Chemical Dosing - Phosphorus Removal

Chemical precipitation is used for phosphorus (P) removal in wastewater treatment, typically using ferric or aluminum salts. This guide focuses on ferric sulphate dosing following UK water industry operational guidelines.

UK Industry Molar Ratio Standards

Fe:P Molar Ratios by Dosing Stage

Dosing Stage	Minimum	Typical	Maximum	Target P Removal
Primary	1.50	1.66	1.80	60-80%
Secondary	1.60	1.80	2.00	70-90%
Tertiary	1.80	2.00	2.20	80-95%

General principle:

• Higher Fe:P ratios → Higher P removal efficiency
• Later stages require higher ratios for diminishing returns
• Typical practice: Use 1.66:1 for primary or secondary dosing

###Alternative Coagulants

Chemical	Formula	Fe/Al Content	Molar Ratio (M:P)	Notes
Ferric sulphate	Fe₂(SO₄)₃	12% Fe	1.5-2.2 (Fe:P)	Most common in UK
Ferric chloride	FeCl₃	38-45% Fe	1.5-2.2 (Fe:P)	Higher Fe content, more corrosive
Aluminum sulphate	Al₂(SO₄)₃	8% Al	1.5-2.5 (Al:P)	Alternative to Fe, common in drinking water
PAC	Al_n(OH)_m(Cl)₃ₙ₋ₘ	10% Al	1.5-2.5 (Al:P)	Polyaluminum chloride, wide pH range

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Design Equations

Core Dosing Calculations

Iron Dose from P Load

Formula:

Fe_dose = P_load × (Fe:P ratio) × (MW_Fe / MW_P)

Where:

• Fe_dose = iron required (kg Fe/d)
• P_load = phosphorus to be removed (kg P/d)
• Fe:P ratio = molar ratio (e.g., 1.66)
• MW_Fe = molar mass of Fe = 56 g/mol
• MW_P = molar mass of P = 31 g/mol
• MW_Fe / MW_P = 56/31 = 1.806

Example:

P_load = 10 kg/d
Fe:P ratio = 1.66 (typical)

Fe_dose = 10 × 1.66 × 1.806 = 29.98 kg Fe/d

Compound Dose from Fe Dose

Formula:

Compound_dose = Fe_dose / (Fe_content / 100)

Where:

• Compound_dose = ferric sulphate required (kg/d)
• Fe_content = % Fe in compound (typically 12% for ferric sulphate)

Example (ferric sulphate at 12% Fe):

Fe_dose = 30 kg Fe/d
Fe_content = 12%

Compound_dose = 30 / 0.12 = 250 kg ferric sulphate/d

Volumetric Dosing Rate

Formula:

Dose_rate = Compound_dose / Density / 24

Where:

• Dose_rate = volumetric dosing rate (L/h)
• Density = solution density (kg/L, typically 1.5 for ferric sulphate)

Example:

Compound_dose = 250 kg/d
Density = 1.5 kg/L

Dose_rate = 250 / 1.5 / 24 = 6.94 L/h

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Dosing Stage Selection

Primary Stage Dosing

Location: Dosed into primary sedimentation tanks

Advantages:

• Simultaneous P and SS removal
• Enhanced primary sludge settleability
• H₂S odor control (bonus benefit)
• Lower overall sludge volume vs later stages

Disadvantages:

• Lower P removal efficiency (60-80%)
• Higher chemical consumption for given removal
• No BOD enhancement

Typical application:

• Works with primary treatment
• Moderate P consents (≤ 2 mg/L)
• Where sludge minimization is priority

Fe:P ratio: 1.66:1 (typical)

Secondary Stage Dosing

Location: Dosed into biofilter effluent or humus tanks

Advantages:

• Higher P removal efficiency (70-90%)
• Improved humus sludge settlement
• Can retrofit to existing works

Disadvantages:

• Chemical sludge separate from primary
• May interfere with biofilter performance if overdosed
• Requires good mixing

Typical application:

• Biofilter works without primary treatment
• Stricter P consents (≤ 1 mg/L)
• Retrofit applications

Fe:P ratio: 1.80:1 (typical)

Tertiary Stage Dosing

Location: Dosed after all biological treatment (polishing)

Advantages:

• Highest P removal (80-95%)
• Fine-tuning to meet very low consents
• No interference with biological processes

Disadvantages:

• Highest chemical consumption per kg P removed
• Additional settling/filtration required
• Separate sludge handling

