A settling chamber is a small tank of 200–500 L between the roof inlet and the main cistern, with the inlet at one end, the outlet at the other and a baffle between them, so that sand and grit sink out before the water reaches storage. At storm flow it holds water for only about 6–15 minutes, which removes coarse particles but not silt, so it is a grit trap and not a clarifier.
Part of Cisterns and Rainwater Storage
A settling chamber is a pre-storage sedimentation tank that slows incoming roof water long enough for heavy particles to fall to the floor. It keeps the main cistern’s floor cleaner and its cleanings less frequent.
A Baffle Forces The Water To Travel Instead Of Short-Cutting
Water entering one side of a small tank and leaving by the other can run straight across and carry sediment with it. A baffle, a wall that forces the water to go under or over it, spreads the flow and lengthens the path. Heavy particles drop out along the way.
How much a chamber catches depends on how fast water passes through relative to how fast a particle sinks. The method used in Settling Basins is surface overflow rate: flow divided by the water-surface area.
What You Need
- A tank of 200–500 L — a drum, a masonry box or a ferrocement cell
- A baffle — a board, slab or sheet across the tank, with a gap
- Pipe fittings — inlet at one end, outlet at the other, with the outlet above the floor
- A drain plug at the bottom for clean-out
- A lid with a screened vent
Method: Place The Baffle So Water Must Go Round
Step 1: Put the inlet and outlet at opposite ends.
Step 2: Fit the baffle between them, either hanging from the top so the water goes under it, or standing on the floor so water goes over it.
Step 3: Set the outlet above the floor, so settled grit stays behind.
Step 4: Clean out the settled grit at intervals through the drain plug, flushing the chamber and not the sediment into the cistern.
Worked Example
Flow from a 40 m² roof in rain of 50 mm/h (an illustrative intensity) is 40 × 50 = 2,000 L/h = 2.0 m³/h = 0.56 L/s.
Retention time = volume ÷ flow. A 200 L chamber: 200 ÷ 0.56 = 360 s = 6 minutes. A 500 L chamber: 500 ÷ 0.56 = 900 s = 15 minutes.
Surface overflow rate. If a 300 L chamber is 0.5 m deep, its surface is 0.3 m³ ÷ 0.5 m = 0.6 m². SOR = 2.0 m³/h ÷ 0.6 m² = 3.3 m/h. A particle settles only if it sinks faster than 3.3 m/h = 0.93 mm/s. From the settling table in the Settling Basins page: coarse sand (0.2 mm) sinks at 25 mm/s and settles; fine sand (0.02 mm) sinks at 0.3 mm/s and does not; silt at 0.003 mm/s passes right through.
So the chamber removes grit and coarse sand. Finer sediment is left to settle slowly in the quiet cistern, where it needs a clean-out every so often, and clay never settles by itself.
Common Mistakes
- No baffle. Water runs straight through.
- Outlet at the floor. It carries settled grit out.
- Never cleaning it. Settled grit builds up, reduces the volume and re-enters the flow in a heavy storm.
- Too small for the roof. At high flow, water passes before anything settles.
What’s Next
- Learn next: Overflow Management — what happens after the tank fills
- Combine with: Inlet Filtering — how the chamber fits with the other stages