Filtering and treating flood waters?

[chem_doc]

This is my first post here... and I'm just getting started in bushcraft, so please, bear with me.

My day job is as a chemist (not as a pharmacist/druggist, but as a scientist type), and I wanted to chime in here on water purification methods.

Neither reverse osmosis (RO) nor deionized water (DI) are going to be 100% pure. If you're so inclined, read on and I'll tell you why. If you're bored already, you can stop reading now.

To understand RO treatment, let's just start of by discussing regular old osmosis. If you had salt water on one side of a membrane that's semi-permeable (meaning some things can flow through, others can't) and pure water on the other... Well, nature pretty much views this as the water all beign in the same pot. Entropy really wants to "even out" the salt concentration so that both sides have the same amount, not one high and one low. In this case, if salts can't get through the membrane, the only thing that can equalize the salt concentration on either side of the membrane is moving water molecules. The water molecules will move from the low salt side, concentrating any salt on that side, to the high salt side, diluting that water. Eventually, one side of the membrane will have a higher water level than the other.

This higher level of water is pushing down and against the membrane. This forces some of the water back through the membrane. At some point, regardless of the concentrations of salt on either side of the membrane, the water flowing back and forth through the membrane is the same. The amount of pressure due to the higher water height is related to Osmotic Pressure.

What RO does is pressurizes the higher water side of the membrane and forces the water back through to the low-ion side. The caveat here is that RO tends to ONLY remove charged salts...those Na+ or Cl- ions, among others, that are dissolved in the water. Something like alcohol, that isn't charged, will more than likely slip through the membrane. Most petrochemicals are uncharged, so they can be carried along through RO membranes.

DI water is water that's (usually) passed through a bed of beads made of two kinds of polymers. One traps positively charged ions and releases H+ ions. The other traps negatively charged ions and releases OH- ions. The H+ and OH- combine to form water molecules.

The caveat again here is that DI water is de-IONIZED. It's not de-petrochemicaled. If there's something like alcohol dissolved in the water, it's not going to be removed by these columns.

Now, since we're talking about water, another thing to keep in mind. Water loves having things dissolved in it. Really clean DI water is pretty corrosive. If it's kept in glass bottles or jars, it pretty quickly pulls ions out of the glass and becomes "contaminated" again. Bear in mind, this contamination probably is fine for consumption, unless you're using something like cobalt glass...

Someone ealier in this thread mentioned activated carbon. Activated carbon would get out some of the petrochemicals. But it doesn't trap them. What happens is that it has a surface that is sticky to them. They pop on and pop off of the surface. Eventually, the number of petrochemical molecules sticking to the surface of the carbon and those popping off stabilize and it's taken out as much petrochemicals as you can get out of it with that filter. Not all activated carbons are created equal, either. Some will hold much more crap than others.

The poster who mentioned distillation would probably be the route I'd go. Let the early boiling off water go. That will also get rid of any petrochemical that boils at a lower temp than the water. Collect the middle half of the water. Let the remaining 1/4 go. That's where all the salt (that doesn't distill) and the stuff that boils at a higer temp than water will be. And you will have killed/removed all the bugs. Nasties don't distill either.

Starting with a good filter system would be a first step in remediating flood waters, but as others pointed out, I still wouldn't drink it.

Wow, that was a long first post! Too much?

 

[Tourist]

This is my first post here... and I'm just getting started in bushcraft, so please, bear with me.

My day job is as a chemist (not as a pharmacist/druggist, but as a scientist type), and I wanted to chime in here on water purification methods.

....................................

Wow, that was a long first post! Too much?

Nah, good post mate. It has tickled the grey matter enough to make me re-visit what I have been told by others.

I [think] I understand what you mean. You are saying that salt will be removed - which is why RO is used on ocean going vessels, and a good thing if you live in a sea water area and need fressh water supplies.

You then move on to saying that petrochemicals and alcohol would not be removed efficiently by RO. I have noticed that many RO systems come with extra [carbon] filtration which would presumably be good for the manufacturers specifified life of the filter in the environment in which it was being applied?

So basically forget the Brita filter and go for distillation then?

 

[chem_doc]

Nah, good post mate. It has tickled the grey matter enough to make me re-visit what I have been told by others.

I [think] I understand what you mean. You are saying that salt will be removed - which is why RO is used on ocean going vessels, and a good thing if you live in a sea water area and need fressh water supplies.

