Review on Energy Saving and Carbon Reduction of Aeration Systems in Wastewater Treatment Plants
By the end of 2020, China had 4,326 municipal-level and above wastewater treatment plants (WWTPs), treating 65.59 billion cubic meters of wastewater annually, with an annual electricity consumption of 33.77 billion kWh, accounting for 0.45% of the national total electricity consumption. In 2020, the unit electricity consumption per cubic meter of water treated was 0.405 kWh/m³ for WWTPs implementing the Grade A standard or above of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918-2002), and 0.375 kWh/m³ for those implementing standards below Grade A. These figures are significantly higher than the average in developed countries. Although the average influent pollutant concentration in Chinese WWTPs is less than 50% of that in developed countries, the unit electricity consumption per pollutant removed is at least 100% higher. Therefore, there remains substantial potential for energy saving and carbon reduction in China's WWTPs.
The carbon emissions from WWTPs include direct and indirect emissions. According to the "Technical Specification for Low-Carbon Operation Evaluation of Wastewater Treatment Plants" (T/CAEPI 49-2022), direct carbon emissions primarily consist of CH₄, N₂O, and CO₂ from fossil fuel combustion. Indirect emissions encompass those associated with purchased electricity, heat, and chemicals. As defined by the Intergovernmental Panel on Climate Change (IPCC), CO₂ emitted from the biological degradation process in wastewater treatment is not included in the carbon emission accounting. Among the various carbon emission elements in WWTPs, electricity consumption contributes the highest proportion. Jiang Fuhai et al., based on a sample of 10 WWTPs, found that the contribution weight of electricity consumption to carbon emissions ranged from 31% to 64%. Hu Xiang et al., analyzing 22 WWTPs in the Chaohu Lake basin, reported that carbon emissions from electricity consumption accounted for 61.55% to 73.56%. The lower the influent concentration and the higher the effluent standard, the higher the proportion of direct carbon emissions, particularly those from electricity consumption. Aeration systems consume over 50% of a WWTP's total electricity. The operational effectiveness of aeration systems directly impacts nitrogen and phosphorus removal. Excessive aeration leads to unnecessary consumption of endogenous carbon sources in the wastewater, reducing the efficiency of biological nitrogen and phosphorus removal, thereby increasing the dosage of external carbon sources and phosphorus removal chemicals, which in turn raises carbon emissions from chemical consumption. Consequently, energy saving in aeration systems is key to carbon reduction in WWTPs, making research on aeration system energy-saving technologies highly significant.
1. Reasons for High Energy Consumption in Aeration Systems of Chinese WWTPs
1.1 Actual Influent Load is Lower Than Design Load
Low influent load includes both low flow rate and low pollutant concentration. It is a primary cause of excessive aeration. Over-aeration not only increases electricity consumption but also excessively depletes endogenous carbon sources in the wastewater and elevates dissolved oxygen concentrations in anaerobic and anoxic tanks, impairing nitrogen and phosphorus removal. This necessitates increased dosages of carbon sources and phosphorus removal chemicals, raising associated carbon emissions.
1.1.1 Low Flow Rate
Typically, in the initial years after a WWTP's construction, influent flow often fails to reach the design capacity due to lagging urban development or sewer network construction. Furthermore, in combined sewer system areas or regions with severe stormwater and sewage mixing, dry-weather flow is significantly lower than wet-weather flow, resulting in large flow fluctuations. This demands more precise regulation and control of aeration rates; otherwise, over-aeration during low-flow periods is common, affecting carbon, nitrogen, and phosphorus removal efficiency and increasing both electricity and chemical consumption. Figure 1 shows the variation in wastewater treatment volume in Changsha City between dry and wet seasons. Wet-season treatment volume is 30%–40% higher than in the dry season. Seasonal fluctuations in treatment volume require more precise aeration system control.
