Short answer: If space and effluent quality are priorities in a new facility installation, prefer MBR. If the capacity of the existing conventional activated sludge (CAS) plant is insufficient, converting to MBR by adding a UF membrane module instead of a final clarifier is generally the most sensible investment — capacity increases by 50-100% without changing the footprint.
What is Conventional Activated Sludge (CAS)?
The activated sludge process is a biological treatment method that aerobically breaks down organic matter (KOİ, BOİ) in wastewater using a suspended mass of microorganisms. Developed in Manchester in 1914, it is the industry standard used in 80% of municipal wastewater treatment plants worldwide today.
Typical configuration: aeration tank → final clarifier → return/waste sludge line. The MLSS value operates between 2,000-4,000 mg/L, with an F/M ratio of 0.2-0.5 kg BOİ/kg MLSS·day.
What is MBR? Difference from Activated Sludge
MBR (Membrane Bioreactor) combines conventional activated sludge with ultrafiltration (UF) or microfiltration (MF) membranes. Instead of a final clarifier, a membrane module is used to retain 100% of the biomass.
This structural change results in three major outcomes:
- MLSS increases 4-5 times (8,000-15,000 mg/L) → 4-5 times the yield in the same volume
- Sludge age increases (SRT 20-40 days) → nitrification is safe, less sludge
- Physical barrier → AKM, bacteria, viruses are 100% retained
Detailed Comparison Based on 12 Criteria
| Criterion | Conventional Activated Sludge | MBR |
|---|---|---|
| MLSS | 2,000-4,000 mg/L | 8,000-15,000 mg/L |
| Reactor volume | Reference (100%) | 40-60% |
| Effluent AKM | 10-30 mg/L | < 1 mg/L |
| Effluent BOİ | 10-25 mg/L | < 5 mg/L |
| Bacteria/virus | 2-3 log (disinfection required) | 5-6 log (physical barrier) |
| Final clarification | Required (large tank) | Not required |
| Bulking risk | High | None (membrane barrier) |
| Sludge production | 0.6-0.8 kg/kg KOİ | 0.3-0.5 kg/kg KOİ |
| Energy consumption | 0.3-0.6 kWh/m³ | 0.8-1.5 kWh/m³ |
| Investment cost | Low | High (membrane) |
| Water recovery | Additional treatment required | Direct (irrigation, cooling) |
| Automation | Manual/semi-automatic | Full automation with SCADA + PLC |
Land Requirement for Treating the Same Wastewater
Land comparison based on a 1000 m³/day urban wastewater example:
- CAS plant: Aeration tank ~500 m² + final clarification ~250 m² + sludge treatment = total ~1,000-1,200 m²
- MBR plant: Reactor + membrane tank ~300 m² + sludge treatment = total ~450-550 m²
MBR uses on average 50-55% less land. This difference is extremely critical in urban facilities, plots within organized industrial zones, and within factories.
Effluent Quality Comparison
| Parameter | Typical CAS Effluent | Typical MBR Effluent | Reuse Limit |
|---|---|---|---|
| KOİ (mg/L) | 40-80 | 15-30 | < 50 |
| BOİ₅ (mg/L) | 10-25 | < 5 | < 10 |
| AKM (mg/L) | 10-30 | < 1 | < 5 |
| NH₄-N (mg/L) | 1-5 | < 0.5 | < 1 |
| Total phosphorus (mg/L) | 1-3 | 0.3-0.5 | < 1 |
| E.coli (CFU/100mL) | 10³-10⁴ | < 10 | < 100 |
As seen, MBR effluent directly meets the EU Water Framework Directive and Turkey's SKKY reuse class A standard. Advanced treatment (sand filter + UV + disinfection) is required for CAS effluent.
Converting an Old CAS Plant to MBR (Retrofit)
When the existing conventional plant capacity is insufficient, there are three options:
- Building a new line — expensive, requires space
- Upgrading with MBBR — capacity +30-50%, effluent quality improves marginally
- Converting to MBR — capacity +50-100%, effluent quality leaps
Typical steps for MBR conversion:
- The existing final clarifier is emptied → converted to a membrane tank
- Submerged UF membrane modules (PVDF or PES) are installed
- Permeate pump, vacuum system, CIP line, NaOCl/citric acid tanks are added
- Aeration system is upgraded (for high MLSS)
- Automation is integrated with SCADA + PLC
- Commissioning + 4-6 weeks of MLSS adaptation
Typical conversion time: 3-5 months. The investment cost is 40-50% lower compared to a new plant.
In Which Cases is CAS Still the Right Choice?
MBR is not always superior. CAS is still sensible under the following conditions:
- Ample land + loose discharge limits: Rural municipalities, receiving environments rivers/seas
- Low investment budget + long ROI: State/local government small plants
- High hydraulic fluctuations: Agricultural seasonal flows (MBR is less tolerant)
- No water recovery target: Treatment solely for discharge purposes
Important note: If water recovery is required in new plants, the total cost of CAS + advanced treatment (UF/RO) is generally higher than direct MBR. Make the comparison based on LCC (Life Cycle Cost).
Decision Framework: 4 Critical Questions
- Have your discharge limits tightened in the last 5 years? Yes → It’s time to transition to MBR.
- Is your current plant capacity insufficient? Yes → CAS→MBR retrofit is the fastest solution.
- Does your water bill account for 5%+ of your operational costs? Yes → Recovery with MBR pays for itself.
- Is the cost of sludge disposal increasing? Yes → MBR sludge production is 30-40% lower.
Conclusion
Conventional activated sludge is still the correct solution for the low cost + ample land equation. However, three trends in the last decade (tightening discharge limits, mandatory water recovery, land constraints) are directing new investments towards MBR. Approximately 60% of municipal and organized industrial zone facilities in Turkey will transition to MBR by 2030.
You can also check our comparison of MBR and MBBR to position the three technologies against each other. Share the characterization of your wastewater, and the Arsistek engineering team will provide a technical + economic comparative report within 72 hours.
Atıksu arıtma uzmanı, çevre mühendisi. Endüstriyel su arıtma projelerinde 20+ yıl saha deneyimi.
