Biofilters - PS211 Design Standards

Biofilters (trickling filters) are fixed-film biological treatment units where wastewater is distributed over a media bed, allowing microbial biofilms to oxidize organic matter and ammonia. They provide robust, low-energy secondary treatment.

UK PS211 Standards

Design Criteria

Parameter	PS211 Limit	Units	Notes
BOD loading rate	≤ 0.07	kgBOD/m³/d	Volumetric loading on media
Ammonia loading rate	≤ 0.015	kgN/m³/d	Volumetric loading on media
Minimum wetting rate	≥ 0.04	m³/m²/h	Hydraulic loading (surface)
Maximum wetting rate	≤ 0.75	m³/m²/h	Hydraulic loading (surface)
Maximum BOD concentration	≤ 130	mg/L	Influent to biofilter
Maximum NH₃ concentration	≤ 25	mg/L	Influent to biofilter

Media Requirements

Media Type	Specific Surface Area	Void Space	Typical Use
Random plastic	80-120 m²/m³	90-95%	General purpose, low rate
Structured plastic	100-150 m²/m³	95-97%	High rate, nitrification
Rock	45-60 m²/m³	40-50%	Traditional, low rate

Typical selection:

Modern works: Structured plastic media (120-150 m²/m³)
Media depth: 2.0-3.0 m (plastic), 1.5-2.0 m (rock)

Design Equations

Geometry Calculations

Filter Surface Area (Plan View)

Circular filter:

A = π × (D/2)² = π × r²

Where:

A = surface area (m²)
D = diameter (m)
Typical: 10-30 m diameter

Media Volume

Formula:

V_media = A × h_media

Where:

V_media = media volume (m³)
h_media = media depth (m)
Typical depth: 2.0-3.0 m for plastic media

Total Media Surface Area

Formula:

A_media = V_media × SSA

Where:

A_media = total biological surface area (m²)
SSA = specific surface area of media (m²/m³)

Example:

V = 1,000 m³, SSA = 120 m²/m³
A_media = 1,000 × 120 = 120,000 m²

Loading Rate Calculations

BOD Volumetric Loading

Formula:

L_BOD = BOD_load / V_media

Where:

L_BOD = BOD loading rate (kgBOD/m³/d)
BOD_load = applied BOD load (kg/d)
V_media = media volume (m³)
PS211 limit: ≤ 0.07 kgBOD/m³/d

BOD load calculation:

BOD_load = (C_BOD × Q) / 1000

Where:

C_BOD = BOD concentration (mg/L)
Q = flow rate (m³/d)

Loading rate classification:

Low rate: 0.08-0.32 kgBOD/m³/d (traditional, 85-90% removal)
High rate: 0.32-1.0 kgBOD/m³/d (modern, 65-85% removal with recirculation)
PS211 limit: 0.07 kgBOD/m³/d (conservative, ensures nitrification)

Ammonia Volumetric Loading

Formula:

L_NH3 = NH3_load / V_media

Where:

L_NH3 = ammonia loading rate (kgN/m³/d)
NH3_load = applied ammonia load (kg/d)
PS211 limit: ≤ 0.015 kgN/m³/d

Ammonia load calculation:

NH3_load = (C_NH3 × Q) / 1000

Design notes:

Nitrification typically controls design (more restrictive than BOD)
Temperature correction required (see below)

Wetting Rate (Hydraulic Loading)

Formula:

W_rate = Q / A / 24

Where:

W_rate = wetting rate (m³/m²/h)
Q = applied flow rate including recirculation (m³/d)
A = plan surface area (m²)

PS211 range: 0.04-0.75 m³/m²/h

Purpose: Ensures adequate moisture distribution over media to prevent dry patches and maintain biofilm activity.

Recirculation Requirements

If influent flow alone produces wetting rate < 0.04 m³/m²/h, recirculation is required.

Recirculation Flow Required

Formula:

Q_total = W_min × A × 24
Q_recir = Q_total - Q_inf
R = Q_recir / Q_inf

Where:

W_min = minimum wetting rate (0.04 m³/m²/h)
Q_recir = recirculation flow (m³/d)
Q_inf = influent flow (m³/d)
R = recirculation ratio (dimensionless)

Typical recirculation ratios:

Low rate filters: 0-1
High rate filters: 1-4
Nitrifying filters: 2-4 (for dilution and wetting)

Applied BOD with Recirculation

Formula:

BOD_applied = BOD_inf / (1 + R × (1 - E))

Where:

E = fractional BOD removal (e.g., 0.85 for 85%)
Recirculation dilutes applied BOD concentration

Note: Recirculation improves performance by:

Maintaining minimum wetting rate
Diluting high-strength influent
Re-seeding with active biomass
Buffering shock loads

Temperature Correction

Biological activity is temperature-dependent. Use correction factor for non-standard temperatures.

