Phosphorus Removal in MBR Treatment: Method, Efficiency, Cost

Phosphorus removal in MBR (Membrane Bioreactor) systems is one of the most critical steps in modern wastewater treatment. The 2 mg/L discharge limit set for total phosphorus (TP) in the Water Pollution Control Regulation can be reduced to 1 mg/L and even 0.5 mg/L levels for sensitive receiving environments. MBR technology is an advanced treatment solution that can easily meet these limits with proper design and operation.

In this guide, we share phosphorus removal methods in MBR systems, real field data, cost analysis, and practical knowledge gained from our 20+ years of industrial wastewater treatment experience.

What is Phosphorus Removal in MBR Systems?

Phosphorus removal in MBR systems is the process of removing both dissolved and particulate phosphorus from wastewater before it is discharged into the receiving environment, using both biological and chemical processes. The ultrafiltration (UF) membrane of the MBR provides a 30-50% higher particulate phosphorus removal efficiency compared to conventional systems.

Typical Performance Values

• Influent total phosphorus (TP): 6-15 mg/L (domestic), 10-50 mg/L (industrial)
• Effluent TP (biological only): 1-3 mg/L
• Effluent TP (biological + chemical): 0.3-0.8 mg/L
• Effluent AKM: < 1 mg/L (thanks to the UF membrane)
• Efficiency: 92-98% total phosphorus removal

Phosphorus Removal Methods in MBR

1. Biological Phosphorus Removal (EBPR – Enhanced Biological Phosphorus Removal)

The EBPR method is based on the principle that phosphate-accumulating organisms (PAO – Phosphate Accumulating Organisms) accumulate phosphorus inside the cell in an environment that alternates between anaerobic and aerobic conditions.

Process flow:

1. Anaerobic zone: PAOs consume VFA (Volatile Fatty Acids) and release intracellular polyphosphate
2. Aerobic zone: The same PAOs then take up phosphorus excessively from the environment (luxury uptake)
3. Sludge discharge: High phosphorus content sludge is removed from the system

Advantages:

• No chemical usage (low operating cost)
• Sludge can be valorized (potential for phosphorus recovery)
• Sustainable (positive in ESG reporting)

Disadvantages:

• Requires operator control (sensitive process)
• Efficiency decreases in cold weather (T < 10 °C)
• Cannot guarantee effluent TP < 1 mg/L alone

2. Chemical Phosphorus Removal (Precipitation)

The chemical method involves precipitating phosphorus with metal salts (aluminum or iron-based). The most commonly used chemicals in MBR systems are:

• Ferric Chloride (FeCl₃): Most common, effective, economical
• Aluminum Sulfate (Al₂(SO₄)₃ – Alum): Effective at low pH
• PAC (Polyaluminum Chloride): Produces less sludge
• Ferrous Sulfate (FeSO₄): Economical but requires oxidation

Chemical Dosage Calculation (Practical Formula)

Stoichiometrically, 1 mg P requires 1.8 mg Fe or 0.87 mg Al. Due to inefficiencies occurring in the field, the dosage is taken 1.5-2 times higher:

• FeCl₃ dosage: 3-5 mg Fe / mg P (to be removed)
• Alum (Al₂(SO₄)₃) dosage: 1.5-2.5 mg Al / mg P

Example calculation: In an MBR facility with a flow rate of 1000 m³/day, if the influent TP = 8 mg/L and the target effluent = 0.5 mg/L, the phosphorus to be removed = 7.5 mg/L = 7.5 kg/day. Accordingly, FeCl₃ requirement: 7.5 × 4 = 30 kg Fe/day (≈ 75 kg FeCl₃ 40% solution).

3. Hybrid Method (Most Common and Recommended)

The most successful results in industrial wastewater treatment are achieved with a biological + chemical hybrid approach:

• Reduce effluent TP to 1-2 mg/L with EBPR
• Use online chemical dosing to bring it down below 0.3-0.5 mg/L during peak moments
• Chemical consumption occurs only when necessary → minimum operating cost

Advantage of MBR in Phosphorus Removal: UF Membrane

While the effluent in classical activated sludge systems is separated by sedimentation, an 0.04-0.4 micron pore size UF membrane is used in MBR. This provides a critical advantage in terms of phosphorus removal:

• Particulate phosphorus is completely retained (5-10% escapes in classical systems)
• Can operate with high MLSS (8-12 g/L) → increases biological activity
• More compact bioreactor → flexible chemical dosing point
• During backwashing, the accumulation on the membrane is removed, ensuring continuous high efficiency

Cost Comparison: Biological vs Chemical vs Hybrid

A 1-year comparison in an industrial MBR facility with a flow rate of 1000 m³/day (example):

• Only biological (EBPR):
  • Investment: 15-20% more expensive (extra zone)
  • Operation: Very low (no chemicals)
  • Efficiency risk: Can drop 20-30% in cold weather
• Only chemical:
  • Investment: Low (dosing pump + tank)
  • Operation: Annual high chemical cost (approximately 6-12 times that of biological) (depending on flow rate)
  • Sludge production: Increases by 20-30% (waste disposal cost)
• Hybrid (Recommended):
  • Investment: Medium
  • Operation: Annual medium chemical cost (approximately 2-4 times that of biological, only during peak times) (only during peaks)
  • Efficiency: Guarantees 0.3-0.5 mg/L
  • 25-35% total cost savings over 3-5 years

7 Common Mistakes Encountered in the Field

1. Incorrect selection of dosing point: FeCl₃ should be dosed at the end of the aerobic zone; it negatively affects PAOs in the anaerobic zone
2. Excessive chemical usage: 30% excess dosage for a 5% efficiency increase → not economical
3. Lack of online TP measurement: Real-time control is not possible with manual sampling; online TP measurement is essential for automatic dosing control
4. Short sludge age in EBPR: If SRT (Sludge Retention Time) < 8 days, PAOs cannot reproduce sufficiently
5. Membrane fouling accumulation: Iron hydroxide accumulates on the membrane; regular chemical cleaning (CIP) is essential
6. Failure to control pH: Optimum pH is 6.0-7.5; if the upper limit is exceeded, metal hydroxide accumulates and does not precipitate
7. High nitrate recirculation: Entry of NO₃⁻ into the anaerobic zone of EBPR → PAO inhibition, decreases phosphorus removal

Which Sectors is MBR + Phosphorus Removal Critical For?

• Dairy and food industry: High phosphorus detergent and milk protein wastes
• Beverage production facilities: High TP after anaerobic fermentation
• Textile finishing facilities: Phosphorus in dye complexes
• Hotels and resort complexes: Near sensitive receiving environments (sea, lake)
• Municipal wastewater facilities: New EU directives are heading towards a 0.5 mg/L limit
• Pharmaceutical and chemical facilities: Specific process wastes

Use in Conjunction with Water Recovery

After phosphorus is reduced to below 0.5 mg/L in MBR, the effluent can be sent to the RO (Reverse Osmosis) system, allowing for 85-95% water recovery. In this case, phosphorus is concentrated in the RO concentrate and becomes a recoverable by-product (for the fertilizer industry).

Conclusion: The Importance of Proper Design

Phosphorus removal in MBR systems is successful not only by adding chemical dosage but also through the integrated design of the entire process. The anaerobic zone volume, SRT, chemical dosing point, online monitoring, and sludge management are interconnected parameters.

As Arsistek Arıtma A.Ş., we have commissioned over 150 projects in the last 20 years. We offer site analysis, wastewater characterization, process design, manufacturing, installation, and long-term operational support in MBR + phosphorus removal. Contact us for consultancy and a free preliminary study for your specific project.