Performance Measurement and Evaluation of Fine Bubble Aeration System in AAO Process during Summer and Winter
Most municipal wastewater treatment plants (WWTPs) in China utilize aerobic biological processes to remove organic matter, nitrogen, phosphorus, and other pollutants from wastewater. The supply of dissolved oxygen (DO) in water is a prerequisite for maintaining microbial life demand and treatment efficiency in the aerobic biological process. Consequently, the aeration unit is the core of the aerobic biological wastewater treatment. Simultaneously, the aeration system is also the main energy-consuming unit in WWTPs, accounting for 45% to 75% of the total plant energy consumption. Besides operational conditions, aeration system energy consumption is influenced by factors such as wastewater quality and environmental conditions. Most regions in China have distinct four seasons, abundant rainfall, and significant seasonal temperature variations. Summer rainfall dilutes the influent pollutant concentration of WWTPs, while low winter temperatures affect microbial activity, thereby impacting effluent quality. Fluctuations in influent flow rate and quality also pose challenges for the precise control of the aeration system in WWTPs. Without sufficient understanding of the changes in the oxygen transfer performance of fine bubble diffusers and their maintenance during operation, the advantage of high oxygen transfer efficiency (OTE) of fine bubble aeration systems cannot be fully utilized, leading to energy waste.
The most widely used type currently is the fine bubble diffuser, whose performance is directly related to the operational energy consumption of the aeration system. Methods for measuring the oxygen transfer performance of fine bubble diffusers include static tests (such as the clean water test) and dynamic tests (such as the off-gas analysis method). Research on static tests mostly focuses on laboratory-scale simulations, while dynamic test methods are rarely reported due to factors like test site requirements and field testing constraints. Currently, China has only established relevant standards for the clean water test method. During actual operation, the oxygen transfer performance of diffusers is affected by factors such as influent quality, sludge characteristics, operational conditions, and diffuser fouling. The actual performance differs significantly from clean water test results, leading to considerable deviations when using clean water data to predict the actual air supply requirement. The lack of effective monitoring methods for aeration system energy efficiency performance in WWTPs results in energy waste. Therefore, it is necessary to measure and evaluate the oxygen transfer performance of diffusers during actual operation to guide timely adjustments of aeration strategies and help achieve energy savings and consumption reduction in aeration systems. This study takes a municipal WWTP in Shanghai as an example. Through field measurements of pollutant concentration in the aerobic tank and the variation patterns of OTE along the pathway of the fine bubble aeration system in summer and winter, the pollutant removal efficiency and aeration system performance were systematically measured and evaluated. The aim is to explore the influence of seasonal changes on the oxygen transfer performance of the aeration system, providing guidance for precise control and energy-saving operation of aeration systems in wastewater treatment.
1. Materials and Methods
1.1 WWTP Operational Overview
The Shanghai municipal WWTP employs a process combination of pretreatment + AAO process + deep bed fiber filter + UV disinfection. The treatment capacity is 3.0×10⁵ m³/d. The main process flow of the WWTP is shown in Figure 1. The influent is primarily domestic sewage, and the effluent meets the Grade A standard of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918-2002) before being discharged into the Yangtze River. The hydraulic retention times (HRT) for the anaerobic tank, anoxic tank, and aerobic tank of the biological tank in this plant are 1.5 h, 2.7 h, and 7.1 h, respectively. The internal reflux ratio and external reflux ratio are both 100%. The sludge age is controlled between 10-15 days. The plant has a total of 8 aerobic tanks. A single aerobic tank measures 116.8 m × 75.1 m × 7.0 m (L × W × H), with a volume of 11,093 m³. The mixed liquor suspended solids (MLSS) concentration is controlled at around 4 g/L. The bottom is equipped with Ukrainian Ecopolemer polyethylene tubular fine bubble diffusers, sized at 120 mm × 1,000 mm (D × L). The air-to-water ratio is 5.7:1. Each aerobic tank consists of 3 channels (Zone 1, Zone 2, and Zone 3). Based on the DO concentration measured by gas flow meters within the channels, the guide vanes of single-stage centrifugal blowers (4 operational, 2 standby) are adjusted to maintain the DO concentration in the aerobic tank between 2-5 mg/L. Each blower has a rated air flow rate of 108 m³/min, a pressure of 0.06 kPa, and a power of 160 kW. Each channel is controlled separately using gas flow meters. Combined with DO reading feedback, the actual air supply is controlled by adjusting the guide vanes of the single-stage centrifugal blowers to maintain the average DO in the aerobic tank between 2-5 mg/L. The designed influent/effluent quality and the 2019 influent quality of the plant are shown in Table 1.
