Case Study: Polymer Cation for Aquaculture Wastewater Treatment — 30% Reduction in Sludge Disposal Costs

Date: · Author: Hóa chất Lộc Thiên

1. Introduction

The seafood processing industry is one of the sectors with high wastewater flow rates and complex pollution characteristics. Wastewater from the raw material washing, preliminary processing, and fish/shrimp processing stages contains high levels of organic matter (BOD, COD), oil and grease, dissolved proteins, and, in particular, very high levels of total suspended solids (TSS)—which can reach 1,000–2,500 mg/L.

For medium- and large-scale seafood processing plants in the Mekong Delta region, the cost of treating sludge has always been a significant burden. Pressed sludge typically has a high moisture content (82–87%), resulting in large volumes of sludge generated and significantly increased transportation and treatment costs.

This article presents a real-world case study: the application of cationic polymers combined with PAC in the seafood wastewater treatment process, which helps reduce the moisture content of the sludge cake and significantly cuts transportation and treatment costs.

2. Plant Background

2.1. Scale and Characteristics

• Industry: Frozen seafood processing (catfish, shrimp)
• Wastewater treatment capacity: ~800 m³/day
• Region: Mekong Delta
• Existing wastewater treatment system: Collection tank → Grease separator → Equalization tank → Coagulation-flocculation tank → Settling tank → Aerotank → Secondary settling tank → Disinfection → Discharge
• Sludge dewatering equipment: Belt press (belt press)

2.2. Characteristics of Inflow Wastewater

With high pollution loads, the amount of sludge generated from the coagulation tank and the sedimentation tank ranges from 2.5 – 3.5 metric tons of wet sludge per day.

3. Issues Before Using Polymer

3.1. Old Process: Use of PAC Only

Before the upgrade, the plant used only PAC (Polyaluminum Chloride) as the sole coagulant. The PAC dosage was high—250–350 mg/L—to achieve the desired TSS removal efficiency.

After the flocculation and floc formation process, the sludge was fed into a belt press. However, because only PAC was used, the resulting flocs were small, weak, and difficult to dewater during the pressing process.

3.2. Consequences

Total monthly sludge costs (30 days):

• Transportation: 1,950,000 × 30 = 58,500,000 VND/month
• On-site treatment: 3.25 metric tons × 800,000 × 30 = 78,000,000 VND/month
• Total: ~136,500,000 VND/month

4. Solution: PAC + Cationic Polymer

4.1. Polymer Selection

After surveying and analyzing the wastewater characteristics, the technical team proposed adding Cationic Polymer (C-PAM) to the flocculation and sludge dewatering stages. Cationic Polymer was selected because:

• Aquaculture sludge is rich in protein and negatively charged organic matter → Cationic Polymer, which is positively charged, effectively neutralizes these charges
• Forms polymer bridges → larger, firmer flocs that are easier to dewater
• It is highly compatible with PAC within the same process

Polymer Specifications:

4.2. New Process

Wastewater → Coagulation tank (PAC: 200 mg/L) → Flocculation tank (Cationic polymer: 1.5–2.0 mg/L)
→ Settling tank → Settled sludge → Sludge thickening tank (Additional cationic polymer: 2–3 mg/L)
→ Belt press → Drier sludge cake

Key Improvements:

1. 30% reduction in PAC dosage: From 300 mg/L to 200 mg/L. Cationic polymer aids floc formation, so a high PAC dosage is no longer required as before
2. Cationic polymer in the flocculation tank: 1.5–2.0 mg/L — helps form larger flocs that settle faster
3. Additional cationic polymer before sludge dewatering: 2–3 mg/L — improves the sludge’s dewatering capacity during the dewatering process

4.3. Operation and Adjustment

During the first two weeks of implementation, the technical team conducted daily jar tests to optimize dosages. The jar test results showed:

• At a cationic polymer dosage of 1.5 mg/L: flocs formed well, with average size
• At a dosage of 2.0 mg/L: large, firm flocs; the water was clear after 15 minutes of settling
• At a dosage of ≥ 2.5 mg/L: signs of “overdosing” began to appear—flocs became sticky, partially floated, and the water became turbid again

→ Optimal dosage determined: 2.0 mg/L for the flocculation tank, and 2.5 mg/L for the sludge thickening tank prior to pressing.

