Application of Online Chemical Cleaning Technology for Fine Bubble Aerators in Wastewater Treatment Plants
Fine bubble aerators are widely used as aeration equipment in wastewater treatment plants due to their simple structure, high oxygen utilization efficiency, reliable performance, clogging-resistant pores, prevention of wastewater backflow, uniform circumferential stress distribution, long service life, easy installation and maintenance, and low system cost. As a key component for oxygen supply in wastewater treatment, fine bubble aeration systems are prone to clogging by fouling and biofilm during long-term operation, posing significant challenges to maintaining their performance. Online chemical cleaning technology provides an effective solution to this problem.
1. Formation and Hazards of Fine Bubble Aerator Clogging
After prolonged operation, fine bubble aerators are susceptible to clogging, typically categorized as "internal clogging" and "external clogging" based on the form of pollutant blockage. "Internal clogging" refers to the deposition of fine particles such as colloidal particles and solute macromolecules from the mixed liquor within the pores, leading to pore blockage. "External clogging" refers to the deposition of scaling substances on the membrane surface facing the water side. This type of blockage tends to continuously increase the air discharge resistance of the membrane, leading to increased pressure on the membrane and a gradual enlargement of the pore size. Over time, this can easily cause membrane tearing. Once the membrane tears, the impact extends from the destruction of aeration efficiency to structural damage of the system, potentially necessitating shutdown for maintenance or aerator replacement.
Clogging issues in fine bubble aerators bring increased operational risks:
- From an electricity consumption cost perspective: As aerators clog, pipeline pressure rises, forcing blowers to operate under high-load, high-energy-consumption conditions. This increases power consumption and also affects blower lifespan.
- From an environmental risk perspective: Uneven aeration reduces oxygen transfer rates, limits process control flexibility, and can severely affect effluent quality in serious cases.
- From an economic cost perspective: The cost of manual cleaning after emptying tanks is high.
- From a safety perspective: Manual cleaning after emptying requires entering tanks for sludge removal, involving confined space entry and temporary electrical work, thereby increasing risks of electrical and personal safety hazards. Figure 1 shows the sludge accumulation phenomenon from aerator clogging.
Therefore, regular maintenance and cleaning of fine bubble aerators is crucial to ensure their operational performance. Traditional aerator maintenance and cleaning methods require complete emptying of the biological reaction tanks. Large-scale maintenance and cleaning of wastewater treatment facilities may affect normal wastewater treatment and discharge, or require approval from relevant government departments if conducted in specific locations (such as areas covered by urban drainage networks or drinking water source protection zones). This process involves multiple hazardous operations (e.g., confined space entry) with numerous risks and disadvantages, imposing significant economic burdens and potential costs (e.g., coordinating with government relations, reduced treatment capacity during maintenance, water quality adjustment, safety risks) on wastewater treatment plants. The pressure and challenges posed by emptying for maintenance render the feasibility of regular emptying for aerator cleaning relatively weak.
Given the numerous drawbacks of traditional manual cleaning after emptying-high cost, high operational risk, and suboptimal cleaning effectiveness-research on online cleaning of fine bubble aerators using online chemical dosing devices under normal aeration conditions is particularly important.
This study selected a plant project as the field test site for the online chemical cleaning technology. The plant has a total wastewater treatment capacity of 600,000 tons per day, constructed in four phases. The third-phase project has a treatment capacity of 100,000 tons per day, using an AAO process; the fourth-phase project has a treatment capacity of 200,000 tons per day, using an MBR process. The effluent quality meets the Grade A standard of GB 18918-2002 "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants". Online cleaning was performed on fine bubble aerators in the aerobic tanks of the third and fourth phases, which had been in operation for 6-7 years.
2. Principle of Online Chemical Cleaning Technology
Online chemical cleaning technology involves adding specific chemical agents to the aeration system to dissolve or disperse clogging substances through chemical action. These agents can be acidic, alkaline, oxidizing, or chelating. For example, some acidic agents can dissolve alkaline precipitates like calcium carbonate, while oxidizing agents can decompose organic blockages produced by microorganisms.
2.1 Analysis of Common Pollutants
Pollutants adhering to aerator surfaces are diverse, and their composition is closely related to wastewater characteristics, treatment processes, and operational conditions. Common pollutants are analyzed as follows:
- Inorganic Pollutants: Include calcium and magnesium compounds, sulfides, metal oxides, and hydroxides, mainly originating from chemical precipitation and ion supersaturation. Their primary impacts on aerators include pore clogging, reduced aeration efficiency, increased system energy consumption, increased aeration resistance, and decreased oxygen transfer efficiency.
