How To Select A Rainwater Harvesting System


Published: April 27, 2023 | by Wahaso Grey Water Systems

How To Select A Rainwater Harvesting System

Rainwater harvesting is becoming an increasingly popular method of capitalizing on a valuable resource, conserving water and reducing reliance on municipal potable water. When looking into this as an option, there are a few key things to consider to ensure the selection of the most suitable system.

Determining Water Demands

Before a rainwater harvesting system design may proceed, it’s key to determine the relationship between water source volume and water demand volume for each particular project. This will depend on the size of a property, the amount of local rainfall, and the type and volume of water demand for which the water will be treated. If intended for watering a large-scale irrigation area or serving cooling towers, a larger system may be called for than one for simply indoor flush fixtures or site washdown.

Selecting the Right Cistern

Once water demands have been determined, cistern sizing may proceed. Cisterns (tanks) come in a wide range of volumes and materials, including HDPE, Fiberglass, Concrete, and corrugated steel. Plastic indoor or exterior tanks are often the most common and low-cost option, while concrete and subterranean HDPE tanks are often more durable, and more practical relative to space efficiency and Civil infrastructure. With the aid of historical rainfall data from NOAA, an optimal range of cistern volumes may be presented to design team and client, and a practical volume ultimately chosen. Cistern design and production can then begin, taking into account location, material and structure, and location on site.

Considering Cistern and System Location

A rainwater harvesting system’s location is crucial for a number of reasons. The project itself should ideally be located in an area with adequate annual rainfall relative to demand, the cistern should be sited according to building and property space constraints, and the treatment system should be as feasibly close in proximity as possible, to the cistern and the water demand system (flush fixtures, irrigation system, cooling towers, etc). This will help ensure that the pressurization system (pump array) may be properly sized, so the water is conveyed from cistern, to treatment skid, and all the way to end-use.

Envisioning the System Design

Depending on the type(s) of water demand on a project, a rainwater harvesting system will include varying treatment methodologies and components. Even if simply harvested for irrigation use, it is strongly recommended to filter rainwater to prevent rooftop residue or sediment from accumulating in the cistern or even the irrigation system itself. And if the water is to be used for cooling tower makeup, toilet flushing and/or spray irrigation, a third-party certified system with advanced filtration and sanitization technology is typically required.

Considering Operations and Maintenance

As with any complex mechanical system, a rainwater harvesting system requires maintenance. The components (filters, pumps, gauges, controls, sanitization system, etc) must be monitored and serviced periodically, to ensure proper operation and ongoing reduction of municipal water consumption. This may be managed by the property staff and/or a third-party technical team focused on water quality testing.

In Summary

To sum up, to select a rainwater harvesting system for a project, it’s key to consider: water challenges, water demands, cistern size, project location, system design, and maintenance requirements. With the right system in place, and a well trained team to maintain it, rainwater harvesting can be a long lasting water conservation and cost reduction tool for many building types.


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