Primary clarifiers — circular or rectangular gravity tanks that settle the readily-settleable solids and float scum ahead of biological treatment.
Sedimentation & Clarification — in depth
Primary clarifiers are the gravity workhorse of solids removal. Wastewater slows in a circular or rectangular tank so settleable solids fall to a scraped hopper and scum is skimmed off the top — removing 50–70% of suspended solids and a third of the BOD, cutting the load on downstream processes.
What matters in practice
Tank geometry to suit the site.
Settleable solids fall to the floor.
Mechanical sludge and scum removal.
Surface loading sets capture.
| Parameter | Typical | Note |
|---|---|---|
| SS removal | 50–70% | Settleable |
| BOD removal | 25–40% | Particulate |
| Overflow rate | 1–2.5 m/h | Design |
| Detention | 1.5–2.5 h | Typical |
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Fundamentals, design drivers and practical guidance
Primary clarifiers — circular or rectangular gravity tanks that settle the readily-settleable solids and float scum ahead of biological treatment.
Ballasted clarification accelerates settling by attaching micro-sand (or recycled ballast) to the floc, sharply increasing its settling velocity and allowing very high overflow rates in a compact, fast-starting plant — ideal for variable or storm flows. Across all types, inlet/outlet hydraulics, weir loading and sludge-collection design determine whether the theoretical overflow rate is actually achieved or short-circuited away.
Reynolds & Bauhm sizes clarification on settling velocity and surface overflow rate — selecting conventional, lamella or ballasted systems and the inlet, weir and sludge-collection detail that makes the design overflow rate real, not theoretical.
Sedimentation removes settleable solids by letting gravity do the work, and its performance is governed by surface overflow rate — the design flow divided by the clarifier's plan area — rather than by depth or retention time alone. A particle is captured only if its settling velocity exceeds the upflow (overflow) rate, which is why plan area, not tank volume, is the master sizing variable.
What our engineers assess on every scope of this type
| Parameter | Typical basis | Why it matters |
|---|---|---|
| Overflow rate | Flow / plan area | Captures particles above it |
| Plan area | Master sizing variable | Not depth or volume |
| Lamella | Inclined-plate area | Big area, small footprint |
| Ballasted | Micro-sand floc | High rate, fast start |
| Inlet | Energy dissipation | Stops short-circuiting |
| Sludge | Collection + withdrawal | Removes settled solids |
Common questions on sedimentation and clarification
Because poor inlet hydraulics create currents and short-circuiting that let flow bypass the settling zone, so the tank never achieves its theoretical overflow rate. Energy dissipation and even distribution are essential to realise the design.
Sludge-collection mechanisms scrape settled solids to a hopper from which they are withdrawn at a controlled rate. Proper collection design keeps the clarifier in balance and prevents solids re-suspension.
Surface overflow rate — the flow divided by plan area. A particle settles out only if its settling velocity exceeds that rate, so Primary Clarifiers is sized on plan area and settling velocity, not on tank depth or retention time alone.
By stacking inclined plates or tubes, they present the projected settling area of a much larger basin within a small footprint. This area multiplication is why they suit land-constrained sites while keeping the effective overflow rate low.
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