Where the electricity in an activated sludge plant goes, and how to find out how much of it you do not need
The blower nameplate tells you what the plant can consume. The load tells you what it needs to. The gap between those two is the whole subject.
On an activated sludge plant, aeration is the load worth looking at first, and it is often running further than it needs to. Not because anyone decided to waste it, but because the blower was sized for a load that has not arrived, and nothing in the plant is telling it to stop.
Working out how much air the process actually calls for is a four-step calculation, and every step is a place where a real plant diverges from the one on the drawing.
Step one: the oxygen the process actually demands
Oxygen demand is set by the load, not by the tank. Three terms:
- Carbonaceous demand — the oxygen consumed oxidising the biodegradable COD, less the fraction that leaves as new biomass rather than as carbon dioxide. Sludge production and oxygen demand are two sides of one balance: a process that wastes more sludge consumes less oxygen for the same load.
- Nitrogenous demand — the oxygen consumed nitrifying ammonium. Per unit of nitrogen this is a large number, which is why a plant asked to nitrify has a materially different air demand from one that is not, at the same COD.
- Credit for denitrification — nitrate used as an electron acceptor in an anoxic zone returns part of the oxygen already spent making it. A plant with a properly fed anoxic zone genuinely needs less air.
All three depend on the influent characterisation, which is why the energy answer and the fractionation are the same question asked twice. And the demand must be evaluated at the peak, not at the average — the blower has to meet the worst four hours of a Monday, and the plant spends the rest of the week not needing that.
Step two: standard is not actual
Diffuser performance is published at standard conditions: clean water, 20 °C, one atmosphere, zero dissolved oxygen. Your plant is none of those. The correction from the standard rate to the rate you will actually get is where most of the optimism in an aeration design lives.
| Factor | What it accounts for | Why it bites |
|---|---|---|
| Alpha | Transfer in your mixed liquor relative to clean water. | Surfactants and dissolved organics suppress transfer. Industrial effluents can sit well below municipal values, and alpha also falls as diffusers foul in service — so the design value and the year-three value are not the same number. |
| Beta | Saturation in your water relative to clean water. | Dissolved solids lower the saturation concentration. |
| Temperature | Both the transfer coefficient and the saturation value. | These move in opposite directions. Warm water transfers faster but holds less, and the summer case is not automatically the easy one. |
| Operating dissolved oxygen | The driving force you have left. | Transfer is driven by the deficit between saturation and the concentration you hold. Holding a higher setpoint shrinks that deficit and costs air for no treatment benefit — this is the single most common operational overspend. |
| Submergence and pressure | Where the diffuser sits and what the blower must overcome. | Depth raises the effective saturation, which helps transfer, and raises the discharge pressure, which costs power. |
Why transfer efficiency per metre is not transfer efficiency
Diffuser data is often quoted as a standard transfer efficiency per metre of submergence. That figure exists so that performance can be compared across tanks of different depth. It is not the efficiency of your installation, and multiplying it by your depth is not a general result — the relationship is not linear over the whole range, and the quoted per-metre figure carries its own basis conditions.
Two errors follow from treating them as interchangeable, and they push in the same direction: an overstated transfer efficiency produces an understated air demand, which produces a blower that cannot meet the peak. The plant then runs at full output permanently, which is both the worst place on the curve and the reason the energy bill is what it is.
Step three: the blower, honestly
Shaft power follows from the mass flow of air, the pressure ratio the blower works across, and its efficiency. The pressure ratio has to include everything: diffuser submergence, diffuser headloss — clean and fouled — piping, and the filter.
The check worth doing is a comparison rather than a calculation: does the blower's duty match the process air the reactors downstream actually call for? Those two are specified by different people at different times, and they disagree more often than anyone expects.
We found exactly that in our own simulator. The blower block took a discharge pressure and an efficiency, used neither, and reported only air flow — while the correct equation was already implemented a hundred lines away for a different block. It now computes shaft power, pressure ratio, air mass flow and specific power, and it compares its duty against what the reactors demand. That item, and eleven others, are published in the verification.
Step four: get to a number you can compare
Two metrics make aeration energy legible, and a plant should know both:
- kWh per cubic metre treated. Useful against your own history and against your water bill.
- kWh per kilogram of BOD removed. The one that allows comparison between plants, because it normalises out the fact that some plants treat weak water and some treat strong water. A plant looking efficient per cubic metre may simply have dilute influent.
The second is the one worth tracking, and it is also the one with no public benchmark behind it: there is no published figure in this region that a plant manager can hold their own number up against, which is why so many plants have no idea whether theirs is good.
How to find the gap on your own plant
Before any capital is spent, four things are worth doing, in order:
- Plot the dissolved oxygen against time. If it never drops, the plant is aerating past the demand for part of every day.
- Compute the demand from the load, hour by hour, from a real characterisation — then compare it with what the blowers are delivering.
- Check the control. Fixed-speed blowers against a varying load can only be right at one point on the day. Everything else is either over-aeration or a deficit.
- Check the setpoint. A dissolved-oxygen setpoint set high "for safety" costs air continuously, and is usually the cheapest change available.
None of that needs capital. All of it needs the plant modelled against its own water rather than against a nameplate — which is what the simulator is for.