Inlet filtering keeps the dirt out of a cistern in three stages: a leaf screen with 2–3 mm openings stops leaves and twigs, a first flush diverter throws away the first 1 L of runoff per m² of roof, and a settling chamber of 200–500 L drops out sand and grit. These parts reduce what enters the tank, but they do not make the water safe to drink; stored rainwater still needs treatment.
Part of Cisterns and Rainwater Storage
Inlet filtering is the set of devices between the roof and the tank that remove debris and the dirtiest water before storage. Everything that gets past them settles on the bottom of the tank or dissolves in the water.
Dirt Is Cheaper To Stop At The Gutter Than To Remove From The Tank
The first rain after a dry spell washes dust, bird droppings, leaves and insects off the roof. Whatever enters the tank stays there, feeds bacteria and algae, and has to be removed by cleaning. Each stage deals with a different kind of dirt:
| Stage | What it removes | Page |
|---|---|---|
| Leaf screen | Leaves, twigs, large debris | Leaf Screens |
| First flush diverter | The dirtiest first runoff | First Flush Diverter |
| Settling chamber | Sand and grit | Settling Chamber |
Water flows through them in that order: gutter, screen, diverter, settling chamber, tank.
What You Need
- A mesh screen with 2–3 mm openings
- A length of pipe, capped at the bottom, for the diverter
- A small tank of 200–500 L with a baffle, for the settling chamber
- A cleaning routine — the screen needs cleaning monthly
Method: Fit And Maintain The Three Stages
Step 1: Fit the screen at the gutter entrance or at the tank inlet.
Step 2: Size the diverter from the roof area: 1 L for every m² of roof. A 40 m² roof needs a diverter that holds 40 L.
Step 3: Place the settling chamber between the diverter and the tank, so water enters at one end and leaves from the other past a baffle.
Step 4: Clean the screen monthly, empty the diverter’s drip hole of grit, and clear sediment from the chamber at intervals.
Step 5: Treat the water before drinking (Water Purification).
Common Mistakes
- Skipping the diverter. Contaminated water enters the tank.
- Never cleaning the screen. It clogs and the gutter overflows past it.
- Treating the filters as purification. They do not remove bacteria or dissolved chemicals.
What’s Next
- Learn next: First Flush Diverter — the single most effective part
- Combine with: Overflow Management — what happens when the tank is full