First flush device
A first flush device is a method of reducing or eliminating the first flush of bird droppings, dust, and other airborne contaminants from rainwater harvesting from rooftops, especially in urban and arid remote areas.
Types
There are several methods, including but not limited to:
- Float valve type
- Overflow type
- Flow rate type
- Electronic conductivity type
Overflow example
An example of an overflow type first flush device for an area with a high pollution load, such as one with birds, leaves, seeds, and moss in an urban setting, can be constructed from PVC downspouts in the following manner. The distinguishing feature of this method is that the collection pipe is submerged before any water is collected. This prevents 90% of moss, algae, lichen, small seeds, and partially digested bird droppings from entering the storage system. This is an important modification to most commercially available systems. Another important feature is the inner tube and its length. This inner tube helps settle sediment by keeping the agitation zones away from the collection point. This prevents or greatly reduces the agitated particles from being collected.
It is a large-diameter pipe (around 6 in or 150 mm) used in a vertical orientation with a cap at its lower end, a normal downspout-sized inner pipe, and a collection pipe.
This cap has a small bleed hole (or ball valve), and cleaning access.
A hole towards the top of this pipe is provided for the water to exit into the water storage. This collection pipe extends into the outer pipe and has a 90-degree bend, and extends downward between the inner and outer pipes about one-sixth the length of the outer pipe, up to 6 in (150 mm). The collection pipe has a removable gauze to prevent leaves from exiting the pipe and insects and their larvae from breeding in the pipe(s) and entering the water collection tank(s).
An inner pipe of a smaller diameter extends from the top of the large pipe (coaxially) about one-third the length of the outer larger pipe, down past the exit point.
The water enters from the top and flows down the inner tube. At this point, the water starts to bleed out the bleed hole, which goes into the greywater system, a garden watering system, or a drain. As the flow from the roof exceeds the bleed amount, the tube fills up, depositing any heavy material in the lower part of the tube, most of which flushes out the bleed hole. Some debris floats (such as seeds, leaves, insect bodies) and remains floating between the inner and outer pipe. It is prevented from entering the collection pipe because the entry of the collection pipe is submerged under water.
Sizing
The capacity can be increased by extending the pipe nearly horizontally or even in a "T" shape at the bottom, provided the bleed holes allow complete draining to prevent water pooling and insect breeding. Inspection and cleaning caps must also be accommodated. Some first flush devices are constructed from large plastic 200-liter drums. Drinking water requires a larger flush than water used for washing clothes. Yet other sources, including the "Texas Water Development Board's Texas Guide to Rainwater Harvesting"[1] recommend 1 to 2 gallons per 100 square feet (9.3 m2) of roof collection area.
The size of the first flush is dependent on the rubbish load in the area. A rule of thumb for a heavy pollution load is for the first 2 mm of rain to be contained in the tube or drum if intended for drinking water. Size is roughly given by 2 x Area (in meters squared) = volume of first flush device in Litres.
Drinking water
Water collected in this manner will need treatment for waterborne pathogens and dissolved toxins. A two-stage 0.5-micrometer activated carbon filter should also be considered, among other available methods. Blue-Green Algae (Cyanobacteria) can be toxic to humans and animals, and its toxin can be released by the process of boiling water, thus it must be removed before any other treatment via filtration.
References
- ↑ "Archived Copy" (PDF). Archived from the original (PDF) on 2009-01-06. Retrieved 2017-10-30.CS1 maint: Archived copy as title (link)
- Stormwater: Designing first flush systems Department of Environment and Climate Change
- Introduction to domestic roofwater harvesting Warwick DTU U.K.
- Blue-Green Algae (Cyanobacteria) and their Toxins Health Canada
- Texas Guide to Rainwater Harvesting The Texas Water Development Board Archived 2009-01-06 at the Wayback Machine
- Electrical Conductivity of Rainwater with High Resolution in Space and Time State University of New York at Albany, Albany, New York
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