Rainwater harvesting is the process of collecting and storing rainwater runoff from a surface for later use. This practice is experiencing a rise in popularity as people look for ways to enhance household sustainability and reduce dependence on municipal water supplies. By capturing precipitation that would otherwise become stormwater runoff, homeowners can effectively lower their utility bills and gain a degree of water independence.
Designing the Rainwater Collection System
The foundation of any successful rainwater harvesting setup is the proper selection and integration of its three main components: the catchment surface, the conveyance system, and the storage vessel. The catchment surface is typically a rooftop, with smooth, non-toxic materials like metal or clay tiles being preferable, as they minimize chemical leaching and debris accumulation compared to materials like asphalt shingles. The total surface area of the roof directly dictates the potential volume of water that can be collected, which is a primary factor in system sizing.
The conveyance system includes the gutters and downspouts that channel the collected water toward the storage tank. Gutters must be appropriately sized and sloped to handle the maximum expected rainfall intensity without overflowing and ensure efficient flow. A coarse mesh screen should be installed at the downspout inlet to exclude large debris like leaves and twigs.
Sizing the storage vessel, often a cistern or large barrel, requires balancing the catchment area, local rainfall data, and the intended water usage. Storage vessels can be made from various materials, including food-grade plastic, reinforced concrete, or galvanized steel, but must be designed to be fully opaque to prevent light penetration. Proper placement involves installing the tank on a stable, level surface, ideally in a shaded location, to maintain a cooler water temperature and inhibit microbial growth.
Maintaining Water Quality
Maintaining the quality of harvested rainwater begins with minimizing the contamination entering the system. A first-flush diverter is a simple, mechanical accessory that channels the initial runoff away from the storage tank. This initial water tends to contain the highest concentration of contaminants, such as dust, bird droppings, and accumulated debris from the roof surface. Diverting this “first flush” significantly improves the overall quality of the water that ultimately enters the cistern.
After the first flush is diverted, the remaining water passes through pre-storage filters, such as a debris excluder or a mesh screen, to remove any remaining physical particulates before storage. The storage tank itself should be completely sealed and opaque to block sunlight, which is necessary to prevent the growth of algae and other photosynthetic microorganisms. All inlet and overflow openings must be screened with a fine mesh to prevent insects, especially mosquitoes, from entering and breeding inside the water.
Regular cleaning and inspection are fundamental to long-term water quality. Gutters and downspouts should be inspected and cleared of debris every few months, particularly before and after the rainy season. The storage vessel itself requires periodic cleaning, usually every one to three years, to remove the sediment and sludge layer that naturally accumulates at the bottom. For systems used in sensitive applications, water quality testing for bacteria and pH levels should be conducted regularly to ensure safety.
Practical Uses for Stored Rainwater
Stored rainwater is generally categorized for either non-potable or potable uses, and the required treatment level depends entirely on the intended application. Most residential systems are designed for non-potable uses, which include any application where the water is not consumed by humans or pets. Common non-potable uses include:
- Landscape and garden irrigation.
- Washing vehicles.
- Supplying water for flushing toilets.
- Washing clothes, though this may require slightly more filtration to protect the washing machine and clothes.
Achieving potable, or drinking-quality, water requires a much more rigorous and multi-stage treatment process to eliminate biological and chemical risks. Even though rainwater is inherently soft, it can still contain pathogens like bacteria and viruses, as well as chemical contaminants picked up from the atmosphere and the roof surface. Simple filtration alone is insufficient for making water safe for consumption.
For potable applications, the water must first pass through a series of filters, often including a sediment filter and a finer carbon filter, to remove particulates and improve taste and odor. This is followed by a disinfection stage, which is essential for health safety. Disinfection is typically achieved using ultraviolet (UV) light sterilization, which scrambles the DNA of microorganisms, or by chemical means, such as shock chlorination. Without these advanced steps, using stored rainwater for drinking or cooking carries significant health risks.