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Old 11/11/2005, 11:41 AM   #1
WaterKeeper
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Filter this Through that Thick Skull of Your's-Newbie!!

All right Newbies let's settle down, light em if you got em (filtered of course), as we have a long march ahead of us.

Filters, we talk a lot about them in this hobby but everyone has their own ideas about why they are needed and how they work. Over the next several weeks we'll be talking about the filtration methods used in this hobby and some of the principles behind them. Hopefully, although that is pushing it with Newbies , you will have a better understanding of what equipment you may need and its function in your aquarium.

If we wanted a definition of what exactly is a filter we could say, a filter is a devise that acts as a barrier to remove unwanted materials from a fluid.

That simple definition takes on a lot of area as filters can be-
  • Mechanical
  • Chemical
  • Biological
  • A combination of any and all of the above

I’ll start off with the simplest, for your simple minds , the mechanical filters. We all know about mechanical filters, as they are everywhere. They are on your dryer, in the coffee pot, in your car's air cleaner and oil filter. Heck, last night it got cold here and I was reminded to change out the furnace filter, something I have been saying I would do for the past seven years.

With mechanical filters there are five basic ways in which they function. Sieving, interception, impaction, diffusion and electrostatic bonding are those method of filtration. Quite a list isn't it and probably somewhat meaningless at this point. Let's tackle them one at a time as we go on.

Most of the around the house type filters that I mentioned above rely on sieving. Sieving is the most commonly visualized concept of filtration. Take a bucket and fill it about a quarter of the way up with marbles and fill the rest with BB's. Get a kitchen colander with holes in it smaller that the marbles but larger than the BB's. Now pour the mixture in and, after some shaking, we end up with all the marbles in the colander and all the BB's separated out.

Darn it Newbie!!! Yeah, you in the third row near the aisle, the idea was to hold the colander over the bucket not the floor. Now drop down and get everyone of those BB's up before someone slips and breaks his or her can.


Sieving is very straightforward. If the holes in a sieve are of size X the sieve retains all particles larger than size X while letting things smaller than X pass by. Now with our colander we have a surface type filter. That is, all the trapped materials stay on the surface while the smaller materials pass completely through. To keep the holes from plugging up with marbles we agitate the colander to keep the mixture from filling the holes and blocking the exit of the BB's. The only pressure on this filter is that of gravity so it is easy to keep the holes open with simple agitation.

In an aquarium most mechanical filtration is done under pressure. That is, there is force applied to move the material through the filter. In a Mr. Coffee machine a small amount of water pressure, called static head, helps speed the passage of the water through the filter. I once told the makers of Mr. Coffee that I could make their pots work faster by increasing this head. Imagine my surprise when some fool in their marketing department said that the public was just not ready for 12-foot tall Mr. Coffee machines in their kitchens. Go figure.

In pressure applications it is far more difficult to keep the holes is the sieve type filter from plugging up. The pressure filters hold the trapped material against the filter surface and eventually the filter clogs up and flow stops. It is this phenomena that I believe is the problem with my furnace.

Anyway, about the only way to improve the life of a surface filter is to increase the surface area and provide more holes. In our Mr. Coffee, one sees the filters are pleated just like the oil filter in our cars. Pleats provide more surface area and increase the life of our filter.

Surface filters are usually limited in cleaning fluids. They work OK in a coffee pot but really don't have the capacity to filter large volumes of liquids or gasses. For this we need a depth filter. Depth filters allow particles to pass into the filter media. The very largest are trapped at the surface but smaller particles can get by where they are caught deeper in the filter media. Not only is sieving being applied but interception is now coming into play. Now interception is mathematically defined by-

Equation 1
ER= (Dp/Df)2 / Ku

What's that Newbie? You didn't think you needed a calculator for this lesson? Yeah you do and Ku is Kuwabara's hydrodynamic constant if you don't know already. Now drop down and help get those darn BB's off the #*&&@% floor. Geez, next you'll want me to define Stokes number.

All right folks, I'll drop the math as it isn't going to help you, or your fish, very much. I just throw it in so those gurus on the Lighting and Filtration Forum will have something to do when they talk about which foam filter block is best in their HOB Skilters.

