suspended solids, heavy metals, and some organic substances, as well as providing stable and safe water quality after treatment. That is why PAC is an excellent choice for swimming pool water treatment. In addition, it is also widely used in municipal drinking water systems, industrial wastewater treatment, papermaking, mining, food processing, and cooling water systems.
On the other hand, due to its low cost and wide applicability, aluminum sulfate is commonly used as a coagulant in drinking water treatment, paper mills, textile finishing, and municipal wastewater treatment.
