Rainwater harvesting is a highly customizable technology. Myriad solutions exist across a global context. Rainwater harvesting systems can usually be categorized as either passive or active types. A passive type uses almost no mechanical means to capture, convey, or treat the caught rainwater. An active type uses mechanical and/or electrical means to capture, convey, and/or treat rainwater.
I often avoid using the terms passive versus active and instead refer to landscape versus built types. Landscape type systems of rainwater harvesting, as shown in Figure 1-1, use landscape features to slow, absorb, and/or store rainwater. Landscape type systems are usually considered passive.
A built type system, as shown in Figure 1-2, uses mechanical and/or electrical means to capture, convey, and/or treat rainwater. Built type systems are usually considered active. Landscape types have the advantage of lower cost per volume of water. Built types have the advantage of providing cleaner water. Many systems incorporate aspects of both, for example conveying the overflow from a built tank into local landscaping.
Built type rainwater harvesting systems can be further categorized into dry versus wet systems (Figure 1-3). A dry system is one in which the conveyance system (e.g., downspouts) is evacuated and dry between rains, whereas a wet system is one in which the conveyance system remains filled with water between rains. A dry system often has pipe overhead, whereas a wet system often has pipe below ground. A dry system has the advantage of less clogging and fewer insects. A wet system has the advantage of being able to pipe the water underground and back up into a tank to keep the piping out of the way.
A characteristic important to all rainwater harvesting systems is the source of pressure. Pressure is what moves water from one point to another. The source of pressure can be gravity or pumps. Gravity systems have the benefit of lower cost, lower maintenance, and being generally more robust but they rely on elevation change. Pump systems have the benefit of being able to generate the needed pressure independent of available terrain, but are higher cost and require energy input.
A final important aspect of rainwater harvesting systems is their scope and/or size. Rainwater harvesting systems can serve a small garden, a house, a school, a community, or an even larger project. The larger the scope and size, the more costs are involved and the more water can be utilized. This book focuses primarily on built, dry, gravity-fed systems for garden, household, and small organizations.