Blue Mountain WDDP01 90mm First Flush Downpipe

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In order to help prevent the first few millimetres of contaminated rainfall from entering your water tank, you can install the Blue Mountain WDDP01 90mm First Flush Downpipe to divert that water washing off of your roof. By using this flush to get rid of the more contaminated water, the First Flush Downpipe helps keep hazardous bacteria and fine particles out of your tank, improving the overall quality of the rainwater you collect.

The First Flush can be installed anywhere between your roof catchment area and your rainwater tank to isolate and capture the first "flush" of dirty water at the beginning of a rain event. The slow release valves automatically drain the captured dirty water. The draining water can then feed your garden irrigation system or be discarded into stormwater.

In general terms, the more water you divert, the better the quality of the rainwater that ends up in your tank. However, diverting more rainwater than necessary can lead to volume shortages, so it's important to calculate your diversion needs, and divert the water accordingly. The following formulas offer general guidelines for calculating how much water you should divert: areas with low pollution (open fields, no trees, no bird droppings or animal matter, clean environment) should divert 0.5 litres of water per square metre of roof; areas with higher pollution (leaves and debris, bird droppings, animal matter/carcasses, pollution) should divert at 2 litres of water per square metre of roof. This calculation will allow you to determine what length and size pipe or pipes you need for your diversion chambers.

PDF Document Blue Mountain WDDP01 First Flush Downpipe Installation Guide


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