Trickling Filter

Attached growth biological reactor where wastewater is distributed over a bed of media supporting a microbial biofilm for BOD oxidation and nitrification.

Overview

The trickling filter is one of the oldest and most reliable biological treatment processes. Wastewater trickles over media colonized by microorganisms that consume organic matter and nutrients.

When to Use

• Small to medium plants (<20,000 PE)
• Roughing treatment for high-strength waste
• Nitrifying filters (low BOD applications)
• Energy-efficient treatment (low energy)
• Remote/rural applications

Key Characteristics

• Low energy consumption
• Simple operation
• Robust to shock loads
• Produces humus sludge
• Natural ventilation possible

Biofilm Theory

Eckenfelder Model

$$\frac{S_e}{S_0} = \exp\left(-\frac{K \times D}{(Q/A)^n}\right)$$

Where:

• S_e, S_0 = effluent/influent substrate (mg/L)
• K = treatability constant (m/d)^n
• D = media depth (m)
• Q/A = hydraulic loading rate (m³/m²/d)
• n = exponent (typically 0.5)

NRC Formula (Single Stage)

$$E = \frac{100}{1 + 0.44 \times \sqrt{W/(V \times F)}}$$

Where:

• E = BOD removal efficiency (%)
• W = BOD load (kg/d)
• V = media volume (m³)
• F = recirculation factor

Recirculation Factor

$$F = \frac{(1 + R)}{(1 + 0.1 \times R)^2}$$

Where R = recirculation ratio

R	F
0	1.0
0.5	1.3
1.0	1.6
2.0	2.0
4.0	2.4

Two-Stage Formula

$$E_{total} = E_1 + E_2 \times (1 - E_1)$$

Second stage (reduced loading):

$$E_2 = \frac{100}{1 + 0.44/\sqrt{1-E_1} \times \sqrt{W_2/(V_2 \times F_2)}}$$

Design Equations

Organic Loading Rate (OLR)

$$OLR = \frac{Q \times BOD}{V}$$

Units: kg BOD/m³/d

Hydraulic Loading Rate (HLR)

$$HLR = \frac{Q \times (1+R)}{A}$$

Units: m³/m²/d

Wetting Rate

$$WR = \frac{HLR}{24}$$

Units: m³/m²/h

Minimum: 0.5 m³/m²/h for biofilm maintenance

Specific Surface Area

Rock media: 40-80 m²/m³ Plastic media: 100-250 m²/m³

Design Criteria

Low-Rate (Standard)

Parameter	Value
OLR	0.08-0.4 kg BOD/m³/d
HLR	1-4 m³/m²/d
Depth	1.8-2.5 m
Media	Rock
Recirculation	0
BOD removal	80-90%
Nitrification	Yes (if BOD low)
Humus sludge	0.5 kg/kg BOD

High-Rate

Parameter	Value
OLR	0.4-2.4 kg BOD/m³/d
HLR	10-40 m³/m²/d
Depth	1-2 m
Media	Plastic
Recirculation	1-4
BOD removal	60-80%
Nitrification	Limited
Humus sludge	0.3 kg/kg BOD

Roughing Filters

Parameter	Value
OLR	2.4-8.0 kg BOD/m³/d
HLR	40-100 m³/m²/d
BOD removal	40-65%
Application	Pre-treatment

Nitrifying Filters

Parameter	Value
OLR	<0.1 kg BOD/m³/d
NH₄-N loading	0.1-0.5 g N/m²/d
BOD:TKN ratio	<1
DO	>2 mg/L

Nitrification Rate

$$r_N = k_N \times \frac{NH_4}{K_N + NH_4} \times \frac{DO}{K_O + DO} \times \frac{A}{V}$$

Limited by:

• Temperature: θ = 1.10
• BOD competition: inhibition at OLR > 0.8 kg/m³/d
• pH: optimal 7.5-8.5

Parameters

Media Parameters

Parameter	Unit	Rock	Plastic
Size	mm	50-100	modules
Specific area	m²/m³	40-70	100-250
Void ratio	%	40-50	90-97
Density	kg/m³	1500	30-100
Depth	m	1.8-2.5	4-12

