Floating Aerators IrelandHow do you size a wastewater aerator?
A wastewater aerator is sized twice and the larger answer wins: once for oxygen (the oxygen the load needs each day, divided by what the aerator delivers per kWh in real wastewater) and once for mixing (watts per cubic metre to keep the tank moving).
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Step 1: how much oxygen does the load need?
The oxygen needed is typically about 1.5 to 2.0 times the BOD load. A tank receiving 200 kg of BOD a day therefore needs roughly 300 to 400 kg of oxygen a day.
Step 2: how much oxygen does an aerator deliver?
Manufacturers quote about 1.2 to 2.4 kg of oxygen per kWh for surface aerators in clean water tests. In real wastewater the figure is usually 50 to 70 percent of that, so sizing uses a field value, often around 1 kg of oxygen per kWh.
Worked example: 400 kg of oxygen a day at 1 kg per kWh is 400 kWh a day, or about 17 kW running around the clock.
Step 3: is there enough power to mix the tank?
Partial mixing needs about 1 to 2 W per m3; full mixing, where all solids stay in suspension, needs about 5 to 8 W per m3. A 5,400 m3 tank at partial mixing needs about 5.4 to 10.8 kW just to keep moving.
Install whichever is larger, the oxygen figure or the mixing figure, split across enough units to cover the whole tank without dead zones.
Sources
US Department of Energy, Better Plants training: Aeration Systems (bptraining.ornl.gov). Wisconsin Administrative Code NR 110.25, design standard of about 2.0 lb oxygen per hp hour (docs.legis.wisconsin.gov).
Frequently asked questions
What is a rule of thumb for aerator power?
About 1 to 2 W per m3 for partial mixing and 5 to 8 W per m3 for full mixing, checked against the oxygen the BOD load needs.
Why is field oxygen transfer lower than the brochure figure?
Brochure figures come from clean water tests. Wastewater, temperature and the dissolved oxygen you need to hold all reduce transfer, typically to 50 to 70 percent of the clean water figure.