Primary Tanks - PS208 Design Standards

Primary sedimentation tanks (PSTs) are the first stage of treatment following preliminary screening. They remove settleable solids and floating material through gravity settling, typically achieving 30-60% SS removal and 30-40% BOD removal.

UK PS208 Standards

Design Criteria

Parameter	PS208 Limit	Units	Notes
Upflow velocity (DWF)	≤ 1.2	m/h	All tanks in service
Upflow velocity (FFT)	≤ 2.0	m/h	All tanks in service
Hydraulic retention time	≥ 2.0	hours	At DWF minimum
Sidewater depth (SWD)	≥ 2.5	m	Minimum operating depth
Weir loading (DWF)	≤ 150	m³/m/d	Effluent weir overflow rate
Minimum tanks	2	-	For N-1 compliance

N-1 Compliance

With one tank out of service for maintenance:

• Upflow velocity at DWF ≤ 1.2 m/h
• Upflow velocity at FFT ≤ 2.0 m/h
• HRT at FFT ≥ 2 hours

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Design Equations

Geometry Calculations

Radial (Circular) Tanks

Surface area:

A = π × (D/2)² = π × r²

Where:

• A = surface area (m²)
• D = diameter (m)
• r = radius (m)

Volume:

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

where: h_cone = r × tan(θ)

Where:

• SWD = sidewater depth (m)
• θ = floor slope angle (degrees)
• h_cone = height of conical section (m)

Typical dimensions:

• Diameter: 15-40 m
• SWD: 2.5-4.0 m
• Floor slope: 8-12° (typical 10°)

Rectangular Tanks

Surface area:

A = L × W

Volume:

V = L × W × SWD

Where:

• L = length (m)
• W = width (m)

Typical dimensions:

• Length: 20-60 m
• Width: 5-15 m
• Length:width ratio: 3:1 to 6:1

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Hydraulic Calculations

Upflow Velocity (Surface Overflow Rate)

Formula:

V_up = Q / A / 24

Where:

• V_up = upflow velocity (m/h)
• Q = flow rate (m³/d)
• A = total surface area (m²)
• 24 = hours per day conversion

PS208 limits:

• At DWF: ≤ 1.2 m/h
• At FFT: ≤ 2.0 m/h

Physical meaning: Upflow velocity must be less than particle settling velocity for effective removal. Typical settling velocities:

• Coarse solids: 3-5 m/h
• Fine solids: 0.5-1.5 m/h
• Design margin ensures capture of finer particles

Hydraulic Retention Time

Formula:

HRT = (V / Q) × 24

Where:

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

PS208 minimum: 2 hours at DWF

Typical values:

• At DWF: 2-4 hours
• At FFT: 1-2 hours

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Removal Efficiency

Without Chemical Dosing

Parameter	Removal (%)	Influent → Effluent
BOD	30	300 → 210 mg/L
SS	50	350 → 175 mg/L
NH₄-N	0	37.5 → 37.5 mg/L
TP	15	12.5 → 10.6 mg/L

With Chemical Dosing (Fe or Al)

Parameter	Removal (%)	Influent → Effluent
BOD	40	300 → 180 mg/L
SS	60	350 → 140 mg/L
NH₄-N	0	37.5 → 37.5 mg/L
TP	70	12.5 → 3.75 mg/L

Effluent concentration:

C_eff = C_inf × (1 - E/100)

Where:

• C_eff = effluent concentration (mg/L)
• C_inf = influent concentration (mg/L)
• E = removal efficiency (%)

Effluent load:

Load_eff = (C_eff × Q) / 1000

Where:

• Load_eff = effluent load (kg/d)
• Q = flow (m³/d)

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Sludge Production

Primary Sludge Without Chemicals

Formula:

Sludge_dry = TSS_removed = TSS_inf × (E_SS / 100)
Sludge_flow = (Sludge_dry / 1000) / (DS% / 100)

Where:

• TSS_removed = solids removed (kg/d)
• TSS_inf = influent TSS load (kg/d)
• E_SS = SS removal efficiency (%)
• DS% = dry solids concentration (%, typical 3-5%)

Typical primary sludge characteristics:

• Dry solids: 3-5% DS
• Volatile solids: 65-75% of DS
• Specific gravity: 1.02-1.05

With Chemical Dosing

Chemical sludge production adds to primary sludge (see Chemical Dosing):

Total_sludge = Primary_sludge + Chemical_sludge
Chemical_sludge = Fe_dose × 2.3 (kgSS/d)

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Worked Example

Design Basis

• Population: 5,000 PE
• DWF: 600 m³/d
• FFT (Formula A): 1,800 m³/d
• Influent: BOD 300 mg/L, SS 350 mg/L

Step 1: Determine Number and Size of Tanks

Try 2 radial tanks, D = 25 m each:

Single tank area:

A = π × (25/2)² = π × 12.5² = 490.9 m²
Total area = 490.9 × 2 = 981.7 m²

Step 2: Check Upflow Velocity

At DWF (all tanks):

V_up = 600 / 981.7 / 24 = 0.025 m/h ✓
(Well below 1.2 m/h limit)

At FFT (all tanks):

V_up = 1800 / 981.7 / 24 = 0.076 m/h ✓
(Well below 2.0 m/h limit)

N-1 check (one tank out, area = 490.9 m²):

