What Size Rotary Screw Blower Does a Wastewater Treatment Plant Need?

Short answer: Most municipal aeration basins need a rotary screw blower sized for a duty point between 0.5 and 0.8 bar, running 24 hours a day — in our WindForce series that is typically a WF-BL-50 to WF-BL-100 package (37–75 kW, airflow 20–100 m³/min at 0.3–1.0 bar). Size the machine from basin depth, diffuser submergence and design flow, never from a nameplate range; internal compression typically cuts absorbed power by up to 20–30% versus a Roots blower on the same duty (per our published WindForce planning data — a planning benchmark, not a guarantee).

By Johnny Wayne, General Manager, AirSpace Machinery Co., Ltd. — 20 years in industrial compressed air systems. Reviewed by our engineering team.

Wastewater treatment is the largest single installed base for low-pressure blowers, and it is also the application where the choice between blower technologies shows up fastest on the electricity bill. When plant operators ask us what a screw blower actually does for them — we cover the basics in our main screw air blower page — the honest answer starts with the duty point, not the model list. This guide walks through how the aeration duty determines the machine, what the WindForce planning range covers, where the energy savings come from, and exactly what information our engineers need from you before we will quote a flow figure.

What does the aeration duty actually ask of a blower?

Biological treatment is an aerobic process: the bacteria that consume the organic load need dissolved oxygen, and that oxygen arrives as air blown through diffusers at the bottom of the basin. Three features of this duty shape the whole selection:

Pressure is set by water depth, not by the process. Aeration duty typically sits between 0.5 and 0.8 bar, driven mostly by diffuser submergence depth plus pipework and diffuser losses — per the sizing method in our screw blower specification guide.
The load runs 24 hours a day, all year. That is 8,760 operating hours annually, so every point of specific power saved is multiplied relentlessly on the electricity bill.
The demand is never constant. Inflow and therefore biological oxygen demand vary through the day and across seasons, so a machine sized for peak flow and run at fixed speed blows surplus air — and surplus kilowatts — around the clock.

Aeration typically accounts for the majority of a treatment plant’s electricity consumption, which is why the blower decision deserves the same rigor as the main pump station, not the same rigor as a utility purchase.

Which WindForce model matches your basin?

Our WindForce screw blower series is planned across five frame sizes, all rated for a discharge pressure of 0.3–1.0 bar:

Model	Power	Discharge pressure	Airflow volume	Primary application
WF-BL-30	30 HP / 22 kW	0.3–1.0 bar	10–25 m³/min	Small plants, package plants, conveying
WF-BL-50	50 HP / 37 kW	0.3–1.0 bar	20–45 m³/min	Municipal wastewater, mid-size basins
WF-BL-75	75 HP / 55 kW	0.3–1.0 bar	40–70 m³/min	Large aeration basins
WF-BL-100	100 HP / 75 kW	0.3–1.0 bar	65–100 m³/min	Industrial treatment, large works
WF-BL-150+	150+ HP / 110+ kW	0.3–1.0 bar	95–160+ m³/min	Large-scale process aeration

Airflow figures are planning values at standard intake conditions; final FAD must be verified at your stated pressure through an RFQ, per the acceptance-test logic of ISO 1217.

Worked example — a mid-size municipal works. Suppose your basin needs 35 m³/min of FAD at 0.7 bar gauge, continuous duty. Here is how the selection lines up, step by step:

Pressure check: 0.7 bar sits inside the 0.3–1.0 bar WindForce envelope, with margin for diffuser fouling over the maintenance cycle.
Flow check: 35 m³/min falls in the middle of the WF-BL-50 range (20–45 m³/min) — mid-range placement matters, because a variable-speed machine needs headroom in both directions to follow the oxygen curve.
Redundancy check: for a continuous biological duty, the standby plan usually decides the machine count, not the flow calculation. Two WF-BL-50 units (duty plus standby, or two machines sharing the load) typically beat one larger unit on both risk and part-load efficiency.
Energy check: see the next section for what the power difference versus a Roots installation is worth per year at this size.

We would not finalize that selection without your measured duty point — our standing rule, which we also apply to compressor work, is that we do not quote a flow figure without the pressure it was measured at.