1.1.2 Low Influent Concentration
The actual influent pollutant concentrations in China's municipal WWTPs are generally much lower than design values. In WWTP design, influent quality is usually based on mid-to-long-term projections with complete sewer networks. According to the "Standard for Design of Outdoor Wastewater Engineering" (GB 50014-2021), the five-day biochemical oxygen demand (BOD₅) for domestic wastewater is calculated at 40–60 g/(person·d), generally taking 40 g/(person·d). With a per capita wastewater discharge of 200–350 L/(person·d) in most cities, the design BOD₅ concentration typically ranges from 110 to 200 mg/L. Statistics show that 68% of WWTPs in China have an actual annual average influent BOD₅ below 100 mg/L, with 40% having an annual average below 50 mg/L. From the perspective of influent concentration versus required aeration, most Chinese WWTPs have aeration systems designed with an "oversized motor for a small cart" situation-configured with high-capacity blowers while actual air demand is low. This configuration easily leads to over-aeration and increased energy consumption.
1.2 Unreasonable Configuration of Aeration Equipment Quantity
Many WWTPs have unreasonably configured the number of aeration equipment units due to not accounting for frequent low-load operational conditions. For example, many small and medium-sized WWTPs typically configure blowers in a "2 duty + 1 standby" (total 3) setup in the blower room design, which is optimal under design flow and quality conditions. However, under low influent load conditions, operating even one blower at its minimum output may cause over-aeration and increased power consumption. While installing variable frequency drives (VFDs) or other means to reduce air supply can avoid over-aeration, these measures can shift blower operation away from its high-efficiency zone, reducing efficiency and wasting energy. Given the generally low influent concentrations, strategies like increasing the number of blowers while reducing individual unit capacity should be considered to meet air demand regulation needs during low-load periods. Historically, limited budgets and the high cost of imported high-performance blowers led to fewer-unit configurations. With the maturation of domestic high-performance blower technology and reduced costs, conditions are now favorable for optimizing blower configurations to achieve energy saving and carbon reduction.
1.3 Low Efficiency of Aeration Equipment
Some older WWTPs, built with the technology of their time, employ low-efficiency, high-energy-consumption aeration equipment. By current technological and energy efficiency standards, equipment like Roots blowers, multi-stage low-speed centrifugal blowers, disc aerators, and brush aerators are considered low-efficiency, typically ranging from 40% to 65% efficiency-15% to 40% lower than modern high-speed centrifugal blowers. Furthermore, in WWTPs using fine-bubble diffused aeration in Anaerobic-Anoxic-Oxic (A₂/O) or Anoxic-Oxic (A/O) processes, aging or clogging of diffusers reduces oxygen transfer efficiency and increases resistance, thereby raising blower energy consumption.
1.4 Unreasonable Configuration of Mixers in Biological Tanks
In oxidation ditches with surface aerators, the equipment serves both aeration and mixing/pushing functions. This is a reasonable design under design load conditions. However, under low-load conditions, reducing or stopping aeration may be necessary, but to prevent sludge settling or liquid-solid separation, sufficient flow velocity must be maintained, forcing continued operation of aerators and causing over-aeration, poor nutrient removal, and energy waste. For more energy-efficient operation at low loads, oxidation ditches should be equipped with properly configured submersible mixers.
In A₂/O and A/O processes, aerobic tanks are typically fully covered with fine-bubble diffusers without dedicated mixers, relying on sufficient aeration to prevent settling. Under low loads, reducing aeration or implementing intermittent aeration to avoid over-aeration can easily lead to sludge settling, affecting treatment. To operate more efficiently at low loads, A₂/O and A/O aerobic tanks should consider adding appropriate mixers.
