These last steps are crucial in completing the rainwater harvesting process. (Part 4 of this four-part series).
In my article in pme’s June issue, “Cistern Management for Rainwater Harvesting” (Part 3 of this series), I discussed the importance of cistern inlets, overflows, sizing and floating pump inlets. The central premise has always been that the overall health of a rainwater harvesting system starts with the cistern. The final installment here in my four-part series will discuss methods engineers can employ to design effective and cost-efficient final treatment components. With a good cistern design, final treatment is simple.
When transferring water into a building for final use as flushing water for toilets and urinals, one of the first important principles to understand is that an interior so-called “day” or “buffer” tank is necessary to provide storage and treatment (See Figure 1). Think of the exterior cistern as bulk storage and the interior tank as a treated, ready supply. Such a tank is typically sized to meet the water supply needs for a couple hours. An efficient size is about 500 to 750 gal., and such tanks constructed of polyethylene or fiberglass are readily available off the shelf (See Figure 2).
The buffer tank is central to a flushing water system and serves several purposes.
First, it provides a means of adding supplemental city water during dry periods by means of an air gap. Some jurisdictions may allow a hard-piped connection through an approved reduced-pressure zone backflow preventer, which can directly feed the flushing water system when rainwater is not available, but other jurisdictions may require an air gap.
Second, the tank allows for the collection of other water sources such as HVAC condensate and clear water waste streams such as reverse osmosis reject water. Most jurisdictions would not allow the comingling of rainwater with tap water sources of waste such as reverse osmosis water so it would not be possible to use the cistern, which overflows to the storm sewer, to collect tap water waste streams.
Third, the tank allows for the proper mixing and contact time of disinfectants before the water is distributed to the building fixtures.
As shown in Figure 1, rainwater is transferred from the cistern to the day tank. As discussed in Part 3, this transfer would be made by means of a small transfer pump coupled to a floating inlet. The transfer rate is typically between 20 to 30 gpm for most systems, which allows for a steady transfer of water to the tank for a fill time of 10 to 15 minutes. This transferred water passes through a set of fine filters, typically 5 microns, to remove any floating particulates that may be present.