Humus Tanks - PS221 Design Standards

Humus tanks (also called secondary settling tanks or final clarifiers in biofilter systems) capture sloughed biomass from biofilters. They provide final clarification before discharge or further treatment.

UK PS221 Standards

Design Criteria

Parameter	PS221 Limit	Units	Notes
HLR (all tanks in service)	≤ 1.0	m³/m²/h	Hydraulic loading rate at FFT
HLR (N-1, one tank out)	≤ 2.0	m³/m²/h	With one tank OOS for maintenance
Minimum HRT	≥ 2.0	hours	At maximum flow (FFT)
Minimum SWD	≥ 2.0	m	Sidewater depth
Minimum tanks	2	-	For N-1 redundancy

Comparison with Primary Tanks (PS208)

Parameter	Primary (PS208)	Humus (PS221)	Reason for Difference
Max HLR	1.2 m/h (DWF)	1.0 m/h (FFT)	Humus sludge settles slower than primary
Min HRT	2 hours	2 hours	Same settling time requirement
Min SWD	2.5 m	2.0 m	Humus tanks can be shallower
Sludge %DS	3-5%	1-2%	Humus sludge is lighter, less dense

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Design Equations

Geometry Calculations

Radial (Circular) Tanks

Surface area:

A = π × (D/2)²

Where:

• A = surface area (m²)
• D = diameter (m)
• Typical: 12-25 m diameter

Volume:

V_cylindrical = A × SWD
V_conical = (1/3) × π × r² × h_cone
V_total = V_cylindrical + V_conical

Where:

• SWD = sidewater depth (m), typically 2.0-3.0 m
• h_cone = conical hopper depth (m), typically 0.5-1.5 m

Rectangular Tanks

Surface area:

A = L × W

Volume:

V = L × W × SWD

Where:

• L = length (m), typical 15-40 m
• W = width (m), typical 4-10 m
• L:W ratio: 3:1 to 5:1

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Hydraulic Calculations

Hydraulic Loading Rate (HLR)

Formula:

HLR = Q / A / 24

Where:

• HLR = hydraulic loading rate (m³/m²/h)
• Q = flow rate (m³/d)
• A = surface area (m²)

PS221 limits:

• All tanks in service: ≤ 1.0 m³/m²/h at FFT
• N-1 (one tank out): ≤ 2.0 m³/m²/h at FFT

Design notes:

• Check HLR at DWF, ADF, and FFT
• FFT typically controls design
• N-1 condition must be verified

Hydraulic Retention Time (HRT)

Formula:

HRT = (V / Q) × 24

Where:

• HRT = hydraulic retention time (hours)
• V = liquid volume above SWD (m³)
• Q = flow rate (m³/d)

PS221 minimum: 2 hours at FFT

Typical values:

• At DWF: 6-12 hours
• At ADF: 4-8 hours
• At FFT: 2-4 hours

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Settling Velocity Considerations

Humus sludge has lower density and slower settling than primary sludge.

Typical Settling Velocities

Sludge Type	Settling Velocity	Density	Notes
Primary sludge	1.5-3.0 m/h	1.02-1.05	Coarse, dense solids
Humus sludge	0.5-1.5 m/h	1.005-1.02	Light biological floc
WAS (activated sludge)	0.8-2.0 m/h	1.003-1.01	Flocculated biomass

Design implications:

• Lower HLR limit (1.0 vs 1.2 m/h) accounts for slower settling
• Longer retention time aids flocculation
• Gentle flow regime prevents floc breakup

Upward Velocity vs Settling Velocity

For effective capture:

V_upward < 0.7 × V_settling (safety factor)

PS221 HLR of 1.0 m/h provides adequate safety margin:

V_upward = 1.0 m/h
V_settling = 1.5 m/h (typical humus)
Safety factor = 1.0 / 1.5 = 0.67 ✓

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N-1 Compliance

With one tank out of service:

Required area calculation:

A_required = Q_FFT / HLR_max / 24
A_provided = n × A_single
A_N-1 = (n-1) × A_single

For compliance: A_N-1 ≥ A_required (at HLR = 2.0 m/h)

Design approach: For 2 tanks (minimum):

• Each tank must handle FFT/2 at ≤ 1.0 m/h (both in service)
• Single tank must handle FFT at ≤ 2.0 m/h (N-1 condition)

Example:

FFT = 1,800 m³/d

All tanks in service (n=2):
A_required = 1800 / 1.0 / 24 = 75 m² per tank minimum
A_total = 150 m²

N-1 (one tank out):
A_single = 75 m²
HLR_N-1 = 1800 / 75 / 24 = 1.0 m/h ✓ (< 2.0 m/h limit)

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Humus Sludge Capture

Capture Efficiency

Typical humus sludge capture:

Capture_efficiency = 85%

This means 85% of biofilter biomass production is captured; 15% passes to effluent as dispersed solids.

