Your discharge permit is tightening. Effluent limits are dropping to 5 mg/L BOD and TSS — sometimes lower.
Your engineering team is split. Half are pushing for MBR (Membrane Bioreactor) because “it’s the best available technology.” The other half want conventional activated sludge with tertiary filtration because “MBR is expensive and complicated.”
Both sides have valid points. And that’s exactly the problem.
MBR can deliver exceptional effluent quality in a compact footprint — but it comes with higher CAPEX, membrane replacement costs, and operational complexity. Conventional treatment is proven and simpler — but requires more space and may struggle to meet stringent limits without extensive upgrades.
This article cuts through the marketing noise and lays out when MBR makes financial and technical sense, and when conventional treatment is the smarter choice.
What MBR Actually Does Differently
The Core Difference
Both MBR and conventional activated sludge use biological treatment — microorganisms consume organic pollutants in an aeration tank.
The difference is how solids are separated from treated water:
Conventional system:
Mixed liquor flows to a secondary clarifier where solids settle by gravity. Clarified water overflows, and settled sludge is returned to the aeration tank.
MBR system:
Mixed liquor is filtered directly through submerged or external membranes (typically 0.04-0.4 micron pore size). Membranes physically block all suspended solids and bacteria, producing ultra-clear effluent.
The result:
- MBR effluent: Consistently <1 mg/L TSS, <5 mg/L BOD, near-zero turbidity
- Conventional effluent (secondary only): 10-30 mg/L TSS, 10-20 mg/L BOD
If your discharge limits are tight, MBR can meet them without tertiary filtration. If limits are moderate, conventional treatment with proper design is sufficient.
When MBR Makes Sense
1. Stringent Effluent Requirements
Choose MBR if:
- BOD/TSS limits are ≤5 mg/L
- Discharge to sensitive receiving waters (rivers, lakes, coastal zones)
- Water reuse is required (irrigation, industrial reuse, indirect potable reuse)
- Nutrient limits are strict (MBR supports high MLSS concentrations, improving nitrogen removal)
Why it works:
MBR membranes provide an absolute barrier to solids. No secondary clarifier upsets, no solids carryover during peak flows or temperature swings.
2. Severe Space Constraints
Choose MBR if:
- You’re retrofitting an existing plant with no room for clarifiers
- You’re building in dense urban areas where land costs are high
- Your site has challenging terrain (steep slopes, irregular shapes)
Footprint comparison (typical 5,000 m³/day plant):
- Conventional activated sludge + clarifiers: 1,500-2,000 m²
- MBR: 800-1,200 m² (40-50% smaller)
3. High Variability in Influent Flow or Load
Choose MBR if:
- You have significant diurnal or seasonal flow variations
- Influent strength varies widely (industrial discharges, tourist areas)
- Peak hydraulic loads exceed 2-3× average flow
Why it works:
MBR performance is decoupled from hydraulic loading. Clarifiers struggle during peak flows (shorter settling time = solids carryover). Membranes maintain effluent quality regardless of flow spikes.
When Conventional Treatment Is the Better Choice
1. Moderate Effluent Limits and Adequate Space
Stick with conventional if:
- Discharge limits are 10-20 mg/L BOD/TSS (achievable with conventional secondary + filtration)
- Land is available and affordable
- CAPEX budget is limited
Why it works:
Conventional systems are 30-50% cheaper to build. If space is not a constraint and discharge limits are not extreme, the cost premium for MBR is hard to justify.
2. Limited Operational Expertise
Stick with conventional if:
- Your operators have limited experience with membrane systems
- Local technical support for membrane suppliers is weak
- Spare parts logistics are challenging (remote locations, long lead times)
Why it works:
Conventional clarifiers are forgiving. Operators can visually monitor sludge blankets and make adjustments. MBR requires understanding transmembrane pressure (TMP), chemical cleaning protocols, and membrane integrity testing.
3. High Energy Costs
Stick with conventional if:
- Electricity costs are very high (>€0.15-0.20 per kWh)
- Energy availability is unreliable
Energy comparison (typical per m³ treated):
- Conventional activated sludge: 0.3-0.5 kWh/m³
- MBR: 0.6-1.0 kWh/m³ (due to membrane aeration and permeate pumping)
Over 10-15 years, higher energy costs can offset MBR’s space savings.
The Real Cost Comparison
CAPEX: MBR Costs 30-50% More
Typical costs for 5,000 m³/day plant:
- Conventional (aeration + clarifiers + filtration): €1.5-2.5 million
- MBR: €2.0-3.5 million
The difference is driven by:
- Membrane modules (initial supply)
- Higher-capacity blowers for membrane scouring
- More sophisticated control systems
OPEX: Where the Math Gets Tricky
MBR advantages:
- Smaller footprint = lower land costs (if land is expensive)
- No tertiary filtration = simpler process, fewer chemicals
- Better sludge settleability = potentially lower sludge production
MBR disadvantages:
- Membrane replacement: €50,000-150,000 every 7-10 years
- Higher energy consumption: +50-100% vs. conventional
- Chemical cleaning: Periodic citric acid, sodium hypochlorite, or caustic cleaning
Typical 10-year lifecycle cost (5,000 m³/day plant):
- Conventional: €3.5-4.5 million (CAPEX + OPEX)
- MBR: €4.0-5.5 million (CAPEX + OPEX)
MBR is rarely cheaper over the lifecycle — but it may be the only way to meet discharge limits or fit into available space.
