Fouling Characterization and Aeration Performance Recovery of Fine-pore Diffuser in Wastewater Treatment Plants
As a critical step in the activated sludge process of municipal wastewater treatment plants (WWTPs), aeration for oxygen supply not only provides sufficient oxygen to sustain the fundamental life activities of microorganisms but also keeps the sludge suspended, facilitating the adsorption and removal of pollutants. Aeration is also the most energy-consuming unit in WWTPs, accounting for 45% to 75% of the plant's total energy consumption. Therefore, the performance of the aeration system directly affects the treatment efficiency and operational costs of the WWTP. Aeration equipment is a key component of the aeration system, with fine bubble aerators being the most commonly used in municipal WWTPs due to their high oxygen transfer efficiency (OTE). However, during long-term operation, pollutants inevitably accumulate on the surface and within the pores of the aerators. To ensure effluent quality, additional air supply from blowers is required, leading to increased energy consumption. Furthermore, pollution exacerbates pore clogging and alters the aerator material. The pressure loss (dynamic wet pressure, DWP) of the aerator components increases over extended operation, raising the blower's outlet air pressure and causing further energy wastage.
Pollutants accumulating on the surface and inside the pores of fine bubble aerators include biological, organic, and inorganic fouling. Organic fouling results from the adsorption and precipitation of organic matter and the deposition of microbial secretions. Inorganic fouling typically consists of chemical precipitates formed by polyvalent cations, such as metal oxides. Based on whether they can be removed by physical cleaning, pollutants can be categorized as physically reversible or physically irreversible fouling. Physically reversible fouling can be removed by simple physical methods like mechanical scrubbing, as these pollutants are loosely attached to the aerator surface. Physically irreversible fouling cannot be eliminated by physical cleaning and requires more thorough chemical cleaning. Within physically irreversible fouling, pollutants that can be removed by chemical cleaning are termed chemically reversible fouling, while those that cannot be removed even by chemical cleaning are considered irrecoverable fouling.
Currently, fine bubble aerators used domestically include traditional rubber materials such as ethylene propylene diene monomer (EPDM) and newer materials like high-density polyethylene (HDPE). The gas distribution layer of HDPE aerators is formed by coating the inner air delivery pipe with molten polymer, with pore diameters approximately (4.0 ± 0.5) mm. HDPE offers good chemical, mechanical, and impact resistance properties and a long service life. However, its pore sizes are inconsistent and unevenly distributed, making them prone to pollutant deposition. EPDM material is highly flexible, with pores created by mechanical cutting. EPDM aerators have a higher number of pores per unit area, producing smaller bubbles (minimumc an be 0.5 mm). The hydrophilic nature of the rubber membrane also favors bubble formation. However, microorganisms tend to attach and grow on EPDM surfaces, utilizing plasticizers as a substrate. Concurrently, the consumption of plasticizers causes the aerator material to harden, ultimately leading to fatigue damage and shortened service life. Therefore, it is necessary to investigate the pollutant accumulation patterns on these two materials and the consequent changes in oxygen transfer efficiency and pressure loss.
This study took fine bubble aerators replaced after years of operation from two municipal WWTPs with similar process conditions as research subjects. Pollutants on the aerators were extracted and characterized layer by layer to identify their main components. Based on this, the effectiveness of cleaning methods in recovering the oxygen transfer efficiency of the aerators was evaluated, aiming to provide fundamental data and technical references for the long-term optimized and stable operation of fine bubble aeration systems.
1 Materials and Methods
1.1 Introduction to the Wastewater Treatment Plants
Both WWTPs are located in Shanghai and use the Anaerobic-Anoxic-Oxic (AAO) process as the core treatment. WWTP A employs a vortex grit chamber + conventional AAO + high-efficiency fiber filter + UV disinfection process. WWTP B uses an aerated grit chamber + conventional AAO + high-efficiency sedimentation tank + UV disinfection process. Both plants stably meet the Grade A standard of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918-2002). Specific design and operational parameters are shown in Table 1.
