Technical Fundamentals & Operational Management of a Low-Load Blower Aeration Basin
1. Overview
1.1 Operating Principle of Blower Aeration Basins
Blower aeration, commonly used in China, primarily includes diffused, spiral, and microporous aeration types. An aeration basin typically comprises an aeration system, the basin structure, and inlet/outlet ports, serving as a key structure in activated sludge wastewater treatment. Common aeration methods are mechanical and blower aeration. Blower aeration systems generally consist of specialized aerators and blowers. The basins are often divided into multiple compartments, each capable of independent influent feed. Wastewater enters the basin and exits at the opposite end. During this process, air is delivered via compressors to diffusers at the basin bottom and released as bubbles.
1.2 Related Research on Blower Aeration Basins
Research by Cheng Dandan et al. found that in China's municipal wastewater treatment plants (WWTPs), aeration blowers consume approximately 60% of the total energy. Integrating the aeration system with intelligent PID closed-loop control for dissolved oxygen (DO) and implementing blower energy-saving strategies can effectively address high energy consumption in WWTP aeration systems, reducing it by over 30%.
Liu Xiaoqi et al. employed dispersed flow aerators to increase oxygen content in wastewater during treatment while lowering energy consumption. This also achieved uniform water-air mixing and distribution, reducing the precision requirement for aerator installation leveling.
Chang Kai et al. improved conventional aeration basin system performance by modifying the original air collection mode. They replaced traditional microporous aerators with high oxygen-transfer efficiency silicone plate microporous aerators and substituted single-pass straight-flow aeration basins with three-pass serpentine flow basins. Incorporating precise aeration control further enhanced the system, addressing issues of high energy consumption, low efficiency, and poor mass transfer in traditional blower aeration methods.
1.3 Operational Management of Blower Aeration Basins
Blower aeration basins are widely used in wastewater treatment. Following the principle of "separate treatment for different waste streams," a specific WWTP's saline wastewater treatment unit primarily handles electric desalination wastewater from atmospheric-vacuum distillation, stripped purified water, alkylation neutralization wastewater, and some supernatant and high-salinity effluent. This unit features a three-stage biological treatment system, with the blower aeration basin as the secondary stage. Its influent average Chemical Oxygen Demand (COD) is consistently below 100 mg/L, classifying it as a low-load activated sludge process. Beyond equipment upgrades, maintaining optimal operation requires careful control and adjustment of process parameters.
2. Facility Overview
2.1 Saline Wastewater Treatment Unit Process Flow
The unit employs a "Equalization + Oil Separation + Two-Stage Flotation + Three-Stage Biological Treatment" process, with treated effluent sent to a polishing unit. The oil separator uses a combined horizontal flow and inclined plate design. The two flotation stages utilize Vortex Cavitation Air Flotation (CAF) and Partial Reflux Pressurized Dissolved Air Flotation (DAF), respectively. The three biological stages are sequentially: Pure Oxygen Aeration Tank III, Blower Aeration Tank, and Secondary Biochemical Tank (EM-BAF). The process flow is shown in Figure 1.
2.2 Blower Aeration Basin Description
The blower aeration basin is a repurposed facility originally built in 1995 as part of an oily wastewater treatment unit. It uses a traditional plug-flow aeration design with an effective volume of 3,888 m³ and a current hydraulic retention time (HRT) of approximately 17.6 hours. The basin operates in two parallel trains, each with four compartments. Aerators are installed at the bottom, supplied by centrifugal blowers to provide oxygen for activated sludge metabolism. It is also equipped with two secondary clarifiers (Φ18m x 5m).
Within the three-stage biological system:
- Stage 1 (Pure Oxygen Aeration Tank III): Primary function is COD removal.
- Stage 2 (Blower Aeration Tank): Primary function is ammonia nitrogen (NH₃-N) removal, secondary function is further COD removal.
- Stage 3 (Secondary Biochemical Tank - EM-BAF): Functions to further polish effluent COD and NH₃-N, ensuring final water quality.
