5.8 Aeration Systems: Blowers, Diffusers, Surface Aerators, Oxygen Transfer & Altitude Effects

Delivering oxygen is the biggest energy bill in the plant

WPI's Wastewater Treatment equipment outline lists aeration basins and aeration systems including blowers, surface aerators, and diffusers as separate equipment items. Aeration typically consumes 50 to 60 percent of a wastewater plant's total electricity, so the operator who understands it controls both treatment and the largest line in the operating budget.

Two jobs at once

An aeration system must simultaneously transfer oxygen into the mixed liquor and keep the solids in suspension. These are different requirements, and they set a floor on air delivery. Even when biological demand is low — overnight, on a weekend, at a lightly loaded plant — air cannot be turned down below the rate needed for mixing, or solids will settle in the basin and go anaerobic. A common design mixing requirement is roughly 20 to 30 standard cubic feet per minute per 1,000 cubic feet of basin volume for diffused systems.

Diffused aeration

Air is delivered by blowers through headers and dropped into the basin through diffusers.

Fine bubble diffusers transfer more oxygen because smaller bubbles have far more surface area per unit volume and rise more slowly, giving longer contact time. That advantage is exactly why they foul: the fine pores plug from the inside with oil, iron, and biological growth, and from the outside with mineral scale and biofilm. Fouling shows up as rising header pressure at constant airflow and falling oxygen transfer. Cleaning is by air bumping, hosing, or acid gas injection per the manufacturer, on a routine schedule.

Submergence matters. Oxygen transfer improves with depth because contact time and partial pressure both increase, which is why diffusers are placed on the basin floor rather than partway down.

Blowers

Two blower cautions are heavily tested. A positive displacement blower must never be throttled on its discharge; it will simply build pressure until the relief valve lifts or something fails, so flow is controlled by speed. A centrifugal blower must not be operated in surge, the unstable condition at low flow and high pressure where flow momentarily reverses, producing loud pulsation and rapid damage. Surge is avoided by staying right of the surge line on the blower curve, using blow-off or inlet vanes as needed.

Blower auxiliaries an operator maintains: inlet filters (a plugged filter starves the blower and raises energy use), check valves, relief valves, discharge silencers, cooling systems, and lubrication. Blower discharge air is hot, often 150 to 250 degrees Fahrenheit, so piping expansion joints and burn hazards are real.

Mechanical surface aeration

Surface aerators mix and aerate by violently agitating the surface, entraining atmospheric air.

• Low-speed vertical turbine aerators with a gear reducer are robust and produce large droplets.
• High-speed floating aerators are cheap and portable, common in lagoons.
• Brush rotors and disc aerators in oxidation ditches both aerate and propel the liquor around the channel.

Advantages: no blowers, no diffusers to foul, simple. Disadvantages that matter in Colorado: icing on the units and surrounding walkways in winter, aerosol drift carrying pathogens and odors, significant heat loss from the basin in cold weather, and lower transfer efficiency than fine bubble diffusion.

Dissolved oxygen control

• Target dissolved oxygen in a conventional aerobic zone is 1.5 to 3.0 mg/L, most commonly held near 2.0 mg/L.
• Below about 0.5 to 1.0 mg/L, filamentous organisms adapted to low oxygen gain a competitive advantage and low-DO filamentous bulking results. Nitrification also stalls, since nitrifiers need roughly 2.0 mg/L to proceed reliably.
• Above about 3 to 4 mg/L, there is no treatment benefit. The extra air is wasted energy, it can shear floc into pin floc, and it drives dissolved oxygen into the return sludge and the anoxic zone where it destroys denitrification.
• In a plug flow basin, dissolved oxygen is naturally low at the head and rises toward the outlet. Tapered aeration matches diffuser density to that profile.
• Automatic DO control uses in-basin probes to modulate blower output or valve position. The most efficient arrangement is most-open-valve control, which trims blower discharge pressure so that the most-demanding zone's valve sits nearly wide open, eliminating throttling losses.

Oxygen transfer terminology

• SOTR (standard oxygen transfer rate) is measured in clean water at 20 degrees Celsius, zero dissolved oxygen, and one atmosphere.
• AOTR (actual oxygen transfer rate) is what is achieved in mixed liquor at field conditions, always lower.
• Alpha is the ratio of oxygen transfer in wastewater to that in clean water, typically 0.4 to 0.8, and it falls with surfactants and high MLSS.
• Beta corrects for dissolved solids effects on saturation, typically about 0.95.
• Altitude matters in Colorado. Atmospheric pressure falls with elevation, so dissolved oxygen saturation is lower and blowers move less mass of air per unit volume. At Denver's elevation the atmospheric pressure is roughly 83 percent of sea level, and at mountain communities near 9,000 feet it is closer to 72 percent. A blower sized on sea-level tables will underdeliver oxygen at altitude, and saturation concentrations from standard tables must be corrected. This is one of the genuinely Colorado-specific operating realities in this subject.

Air requirement estimation

A common planning figure is roughly 1,000 to 1,500 cubic feet of air per pound of BOD removed for fine bubble diffusion in a conventional plant. Nitrification adds substantially: oxidizing ammonia consumes about 4.6 pounds of oxygen per pound of ammonia nitrogen, on top of roughly 1.0 to 1.5 pounds of oxygen per pound of BOD removed.

Worked example. A plant removes 3,200 lbs BOD per day and nitrifies 420 lbs of ammonia nitrogen per day.

• Carbonaceous demand: 3,200 x 1.2 = 3,840 lbs O2/day
• Nitrogenous demand: 420 x 4.6 = 1,932 lbs O2/day
• Total: 5,772 lbs O2/day

That single calculation explains why a permit change adding an ammonia limit can increase a plant's aeration energy by half.

Troubleshooting