Where a treatment plant's electricity goes: aeration and ways to reduce its share

At a biological wastewater treatment plant, the largest consumer of electricity is neither the pumps nor sludge dewatering but the aeration tank blowers. Aeration typically accounts for between half and three quarters of the plant's entire power bill. Let us look at what makes up that share, where air is wasted and which measures save energy without harming treatment quality.

Why an aeration tank needs so much air

Activated sludge is a community of microorganisms that oxidise the pollutants in wastewater using oxygen. Oxygen is needed for two processes. The first is oxidation of organic matter: the more of it arrives with the wastewater, the more oxygen is needed. The second is nitrification, the conversion of ammonium nitrogen to nitrate. It is especially demanding: oxidising one gram of ammonium nitrogen takes about 4.6 grams of oxygen.

The oxygen has to be dissolved in water, and it dissolves poorly. A blower delivers air to the bottom of the tank, the bubbles rise, and during the rise part of the oxygen passes into the water. The rest escapes to the atmosphere. The fraction transferred — the transfer efficiency — determines how much air, and therefore how much electricity, is needed for each kilogram of oxygen the sludge consumes.

Air in an aeration tank also does a second job — mixing. It keeps sludge flocs in suspension and stops them settling to the bottom. These two duties need different amounts of air, and much of the potential for savings rests on that difference.

Why a fine bubble beats a coarse one

Oxygen transfer efficiency depends on the contact area between air and water and on contact time. A fine bubble gives a large surface per unit volume and rises more slowly. Fine-bubble aeration through membrane diffusers therefore transfers two to three times more oxygen per cubic metre of air than coarse-bubble aeration through perforated pipes. The second factor is depth: the deeper the diffusers, the longer the bubble stays in the water and the higher the transfer — but also the higher the pressure the blower must deliver.

Fine bubbles come with caveats. Their efficiency in real wastewater is noticeably lower than in clean water: surfactants in the wastewater hinder transfer, and design calculations account for this with a correction factor that for fine-bubble systems often lies between 0.4 and 0.7. In addition, diffuser membranes foul and lose elasticity over time. Transfer falls while air main pressure rises — the plant pays more for the same result, and none of this is visible from outside.

A separate approach is bubbleless aeration, in which oxygen passes to the biofilm through the wall of a gas-permeable membrane and is almost not lost to the atmosphere. This approach underlies, for example, SOLVIA CryoMABR.

Where air is wasted

Excess air consumption at operating plants almost always comes down to the same causes:

✓The blower runs at constant output around the clock, although the night-time load is several times lower than the daytime one.
✓The dissolved oxygen set point has been chosen with a large "just in case" margin.
✓Air flow is controlled by throttling valves rather than by speed: the blower spends energy overcoming the resistance of its own valves.
✓Diffusers are fouled or partly damaged: pressure rises, transfer falls.
✓Air is distributed unevenly along the tank: oxygen shortage at the inlet, surplus at the end.
✓Air supply is sized for oxygen while mixing is ensured "with a margin", or the other way round.

The oxygen set point: a small number with a big price

Dissolved oxygen in an aeration tank is usually held at 1.5–2 mg/L. Keeping it higher feels safer, but it is expensive, and the reason is the physics of transfer. Oxygen passes into water faster the larger the difference between the saturation concentration and the actual concentration. With saturation at about 9 mg/L, raising the set point from 2 to 3 mg/L reduces that difference from 7 to 6 mg/L, and transferring the same amount of oxygen then takes roughly 15–20 % more air. Treatment quality does not improve: the sludge simply gets more oxygen than it needs.

The opposite mistake happens too. A set point that is too low at the tank inlet, where the load is highest, slows nitrification and can encourage filamentous bacteria, which impair sludge settling in the secondary clarifier.

Measures that save energy

Dissolved oxygen control. Oxygen sensors in the tank and a controller that adjusts air supply to actual demand. This is the basic measure; the others do not work without it.

Variable-speed blowers. Air flow is varied by speed, not by a valve. It pairs well with "most-open-valve" logic: the system keeps header pressure at the minimum at which the most heavily loaded branch still receives its air.

Ammonia-based control. An ammonium nitrogen sensor at the tank outlet sets the oxygen set point: if the ammonium has already been oxidised, the oxygen set point is lowered. This is especially useful under variable load — at hotels, resorts and plants working in shifts.

Separating oxygen supply from mixing. When air supply is cut to what the sludge needs, at night there may not be enough air for mixing, and the sludge starts to settle. The solution is a separate mixing loop that switches on only when the oxygen air is not enough for this. The SmartAeration control system is built on this principle.

Denitrification. If the scheme includes an anoxic zone, bacteria there use the oxygen bound in nitrate to oxidise organic matter. About 2.9 grams of oxygen per gram of nitrate nitrogen reduced is returned to the process, and a corresponding part of the organic load no longer needs to be oxidised with air.

Diffuser maintenance. Regular monitoring of air main pressure and scheduled cleaning or replacement of diffuser membranes. A pressure rise at the same air flow is the earliest sign of fouling.

What next

If you run a biological plant and want to know how much it overpays for air, collect the data for an energy survey: the daily profile of inflow and of organic and nitrogen load, blower power consumption, air flow and header pressure, dissolved oxygen readings along the tank if sensors exist, and the type and age of the diffusers. Note separately how air supply is controlled today — by valve, by timer, or not at all.

This data shows what the plant is short of: control, air distribution or oxygen transfer itself. Air supply control and loop separation are what SmartAeration is designed for; membrane bubbleless aeration addresses oxygen losses. Equipment for the other stages — from flotation to sludge dewatering — is collected in the industrial wastewater treatment section.