Poly Aluminum Chloride (PAC) is a chemical compound that is commonly used in water treatment for the purification of drinking water and wastewater. It is a coagulant, which means it helps to remove suspended particles and impurities from water by promoting the aggregation and settling of these particles.

When PAC is added to wastewater, several chemical reactions and processes take place.

Coagulation:

PAC contains positively charged aluminum ions. In wastewater, there are often negatively charged particles such as organic matter, colloids, and other contaminants. The positively charged aluminum ions neutralize the negative charges on these particles, causing them to come together or coagulate.

Formation of Precipitates:

As the aluminum ions react with the negatively charged particles in the wastewater, insoluble aluminum hydroxide (Al(OH)₃) precipitates form. These precipitates entrap impurities, making them larger and heavier.

Flocculation:

The process of flocculation follows coagulation, where the destabilized particles start to come together to form larger aggregates called flocs. This is usually achieved by gentle stirring or mixing. The flocs are easier to settle or filter out of the water.

Settling:

Due to their increased size and weight, the flocs settle to the bottom of the water, forming a sludge layer. This sludge can be easily separated from the treated water.

Filtration:

The settled flocs can be further removed from the water by filtration. This step helps to ensure the removal of fine particles that may still be present.

Neutralization and pH Adjustment:

The addition of PAC can have an impact on the pH of the water. Aluminum hydroxide formation consumes hydroxide ions, potentially leading to a decrease in ph. pH adjustment may be necessary to ensure that the treated water meets regulatory standards.

The overall result of treating wastewater with PAC is the removal of suspended solids, colloids, and other impurities, leading to clearer and cleaner water. The effectiveness of PAC in wastewater treatment depends on various factors, including the characteristics of the wastewater, the dosage of PAC, and the specific contaminants present in the water. It's important to carefully control the dosage and conditions to optimize the treatment process.