1.2 Extraction and Characterization of Aerator Pollutants
The fine bubble aerators used in the experiments were a tubular HDPE aerator (Ecopolemer, Ukraine) collected from Plant A and a tubular EPDM aerator (EDI-FlexAir, USA) collected from Plant B. Photos of both are shown in Figure 1. The old HDPE tube had been in operation for 10 years, with dimensions D×L=120 mm×1000 mm and pore diameter of (4±0.50) mm, capable of producing fine bubbles of 2~5 mm. The old EPDM tube had been in operation for 3 years, with dimensions D×L=91 mm×1003 mm, producing fine bubbles of 1.0~1.2 mm, with a minimum bubble diameter of 0.5 mm.
The old HDPE and EPDM tubes were retrieved from the aerobic tanks, placed on cling film, and rinsed with deionized water. Mechanical scrubbing was performed using a flame-sterilized blade to scrape off pollutants attached to the aerator surface.
To further study the impact of fouling on the oxygen transfer performance, chemical cleaning was performed on the HDPE tube. After mechanical scrubbing, the HDPE tube was soaked in 5% HCl and 5% NaClO solutions for 24 hours respectively. The old tubes, mechanically scrubbed tubes, and chemically cleaned tubes were dried in a 60°C oven (model XMTS-6000) for 60 hours. Their surfaces were then examined using scanning electron microscopy (SEM, model JSM-7800F, Japan), energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments, UK), and confocal laser scanning microscopy (CLSM, model TCS SP8, Germany). The HCl cleaning solution was filtered through a 0.45 μm membrane, and quantitative analysis of polyvalent cations (including Ca, Mg, Al, Fe ions, etc.) was performed using inductively coupled plasma optical emission spectrometry (ICP, model ICPS-7510, Japan). As HCl and NaClO can cause denaturation and aging of the EPDM membrane, chemical cleaning was not performed on the EPDM tube. The EPDM tube was cut into 5 cm × 5 cm membrane pieces and soaked in HCl for quantitative analysis of polyvalent cations in the solution.
1.3 Testing Apparatus and Method for Aerator Oxygen Transfer Performance
The oxygen transfer performance of the fine bubble aerators was tested according to the "Determination of Clean Water Oxygen Transfer Performance of Fine Bubble Aerators" (CJ/T 475-2015). The test setup is shown in Figure 2.
The apparatus is a stainless-steel structure measuring 1.2 m × 0.3 m × 1.4 m, with organic glass viewing windows on both sides. The aerator was fixed at the center bottom using a metal support, with a submergence depth of 1.0 m. A multi-parameter water quality analyzer (Hach HQ30D, USA) was used to monitor dissolved oxygen (DO) concentration in real-time. Sodium sulfite anhydrous was used as the deoxygenation agent, and cobalt chloride as the catalyst. The pressure gauge reading represented the aerator's dynamic wet pressure (DWP, kPa). Measurement results were corrected for temperature, salinity, and DO. The standardized oxygen transfer efficiency (SOTE, %) was used as the evaluation index.
Blower energy consumption is related to both air supply flow rate and outlet air pressure, which are influenced by the aerator's SOTE and DWP, respectively. Therefore, an aeration energy consumption index J (kPa·h/g), representing the combined effect of SOTE and DWP, was used to assess aerator performance. It is defined as the pressure loss the aerator must overcome per unit mass of oxygen transferred. J is calculated from the slope of the linear regression fit between DWP/SOTE and the air flow rate (AFR), as shown in the following equation:
Where:
AFR is the air flow rate, m³/h;
ρair is the air density, taken as 1.29 × 10³ g/m³ at 20°C;
yO2 is the oxygen content in air, taken as 0.23 g O₂/g air.
2 Results and Analysis
2.1 Oxygen Transfer Performance of New, Old, and Cleaned Aerators
Figure 3 shows the SOTE and DWP of the aerators at different air flow rates.
