Rainwater harvesting and its treatment

Rainwater harvesting (RWH) is the collection and storage of rain falling on roofs or the ground for later use, or for recharging groundwater, instead of letting it run off.

It is mainly done from rooftops: rain falls on the roof, is collected by gutters, passes through a first-flush device and a filter, and is stored in a tank or sent to recharge pits. It is needed because of water scarcity and falling groundwater levels, especially in urban areas like the Kathmandu valley.

Treatment of rainwater

Rainwater is relatively pure, but it picks up dust, leaves, bird droppings and microbes from the roof and the air. Treatment steps:

1. Screening: mesh at the gutters and inlet removes leaves and large debris.
2. First-flush diversion: discards the first 10-15 minutes of dirty rain.
3. Sedimentation: holding the water in a tank lets suspended particles settle.
4. Filtration: a gravity filter with layers of coarse gravel (bottom), fine gravel, sand and charcoal (top) removes turbidity, odour and colour. A slow sand filter or ceramic candle filter can be used for drinking water.
5. Disinfection: chlorination (a chlorine dose of about 2-4 mg/L, e.g. from bleaching powder, leaving a residual of 0.2-0.5 mg/L), boiling, UV, SODIS (solar disinfection) or ozone, to kill pathogens.
6. Storage care: a closed, dark tank with a mosquito-proof vent, cleaned every year.

 rain -> [mesh] -> [first flush] -> [sedimentation]
        -> [sand-gravel-charcoal filter] -> [disinfection]
        -> storage tank -> use