Reducing stormwater runoff by using rainwater harvesting and re-use
Rainwater harvesting (RWH) is a stormwater management and re-use concept that focuses on water conservation, re-use and reduction in public water supply usage for irrigation. This practice was initially only used on a small residential scale. Using a RWH system for conservation is a good tool to reduce water consumption but it can’t be seen as a cost-savings measure, because the price of water is very low in the US compared to most other nations. It can be an effective tool for guarding against water shortages or water restrictions.
The goals of RWH have evolved and with the expansion of the green movement, RWH systems were used to reduce the environmental impact of development and population growth. This is a Low Impact Development (LID) practice. LID practices reduce impervious area and infiltrate wherever practical in order to provide runoff reduction.
Engineers have to observe requirements when they use RWH as a Best Management Practice (BMP):
- Have storage capacity to catch the next storm event
- Have demand in the water budget to empty the storage cistern
The challenges of RWH during the wet season result from:
- Irrigation demand is low
- Water supply is high
Thus, there is a need to find additional applications to use the harvested runoff
The United States Green Building Council (USGBC) created a plan, the Leadership in Energy and Environmental Design (LEED), for responsible development and reducing the impacts of development. RWH and water conservation strategies are both good means to reach some of the goals and to earn points in order to receive the LEED certification.
RWH is used more frequently as building codes are changing and many include provisions for RWH. New requirements have to be observed including catchment, first flush diversion and pretreatment, storage, installing a re-use water line, and separating the RWH system from municipal supplies, in order to prevent water contamination and reduce runoff and water consumption.
Even as RWH systems are spreading, it is not always easy to regulate harvesting runoff. Stormwater management regulations aim at reducing runoff. But there are water laws as well, which particularly in arid areas, tend to limit upstream runoff reduction to protect the owners of water claims downstream. Stormwater regulations and water law are in conflict. This conflict needs to be clarified in order for RWH to grow as a BMP.
Reducing annual runoff by using harvested water is a particularly common application in irrigation. This practice is frequently used but RWH systems do not have to be limited to this application to reduce annual runoff. Engineers can use harvested water beyond irrigation for:
- Toilet flushing
- Washing machines
- Hose bibs and outdoor washing (vehicles, windows)
- Process water for commercial or industrial projects
- Potable applications, but this is not really the best way to reduce runoff, treatment costs are high and monitoring is required
The components of a RWH system:
A RWH system consists of common building blocks and incorporates:
- Catchment: rooftops contain less sediments and nutrients than hardscape surfaces
- First flush diversion and pre-treatment : diversion structures are required initial runoff building codes. The first flush diversion is useful because runoff from the beginning of a rainfall event is thought to carry more pollutants. Pretreatment cleans the water before the storage, protects downstream pumps, filters and fixtures from damage and keeps pollutants out of cistern and filters. Pretreatment is useful when RWH is employed as a BMP.
- Storage : aboveground cisterns for smaller systems, belowground cisterns for larger sites and additional storage features
- Day tanks, pressure tanks, make-up water : can provide an air gap between potable and re-use water and can ensure the end use application has a consistent water supply
- Pressurization : pumps are used for all combined applications
- Treatment : consisting mostly of filtration, to treat after storage
- Disinfection : UV Radiation, Clorination, Ozone and Reverse Osmosis, possible disinfection processes dependent on end-use.
- Controls : controls of the cistern, back-flushing of filters, disinfection dosing, ongoing monitoring and communication
RWH is an effective way to turn runoff into a valuable resource, to implement sustainable development, to reduce runoff efficiently, to reduce municipal water consumption, and to save energy. By incorporating RWH, engineers can meet stormwater regulations, earn points toward LEED, and reduce demand on municipal water supplies.
If you have any questions about rainwater harvesting or water re-use, please contact John McAllister at firstname.lastname@example.org or at (508) 747 - 7900 x 117.
Information in this article taken from September 2010, article by Greg Kwalsky and Kathryn Thomason, published in CE News.