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Am from a place where the summer heats up to temperatures of 38 C to 44 C and water is a major problem. I thought of building a tank so that water collected from rainy season can be purified and later directed to the tank for reutilizaion in summer.

But that means that the water remains stagnant for over 4-5 months and it is not good for use when summer comes. Can somebody suggest few tips to overcome this situation.

What am trying: Preserve the rain water which can be later used for house hold work and for drinking purpose, in summer season(especially).

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3 Answers 3

up vote 7 down vote accepted

Your catchment and cistern (the part that's open to air and thus to contamination by insects/bacteria) should be as small as it can be while still providing enough capacity to take in the rain as it falls. Once rain enters the cistern from the catchment, it should then be pumped into a holding tank which is air- and water-tight, and from which you draw your water for general use. That way, water doesn't stagnate in the cistern. To make this work properly, you'll want a drain in the cistern. Normally the drain would be kept open to prevent standing water in the cistern between rains. When rains start, the first few minutes' worth should "wash" the catchment and cistern relatively clean of contaminants like dust, pollen, bird doo, etc; then the drain can be plugged and water diverted into the holding tank. You can do this manually, or set up some sort of rain-sensing system that will control the drain for you.

Once water is in the holding tank, a small amount of antibacterial and algaecide agents will keep it clean. Iodine kills both bacteria and algae, and you need to take in a little of it anyway (good for the thyroid), but the water will smell a little funny, will not be suitable for laundry (iodine permanently stains cotton and most fabrics so over time your whites will become yellow) and too much iodine can be poisonous. Chlorine is a no-brainer antibacterial but is less effective against algae except in large (harmful) doses, and will also make for funny-smelling water.

You could consider a saltwater chlorination system. These systems use a small amount of sodium chloride (3.5g/L is the threshold for human perception of salt by taste; saltwater systems generally work with salt concentrations between 2-3g/L) along with electrically-charged plates to create free chlorine ions that disinfect the water. Saltwater chlorination is used for pools to avoid chloramines ("combined" chloride compounds, which give the water that distinctive smell) and similar systems are available for producing potable water.

Understand that any chlorine-based disinfecting system can produce "disinfection by-products" that can be harmful; however these are mostly a concern in large pools where lots of people are shedding organic material (bacteria, dead skin, etc) that forms the raw materials for these by-products. Rainwater that you're not swimming in, and that you're actively keeping new contaminants out of, shouldn't have this problem. Despite that, you can also consider a simple carbon filter coupled with a UV sanitizer; these systems are also commercially available for water sanitization in systems like yours.

Lastly, you'll need to keep the bottom of the tank relatively clear of sediment. The easiest method in an above-ground tank is to have a drain at the bottom which you can open for a short time to flush sediment. The next-easiest way is to vacuum it, just like you would a pool. Every few years you may want to drain the thing completely at the end of the dry season and give it a scrub, to clean off scaling and other build-up.

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Automatic drain can be a first flush divertor –  UNECS Apr 19 '12 at 0:09
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You can use a storage tank with mesh inlet and outlet filters to prevent debris etc entering the tank, also fit a first flush divertor to flush the dirt from the line when it first rains. You can then use a reverse osmosis system to purify your water as needed.

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If you keep water in a tank that is not exposed to light and water tight, then water might stay fresh. There's also probably a way to chlorinate water yourself.

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Stagnant waters are breeding grounds for mosquitoes. Since the water stays down for a long time, am not sure if it'll be good for usage(especially drinking purpose after purifying). –  Anuj Balan Mar 17 '12 at 15:43
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cisterns: google.com/…. Cistern Storage Time and Treatment google.com/… –  Wayfaring Stranger Mar 17 '12 at 16:35
    
Thank you Wayfaring. It looks promising. –  Anuj Balan Mar 19 '12 at 5:44
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