Fault Analysis and Renovation Scheme of Aeration System
Introduction
The aeration system, as one of the components of the biological wastewater treatment system, primarily functions to supply oxygen required for microbial metabolism and regulate the dissolved oxygen (DO) concentration within the biological tank. The vortices generated by rising bubbles and the disturbances caused by their rupture provide effective mixing of the activated sludge, preventing sludge deposition. For contact biological tanks containing media, aeration also promotes the shedding of aged biofilm from the media surface, facilitating biofilm renewal and enhancing its activity.
Studies indicate that changes in the DO concentration within the biological tank lead to alterations in the species, quantity, condition of zoogloea, biological activity, and metabolic types of microbial communities. Consequently, the reaction rates and efficiencies of biochemical processes such as biological carbon removal, biological nitrogen removal, and biological phosphorus removal are affected, changing the removal efficiencies of pollutants like organic matter, ammonia nitrogen, total phosphorus, and total nitrogen in the wastewater. The operational status of the aeration system directly impacts the microbial pollutant removal efficiency, thereby influencing the overall purification performance of the wastewater treatment plant (WWTP).
Therefore, maintaining the aeration system in good working condition is a primary task in WWTP operation and maintenance.
1. Materials and Methods
1.1 WWTP Overview
A WWTP with a design capacity of 15,000 m³/d. The designed influent pollutant indicators are shown in Table 1, and the effluent standards meet the Grade A standard of "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918-2002). The main treatment process is: Preliminary Treatment + Coagulation-Sedimentation + Biological System + Secondary Sedimentation Tank + Advanced Treatment.
Initially, due to underdeveloped collection networks and ongoing construction of surrounding enterprises, the plant operated intermittently due to low inflow. As surrounding enterprises became operational, inflow and pollutant load increased, leading the biological tank aeration system to transition to 24-hour continuous operation, with aeration rates adjusted based on inflow and load. During this period, both the biological tank and aeration system operated stably, with all effluent parameters consistently meeting standards.
1.1.1 Biological Tank Description
The biological system adopts a layout similar to the traditional A²/O process, comprising anaerobic, anoxic, and oxic zones. The anaerobic and anoxic zones are each divided into two Tandem process sections of equal volume, while the oxic zone is divided into four. Six submersible mixers are installed in the anaerobic and anoxic zones. Fixed fine-bubble diffusers are installed at the bottom of sections in the anoxic and oxic zones, with retrievable imitation media attached above the diffusers for microbial growth. The aeration system uses blowers to supply compressed air to the fine-bubble diffusers via laterals. Aeration rate in each lateral is regulated by valves. Three blowers are installed, operating in a 2-duty + 1-standby mode.
1.1.2 Fault Description
After approximately 5 years of stable operation, significant sludge accumulated at the bottom of the anoxic and oxic zones. Blowers frequently experienced high outlet pressure alarms and protective shutdowns. Some fine-bubble diffusers ruptured. As outlet pressure continued to rise, the frequency of blower shutdowns and the number of ruptured diffusers increased. Significant air loss through broken diffusers led to continuously decreasing DO levels in the biological tank, causing a gradual deterioration of effluent quality. To maintain compliance, the number and runtime of operating blowers were increased. This vicious cycle caused frequent damage to blower components like bearings and gears. Ultimately, one blower was severely worn and scrapped. Sludge in the oxic zone turned dark brown, with loose, foul-smelling zoogloea, and effluent quality worsened further.
1.2 Fault Cause Analysis
Reviewing operational records (influent, aeration system, equipment maintenance) and site observations, the causes were analyzed as follows:
1.2.1 Causes of Blower Damage
- Frequent starts/stops due to initial intermittent inflow, causing mechanical wear.
- Restarting blowers under pressure after overload shutdowns, and prolonged operation under overload.
- Increased air demand due to higher flow and ruptured diffusers, leading to extended operation.
- Elevated operating temperatures due to prolonged overpressure.
1.2.2 Causes of High Blower Outlet Pressure & Diffuser Damage
- Incomplete air piping cleaning during construction, leaving debris that clogged diffuser pores.