1.2 Test Point Layout
Two tests of the oxygen transfer performance of the fine bubble aeration system under actual operating conditions were conducted in July (summer) and December (winter). Along the flow direction, 22 test points were set up according to the locations of the inspection ports of the aerobic tank. The distance between two adjacent test points was about 5 m, with 7, 7, and 8 test points in Zone 1, Zone 2, and Zone 3, respectively. The distribution of test points is shown in Figure 2. The actual OTE of the fine bubble diffusers at each point was calculated by measuring the oxygen content in the off-gas escaping the water surface. Simultaneously, the DO concentration and water temperature at each point were measured using a multi-parameter water quality meter (HQ 30d, Hach, USA), and the pollutant concentration at each point was measured and analyzed to obtain its variation pattern along the pathway. To prevent the CODCr in the samples from degrading during transfer, samples taken along the aerobic tank were filtered on-site before measurement.
1.3 Measurement of Oxygen Transfer Performance of Fine Bubble Diffusers under Actual Conditions
The measurement of the oxygen transfer performance of fine bubble diffusers under actual conditions used an off-gas analyzer independently developed by Shanghai University of Electric Power, consisting of a gas collection system, gas analysis system, and signal conversion system. Off-gas was collected using a gas pump (KVP15-KM-2-C-S, Karier, China) and a hood, and delivered to an electrochemical oxygen sensor (A-01, ITG, Germany) for analysis. The signal conversion system converted the sensor's output voltage signal into the oxygen partial pressure in the gas. During off-gas testing, the oxygen partial pressure in the ambient air was measured first. Then the hood was fixed on the water surface of the aerobic tank to collect off-gas and measure its oxygen partial pressure. Data was recorded after the output stabilized for 5 minutes. Parameters obtained via the off-gas analyzer included the oxygen partial pressure in ambient air and off-gas, from which the percentage of oxygen transferred from the gas phase to the mixed liquor, i.e., the OTE of the fine bubble diffuser, was calculated as in Equation (1).
Where:
Y(O₂,air) - Proportion of oxygen in air;
Y(O₂,off-gas) - Proportion of oxygen in off-gas;
AOTE - Value of OTE.
The OTE measured by the off-gas analyzer was corrected for DO, temperature, and salinity to obtain the standard OTE (αSOTE) of the fine bubble diffuser in wastewater under standard conditions, as in Equation (2). The calculation of saturated DO in water is shown in Equation (3).
Where:
θ - Temperature correction coefficient, taken as 1.024, dimensionless;
AαSOTE - Value of αSOTE;
β - Salinity coefficient for the mixed liquor (calculated based on total dissolved solids in mixed liquor), dimensionless, usually taken as 0.99;
α - Ratio of oxygen transfer efficiency of the diffuser in wastewater versus clean water conditions, dimensionless;
C - DO concentration in water, mg/L;
CS,T - Saturated DO concentration in water at temperature T, mg/L;
CS,20 - Saturated DO concentration in water at 20°C, mg/L;
T - Water temperature, °C.
1.4 Calculation Method for Aeration System Energy Consumption
The theoretical oxygen demand of the aerobic tank was calculated according to the Activated Sludge Model (ASM). The oxygen demand was calculated based on CODCr and ammonia nitrogen removal results to determine the total oxygen demand (TOD) of the aerobic tank, as in Equation (4).
Where:
MTOD - Value of TOD, kg O₂/h;
Q - Influent flow rate, m³/d;
ΔCCODCr - Difference between influent and effluent COD Cr concentration, mg/L;
ΔCAmmonia nitrogen - Difference between influent and effluent ammonia nitrogen concentration, mg/L; 4.57 is the conversion factor for ammonia nitrogen to NO₃⁻-N.
The oxygen supply rate of the fine bubble aeration system is calculated as in Equation (5).
Where:
MOTR - Value of actual oxygen supply rate, kg O₂/d;
QAFR - Air flow rate, m³/h;
ŷO₂ - Mass fraction of oxygen in air, 0.276.
Blower power is determined by the actual air supply rate of the blower and the outlet pressure, which in turn is determined by the intake pressure, pressure loss of air in the pipeline, the pressure loss of the fine bubble diffuser itself, and the static water pressure承受 at the tank bottom, as in Equation (6).
