Introduction to Rainwater Harvesting
When properly treated and stored, rainwater harvesting ensures a consistent and independent water supply essential for gardening, cleaning, and drinking. Households that practice rainwater harvesting reduce their reliance on municipal water supplies, lower their utility bills, and support conservation efforts, key components in achieving self-sufficiency.
Critical Components of a Rainwater Harvesting System
Roof Catchment Area
The roof collects rainwater, channeling it into the system. The larger the roof area, the more water can be harvested. In addition to the main house, the roofs of outbuildings like greenhouses, sheds, and barns can serve as efficient catchment areas. Other possibilities for rainwater capture include paved surfaces such as driveways and patios.
First Flush Diverter
A first flush system is an essential component of rainwater harvesting systems. It improves the quality of collected rainwater by diverting the initial flow of water, which carries the highest concentration of dust and other pollutants that have accumulated on the catchment surface.
The first flush diverter is installed between the downspout and the storage tank. It consists of a chamber or pipe that temporarily stores the initial runoff. A ball or flap valve (similar to a toilet tank mechanism) is commonly used as the diverter mechanism.
When it starts raining, the initial contaminated runoff flows into the first flush chamber. Once the chamber is full, the valve closes, and the cleaner water bypasses the diverter and continues to the storage tank. The contaminated water in the chamber is gradually released into the soil through a drain hole or a slow-release valve at the bottom of the chamber.
Storage Tanks and Cisterns
These containers dedicated to storing harvested rainwater come in various sizes and materials, including plastic, concrete, and metal. Homeowners can choose based on capacity needs, budget, and installation space. Impermeable liners are often used within these tanks to ensure water remains uncontaminated and leak-proof.
Filtration and Purification Systems
Filters and purifiers ensure the harvested rainwater is clean and safe by removing impurities and contaminants.
The Role of Impermeable Liners in Rainwater Harvesting
Storage tanks and cisterns are often lined with impermeable geomembranes to ensure that the stored water is clean and to eliminate the risk of loss through seepage or leaks. When the stored water is destined for drinking or cooking, appropriate materials must be used to avoid contamination from the storage structure. BTL’s AquaArmor line offers durable, high-quality liners that are ANSI-certified for storing potable water.
Designing a Rainwater Harvesting System
Calculating Water Needs and Catchment Area
Calculating the household’s water needs is the first step in designing an effective rainwater harvesting system. Start by determining the household’s average daily and monthly water use. Water use includes water for drinking, cooking, cleaning, irrigation, and other purposes. Understanding your water consumption patterns helps design a system that meets your needs.
Calculating Harvestable Water
Several factors must be considered when calculating the amount of water you can expect to harvest from your system, including the size of the catchment area, the average rainfall in your region, and the efficiency of your collection system.
The catchment area can be calculated by multiplying the surface’s length and width to determine the total square footage. The average annual rainfall in your area (in inches) can be obtained from local weather stations, meteorological services, or reputable internet sites. Multiply the catchment area (in square feet) by the average annual rainfall (in inches), then multiply that by 0.623 to get the volume in gallons. Since no system is 100% efficient, you might start by assuming that you’ll lose 15% to 20% of that volume from various factors. Over time, you’ll be able to determine a more precise efficiency coefficient.
Distribution
Plan the distribution system to efficiently deliver water to wherever it will be used. Depending on the layout and elevation of the storage tanks relative to the points of use, this may call for gravity-fed systems or pumps.
Choosing the Right Storage Capacity
You’ll need storage tanks or cisterns large enough to store sufficient rainwater to meet household demand. Based on your calculated water needs and the harvestable water from your catchment area, you calculate the storage capacity you’ll need for daily use plus carryover for periods of low rainfall. A general guideline is to have storage that can hold at least a few weeks’ worth of water supply.
Seasonal Rainfall
However, storage capacity calculations become more complex if your region has distinct seasonal rainfall patterns. If this is the case, you’ll need to estimate the length, in months, of your dry season and multiply it by your monthly water needs to determine your total water needs over the span of a dry season and an entire year.
Next, you’ll calculate the volume of harvestable water during the rainy season. In this calculation, you won’t use the average annual rainfall - instead, find out the average rainfall for your area during those specific months.
Once you have all the numbers, compare the total harvestable amount to household needs on an annual basis. If you cannot harvest enough to meet your needs, you’ll need to reduce consumption, identify alternative water sources, or consider incorporating additional catchment areas.
Finally, you’ll need to plan for enough storage capacity to meet water needs during the dry season. For example, if your household uses 100 gallons per day or 3000 gallons per month, a six-month dry season will require that you divert at least 18,000 gallons to storage. Lined cisterns and underground storage tanks are good options. They help protect the water from contamination and prevent loss from evaporation or seepage.
Incorporating Overflow and Drainage Considerations
Your rainfall harvesting system should be able to handle excess water during heavy rainfall to prevent overflow and potential damage. When storage tanks reach capacity, overflow pipes or channels should direct excess water away from them to avoid flooding and erosion around them. French drains, swales, or other methods for channeling water away from the foundation can help prevent waterlogging.
Filtration and Purification Systems
Maintaining high water quality in your storage system is essential if you use it for drinking or cooking. Pre-filtration strategies such as leaf guards, gutter screens, and first flush diverters will remove larger debris and contaminants before the water enters the storage tanks. Still, a well-maintained purification system should be included for any potable uses. Carbon filters, UV purifiers, or reverse osmosis units perform well in removing harmful contaminants and pathogens. Be sure to keep replacement parts on hand.
Conclusion
Rainwater harvesting is a valuable practice for those seeking self-sufficiency. By maximizing water capture, avoiding water loss through leaks, and ensuring water quality is suitable for human consumption, households can ensure their water needs are safely and sustainably met.
