The Secret to Successful Rainwater Harvesting: Floating Filter Intakes
by Doug Pushard
Why worry about where the water in the tank is pulled from? Isn't it all the same? The simple answer is NO. No matter how good your first flush system, if you have one at all, some particulates are going to enter the tank. Eventually, this material will settle on the bottom of the tank. So taking water off the bottom picks up these particulates. Some may be caught by an outgoing filter but this water may also contain heavy metals or chemicals which, at worst, may not always be filtered out and at best, will clog the intake filter faster.
Some of these incoming materials will float when they first enter, the tank so taking water straight off the top is not a good idea either. This is where a device commonly called either a "floating extractor" or a "floating intaker" comes in.
It is an often overlooked part of most rainwater catchment systems. With all rainwater systems, some particulates will enter the storage tank and either settle to the bottom or float.
As the name implies, a floating extractor floats in the water in the tank and its sole purpose is to intake water from the calm, clean water that is in the middle of the tank. It connects to the pipe outlet, typically near the very bottom of the cistern, where water is drawn for household and irrigation use. On most systems, this intake pipe is normally left uncovered.
To illustrate the difference in water quality, water samples were drawn from the same cistern at the same time. The results of hese tests are shown in the chart.
As is visible from the tests, the cleanest water is from the middle of the tank. These water quality tests were performed at a nationally certified test laboratory ( i.e. Related Topics) with the top water (T) being sampled first, followed by the center (C) and then the very bottom (B). The bottom of the tank was not scraped, which would have given a more extreme result.
Water from the top of the tank has significantly more particulates than the other two tests.
pH - The measure of the water's relative acidity or alkalinity. A pH of 7 is neutral while higher numbers indicate acidity and lower alkalinity.
Turbidity - Measures the impedance of light through water. The particles which cause turbidity can interfere with disinfection by sheltering microbes.
Hardness - inhibits the cleaning action of soaps and detergents. It can also cause deposits of scale on the inside of hot water pipes and cooking utensils. Hardness is caused by calcium and or high levels of magnesium salts.
Additionally, either the top or bottom had the higher mineral and metal content (i.e. Calcium 16% higher, Copper 7.1% higher, Iron 11.6% higher, Magnesium 7.1% higher, and Manganese 16% higher) than the water in the middle. Water from the very bottom and the very top of the tank are clearly the worst. The tested cistern has only been in use for 2 years and it is reasonable to expect that over time the metals, minerals and solids will continue to build up, and as this "stuff" builds up, it will get closer and closer to the intake pipe and will become part of the everyday incoming water supply.
It is critical to note that particulates in the water can also decrease the effectiveness of UV lamps by potentially blocking harmful bacteria from sterilization.
The pH and hardness do not vary much with the depth of the water; however, these readings are lower, and in some cases vastly lower, than nearby well and city water.
So taking water from the middle of the tank is clearly the best solution. Floating extractors are a quick and inexpensive way of getting to this water; providing access to the best water, filters will last longer due to having less to filter, and best of all, they are simple and inexpensive.
You can purchase one for $75-$100 or build a simple one yourself for around $30. Either way they are a great investment and should be included in every system.
Related Webpage: EPA published water standards
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Related Webpage: National Testing Laboratory
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