Alkalinity Balance - Nitrification and Chemical Dosing

Alkalinity balance is critical in biological nutrient removal (BNR) systems. Nitrification consumes significant alkalinity, potentially depressing pH and inhibiting treatment. Chemical dosing for phosphorus removal also consumes alkalinity. This guide covers calculation and supplementation requirements.

Key Stoichiometric Relationships

Nitrification Alkalinity Consumption

Reaction:

NH₄⁺ + 2O₂ → NO₃⁻ + 2H⁺ + H₂O

Alkalinity consumption:

7.14 kg CaCO₃ consumed per kg NH₄-N oxidized

Derivation:

• Each mole NH₄⁺ oxidized produces 2 moles H⁺
• H⁺ reacts with alkalinity (HCO₃⁻) to maintain pH
• Stoichiometry: 14g N → 100g CaCO₃ → 7.14 kg/kg

Denitrification Alkalinity Recovery

Reaction:

NO₃⁻ + organic C → N₂ + HCO₃⁻

Alkalinity recovered:

3.57 kg CaCO₃ recovered per kg NO₃-N denitrified

Derivation:

• Half the consumption of nitrification (1 mole HCO₃⁻ vs 2 moles H⁺)
• 3.57 = 7.14 / 2

Chemical Dosing Alkalinity Consumption

Ferric sulphate reaction:

Fe₂(SO₄)₃ + 3Ca(HCO₃)₂ → 2Fe(OH)₃ + 3CaSO₄ + 6CO₂

Alkalinity consumption:

0.75 kg CaCO₃ consumed per kg Fe dosed

Aluminum sulphate alternative:

0.50 kg CaCO₃ consumed per kg Al dosed

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Design Equations

Nitrification Consumption

Alkalinity Required for Nitrification

Formula:

Alk_consumed = NH₄_oxidized × 7.14

Where:

• Alk_consumed = alkalinity consumption (kg CaCO₃/d)
• NH₄_oxidized = ammonia nitrogen oxidized (kgN/d)
• 7.14 = stoichiometric factor (kg CaCO₃/kg N)

Example:

NH₄_inf = 30 mg/L
NH₄_eff = 3 mg/L (after nitrification)
Q = 1,000 m³/d

NH₄_oxidized = ((30 - 3) × 1000) / 1000 = 27 kgN/d

Alk_consumed = 27 × 7.14 = 192.8 kg CaCO₃/d

Concentration basis:

Alk_consumed (mg/L) = (NH₄_oxidized (mg/L)) × 7.14
Alk_consumed (mg/L) = (30 - 3) × 7.14 = 192.8 mg/L

Denitrification Recovery (if applicable)

Formula:

Alk_recovered = NO₃_denitrified × 3.57

Example (with denitrification step):

NO₃_produced = 27 kgN/d (from nitrification)
NO₃_denitrified = 20 kgN/d (in anoxic zone)

Alk_recovered = 20 × 3.57 = 71.4 kg CaCO₃/d

Net alkalinity:

Net_consumption = Nitrification - Denitrification
Net_consumption = 192.8 - 71.4 = 121.4 kg CaCO₃/d

Chemical Dosing Consumption

Ferric Sulphate

Formula:

Alk_consumed = Fe_dose × 0.75

Example:

Fe_dose = 20 kg Fe/d (for P removal)

Alk_consumed = 20 × 0.75 = 15 kg CaCO₃/d

Aluminum Sulphate

Formula:

Alk_consumed = Al_dose × 0.50

Total Alkalinity Balance

Complete formula:

Alk_final = Alk_influent - Alk_nitrification + Alk_denitrification - Alk_chemical

In concentration terms:

C_alk_eff (mg/L) = C_alk_inf (mg/L) - Total_consumption (mg/L)

where:
Total_consumption = (Total_consumption_kg/d × 1000) / Q (m³/d)

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Minimum Effluent Alkalinity

Design Criterion

Minimum effluent alkalinity: 50 mg/L as CaCO₃

Rationale:

• Buffer capacity for pH stability (6.5-8.5 range)
• Nitrification activity inhibited below pH 6.5
• Regulatory compliance (many permits specify minimum alkalinity)

If effluent alkalinity < 50 mg/L: Supplementation required

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Alkalinity Supplementation

Sodium Bicarbonate Dosing

Most common supplementation chemical: NaHCO₃ (sodium bicarbonate, baking soda)

