Rainwater harvesting systems: uses and limits

Short answer: rainwater harvesting systems collect water from the roof, filter it and store it in a tank for non-drinking uses, such as irrigation, washing and toilet flushing. They can reduce the use of treated water, but collected water should not be used for drinking without proper treatment, and the system requires maintenance. This text is general and does not replace a technical design.

How it works

1. Collection. Rain runs off the roof into gutters and downpipes.
2. Filtering. Screens and filters hold back leaves and dirt. Many systems discard the first flush of rain, which carries dust and impurities.
3. Storage. A tank, buried or above ground, holds the water, preferably protected from light and heat.
4. Distribution. A pump carries the water to the points of use, in separate, labeled piping.

Common uses

• Irrigating gardens and vegetable beds, following local guidance.
• Washing floors, sidewalks and vehicles.
• Flushing toilets.
• Cooling and other non-drinking uses.

Limits

• Variable quality. The water may contain dust, bird droppings, leaves and microorganisms. Drinking or cooking with it requires proper treatment and testing.
• It depends on rain. Dry periods reduce supply, and the tank may run empty.
• Roof area and volume. Small houses collect less water.
• Initial cost. The tank, filters and installation can take a long time to pay off.
• Maintenance. Gutters and filters need cleaning, and the tank must be closed to prevent mosquitoes.

Precautions and standards

• Label non-drinking pipes to avoid cross-connections with the treated-water network.
• Cover the tank and use screens to keep insects out.
• Check your municipality's rules and requirements, which vary.
• In condominiums, consult the rules for works.

Counterpoints

In regions with regular rain and high consumption for gardens or cleaning, the return is better. In places with little rain or where water is very cheap, the savings may be small. It is worth calculating beforehand.

Conclusion

Rain harvesting is a useful measure, when well designed and maintained, for uses that do not need drinking water. It does not replace treatment for human consumption.