- Sludge deposition covering diffusers, clogging pores.
- Condensate in air pipes clogging diffuser pores.
- Intermittent aeration causing frequent expansion/contraction, aging diffuser membranes, and incomplete pore opening, leading to pressure buildup.
- Wastewater/sludge ingress into broken diffusers, dispersing and clogging other diffusers.
1.2.3 Causes of Bottom Sludge Accumulation
- Intermittent inflow and aeration causing deposition.
- Frequent blower faults causing intermittent aeration.
- Reduced aeration in laterals with ruptured diffusers.
- Poor aeration performance increasing deposition of inactive biofilm sloughed from tank and media.
1.3 Renovation Scheme
Addressing the faults and their causes, considering inflow patterns and the need for continuous operation, the following renovation scheme was developed:
The irreparable blower was replaced with a single air suspension blower with higher capacity and pressure rating than design, modifying the outlet piping accordingly.
For the aeration system issues (high pressure, clogging, rupture, uneven aeration), considering process requirements (mixing intensity, air flow, DO control), equipment layout (mixers, piping, media), and the pattern of damaged diffusers, separate renovation schemes were designed for the anoxic and oxic zones.
Anoxic Zone Renovation: Damaged diffusers were concentrated in the middle of Anoxic Sections 1 & 2, coinciding with sludge accumulation. Utilizing the existing media frame for support, a new air lateral connected to the main header was installed within the media bed, with a flow control valve. New downward-oriented perforated pipes were installed at the bottom of the media frame as the new aeration system. The original fixed bottom system was decommissioned. See Figure 1.
Oxic Zone Renovation: Similarly, media was removed in areas with damaged diffusers. A new lateral with valve was installed. New fine-bubble air discs were installed at the bottom of the media frame. Perforated pipes, similar to the anoxic zone, were also installed vertically within the media frame to periodically disturb bottom sludge by switching valves. The original fixed bottom system was decommissioned. See Figure 2.
2. Results and Analysis
Following a pilot-testing approach, the most severely affected sections (Anoxic 1, Oxic 1) were renovated. Key parameters (DO, blower pressure, sludge thickness) were monitored for 30 days pre- and post-renovation. Results are shown in Figure 3 and analyzed in Table 2.
DO (Fig 3a, 3b, Table 2): DO levels improved significantly. In the anoxic zone, DO increased from 0.12-0.23 mg/L (avg. 0.16) to 0.32-0.58 mg/L (avg. 0.46), a 1.88-fold increase. In the oxic zone, DO increased from 0.89-2.22 mg/L (avg. 1.78) to 2.81-5.02 mg/L (avg. 4.17), a 1.34-fold increase.
Blower Pressure (Fig 3c, Table 2): Outlet pressure decreased from 69.2-75.2 kPa (avg. 71.44) to 61.2-63.5 kPa (avg. 62.06), a 0.13-fold reduction.
Sludge Thickness (Fig 3d, Table 2): Bottom sludge thickness decreased from 27.3-33.4 cm (avg. 30.00) to 14.2-28.8 cm (avg. 20.75), a 0.31-fold reduction.
Observing the activated sludge post-renovation showed improved activity, color change, and better zoogloea growth on the media, indicating system recovery. Foul odors ceased.
Effluent quality improved: average Ammonia Nitrogen decreased to 1.49 mg/L (90.5% removal, +17.7%); average Total Phosphorus decreased to 0.19 mg/L (88.9% removal, +12.7%); average Total Nitrogen decreased to 10.28 mg/L (57.9% removal, +16.9%). Blower power consumption decreased from 72.5 kW to 59 kW under similar conditions, saving 18.6% in energy.
3. Conclusion
Analysis identified the causes of blower damage, high pressure, diffuser damage, and sludge accumulation. Targeted renovation schemes for the anoxic and oxic zones were implemented. Pilot testing demonstrated significant improvements: anoxic DO, oxic DO, blower pressure, and sludge thickness were improved by factors of 1.88, 1.34, 0.13, and 0.31, respectively. This provides a sound basis for full-scale renovation.