You then move on to saying that petrochemicals and alcohol would not be removed efficiently by RO. I have noticed that many RO systems come with extra [carbon] filtration which would presumably be good for the manufacturers specifified life of the filter in the environment in which it was being applied?

So basically forget the Brita filter and go for distillation then?

Well... the carbon filter is doing two things: it's removing the chunks of stuff down to a certain size (and I'm going to post a little more on that below) and it's removing stuff that sticks to it. So it's doing double duty. Depending upon the qualities of the activated carbon, it could be doing anything from pretty much just removing particles to removing particles and stuff is really sticking to its surface. (activated carbon, under a microscope, actually looks like a sponge. The more holes/surface area in the AC, the more stuff it can hold, usually. There are some other things in there to consider such as how it was made, what the carbon source was, is it treated in any way, etc.). The upside is that activated carbon is pretty cheap, so it makes sense to use it as a prefilter to prevent the expensive RO membrane from being plugged up.

I would assume that the AC filter would last as long as the RO membrane, but it shouldn't be that expensive to replace.

Oddly, I used to work for the Brita folk. Without sounding too biased, Brita filters are good, but they're pretty much just activated carbon. So that should tell you how well good activated carbon can be.

I mentioned earlier filtering out solids... Bacteria and viruses are solids. They're just really small ones. I've done work with these things and you can sterilize a solution by passing it through a 0.2 micron filter (meaning that the pores in the filter are 0.2 microns in size). This filters out the bacteria... Some viruses are smaller than that, so to completely remove viral contaminants, you should use a 0.004 micron filter.

 

[Tourist]

Well I would assume that the AC filter would last as long as the RO membrane, but it shouldn't be that expensive to replace.

During my Nuke Chem and Bio warfare training I had from the nice chaps at Porton Down I
was taught that the life of an AC filter is dependent on the environment in which it is operating - that is to say dependent on what it is having to filter out. There are set life periods for differing types of contaminant be it what we consider pure air to a full on Nerve Agent environment. These AC filter life durations can be measured from minutes to hours depending on what they are having to cope with and wether the environemnt you are operating in is forced air (tank or other AFV) or normal atmospheric pressure.

In comparison, the 'experts' I have spoken to about RO kit have told me that the life of an RO membrane filter is only dependent on the volume of water passing thru it. I specifically asked them this question, but then I don't suppose any of them had a GCSE never mind a Degree. I was a bit dubious about the answers, my own thinking being that an RO filter membrane would live until it got clogged up with the chunky stuff..........Am I right'ish?

PS I like Brita, would'nt be without one.

 

[chem_doc]

Without knowing specifics (what's in the water, what is the membrane made of, etc.), I'll give you a qualified "yes" to that question.

Here's why:

Scientists call things "salts" besides what's on your dinner table. An example of this is Calcium Phosphate. It's a great salt, but it doesn't dissolve much at all. Seashells are made of this stuff.

All of these "salts," as they dissolve into water, at some point, saturate the water and no more will dissolve. If you put enough table salt into water (and it takes a fair amount), eventually you can get it to the point of saturation; no more will dissolve. Other things take only millionths of a gram to saturate a liter of water.

Since the RO (and regular O) membranes are fiddling with salt concentrations (it's moving the water around), you're in effect increasing the salt concentration on one side of the membrane. At that point, small amounts of those oddball salts may reach their saturation point and start falling out of solution... and onto the membrane/into the pores. It probably won't be massive amounts, but it'll happen. One that happens, there are your "chunks" of stuff.

The massive chunks... things like dirt, etc. probably won't impact the filter much. Think of trying to put a 747 airplane through a garden hose. It simply won't fit into the pores of the filter. But... if you layer enough 747's in front of the garden hose, you can restrict the flow of water. This is what a good prefilter does. It is cheap and sacrifices itself so that those nsaty 747's can't layer on top of the RO membrane.

One last word, because I've done this countless times at work: Be careful with the RO membrane. As it gets the pores blocked, it takes more and more pressure to force the water through. Eventually, you can pressurize the one side enough to actually tear/rip/break the membrane.

 

[TheGreenMan]

...Wow, that was a long first post! Too much?

Nope

It's just the kind of definitive information from a qualified professional I'd hoped for. Thanks for the great posts. Nice work!

Best regards,
Paul.

 

[Tourist]

Marvellous info there Doc, I appreciate it..........it gives me the chance to abuse some potential suppliers, in an academic sort of way.