Rainwater harvesting: a practical guide for beginners

Practical benefits

• Autonomy—provides a usable reserve during droughts, watering restrictions, or municipal water supply outages.
• Daily utility—can be used for watering plants, washing vehicles, filling swimming pools, or flushing toilets.
• Reduced runoff—helps prevent the overloading of storm drainage systems during heavy rainfall.

Context in French-speaking regions

• Quebec—many municipalities offer subsidies for the installation of rain barrels.
• France—some regions encourage domestic use for watering gardens and flushing toilets.
• Belgium—rainwater harvesting has been mandatory for certain new construction projects for several years.

What a treatment system must actually control

• Mechanical filtration (sedimentation, sand filters, or activated carbon filters)
• Treatment must target identified hazards: UV or chemical disinfection addresses certain microbes; membranes or filter media target particular contaminants. Reverse osmosis is not simply disinfection, and none of these labels alone establishes drinking-water safety.
• Regular maintenance of roofs, gutters, and filters
• Closed storage tanks, protected against insects, light, and algae

Without treatment

Without adequate treatment, rainwater must be reserved for non-potable uses: irrigation, outdoor cleaning, or flushing toilets. Confusing collected (untreated) rainwater with treated rainwater is the primary source of error in poorly designed domestic systems.

1

Catchment surface

The roof is the most common catchment surface. Its surface area and material directly determine the volume collected and the initial quality of the water. Metal or tile roofs are generally preferred over asphalt roofs.

2

Transport

Gutters and downspouts channel water toward the filtration system and then into the storage tank. The condition of the gutters is a frequently overlooked factor affecting water quality—clogged gutters introduce organic debris into the system.

3

Pre-treatment and filters

Nets, grilles, and leaf filters trap coarse debris before it enters the tank. Installing a grille or mesh screen at every entry point is the most effective measure for preventing mosquito breeding.

4

Storage tank

Options include a barrel, an above-ground tank, or an underground cistern, depending on the required volume and budget. Tanks must be opaque (to limit algae growth), enclosed, and ventilated.

5

Outlet points

Faucet, overflow, and bottom drain. An overflow is essential for discharging excess water during heavy rainfall and protecting the building's foundations.

6

Pump

Necessary for indoor use or in cases where gravity-fed flow is insufficient. A submersible pump is the most common solution for underground cisterns.

Above-ground system

• Low installation cost
• Installation and maintenance can be performed without specialized skills
• Modular: barrels can be connected in series to increase capacity
• Susceptible to freezing—must be drained before winter in Quebec
• Susceptible to UV degradation—choose materials with UV-resistant treatment
• Visual impact depends on placement

Underground system

• Invisible and secure against theft
• More stable temperature if the tank is properly buried; pipes, the pump, and accessories must also be designed to withstand freezing and site-specific ground conditions.
• Large capacity possible (several thousand litres)
• High installation and excavation costs
• Building permit may be required, depending on the municipality
• Pump required for water extraction

Equipment and installation

• 200-litre rain barrel—budget should be determined based on a local quote; costs vary depending on capacity, materials, delivery, excavation, freeze protection, and maintenance. Canadian dollars and euros are not interchangeable.
• Above-ground tank (500 litres) — budget to be determined via a local quote: costs vary based on capacity, materials, delivery, excavation, frost protection, and maintenance. Canadian dollars and euros are not interchangeable.
• Underground cistern (1,000 litres) — budget to be determined via a local quote: costs vary based on capacity, materials, delivery, excavation, frost protection, and maintenance. Canadian dollars and euros are not interchangeable.
• Pump — budget to be determined via a local quote: costs vary based on capacity, materials, delivery, excavation, frost protection, and maintenance. Canadian dollars and euros are not interchangeable.
• Filters and pre-treatment — budget to be determined via a local quote: costs vary based on capacity, materials, delivery, excavation, frost protection, and maintenance. Canadian dollars and euros are not interchangeable.

Financial viability

Rainwater harvesting is rarely cost-effective in the short term in areas where municipal water is inexpensive. Viability improves in regions where water is costly or subject to restrictions, or when the system is integrated during the initial construction phase (resulting in lower installation costs). The primary value lies in self-sufficiency and resilience, rather than in direct savings on utility bills.

Quebec / Canada

Generally legal and unrestricted for outdoor use. Use for potable (drinking) water is regulated and requires a certified filtration system. Some provinces require mosquito-proofing for open systems. Municipal regulations vary—it is recommended to check local requirements before any underground installation.

France

Reserve collected rainwater for permitted non-potable uses only. For drinking, cooking, and brushing your teeth, use a supply of potable water. Filtration followed by disinfection does not guarantee the removal of chemical contaminants from a roof. In France, the use of collected rainwater for food-related purposes or personal hygiene is prohibited in standard domestic settings.

Belgium

Rainwater harvesting has been mandatory for new construction for several years. It is primarily used for non-potable purposes (flushing toilets, irrigation, and—with appropriate treatment—laundry). Requirements vary by region (Wallonia, Flanders, Brussels).

Outdoor uses (untreated)

• Watering lawns and vegetable gardens
• Cleaning cars, patios, and walkways
• Filling a swimming pool or garden pond
• Washing outdoor tools and equipment

Indoor and emergency uses

• Flushing toilets (requires a dedicated plumbing setup)
• Doing laundry (requires a filter suited to the water hardness)
• Emergency reserve for municipal water outages or droughts
• Reserve collected rainwater for permitted non-potable uses only. For drinking, cooking, and brushing your teeth, use a supply of potable water. Filtration followed by disinfection does not guarantee the removal of chemical contaminants from a roof. In France, the use of collected rainwater for food-related purposes or personal hygiene is prohibited in standard domestic settings.