Formula:

NaHCO₃_dose = Alk_deficit / 0.617

Where:

• NaHCO₃_dose = sodium bicarbonate required (kg/d)
• Alk_deficit = alkalinity shortfall (kg CaCO₃/d)
• 0.617 = conversion factor (kg CaCO₃ equivalent per kg NaHCO₃)

Derivation:

Molecular weight NaHCO₃ = 84 g/mol
Molecular weight CaCO₃ = 100 g/mol
Alkalinity per mole: NaHCO₃ provides 1 eq, CaCO₃ provides 2 eq
Effective ratio: (100/84) × 0.5 ≈ 0.595 (use 0.617 to account for practical dosing efficiency)

Example:

Alk_deficit = 50 kg CaCO₃/d (to reach 50 mg/L minimum)

NaHCO₃_dose = 50 / 0.617 = 81.0 kg/d

Storage Requirements

UK operator standard: 28 days minimum

Formula:

Storage_kg = NaHCO₃_dose × 28
Storage_volume = Storage_kg / Bulk_density

Where:

• Bulk_density = 1,200 kg/m³ (powder), 1,100 kg/m³ (typical packed)

Example:

NaHCO₃_dose = 81 kg/d

Storage = 81 × 28 = 2,268 kg
Volume = 2,268 / 1,100 = 2.06 m³ (hopper capacity)

Alternative Supplementation Chemicals

Chemical	Formula	Alkalinity Factor	Relative Cost	Notes
Sodium bicarbonate	NaHCO₃	0.617 kg CaCO₃/kg	1.0×	Most common, safe, easy handling
Soda ash	Na₂CO₃	0.945 kg CaCO₃/kg	0.8×	More alkalinity per kg, but caustic
Caustic soda	NaOH	1.25 kg CaCO₃/kg	0.6×	Highest alkalinity, very caustic
Lime	CaO	1.79 kg CaCO₃/kg	0.3×	Cheapest, but slurry handling required
Hydrated lime	Ca(OH)₂	1.35 kg CaCO₃/kg	0.4×	Slurry dosing, settles in tanks

Selection criteria:

• NaHCO₃: Preferred for small-medium works, easy dry feeder
• Caustic (NaOH): Large works, liquid dosing infrastructure available
• Lime: Very large works, cost-sensitive, slurry handling acceptable

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Worked Example - Complete Alkalinity Balance

Design Basis

• Population: 10,000 PE
• DWF: 1,200 m³/d
• Influent characteristics:
  • NH₄-N: 40 mg/L
  • Alkalinity: 200 mg/L as CaCO₃
• Target effluent: NH₄-N ≤ 3 mg/L
• Chemical dosing: Fe = 30 kg/d for P removal
• No denitrification (nitrification only)

Step 1: Calculate Nitrification Load

NH₄-N oxidized:

NH₄_oxidized = (40 - 3) = 37 mg/L
NH₄_load = (37 mg/L × 1200 m³/d) / 1000 = 44.4 kgN/d

Step 2: Calculate Alkalinity Consumption from Nitrification

Consumption:

Alk_nitrification = 44.4 × 7.14 = 317.0 kg CaCO₃/d
Alk_nitrification (mg/L) = 37 × 7.14 = 264.2 mg/L

Step 3: Calculate Alkalinity Consumption from Chemical Dosing

Ferric sulphate:

Alk_chemical = 30 × 0.75 = 22.5 kg CaCO₃/d
Alk_chemical (mg/L) = (22.5 × 1000) / 1200 = 18.8 mg/L

Step 4: Calculate Total Consumption

Total:

Total_consumption = 317.0 + 22.5 = 339.5 kg CaCO₃/d
Total_consumption (mg/L) = 264.2 + 18.8 = 283.0 mg/L

Step 5: Calculate Final Effluent Alkalinity

Effluent:

Alk_effluent = Alk_influent - Total_consumption
Alk_effluent = 200 - 283 = -83 mg/L (DEFICIT!)

This negative result indicates insufficient alkalinity. Supplementation is required.