2.3 Blower Aeration Basin Influent and Effluent Quality
Influent to the blower aeration basin comes from Pure Oxygen Aeration Tank III, with pollutant limits: CODcr ≤ 300 mg/L, NH₃-N ≤ 30 mg/L, Suspended Solids (SS) ≤ 50 mg/L.
Its effluent feeds the Secondary Biochemical Tank, with limits: CODcr ≤ 120 mg/L, NH₃-N ≤ 30 mg/L, SS ≤ 50 mg/L.
The final effluent from the Secondary Biochemical Tank must meet: CODcr ≤ 70 mg/L, Petroleum ≤ 5 mg/L, NH₃-N ≤ 3 mg/L.
Throughout 2021, the basin's average influent CODcr was 67.094 mg/L, and average NH₃-N was 23.098 mg/L, both meeting design requirements. However, the notably low influent COD led to carbon source deficiency for the activated sludge, impacting its normal metabolism. Conversely, sufficient ammonia nitrogen and low organic pollutant concentration in the mixed liquor favored nitrification, which proceeded effectively.
3. Operational Influencing Factors and Control Measures
3.1 Impact of Low Influent Load and Sludge Aging
With influent COD at 67.094 mg/L-below both the design limit (≤300 mg/L) and the microbial carbon demand (approx. 100 mg/L BOD₅)-the activated sludge experienced carbon source deficiency. The low load resulted in slow sludge growth, making it prone to aging and forming a loose structure. Aged, dead sludge formed scum floating on the secondary clarifier surface. Lacking scum collection equipment, this scum flowed out with the effluent, causing turbidity, exceeding COD and SS limits, and subsequently overloading the downstream Secondary Biochemical Tank, affecting its final effluent quality.
Countermeasure: The operating team controlled the Mixed Liquor Suspended Solids (MLSS) concentration. Using a 1000 mL graduated cylinder for the 30-minute Sludge Volume Index (SVI) test, they maintained SVI around 20%, corresponding to an MLSS of approximately 2 g/L. This balanced pollutant removal efficiency with preventing sludge aging, floating, and water quality deterioration. The slow sludge growth meant minimal and infrequent sludge wasting, allowing nitrifying bacteria a residence time exceeding their minimum generation time, further promoting nitrification.
3.2 Impact of Dissolved Oxygen (DO) Control
Microorganisms in activated sludge are primarily aerobic, typically requiring DO between 1-3 mg/L. Corporate standards set the DO range for traditional plug-flow aeration basins at 2-4 mg/L, with nitrification requiring DO generally not below 2.0 mg/L. The current low influent load and further reduced MLSS concentration lowered the DO demand, making control challenging. Maintaining full mixing often raised DO above 4 mg/L, while controlling DO within the target range sometimes led to inadequate mixing in some areas, causing sludge settling.
Furthermore, high DO accelerates organic matter decomposition, exacerbating sludge aging. Therefore, in practice, DO is controlled around 3 mg/L. Additionally, all air valves are adjusted approximately monthly to improve mixing uniformity, reactivate dormant flocs, and maintain active biomass.
3.3 Impact of Water Temperature
Temperature significantly affects microbial activity. Suitable temperatures promote activity, while low temperatures inhibit or reduce it, and high temperatures can alter physiology or cause death. In this system, thermophilic bacteria are the main functional groups. For system safety, temperature is typically maintained between 15–35°C, though the suitable range is 10–45°C. Exceeding 30°C can denature nitrifier proteins, reducing their activity. The activated sludge contains both COD-degrading and nitrifying bacteria, with nitrification having a narrower optimal range of 5–30°C.
The saline wastewater influent contains high-temperature streams. Past incidents involved consecutive days of influent temperature exceeding 40°C, leading to sludge disintegration, death of COD-degraders and nitrifiers, and system collapse. Subsequently, a thermometer was installed on the equalization tank effluent line to strictly control discharge temperature not to exceed 40°C, meeting sludge temperature requirements. No similar incidents affecting nitrification occurred in 2021.