Typical application:

• Very strict P consents (≤ 0.5 mg/L)
• Sensitive receiving waters (eutrophic lakes)
• Works with advanced treatment trains

Fe:P ratio: 2.00:1 (typical)

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P Removal Efficiency

Efficiency by Stage and Ratio

Primary dosing:

Efficiency (%) = 60 + 20 × ((ratio - 1.5) / (1.8 - 1.5))

Range: 60-80%

Secondary dosing:

Efficiency (%) = 70 + 20 × ((ratio - 1.6) / (2.0 - 1.6))

Range: 70-90%

Tertiary dosing:

Efficiency (%) = 80 + 15 × ((ratio - 1.8) / (2.2 - 1.8))

Range: 80-95%

Example (primary, Fe:P = 1.66):

Efficiency = 60 + 20 × ((1.66 - 1.5) / 0.3)
Efficiency = 60 + 20 × 0.533 = 70.7%

Effluent P Calculation

Formula:

P_effluent = P_influent × (1 - E/100)

Example:

P_influent = 12 mg/L
Efficiency = 70%

P_effluent = 12 × (1 - 0.70) = 3.6 mg/L

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Chemical Sludge Production

Sludge from Ferric Sulphate

Formula:

Sludge_production = Fe_dose × S_factor

Where:

• Sludge_production = chemical sludge (kgSS/d)
• Fe_dose = iron dosed (kg Fe/d)
• S_factor = sludge factor = 2.3 kgSS/kg Fe (typical)

Derivation:

Fe precipitation: Fe³⁺ + PO₄³⁻ → FePO₄ (solid)
Fe hydrolysis: Fe³⁺ + 3OH⁻ → Fe(OH)₃ (solid)
Combined: 1 kg Fe → ~2.3 kg dry solids

Example:

Fe_dose = 30 kg Fe/d

Sludge = 30 × 2.3 = 69 kgSS/d

Combined with primary sludge:

Total_sludge = Primary_sludge + Chemical_sludge
Total_sludge = 105 + 69 = 174 kgSS/d

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Storage Requirements

Chemical Storage Tank Sizing

UK operator standard: 28 days storage minimum

Formula:

Storage_volume = Compound_dose × Storage_days / Density / 1000

Where:

• Storage_volume = tank capacity (m³)
• Storage_days = typically 28 days (UK operator)
• Density = solution density (kg/L)

Example:

Compound_dose = 250 kg/d
Storage_days = 28 days
Density = 1.5 kg/L

Storage = (250 × 28) / 1.5 / 1000 = 4.67 m³

Delivery Mass

Formula:

Delivery_mass = Storage_volume × Density

Where:

• Delivery_mass = tanker delivery (tonnes)

Example:

Storage = 4.67 m³
Density = 1.5 kg/L = 1,500 kg/m³ = 1.5 tonnes/m³

Delivery = 4.67 × 1.5 = 7.0 tonnes per delivery

Delivery frequency:

Frequency = Storage_days = 28 days (~ monthly)

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Worked Example

Design Basis

• Population: 5,000 PE
• DWF: 600 m³/d
• Influent TP: 12 mg/L
• Target effluent TP: ≤ 2 mg/L (consent)
• Dosing stage: Primary
• Chemical: Ferric sulphate (12% Fe)

Step 1: Calculate P Load

Influent load:

P_inf = (12 mg/L × 600 m³/d) / 1000 = 7.2 kg P/d

Target effluent:

P_eff = 2 mg/L
P_eff_load = (2 mg/L × 600 m³/d) / 1000 = 1.2 kg P/d

P to be removed:

P_removal = 7.2 - 1.2 = 6.0 kg P/d
Removal efficiency = 6.0 / 7.2 × 100 = 83.3%

Step 2: Select Fe:P Ratio

For primary dosing to achieve 83% removal:

Target E = 83%
Primary range: 60-80% typical

Use higher end ratio: Fe:P = 1.75 (between typical 1.66 and max 1.80)

Step 3: Calculate Fe Dose

Formula:

Fe_dose = P_removal × Fe:P × (56/31)
Fe_dose = 6.0 × 1.75 × 1.806
Fe_dose = 18.96 kg Fe/d ≈ 19 kg Fe/d

Step 4: Calculate Compound Dose

Ferric sulphate (12% Fe):

Compound = 19 / 0.12 = 158.3 kg/d

Step 5: Calculate Dosing Rate

Volumetric rate:

