First flush device

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A first flush device is a method of reducing or eliminating first flush of bird droppings, dust, and other airborne contaminants from rainwater harvesting from roof tops especially in the urban and dry remote setting.


There are several methods including but not limited to:

  1. Float valve type
  2. Over flow type
  3. Flow rate type
  4. Electronic conductivity type

Overflow example[edit]

An example of an over flow type first flush device for a high pollution load area, such as where there are birds, leaves, seeds, and moss in an urban setting, can be constructed from PVC down pipes in the following manner. The distinguishing feature of this method is the collection pipe is submerged before any water is collected. This prevents 90% of moss, algae, liken, small seeds, and partially digested bird dropping 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 to settle sediment by keeping the agitation zones away from the collection point. This prevents or vastly reduces the agitated particles from becoming 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 with a normal down pipe sized inner pipe and 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 down ward between the inner and outer pipes about 1/6 of the length of the outer pipe up to 6 in (150 mm). The collection pipe has a removable gauze to prevent leaves exiting the pipe and insects and their larvae 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 (co-axially) about 1/3 of the length of the outer larger pipe, down past the exit point.

The water enters from the top and down the inner tube. At this point the water starts to bleed out the bleed hole which goes into the grey water system, a garden watering system, or 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 due to the entry of the collection pipe being submerged under water.


The capacity can be increased by extending the pipe near horizontally or even in a "T" at the bottom providing 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 Litre drums. Drinking water requires a larger flush than water used for washing clothes. Yet other sources including "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 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[edit]

Water collected in this manner will need treatment for waterborne pathogens and dissolved toxins. A two-stage 0.5-micrometre activated carbon filter should also be considered among other methods available. 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 flitration.