In the world of roof-water collection, first flush devices are about as technical as it gets from a collection cleaning or filtration perspective. Roofs are a good source of relatively clean, free water, generally suitable for outdoor use or for indoor use such as toilet flushing. However as with any surface, roofs and gutters accumulate debris ranging from leaves and dust through to bird droppings. Storing these contaminants within a rainwater tank can produce nasty odours and may require extensive cleaning of the tank after a period of time.
For residential houses a container type first flush device is commonly used, (see image to the right – from Rainwater Harvesting). For small scale sites, or when used in array configurations this solution makes sense for the average homeowner, its easy to maintain and monitor and made of simple components. Most of these are based on an empirical relationship between the area of the roof, and some depth of rainfall, which equates to a volume of first flush. The obvious issue with this relates to rainfall intensity, sometimes it rains really hard, other times it drizzles. In a light rain, the volume required to clean the roof of dust, organic and inorganic debris is going to be considerably higher than for a larger storm event.
However, on a larger site such as a warehouse or industrial building, the stakes go up. A larger roof area means that contaminants have further to travel to the first flush device, and may not have been transported within the allocated volume of first flush. Options include using multiple first flush devices that may need to discharge to the garden, sewer or septic system, or coming up with a different set of criteria.
There is a bit of research available on first flush by flow rate rather than by volume, which seems to make more sense. Still, when you are talking about a 2.8ha of roof area, the first millimetre of rainfall equals 28 cubic meters of first flush that needs to be dealt with, each and every storm. Regulatory bodies and even some clients are adamant that they want to use roof collected rainwater for garden and landscape watering, toilet flushing and for truck wash-down. If this is the aim, my advice would be to narrow down the daily usage of stored water to allow for an optimisation of contributing roof area and storage volume. One client I worked for arbitrarily stated that they wanted 800 cubic meters of underground tank, 40x10x2m. As far as I know there was no consideration for how to keep 800cum of rainwater clean and free of contaminants. My suggestion at development application stage was to include a flow splitter pit with an orifice sized to release a given flow of water straight to the wetland, then as flow increased a weir inside the pit would divert hopefully cleaner water to the storage tank. In a major storm event such as the 1% event, (1 in 100 year storm), a weir at the outlet of the stormwater tank would divert flow straight to the wetland bypass preventing backing up or inundation of the collection and transportation system.
As far as I can tell, there are not too many large scale viable options other than this type of arrangement for private ownership facilities. Most other systems would require a level of maintenance that would be unlikely be kept over time.
I’d be interested to hear of other designs around the place for large scale first flush devices. Building an industrial structure with a green roof would solve many of these problems in the short and long term.