How much can a screw blower save against a Roots blower?

The saving comes from compression physics, not from a bigger motor. A Roots blower moves a fixed volume per revolution and meets the pipe pressure by backflow compression — the surge of air back into the lobe chamber is pure loss, and it is loudest exactly when the machine is working hardest. A screw blower compresses internally between the rotors, so the pressure rise happens progressively inside the air end.

Per our published WindForce planning data, internal compression typically reduces absorbed power by up to 20–30% versus a conventional Roots blower on the same duty (a planning benchmark, not a guarantee). Here is what that means in money for the worked example above, if your existing Roots package draws 90 kW at the 35 m³/min / 0.7 bar duty point:

Power saved: 90 kW × 25% (mid-point of the 20–30% benchmark) = 22.5 kW.
Energy saved per year: 22.5 kW × 8,760 h/yr = 197,100 kWh.
Cost saved per year: 197,100 kWh × 0.08 USD/kWh = about 15,770 USD/yr (based on 0.08 USD/kWh · 8,760 h/yr · 25% delta on a 90 kW Roots baseline — a planning estimate, not a guarantee).

In the field, the ranking order of blower technologies rarely changes: screw machines earn their keep on continuous, variable, energy-dominated duties like aeration; Roots units still make sense where cost per installed kilowatt dominates and the duty is light or intermittent.

How does variable speed follow the oxygen curve through the day?

Biological oxygen demand follows the plant’s load profile — in a typical municipal works the overnight inflow can sit 30% or more below the daytime peak, yet a fixed-speed blower sized for peak flow answers every hour as if it were the peak hour, and the surplus is blown off through valves as noise and heat.

With PMV/VSD control, the machine follows the basin’s dissolved-oxygen setpoint instead of the clock: when the basin needs less air, the blower slows and the power curve follows roughly with the cube of speed in the favorable part of the map — which is why we treat variable speed as more consequential on a blower than on a 7-bar compressor of the same power. Our IE4/IE5 permanent-magnet motor packages are planned for exactly this duty profile, and all packages carry a 50°C full ambient rating, because a machine derating in a 40°C blower room is a machine that no longer meets the duty you bought it for.

What did our commissioning record teach us about pressure stability?

This is the part a data sheet cannot tell you, so here is what our engineers log on site. In a 2025 project in Ho Chi Minh City, we replaced an ageing fixed-speed machine at an iron fabrication plant running 15,000 m² of shop floor with more than 200 employees on 16-hour production days. The complaint was not capacity — it was that system pressure moved by ±0.5 bar across the shift, which disrupted the work and stressed downstream equipment. The full before-and-after record is in our Vietnam PMV installation case study.

Our commissioning procedure is the same whether the machine is a compressor or a blower, and we carry it into every aeration enquiry: we survey the existing system before quoting, we measure the actual duty point rather than trusting the nameplate, we install pressure and flow instrumentation, and we re-measure after commissioning so the plant has a before-and-after record instead of a promise. Performance is verified against the acceptance-test method of ISO 1217, and air quality against ISO 8573-1. When you evaluate suppliers, ask for exactly that pair of measurements from a reference site — the answer tells you more than any brochure.

Why does oil-free delivery matter in biological treatment?

The air stream in an aeration basin is a process input: whatever the blower carries into the basin ends up in the biology you are paid to maintain. Lubricant carry-over from a machine’s gearbox or oil circuit is an effluent-obligation problem as much as a process problem. Our WindForce packages deliver an oil-free air stream by design, and for process-critical duties we plan against ISO 8573-1 Class 0 oil-free air, the strictest air-purity class in the standard — the same Class 0 logic fermentation and food plants apply, where the air is an ingredient rather than a utility.

Can one plant need both a blower and a low-pressure air compressor?

Yes — and it is more common than buyers expect. A treatment works, a textile mill, a ceramic plant or a cement line often runs two distinct low-pressure duties side by side: aeration or conveying at 0.3–1.0 bar, which is blower territory, and plant or process air at 3–5 bar, which is compressor territory. For the second duty we build low-pressure air compressors for the textile, ceramic, glass and cement industries in the 15–300 HP range at 3–5 bar — our Egypt textile installation shows that family working in an adjacent role to the blower. If your site mixes both duties, tell us both duty points; sizing them as one system typically saves a machine and a control layer. The background on why screw technology wins the low-pressure end of either family is in Why a Blower?.

