Written by the HyChron Technical Team — water treatment specialists with over 15 years of field experience in municipal and industrial systems. Last reviewed: April 2026
COD (Chemical Oxygen Demand) is one of the most widely regulated parameters in industrial wastewater discharge permits — and one of the most misunderstood in terms of what coagulation can and cannot achieve.
PAC is effective at removing the colloidal and suspended fraction of COD, which in many industrial wastewater streams represents 40–70% of total COD. However, PAC is not effective at removing dissolved, truly soluble COD — that fraction requires biological treatment, advanced oxidation, or adsorption.
Understanding this distinction is the starting point for using PAC correctly in COD reduction programs — and for knowing when PAC alone is sufficient and when it needs to be combined with other treatment steps.
Understanding COD Fractions
Total COD in wastewater consists of three fractions with very different removal pathways:
Suspended COD: Organic matter associated with suspended particles (fibres, food particles, biological solids). Removed highly effectively by PAC coagulation — typically 80–95% removal of this fraction.
Colloidal COD: Organic matter in colloidal form — too small to settle without coagulation but carrying surface charges that PAC can neutralize. Removed effectively by PAC — typically 60–80% removal of this fraction.
Dissolved COD: Truly dissolved organic molecules (sugars, alcohols, short-chain organics, most pharmaceuticals). PAC has minimal direct effect on this fraction — it requires biological degradation, ozone, Fenton, or activated carbon for removal.
The ratio of these fractions in your effluent determines how much total COD reduction PAC can achieve. Industrial effluents with high suspended and colloidal COD fractions respond better to PAC treatment than effluents dominated by dissolved COD.
PAC vs Other COD Reduction Methods
| Method | COD Fraction Removed | Typical Removal | Limitations |
|---|---|---|---|
| PAC coagulation | Suspended + colloidal | 30–65% of total COD | No effect on dissolved COD |
| Biological (activated sludge) | Dissolved + colloidal | 70–95% of biodegradable COD | Cannot treat non-biodegradable COD |
| Advanced oxidation (Fenton/ozone) | All fractions | 70–99% | High cost, energy-intensive |
| Activated carbon | Dissolved + colloidal | 60–90% | High cost, regeneration required |
| PAC + biological (combined) | All fractions | 80–95% total COD | Requires both treatment stages |
The most cost-effective COD reduction strategy for most industrial applications is: PAC for colloidal/suspended fraction → biological treatment for soluble biodegradable COD.
PAC pre-treatment protects the biological stage from toxic compounds, improves its stability, and reduces the organic loading entering it — improving overall treatment system performance and cost.
Typical COD Reduction Performance by Industry
| Industry | Typical Influent COD | PAC Reduction | Remaining for Biological Treatment |
|---|---|---|---|
| Textile / dyeing | 800–2,000 mg/L | 35–55% | 45–65% |
| Food processing | 500–3,000 mg/L | 40–60% | 40–60% |
| Paper and pulp | 500–2,000 mg/L | 35–55% | 45–65% |
| Petrochemical | 300–1,500 mg/L | 30–50% | 50–70% |
| Municipal sewage | 200–500 mg/L | 40–65% (CEPT) | 35–60% |
Ranges reflect variation in effluent composition and PAC dosage optimization. Jar testing with your specific effluent is required.
Criteria for Selecting PAC as the Primary COD Reduction Step
When PAC is the right first step:
- Effluent contains significant suspended or colloidal COD (food particles, fibres, emulsified organics)
- Biological treatment follows PAC as a second stage — PAC reduces the load entering biological
- COD discharge target is moderate (> 100 mg/L total COD) — achievable with combined PAC + biological
- Effluent contains toxic or inhibitory compounds that would harm biological cultures without pre-treatment
When PAC alone is insufficient:
- Discharge permit requires total COD below 50–80 mg/L — biological or advanced treatment is needed in addition
- Effluent is dominated by dissolved biodegradable COD — biological treatment is more cost-effective than PAC for this fraction
- COD is from non-biodegradable dissolved compounds — requires advanced oxidation or activated carbon
Optimizing PAC for COD Reduction
Dosage Guidelines
| Application | Typical PAC Dose for COD Reduction |
|---|---|
| Municipal sewage (CEPT) | 20–60 mg/L |
| Food processing wastewater | 20–80 mg/L |
| Textile / dyeing effluent | 80–200 mg/L |
| Paper mill effluent | 30–80 mg/L |
| Petrochemical wastewater | 40–100 mg/L |
Higher doses increase colloidal COD removal but do not increase dissolved COD removal. Jar testing confirms the optimal dose for your COD reduction target.
pH Optimization
For COD reduction, the same pH guidelines apply as for turbidity removal — PAC is most effective in the pH 6.0–8.0 range. For effluent with high dissolved organic content (high NOM), slightly acidic pH (5.5–6.5) improves coagulation of humic and fulvic acid fractions, increasing COD removal beyond what neutral pH achieves.
Combining PAC with PAM
Adding anionic PAM after PAC flash mixing improves floc density and settling, which increases capture of the colloidal COD fraction — improving total COD removal by 5–15% compared to PAC alone. For high-COD applications where every percentage point matters, PAM addition is worth evaluating.
For PAC + PAM combination: Using PAC with PAM: Best Practices
Frequently Asked Questions
How do I determine what fraction of my effluent COD is colloidal vs dissolved?
Filter a representative sample through a 0.45 micron membrane filter. Measure COD of the filtered sample (dissolved COD) and the unfiltered sample (total COD). The difference is colloidal + suspended COD — the fraction that PAC can remove. This simple measurement tells you immediately what reduction is theoretically achievable with PAC.
My discharge permit requires COD below 80 mg/L — can PAC achieve this alone?
Possibly, but unlikely as a standalone treatment unless your effluent has very high suspended/colloidal fractions and low dissolved COD. For most industrial effluents with total COD above 300–400 mg/L, achieving 80 mg/L discharge limit requires biological treatment in addition to PAC pre-treatment. PAC reduces the load entering biological treatment, making it more stable and efficient.
Does PAC reduce BOD as well as COD?
Yes — PAC removes the suspended and colloidal organic matter that contributes to both BOD and COD. The BOD/COD ratio of the effluent may change after PAC treatment, as PAC preferentially removes the colloidal organic fraction, which may have different biodegradability characteristics than the dissolved fraction. Monitor both parameters to understand the full impact of PAC treatment on your effluent profile.
Conclusion
PAC is an effective and cost-efficient treatment for the suspended and colloidal COD fractions that make up 30–65% of total COD in most industrial wastewater. Used correctly — at optimized pH and dose confirmed by jar testing — it delivers meaningful COD reduction at lower cost than biological treatment alone for the colloidal fraction, while also protecting biological systems from toxic loads.
Understanding which fraction of your COD responds to PAC coagulation — by measuring dissolved versus total COD before and after treatment — is the most important diagnostic step for designing an effective, cost-efficient combined treatment program.
Contact our technical team today for a free COD reduction assessment, PAC product samples, and a treatment program recommendation for your specific effluent. We respond within 24 hours.