Just about any reefer that has had experience with FW tanks has a pretty good understanding of depth filtration. A UGF uses it and that glass wool, used in may filters, are examples. One of the things about any depth filter is that it gets better at filtering as it ages. Why? Well as particles collect in the material of the filter the void spaces in the filter get smaller and smaller. Particle that originally passed through the media now are trapped as pore spaces tighten. I think most people have seen the spiral wound filters that look like a ball of knitting yarn. These are often used for drinking water filters. As they are used they grow darker and darker in color as particles fill the pores up. Also, since they have fibers, some particles collide with and stick to those fibers, interception and inertial impaction this is called. They can therefore remove much more material than a simple surface filter. The drawback is of course that over time the amount of flow they allow through becomes less and less until they hardly let any water pass at all. Depth filter are very difficult to clean so they are usually just replaced.

Well, that's about all for today. I'll continue whenever I feel good and ready to.

Weekend furlough time Newbie; except for you two picking up those BB's! When at the bar this weekend I'm sure Peclet's number will be in your thoughts as we head toward talking about RO membrane filters.



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Old 11/11/2005, 12:08 PM   #2
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Excellent! Another classic WK thread in the making.

I apologize, I may be jumping ahead in the sylabus, but I'm noticing that my color changing DI is changing at a much more rapid pace than before. Does this mean my RO filters are going downhill and need to be replaced?


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Old 11/11/2005, 01:28 PM   #3
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Yay! A new WaterKeeper thread! Entertainment and information all at once!


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Old 11/11/2005, 02:51 PM   #4
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Thanks Amy and Dave. Dave, that would be my guess that the RO portion of the system is producing a lower grade effluent. Sometimes this is just a matter of re-seating the membrane as they sometimes get a little gap in the seals that allows some water to bypass the membrane. With use the membranes stretch and the pore diameter increases. This allows some ions, that were rejected in the new membrane, to pass through. There is no cure for that except to replace the membrane. It is a good idea to check the TDS on the membrane's effluent when first put into use and periodically thereafter. If there is a consistent rise in TDS with time you probably should replace it when the output degrades by about 10-15% of the original TDS when the membrane was new. In general, a good RO unit reduces TDS by at least 95% of the incoming water. When it falls below 90% it is probably time for a replacement.


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Old 11/11/2005, 03:07 PM   #5
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I can't wait for biological filtration, Can we skip to biological filtration ? Pleeeeeease ... I'll help pick up BB's


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Old 11/11/2005, 03:13 PM   #6
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All things come to he who waits and, in biofiltration, waiting is a virtue.


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Old 11/11/2005, 03:17 PM   #7
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I feel like a young Paduwan Reef Jedi


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Old 11/12/2005, 11:40 AM   #8
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Biological filtration is where the teachings of the Jedi master filter into that incredibly porous structure of the Padawan's grey matter.

Oh, BTW, I notice I did have chemical filtration as the second topic in that list above. I think when I finish mechanical I'll go to biological first as it is more important.


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Old 11/12/2005, 02:25 PM   #9
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When is the mechanical filtration lesson Sir WaterKeeper?


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Old 11/13/2005, 12:29 PM   #10
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I kinda thought we started that with the first post in this thread. However, I am frequently wrong.


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Old 11/13/2005, 03:35 PM   #11
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I wish I had a professor like you in some of my chem classes. I might have had a bit more fun during some lectures.

I'm looking forward to the rest of this thread on filtration, if only for the occasional equation and funny asides.

I'm only a lowly reseach associate, but perhaps some day I can also be a reef master/PhD analytical chemist.


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Old 11/14/2005, 10:07 AM   #12
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Come on back there, Wake up! I know you were out partying and blowing all your cash this weekend but it is time to filter through some more material.

We were talking about mechanical filtration and had discussed how sieving is used to remove particulate matter from a fluid. Now the Mr. Coffee type filter is not generally used in the hobby. Film type filters are just not suited to filtering large volumes of fluid as they have limited loading capacity. From the looks of things a lot of you Newbies are still loaded after the weekend.

There are uses for surface type filters in a large number of industries. Gore-Tex is used as a membrane barrier type filter in areas like pharmaceutical manufacturing. Say one is microencapsulating a medication for timed release. The larger the particle size the slower it dissolves. During the encapsulation process various diameter particles are formed and must be separated. The answer here is using a series of surface filters each with a smaller pore size than the previous filter. Say an initial filter of 50 microns and intermediate filter of 20 microns and a final filter of 5 microns. Gore-Tex films with those exact diameter holes are made and work as sieves to separate the sizes. Having such precise filtration is not really needed in aquarium use. Where we do see such filter trains is in pre-filtration for a RO/DI system. Usually we will have a fairly large pore size barrier filter followed by a couple of finer carbon block filters. This protects the RO membrane from collecting particulate matter and fouling.

