TL;DR
DO below 1 mg/L in the bioreactor during normal loading means the aeration blowers are not delivering enough air. Check blower speed, discharge pressure, and diffuser condition before raising the DO setpoint.
What you might see
- bioreactor DO reading below 1 mg/L during peak loading
- effluent BOD or ammonia above permit limit
- filamentous bulking or foaming in the bioreactor
- sludge volume index (SVI) rising above normal
Likely causes
Aeration blower running below design speed or on partial load due to a VFD fault or mechanical issue
Fine-bubble diffuser fouling increasing the pressure drop and reducing oxygen transfer efficiency
Sudden organic loading increase overwhelming the current aeration capacity
DO controller setpoint too low for the current MLSS concentration or temperature
Required tools
- Dissolved oxygen meter or probe calibration kit
- Aeration blower instrumentation from DCS
- Long-arm sampling pole for mid-basin DO measurement
Safety first
- Bioreactor entry for diffuser maintenance requires confined-space procedures including gas testing and a continuous atmospheric monitor.
- Biological sludge is a potential biohazard. Wear waterproof gloves and eye protection during sampling and handling.
Procedure
- 1
Read the DO sensor output from the DCS and confirm the calibration date. Calibrate the DO probe if it has not been done in the past 2 weeks.[1]
- 2
Check aeration blower speed, inlet pressure, and discharge pressure. Compare to design curves. Low speed on a fixed-speed blower usually means a motor problem; on a VFD blower, check the drive setpoint.
- 3
Measure the dissolved oxygen at multiple points in the bioreactor to determine if the deficiency is localized (one zone) or basin-wide.
Warning: Bioreactor access requires confined-space entry if entry is needed. For surface-level sampling with a long-arm sampler, confined-space entry is not required but guard rails and non-slip precautions still apply. - 4
If a blower is running at full speed but DO is still low, check the fine-bubble diffuser headers for plugging by measuring the static pressure in the diffuser grid versus the baseline. Elevated pressure indicates fouled diffusers.
- 5
Schedule a diffuser acid-clean or pull-and-clean procedure during a basin dewatering event if diffuser fouling is confirmed.
- 6
If loading has spiked, increase WAS rate to lower MLSS and reduce the oxygen demand per unit of aeration.
Sources
Veolia Evoqua Memcor MBR Wastewater Treatment (Clarifier / MBR) general technical documentation, Veolia
MBR bioreactor aeration control, dissolved oxygen management, general water treatment references (general)
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