What does polyaluminium chloride do in a swimming pool?

In swimming pool water treatment chemistry, polyaluminium chloride plays a crucial but often overlooked role as an efficient flocculant. Its main task is not to directly disinfect, but to act as a "beautician" and "cleaner" of water quality, specifically solving sensory problems such as turbidity, whitening, and low visibility. It is a key chemical substance to ensure that swimming pool water presents an enticing and crystal clear feeling.

1. Core mechanism of action: Electric neutralization and flocculation bridging

The reason why swimming pool water is turbid is mainly due to the presence of a large number of tiny particles suspended in the water that are difficult to distinguish with the naked eye, including oil secreted by the human body, sweat, dandruff, cosmetics residue, dust in the air, and disinfection by-products. These particles usually carry negative charges, and due to their repulsion, they cannot self aggregate and combine to sink. Instead, they remain suspended in water for a long time, resulting in a decrease in water transparency and appearing turbid.
The role of polyaluminium chloride in this process is miraculous, and its mechanism mainly includes two steps:
1. Neutralization effect: After dissolving in water, polyaluminium chloride will hydrolyze to produce a series of hydroxyl aluminum complex ions with high positive charges. These positive ions can quickly neutralize the negative charges on the surface of suspended particles, disrupting the repulsive force that keeps the particles dispersed and stable (i.e., the "double layer"). Once the charge is neutralized, the repulsive energy barrier between particles is eliminated, creating a prerequisite for collision and binding between particles.
2. Adsorption bridging and net trapping: After neutralizing the charge, the large and branched aluminum hydroxide colloid formed by the hydrolysis of polyaluminium chloride can, like many "hands", connect and bind multiple unstable small particles together through physical and chemical adsorption, forming a larger, loose and porous floc. This process is called "adsorption bridging". During the formation and sinking process, these flocculent particles also act like a fine net, wrapping and sweeping smaller particles in the water, and settling together.

In the end, countless tiny suspended particles were aggregated into visible, cotton like "alum flowers". These alum flowers have a higher density than water and slowly settle to the bottom of the pool under the action of gravity, thereby "stripping" the majority of suspended impurities from the water, completing the transition from turbidity to clarity.

2. Specific application in swimming pool water treatment process

The application of polyaluminium chloride is a key link in the circulating filtration system of swimming pools, usually following the following steps:
1. Addition: After one day of operation or maintenance in the swimming pool, dilute polyaluminium chloride into a certain concentration solution and evenly sprinkle it into the pool water through a dedicated dosing pump or manual method. The dosage should be accurately calculated based on the turbidity, pH value, and water temperature of the pool water. Excessive dosage may cause the water to turn white or the pool wall to become slippery, while insufficient dosage may result in ineffective results.
2. Circulation and mixing: Turn on the circulation water pump and use the water flow power to fully and quickly mix the polyaluminium chloride agent with the pool water, ensuring that it can contact the suspended particles in every corner.
3. Static settlement: Close the circulation system to keep the pool water still. In the following hours (usually 6 to 8 hours, recommended at night), polyaluminium chloride begins to take effect, and flocs gradually form and slowly sink. At this point, the bottom of the pool will be covered with a thin layer of alum.
4. Sewage suction and discharge: The next morning, before opening the swimming pool, the administrator uses manual sewage suction equipment or fully automatic sewage suction robots to thoroughly remove the alum settled at the bottom of the pool and discharge it into the sewage system. Remember not to use a circulating filtration system for backwashing to remove these alum blooms, as they can quickly clog or even damage expensive sand filter media.

After this series of operations, the swimming pool water will return to a pleasing and clear state.

3. Advantages and precautions of using polyaluminium chloride

1. Advantages:

-Efficient flocculation: Compared to traditional aluminum salts such as aluminum sulfate, polyaluminium chloride has the characteristics of fast floc formation, large particles, high density, and fast settling speed, resulting in higher water purification efficiency.
-Wide applicable pH range: It can maintain good performance in a wide pH range (especially near neutral) and has stronger adaptability.

-Low residual aluminum in effluent: In modern water treatment, there are strict standards for residual aluminum content. High quality polyaluminium chloride products can effectively reduce the residual aluminum concentration in effluent, which is more in line with health requirements.

2. Precautions:

-Strictly follow the dosage: the dosage must be precisely controlled to avoid excess.
-PH control: When using, ensure that the pH value of the pool water is maintained within the ideal range (usually 7.2-7.8), as excessive acidity or alkalinity can affect its flocculation effect.

-Cannot replace disinfectants: It must be clarified that polyaluminium chloride is a flocculant and does not have the function of killing bacteria and viruses. It must be used in conjunction with mainstream disinfectants such as chlorine and bromine to ensure the hygiene and clarity of water quality.

List of the role of polyaluminium chloride in swimming pool water treatment

Role Dimension	Specific Roles and Functions	Mechanism of Action	Final Outcomes
Core Function	High-efficiency flocculant	The positively charged complex ions generated from its hydrolysis neutralize the negative charges of suspended particles and form "flocs" through adsorption and bridging.	Aggregates tiny, suspended turbid substances in the water into larger particles that settle, significantly improving water clarity and light transmittance.
Target Objects	Suspended particulate matter (such as dust, grease, skin flakes, organic debris)	Specifically targets colloidal and suspended pollutants, destabilizing them through physicochemical actions.	Removes the root causes of turbidity, solving issues of whitish, grayish, and non-transparent water.
Position in the Process	Physical purification enhancer, auxiliary to the filtration system	Prior to filtration, it converts micron-sized particles that are difficult to filter into millimeter-sized flocs that are easy to settle.	Reduces the burden on the filtration system, enhancing the efficiency and effectiveness of the entire circulation and filtration system.
Relationship with Disinfectants	Synergistic and complementary functions	After removing suspended solids, the water becomes clearer, reducing the chances of disinfectants being consumed by organic impurities and allowing deeper penetration of ultraviolet light (if applicable).	Indirectly improves disinfection efficiency, helping to maintain a more stable and effective residual level of disinfectants.
Operational Key Point	Subsequent pool bottom sludge suction is required	The formed flocs settle on the pool bottom and need to be physically removed through manual sludge suction or automatic robots.	Completes the final step of water quality purification, achieving thorough removal of impurities from the pool.
Operational Key Point	1. Excellent flocculation effect and fast speed 2. Forms large and solid flocs that settle quickly 3. Has a wider applicable pH range compared to traditional aluminum salts	Due to its pre-polymerized form, it exhibits superior charge neutralization and bridging capabilities in water.

In summary, polyaluminium chloride, through its excellent flocculation ability, transforms invisible suspended threats in water into easily removable sediment, making it the hero behind creating a pool of rippling and clear water.