- Organic Pollutants: Include microbial biofilm, suspended organic particles, fats/oils, and organic colloids. Microbial biofilm primarily forms due to microbial colonization and extracellular polymeric substance (EPS) adhesion. Its hazards include creating anaerobic microenvironments and releasing toxic gases (e.g., H₂S). Organic colloids form due to hydrophobic interactions and electrostatic adsorption, creating hydrophobic layers that hinder gas release and affect aeration uniformity.
- Composite Pollutants (Inorganic-Organic Mixed Scale): Include biological-chemical mixed scale and sludge particle attachment, mainly formed through physical entrapment and chemical bonding. Their effects include covering the aerator surface, reducing effective aeration area, accelerating equipment aging, and shortening maintenance cycles.
Through maintenance inspections of the plant's aeration system, the following issues were identified: ① Prolonged underwater operation of aerators, coupled with increasing service life, led to significant aging of O-ring seals at connection points, resulting in gas leakage; ② During operation, continuous sludge deposition and adjustments in production process control resulted in higher sludge concentrations in certain areas, indirectly causing severe scaling on aerator membrane surfaces, as shown in Figure 2; ③ When sludge concentration in the biological reaction tanks is too high, the sludge age extends, increasing the dissolved oxygen required for normal microbial activity and raising demands on the oxygen supply system; ④ Increased mixed liquor density in aeration tanks increases resistance, leading to higher power consumption for mechanical or blower aeration; ⑤ Some fouling had penetrated into the aeration pores, affecting the system's aeration, as shown in Figure 3. Based on the causes of pollutant formation, it was determined that the scale on the aerator surfaces contained inorganic pollutants, organic matter, proteins, etc.
2.2 Selection of Cleaning Agents
For the types of membrane pollution, suitable chemical cleaning agents need to be selected. These agents can penetrate through the aeration pores in the pipe wall to the space between the membrane and the pipe wall, achieving cleaning of the membrane surface and its pores. The selection of cleaning agent type should be based on the actual physicochemical properties of the membrane, the types of pollutants, and the degree of fouling. The cleaning agent should be biodegradable and non-toxic to organisms, capable of effectively removing inorganic scale from the air pipe walls and inside the diffusers. It should have good cleaning efficacy against blockages (also known as "gas-phase clogging") caused by contaminants, particles, or dust in the inlet air of blower aeration systems, oil leaks from blowers, and rust from internal air piping.
Alkaline cleaning agents include sodium hydroxide, sodium carbonate, sodium phosphate, sodium silicate, potassium hydroxide, etc. Sodium hydroxide is a common chemical agent in wastewater treatment processes for raising wastewater pH, so it can be selected as the alkaline cleaning agent.
Acidic cleaning agents include sulfuric acid, hydrochloric acid, nitric acid, citric acid, oxalic acid, phosphoric acid, etc. Given that citrate has strong chelating ability for ions such as manganese and iron, and in practice, compared to mineral acids, citric acid is relatively weak, less corrosive to equipment, safer, and easily biodegradable by microorganisms, citric acid was selected as the acidic cleaning agent.
Table 1 shows the categories and performance of cleaning agents commonly used for membrane fouling.
2.3 Design of Online Cleaning Device
Given the pressure in operating fine bubble aeration systems and the numerous branch pipes, designing a suitable online dosing device for fine bubble aerators is particularly important. The dosing cleaning device designed in this study includes a dissolution/dilution unit and a dosing unit, as shown in Figure 4.
The dissolution/dilution unit mainly consists of a preparation tank, agitator, and level gauge, used for dissolving and diluting the agents. By injecting a certain amount of water into the preparation tank, adding the agent, and starting the agitator, an agent of specific concentration can be prepared for use by the dosing unit.
The dosing unit mainly consists of a dosing tank, exhaust valve, dosing valve, balance valve, feed valve, and some piping systems. The bottom of the dosing tank is connected to a dosing pipe, which further branches into multiple dosing sub-pipes. All dosing sub-pipes are connected one-to-one with multiple aeration branch pipes, which in turn are connected to several fine bubble aerators, thus achieving the purpose of cleaning the fine bubble aerators.
During implementation, a Φ15 mm hole was drilled in each aeration branch pipe of the biological reaction tanks as a dosing port, through which a nylon dosing pipe was installed to deliver the agent to the fine bubble aerators, reducing agent loss. Simultaneously, an additional hole was drilled in the aeration branch pipe as a balance gas pipe to equalize pressure between the dosing tank and the aeration branch pipe. The holes drilled in the aeration branch pipes are sealed with plugs during normal operation, and quick-connect terminal fittings are installed during dosing to enable rapid installation and removal.
