Aerobic wastewater treatment needs dissolved oxygen (DO) of at least 2 ppm so bacteria can break down organic pollutants. Air blowers struggle to hold that level when the organic load rises, because air is only 20.9% oxygen. A PSA oxygen generator supplies 90–95% oxygen on site, about 4.5 times richer than air, so DO rises faster, odours drop, and the plant can treat more load in the same tanks.
This guide explains why DO falls, how pure oxygen compares with aeration, and how to estimate the size of oxygen generator a treatment plant needs.
Why dissolved oxygen matters in wastewater
Aerobic bacteria do the work in biological treatment. They consume organic matter and need oxygen to do it. When DO is too low:
- Treatment slows down. Bacteria cannot break down pollutants fast enough, so effluent quality drops.
- Odours appear. Oxygen-starved zones turn anaerobic and release foul-smelling gases.
- Capacity is lost. The plant cannot take extra load without new tanks.
Measure DO with a dissolved oxygen meter by placing the probe in the aeration tank. Keep it above 2 ppm for good treatment.
Why DO falls
- High organic load: more pollutants mean more oxygen consumed by bacteria.
- More biomass: a larger bacterial population uses more oxygen.
- Peaks and shock loads: industrial discharges or seasonal peaks can outrun the blowers.
Air blowers vs pure oxygen
Conventional aeration blows large volumes of air into the tank. Because only about a fifth of that air is oxygen, the blowers move a lot of gas for each kilogram of oxygen dissolved, which costs power and time.
Pure oxygen from a PSA generator is injected through diffusers. With about 4.5 times more oxygen in every bubble, DO rises faster and less gas is moved for the same oxygen transfer. Fine-bubble diffusers and newer nanobubble systems increase the contact area and keep bubbles in the water longer, so more of the oxygen dissolves.
Pure oxygen is used in three ways:
- Main oxygen supply for new high-rate plants.
- Top-up alongside existing blowers, to handle peaks without building new tanks.
- Odour and septicity control in sumps, rising mains and holding tanks.
How much oxygen does a treatment plant need?
A practical estimate starts from the BOD load the plant removes each day.
- Daily BOD removed (kg/day) = flow (m³/day) × BOD removed (mg/L) ÷ 1,000.
- Oxygen demand: a common planning figure is 1.1–1.5 kg of oxygen per kg of BOD removed. Plants that also remove ammonia need extra oxygen for nitrification.
- Transfer efficiency: not all oxygen supplied dissolves. It depends on the diffusers and tank depth.
- Generator size: 1 Nm³ of 93% oxygen carries about 1.33 kg of oxygen.
Worked example:
| Step | Value |
|---|---|
| BOD removed | 500 kg/day |
| Oxygen demand at 1.2 kg O₂ per kg BOD | 600 kg/day, or 25 kg/hr |
| Oxygen to supply at 70% transfer efficiency | about 36 kg/hr |
| Generator size (÷ 1.33 kg per Nm³) | about 27 Nm³/hr |
If pure oxygen is only topping up existing blowers, size it for the shortfall at peak load, not the full demand.
Benefits of an on-site PSA oxygen generator
- Reliable supply: oxygen is produced 24×7 at the plant, with no deliveries of liquid oxygen or cylinders.
- Lower operating cost: no bulk oxygen purchases or transport.
- Control: oxygen output can be matched to the process as the load changes.
- Lower emissions: no delivery trucks.
- Purity: 90–95% oxygen, which is ideal for biological treatment and as feed gas for ozone systems.
To see how the generator itself works, read the PSA oxygen generator engineering guide.
Get a system sized for your plant
See our oxygen generators for wastewater treatment or the Idos industrial oxygen generator range. Email info@absstem.com or call 1800 547 0506.