Preserving Water/Rainwater for Future Dry Days

In arid parts we have been saving rain water for ages. Rain water from the roof is stored in a tank. Used as required. Purifying procedures are normally not required but sand, dust, pollen must be avoided.

There are a few bigger schemes going as well.

The Presidency built in the 1890's in Bloemfontein had a huge storage area under the double story house. It was re-discovered some years ago.

Your intention seems to be for bigger quantities.

A few considerations must be taken into account.

The water available from the roof can be calculated using the roof area (horizontal plane) x the rainfall. Example: 250 square m x 800 mm/year = 200 cubic metre.

The average water consumption must be calculated. Water supply accounts will give a good indication. At a stingy use of 50 cubic metres per month the storage will last 4 months without additional water supply.

A 2 meter deep basement under the house will hold more than 400 m3.

This can only work with new houses. The normal stuff in the basement will have to move. A double story basement might be a solution.

The next step is to calculate the costs. My personal opinion is that the cost will exceed the value.

Purification: the water used for washing etc may only needs to be filtered. Drinking water will need disinfecting.

Most drinking water starts out as rain, and that which doesn't will probably start from brackish or seawater via a membrane or evaporation process.

Water achieves various levels of purity. Treatment is about removing suspended and dissolved solids, and making the water biologically safe to drink. The one thing that is certain is that, if it hangs about too long in an unprotected state, viable bugs will grow in it at a rate of 2^kt (k varying with temperature) rendering the water unfit beyond certain levels. Raising the temperature above, say, 60degC is one of the ways of preventing bacteria growth. Maintaining a free chlorine level or ozonating it is another, though these practices are prohibited in some locations.

There are World Health Organisation guidelines for drinking water.

There are guidelines for the provision of drinking water to dwellings in the UK, the requirements becoming more stringent the greater the number of homes supplied from a particular source. In the event of the quality of a paricular supply being too low, Local Authorities have the power to requisition changes and additions to a domestic supply, invoking modifications to the local water supply utility company's network.

Local conditions and avaliability determine the regime to be pursued to produce a potable water. In upland areas in the UK, for example, no treatment may be needed and water for individual homes might be drawn straight from a stream; there are areas of the Lake District where this takes place. In the north of Scotland, apart from a slight reddish colouration indicating the presence of organics from decaying vegetation, water is distributed to homes with little or no treatment via a public network. However the river Lee, draining through Hertfordshire and north east London, is reputedly wholly consumed five times before it reaches the Thames, passing through a number of treatment works en route. In Africa various low-tech solutions are employed for the capture and distribution of drinking water, some involving the capture of power from the sun and wind. In the Himalayas, mountain fog can be coalesced onto fine mesh suspended from a simple structure, from where droplets are collected for use as a potable water without further treatment.

The WaterAid website might prove an interesting read.

There are a large number of homes in the UK that have no mains water supply. At these particular co-ordinates, for example, which are close to a navigable river, a well collects water from about 1.5m below the surface, it having arrived by percolation through the local strata. The well structure is formed from a number of reinforced concrete rings 1.2m in diameter that are simply sunk into the ground on-end, one on top of the other. The centre of the rings is filled with clean shingle and then single-size sand. A submersible pump lifts water from this well to a 0.1m³ storage tank in the building at about 5.5m above surface. The pump is controlled by two level switches, one in the well and one in the storage tank. The water is filtered to below 5μm using a single renewable cartridge depth filter in a 400mm housing, and that part of the water that is intended for drinking passes a UV steriliser, where any bugs are zapped immediately prior to the water being consumed. All tests carried out by the Local Authority have indicated that the water at the kitchen tap is fit to drink, the total dissolved solids being below 500ppm and the viable bug count being zero. Maintaining this level requires periodic attention to the equipment, which is carried out annually. As a matter of routine a few millilitres of concentrated bleach is added to the well each time the cover is lifted as part of this practice.

For tanked rainwater, certainly some settlable solids removal needs to take place; one particular installation in Australia simply uses a fine filter on the inlet to the tank to take out birds' feathers, leaves and twigs. Storage temperature and time is an important consideration. The admissibility of introducing and maintaining chlorine levels is another; chlorine may need to be supplied either as a bottled additive (with storage safety challenges) or generated locally by electrolysis of NaCl, for which a power source and additional equipment is required, as is space to house it. Materials of construction of the storage and distribution system is another factor. Any requirements by the Local Authority regarding sampling and testing need to be taken on board.

The various vast public drinking water cisterns located around the city of Istanbul in Turkey, now significant tourist attractions, are well worth a visit, many having survived various earthquake episodes over a period of >1.5 millenia.

In summary, there are no hard and fast rules about the practices that are needed to produce and maintain a potable water. Each case needs to be considered on its merits.

Every single drop of 'fresh' water on this planet, has microbes in it. There isn't anyway possible to 'clean' it, just minimize the impurities. You can reduce the impurities by filtration, and reduce the 'livability' of the water by chemical treatment, but the only way to effectively purify, and store the water without the worry of 'bug' growth, is to irradiate it, boil it, or chemically sanitize it. Water on this planet has survived forever underground without any treatment at all, other than natural filtration. W

Indeed. Water is an excellent environment for biology to reside and flourish, and this is uppermost in the mind of the designer when considering equipment for safe potable water systems.