Where:
ρair - Air density, g/L, taken as 1.29 g/L;
N - Blower power, kW;
R - Universal gas constant, 8.314 J/(mol·K);
Tair - Atmospheric temperature, °C;
B - Blower conversion coefficient, taken as 29.7;
γ - Specific heat ratio of gas, taken as constant 0.283;
η - Combined efficiency of motor and blower, taken as constant 0.8;
Pi - Blower intake pressure, Pa;
Z - Immersion water pressure on diffuser, Pa;
Ploss - Pressure loss of the fine bubble diffuser itself, Pa;
hL - Pressure loss of air in the pipeline, Pa.
Under test conditions, the amount of oxygen transferred into the water per unit electrical energy consumed by the diffuser [kg/(kW·h)] is the Standard Aeration Efficiency (SAE), as in Equation (7). The SAE value can be used to evaluate the actual usage efficiency of the fine bubble diffuser.
Where:
ASAE - Value of SAE.
1.5 Conventional Indicator Measurement Methods
Mixed liquor samples were filtered through qualitative filter paper. Soluble CODCr (SCODCr ), ammonia nitrogen, NO₃--N, and TP were measured using national standard methods.
2. Results and Discussion
2.1 Pollutant Removal Efficiency
The influent quality of main pollutants in summer and winter at the WWTP is shown in Figure 3. The average treatment flow rates in summer and winter were 3.65×10⁵ m³/d and 3.13×10⁵ m³/d, respectively. The summer influent CODCr and ammonia nitrogen concentrations were (188.38 ± 52.53) mg/L and (16.93 ± 5.10) mg/L, respectively. The winter influent CODCr and ammonia nitrogen concentrations were (187.94 ± 28.26) mg/L and (17.91 ± 3.42) mg/L, respectively. Higher summer rainfall leads the WWTP to operate in a "high hydraulic load - low pollutant load" mode. The increase in hydraulic load shortens the system's HRT, reducing the reaction time in the biological tank and affecting pollutant removal. Low influent pollutant load in WWTPs can easily lead to excessively low sludge loading, causing over-aeration and sludge disintegration. WWTPs should timely adjust sludge loading and air supply rates to mitigate the impact of low pollutant load operation. The summer water temperature was (27.32 ± 1.34)°C, significantly higher than the winter temperature of (17.39 ± 0.75)°C. Temperature is one of the important factors affecting the pollutant removal capacity of the system. The tolerance of filamentous bacteria is higher than that of floc-forming bacteria, making them prone to proliferate in low-temperature environments, causing sludge bulking. Lower temperatures also reduce the enzyme activity of microorganisms in the activated sludge, decreasing the substrate degradation rate and endogenous respiration rate, leading to reduced pollutant removal efficiency. WWTPs can take measures such as increasing sludge age and MLSS in the biological tank to alleviate the negative impact of low temperature on pollutant removal. As the hydraulic load in winter is lower than in summer, the HRT in the aerobic tank is slightly extended with sufficient aeration, offsetting the negative impact of low temperature on nitrification. Therefore, the effluent quality in both summer and winter met the Grade A standard of GB 18918-2002.
2.2 Variation Patterns of Pollutant Forms along the Aerobic Tank
On the test days, the influent SCODCr concentrations in summer and winter were 186.76 mg/L and 248.42 mg/L, respectively, and the ammonia nitrogen concentrations were 22.05 mg/L and 25.91 mg/L, respectively. Possibly due to combined sewer overflow and groundwater infiltration, the influent quality was lower than the design values. The variation of pollutants along the aerobic tank is shown in Figure 4.
