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There is an 8000 liter water tank on top of my house which provides water for the house.

Bacteria, fungi, algae, etc. often start developing in the tank, giving unpleasant smells, etc.

The water is ground water that we pump up there with our own pump.

The tank is made of concrete and covered with a steel lid.

Nobody is drinking or cooking with the water. It's used for stuff like laundry, showers, washing dishes, etc.

What can I do to prevent these unpleasant qualities of the water? It's very hot here (like 40 celsius in the shade), and the water tank is in the baking sun.

My landlord is unlikely to want to make any large investments. Therefore, cheap solutions are a plus.

Edit: Several answers have suggested chlorination, and it seems like the best option. But I live in rural India and it's difficult to find chlorine here. Bleach, though is readily available. Does it seem suitable? I've asked this as a separate question on Chemistry SE.

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    The conventional solution is to treat the water with chlorine additive. – Michael Karas♦ Jun 2 '16 at 19:15
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    Also helpful would be filtering and a UV bulb, but chlorine would still be the answer. The real issue is nitrogen from the rainwater provides nutrient for stuff to grow. – Ben Welborn Jun 2 '16 at 20:14
  • Oh! I made a slight mistake, I was thinking that it was a cistern... but if you have bacteria or other stuff growing in it, then still yes, nitrogen (microbial food) is why. Water that does not have any nutrients cannot support microbial growth. Normally, the ground does filter out nitrogen. Speculatively, nitrogen may be comming from farm runoff, or sewage, or maybe rain water is getting into the ground water too quickly to be filtered out. I'm sure that if you get it tested for nitrogen, you will see that it's high. – Ben Welborn Jun 2 '16 at 20:32
  • A carbon filter will help reduce nitrogen... chlorine will react with nitrogen. – Ben Welborn Jun 2 '16 at 20:37
  • I will give you a hint: snark in the face of free advice is not likely to get you any further advice. – bobfandango Jun 5 '16 at 5:04
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Even though you are not drinking this water, you are using it for showers and for washing dishes - so it probably needs to be treated nearly as seriously as drinking water.

The main methods are filtration, treatment with floculants, thermal treatment and chlorination.

Chlorination is a widely used method. Chlorine is present in most disinfected drinking-water at concentrations of 0.2–1 mg/litre

The World Health Organisation (WHO) publish "A toolkit for monitoring and evaluating household water treatment and safe storage programmes" (HWTS):

Several HWTS methods have been proven to significantly improve drinking-water quality in the laboratory and in field trials in developing countries . These HWTS methods include filtration, chemical disinfection, disinfection with heat (boiling, pasteurization) and flocculants/ disinfectants. In addition, a combination of these methods may be used to increase the efficacy of treatment.

...

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Note that WHO cite "low cost" as an advantage of chlorination.

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Besides the suggestions above some things you might try:

Oxygenation: a small pump set up in the tank pumping the water into the air gap above water float level you have or an air pump pumping air into the water might help with some of the issues. The pump can be placed on a timer to cut down the electrical expense.

Refreshing the water on a regular basis. A tank I set up for a Solar off-the-grid type situation pumped continuously into the tank during the day, (but with solar powered pump, so pumping was free), and the tank had an overflow outlet near the top which could be tapped into drip irrigation system. Water was exchanged continuously, keeping growth down. An alternative to this would be flushing and refilling the tank on a regular maintenance schedule.

Lastly, and possibly the best solution, would be simply placing a filter canister with a taste/odor filter cartridge between your tank and rest of the home.

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You should treat it like a (small) swimming pool. Maintain the free available chlorine (FAC) level between 1 - 3 parts per million (PPM). PPM is basically the same as milligrams per liter. So for 8000 liters you need 8 - 24 grams of chlorine. I would use calcium hypochorite powder/granules, which is commonly 65% Ca(ClO)2 by weight; so you will need 12.3 - 36.9 grams to treat your tank (see video about adding Ca(ClO)2 to pools).

Of course this is a little over simplified. You will need to add chlorine after you add water and as chlorine evaporates. Monitoring chlorine levels and keeping everything balanced takes some extra equipment and it's too lengthy to write all of it up here, but you should have a pool pump and filter and a pool chemistry test kit (see video for info about the kit). This may seem daunting, but I believe that this tank will be easier to maintain than an actual swimming pool because you will be using/replacing the water, so you will get familiar with adding chlorine once in a while or as you fill the tank.

Regarding a pool pump, I don't think that you need something heavy duty... for example, a cheap and easy pool pump like this (Intex Cartridge Filter Pump) should work fine:

enter image description here

You should probably make a shelter or use a dog-house if you decide to get this (particularly cheap) pump; keeping it from being exposed to weather will make it last much longer. Also, setting it on a timer (like 4 hours per day) would probably be enough to keep the water circulated and again, extend the life of the pump. And be sure to keep an eye on the filter; I'm not sure how often it will need to be changed, but I would guess (probably) twice a year.

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  • Thanks. I guess I'll experiment with different chlorine levels to see what works. BTW, what exactly does the pump do? – Revetahw says Reinstate Monica Jun 7 '16 at 16:54
  • I live in rural India and it's difficult to find chlorine here. But this product is available. Would it be suitable? thecloroxcompany.com/products/ingredients-inside/en-us/clorox/… – Revetahw says Reinstate Monica Jun 7 '16 at 17:08
  • @Fiksdal The pump circulates the water (which helps mix the chlorine) and it has a filter to catch any buildup. You don't really need to experiment with chlorine levels. I would add ~37 grams of 65% Ca(ClO)2 to the full 8000 gallon tank. Then keep it maintained at 1 - 2 ppm... which means you need to check it occasionally with a kit. PPM = mg per liter. If you need to add 1 ppm then you add 12.3g of 65% Ca(ClO)2 (into 8000 liters). – Ben Welborn Jun 7 '16 at 17:17
  • I see. The pump sounds useful, as dead bugs often accumulate and decompose in the tank. Did you see my other comment? I don't have access to these exact chemicals. Any thoughts on the product I linked to? – Revetahw says Reinstate Monica Jun 7 '16 at 17:19
  • @Fiksdal regular chlorox is 5% or 6% NaOCl. So the amount you need to add would be about 160 ml per each ppm. – Ben Welborn Jun 7 '16 at 17:20
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Since this is for a "not for drinking", unfiltered, untreated, non-potable water storage tank. It seems your goal is just to reduce the levels of bacteria/fungi/algae in the water.

If you can get the interior water temperature over 50 deg C, you start killing off the stuff living in the water. With 40 deg C in the shade, it sounds like you have lots of free solar energy to heat the storage tank with, something like a thermosiphon could work.

Another thing that should help is putting a large coil of bare copper wire in the water, when the large surface area of copper slowly starts corroding, it will flood the water with copper ions known to kill off bacteria and algae.

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  • There are many (common) organisms that grow at 50 - 60 °C. Bacillus subtilis comes to mind. 60 °C will kill non-spore formers, but the amount of time depends on how much bacteria was present to begin with. Most spore forming bacteria will survive for days at 70-80 °C. I wouldn't trust water held at 90 °C for two weeks to be free from bacteria (with or without copper). Boiling for 30 minutes is fine for a pot of relatively clean water, large amounts of water (180 liters) need to be boiled for days, depending on the bacterial load. For 8000 liters, boiling would be impractical and insufficient. – Ben Welborn Jun 2 '16 at 20:10

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