A free process control resource from Wastewater Solutions, Inc.help@ActivatedSludge.com  |  503-701-2766
ActivatedSludge.comPractical wastewater process control

Aeration

Aeration supplies the oxygen the biomass needs and keeps the mixed liquor in suspension. It is usually the biggest energy user in the plant.

Types of aeration

  • Fine bubble diffused air: membrane discs, tubes or panels near the floor. High efficiency (SOTE often 25 to 35% at 14 to 15 ft submergence). White River uses 3,600 nine inch EPDM discs.
  • Coarse bubble diffused air: lower efficiency but less fouling; common in channels, digesters and aerobic holding.
  • Mechanical surface aerators: common in oxidation ditches and lagoons.
  • Jet aeration and others.

How oxygen transfers

Oxygen transfer basicsOxygen moves from an air bubble through gas and liquid films into the mixed liquor. The driving force is the gap between saturation and the operating DO. Fine bubbles transfer more oxygen than coarse bubbles.Air bubble21% oxygenGas film and liquid film at the surfaceDriving forceC* (saturation)DO in tankTransfer ∝ alpha x (beta C* - DO)Fine bubblemore surface, SOTE 20 to 35%Coarse bubbleless surface, SOTE 8 to 15%Lower DO in the tank means more driving force but less margin for the bugs.
Oxygen transfer from a rising bubble: oxygen moves across the bubble surface into the liquid, driven by the difference between saturation and actual DO.

Transfer depends on bubble size (smaller bubbles, more surface), depth (more contact time and pressure), the gap between saturation and actual DO, temperature, and contaminants (alpha factor).

AOR is the oxygen the process actually needs in the field. SOR is the equivalent oxygen in clean water at standard conditions, the number diffusers and blowers are rated for. The ratio AOR/SOR is often 0.4 to 0.55 for fine bubble.

Air (scfm) = SOR / (SOTE x 0.0173 x 1,440)

White River design: AOR 15,745 lb/d, AOR/SOR 0.50, SOTE 29% gives about 4,360 scfm.

Blowers

White River uses four 2,500 scfm high speed turbo blowers at 8.5 psig (three duty, one standby), firm capacity 7,500 scfm. Other common types: multistage centrifugal, single stage geared centrifugal, positive displacement (rotary lobe, screw).

Key operating data: discharge pressure, airflow, power, inlet temperature, surge margin.

DO control

  • Setpoints by zone: White River tapers 2.0, 1.5 and 1.0 mg/L across grids 1 to 3.
  • Most open valve pressure control: lowers header pressure to save energy.
  • Ammonia based aeration control: sets DO by ammonia load. White River cascades ABAC to DO setpoints.
  • Minimum air for mixing: check manufacturer limits per diffuser (White River: 0.5 to 2.5 scfm each).

Energy

Aeration is often 50 to 60% of plant power. The biggest savings usually come from lower DO setpoints where the process allows, clean diffusers, good valve control, and blower turndown. Track kWh per million gallons and per pound of BOD and ammonia removed.

Maintenance

  • Trend header pressure for diffuser fouling.
  • Drain moisture from air headers.
  • Inspect for boils and dead zones.
  • Clean or replace membranes on schedule.
  • Calibrate DO probes weekly or as the manufacturer recommends.