Bigyin wrote on Aug 16^th, 2012 at 12:17am:

I'm fascinated to find out how many people on here rely on rainwater only for their supplies.  I'm one of the lucky ones who has a well and haven't had to worry about having enough water.  I don't know if you're living here yet Coldalba or just planning ahead ? The rainfall here is erratic to say the least.  It's not unusual to have not a drop from say April to September and then when it does come it's an almighty torrent.  The average annual rainfall is about 600mm which equates to 600 litres/sqm of catchment area.  Now I estimate that I use about 500 litres a day for everything except drinking.  I realise that may sound extravagant to some people (but we don't have a shortage).  That comes out at about 182,000 litres a year or 182 cubo's and would require over 300sqm of catchment.   That's a lot of storage space and catchment area.  Then when you've collected all of that you've got to try and keep it sweet.  If I'm talking rubbish, please let me know.  :o

This is exactly the problem with holding water in cisternas. It is not easy to keep sweet and clean. Even in small cisternas the water will go green from algae and once it's in, you can not completely eliminate it.

It is far easier therefore to have a large holding tank and feed the water in it through a cleaning system into a much smaller holding tank which is easier to keep clean because it can be used before it has a chance to go green again.

There are ways to keep even large amounts of water clean but it is not easy and it is not cheap. Chlorination is one way but it evaporates and has to be continually replaced. Lime is fine in wells but causes problems in cisternas (concentration levels, having to settle out etc.) The best way is to only clean your immediate needs completely and let the rest just filter through a slow sand filter to keep it fairly clean. please note the term 'SLOW' sand filter as opposed to any old sand filter.

If you want to see how effective sand is at cleaning water, try this:

Take a plastic 2 litre bottle. Put a small hole (1-2 mm diameter) in the lid. Cut the bottom of the bottle off. Turn the bottle upside down so the open end is at the top. Place some (a few) small clean pebbles in the bottle. cover the pebbles with small pea gravel to a depth of about 1 inch. Cover with washed fine sharp sand (washed river sand not building sand).  Take a bucket and place a shovelfull of earth in it. top it up with water and mix thoroughly. Let it stand so the big lumps settle out a bit. Slowly pour the dirty water onto the top layer of sand and let the bottle fill slowly. catch the CLEAN water coming out of the hole in the bottle lid. Amazed? I was when I first saw it. Now scale it up and you have a sand filter.

NOTE:
This is NOT a 'slow' sand filter and is still very crude at this stage. The water may look clean but it is still full of germs, chemicals, and fine sand at this stage. Do NOT drink it.  :(

John