Humus Sludge Production

Formula:

Sludge_solids = Biomass_biofilter × (E_capture / 100)
Sludge_flow = Sludge_solids / (DS% × 10)

Where:

• Biomass_biofilter = biofilter biomass production (kgSS/d)
• E_capture = capture efficiency (%, typically 85%)
• DS% = humus sludge dry solids (%, typically 2%)

Humus sludge characteristics:

• Dry solids: 1.5-2.5% (typically 2%)
• Volatile solids: 70-80% of DS
• Specific gravity: 1.005-1.015
• Color: Brown (aerobic) or grey-black (partly anaerobic)

Example:

Biofilter biomass = 100 kgSS/d
Capture = 85%
DS = 2%

Sludge_solids = 100 × 0.85 = 85 kgSS/d
Sludge_flow = 85 / (2 × 10) = 4.25 m³/d

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Worked Example

Design Basis

• FFT: 1,800 m³/d
• DWF: 600 m³/d
• ADF: 960 m³/d (DWF × 1.6)
• Biofilter biomass production: 48 kgSS/d

Step 1: Size for HLR Compliance

All tanks in service (at FFT):

A_required = Q_FFT / HLR_max / 24
A_required = 1800 / 1.0 / 24 = 75 m²

For 2 tanks: A_per_tank = 37.5 m²

Try D = 15 m per tank:

A = π × (15/2)² = 176.7 m² per tank
A_total = 176.7 × 2 = 353.4 m²

This is conservative (much larger than minimum 75 m²).

Step 2: Check HLR at All Flows

At DWF:

HLR = 600 / 353.4 / 24 = 0.071 m/h ✓
(Well below limit)

At ADF:

HLR = 960 / 353.4 / 24 = 0.113 m/h ✓

At FFT:

HLR = 1800 / 353.4 / 24 = 0.212 m/h ✓
(Well below 1.0 m/h limit)

Step 3: Check N-1 Compliance

With one tank out (area = 176.7 m²):

HLR_N-1 = 1800 / 176.7 / 24 = 0.424 m/h ✓
(Well below 2.0 m/h limit)

Step 4: Determine Tank Depth

Choose SWD = 2.5 m, conical depth = 1.0 m

Liquid volume (cylindrical):

V_liquid = A × SWD
V_liquid = 353.4 × 2.5 = 883.5 m³

Total volume (including conical hoppers):

V_cone_each = (1/3) × π × (7.5)² × 1.0 = 58.9 m³
V_total = 883.5 + (2 × 58.9) = 1,001.3 m³

Step 5: Check HRT

At DWF:

HRT = (883.5 / 600) × 24 = 35.3 hours

At ADF:

HRT = (883.5 / 960) × 24 = 22.1 hours

At FFT:

HRT = (883.5 / 1800) × 24 = 11.8 hours ✓
(Well above 2 hour minimum)

N-1 at FFT:

V_single = 176.7 × 2.5 = 441.8 m³
HRT_N-1 = (441.8 / 1800) × 24 = 5.9 hours ✓

Step 6: Calculate Humus Sludge Production

Capture from biofilter:

Sludge_solids = 48 × 0.85 = 40.8 kgSS/d

Sludge flow (at 2% DS):

Sludge_flow = 40.8 / (2 × 10) = 2.04 m³/d

Design Summary

Parameter	Value	Compliance
Configuration	2 × 15 m diameter	✓ N-1 capable
SWD	2.5 m	✓ > 2.0 m
Total surface area	353.4 m²
Liquid volume	883.5 m³
HLR at DWF	0.071 m/h	✓
HLR at FFT	0.212 m/h	✓ < 1.0 m/h
HLR N-1 at FFT	0.424 m/h	✓ < 2.0 m/h
HRT at DWF	35.3 hours	✓
HRT at FFT	11.8 hours	✓ > 2 hours
HRT N-1 at FFT	5.9 hours	✓ > 2 hours
Humus sludge	2.04 m³/d at 2% DS

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UK Water Company Variations

UK Industry Practice

• HLR: 0.8 m/h preferred at FFT (more conservative than 1.0)
• SWD: 2.5-3.0 m typical
• Minimum 2 tanks, prefer 3 for larger works (> 10,000 PE)

UK Industry Practice

• HLR: 1.0 m/h at FFT (as per PS221)
• Rectangular tanks common for large works
• Combined biofilter/humus tank duty in process design

UK Industry Practice

• N-1 strictly enforced for all new works
• HRT: 3 hours minimum preferred at FFT
• Covered tanks for odor control in urban catchments

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Design Considerations

Tank Configuration

Radial tanks advantages:

