What it shows
The first rain after a dry spell washes dust, bird droppings and leaves off a roof. A first flush diverter fills a side pipe with that dirty water before clean water reaches the tank. The project builds one and tests turbidity before and after to show how it protects stored water.
The science
The pipe is a vertical chamber below the downpipe. Rainwater coming down fills the chamber first because the chamber has the easiest path. When it is full, the ball floats up and seals the top, so the later cleaner water goes sideways into the tank. The small hole at the bottom empties the chamber slowly after the rain. Dirty first flush carries most of the pollution load, so removing it cleans the tank water.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 1 m piece of 75 mm PVC pipe with a cap, T-joint and a float ball (a table-tennis ball)
- A drip hole made by an adult in the cap, 3 mm
- A model roof from a tray or a plastic sheet on a slope
- Garden dust and dry leaves
- A watering can or mug for rain
- Two clear bottles for samples and a turbidity tube (a clear tube with a black cross at the bottom)
- A rubber seal or tape
Safety
For this project: Do not drink any collected water. An adult must cut the pipe and drill the hole.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Measure the roof area of the model and calculate the first flush volume as 1 to 2 litres per 10 square metres of roof, then scale to your model.
Choose the pipe length to give the required volume using volume = area x length.
Cut the pipe, fit the cap with a small drip hole at the lower end and the T-joint at the top.
Place the table-tennis ball in a cage made of stiff wire or tape so it can float but not escape.
Connect the downpipe from the roof to the T-joint, and the outlet side to the collection bottle.
Dust the roof with garden dust and leaves, then pour water gently to simulate 10 mm of rain.
Collect the first litre of outlet water in a bottle labelled A and the later water in B.
Measure turbidity of both samples with the turbidity tube or a simple torch and phone-light meter test.
Run a test with the diverter blocked, and compare the turbidity of the tank water in each case.
Write the report with the sizing calculations, results, and how often the chamber must be cleaned.
What you should see
The first flush sample is much muddier than the later sample, and the tank water with the diverter is visibly clearer than without it. A turbidity tube shows a clear difference in the depth at which the cross disappears.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is first flush?
The initial part of a rainfall that washes accumulated dirt from the roof.
2How is the volume of the chamber chosen?
About 1 to 2 litres for every 10 square metres of roof area, adjusted to local dust.
3Why is there a hole in the cap?
To let the chamber drain slowly so it is ready for the next rain.
4Does the diverter make the water safe to drink?
No. It reduces dirt but the water still needs filtering and disinfection.
Take it further
Test for different roof materials such as tile, tin and concrete and compare the amount of dirt in the first flush.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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