Sedimentation & Clarifier Operation Explained
Sedimentation sits between the chemistry (coagulation/flocculation) and the filters. Get the floc to settle here and the filters run clean and long; let it carry over and the filters plug fast. The exam tests both the process and three pieces of loading math — here they are together.
Key takeaways
- Sedimentation lets the heavy floc settle out as sludge so the filters only have to polish.
- The three numbers operators live by: detention time, surface overflow rate, and weir loading (overflow) rate.
- Good settling depends on well-formed floc, slow steady flow, and no short-circuiting (temperature/density currents, wind, inlet design).
- Pairs with coagulants compared, jar testing, and filter operation. Practice on the coagulation test and Level 1–4 tests.
What the basin does
After flocculation, water enters a large, quiet basin where velocity drops so low that the floc settles to the bottom under gravity, forming sludge that's scraped and removed. Clarified water flows out over weirs at the far end (or around the rim of a circular clarifier) on its way to the filters. A well-run basin can remove the large majority of the suspended solids before filtration.
Common types: rectangular (horizontal flow), circular/upflow (center feed, rim weirs), and basins fitted with tube or plate settlers that add settling surface in a smaller footprint.
The three loading numbers
1. Detention time — how long, on average, water stays in the basin:
Detention time (hr) = basin volume (gal) ÷ flow (gph)
Typical conventional basins run a few hours. Too short and the floc doesn't have time to settle.
2. Surface overflow rate (SOR) — flow divided by the basin's surface area:
SOR (gpd/sq ft) = flow (gpd) ÷ surface area (sq ft)
This is really a "settling velocity" — particles that settle faster than the overflow rate are captured; slower ones carry over. Lower SOR = better clarification.
3. Weir overflow (loading) rate — flow divided by the total length of weir the water spills over:
Weir loading (gpd/ft) = flow (gpd) ÷ weir length (ft)
If weir loading is too high, the water moves too fast near the weir and pulls settled floc up and over. Plants add more weir length (launders, finger weirs) to keep this rate down.
Worked example: A circular clarifier 60 ft in diameter treats 2.0 MGD.
- Surface area = 0.785 × 60² = 2,826 sq ft.
- SOR = 2,000,000 ÷ 2,826 ≈ 708 gpd/sq ft.
- If the rim weir is π × 60 = 188 ft, weir loading = 2,000,000 ÷ 188 ≈ 10,600 gpd/ft.
What hurts settling (the troubleshooting list)
- Weak or pin floc — the chemistry upstream is off; fix coagulation, not the basin.
- Short-circuiting — water finds a fast path from inlet to outlet, cutting effective detention. Caused by poor inlet baffling, density/temperature currents (cold dense water diving along the floor), or wind on open basins.
- Too-high overflow or weir rate — basin is overloaded for the flow; floc carries over.
- Sludge buildup — not removing sludge often enough; it can go septic, float, and degrade water quality.
A telltale exam clue: if settled-water turbidity is high but the floc looked fine, suspect short-circuiting or overloading; if the floc itself is poor, look back at coagulation.
Where the exam goes
Expect questions on what sedimentation removes, the three loading calculations, why high weir loading carries floc over, and short-circuiting causes. Build the math with the water operator math formulas guide, then take a Level practice test and review every explanation.