Temperature Correction Factor

Formula:

k_T = k_20 × θ^(T - 20)

Where:

k_T = reaction rate at temperature T (°C)
k_20 = reaction rate at 20°C (baseline)
θ = temperature coefficient (typically 1.035 for biofilters)
T = operating temperature (°C)

Typical UK design temperatures:

Summer: 15-18°C
Winter: 8-12°C
Design basis: 10-12°C (conservative)

Impact on removal efficiency:

E_T = E_15 × θ^(T - 15)

Where:

E_15 = efficiency at 15°C (base case)
At 10°C: ~15% reduction in efficiency
At 20°C: ~18% increase in efficiency

Removal Efficiency

Base Efficiency (15°C, optimal loading)

Parameter	Removal (%)	Notes
BOD	85	At loading ≤ 0.05 kgBOD/m³/d
NH₄-N	85	At loading ≤ 0.015 kgN/m³/d
TP	5	Minimal P removal (biomass uptake only)

Loading Rate Adjustment

High loading reduces efficiency:

If L_BOD > 0.05 kgBOD/m³/d:
  Adjustment = max(0.7, 1 - (L_BOD - 0.05) / 0.1)
  E_BOD = E_base × Adjustment

Example:

At 0.06 kgBOD/m³/d: E = 85% × 0.9 = 76.5%
At 0.07 kgBOD/m³/d: E = 85% × 0.8 = 68%

Effluent Quality

Formula:

C_eff = C_inf × (1 - E/100)
Load_eff = (C_eff × Q) / 1000

Typical effluent from biofilter:

BOD: 15-30 mg/L (after primary treatment influent ~150 mg/L)
NH₄-N: 3-8 mg/L (after primary influent ~35 mg/L)
TSS: 15-30 mg/L (sloughed biomass, requires humus tank)

Biomass Production

Biomass Yield

Formula:

Biomass_production = BOD_removed × Y

Where:

Biomass_production = excess biomass (kgSS/d)
BOD_removed = BOD oxidized (kg/d)
Y = yield coefficient (kgSS/kgBOD)

Typical yield for biofilters:

Y = 0.7 kgSS/kgBOD (higher than activated sludge due to fixed film)

Sloughed biomass: Biomass periodically sloughs from media and is captured in humus tanks as humus sludge.

Worked Example

Design Basis

Population: 5,000 PE
Flow (DWF): 600 m³/d
Primary effluent: BOD 150 mg/L, NH₄-N 30 mg/L
Design temperature: 12°C

Step 1: Calculate Applied Loads

BOD load:

BOD_load = (150 mg/L × 600 m³/d) / 1000 = 90 kgBOD/d

Ammonia load:

NH3_load = (30 mg/L × 600 m³/d) / 1000 = 18 kgN/d

Step 2: Size Media Volume

BOD-based sizing:

V_BOD = BOD_load / L_max
V_BOD = 90 / 0.07 = 1,286 m³

Ammonia-based sizing (controlling):

V_NH3 = NH3_load / L_max
V_NH3 = 18 / 0.015 = 1,200 m³

Select: V = 1,300 m³ (ammonia controls, add 8% margin)

Step 3: Determine Filter Configuration

Try 2 filters, D = 20 m each:

A_per_filter = π × (20/2)² = 314.2 m²
A_total = 314.2 × 2 = 628.3 m²

Media depth required:
h = V / A_total = 1300 / 628.3 = 2.07 m

Select h = 2.5 m (provides extra volume)
V_actual = 628.3 × 2.5 = 1,570.8 m³

Step 4: Check Loading Rates

BOD loading:

L_BOD = 90 / 1570.8 = 0.057 kgBOD/m³/d ✓
(Below 0.07 limit)

Ammonia loading:

L_NH3 = 18 / 1570.8 = 0.011 kgN/m³/d ✓
(Below 0.015 limit)

Step 5: Check Wetting Rate

Without recirculation:

W_rate = 600 / 628.3 / 24 = 0.040 m³/m²/h ✓
(Just meets 0.04 minimum)

Recommendation: Add recirculation for operational flexibility.