3.4 Impact of Alkalinity
According to relevant enterprise standards, when using activated sludge for ammonia removal, the influent total alkalinity to ammonia nitrogen ratio should not be less than 7.14; otherwise, alkalinity must be supplemented. With a design influent NH₃-N of 30 mg/L and an actual average of 23.098 mg/L, the required total alkalinity is not less than 214.2 mg/L. Currently, the influent alkalinity is insufficient, requiring daily soda ash (Na₂CO₃) addition to meet process demands.
3.5 Impact of pH and Toxic Substances
Activated sludge microorganisms thrive in a pH range of 6.5–8.5. Below pH 4.5, protozoa largely disappear, most microbial activity is inhibited, fungi become dominant, floc structure is destroyed, and sludge bulking can occur. Above pH 9, metabolism is severely affected, causing floc disintegration and bulking. Wastewater with pH >10 or <5 should be neutralized before entering the aeration basin.
Aerobic microbial metabolism can moderately buffer pH changes. For instance, nitrogen compound utilization can lower pH during nitrification, while decarboxylation produces alkaline amines, raising pH. This allows for long-term acclimation to mildly acidic/alkaline wastewater. The inherent alkalinity of the wastewater also helps inhibit pH drop.
However, drastic pH shifts (e.g., sudden alkaline inflow into an acidic system) significantly impact microbes and can disrupt operation. Therefore, neutralization necessity depends on the specific case. Minor, consistent pH fluctuations, especially with weak acids/bases, may not require neutralization. Larger fluctuations necessitate pH adjustment to neutral.
Nitrifying bacteria are highly pH-sensitive, with optimal nitrification at pH 7.2–8.0, while general microbes prefer 6.5–8.5. For specific industrial wastewaters, toxic substance types are often fixed, but concentrations and discharge volumes fluctuate. Besides equalization, influent toxic substance levels must be monitored and controlled. After sludge acclimation, a maximum influent concentration limit should be established based on acclimation degree and operational experience. Prolonged exceedance requires measures like reducing inflow, increasing sludge recycle, or enhancing oxygenation to prevent microbial poisoning and treatment failure. Currently, no toxic substances causing microbial poisoning have been detected in the basin's influent.
3.6 Impact of Influent Shock Loads
The influent COD remains stably low with minor fluctuations, and NH₃-N and Total Nitrogen (TN) also stay within relatively stable ranges over long periods. The nitrifier population remains relatively fixed. However, due to their slow growth rate, a sudden, significant increase in influent NH₃-N or TN can saturate the basin's removal capacity, compromising effluent NH₃-N and TN quality.
Theoretically, microbial N and P demand follows a BOD₅:N:P ratio of 100:5:1. However, N and P content varies greatly with industrial wastewater type. Some wastewaters are high in N and P, requiring removal to meet standards. Others are deficient, necessitating supplementation to avoid limiting metabolism. For operating basins treating low N/P wastewater, influent levels of about 10 mg/L NH₃-N and 5 mg/L phosphate can基本 meet microbial needs. Prolonged levels below these require increased N/P dosing.
Daily operation requires close monitoring of NH₃-N and TN in all influent streams and the equalization tank effluent, as well as in recycle flows from adjustment tanks, to prevent overloading the downstream polishing unit and threatening final discharge water safety.
4. Conclusion
As the core nitrification reactor in the saline wastewater treatment unit, the blower aeration basin requires close daily monitoring of water temperature, influent NH₃-N, and TN. Strict control of MLSS concentration, maintaining DO around 3 mg/L, and ensuring adequate alkalinity addition are essential. Under these optimized measures, the system runs stably with excellent effluent quality: average COD of 54.213 mg/L, NH₃-N of 9.678 mg/L, and SS of 23.849 mg/L, fully meeting the Secondary Biochemical Tank's influent requirements. Ongoing testing, summarization, and optimization from multiple aspects are also crucial to further ensure equipment reliability and system treatment efficiency.