Due to phosphorus release in the anaerobic tank, denitrification in the anoxic tank, and dilution by sludge return, the pollutant concentration significantly decreased before entering the aerobic tank. The SCODCr concentrations at the aerobic tank inlet in summer and winter were 30.32 mg/L and 52.48 mg/L, respectively, and the ammonia nitrogen concentrations were 3.90 mg/L and 4.62 mg/L, respectively. The TN concentrations at the aerobic tank inlet in summer and winter were 4.86 mg/L and 6.16 mg/L, respectively, decreasing slightly to 4.46 mg/L and 5.70 mg/L in the effluent, indicating a relatively low proportion of simultaneous nitrification and denitrification occurring in the aerobic tank. The SCODCr concentration decreased significantly in Zone 1 to 19.36 mg/L and 30.20 mg/L in summer and winter, respectively; the ammonia nitrogen concentration decreased to 1.75 mg/L and 2.80 mg/L. The decreasing trend of pollutant concentration slowed down in Zone 2, indicating that small molecular organic matter had been fully degraded and nitrification was complete. The pollutant concentration at the end of Zone 2 already met the effluent discharge standard. The pollutant concentration remained almost unchanged in Zone 3, but the DO value in the mixed liquor increased, indicating that most of the oxygen supplied in this zone dissolved into the sludge mixed liquor and was not used for CODCr oxidation and ammonia oxidation. The effluent SCODCr concentrations from the aerobic tank in summer and winter were 15.36 mg/L and 26.51 mg/L, respectively, and the effluent ammonia nitrogen concentrations were 0.17 mg/L and 0.50 mg/L, respectively. The higher ammonia nitrogen removal rate in summer was due to higher water temperature enhancing the nitrification-denitrification activity of microorganisms. Zhang Tao et al. found that low winter temperatures reduce the abundance of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, decreasing the ammonia nitrogen removal rate in WWTPs.
2.3 Off-Gas Test Results along the Aerobic Tank
Field tests of the oxygen transfer performance of the fine bubble aeration system were conducted along the aerobic tank in summer and winter using the off-gas analyzer. The results are shown in Figure 5. The DO concentration in the aerobic tank gradually increased along the flow direction. The DO concentration in the mixed liquor depends on the amount of oxygen transferred from the gas phase to the liquid phase by the diffusers (i.e., OTR) and the oxygen consumed by microorganisms (i.e., OUR). The substrate is abundant at the front end of the aerobic tank, and microorganisms require more oxygen to degrade the substrate. Therefore, the DO concentration was lowest in Zone 1 in both summer and winter, at (1.54 ± 0.22) mg/L and (1.85 ± 0.31) mg/L, respectively. The DO concentration increased to (2.27 ± 0.45) mg/L and (2.04 ± 0.13) mg/L in Zone 2, respectively. In Zone 3, the DO concentration was (4.48 ± 0.55) mg/L and (4.53 ± 1.68) mg/L, respectively. The variation pattern of DO along the pathway is consistent with that of pollutant concentration. Organic matter degradation and nitification were basically completed in Zone 2. The organic matter content in Zone 3 is lower, reducing the oxygen demand, leading to oxygen not being fully utilized and being stored in the water phase as DO, causing the DO concentration to rise to excessively high levels. The average DO in Zone 3 was significantly higher than 2.0 mg/L, indicating over-aeration at the end of the aerobic tank. Endogenous respiration of activated sludge reduces sludge activity and can easily cause sludge bulking, while also wasting energy. The excessively high DO concentration at the end of the aerobic tank also results in a higher DO concentration in the return liquor, which not only increases the DO concentration entering the anoxic tank via external reflux but also reduces the amount of available COD Cr , thereby lowering denitrification efficiency. Therefore, it is recommended to reduce the air supply in Zone 3, maintaining only the necessary mixing intensity, to save aeration energy consumption.
As shown in Figure 5, significant differences exist in the oxygen transfer performance of diffusers in different channels during actual operation between summer and winter. The average OTE measured in winter was 9.72%, lower than the result measured in summer (16.71%). This is because the decrease in water temperature reduces the activity of microorganisms in the aerobic tank of the WWTP, leading to lower oxygen utilization rate. After correction for temperature, salinity, and DO, the average αSOTE values in summer and winter were 17.69% and 14.21%, respectively. The summer αSOTE was slightly higher than in winter, possibly because prolonged operation exacerbated diffuser fouling, blocking pores, and reducing the oxygen transfer performance of the diffuser.
2.4 Analysis of Energy Optimization Potential for the Aerobic Tank Aeration System
According to Equations (3) and (4), the oxygen demand, oxygen supply rate, and blower power for each channel of the aerobic tank in summer and winter were calculated, as shown in Table 2. The total oxygen demand of the aerobic tank in winter was about 34.91% higher than in summer, caused by the higher influent CODCr and ammonia nitrogen pollutant load in winter compared to summer. The oxygen demand in each zone of the aerobic tank decreases as influent pollutants are degraded along the pathway. Zone 1 has the highest pollutant concentration and sufficient substrate, resulting in higher microbial activity, hence its oxygen demand is the highest. As pollutants are continuously degraded, the oxygen demand in Zone 2 and Zone 3 gradually decreases. In summer, the oxygen demand proportions of the three zones were 72.62%, 21.65%, and 5.73% of the total aerobic tank oxygen demand, respectively. In winter, the proportions were 72.84%, 24.53%, and 2.63%, respectively. In conventional activated sludge reactors, the oxygen demand for the front section is 45%-55%, the middle section 25%-35%, and the rear section 15%-25%. The treatment load at the end of this aerobic tank is lower than conventional values. The air supply at the front end could be appropriately reduced, allowing some pollutants to be degraded in the rear sections.