2. Technical Approaches for Energy Saving and Carbon Reduction in WWTP Aeration Systems
2.1 Replacement with High-Efficiency Aeration Equipment
WWTPs still using low-efficiency equipment like Roots blowers, multi-stage low-speed centrifugal blowers, disc aerators, or brush aerators, or those with severely aged and inefficient equipment, should conduct energy efficiency evaluations from an energy-saving and carbon-reduction perspective and timely replace them with new, high-efficiency models. Currently, high-speed blowers like single-stage high-speed centrifugal blowers, magnetic bearing blowers, and air bearing blowers used in large WWTPs typically boast efficiencies between 80% and 85%. However, the market currently lacks small-capacity high-speed centrifugal blower products. WWTPs with capacities below 2,000 m³/d still rely on less efficient equipment like Roots blowers, with efficiencies generally between 40% and 65%, indicating significant potential for improvement. Therefore, developing more efficient small-scale aeration equipment is meaningful for energy saving and carbon reduction in small WWTPs.
2.2 Conversion from Surface Aeration to Fine-Bubble Diffused Aeration
Given suitable water depth, fine-bubble diffused aeration is more energy-efficient than surface aeration. Converting oxidation ditches from surface to fine-bubble diffused aeration can yield good energy-saving results. From implemented retrofit projects, such conversions not only achieve significant energy savings but also improve biological nutrient removal efficiency. Chen Chao's study noted that after one WWTP converted, total electricity consumption decreased by 24.7%, while removal rates for ammonia nitrogen, COD, and total phosphorus increased by 30.39%, 5.39%, and 2.09%, respectively. Xie Jici et al. reported energy savings of 0.09–0.12 kWh/m³ after a similar conversion, with significant improvement in biological nutrient removal efficiency. In fine-bubble aeration, oxygen transfer efficiency is linearly positively correlated with water depth. Below a certain critical depth, its efficiency can be lower than surface aeration. Generally, a water depth greater than 4 m is considered a suitable condition for converting oxidation ditches to fine-bubble diffused aeration.
3. Technical Approaches for Energy Saving and Carbon Reduction in WWTP Aeration Systems
3.1 Replacement with High-Efficiency Aeration Equipment
WWTPs still using low-efficiency equipment like Roots blowers, multi-stage low-speed centrifugal blowers, disc aerators, or brush aerators, or those with severely aged and inefficient equipment, should conduct energy efficiency evaluations from an energy-saving and carbon-reduction perspective and timely replace them with new, high-efficiency models. Currently, high-speed blowers like single-stage high-speed centrifugal blowers, magnetic bearing blowers, and air bearing blowers used in large WWTPs typically boast efficiencies between 80% and 85%. However, the market currently lacks small-capacity high-speed centrifugal blower products. WWTPs with capacities below 2,000 m³/d still rely on less efficient equipment like Roots blowers, with efficiencies generally between 40% and 65%, indicating significant potential for improvement. Therefore, developing more efficient small-scale aeration equipment is meaningful for energy saving and carbon reduction in small WWTPs.
3.2 Conversion from Surface Aeration to Fine-Bubble Diffused Aeration
Given suitable water depth, fine-bubble diffused aeration is more energy-efficient than surface aeration. Converting oxidation ditches from surface to fine-bubble diffused aeration can yield good energy-saving results. From implemented retrofit projects, such conversions not only achieve significant energy savings but also improve biological nutrient removal efficiency. Chen Chao's study noted that after one WWTP converted, total electricity consumption decreased by 24.7%, while removal rates for ammonia nitrogen, COD, and total phosphorus increased by 30.39%, 5.39%, and 2.09%, respectively. Xie Jici et al. reported energy savings of 0.09–0.12 kWh/m³ after a similar conversion, with significant improvement in biological nutrient removal efficiency. In fine-bubble aeration, oxygen transfer efficiency is linearly positively correlated with water depth. Below a certain critical depth, its efficiency can be lower than surface aeration. Generally, a water depth greater than 4 m is considered a suitable condition for converting oxidation ditches to fine-bubble diffused aeration.