From Figures 3(a) and (b), the SOTE values for the new HDPE and new EPDM tubes were (7.36±0.53)% and (9.68±1.84)%, respectively. The EPDM tube produces smaller bubbles with a larger specific surface area, increasing the gas-liquid contact area and residence time, thus resulting in higher SOTE. The SOTE of both aerators decreased with increasing AFR because a higher AFR increases bubble number and initial velocity, leading to more bubble collisions and the formation of larger bubbles, which hinders oxygen transfer from gas to liquid phase. The SOTE of the EPDM tube showed a more pronounced decreasing trend with increasing AFR compared to the HDPE tube. This is because the pores of the HDPE aerator are rigid and do not change with AFR, while the pores of the EPDM aerator are flexible and open wider with increased AFR, forming larger bubbles and further reducing SOTE.
After long-term operation, the SOTE of the HDPE tube dropped to (5.39±0.62)%, a reduction of 26.7%, mainly due to pollutant accumulation clogging the pores and reducing the number of effective pores for bubble generation. Mechanical scrubbing increased the SOTE of the HDPE tube to (5.59±0.66)%, but the recovery was not significant, possibly because pollutants on the HDPE tube were not only attached to the surface but also deposited inside the pores, making them difficult to remove by mechanical scrubbing. Jiang et al. found that NaClO can effectively remove pollutants from HDPE tubes and restore their aeration performance. After NaClO cleaning, the SOTE of the HDPE tube recovered to (6.14±0.63)%, which is 83.4% of the new tube's level, still unable to fully recover. This is because, over prolonged operation, pollutants become tightly attached, altering the pore structure, obstructing airflow, increasing bubble coalescence, reducing bubble specific surface area and residence time, and thus hindering oxygen transfer. Simultaneously, fouling causes uneven air distribution, degrading overall performance.
The SOTE of the old EPDM tube dropped to (9.06±1.75)%, a reduction of 6.4%. Besides pore clogging from pollutant accumulation, biological fouling consumes plasticizers in the material, hardening the aerator and deforming the pores. The deformed pores cannot revert to their original state, producing larger bubbles and lowering SOTE. Mechanical scrubbing increased the SOTE of the EPDM tube to (9.47±1.87)%, almost restoring it to the level of the new tube, indicating that pollutants on the EPDM tube were loosely attached to the surface and could be mostly removed by mechanical scrubbing.
From Figures 3(c) and (d), the DWP of the new EPDM tube was (6.47±0.66) kPa, significantly higher than that of the new HDPE tube [(1.47±0.49) kPa]. This is because the pore diameter of the EPDM tube is smaller than that of the HDPE tube, resulting in greater resistance when bubbles are squeezed through. After long-term operation, the DWP of the old HDPE tube increased to (4.36±0.56) kPa, 2.97 times that of the new tube. The increase in DWP is related to both the degree of pore clogging and material changes. Mechanical scrubbing reduced the DWP of the HDPE tube to 2.25 times that of the new tube. NaClO cleaning further reduced it to (2.04±0.45) kPa, 1.39 times that of the new tube. This again indicates that most pollutants on the HDPE tube were deposited inside the pores and could not be effectively removed by mechanical scrubbing, requiring NaClO cleaning to restore performance. The DWP of the old EPDM tube increased to (8.10 ± 0.94) kPa, 1.25 times that of the new tube, and decreased to 1.10 times after mechanical scrubbing.
Figure 4 shows the change of DWP/SOTE (denoted as DWP') with AFR for the aerators.
A linear regression equation was used to fit DWP' versus AFR, and the energy consumption parameter J was obtained from the slope. The J values for the new HDPE and new EPDM tubes were 0.064 and 0.204 kPa·h/g, respectively, indicating that per unit mass of oxygen transferred, the EPDM tube must overcome greater pressure loss. At the time of replacement, the J values for the HDPE and EPDM tubes increased to 0.251 and 0.274 kPa·h/g, respectively. Aerator fouling leading to increased pressure loss may affect blower safe operation. After mechanical scrubbing, the J values for the HDPE and EPDM tubes decreased to 0.184 and 0.237 kPa·h/g, respectively. Changes in J can be used for quantitative analysis of aerator pollutants. The difference in J between the old tube and the mechanically scrubbed tube is caused by physically reversible fouling. The difference between the mechanically scrubbed tube and the new tube is caused by physically irreversible fouling. The difference between the mechanically scrubbed tube and the chemically cleaned tube is caused by chemically reversible fouling, while the difference between the chemically cleaned tube and the new tube is caused by irrecoverable fouling. Figure 5 shows the changes in the energy consumption parameter J for the aerators.