Dose_rate = 158.3 / 1.5 / 24 = 4.40 L/h

Daily volume:

Daily_volume = 4.40 × 24 = 105.6 L/d

Step 6: Calculate Storage

28-day storage:

Storage = (158.3 × 28) / 1.5 / 1000 = 2.95 m³

Select: 3.0 m³ tank (with 2% margin)

Delivery mass:

Delivery = 3.0 × 1.5 = 4.5 tonnes per month

Step 7: Calculate Chemical Sludge

Sludge production:

Chem_sludge = 19 × 2.3 = 43.7 kgSS/d

Impact on primary sludge: Assuming 105 kgSS/d from primary alone:

Total_primary = 105 + 43.7 = 148.7 kgSS/d
Increase = 43.7 / 105 × 100 = 42% increase

Combined sludge flow (3% DS):

Sludge_flow = (148.7 / 1000) / 0.03 = 4.96 m³/d
(vs 3.5 m³/d without chemical)

Step 8: Verify Compliance

Predicted effluent TP: Using Fe:P = 1.75 for primary:

Efficiency ≈ 78% (from correlation)
P_eff = 12 × (1 - 0.78) = 2.64 mg/L

This is above target of 2.0 mg/L.

Adjustment: Increase to Fe:P = 1.80 (max for primary)

Fe_dose = 6.0 × 1.80 × 1.806 = 19.5 kg Fe/d
Efficiency ≈ 80%
P_eff = 12 × (1 - 0.80) = 2.4 mg/L

Still above target. Consider secondary dosing instead:

Using secondary dosing with Fe:P = 1.80:

Efficiency ≈ 80% (secondary range: 70-90%)
P_eff = 12 × (1 - 0.80) = 2.4 mg/L

Or Fe:P = 1.90:

Efficiency ≈ 85%
P_eff = 12 × (1 - 0.85) = 1.8 mg/L ✓

Revised Design Summary

Parameter	Value	Notes
Dosing stage	Secondary (humus tanks)	For higher efficiency
Fe:P molar ratio	1.90	Between typical 1.80 and max 2.00
P removal efficiency	85%
Fe dose	20.6 kg Fe/d
Compound dose	171.7 kg/d	Ferric sulphate (12% Fe)
Dosing rate	4.77 L/h
Storage tank	3.2 m³	28 days @ 1.5 kg/L density
Delivery frequency	Monthly	4.8 tonnes per delivery
Chemical sludge	47.4 kgSS/d
Predicted effluent TP	1.8 mg/L	✓ Meets 2.0 mg/L consent

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UK Water Company Variations

UK Industry Practice

• Fe:P ratios as specified above (1.5-2.2)
• 28-day chemical storage mandatory
• Prefer secondary dosing for P removal
• Monthly site visits for performance verification

UK Industry Practice

• Similar Fe:P ratios
• Often use aluminum sulphate in some catchments
• Require detailed mass balance for consent compliance
• Continuous P monitoring on larger works

UK Industry Practice

• Use ferric chloride in some areas (higher Fe%)
• Prefer primary dosing where compatible with sludge treatment
• Strict reporting on chemical consumption

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Design Considerations

Dosing Point Selection

Primary tanks:

• Dose at inlet channel or distribution chamber
• Good mixing from inlet turbulence
• Rapid precipitation and co-settlement
• Monitor primary sludge density (target 4-6% DS)

Biofilter effluent:

• Dose before humus tanks
• Ensure good mixing (baffled channel or static mixer)
• Allow 10-20 minutes contact time
• Monitor humus sludge for Fe accumulation

Tertiary (after humus):

• Dose into separate contact tank
• Provide flocculation (gentle mixing, 15-30 min)
• Followed by settlement or filtration
• Separate sludge handling required

Mixing Requirements

Rapid mix:

• G value: 300-600 s⁻¹
• Time: 30-60 seconds
• Purpose: Distribute chemical, initiate precipitation

Flocculation:

• G value: 20-60 s⁻¹
• Time: 15-30 minutes
• Purpose: Grow settleable flocs

Equation for mixing intensity:

G = √(P / μ / V)

Where:

• G = velocity gradient (s⁻¹)
• P = power input (W)
• μ = dynamic viscosity (Pa·s)
• V = tank volume (m³)

pH Considerations

Optimal pH ranges:

• Ferric salts: pH 5.0-8.5 (optimal 6.0-7.5)
• Aluminum salts: pH 5.5-7.5 (optimal 6.0-7.0)

pH depression from dosing: Ferric sulphate is acidic and consumes alkalinity:

Alkalinity consumed = Fe_dose × 0.75 kg CaCO₃/kg Fe

See Alkalinity Balance for full calculation.

