Thursday, December 1, 2011

Water, Chlorine and other Purification Methods

I work as a water engineer in emergencies – refugee camps, war zones, etc.. There is basically nothing wrong in using chlorine – indeed this has been the standard sterilization route for water in the West for the last century. Very small systems use high concentration powder, medium systems liquid and large urban systems normally use gas chlorine. There are limits with regards residuals after treatment. These are designed so as to sterilize pipes in case of leakages sucking dirty ground water back into the pipe system – something which happens frequently, and to allow sterilization of any contaminated outlets – i.e. taps. If the water smells strongly of chlorine at the tap, allow it to sit for a while and the chlorine will eventually gas off. Normal contact time for the chlorine from being added to the water to sterilization is usually 30 minutes. Another reason for there being a residual of chlorine after this process is to ensure that enough chlorine was added to begin with to actually kill all bacteria. Testing for chlorine residual is simple – a basic swimming pool tester is more than adequate.
Ceramic filters are good but need to be maintained. The real problem with them is that they produce water only very slowly. The candles can be removed and gently scrubbed to clean them. I have kept the same filters going for considerable lengths of time by doing this. They can even be put into lightly chlorinated water with no damage to them – just rinse them off in fresh water after.
A good, standard way of producing drinking water which is very basic, and which can produce fairly large quantities quickly, is a slow-sand filter. This works by a biological filter developing on the surface of the filtrate medium – the sand – which actually eats any bacteria in the water. These are standard filters used around the world – in the case of dirty or turbid (meaning not clear) water, a gravel roughing filter and/or also a rapid sand filter can be used – these basically take out any suspended solids in the water. Removing turbidity if it is dissolved solids, especially organics, is more complicated and can require aeration as well as filtering, before going into the slow-sand filter.
Slow-sand filters can be built from household size up to large, village level sizes.
Information on design and use of slow-sand filters is widely available on the web. They are easy to construct, easy to use and pretty much low maintenance.
Another, very basic way of sterilizing water of bacteria is of course to boil it – at least 3 minutes at a low boil. This is not sustainable in the long term, but can produce drinkable water in small quantities relatively quickly – even turbid, or dirty water can be treated this way. It might not taste perfect but, unless it has a load of chemicals in it, it won’t harm you. And don’t forget, ceramic filters will not remove that many dissolved chemicals either!
    Don’t rely on a TDS meter for water potability. It is what it says – a total dissolved solids meter. Basically it detects salts and other minerals dissolved in the water. It does not give any indication of bacteria such as cholera, or viruses. If you are using ground eater, it could indicate if there is arsenic present – though it will not distinguish between arsenic and common salt – only tell you there is a presence of dissolved solids. Primarily, they are used as an indicator of acceptability of water with regards taste, etc. If you check on any website dealing with water quality, you will be given acceptable limits with regards TDS for drinking water.
Hope this is of use.
Thanks for the information!
About chlorine, you say there´s nothing wrong with it but what would be a safe amount of it in water?
For example in Buenos Aires, water doesnt just taste like chlorine and smells like it, it even itches after taking a bath and sometimes you need to use body cream to calm it down. People coming from Europe or USA that arent used to it suffer it even more.

Join the forum discussion on this post!



David said...

Some people think water will be THE big concern in coming years. I am no expert, so I gather others' views.

Here's one:

Anonymous said...

I am not an expert, but I play one on TV :)More seriously, I am tangentially involved with the treatment and delivery of safe drinking water.

I would guess that the problem you guys have in Argentina is that your combined chlorine is to high. But that is just a wild ass guess that is not really germane to your question.

What is germane to your question is New York State's Department of health regs which limit you to a 4 ppm averaged over a month. You should note that the New York Regs are a lot stricter when it comes to dropping below the minimum. If you drop below the minimum for more than 4 hours it becomes a major health event and you are in big trouble. But you can spike the chlorine level over the limit for an entire week and still be all right in the eyes of the law as long as your average for the entire month is 4.0.

There is a reasonably good reason for this. Over chlorinating could harm people's health over the long term. Under chlorinating could get a lot of people very sick over night.

As for me personally, I like my everyday drinking water to be without chlorine (I have a well). But if I was in a disaster situation and I was dealing with water of unknown providence, I would error on the side of over chlorinating. Especially if I needed the water soon since Chlorine disinfectant properties are related to time as well as to dose.

Anonymous said...

I like the idea of a sand filter, but I'm concerned about chemicals in the water. I live surrounded by farms, so if I dipped water from a stream or river to filter it, it still might be full of fertilizer, pesticides, etc.

Is there a filter that takes out chemicals, along with the rest?

Lou said...

Any thoughts on how to adapt swimming pool filter hardware in an emergency to provide fresh water? I imagine you could rig a settling tank, which would gravity feed into the sand filter, and then you could use chlorine to treat the output? If so, how much pool chlorine would you use? How long would it take to gas off?

Vincent said...

I have to say there are so many contaminants in our water. I don't drink the tap water directly but use a water filter to get rid of them.