Design Parameters

Parameter	Unit	Default	Range	Description
Diameter	m	20	5-50	Filter diameter
Depth	m	6	2-12	Media depth
HLR	m³/m²/d	20	1-100	Including recirc
OLR	kg/m³/d	1.0	0.1-8	Organic loading
Recirculation	-	1.0	0-4	Ratio
Ventilation	-	natural	natural/forced	Air supply

Biofilm Parameters

Parameter	Unit	Default	Description
Biofilm thickness	mm	1-3	Active zone
Sloughing rate	g TSS/m²/d	5-20	Varies with OLR
Growth yield	g VS/g BOD	0.4-0.6	Biomass production

Media Types

Rock/Stone

• Granite, limestone, slag
• Durable, inexpensive
• Heavy, limits depth
• Lower specific area

Random Plastic

• PP, HDPE rings/balls
• Lightweight
• High void ratio
• Self-supporting

Structured/Modular

• Cross-flow or vertical
• Maximum air flow
• Consistent distribution
• Easy installation

Ventilation

Natural Draft

Air flow driven by temperature difference:

$$\Delta P = 0.0342 \times D \times \left(\frac{1}{T_{air}} - \frac{1}{T_{water}}\right)$$

Requirements:

• Underdrains with air slots
• Ventilation ports at base
• Open top or louvers
• ΔT > 5°C for reliable flow

Forced Ventilation

Required when:

• High OLR (>1.5 kg/m³/d)
• Deep filters (>6m)
• Inadequate natural draft

Air flow: 0.1-0.3 m³/m³/min

Humus Tank

Primary settler for biofilm sloughing:

• Surface area loading: 20-30 m³/m²/d
• Retention time: 1.5-2.5 hours
• Sludge concentration: 2-4% DS

Configuration

Example Configuration

{
  "type": "tricklingFilter",
  "id": "filter-1",
  "parameters": {
    "diameter": 25,
    "depth": 6,
    "mediaType": "plastic-structured",
    "specificArea": 150,
    "voidRatio": 0.94,
    "recirculationRatio": 1.5,
    "distributorType": "rotary",
    "ventilation": "natural",
    "humusTank": {
      "surfaceArea": 200,
      "depth": 3,
      "sludgeRecycle": false
    }
  }
}

Worked Example

Given

• Flow: 5,000 m³/d
• Influent BOD: 200 mg/L
• Target: 25 mg/L (87.5% removal)
• Temperature: 15°C

Design (Two-Stage)

Step 1: First Stage (Roughing) Target E₁ = 70%

Using NRC with R=1, F=1.6: $$0.70 = \frac{100}{1 + 0.44\sqrt{W/(V \times 1.6)}}$$

Solving: V₁ = 385 m³

Step 2: Second Stage (Polishing) Remaining: E₂ = (87.5-70)/(100-70) = 58%

At reduced load: V₂ = 290 m³

Step 3: Dimensions Stage 1: D=10m, H=5m → V=393 m³ ✓ Stage 2: D=8m, H=6m → V=302 m³ ✓

Step 4: Loading Checks Stage 1: OLR = 5000×0.2/393 = 2.5 kg/m³/d (high-rate) ✓ Stage 2: OLR = 5000×0.06/302 = 1.0 kg/m³/d ✓

Troubleshooting

Issue	Cause	Solution
Ponding	Clogged media	Increase flushing
Filter flies	Low wetting	Increase HLR
Odors	Anaerobic zones	Improve ventilation
Poor nitrification	High BOD	Reduce loading
Icing	Cold climate	Cover, recirculate

Related Topics

• MBBR - Moving Bed Biofilm
• BAF - Biologically Active Filter
• Humus-Tank
• Two-Stage-Filters

References

1. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
2. WEF (2000). Aerobic Fixed-Growth Reactors. MOP FD-13.
3. Eckenfelder, W.W. (2000). Industrial Water Pollution Control, 3rd Ed.
4. NRC (1946). Sewage Treatment at Military Installations.