From Figure 5, for the HDPE tube, physically reversible and physically irreversible fouling accounted for 35.8% and 64.2% of the total fouling, respectively. Within the physically irreversible fouling, chemically reversible and irrecoverable fouling accounted for 42.8% and 21.4%, respectively. For the EPDM tube, physically reversible and physically irreversible fouling accounted for 52.9% and 47.1%, respectively. Irrecoverable fouling does not appear initially but accumulates over time, ultimately determining the aerator's service life. Therefore, reasonable cleaning schedules should be established to slow the transition from reversible to irreversible fouling and minimize the accumulation of irrecoverable fouling.
2.2 SEM Observation of New, Old, and Cleaned Aerators
Figure 6 shows SEM images of the surfaces of new, old, and mechanically scrubbed aerators. The porous structure of the new HDPE tube is clearly visible, while the surface of the new EPDM tube is smooth with clean-cut pores. After several years of operation, the surface morphology of both aerators changed significantly. Uneven rod-like and blocky pollutants completely covered the surface, with pollutant aggregates around and inside the pores, hindering oxygen transfer and increasing pressure loss. After mechanical scrubbing, most pollutants on the EPDM tube surface were removed, but pores remained clogged. For the HDPE tube, the pollutant layer thickness decreased, but pores were still covered.
2.3 Inorganic Fouling Analysis of New, Old, and Cleaned Aerators
EDX was used to further analyze the main elemental composition of the aerator surfaces, with results shown in Table 2. Carbon, oxygen, iron, silicon, and calcium were detected on both HDPE and EPDM surfaces. The HDPE tube also contained magnesium, while the EPDM tube contained aluminum. It is inferred that inorganic pollutants on the HDPE tube were silicon dioxide, calcium carbonate, magnesium carbonate, and iron phosphate, while those on the EPDM tube were silicon dioxide and aluminum oxide. These inorganic precipitates formed when the concentrations of inorganic ions from municipal wastewater and activated sludge reached saturation on the aerator surface. After mechanical scrubbing, the inorganic elements on the aerator surfaces showed little difference compared to the old tubes, indicating that mechanical scrubbing cannot effectively remove inorganic pollutants. Kim et al. found that after long-term operation, inorganic pollutants become covered by organic pollutants, tightly adhering to the surface and inside the pores, making them difficult to remove by mechanical scrubbing.
After HCl cleaning, metal ions on the aerator surfaces were completely removed. HCl corroded part of the organic layer covering the surface, penetrated it, and reacted with metal ions, removing inorganic precipitates through neutralization and decomposition. The HCl cleaning solution used for soaking the aerators was analyzed by ICP to calculate the content of inorganic pollutants. The Ca, Mg, and Fe contents for the HDPE tube were 18.00, 1.62, and 13.90 mg/cm², respectively, while for the EPDM tube, the Ca, Al, and Fe contents were 9.55, 1.61, and 3.38 mg/cm², respectively.
2.4 Organic Fouling Analysis of New, Old, and Cleaned Aerators
To quantitatively examine the distribution of organic pollutants, Image J software was used to calculate the biovolume and substrate coverage ratio of total cells, polysaccharides, and proteins from CLSM micrographs, with averages taken as final results (Figure 7).
