Upgrading and Efficiency Gains of Fine Bubble Diffuser Membranes in Municipal Wastewater Treatment Plants
The aeration system, a core component of the activated sludge wastewater treatment process, directly impacts treatment efficacy and operational costs. Statistics show that aeration can account for 40% to 60% of a typical WWTP's total energy consumption. The diffuser membrane, the key medium for oxygen transfer, determines the oxygen transfer efficiency (OTE) and energy consumption level. Over time, membranes commonly suffer from aging, clogging, and damage, leading to decreased OTE and significantly increased energy use.
China has over 4,000 municipal WWTPs with an annual treatment capacity exceeding 60 billion m³. The annual electricity consumption of aeration systems exceeds 100 billion kWh. Therefore, optimizing aeration systems and improving OTE is crucial for achieving "Dual Carbon" goals. However, empirical studies on diffuser membrane replacement in domestic municipal WWTPs are scarce, particularly regarding comprehensive assessments of energy consumption and treatment efficiency.
1. Research Status of Aeration System Optimization
International research focuses on membrane material improvement and aeration method innovation. For instance, Germany's Supratec developed EPDM membranes with an oxygen transfer efficiency of 0.33, and US EPA studies indicate micro-bubble aeration saves over 30% energy compared to traditional methods. Domestic researchers like Hu Peng found optimization could reduce plant energy use by 15%–25%.
However, existing research has shortcomings: predominance of lab studies over real-world cases, focus on short-term effects over long-term stability, and analysis of single indicators over comprehensive benefits. This study, through long-term monitoring, systematically evaluates the comprehensive impact of membrane replacement on treatment efficiency and energy consumption, addressing a research gap.
2. Research Content and Methodology
This study used a comparative analysis of operational data before and after membrane replacement (June 2020 – March 2022) at a WWTP in Dongguan, Guangdong. Key research areas included: changes in pollutant removal efficiency, aeration system energy consumption characteristics, OTE improvement mechanisms, and techno-economic analysis. Methods involved field monitoring and lab analysis.
2.1 Subject Overview
The case WWTP has a design capacity of 20,000 m³/d, uses an A²/O process for municipal sewage, serves approximately 150,000 people, and has an actual daily flow of 18,000–24,000 m³. The original rubber fine bubble diffusers had been in operation for 8 years, showing significant aging.
2.2 Upgrade Plan Design
2.2.1 Oxygen Demand Calculation
Based on water quality/quantity, the aerobic zone's daily oxygen demand was >275 kg/h. Considering service area, oxygen supply capacity, and potential clogging, the required air supply was calculated to be 2,400–4,800 m³/h (influent 1,200 m³/h, Air-to-Water Ratio 2–4). This equated to 480 meters of diffuser tubing (air supply 5–10 m³/h per meter), with a service area under 2.5 m² per meter, allowing a maximum oxygen supply exceeding 380 kg/h.
2.2.2 Membrane Selection
Based on performance comparison (Table 1), considering OTE, airflow range, and cost, EPDM fine bubble membranes were selected. Key parameters: OTE 0.33 (higher than original), airflow 2–15 m³/h, service life 5–8 years, and a cost-effective unit price.
2.2.3 Manufacturer Selection
After consulting domestic suppliers and considering local experience, paddle-type EPDM diffusers were chosen for their comprehensive advantages in oxygen supply, installation structure, and price. A total of 484 meters were installed across two biological tanks. Technical parameters of different models are shown in Table 2.
2.2.4 Replacement Implementation
The replacement in June 2021 took 7 days, involving 484 meters of paddle-type diffusers. The plant maintained continuous operation by running at reduced capacity on one side. The new membranes, designed for 5 m³/h, operated at 4–8 m³/h.
2.3 Data Collection and Analysis
22 months of operational data were collected before and after replacement across four categories: water quality (influent/effluent COD, NH₃-N), operational parameters (total air volume, pressure, DO), energy consumption (aeration system electricity, aeration kWh/m³), and efficiency (OTE, Air-to-Water Ratio).
3. Changes in Pollutant Removal Efficiency
3.1 COD Removal
Post-replacement, COD removal improved significantly. Effluent COD decreased from 14.2 mg/L to 12.4 mg/L, and removal rate increased from 93.5% to 96.0%. The new system also demonstrated better stability despite fluctuating influent COD (117–249 mg/L) (Figure 1).
3.2 NH₃-N Removal
Improvement was more pronounced for NH₃-N. With stable influent levels, effluent NH₃-N decreased from an average of 2.3 mg/L to 0.85 mg/L, and removal rate reached 94.1% (Figure 1). This is attributed to more uniform aeration distribution, promoting nitrifier growth and activity, ensuring stable NH₃-N compliance.
4. Aeration System Energy Consumption Characteristics
4.1 Air-to-Water Ratio
The Air-to-Water Ratio decreased from 3.4 to below 2.0, while aerobic tank DO remained stable at 0.5–1 mg/L (Figure 2), indicating higher efficiency and stability.
4.2 Aeration Energy per Cubic Meter of Water
Aeration energy consumption decreased from 0.073 kWh/m³ to 0.052 kWh/m³, a reduction of 28.3%. The Energy saving effect was stable across months (Figure 3), showing consistent reliability.
4.3 Energy Consumption per Unit of Pollutant Removed
This metric decreased from 0.32 kWh/kg to 0.24 kWh/kg, a 25% reduction (Figure 4). This indicates the new membranes not only reduced absolute energy use but also improved the efficiency of energy use for pollutant removal.
5. Mechanisms for Improved Oxygen Utilization Efficiency
5.1 Change in Oxygen Transfer Efficiency
OTE increased from 15.10% to 24.75%, a 63.9% improvement (Figure 5). This is due to the optimized micro-pore structure and more uniform bubble distribution of the new membranes, enhancing oxygen mass transfer. Advanced nanotechnology allowed finer, more uniformly distributed pores, increasing diffusion and solubility.
5.2 Optimization of Operational Parameters
As shown in Table 3, post-replacement, total air volume decreased by 18.4% while maintaining DO between 0.5–1 mg/L. The Air-to-Water Ratio reduced from 3.4:1 to 2.0:1, OTE increased by 63.9%, and aeration energy per m³ decreased by 28.3%. These comprehensive optimizations improved energy use, operational efficiency, and water quality.
6. Techno-Economic Analysis
6.1 Investment Payback Period
The total investment was 163,900 CNY (membranes, transport, installation, commissioning). Based on energy savings of 0.021 kWh/m³, an electricity price of 0.7 CNY/kWh, and an average daily flow of 24,000 m³, annual electricity savings are 128,800 CNY. The simple payback period is approximately 15 months, indicating significant economic benefits.
6.2 Environmental Benefits
Based on annual treatment of 8.76 million m³, annual electricity savings are 184,000 kWh, equivalent to reducing CO₂ emissions by 184 tons. Improved pollutant removal enhances environmental benefits and ensures more stable effluent compliance, reducing environmental risks.
7. Conclusion
Replacing with EPDM fine bubble diffuser membranes significantly increased OTE to 24.75% and reduced aeration energy consumption by 28.3%, demonstrating good techno-economic performance. The new system increased COD and NH₃-N removal rates to 96.0% and 94.1%, respectively, enhanced system resilience to load fluctuations, and achieved a simple payback period of about 15 months. This approach is suitable for energy-intensive municipal WWTPs seeking quality and efficiency improvements, showing significant promotional value.