PWSlack is pretty comprehensive - but one other comment on storage of water that is not necessarily intended for drinking. My rainwater butt is rapidly colonised by mosquito larvae. That isn't a problem (yet?) here in East Anglia, as these mosquitoes don't attack humans. But anyone living in parts of the world where malaria is prevalent would need to take additional precautions against this. (Curiously, they don't colonise the pond, in spite of it being kept free of fish so the reptiles can breed in the spring)

Hi Physicist?

You are spot on with the mosquito problem and it's not just malaria that is the problem. There are a whole raft of diseases that are spread by mosquitoes like, West Nile fever, Ross River fever, yellow fever, dengue fever, certain types of encephalitis, plus a host of parasites and so on and so on. It only takes one slack bastard that doesn't bother to look after the rainwater tanks to cause an outbreak and a single bad tank can be a real bugger to find.

Until recently I lived in Adelaide, South Australia which is dubbed the capital of the driest state in the driest continent on Earth. People love rainwater tanks there and there are frequent outbreaks of some sort of mosquito transmitted disease or other. The department of health usually manage to get on top of them fairly quickly but a few years back there was an outbreak of Ross River fever that they never managed to find the original source to. The did trap several infected mosquitoes in one of the suburbs were there were several victims but they never managed to find where they were breeding. The only reason the outbreak stopped was a cold snap in the weather that killed off most of the mosquitoes.

We have just gone through the worst drought on record in most of Australia and there is a push in Sydney and other cities to get people to install rainwater tanks. Sydney is further north than Adelaide and has a semi tropical climate and if they do start putting in rainwater tanks everywhere there will be outbreaks of mosquito transmitted diseases.

One of the hassles is the speed at which people travel now and it's very easy for an infected mosquito to hitch a ride from the tropics to a place like Sydney and then start breeding. Thy do fumigate international flights arriving in Australia but you can get an asymptomatic infected passenger that gets bitten by a local mosquito start off an epidemic.

Mosquito born diseases are a real problem throughout the world and a lot of people in the developed world seem to forget about them, however, they have the potential to be just as deadly and all it takes is somewhere for the mosquitoes to breed.

I often run test at work with peristaltic pumps running for long periods circulating tap water... this starts getting strands of black slime growing surprisingly quickly.

...for some tests where I need relatively 'pure' water I just by bottled spring water, this lasts for ages.

Conversely the water in my rain water butt stinks as bad as my butt!

I can't compete with PW's brill' answer but I wanted to contibute as water is such fun stuff...I get to play with LEDs and pumps too at work..if only I could find an excuse to add a sand pit I'd have the perfect job!

Hi Del the cat,

Tell your boss that you need the sand pit to filter the water so that it stops growing those slimy black things or eliminate the need for the expensive bottled water.

Nice one!

While I am in agreement with many of the others posting on this Forum, you havent said what the water is needed for. If the water is for human consumption, the WHO guidelines and others must be considered. For irrigation, thats another matter.

water i was talking about consumption as including for cooking. yes, for irrigation and washing purposes quality of water is of no consequence. i am seeking information more about rainwater. bottled water in many countries including india is widely used where people have doubts about municipal water supply or other sources. good bottled water should, theoretically keep for ever.

good bottled water should, theoretically keep for ever.

Practically, until the effects of the ozonation within the container wear off, which is why commercial suppliers put a 'use by' date on their containers so as to avoid commercial repercussions.

Bottled water comes in many flavors. Fresh, spring, pure, aquafer and others. Are they all treated water? Treated by any of the number of methods mentioned above? Are they required to say if they are treated or not?

In a related story I have recently heard that bottled water that is frozen or microwaved can cause the plastic to release toxins into the water. Is this misinformation by some company like Pyrex to cause a return to glass or ceramics?

In a related experience a 1 gallon plastic bottle of water was left outside sitting in the sun for over a year. After this period of time I inadvertantly hit the bottle with the end of a piece of wood and instead of finding it the soft maleable bottle I had known, the bottle cracked and shaterred into several large pieces. The plastic had become very brittle due to its daylight bombardment of heat and UV from the sun I would guess. Would the chemical / physical changes in the plastic have caused the release of toxins into the water?

Coca Cola Corp got their fingers burned trying to sell 'Desani' to the British public...this was just London tap water from the utility company put into bottles! Quite legally!

Ok the consumer is gullible...but not THAT gullible.

Don't spose anyone got fired!

it depends on your definition of purity, are you talking about sterile i.e. no life, or impurities? Storing large quantities of water is always tricky. As other contributors have said it's tricky to remove all life. If your water is aerated you will get aerobic bacteria, normally these are good things. If it goes stagnant, you will get anaerobic bacteria, these are bad things they advertise the fact by producing a smell. If you keep it in the dark, you are unlikely to get green things (plant life). Keep things simple, filtration is expensive do only fliter down to the level you need. Keep it fresh with air, a wind-powered air pump bubbling air. Keep it in the dark, keep insects out with mesh traps / guards. If you are going to drink it then it needs more treatment, which is very expensive, so only treat the amount you need to drink when you need to drink it. Watch out for dissolved substances which will depend on what sort of air you have (pollution etc) and what your roof is made of etc. You can store it in a balanced pond and treat it from there but in a hot climate you might lose a fair bit by evaporation. Water is a solvent.