Depth filtration is more often used on the tank itself. Depth filters allow higher flow and loading rates. Now in a sieve the holes are fairly uniform. With depth filters direct interception become the major mechanism of filtration. Picture a funnel with a top diameter of about 4? If we place softball into it that ball is caught with most of the ball outside the funnel. Use a hardball and it goes deeper into the funnel and use a marble and it goes to nearly the bottom of the funnel. A BB passes completely through. This is a generalized description of interception and contrary to what you were thinking has nothing to do with cornerbacks in the NFL.

The two most common depth filters are granular media and fiber. Sand filters are granular media. They are often used in water and wastewater treatment. In the early days of wastewater treatment in England Sir Edward Franklin introduced slow sand filters for purifying wastewater. A bed of sand of a foot or two deep was laid out over an underdrain. Wastewater flowed over the top of the filter and, as it passed through, particulate matter was trapped on the sand, mainly on the surface. As people studied such filters they found their filtration ability improved with age (something which doesn't happen with yours truly). It was found that over time microorganisms would colonize the surface of the sand. These microorganisms secrete sticky sugars and proteins to cement themselves to the sand, help catch food, and to provide a protective barrier against the environment. By having bacteria on the surface this same sticky glue closed the diameter of the pores in the sand and also provided a means to capture particles directly on the surface of the sand; this biofilm layer is called the Schmutzedecke, German for dirty skin. Now sand is too hard for most particles to penetrate but this soft biological glue allowed for particles to penetrate and stick, much like throwing darts in to a cork board. This process is called a sticky wicket in England but impaction by the rest of the world. In the mechanisms of filtration, sieving retains mainly larger particles, interception intermediate and impaction very small particles. With early slow sand filters having the dirt mainly on the top of the filter allowed them to be cleaned. Every so often a group of men would take boards and scrape off the top layer of sand. The filter was them allow to sit submerged for awhile to allow a new population of bacteria and other microorganism to grow before the filter was placed back in service. This process is called ripening the filter. In New Haven, Connecticut the water treatment plant there is one of the few in the U.S. to use slow sand filter. During testing on those filters it was also found that incoming water with organic compounds dissolved in it had markedly reduced amounts after passing through these filters. Clearly the filters had hidden benefits that we will be discussing later on in this thread.

The problem with slow sand filters are the same ones that plaque any surface filtration type. To get much output out of them we need a lot of filter surface area; slow sand filters can take up acres of grounds. To decrease the overall size the rapid sand filter, a type of depth filter, was introduced. This is the type filter generally used in water treatment plants today. Rather than a surface filter it is a depth filter with layers of gravel topped by increasingly finer size sands. Now this it the inverse of our funnel type model, the finest sand is at the top. There is a reason to do this, as I said in the first post in this thread, a depth filter is tougher to clean than a surface filter. In a rapid sand filter flow rates are several times that of slow sand and, even with the fine sand on top, particles penetrate deeper into the bed. This makes scraping the surface of impractical, as too much sand would be removed in the cleaning process. To clean these filter a process known as backwashing is used. Water is flushed at a fairly high rate back up through the sand bed. The rate of flow is enough to suspend the sand particles (fluidize the bed) and increases the space between the sand grains. This un-traps the captured material and allows the backwash water to carry off the materials that were collected in filtration process. Once washed the filter is returned to service. Rapid sand filters do not have much of a biological film on the sand as the flow rate is so high, so no ripening is needed.

What do you mean-So what? Drop and give me 20 mister! Much of the technology that we use in our tanks comes directly from the water and wastewater industry. There are some pretty bright minds studying your poop buddy.

Anybody that has much experience in keeping tropical fish knows about the under gravel filter, UGF. It is used in most FW aquariums and its introduction into the hobby was a major advancement. It is a sand deep sand filter by definition but, as it uses fairly course gravel, it really doesn't trap much suspended material. When I used one on my FW tanks I used a HOB type mechanical filter with some filter pads and a foam filter block to power the UGF and remove fine particulate matter from the gravel filtered water. The real purpose of a UGF is not remove particulate matter and, as we will see, that collection of particulate is somewhat undesirable to keep one functional.

