Appearance
Chemically Enhanced Primary Treatment (CEPT)
Primary clarification enhanced with coagulants and flocculants
Overview
CEPT adds chemicals (alum, ferric chloride, polymers) to primary clarifiers, improving TSS removal from 60% to 80-90% and BOD removal from 30% to 50-70%. Reduces downstream biological loading, captures phosphorus (70-90%), and produces more sludge. Cost-effective for plant upgrades and peak flow treatment.
Theory - Basic Mode
$$\text{TSS}{out} = \text{TSS} \times (1 - \eta_{TSS,enhanced})$$ $$\text{TP}{out} = \text{TP} \times (1 - \eta_{TP,enhanced})$$
Enhanced removal:
- TSS: 80-90% (vs. 60% conventional)
- BOD: 50-70% (vs. 30% conventional)
- TP: 70-90% (vs. 10% conventional)
Theory - Advanced Mode
Chemical Dose:
$$\text{Dose}{metal} = \text{TP} \times \text{molar ratio} \times \frac{MW{metal}}{MW_P}$$
For TP removal: Molar ratio 1.5-2.5 (Metal:P)
Sludge Production:
$$\text{Sludge}{CEPT} = \text{Sludge} + \text{Chemical precipitate}$$
Chemical precipitate ≈ 3-4 × chemical dose (includes hydroxide formation)
Parameters
| Parameter | Conventional | CEPT | Units |
|---|---|---|---|
| TSS Removal | 55-65 | 80-90 | % |
| BOD Removal | 25-35 | 50-70 | % |
| TP Removal | 5-15 | 70-95 | % |
| Alum Dose | - | 50-150 | mg/L as Al₂(SO₄)₃ |
| FeCl₃ Dose | - | 30-100 | mg/L |
| Polymer Dose | - | 0.5-3.0 | mg/L |
| Sludge Production | 1.0× | 1.5-2.5× | relative |
Worked Example
Given: CEPT for TP removal
- Flow: 20,000 m³/d
- Influent TP: 8 mg/L
- Target TP: 1 mg/L
- Use alum (Al₂(SO₄)₃·14H₂O)
Calculate: Alum dose
- TP removal: 8 - 1 = 7 mg/L
- Molar ratio Al:P = 2:1
- Alum dose = 7 × 2 × (27/31) × (594/54) = 134 mg/L (as alum)
- Daily usage: 20,000 × 0.134 = 2,680 kg/d
References
- Metcalf & Eddy (2014)
- WEF (2018). Phosphorus Removal, MOP 29