At DWF: V_up = 600 / 490.9 / 24 = 0.051 m/h ✓
At FFT: V_up = 1800 / 490.9 / 24 = 0.153 m/h ✓

Step 3: Determine Tank Depth

Choose SWD = 3.0 m, floor slope = 10°

Volume per tank:

V_cylindrical = 490.9 × 3.0 = 1,472.7 m³

h_cone = 12.5 × tan(10°) = 2.20 m
V_conical = (1/3) × π × 12.5² × 2.20 = 359.4 m³

V_total = 1,472.7 + 359.4 = 1,832.1 m³ per tank

Total volume = 1,832.1 × 2 = 3,664.2 m³

Step 4: Check HRT

At DWF:

HRT = (3664.2 / 600) × 24 = 146.6 hours ✓
(Well above 2 hour minimum)

At FFT:

HRT = (3664.2 / 1800) × 24 = 48.9 hours ✓

N-1 at FFT:

HRT = (1832.1 / 1800) × 24 = 24.4 hours ✓

Step 5: Calculate Sludge Production

Influent loads:

SS_load = (350 mg/L × 600 m³/d) / 1000 = 210 kgSS/d

Primary sludge (50% SS removal, 3% DS):

SS_removed = 210 × 0.50 = 105 kgSS/d
Sludge_flow = (105 / 1000) / 0.03 = 3.5 m³/d

Design Summary

Parameter	Value	Compliance
Configuration	2 × radial tanks	✓ N-1 capable
Diameter	25 m each
SWD	3.0 m	✓ > 2.5 m
Total area	981.7 m²
Total volume	3,664 m³
Upflow (DWF)	0.025 m/h	✓ < 1.2 m/h
Upflow (FFT)	0.076 m/h	✓ < 2.0 m/h
HRT (DWF)	146.6 hours	✓ > 2 hours
Sludge production	3.5 m³/d at 3% DS

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UK Water Company Variations

UK Industry Practice

• Upflow velocity: 1.0 m/h preferred (DWF)
• HRT: 3 hours minimum preferred
• Chemical dosing common for P removal

UK Industry Practice

• Upflow velocity: 1.0-1.2 m/h (DWF)
• Rectangular tanks preferred for large works
• Storm tanks integrated with primary treatment

UK Industry Practice

• SWD: 3.0 m typical minimum
• N-1 compliance strictly enforced
• Combined primary/storm tank arrangements

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Design Considerations

Tank Configuration

Radial tanks advantages:

• Efficient use of space (circular footprint)
• Good for smaller-medium works (PE < 20,000)
• Simple sludge collection with rotating scraper
• Conical floor aids sludge thickening

Rectangular tanks advantages:

• Better for large works (PE > 20,000)
• Can accommodate multiple lanes
• Easier to construct in linear site layouts
• Simpler to cover for odor control

Inlet Arrangements

Radial tanks:

• Central feed well with energy dissipation
• Diameter = 15-20% of tank diameter
• Inlet velocity < 0.3 m/s

Rectangular tanks:

• End-feed with baffle wall
• Cross-flow velocity < 0.15 m/s
• Flow distribution across width

Outlet Arrangements

Weir loading:

W_load = Q / L_weir

Where:

• W_load = weir loading (m³/m/d)
• L_weir = total weir length (m)
• PS208 limit: ≤ 150 m³/m/d

Radial tanks:

• Peripheral weir (circumference = πD)
• V-notch weirs for flow distribution

Rectangular tanks:

• End weir across full width
• Adjustable weirs for level control

Sludge Collection

Radial tanks:

• Rotating scraper bridge (0.02-0.05 rpm)
• Sludge withdrawn continuously or intermittently
• Scraper torque monitoring for density control

Rectangular tanks:

• Traveling bridge or chain-and-flight scrapers
• Hopper spacing: 3-5 m
• Sludge pumping cycles: 15-30 min intervals

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Common Design Issues

Insufficient N-1 Capacity

Problem: Works designed with 2 tanks barely meeting criteria with both in service.

Solution: Size for N-1 from the outset. With 2 tanks, each must handle full DWF within limits.

High Weir Loading

Problem: Excessive weir overflow causing solids carryover.

Solution:

• Increase weir length (use radial peripheral weir)
• Add V-notch weirs for better distribution
• Consider adjustable weirs

Poor Sludge Density

Problem: Thin sludge (< 2% DS) causing excessive volumes.

Solution:

• Increase scraper frequency
• Add sludge blanket monitoring
• Consider pre-thickening
• Check for septicity (add Fe for odor control)

Septicity and Odor

Problem: Anaerobic conditions in sludge causing H₂S.

Solution:

• Reduce HRT if excessive (< 4 hours preferred)
• Add ferric chloride (dual benefit: P removal + H₂S control)
• Frequent sludge withdrawal
• Consider covered tanks with odor treatment

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Related Calculations

• Flow and Load: Influent flow and load determination
• Chemical Dosing: Fe/Al dosing for enhanced P removal
• Sludge Production: Primary sludge handling and storage

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References

• UK Sewers for Adoption (7th Edition): Appendix PS208 - Primary Sedimentation Tanks
• UK operator Design Standards: Primary treatment design criteria
• CIWEM/WaPUG: Urban wastewater treatment design guidance

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