With R = 2:

Q_total = 600 + (2 × 600) = 1,800 m³/d
W_rate = 1800 / 628.3 / 24 = 0.119 m³/m²/h ✓
(Within 0.04-0.75 range, good operating point)

Step 6: Calculate Removal Efficiency

Temperature correction (12°C):

θ = 1.035
T = 12°C
Factor = 1.035^(12 - 15) = 1.035^(-3) = 0.90

BOD removal (at 0.057 loading):

E_base = 85% (15°C, good loading)
E_12C = 85 × 0.90 = 76.5%

Ammonia removal:

E_base = 85% (15°C, good loading)
E_12C = 85 × 0.90 = 76.5%

Step 7: Calculate Effluent Quality

BOD:

C_eff = 150 × (1 - 0.765) = 35.3 mg/L
Load_eff = (35.3 × 600) / 1000 = 21.2 kgBOD/d

Ammonia:

C_eff = 30 × (1 - 0.765) = 7.1 mg/L (before humus tank)
Load_eff = (7.1 × 600) / 1000 = 4.3 kgN/d

Note: Humus tank provides additional polishing (15-25% further BOD/NH₃ removal).

Step 8: Calculate Biomass Production

BOD removed:

BOD_removed = 90 - 21.2 = 68.8 kgBOD/d

Biomass production:

Biomass = 68.8 × 0.7 = 48.2 kgSS/d

This biomass sloughs and is captured in humus tanks.

Design Summary

Parameter	Value	Compliance
Configuration	2 × 20 m diameter
Media depth	2.5 m
Media volume	1,571 m³
Media type	Structured plastic, 120 m²/m³
BOD loading	0.057 kgBOD/m³/d	✓ < 0.07
NH₃ loading	0.011 kgN/m³/d	✓ < 0.015
Wetting rate (no recirc)	0.040 m³/m²/h	✓ = 0.04 min
Recirculation ratio	2:1	Recommended
Wetting rate (with recirc)	0.119 m³/m²/h	✓ within range
BOD removal	77% @ 12°C
NH₃ removal	77% @ 12°C
Biomass production	48.2 kgSS/d

UK Water Company Variations

UK Industry Practice

Prefer structured plastic media (120-150 m²/m³)
Minimum 2 filters for N-1 redundancy
Recirculation ratio 2-3:1 standard
Design temperature: 10°C (conservative)

UK Industry Practice

BOD loading: 0.05 kgBOD/m³/d preferred (more conservative)
Media depth: 2.5-3.0 m typical
Forced ventilation common for larger works

UK Industry Practice

NH₃ loading controls in sensitive catchments
Temperature correction mandatory
Covered filters for odor control in urban areas

Distribution Systems

Rotary Distributors

Design parameters:

Rotation speed: 0.5-2 rpm (adjustable)
Dosing interval: 30-120 seconds per point
Arm spacing: 4 or 6 arms typical

Advantages:

Simple, reliable (gravity-driven)
Self-adjusting to flow variations
No external power required

Limitations:

Minimum flow for rotation (~10-15% of design)
Requires elevated inlet (1-2 m head)

Fixed Spray Nozzles

Design parameters:

Nozzle spacing: 1.5-2.5 m grid
Operating pressure: 0.5-2.0 bar
Spray pattern: 90-180° cone

Advantages:

Even distribution at all flows
No moving parts on media
Suitable for rectangular filters

Limitations:

Requires pumping (energy cost)
Nozzle clogging potential (needs screening)

Ventilation

Adequate oxygen supply is critical for aerobic oxidation.

Natural Ventilation

Design:

Open underdrains with cross-flow ventilation
Inlet/outlet ventilation ports in tank walls
Minimum 50% void space in media

Oxygen demand:

O2_demand = 1.5 × BOD_removed + 4.6 × NH3_oxidized

Where:

BOD oxidation: 1.5 kg O₂/kg BOD
Nitrification: 4.6 kg O₂/kg NH₄-N

Example:

BOD_removed = 68.8 kg/d
NH3_oxidized = 13.7 kg/d
O2_demand = 1.5 × 68.8 + 4.6 × 13.7 = 166.2 kg O₂/d

Forced Ventilation

Required when:

Media depth > 3 m
High loading rates (> 0.05 kgBOD/m³/d)
Enclosed/covered filters
Ammonia effluent > 5 mg/L indicates oxygen limitation

Fan sizing:

Air flow: 0.3-1.0 m³ air/m³ media/min
Typical: 0.5 m³/m³/min for nitrifying filters

Common Design Issues

Insufficient Wetting Rate

Problem: Dry patches on media, reduced performance, odors.

