Case Study: PAC Water Treatment System for a 5,000 m³/day Plant — Turbidity Reduced by 98%
Case Study: PAC Water Treatment Plant with a Capacity of 5,000 m³/Day — 98% Reduction in Turbidity
1. Background and Real-World Challenge
A water treatment plant in Central Vietnam, with a design capacity of 5,000 m³/day, draws its source water from a local river. During the dry season, the quality of the source water is relatively stable, with turbidity ranging from 30–60 NTU. However, whenever the rainy season begins (September–November), turbidity spikes to 200–400 NTU, with peak levels reaching 520 NTU at times. Total suspended solids (TSS) levels also increase 4–6-fold, leading to a marked deterioration in COD levels and water color.
Previously, the plant used aluminum sulfate (Al₂(SO₄)₃·18H₂O) as the primary coagulant. During the flood season, the plant was forced to increase the alum dosage to 60–80 mg/L to meet post-sedimentation water quality standards. As a result:
- Chemical costs skyrocketed by 40–50% during the three-month flood season.
- Sludge volume doubled, overloading the sludge treatment system.
- The pH of the post-flocculation water dropped sharply below 5.5, requiring the addition of lime (CaO) for adjustment—incurring a significant additional cost.
- The water after sedimentation is unstable, with some days still showing 10–15 NTU, exceeding the permissible threshold before entering the filter tank (recommended standard ≤ 5 NTU).
2. Solution: Switch to 10% Liquid PAC Combined with Anionic Polymer
After a two-week survey and jar-test evaluation, the operating unit decided to switch the flocculation technology as follows:
| Item | Before | After Improvement |
|---|---|---|
| Primary Flocculant | Powdered aluminum sulfate | 10% liquid PAC (Al₂O₃) |
| Coagulation Aid | None used | Anionic Polymer (0.05–0.1 mg/L) |
| Dry Season Dosage | 30–40 mg/L | 15–20 mg/L PAC |
| Dosage during the flood season | 60–80 mg/L | 25–35 mg/L PAC |
| pH adjustment | Lime (CaO) 10–15 mg/L | Not required (stable pH 6.5–7.0) |
Why 10% liquid PAC?
PAC (Polyaluminum Chloride) has an Al₂O₃ content of ≈ 10% and a basicity of 45–60%, providing faster flocculation and forming larger, more robust flocs than traditional aluminum sulfate. The liquid form allows for precise dosing via a metering pump, eliminating the need to dissolve it like powdered alum—saving on labor and mixing equipment. Notably, PAC causes minimal pH reduction in the water, reducing or completely eliminating the need for lime.
Role of Anionic Polymer:
Anionic polymer is dosed in very small amounts (0.05–0.1 mg/L) immediately after PAC, acting as a “bridge” to help PAC flocs aggregate into larger flocs that settle more quickly. Jar-test results show that the settling rate increases by 30–40% when Anionic Polymer is used compared to PAC alone.
3. Results After 3 Months of Operation (flood season)
After the conversion and three months of stable operation during the flood season, the following results were obtained:
| Parameter | Before (Aluminum Sulfate) | After (PAC + Polymer) | Improvement |
|---|---|---|---|
| Raw water (turbidity) | 200–400 NTU | 200–400 NTU | — |
| Post-sedimentation water (turbidity) | 8–15 NTU | 2–5 NTU | ↓ 60–70% |
| Turbidity removal efficiency | ~90–95% | 97–98% | ↑ 3–8% |
| Sludge production (kg/m³) | 0.35–0.50 | 0.20–0.30 | ↓ 35–40% |
| pH after flocculation | 5.0–5.5 (requires adjustment) | 6.5–7.0 (no adjustment required) | Stable |
| Chemical cost (VND/m³) | 1,200–1,600 | 900–1,100 | ↓ ~25% |
| Settling time (minutes) | 30–40 | 20–25 | ↓ 30% |
The biggest highlight: turbidity after settling remains stable at 2–5 NTU even when the source water reaches a peak of 520 NTU — a 98% reduction compared to the influent. This allows the sand filter to operate more efficiently, extending the backwash cycle from 24 hours to 36–48 hours, thereby saving backwash water and reducing labor costs.
4. Cost Analysis — How Does It Save 25%?
Calculation for 5,000 m³/day during the flood season (90 days):
| Item | Aluminum Sulfate | PAC + Polymer | Savings |
|---|---|---|---|
| Primary Flocculant (VND/month) | 180–240 million | 135–165 million | 25–31% |
| Lime for pH adjustment (VND/month) | 9–13.5 million | 0 | 100% |
| Sludge treatment (VND/month) | 22–30 million | 12–18 million | 40–45% |
| Labor + operations | Basic | 10–15% reduction | — |
| Total Cost/Month | ~211–283 million | ~147–183 million | ↓ ~30–35% |
Total estimated savings during the 3-month flood season: 190–300 million VND. With an initial conversion investment (PAC dosing pump, storage tank, piping) of only about 80–120 million, the payback period is less than 2 months.
5. Lessons Learned
- Liquid PAC is not just an “alternative flocculant”— it is a comprehensive, optimal solution for water quality, operations, and costs.
- Anionic polymer is a low-cost “catalyst” that delivers superior results when combined with PAC, especially in highly turbid water conditions.
- Jar testing is an essential step — each water source has unique characteristics, so testing is necessary to optimize dosages for each season.
- Calculating TCO (Total Cost of Ownership) rather than just looking at the unit price per cubic meter helps water treatment plants make the right decisions.
📞 Loc Thien Chemicals is a supplier of 10% liquid PAC, Anionic/Cationic Polymers, and specialized water treatment chemicals for drinking water plants and industrial wastewater treatment facilities nationwide. We offer free jar tests, consult on optimal solutions, and provide door-to-door delivery.
☎️ Hotline: 0979 891 929