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Basin Design

5 min read

Parent
🪨
Sedimentation Basins
Letting particles settle out
Current
📐
Basin Design
Sizing and flow rate
Sub-Topics
⏱️
Retention Time
How long water needs to sit

Size a settling basin on two tests and build to whichever gives the larger basin: retention time (volume divided by flow, at least 2-4 hours) and surface overflow rate (flow divided by surface area, which must be below the settling speed of the smallest particle to remove). At a village flow of 2,000 liters a day the retention test sets the volume at only 167-333 liters, so the real size is set by shape, baffles and access to clean it, with a length to width ratio of 3:1 to 5:1.

Part of Water Treatment at Scale

Basin design is the sizing and shaping of a sedimentation basin: its volume, surface area, depth, length to width ratio, inlet baffle and outlet, so that particles settle before water leaves.

Two Different Tests Govern, And At Village Flows The Retention Test Usually Wins

The sizing method of Settling Basins is used here. A particle of settling velocity v_s is held back if the surface overflow rate (SOR), flow divided by surface area, is below v_s. Retention time is volume divided by flow. The carrier requires at least 2-4 hours. Compute both and keep the larger area.

For small flows the SOR test gives an absurdly small basin, so the settling-basins page builds from retention time. The Stokes velocities that page tabulates are 0.3 mm/s for 0.02 mm fine sand and 0.003 mm/s for 0.002 mm silt.

What You Need

  • Design flow in m³/h, set by the filter’s steady rate and not by the peak
  • The smallest particle to remove, and its settling velocity from Stokes’ law
  • A retention time: 2-4 hours from the carrier
  • A site that allows a 3:1 to 5:1 rectangle, an inlet baffle, an outlet weir and a drain

Method: Run Both Tests, Then Shape The Basin

Step 1: Flow. Convert liters per day to m³/h: ÷ 24 ÷ 1,000.

Step 2: Retention test. Volume = flow × retention time. Divide by a depth of 1.0-2.0 m to get area.

Step 3: SOR test. Area = flow ÷ v_s, with v_s in m/h (1 mm/s = 3.6 m/h).

Step 4: Take the larger area. Then shape it 3:1 to 5:1 long to short, so the water travels along the basin and does not short-circuit.

Step 5: Add the baffle about 0.3 m from the inlet, with the outlet on a weir. Make the drain reachable without entering.

Worked Example: The Carrier’s Basin At 2,000 L/day

Inputs: 2,000 L/day = 2 m³/day; basin 2 m × 4 m × 1.5 m = 12 m³, area 8 m².

  • Flow = 2 ÷ 24 = 0.0833 m³/h.
  • Retention test: volume = 0.0833 × 2 = 0.167 m³ up to 0.0833 × 4 = 0.333 m³, 167-333 L. At 1.0 m deep that is under 0.4 m² of surface.
  • SOR of the carrier basin = 0.0833 ÷ 8 = 0.0104 m/h = 0.0029 mm/s.
  • Silt of 0.002 mm settles at 0.003 mm/s = 0.0108 m/h. That is above 0.0104, so in an ideal basin even this silt is held. Ideal means perfectly even flow, no short-circuiting and no resuspension. Real basins fall short of that, so treat this as the upper bound on what the basin can do, not the result.
  • Flow the basin can take: retention-limited at 4 hours, 12 ÷ 4 = 3 m³/h = 0.83 L/s = 72 m³/day. SOR-limited for fine sand of 0.05 mm (7.2 m/h): 7.2 × 8 = 57.6 m³/h = 16 L/s. Retention governs by a factor of about 19.

Shape. The carrier basin is 2 m by 4 m, a ratio of 2:1, below the 3:1 to 5:1 that Settling Basins recommends. At the same 8 m², a 3:1 rectangle is √(8 ÷ 3) = 1.63 m wide and 3 × 1.63 = 4.9 m long.

Parallel basins. Two basins side by side, as the carrier suggests so one can be cleaned, each take half the flow, which doubles retention in each. The series arrangement in the settling-basins page does something different: it settles coarse particles first and fine ones second. Choose parallel for cleaning access and series for performance.

Sludge. This uses assumed inputs. If raw water carries 100 mg/L of suspended solids, 80% settle, and the sludge is 5% solids at 1,100 kg/m³: mass per day = 2 m³ × 0.1 kg/m³ × 0.8 = 0.16 kg; volume = 0.16 ÷ (0.05 × 1,100) = 0.0029 m³/day, about 1.06 m³ a year, a rise of 1.06 ÷ 8 = 0.13 m. A storm that makes the water ten times muddier brings ten times more. Plan desilting from the measured depth of sludge, not the calendar, as Settling Basins does (monthly or when depth falls below 0.5 m).

Failure Modes

  • Water short-circuits along one wall: no baffle, or the inlet is too fast. Dye or a floating chip shows it (Retention Time).
  • Settled mud returns to the outflow: sludge too deep, or a flow surge.
  • Scum at the outlet: the pipe is at the surface and not 30 cm below it.
  • Clay stays cloudy: physical settling alone cannot remove it (Sedimentation).

Common Mistakes

  • Sizing on the SOR test alone at a village flow. The basin is then too small to enter or hold sludge.
  • Mixing mm/s and m/h. 2 mm/s is 7.2 m/h, not 0.12 m/h.
  • Designing to peak flow. The basin feeds a filter that runs at a steady rate.

Safety

A deep basin is a drowning and confined-space hazard

Never enter a drained basin without ventilation, a harness on a line held by a person outside, and a rule that the person outside never goes in after a collapsed worker. Sludge can release gas. Do not test the air with a flame. Fence the basin and cover it so children and animals cannot fall in.

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