3. Application of the Online Dosing Cleaning Device
In this online dosing cleaning experiment, the fine bubble aerators were placed in the biological tanks. Specific cleaning solution was injected into the fine bubble aerator membranes through the aeration branch pipes, allowing it to flow toward the feed side to decompose organic matter adhering to the membrane surface, thereby restoring the transmembrane pressure difference and achieving the cleaning effect. The experimental design was based on three variables: agent type, agent concentration, and cleaning time. The test scheme is shown in Table 2.
3.1 Analysis of Online Dosing Cleaning Effect
After cleaning, sensory observation of the aeration surface at the site showed smaller bubble sizes escaping from the aeration tank surface and more uniform aeration. Figure 5 shows the sensory appearance of aeration before and after cleaning.
After cleaning with different agent types and concentrations, the aerators consistently showed increased flow rate and decreased pipeline pressure, with flow rates restored. Aeration efficiency was restored to varying degrees after treatment with different cleaning methods. Combined data on increased air flow and decreased pipeline pressure indicate that different agent types, concentrations, and cleaning times have varying effects on aerator restoration. Figures 6 and 7 show the changes in flow rate and pressure before and after cleaning, respectively.
The restoration efficiency of aerators after sodium hydroxide cleaning was slightly lower than that after citric acid. The high solubility of sodium hydroxide in water leads to significant heat release upon dissolution. Coupled with its strong hygroscopicity, alkalinity, and corrosiveness, these properties necessitate taking extra precautions in practical operations. From the perspective of cleaning operation safety, sodium hydroxide is not the preferred cleaning agent. Therefore, when selecting cleaning agents, their safety and operational convenience should be carefully evaluated to ensure operator safety and optimal cleaning effectiveness.
Test results showed that after online dosing cleaning, aeration in the biological tanks became more uniform, the flow rate of fine bubble aerators increased, pipeline pressure decreased significantly, and the cleaning effect was remarkable.
3.2 Technical Advantages
- Reduces Downtime: Compared to traditional disassembly cleaning, online dosing cleaning does not require stopping the aeration system, avoiding interruptions in the wastewater treatment process and reduced treatment efficiency caused by shutdowns.
- Improves Cleaning Efficiency: Agents can penetrate deep into the pores, effectively cleaning hard-to-reach clogged areas. After application in some domestic wastewater treatment plants, aeration uniformity improved noticeably, and oxygen transfer efficiency increased significantly.
- Reduces Labor Intensity and Costs: Eliminates the need for manual disassembly and reassembly of aerators, reducing manual labor and the risk of equipment damage from frequent disassembly, thus saving maintenance costs. The cost of online chemical cleaning for fine bubble aerators is 0.47 RMB/ton, whereas the cost of traditional manual cleaning for old aerators is 13.3 RMB/ton. It is estimated that annual savings on fine bubble aerator cleaning costs amount to 515,000 RMB. Compared to traditional manual cleaning of old aerators, online chemical cleaning offers significant economic advantages.
- Extends Aeration Equipment Lifespan: Through online chemical cleaning, the aeration effect of fine bubble aerators is effectively improved, enhancing aerator performance and, to some extent, extending the service life of aeration equipment, effectively reducing blower load.
- Provides More Options for Production Scheduling and Maintenance Plans: Through online chemical cleaning, bubble distribution becomes more uniform, air pipe pressure is effectively reduced, flow rate significantly increases, greatly improving oxygen transfer rates and providing a solid guarantee for water quality regulation.
4. Conclusion
Online chemical cleaning technology for fine bubble aerators holds significant application value in wastewater treatment plants. Through its rational application, clogging issues in fine bubble aerators can be effectively resolved, aeration system performance improved, downtime and operational costs reduced, and stable, efficient operation of wastewater treatment plants ensured. The limitations of traditional manual cleaning will drive the industry towards online cleaning. The emergence of new equipment and intelligent control systems significantly reduces the operational difficulty of online cleaning. Coupled with policy and environmental regulations emphasizing carbon neutrality and water resource recycling, which will indirectly promote the application of online cleaning technology. In the future, agent formulations can be optimized, and multi-agent synergistic cleaning technologies can be researched. Additionally, dosing control strategies and research into equipment intelligence can be pursued to better adapt to the needs of different wastewater treatment plants.