Between 500 KLD and 1 MLD, an STP blower duty grows from roughly 400 m³/hr to 800 m³/hr, and the redundancy pattern shifts from a single duty machine to two-plus-one. This is the band where parallel blowers start to make sense — for turndown, for energy and for redundancy. Here is the sizing worked for three plant sizes, with the models to specify.
The common framework
Every size uses the same steps from the complete STP blower guide:
- BOD load = Q × BOD removed ÷ 1000
- O₂ demand = BOD load × f (≈2.0)
- Air flow = O₂ demand ÷ (0.28 × SOTE_field), SOTE ≈ 0.18 fine bubble
- Add ~30% margin for peak, fouling and mixing
Standard assumptions: influent BOD ≈ 280 mg/L, treated ≈ 15 mg/L, so BOD removed ≈ 265 mg/L.
Worked sizing at three plant sizes
| Plant | Q (m³/day) | BOD load (kg/day) | O₂ demand (kg/day) | Air from oxygen (m³/hr) | Duty air with margin (m³/hr) |
|---|---|---|---|---|---|
| 500 KLD | 500 | 132 | 265 | ~219 | ~350–450 |
| 750 KLD | 750 | 199 | 397 | ~328 | ~500–650 |
| 1 MLD | 1000 | 265 | 530 | ~438 | ~700–800 |
Pressure by submergence
STP tanks in this band are commonly 3.5–5 m deep, which usually pushes the selection into double-stage. Applying submergence × 98 mbar plus losses:
| Submergence | Required discharge | Class |
|---|---|---|
| 3.5 m | ~418 mbar | Double-stage |
| 4.5 m | ~525 mbar | Double-stage / HP Series |
| 5.0 m | ~580 mbar | YEBL-DS-320P (570 mbar) |
Model selection per plant
500 KLD (~400 m³/hr, ~3.5–4.5 m)
- Shallow-ish: YEBL-1-420 (420 m³/hr) can serve if pressure allows a single-stage, but at ~4 m the pressure demand rules it out.
- Typical deep tank: YEBL-DS-320P (320 m³/hr, 570 mbar) — or run 2 × YEBL-DS-230 in parallel for turndown, with a third as standby.
750 KLD (~500–650 m³/hr)
- YEBL-DS-520V (520 m³/hr, 400 mbar) for moderate depth, or the higher-pressure YEBL-DS-520 (660 mbar) for deep tanks.
- Consider 2 duty + 1 standby of a mid-size model for better night turndown.
1 MLD (~700–800 m³/hr)
- YEBL-DS-900 (900 m³/hr) as a single large duty, or 2 × YEBL-1-700 / YEBL-DS-520 in parallel plus one standby.
The pressure maths behind these is in diffused aeration blower sizing.
Combined air-and-pressure worked example
Take a 1 MLD plant at 4.5 m submergence to show both checks landing on one model:
| Step | Calculation | Result |
|---|---|---|
| BOD load | 1000 × 265 ÷ 1000 | 265 kg/day |
| O₂ demand | 265 × 2.0 | 530 kg/day = 22.1 kg/hr |
| Air from oxygen | 22.1 ÷ (0.28 × 0.18) | ~438 m³/hr |
| Duty air (+30%) | 438 × 1.3 | ~570 m³/hr per stream, ~740 total |
| Static head | 4.5 × 98 | 441 mbar |
| + losses | — | ~85 mbar |
| Required discharge | 441 + 85 | ~526 mbar |
At ~740 m³/hr and ~526 mbar, the clean options are a single YEBL-DS-900 (900 m³/hr, capacity to spare) or 2 × YEBL-DS-320P (320 m³/hr, 570 mbar each) running in parallel plus a third as standby.
Single large machine vs parallel: the trade
| Factor | One large blower (+1 standby) | Two smaller (2+1) |
|---|---|---|
| Capital | Lower unit count | Higher unit count |
| Turndown | Throttle or VFD only | Shut one off at night |
| Redundancy on fault | Full duty lost until standby starts | Half duty stays online |
| Maintenance | Whole duty offline | Service one, run the other |
| Energy at part load | Poorer | Better |
For steady-load municipal plants a single large duty is simple; for variable load or where continuity is critical, 2+1 usually wins on running cost and resilience. The energy side is quantified in energy cost of STP aeration.
Why parallel blowers earn their place here
Above 500 KLD, splitting the duty across two smaller blowers instead of one large machine gives real advantages:
- Turndown: drop to one blower at night when load falls, saving energy — see energy cost of STP aeration.
- Redundancy: a failed unit is half the plant, not all of it.
- Maintenance: service one while the other runs.
Plan for phased capacity
Many STPs in this band are built for a design population that fills up over years, so the load at commissioning is a fraction of the design load. A single large blower run at a quarter-load wastes energy and may short-cycle. A 2+1 (or staged) arrangement lets you:
- Run one small blower in the early years and bring the second online as the load grows.
- Avoid throttling a big machine down to a fraction of its duty.
- Match the blower fleet to the actual population curve, not just the ultimate design.
This staged approach pairs naturally with the parallel philosophy and usually beats a single oversized machine on lifetime energy cost — quantified in energy cost of STP aeration.
Air header and diffuser distribution
At 500 KLD to 1 MLD the aeration tank is large enough that even air distribution across the diffuser grid matters. Undersized or poorly balanced headers create dead zones where sludge settles and lively zones where air is wasted. Size the air main and drop legs for low friction loss (that loss adds to the blower's pressure duty), and balance the grid so every diffuser sees similar flow. The pressure-loss allowance folds into the sizing in diffused aeration blower sizing.
Redundancy pattern
At this scale the standard shifts to N+1 with N≥2: for example, two duty blowers meeting peak demand plus one identical standby (2+1). Auto-changeover and weekly rotation as always. Detail in blower redundancy: duty/standby for STPs.
Watch the assumptions
The table figures use a 265 mg/L BOD-removed default. Stronger sewage (institutional, mixed commercial) raises oxygen demand proportionally, and a tighter effluent norm or an ammonia limit raises the oxygen factor f further — both push the blower up a size. Recompute from measured influent and your consent conditions; the compliance link is in wastewater aeration standards & CPCB.
FAQ
Single-stage or double-stage at 1 MLD? Almost always double-stage or larger, because tanks in this band are deep enough to demand 400–600 mbar. A single-stage YEBL-1-700 only suits a shallow tank.
Is a VFD worth it at this scale? Yes — DO-based speed control on the lead blower cuts part-load energy across 8,760 hours, and the payback on a machine this size is quick.
How many standby units for 2 MLD-plus? Larger and more critical plants often move to 2+2 or more; match the redundancy to the consequence of a discharge breach.
For smaller plants, see blower for a 100 KLD STP. All of this serves sewage treatment plants and, at higher loads, effluent treatment plants.
Get your plant sized
Send flow (500 KLD to 1 MLD), influent and target BOD, and tank depth. We will return a single-machine and a parallel option so you can compare capital, energy and redundancy.
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- Email: sales@yashblowers.org