What information should you send for a quotation?

Our engineers can size a WindForce package from six numbers:

Basin or duty volume and water depth (for aeration) — or conveying distance and material (for conveying).
Target dissolved-oxygen setpoint, or the process pressure the duty actually needs.
Diffuser type and submergence, and the pressure rise across diffusers plus pipework.
Design flow in m³/min, and the expected minimum and maximum of the load profile.
Ambient conditions at the installation site — the 50°C full rating only helps if we know the real blower-room temperature.
Voltage, frequency, and whether the unit is duty, standby, or part of an N+1 group.

Send those with a one-line description of the plant, and the quotation will come back with the flow figure stated at its pressure — which is the pair of numbers our specification guide argues every blower quote should carry.

Key takeaways

Aeration duty typically sits at 0.5–0.8 bar and runs 8,760 hours a year, so specific power — not purchase price — dominates the total cost.
The WindForce series covers 30–150+ HP (22–110+ kW) at 0.3–1.0 bar, with airflow from 10 to 160+ m³/min across five frame sizes.
Internal compression typically cuts absorbed power by up to 20–30% versus a Roots blower on the same duty (per our published WindForce planning data — a planning benchmark, not a guarantee); on a 90 kW duty that is worth roughly 15,770 USD/yr at 0.08 USD/kWh.
Size from basin depth, diffuser submergence and design flow; a flow figure quoted without its pressure is not a specification.
Oil-free delivery (planned against ISO 8573-1 Class 0 for process-critical duties) protects both the biology and the effluent obligation.

Frequently asked questions

What pressure does a wastewater aeration blower need?

Typically 0.5–0.8 bar, set by diffuser submergence depth plus pipework and diffuser losses. The WindForce envelope of 0.3–1.0 bar covers conventional municipal basins with margin for diffuser fouling; measure your actual duty point before selecting the model.

Can a screw blower run continuously, 24 hours a day?

Yes — aeration is precisely the continuous duty screw blowers are built for. Packages carry a 50°C full ambient rating and, with PMV/VSD control, follow the oxygen demand curve rather than running at full power around the clock.

Is the air from a screw blower oil-free?

Our WindForce packages deliver an oil-free air stream by design. For process-critical duties — biological aeration, fermentation, food processing — we plan the installation against ISO 8573-1 Class 0, the strictest purity class in the standard.

How do I size a blower for my aeration basin?

Send basin volume and depth, target dissolved oxygen, diffuser type and submergence, design flow, and the pressure rise across diffusers plus pipework. Our engineers will return a model selection with the FAD stated at its pressure, verified per ISO 1217 test logic.

Which is cheaper to run, a screw blower or a Roots blower?

A screw blower, on continuous duties — internal compression typically reduces absorbed power by up to 20–30% versus a Roots blower on the same duty (a planning benchmark, not a guarantee). Roots machines remain competitive where installed cost dominates and the duty is light or intermittent.

Sources and standards

ISO 1217 — displacement compressors, acceptance test method; the logic we apply to verify delivered FAD at a stated pressure.
ISO 8573-1 — compressed air purity classes, including Class 0 for oil-free process air.
ISO 9001:2015 / CE — AirSpace Machinery quality system and product marking.
Published WindForce planning data: Screw Air Blower series page.
Specification method: How Do You Specify a Screw Blower So the Quoted Flow Is Actually Achievable?
Field record: Vietnam 75 kW PM VSD installation case study.

About the author

Johnny Wayne is General Manager of AirSpace Machinery Co., Ltd., a 4,000 m² compressor manufacturing facility in Shanghai, ISO 9001:2015 certified and CE marked, building industrial screw air compressors from 2 to 540 HP, oil-free Class 0 packages, and the WindForce screw blower series for 0.3–1.0 bar duties. He has spent 20 years in industrial compressed air systems, and the commissioning procedure described in this guide — survey, measure, install instrumentation, re-measure — is the same one his team applied on the Ho Chi Minh City installation referenced above.