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Trickling Filter
Attached-growth biological treatment using fixed media
Overview
Trickling filters use plastic or rock media (1-3m deep) for biofilm growth. Wastewater is distributed over the media surface, and organic matter is removed as it trickles through. BOD removal: 65-90%. Common in small-to-medium WWTPs due to simplicity and low energy.
Theory - Basic Mode
$$\text{BOD}{out} = \text{BOD} \times (1 - \eta)$$
Typical removal: 70-85% (single-stage), 85-95% (two-stage)
Theory - Advanced Mode
NRC (National Research Council) Equation:
$$\eta = \frac{100}{1 + 0.0085 \times \sqrt{\frac{L_v}{F}}}$$
Where:
- L_v = organic loading (kg BOD/m³·d)
- F = recirculation factor = (1 + R) / (1 + R/10)²
- R = recirculation ratio
Eckenfelder Equation:
$$\frac{S_e}{S_0} = \exp\left(-\frac{K \times D}{q^n}\right)$$
Where K = treatability constant, D = depth, q = hydraulic loading, n = 0.5-1.0
Parameters
| Parameter | Low-Rate | High-Rate | Units |
|---|---|---|---|
| Organic Loading | 0.1-0.4 | 0.4-2.4 | kg BOD/m³·d |
| Hydraulic Loading | 1-4 | 10-40 | m³/m²·d |
| Depth | 1.8-3.0 | 0.9-2.5 | m |
| Recirculation | 0 | 1-3 | ratio |
| BOD Removal | 80-90 | 65-85 | % |
Worked Example
Given: High-rate filter, BOD=200 mg/L, Flow=5,000 m³/d Calculate: 70% removal efficiency media volume
Using NRC: Assume R=1, F=1.64, η=70%
- Solve: L_v = 0.75 kg/m³·d
- Volume = (5,000 × 0.20) / 0.75 = 1,333 m³
References
- Metcalf & Eddy (2014)
- WEF MOP 8 (2018)