Last Updated on May 11, 2025 by Kevin Chen
Low dissolved oxygen (DO) concentration is a common and critical operational issue in wastewater treatment systems. Insufficient DO affects the regular metabolic activity of microorganisms and often leads to the overgrowth of filamentous bacteria, causing sludge bulking, elevated suspended solids in effluent, and reduced treatment efficiency. Compared to conventional processes, Membrane Bioreactor (MBR) systems offer superior regulation capabilities and adaptability in handling such issues. This article provides a systematic overview of the causes of low DO and effective strategies for mitigation in MBR operations.
Causes and Impact of Low Dissolved Oxygen
In actual operations, low DO concentrations are typically caused by:
- Aeration system malfunction or inefficiency: such as clogged diffusers or insufficient blower capacity.
- Sharp load fluctuations: result in rapid oxygen consumption due to high organic loading.
- Elevated water temperature: reduces the solubility of oxygen.
- Severe sludge bulking: leads to poor oxygen transfer within flocs.
When DO remains below 2 mg/L for an extended period, filamentous bacteria tend to dominate. This results in increased sludge volume index (SVI), poor sludge settling, turbid effluent, foam formation, and even system failure.
How to Fix Low Dissolved Oxygen in MBR Systems?
While MBR systems have self-regulating capacity, persistent low DO conditions require timely intervention through the following measures:
1. Optimize Aeration System
- Routine inspection and maintenance: Regular cleaning of diffusers and performance checks on blowers.
- Increase aeration intensity: Especially during peak inflow hours to maintain DO levels between 2–4 mg/L.
- Segmented or intermittent aeration: Improves oxygen transfer and avoids local anoxia.
2. Adjust Sludge Recirculation and Wasting
- Increase return sludge ratio: Typically 50–100%, to boost microbial density in the aeration tank.
- Enhance sludge wasting: Reduce sludge retention time (SRT) to 5–10 days, minimizing filamentous bacteria accumulation.
3. Apply Targeted Chemical Dosing
- Chlorination or H₂O₂ addition: Low doses of sodium hypochlorite or hydrogen peroxide can suppress filamentous bacteria growth.
- Coagulant addition: Polyaluminum chloride (PAC) or aluminum sulfate enhances sludge flocculation and settling.
- Inert material dosing: Adding materials like diatomaceous earth or clay increases sludge density and settling performance.
4. Balance Nutrient Ratios
- Supplement nitrogen and phosphorus: Maintain the optimal BOD₅:N:P ratio of 100:5:1.
- Carbon source dosing: For influents with a low C/N ratio, add methanol or sodium acetate to support denitrifiers and regulate microbial balance.
5. Control pH and Temperature
- Maintain pH between 6.5 and 8.5 using acid or alkali as needed.
- Keep temperature within 20–35°C by insulating or cooling systems as necessary.
6. Introduce Biological Competition
- Seeding with beneficial strains: Reinforces healthy microbial dominance over filamentous species.
- Add bio-carriers: Provides attachment surfaces for beneficial microbes, reducing filamentous bacteria’s niche space.
Advantages of MBR in DO Regulation
Compared to conventional activated sludge processes, MBR systems offer the following advantages:
1. Higher Biomass Concentration
MBR systems operate with significantly higher MLSS, enhancing organic removal capacity and reducing oxygen stress during load peaks.
2. Longer Sludge Age and Lower Organic Loading
The extended sludge age in MBR promotes a stable microbial community structure, limiting the overgrowth of filamentous bacteria and improving process resilience.
3. Membrane Filtration Replaces Sedimentation
Even if moderate sludge bulking occurs, membrane separation ensures effluent quality and prevents sludge washout.
4. Advanced Monitoring and Automation
MBR systems often have online DO sensors and variable aeration control systems for real-time adjustments and precise regulation.
Conclusion
Low dissolved oxygen is a controllable challenge with proper system design and operational strategies. With their inherent stability and advanced automation, MBR systems offer a robust solution for consistent and high-efficiency wastewater treatment. Understanding the mechanisms and implementing timely interventions are key to maintaining optimal performance and preventing sludge bulking issues.