From Figure 7(a), proteins and total cells were the main components of organic pollutants on the HDPE and EPDM tubes, respectively, with maximum total volumes reaching 7.66×10⁵ and 7.02×10⁵ μm³. The total cell volume on the EPDM tube was 2.5 times that on the HDPE tube, consistent with findings by Garrido-Baserba et al., who reported higher total DNA concentration on old EPDM aerators compared to other materials. Wanger et al. found that when microorganisms attach to EPDM tubes, if the surrounding environment lacks sufficient organic substrate, they turned to use EPDM membrane plasticizers. Microorganisms can utilize plasticizers as a carbon source, accelerating growth and reproduction, thereby intensifying biological fouling on the EPDM surface. The polysaccharide and protein contents on the EPDM tube were much lower than those on the HDPE tube, possibly due to the higher sludge age in Plant B compared to Plant A, leading to lower extracellular polymeric substance (EPS) concentration. As main components of EPS, proteins and polysaccharides secreted by microorganisms became significant sources of organic pollutants on the HDPE tube surface in Plant A.
After mechanical scrubbing, the quantities of total cells, polysaccharides, and proteins on the HDPE tube decreased by 1.49×10⁵, 0.13×10⁵, and 1.33×10⁵ μm³, respectively. On the EPDM tube, the corresponding decreases were 2.20×10⁵, 1.88×10⁵, and 2.38×10⁵ μm³, respectively. This indicates that mechanical scrubbing can reduce organic fouling to some extent.
However, for the HDPE tube, the substrate coverage area of polysaccharides and proteins increased after mechanical scrubbing-from 2.75% and 6.28% to 4.67% and 7.09%, respectively [Figure 7(b)]. This occurred because the extracellular polymeric substances (EPS) possess high viscosity. Consequently, mechanical scrubbing had the counterproductive effect of spreading proteins, polysaccharides, and inorganic pollutants more widely across the HDPE tube's surface, leading to greater area coverage. This likely explains why mechanical scrubbing failed to significantly restore the aeration efficiency of the HDPE tube.
After NaClO cleaning, total cells, polysaccharides, and proteins on the HDPE tube decreased by 2.34×10⁵, 3.42×10⁵, and 4.53×10⁵ μm³, respectively, showing significantly higher removal efficiency than mechanical scrubbing. NaClO oxidizes functional groups of organic pollutants into ketones, aldehydes, and carboxylic acids, increasing the hydrophilicity of the parent compounds and reducing pollutant adhesion to the aerator. Furthermore, sludge flocs and colloids can be decomposed by oxidants into fine particles and dissolved organic matter.
3 Conclusions
① The SOTE values for the new HDPE and new EPDM tubes were (7.36±0.53)% and (9.68±1.84)%, respectively. The SOTE of the EPDM tube showed a more pronounced decreasing trend with increasing AFR compared to the HDPE tube. This is because the pores of the HDPE aerator are rigid and do not change with AFR, while the pores of the EPDM aerator are flexible and open wider with increased AFR, forming larger bubbles and further reducing SOTE.
② Due to pollutant accumulation on the surface and inside pores, the oxygen transfer efficiency of the HDPE tube decreased by 26.7%, and its pressure loss increased to 2.97 times that of the new tube. As most pollutants on the HDPE tube were deposited inside the pores, mechanical scrubbing was not effective. After chemical cleaning, the SOTE of the HDPE tube recovered to 83.4% of the new tube's level, and DWP decreased to 1.39 times that of the new tube, showing significant performance improvement. However, due to pollutant deposition, it could not fully recover to its original state. For the HDPE tube, physically reversible, chemically reversible, and irrecoverable fouling accounted for 35.8%, 42.8%, and 21.4%, respectively.
③ After long-term operation, the oxygen transfer efficiency of the EPDM tube decreased by 6.4%, and its pressure loss increased to 1.25 times that of the new tube. After mechanical scrubbing, the aeration performance of the EPDM tube was almost restored to the level of the new tube, indicating that pollutants on the EPDM tube were loosely attached to the surface and could be largely removed by mechanical scrubbing. For the EPDM tube, physically reversible and physically irreversible fouling accounted for 52.9% and 47.1%, respectively.
④ Proteins were the main component of organic pollutants on the HDPE tube, while total cells were the main component on the EPDM tube. This is because microorganisms utilize plasticizers in the EPDM material as a carbon source, accelerating their growth and reproduction, thereby intensifying biological fouling on EPDM material aerators.