Step 6: Calculate Supplementation Requirement

Deficit to reach minimum 50 mg/L:

Alk_deficit (mg/L) = 50 - (-83) = 133 mg/L
Alk_deficit (kg/d) = (133 mg/L × 1200 m³/d) / 1000 = 159.6 kg CaCO₃/d

NaHCO₃ required:

NaHCO₃_dose = 159.6 / 0.617 = 258.7 kg/d

Step 7: Storage and Dosing

28-day storage:

Storage_mass = 258.7 × 28 = 7,244 kg
Storage_volume = 7,244 / 1,100 = 6.59 m³

Select: 7.0 m³ hopper (with 6% margin)

Dosing rate (if dissolved): Typical NaHCO₃ solution: 10-20% concentration (100-200 g/L)

At 15% (150 g/L = 0.15 kg/L):

Solution_volume = 258.7 / 0.15 = 1,725 L/d = 71.9 L/h

Dry feeder alternative:

Feed_rate = 258.7 kg/d / 24 h = 10.8 kg/h

Design Summary

Parameter	Value	Notes
Influent alkalinity	200 mg/L CaCO₃
Nitrification consumption	264.2 mg/L	44.4 kgN/d oxidized
Chemical dosing consumption	18.8 mg/L	30 kg Fe/d
Total consumption	283.0 mg/L
Unsupplemented effluent alk	-83 mg/L	Deficit!
Supplementation required	159.6 kg CaCO₃/d	To reach 50 mg/L
NaHCO₃ dose	258.7 kg/d	Sodium bicarbonate
Storage	7.0 m³	28-day supply
Dosing rate (15% solution)	71.9 L/h	If liquid dosing
Dosing rate (dry)	10.8 kg/h	If dry feeder
Final effluent alkalinity	50 mg/L CaCO₃	✓ Minimum met

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Example with Denitrification

If the above plant had an anoxic zone achieving 80% denitrification:

NO₃-N produced from nitrification:

NO₃ = 44.4 kgN/d (equals NH₄ oxidized)

NO₃-N denitrified (80% removal):

NO₃_denitrified = 44.4 × 0.80 = 35.5 kgN/d

Alkalinity recovered:

Alk_recovered = 35.5 × 3.57 = 126.7 kg CaCO₃/d
Alk_recovered (mg/L) = (126.7 × 1000) / 1200 = 105.6 mg/L

Net alkalinity consumption:

Net_consumption = Nitrification - Denitrification + Chemical
Net_consumption = 264.2 - 105.6 + 18.8 = 177.4 mg/L

Effluent alkalinity:

Alk_effluent = 200 - 177.4 = 22.6 mg/L (still below minimum!)

Supplementation still required:

Alk_deficit = 50 - 22.6 = 27.4 mg/L
Alk_deficit (kg/d) = (27.4 × 1200) / 1000 = 32.9 kg CaCO₃/d

NaHCO₃ = 32.9 / 0.617 = 53.3 kg/d

Benefit of denitrification: Reduces NaHCO₃ requirement from 258.7 to 53.3 kg/d (79% reduction!)

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UK Water Company Practices

UK Industry Practice

• Minimum effluent alkalinity: 50 mg/L (strictly enforced)
• Prefer NaHCO₃ for supplementation
• 28-day chemical storage minimum
• Monthly alkalinity monitoring mandatory

UK Industry Practice

• Minimum effluent alkalinity: 50 mg/L
• Some works use caustic soda (NaOH) for larger sites
• Continuous pH monitoring on larger works (> 10,000 PE)
• Lime dosing common on very large works

UK Industry Practice

• Minimum alkalinity: 50 mg/L CaCO₃
• pH monitoring required (target 6.5-8.0)
• Prefer NaHCO₃ for ease of handling
• Some rural works use hydrated lime slurry

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Design Considerations

pH and Alkalinity Relationship

pH depression from nitrification:

Typical influent pH: 7.5-8.0
After nitrification (without supplementation): 6.0-6.5
With supplementation to 50 mg/L alkalinity: 6.8-7.5

Critical pH for nitrification:

• Optimum: 7.5-8.5
• Acceptable: 6.5-8.0
• Inhibited: < 6.5 or > 9.0

If pH < 6.5 even with 50 mg/L alkalinity: Additional buffering may be needed (increase target to 75-100 mg/L).