Compared to summer, the oxygen demand of the biological treatment process in winter is higher, and the oxygen transfer efficiency of the fine bubble aeration system is lower, leading to a higher required air supply. According to the operational data of the WWTP, the total blower air supply rates in summer and winter were 76.23 m³/h and 116.70 m³/h, respectively. The air supply was highest in Zone 1, while the air supply in Zone 2 and Zone 3 were similar but lower than in Zone 1. The oxygen supply in summer was 38.99% higher than the oxygen demand, indicating significant energy-saving potential. The oxygen supply in both Zone 2 and Zone 3 exceeded the actual oxygen demand. The oxygen supply in winter was 7.07% higher than the oxygen demand. The oxygen supply and demand in Zone 1 and Zone 2 were matched, while over-aeration occurred in Zone 3. Blower power is proportional to the air supply rate, as in Equation (6). The power consumption of the blowers in summer and winter was 85.21 kW and 130.44 kW, respectively. Henkel suggests that an increase in air temperature reduces the power of blowers in aeration systems. In response to the differences in oxygen demand among different channels, WWTPs should take corresponding aeration adjustment measures, such as tapered aeration. This could involve fully opening the air supply branch pipes at the front end, opening those at the middle end halfway, and adjusting the branch pipes at the end to the minimum opening to save air supply and aeration energy consumption.
Further quantifying the actual usage efficiency of the fine bubble diffusers, the Standard Aeration Efficiency (SAE) in the aerobic tank in summer was 2.57 kg O₂/kW·h, which is 32.29% higher than in winter. Differences in influent water quality, quantity, and temperature between summer and winter cause significant variations in the operation and control of the aeration system in the WWTP. Energy waste was more severe in summer than in winter, and the aeration system achieved a better supply-demand balance in winter. Considering the influent flow rate and quality, the air supply could be appropriately reduced in summer while ensuring effluent quality and adequate mixing in the aerobic tank. In winter, to mitigate the impact of high influent pollutant load and low temperature, sufficient aeration should be ensured. However, it is important to note that during long-term operation, pollutants accumulate on the surface and inside the pores of the diffusers, gradually blocking the pores, and the oxygen transfer efficiency will decrease. If diffuser cleaning is not timely, it can lead to insufficient oxygen supply by the aeration system, affecting effluent quality.
The WWTP employs a DO-blower air flow control strategy. The goal of the aeration control system is to provide a stable DO environment for microorganisms in the aerobic tank and ensure effluent compliance. However, the DO feedback mechanism alone cannot assess the energy-saving potential of the aeration system. Field testing the oxygen transfer performance of the aeration system allows precise calculation of the actual oxygen supply rate of the aeration system and describes its variation pattern along the pathway. Combined with oxygen demand data, this enables precise control of the aeration system to achieve a supply-demand balance and the goal of energy saving and consumption reduction.
3. Conclusion
- Higher summer water temperatures enhance microbial nitrification activity and denitrification, resulting in higher effluent COD Cr and ammonia nitrogen in winter compared to summer. However, due to lower hydraulic load in winter than summer, the extended HRT in the aerobic tank and sufficient aeration offset the negative impact of low temperature on nitrification. Therefore, effluent quality in both summer and winter met the Grade A standard of GB 18918-2002.
- Compared to summer, the oxygen demand of the biological treatment process in winter is higher, the oxygen transfer efficiency of the fine bubble aeration system is lower, leading to a higher required air supply rate and lower aeration efficiency.
- The oxygen supply in summer and winter was 38.99% and 7.07% higher than the oxygen demand, respectively, indicating greater energy-saving potential in summer. Pollutant concentration decreases gradually along the aerobic tank, remaining almost constant at the end, while the DO concentration at the end is much higher than at the front. This indicates that most of the oxygen supplied at the end dissolves into the sludge mixed liquor and is not used for CODCr oxidation and ammonia oxidation, suggesting over-aeration. Therefore, the air supply at the end of the aerobic tank can be appropriately reduced while ensuring effluent quality and adequate mixing.