• Simple sludge collection (rotating scraper)
• Good for small-medium works (PE < 20,000)
• Compact footprint
• Well-proven technology

Rectangular tanks advantages:

• Better for large works (PE > 20,000)
• Multiple lanes possible
• Chain-and-flight scrapers robust
• Easier integration with rectangular biofilters

Inlet Arrangements

Radial tanks:

• Central feed well with stilling chamber
• Diameter = 10-15% of tank diameter
• Inlet velocity < 0.3 m/s
• Radial baffles to dissipate energy

Rectangular tanks:

• End-feed with baffle wall
• Full-width distribution channel
• Submerged inlet (avoid surface turbulence)

Outlet Arrangements

Weir configuration:

• Peripheral weir for radial tanks (full circumference)
• End weir for rectangular tanks (full width)
• V-notch weirs for even flow distribution

Weir loading:

W_load = Q / L_weir

Typical: < 150 m³/m/d (same as primary tanks)

Sludge Collection

Radial tanks:

• Rotating scraper bridge: 0.02-0.05 rpm
• Hopper diameter: 1.5-3.0 m
• Continuous or intermittent withdrawal

Rectangular tanks:

• Chain-and-flight scrapers
• Scraper speed: 0.6-1.2 m/min
• Hoppers at inlet end

Sludge withdrawal:

• Frequency: Every 2-4 hours typical
• Duration: 10-15 minutes per cycle
• Desludging rate: Balance concentration vs volume

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Common Design Issues

Poor Sludge Settlement

Problem: High effluent SS, blanket rising.

Causes:

• Excessive HLR (> 1.2 m/h)
• Biofilter overloading (poor floc formation)
• Denitrification in sludge blanket (N₂ gas flotation)
• Septicity (H₂S production, gas bubbles)

Solutions:

• Reduce HLR (add tank area or reduce flow to FFT)
• Optimize biofilter loading
• Increase sludge withdrawal frequency
• Avoid long sludge retention (< 4 hours in tank)

Denitrification in Blanket

Problem: Sludge blanket floats due to N₂ gas from nitrate reduction.

Mechanism:

NO₃⁻ + organic carbon → N₂ (gas) + CO₂

Gas bubbles attach to sludge floc, causing flotation.

Solutions:

• Increase sludge withdrawal rate (reduce blanket depth)
• Reduce nitrate in biofilter (lower recirculation)
• Avoid high sludge blanket levels (< 0.5 m typical)
• Consider de-nitrification filter upstream

Insufficient N-1 Capacity

Problem: Works cannot operate with one tank out.

Solution:

• Size for N-1 from start (each tank handles full FFT at 2.0 m/h)
• For 2-tank configuration, this means 50% oversizing vs nominal
• Or use 3 tanks with each at 67% capacity

Hydraulic Overloading in Storms

Problem: Solids carryover during storm flows.

Solutions:

• Storm flow diversion upstream (limit to FFT = 6 DWF)
• Storm tanks to attenuate peak flows
• Chemically-assisted settlement (PAC dosing)

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Effluent Polishing

Humus tanks provide additional BOD/NH₃ polishing beyond biofilter.

Additional Removal in Humus Tank

Parameter	Biofilter Effluent	Humus Tank Effluent	Additional Removal
BOD	30-40 mg/L	15-25 mg/L	15-25%
SS	30-50 mg/L	15-25 mg/L	50% (biomass capture)
NH₄-N	6-8 mg/L	5-7 mg/L	10-15%

Mechanisms:

• SS removal: Gravity settling of sloughed biomass
• BOD removal: Continued oxidation in biofilm on settled solids
• NH₄-N removal: Nitrification in thin biofilm on tank surfaces

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Liquor Return from Sludge Thickening

Humus sludge is often thickened before disposal, producing a liquor return stream.

Typical Liquor Characteristics

Parameter	Concentration (mg/L)	Return Load Contribution
TSS	500	Low (< 5% influent)
BOD	750	Medium (10-15% influent)
NH₄-N	100	High (20-30% influent)
TP	10	Low (5-10% influent)

Design impact:

• Return liquor to inlet works (before primary tanks)
• Account for loads in overall mass balance
• NH₄-N return can be significant (consider in alkalinity balance)

Liquor return calculation:

Load_return = (C × Q_liquor) / 1000

Where:

• C = pollutant concentration in liquor (mg/L)
• Q_liquor = liquor flow rate (m³/d)

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Related Calculations

• Biofilters: Biomass production feeding humus tanks
• Sludge Production: Humus sludge handling and storage
• Alkalinity Balance: Return liquor NH₄-N impact

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References

• UK Sewers for Adoption (7th Edition): Appendix PS221 - Humus Tanks
• UK operator Design Standards: Secondary settling design criteria
• WRc/CIRIA: Final clarifier design for biological treatment

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