Key Design Choices That Make or Break MBR Performance
1. Membrane Configuration
Submerged (immersed) membranes:
- Membranes installed directly in aeration tank or separate membrane tank
- Lower energy (gravity-driven permeate flow)
- Easier maintenance (modules can be lifted out for cleaning)
- Most common choice for municipal and industrial MBR
External (sidestream) membranes:
- Mixed liquor pumped through external membrane modules
- Higher energy (pumping required)
- More compact, but more complex piping
- Used for high-solids applications or retrofits with limited tank space
2. Membrane Material
PVDF (polyvinylidene fluoride):
- Most common
- Good chemical resistance, suitable for chlorine cleaning
- Typical pore size: 0.04-0.1 microns
PES (polyethersulfone):
- Higher flux rates
- Less chlorine tolerance
- Slightly lower cost
PTFE (polytetrafluoroethylene):
- Premium option, excellent chemical resistance
- Higher cost
3. Aeration System Design
Biological aeration:
Provides oxygen for microorganisms (same as conventional systems).
Membrane scouring aeration:
Coarse bubble air scours membrane surfaces to prevent fouling. This is 50-70% of total MBR energy consumption.
Critical design tip:
Undersized membrane scouring leads to rapid fouling and frequent cleanings. Oversized aeration wastes energy. Proper design balances flux rate, MLSS concentration, and air flow.
4. Flux Rate Selection
Flux: The rate at which water passes through the membrane (L/m²/hr or LMH).
- Conservative design: 15-20 LMH (municipal wastewater)
- Aggressive design: 25-30 LMH (requires excellent pre-screening and more frequent cleaning)
Lower flux = longer membrane life, less fouling, but more membrane area required (higher CAPEX).
What to Watch For When Evaluating MBR Suppliers
Red Flag #1: Overly Aggressive Flux Rates
Some suppliers quote 30-35 LMH to reduce membrane area and win on price. These numbers work in pilot tests with ideal conditions — but lead to chronic fouling in real plants.
Ask:
- What is the design flux under average and peak conditions?
- What are the actual operating flux rates at their reference plants?
Red Flag #2: Membrane Replacement Costs Not Disclosed
Membranes degrade over time (7-10 years typical lifespan). Replacement costs can be €50,000-150,000 for a mid-size plant.
Ask:
- What is the cost per m² for replacement membranes?
- What is the expected membrane lifespan under your operating conditions?
- Are replacement membranes readily available, or do they require long lead times?
Red Flag #3: Inadequate Pre-Screening
Hair, fibers, and debris cause membrane fouling. If the supplier does not specify fine screening (1-3 mm) upstream of the MBR, expect problems.
Ask:
- What screening is included in the system design?
- Is there a grit removal system?
Red Flag #4: No Local Technical Support
MBR systems require periodic optimization: TMP monitoring, cleaning protocols, flux adjustments. If the supplier has no local service team, you will struggle.
Ask:
- Where is the nearest service center?
- What is the response time for technical support?
- Do they offer remote monitoring and troubleshooting?
How G-LINK Helps You Make the Right Choice
We do not default to MBR just because it is “advanced technology.” We help you choose the right biological treatment approach for your specific situation.
Our Process:
1. Requirements Analysis
We start by clarifying:
- What are your actual discharge limits (not just “tight,” but specific numbers)?
- What is your available site footprint?
- What is your CAPEX and OPEX budget over 10-15 years?
- What is your team’s operational experience?
Based on this, we recommend MBR, conventional treatment, or hybrid options (e.g., conventional + tertiary filtration).
2. Lifecycle Cost Modeling
We build a 10-15 year cost model comparing:
- CAPEX (equipment, civil, installation)
- OPEX (energy, chemicals, membrane replacement, labor)
- Risk factors (land availability, regulatory changes, future capacity expansion)
This shows the true cost difference, not just equipment prices.
3. Technology and Supplier Comparison
If MBR is the right choice, we compare offerings from multiple membrane suppliers:
- Module design and materials
- Flux rates and fouling control strategies
- Spare parts availability and costs
- Local service and support capabilities
4. Complete System Integration
MBR is not just membranes. A functional system includes:
- Pre-treatment: Fine screens, grit removal, equalization
- Biological treatment: Aeration system, nutrient removal zones (if needed)
- Membrane system: Modules, permeate pumps, backwash system, chemical cleaning setup
- Post-treatment: Disinfection (UV or chlorine), pH adjustment
- Automation: PLC-based control, TMP monitoring, cleaning-in-place (CIP) automation
We ensure all components are properly sized and integrated — not just a collection of separate equipment.
5. Commissioning and Performance Optimization
MBR systems require startup optimization:
- Gradual flux increase during biomass acclimation
- TMP monitoring and cleaning frequency adjustments
- MLSS concentration optimization
We provide technical guidance and remote support to help achieve target performance without costly trial-and-error.
Final Thoughts
MBR is not inherently better than conventional treatment — it is a tool that solves specific problems.
Use MBR when:
- Discharge limits are very tight
- Space is severely constrained
- Water reuse is required
Use conventional treatment when:
- Discharge limits are moderate
- Space is available
- CAPEX and operational simplicity are priorities
The worst mistake is choosing technology based on trends or supplier pressure rather than your actual project requirements.
If you are evaluating biological treatment options, the most important step is understanding the true lifecycle costs and operational implications of each approach.
Trying to decide between MBR and conventional treatment? Contact us to discuss your discharge requirements and site conditions, or explore our complete biological treatment solutions.
Related resources:
- Nutrient Removal in MBR: Do You Need Separate Anoxic Zones? (coming soon)
- MBR Fouling Control: Operational Strategies to Extend Membrane Life (coming soon)