HOB type mechanical filters are pretty common in SW use. They provide circulation and some water polishing. Even better results can be obtained using a higher-pressure canister filter. I often see threads saying, "ditch the canister". That is because they are being used as bio-filters and that can cause problems. When used as mechanical particulate filters they are unsurpassed. A Marineland Magnum filter using a micron-polishing filter inside can quickly clear a cloudy tank. When the mechanical filtering is finished they can be loaded with carbon and used as a chemical filter. They are very versatile. I have an older Aqua-Tec that I use whenever I have fish in quarantine. I came upon some 0.25 micron filters, I think from U.S. filter, which are fine enough to remove any parasite and almost all bacteria. They fit the Aqua-Tec chamber and using it provides almost a biologically sterile Q-tank environment. Even the 1 micron pleated filter sold for the Magnums will remove ich and velvet protozoa's from ones tank and will do it faster than a UV system as they can process a lot more water in a shorter time. With their low cost they are a handy investment for the hobbyist.

One little tip about these pleated polishing filters. They are really a fiber, depth type filter and back flushing really doesn't clean them very well. They are tolerant of chlorine however. Soaking them in a 1 to 3 part bleach-water solution removes captured detritus. If you use aragonite sand for your bed they then can be rinsed well and soaked in a 50:50 vinegar solution. That will dissolve aragonite fines that have become trapped. You can reuse them quite often that way.

Well, I think that is enough for today. We'll wrap the mechanical part up and switch to biology next time. I know biology is mainly on your newbie minds anyway.


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Old 11/14/2005, 10:50 AM   #13
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Must have moreeeeeee! Cookie Monster Hungry for more Water Lessons.

I was also wondering if you could advise me on which is better. RO water or Steam Distilled ... My local "Pure Water" shop tells me steam water is virtually 100% Pure as opposed to RO water ... I purchased a 100 gallon card for $45 and can pick up any water at any time...

... And I'm still picking up marbles from the first lesson


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Old 11/14/2005, 11:43 AM   #14
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Chocolate, from what I know, distilation allows water molecules to evaporate, leaving all the rest behind. So, when steam condences you will have pure water. However, there is some water contamination still occur in this process. For instance, some commersial distilate equipment uses copper pipes, so there is very small copper leak. Over time, that copper could accumulate in your tank and create big problems.

On the other hand consider this, RO/DI unit 125GPD from eBay costs $200 w/shipping. Given the membrane live span 2 years, your cost per 100 gallons of water is 0.9 of a cent!!!!


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Old 11/14/2005, 12:59 PM   #15
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9 cents a gallon! I wish my car ran on RO/DI water Unfortunately I don't have an extra $200 lying around at the moment since I am on a budget putting together my new tank ... and I spent my extra money on a snorkeling trip to Egypt in Jan. But the per gallon price info is good to know!

Thanks for the info TekCat ... I'm still patiently awaiting WaterKeepers response.


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Old 11/14/2005, 01:55 PM   #16
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not 9 cents, 0.9 of a cent!!

Good luck snorkeling


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Old 11/14/2005, 02:29 PM   #17
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oh snaps ... I misread that ... my bad!

I don't need LUCK snorkeling I need LUCK to keep those Arabs from planting a bomb at the 5 Star Hotel I'm staying at! I'm going to keep my butt in the water as much as I can and away from the hotel. Thanks TekCat!


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Old 11/14/2005, 03:39 PM   #18
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Right you are TekCat, distilled water is good, especially that distilled in all glass or quartz stills, but it is not as good as RO/DI. Good DI water has a resistance of close to 18 million ohms with 18.33 million being theoretically pure. The best stills top out around 1 million ohms. Either is fine for your tank but buying an RO/DI will pay off in the long run unless you have a nano in a Mason Jar.

See [rodifaq] for more on the subject.


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Old 11/14/2005, 04:33 PM   #19
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Thanks WaterKeeper ... for now I think I am gonna go with the steam distilled because the shop said it's 99.9% pure as opposed to their RO/DI which was only 75-85% pure depending on which nutrients / minerals. Since I already purchased a 100 gallons from them I will consider the RO/DI once the 100 gall's are up! I will make sure to ask that they don't use any copper piping in the steam distillation process if so by some chance ... I will go with their RO/DI


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Old 11/14/2005, 04:43 PM   #20
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Sweet......the only reason my reef seems to still be living is b/c of your loooonnnngggg threads.....excellent reading....keep it up WK


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Old 11/14/2005, 11:59 PM   #21
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Cookie Monster Hungry For BIO info!

This thread needs to be bumped for the noob's!


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Old 11/15/2005, 12:12 AM   #22
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great thread WK.

just when i think i am getting the hang of knowing things you have to jump in and teach me something more about the stuff i thought i had already knew

but all good i love to learn. (about reefkeeping that is)


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Old 11/15/2005, 08:35 AM   #23
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All these threads are in the sticky thread entitled "Please Look Here Before Posting" at the top of the New to the Hobby Forum. The list is the second Post Down


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Old 11/15/2005, 03:04 PM   #24
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Quote:
Originally posted by aschke
I wish I had a professor like you in some of my chem classes. I might have had a bit more fun during some lectures.

