Creating a First Flush System for Your Water Harvesting System.

Creating a First Flush System for Your Water Harvesting System.

Rain harvesting for a homestead can be as simple as a method to capture rain water for growing and agriculture purposes – or as advanced as a system designed to harvest water that is intended for drinking and cooking.     Regardless though, you may want to look into installing what is called a first flush system which is designed to divert the initial flow of rainwater, which is typically loaded with pollutants and debris accumulated on the catchment surface away from the storage tank which in turns reduces the contaminants that make it into the primary water supply.

For a lot of homesteaders and preppers, you might not have even given though to a first flush system – or the value of it.   It’s a simple concept – when it rains, the first flush of water from the roof or catchment area contains the highest level of contaminants which makes sense as this first flow of water washes away the dust, bird droppings, leaves, and other debris that have settled on the surface since the last rainfall.  With that said, a first flush system is designed to capture this initial, dirtier flow of water and divert it away, ensuring that it does not enter the storage tank.

At the heart of a First Flush System is a diverter, which is a valve or mechanism that directs the initial flow of water to a separate drainage or storage area. After a predetermined amount of water has been diverted (which is often adjustable based on the size of the catchment area and the amount of debris typically accumulated), the diverter automatically closes, and subsequent cleaner rainwater is allowed to flow into the storage tank/s.   By removing the first flush of water, the overall quality of the stored rainwater is significantly improved which not only is important for potable purposes, but it’s also beneficial for non-potable uses, as it reduces the burden on filtration systems and preserves the cleanliness of the storage tank while also removing man made and environmental pollutants such as those associated with smog and acid rain.

The basics of the system work in conjunction with your current water harvesting system which for many homesteaders is to simple divert their eavestrough downspouts to their rain barrels.  With a First Flush system, you are going to split your storage system to have a reservoir to collect the dirty water that comes off during the initial rain.   The standard formula for this is to flush a half gallon of water per ten square feet of roof surface.   A 1000 square foot roof surface then would mean you would want to plan to flush the first 50 gallons of water.

First flush systems are connected vertically down from the main water drain via a T-joint fitting. Use a suitable PVC pipe (for example, a 90mm or 100mm diameter) for the diverter. The pipe should be fitted with a ball float which will rise as the pipe fills with water, rerouting the cleaner water into the tank after the initial dirty water has been diverted.  Sites like Amazon have numerous options for buying diverter kits around the $100 mark.

It is important to remember that the collected ” Dirty Water ” from the first flush is going to have a far higher contaminant level then a basic water barrel collection system for the simple reason that the water in this reservoir isn’t going to be diluted with larger volumes of fresh clean rain water.   Although some look at this water as will usable, I lean towards rating its use as limited.  I wouldn’t recommend using it on your garden due to it’s high volume of contaminants – and if you were to use it on your lawn it still would be potentially contaminating your ground water system.  Although honestly compared to a civic water system filled with chlorine and fluoride,  its a toss up for which is worse.

That isn’t to say there isn’t value for this water including such uses as a fire suppression reserve or to use for cleaning outdoor areas.   This dirty water can also be used for wildlife habitats thanks to it’s high volume of organic material but I would recommend running it through a gravel bed filter to remove as much contaminants as possible.   This is the process of using gravel or a rock bed to run the water through it which can be a very effective means to treat polluted water.

An outdoor basic design involves using a bed of gravel or crushed rock of stones in various sizes can be made by making a 2 to 3 foot wide trough of rock a minimum of 15 feet long ( the longer the better ) where the dirty water is drained across.  This requires a downhill slope as you don’t want the water pooling or sitting still in the trough.  The water at the other end of the bed will have a vast majority of contaminants removed and far more safe / useful for uses such as non food based agriculture growing.

You can also create a system that uses barrels filled with rock to treat the water and in a more elaborate design, could feasible use various sizes of rock in a consecutive barrel system while keeping the lower valve / exit spout a few inches higher than the bottom of the barrel so that the contaminants can rest at the bottom of the rock bed and not be transported to the next filtering barrel.  A drain spout at the bottom of the barrel allows you to let the debris and pollutants to be cleaned out on a periodic basis.

The main point in this discussion though is the importance of using a first flush system in water harvesting to ensure that the dust, bird crap and other debris on the roof of your buildings isn’t directly put into your harvested water.

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