C-310Rectangular Horizontal-Flow Sedimentation

How it works

1. The layout — Flow enters across the full width through a perforated baffle, travels the length of the basin, and leaves over a weir at the far end. Simple, and the sharing of walls between basins is why rectangular is cheaper per m² than circular once you need more than two.
2. Inlet distribution — The inlet baffle exists to convert a pipe jet into plug flow across the whole cross-section. Without it the flow forms a jet along one wall, the effective volume collapses, and a basin sized for two hours of retention delivers twenty minutes of it. This is the most common defect in real basins.
3. Settling — A particle enters at the surface and must reach the floor before the flow carries it to the weir. That is the same Hazen criterion as the circular tank — capture depends on overflow rate, Q over plan area, and not on depth.
4. Scour — Depth is not free of consequence, though. Reduce it and the horizontal velocity rises for the same flow. Once it approaches the critical scour velocity, settled sludge is lifted back into suspension and you lose the removal you had already achieved. Watch the flag turn as you shrink the depth.
5. What governs it — Two checks, pulling against each other. Overflow rate wants a large plan area; scour wants enough depth to keep the horizontal velocity modest. Length-to-depth ratio between 6 and 20 keeps you clear of both, plus far enough from the inlet for the flow to settle down.
6. Chain-and-flight — Flights on a chain sweep the floor toward a hopper and the surface toward a scum box, in one mechanism. It is cheap per square metre and it works. What it costs you is submerged wearing parts: sprockets, pins and shoes that are inspected only by taking the basin out of service and draining it.

Design parameters

The panel opposite runs the same correlations as the C-310 design sheet, so the animation and the calculator cannot disagree. Every parameter below is live — drag one and the picture responds.

Open the full C-310 design sheet for the other four inputs, all twenty-three results with their envelopes, the six equations printed beside the numbers they produce, and a calculation record you can export and put in a design file.