In aquaculture wastewater treatment, polyaluminum chloride (PAC) is an inorganic polymer coagulant that primarily removes suspended solids, colloids, some soluble pollutants, and nutrients from wastewater through coagulation and sedimentation. It is a key agent in pretreatment and advanced treatment stages. Its specific role can be analyzed from the following perspectives:
1. Core Function: Coagulation and sedimentation to remove suspended solids (SS) and colloids:
Aquaculture wastewater contains large amounts of suspended solids (SS) and colloids, such as undigested feed residues, animal feces, hair, and dander, as well as colloidal substances produced by microbial metabolism (e.g., bacteria and algae). These particles are small (typically <1μm) with negative surface charges, which repel each other and hinder natural sedimentation.
The mechanism of action of PAC is:
Electroneutralization: PAC hydrolyzes to form polynuclear hydroxyaluminum ions (e.g., [Al₁₃O₄(OH)₂₄]⁷⁺). These highly charged positive ions neutralize the negative surface charge of suspended solids/colloids, reducing the zeta potential and destabilizing the particles. Adsorption Bridging: Destabilized particles adhere to each other through the long chains of PAC, forming larger flocs (alum flocs).
Netting and Sweeping: The Al(OH)₃ gel generated by PAC hydrolysis captures fine particles and accelerates sedimentation.
Finally, suspended solids and colloids are removed through sedimentation, significantly reducing wastewater turbidity and suspended solids concentration (SS).
2. Assisted Removal of Dissolved Organic Matter (COD):
Aquaculture wastewater contains large amounts of dissolved organic matter (such as protein, fat, and urea), which is a major source of COD (chemical oxygen demand). The coagulation effect of PAC can partially remove these pollutants:
Adsorption: The surface of the hydroxyaluminum polymer of PAC has numerous active groups that can adsorb dissolved organic matter (especially large organic molecules) and remove them through floc co-precipitation.
Complexation Sedimentation: Some organic matter (such as compounds containing -NH₂ and -OH) can form complexes with Al⁺ and settle with the flocs.
However, it should be noted that PAC has limited removal effectiveness for small organic molecules (such as volatile fatty acids), and further COD degradation is usually required in conjunction with biochemical treatment (such as the A/O process).
3. Phosphorus Removal:
The main sources of phosphorus in aquaculture wastewater are phosphorus added to feed (unabsorbed by the animal) and organic phosphorus in feces. PAC can remove phosphorus through chemical precipitation:
The Al⁺ in PAC reacts with phosphate (PO₄³⁻) in wastewater to form an insoluble AlPO₄ precipitate:
Al₃++PO₄⁻→AlPO₄↓
At the same time, the Al(OH)₃ flocs formed by PAC hydrolysis can adsorb colloidal phosphorus (such as organic phosphorus decomposition products), further reducing total phosphorus (TP) concentrations.
Compared to traditional phosphorus removal agents (such as aluminum sulfate), PAC has a higher aluminum content, better phosphorus removal efficiency, and is applicable over a wide pH range (5-9).
4. Adsorption of Heavy Metals and Pathogens:
Aquaculture wastewater may contain small amounts of heavy metals (such as Cu and Zn, from feed additives) and pathogens (such as E. coli and parasite eggs). The porous structure and high specific surface area of PAC flocs enable the removal of these pollutants through physical adsorption or co-precipitation:
Heavy metal ions can compete with Al⁺ for adsorption sites or be encapsulated and precipitated by the Al(OH)₃ flocs. Pathogens (such as bacteria and viruses) are captured by the flocs and removed with sedimentation, reducing the biological risk of wastewater.
5. Advantages: Strong adaptability and manageable costs:
PAC forms flocs quickly (typically 5-15 minutes) and has excellent settling performance, shortening treatment time.
Wide adaptability: It effectively treats aquaculture wastewater with high SS (1000-5000 mg/L), high COD (2000-10000 mg/L), and a wide pH range (5-9).
Manageable costs: Compared to organic coagulants (such as polyacrylamide), PAC poses no secondary pollution risk and requires a lower dosage (typically 30-100 mg/L), resulting in a lower overall cost. In aquaculture wastewater treatment, PAC mainly removes suspended solids, colloids, some organic matter and phosphorus through coagulation and sedimentation, while assisting in the adsorption of heavy metals and pathogens. It is a key agent in the pretreatment or deep treatment stage, but its removal of small molecular organic matter is limited. It usually needs to be used in combination with biochemical processes (such as anaerobic-aerobic) to achieve standard discharge or reuse.
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