Solution:

Increase recirculation ratio (typically 2-3:1)
Check distributor rotation speed
Verify distributor arms not blocked

Ponding

Problem: Flooding of media surface, anaerobic conditions.

Causes:

Excessive loading (> 0.10 kgBOD/m³/d)
Plugged media (biomass or debris)
Inadequate ventilation

Solutions:

Reduce loading rate or add media volume
Increase recirculation to dilute
High-pressure washing to clear media
Install forced ventilation

Poor Nitrification

Problem: High effluent NH₄-N.

Causes:

Ammonia loading > 0.015 kgN/m³/d
Low temperature (< 10°C)
Oxygen limitation
Low pH (< 7.0)

Solutions:

Increase media volume
Add forced ventilation
Check alkalinity (nitrification consumes alkalinity)
Warm weather will improve (seasonal variation expected)

Odor Issues

Problem: H₂S odors from anaerobic zones.

Solutions:

Improve ventilation (natural or forced)
Reduce loading rate
Cover filters with odor treatment (biofilter/scrubber)
Add ferric chloride to primary tank (controls H₂S)

Primary Tanks: Pre-treatment before biofilter
Humus Tanks: Post-biofilter settling
Alkalinity Balance: Nitrification alkalinity consumption

References

UK Sewers for Adoption (7th Edition): Appendix PS211 - Biological Filters
UK operator Design Standards: Biofilter design criteria
WRc/CIRIA: Trickling filter design and operation

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Biofilters - PS211 Design Standards ​

UK PS211 Standards ​

Design Criteria ​

Media Requirements ​

Design Equations ​

Geometry Calculations ​

Filter Surface Area (Plan View) ​

Media Volume ​

Total Media Surface Area ​

Loading Rate Calculations ​

BOD Volumetric Loading ​

Ammonia Volumetric Loading ​

Wetting Rate (Hydraulic Loading) ​

Recirculation Requirements ​

Recirculation Flow Required ​

Applied BOD with Recirculation ​

Temperature Correction ​

Temperature Correction Factor ​

Removal Efficiency ​

Base Efficiency (15°C, optimal loading) ​

Loading Rate Adjustment ​

Effluent Quality ​

Biomass Production ​

Biomass Yield ​

Worked Example ​

Design Basis ​

Step 1: Calculate Applied Loads ​

Step 2: Size Media Volume ​

Step 3: Determine Filter Configuration ​

Step 4: Check Loading Rates ​

Step 5: Check Wetting Rate ​

Step 6: Calculate Removal Efficiency ​

Step 7: Calculate Effluent Quality ​

Step 8: Calculate Biomass Production ​

Design Summary ​

UK Water Company Variations ​

UK Industry Practice ​

UK Industry Practice ​

UK Industry Practice ​

Distribution Systems ​

Rotary Distributors ​

Fixed Spray Nozzles ​

Ventilation ​

Natural Ventilation ​

Forced Ventilation ​

Common Design Issues ​

Insufficient Wetting Rate ​

Ponding ​

Poor Nitrification ​

Odor Issues ​

Related Calculations ​

References ​

Biofilters - PS211 Design Standards

UK PS211 Standards

Design Criteria

Media Requirements

Design Equations

Geometry Calculations

Filter Surface Area (Plan View)

Media Volume

Total Media Surface Area

Loading Rate Calculations

BOD Volumetric Loading

Ammonia Volumetric Loading

Wetting Rate (Hydraulic Loading)

Recirculation Requirements

Recirculation Flow Required

Applied BOD with Recirculation

Temperature Correction

Temperature Correction Factor

Removal Efficiency

Base Efficiency (15°C, optimal loading)

Loading Rate Adjustment

Effluent Quality

Biomass Production

Biomass Yield

Worked Example

Design Basis

Step 1: Calculate Applied Loads

Step 2: Size Media Volume

Step 3: Determine Filter Configuration

Step 4: Check Loading Rates

Step 5: Check Wetting Rate

Step 6: Calculate Removal Efficiency

Step 7: Calculate Effluent Quality

Step 8: Calculate Biomass Production

Design Summary

UK Water Company Variations

UK Industry Practice

UK Industry Practice

UK Industry Practice

Distribution Systems

Rotary Distributors

Fixed Spray Nozzles

Ventilation

Natural Ventilation

Forced Ventilation

Common Design Issues

Insufficient Wetting Rate

Ponding

Poor Nitrification

Odor Issues

Related Calculations

References