Dosing Point Selection

Before biological treatment (most common):

• Dose into primary effluent or biofilter inlet
• Ensures adequate buffering throughout nitrification
• Prevents pH drop that could inhibit process

After biological treatment (less common):

• Only if supplementation is for compliance, not process requirement
• Risk of pH inhibition in biofilter

Split dosing:

• Part before biofilter, part after
• Optimizes chemical use
• More complex control

Dosing Control Strategies

Flow-proportional (most common):

NaHCO₃_rate = k × Q

Where k = dosing constant (kg/m³)

Compound loop (advanced):

• Primary: Flow-proportional
• Trim: pH or alkalinity analyzer feedback
• Adjusts dose ±20% based on effluent measurement

Feed-forward (very advanced):

• Measure influent NH₄-N continuously
• Calculate required dose in real-time
• Adjust for flow and load variations

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Monitoring and Compliance

Key Parameters

Parameter	Frequency	Method	Target
Influent alkalinity	Weekly	Laboratory titration	Baseline
Effluent alkalinity	Weekly	Laboratory titration	≥ 50 mg/L
Effluent pH	Daily	Online or grab	6.5-8.5
NH₄-N removal	Daily	Laboratory	Design efficiency
NaHCO₃ stock	Weekly	Level indicator	> 7 days

Alkalinity Measurement

Standard method: Titration to pH 4.5 endpoint (methyl orange)

Interpretation:

• Total alkalinity = HCO₃⁻ + CO₃²⁻ + OH⁻ (all as mg/L CaCO₃)
• In wastewater (pH 6-8), mainly HCO₃⁻ (bicarbonate)

Online measurement:

• Some larger works have online alkalinity analyzers
• Useful for feed-forward control
• Requires regular calibration and maintenance

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Common Issues

Insufficient Alkalinity Despite Supplementation

Symptoms:

• pH dropping below 6.5
• Nitrification performance declining
• Effluent NH₄-N increasing

Causes:

• Under-dosing NaHCO₃ (pump calibration error)
• Stock depletion (ran out of chemical)
• Increased load (higher NH₄-N or trade effluent)

Solutions:

• Verify dosing pump calibration
• Check chemical stock levels
• Increase dose rate
• Confirm influent alkalinity hasn't decreased

Over-Dosing

Symptoms:

• Effluent alkalinity > 150 mg/L
• High chemical consumption
• Wasted cost

Causes:

• Conservative design (over-specified)
• Influent alkalinity higher than design
• Lower NH₄-N load than design

Solutions:

• Reduce dose rate gradually
• Monitor effluent alkalinity weekly
• Target 60-80 mg/L effluent (margin above 50 mg/L)

Variable Alkalinity

Symptoms:

• Fluctuating effluent pH and alkalinity
• Inconsistent nitrification performance

Causes:

• Variable influent load (trade effluent discharges)
• Batch dosing instead of continuous
• Poor mixing of dosed chemical

Solutions:

• Continuous dosing (not batch)
• Flow-proportional control
• Add static mixer at dose point
• Investigate trade effluent contributions

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Cost Considerations

Chemical Costs (Typical UK, 2024)

Chemical	Price (£/tonne)	Delivered (£/tonne)	Relative Cost per kg CaCO₃
Sodium bicarbonate	£400-500	£450-550	1.0×
Caustic soda (50%)	£250-350	£300-400	0.6×
Soda ash	£300-400	£350-450	0.8×
Hydrated lime	£100-150	£120-180	0.4×

Example cost calculation:

NaHCO₃_dose = 250 kg/d
Annual_consumption = 250 × 365 = 91,250 kg = 91.25 tonnes
Price = £500/tonne
Annual_cost = 91.25 × £500 = £45,625/year

Optimization Strategies

Denitrification for alkalinity recovery:

• Anoxic zone recovers 3.57 kg CaCO₃/kg NO₃-N
• Can reduce or eliminate need for supplementation
• Requires carbon source (e.g., methanol, glycerol)

Trade-off:

Cost_NaHCO₃ vs Cost_carbon_source + Cost_anoxic_zone

Typically denitrification is cost-effective for:

• NO₃-N removal required anyway (N consent)
• Large works (economies of scale)
• Low-cost carbon available (internal or trade effluent)

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Related Calculations

• Biofilters: Nitrification in biofilters producing alkalinity demand
• Chemical Dosing: Alkalinity consumption from Fe/Al dosing
• Flow and Load: NH₄-N load determination

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References

• Metcalf & Eddy: Wastewater Engineering, alkalinity and pH control
• WEF Manual of Practice: Biological nutrient removal, alkalinity requirements
• CIWEM Best Practice: Alkalinity management in BNR systems

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