Example:

Fe_dose = 20 kg Fe/d
Alk consumed = 20 × 0.75 = 15 kg CaCO₃/d

If this drives effluent pH < 6.5 or alkalinity < 50 mg/L, add lime or caustic soda.

Alkalinity and Bicarbonate Dosing

If pH falls below optimal:

Lime addition:

Lime_dose = Alkalinity_deficit / 0.74

Where 0.74 converts CaCO₃ equivalent to CaO.

Caustic soda alternative:

NaOH_dose = Alkalinity_deficit / 1.25

Where 1.25 converts CaCO₃ to NaOH equivalent.

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Dosing Equipment

Dosing Pumps

Types:

• Diaphragm metering pumps (most common)
• Peristaltic pumps (lower capacity)
• Progressive cavity pumps (higher capacity)

Sizing:

• Max flow: 150-200% of average dose rate (turndown 10:1)
• Example: Average 5 L/h → Select 10 L/h max pump
• Redundancy: Minimum 2 pumps (duty + standby)

Control:

• Flow-proportional dosing (most common)
• Compound loop (P analyzer feedback, optional)
• Manual trim adjustment (±20%)

Storage and Handling

Tank material:

• Ferric sulphate: HDPE, GRP (not mild steel)
• Level indication: Ultrasonic or pressure transmitter
• Bunding: 110% of tank capacity

Safety:

• Ferric sulphate: Corrosive, pH ~2
• PPE: Goggles, gloves, apron
• Spillage: Neutralize with lime, wash down
• Ventilation: Adequate for fumes (acidic vapor)

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Monitoring and Control

Key Parameters

Parameter	Frequency	Method	Target
Influent TP	Daily	Laboratory	Baseline
Effluent TP	Daily	Laboratory	< Consent limit
Fe dose rate	Continuous	Pump stroke counter	Per design
Chemical stock	Weekly	Level gauge	> 7 days
Sludge %DS	Weekly	Laboratory	4-6% (primary)

Optimization

Jar testing:

• Periodic jar tests to verify optimal Fe:P ratio
• Especially after significant influent changes
• Test range: ±20% around design ratio

Dose trimming:

If effluent TP consistently < target: Reduce dose by 10%
If effluent TP > target: Increase dose by 10%
Repeat until stable at target ± 15%

Cost optimization:

Chemical_cost = Compound_dose × Unit_price

Typical ferric sulphate: £150-200/tonne

Example:

Dose = 170 kg/d = 5.1 tonnes/month
Price = £180/tonne
Monthly cost = 5.1 × £180 = £918/month

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Common Dosing Issues

Over-Dosing

Symptoms:

• Effluent TP well below consent (wasted chemical)
• Yellow/brown color in effluent (residual Fe)
• Excessive sludge production

Solutions:

• Reduce Fe:P ratio
• Check influent TP (may have decreased)
• Optimize based on effluent monitoring

Under-Dosing

Symptoms:

• Effluent TP exceeds consent
• Poor sludge settlement (insufficient coagulation)

Solutions:

• Increase Fe:P ratio
• Check for short-circuiting (poor mixing)
• Verify pump calibration and chemical strength

Poor Mixing

Symptoms:

• Variable effluent TP
• Poor sludge settlement (incomplete precipitation)
• Chemical accumulation in corners

Solutions:

• Add static mixer or baffle
• Increase dosing point turbulence
• Relocate dose point to higher energy zone

Sludge Handling Issues

Symptoms:

• Sludge difficult to thicken (Fe-rich sludge)
• Dewatering problems (high Fe binds water)

Solutions:

• Optimize Fe dose (avoid over-dosing)
• Add polymer for dewatering (5-15 kg/tonne DS)
• Consider lime stabilization before dewatering

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Related Calculations

• Alkalinity Balance: Alkalinity consumption from Fe dosing
• Sludge Production: Combined primary and chemical sludge
• Primary Tanks: Enhanced primary treatment with Fe

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References

• UK operator Operational Guidelines: Chemical dosing for phosphorus removal
• CIWEM Best Practice: Ferric and aluminum dosing in wastewater treatment
• WRc Technical Guidance: Chemical phosphorus removal

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