5. Results After 3 Months of Operation

5.1. Before-and-After Comparison

5.2. Economic Analysis

New chemical costs (monthly):

• PAC: 200 mg/L × 800 m³ × 30 days = 4,800 kg/month × 8,500 VND/kg = 40,800,000 VND
• Cationic Polymer: (2.0 + 2.5) mg/L × 800 m³ × 30 days = 108 kg/month × 85,000 VND/kg = 9,180,000 VND
• Total cost of new chemicals: 49,980,000 VND/month

Cost of old chemicals:

• PAC: 300 mg/L × 800 m³ × 30 days = 7,200 kg/month × 8,500 VND/kg = 61,200,000 VND/month

→ Chemical savings: 61,200,000 − 49,980,000 = 11,220,000 VND/month

New Sludge Costs:

• Transportation: 2.47 metric tons × 600,000 VND/trip × 30 days = 44,460,000 VND/month
• Treatment: 2.47 metric tons × 800,000 VND × 30 days = 59,280,000 VND/month
• Total cost of new sludge: 103,740,000 VND/month

→ Sludge cost savings: 136,500,000 − 103,740,000 = 32,760,000 VND/month (~24%)

Total monthly savings: 11,220,000 + 32,760,000 = 43,980,000 VND/month

Total annual savings: ~527,760,000 VND/year

5.3. Non-Financial Benefits

• Significant reduction in odor: Drier sludge cakes → less anaerobic decomposition during storage and transportation
• Extended service life of sludge dewatering equipment: Sludge dewater more easily → reduced stress on conveyor belts and press shafts
• Improved effluent water quality: TSS reduced by 50% → reduced load on the downstream Aerotank → savings on aeration energy
• More stable operation: Reduced PAC dosage → fewer pH shocks in the coagulation tank

6. Lessons Learned

6.1. The jar test is an essential step

Each plant has different wastewater characteristics depending on the season and the type of processed materials. Jar tests should be conducted regularly (at least once a week) to adjust the polymer dosage to the optimal level, avoiding waste or under-dosing.

6.2. The polymer mixing equipment plays a crucial role

Powdered cationic polymer must be mixed to the correct concentration (0.1%) and allowed sufficient aging time (45–60 minutes) to achieve maximum effectiveness. An automatic dosing system is a worthwhile investment, helping to:

• Ensure precise concentration
• Prevent clumping (fish-eye) that occurs during manual mixing
• Reduce labor costs

6.3. The polymer injection point must be in the correct location

Cationic polymer must be injected into an area with turbulent flow to ensure rapid and uniform dispersion. If injected into a stagnant water zone, the polymer will not disperse evenly, leading to waste and reduced efficiency.

6.4. PAC Should Not Be Completely Replaced

Cationic polymer cannot completely replace PAC in aquaculture wastewater treatment. PAC plays the role of coagulant—neutralizing charges and breaking down colloidal systems. The polymer serves the role of flocculation—linking the colloidal particles that PAC has destabilized into larger flocs. The two chemicals complement each other; they do not replace one another.

7. Conclusion

This case study demonstrates that combining Cationic Polymer with PAC in the aquaculture wastewater treatment process delivers dual benefits: it both improves the quality of the effluent and significantly reduces operating costs. With savings of up to ~44 million VND per month, the payback period for investing in a polymer dosing system is only 2–3 months.

If your plant is facing similar issues—wet sludge, high transportation costs, and unstable effluent quality—the cationic polymer solution could be the key.

📞 Call 0979 891 929 for a consultation on polymer solutions for your plant.

Loc Thien Chemicals — Your trusted partner in industrial wastewater treatment.