3.3 Intermittent Aeration Technology
For WWTPs with low influent concentrations, continuous-flow intermittent aeration effectively addresses issues of poor nutrient removal and high energy consumption caused by over-aeration. It involves continuous influent and effluent flow while the aeration system operates in cycles of aeration on/off. Following ARAKI et al.'s 1986 research on intermittent aeration for nitrogen removal in oxidation ditches, many scholars have conducted experimental studies. Hou Hongxun et al. conducted a full-scale trial in a 100,000 m³/d WWTP using continuous-flow intermittent aeration in an oxidation ditch, achieving a 20% increase in total nitrogen removal, a 49% increase in total phosphorus removal, and a 21% reduction in total plant energy consumption. He Quan et al., in a 40,000 m³/d WWTP oxidation ditch trial using a 2-hour on/2-hour off cycle, found that compared to continuous aeration, intermittent aeration saved 42% in aeration energy, increased total nitrogen removal by 9.6%, and total phosphorus removal by 6.9% under winter low-temperature conditions. Zheng Wanlin et al., in a 40,000 m³/d WWTP A₂/O process trial using a 3-hour on/3-hour off cycle, maintained stable standard-compliant effluent quality while saving 18.3% in electricity consumption. Currently, full-scale applications of continuous-flow intermittent aeration are still limited, with several technical challenges remaining.
For A₂/O processes using fine-bubble aeration, two factors limit the wide application of intermittent aeration. First, high-speed centrifugal blowers generate high-decibel, sharp noise upon startup; frequent cycling for intermittent operation creates noise pollution. Second, frequent start-stop cycles for magnetic/air bearing blowers cause the non-contact bearings to repeatedly contact the housing, easily leading to bearing damage, increased failure rates, and reduced lifespan.
When applying intermittent aeration to oxidation ditches or A₂/O processes, sufficient mixing velocity during non-aeration periods must be ensured, potentially requiring additional mixers to prevent sludge settling. Ammonia nitrogen concentrations can rise rapidly during non-aeration, risking instantaneous exceedance. Therefore, further research is needed to scientifically set and adjust aeration cycles, better improving energy savings and pollutant removal while avoiding instantaneous ammonia nitrogen exceedance.
WWTPs' concern about potential instantaneous ammonia nitrogen exceedance is a major barrier to wide application of intermittent aeration. In January 2022, the Ministry of Ecology and Environment issued a consultation on a draft amendment to GB 18918-2002, primarily proposing to add maximum allowable limits for single measurements. These proposed single measurement limits are significantly higher than the original daily average limits, while daily averages remain unchanged. For example, for Grade A standard, a single measurement below 10 mg/L (15 mg/L below 12°C) would be acceptable if the daily average remains below 5 mg/L (8 mg/L below 12°C). If implemented, this amendment could help address regulatory concerns regarding instantaneous exceedance from intermittent aeration, facilitating its application in oxidation ditch processes.
3.4 Precise Aeration Technology
WWTP flow rates and influent concentrations fluctuate significantly, even throughout the day, causing variable air demand. Relying solely on manual experience-based adjustment makes precise control difficult and can compromise effluent quality stability. With advances in big data and artificial intelligence, the concept of precise aeration has emerged. Precise aeration technology has been applied in some WWTPs, typically achieving 10%–20% energy savings in aeration systems. Combining precise aeration with other process modifications can yield better results. Zhu Jie et al. implemented precise aeration retrofitting in a multi-stage A/O process WWTP, achieving 49.8% energy savings in the aeration system. Precise and intelligent aeration represent important future directions for energy saving and carbon reduction. Current limitations exist in the real-time capability and accuracy of data acquisition and analysis for these systems. More technological breakthroughs are needed in real-time precise control of blowers and valves and accurate air distribution.
4. Conclusion
Energy saving in aeration systems is key to carbon reduction in WWTPs. The primary reason for high energy consumption in Chinese WWTP aeration systems is low influent load, which easily leads to over-aeration, wasting electricity and increasing carbon emissions from both power and chemicals. Other reasons include aging/low-efficiency equipment and unreasonable configuration of aeration and mixing equipment. Effective means to achieve energy saving and carbon reduction include replacing low-efficiency with high-efficiency aeration equipment, converting surface to fine-bubble diffused aeration, and applying technologies like continuous-flow intermittent aeration and precise aeration.