I'm looking forward to the rest of this thread on filtration, if only for the occasional equation and funny asides.

I'm only a lowly reseach associate, but perhaps some day I can also be a reef master/PhD analytical chemist.


I'll have you know, you novice in the black arts of Alchemy, that the lab is not place for levity. If you are creating the next miracle of modern science, such as Oxy-Clean or making the Un-cola red, with a cherry flavor, it takes firm dedication on the part of researchers involved.

One of the areas where filtration in often overlooked on one's tank is air introduction devises. These are things like skimmer venturi intakes and air introduction lines attached to powerheads. Room air has dust, oils, smoke and other vapors that find their way into our tanks unless removed by filtration. Placing a barrier air filter on such lines greatly reduces the introduction of such substances into our tanks. Since air is less viscose than water, allowing it to flow more freely through a loaded filter, the filters are generally small and inexpensive ( my furnace seems to have forgotten this rule). Adding one is an area that should not be overlooked in proper tank husbandry.

I see that some people have noticed this thread and my love of Canister Filters and started a thread over on the Reef Discussions forum. It appears that most people over there have more important things to spend their money on, like Algae-Out herbicide, reef formula. The canisters are losing in that poll but I will demand that the ballots be rechecked for hanging chad when the polls close.

I mentioned above that I found some 0.25 micron pleated filters that I can use to remove anything but the very smallest bacteria. You may need to look around for commercial suppliers to find them but most canister filters use standard sizes of filters and a little searching can locate one that fits your housing.

Now a 0.25 micron filter is just about as small a pore size that is considered "normal particulate filtration". Just below that range the term switches to such things as ultra-filtration, nano-filtration and, even, reverse osmosis. These are filters that are so fine that they can remove things from a fluid that were not possible until a few decades ago. Using ultra or nano filtration Coke can fight back and make crystal clear cola, with no change in the Coke favor, to take on the challenge presented to them by Cherry 7-Up.

All classes of these filters use membrane technology. Now when we think about membranes we think surface filters but, when the particle is only a few angstroms (0.0001 microns) or a 1000 Daltons in size, a very thin membrane is a very dense area to cover. To molecules, a hairs width is a long journey indeed. Surprisingly, the oldest of this class of filters is also the "finest".In the early 1960's the first RO filters were made by Doctor David Sourirajan in his lab at UCLA. The more open structured filters where developed somewhat latter. Now with my 0.25 micron filter, direct filtration is used. That is, all the water passes through it leaving the particulate matter behind. This poses a problem as it severely limits the filters ability to pass flow. Take a 0.25 micron filter and use it on a tank during the sandstorm created when introducing LS and it works great....for about 3 minutes. Even a larger, 1 micron, filter bogs down in fairly short order. With filters this fine a pre-filter is a must. With my pleated polishing system I use an outer foam filter sleeve. This larger diameter sleeve takes some of the burden off the inner polishing stage.

Time for another quick tip. If you are cleaning up a sandstorm using such a system give it at least a day or two for things to settle on their own. Then, when the system is placed on-line, the inner filter clogs up rapidly. When that happens, don't clean the foam filter sleeve. Give the inner filter a vinegar soak and, after rinsing, place it back in the dirty sleeve. Why? As I said earlier, filtration ability improves with age. The now seasoned outer filter will trap finer material now and lessen the load on the polishing stage. Even so, be prepared to do a lot of vinegar soaks at first.

With membrane filter this same effect would be far worse except there was a solution also developed at UCLA, Cross current filtration (CCF). Now these filters don't work on the principles of particulate filters. Sieving, interception, and impaction probably don't even enter the picture. If one looks at the RO membranes under the best electron microscopes there is no discernable pore structure. In fact, to this day, there is a huge debate over how they really do work. Most theories figure it is a blend of diffusion and electrostatic repulsion but nobody is really sure. Anyway, with CCF, flow is directed laterally across the membrane surface. Whatever the mechanism, it allows part of the water to be filtered while the water on the intake side of the membrane continually flushes the membrane surface sweeping away the materials as they accumulate and sends them off to waste. This helps prevent the membrane from clogging up after just a few gallons of use. Almost all forms of ultra filters use this method.

RO is the main type of filtration of concern to the hobbyist. With it, one is saved the expense of continually replacing large amounts of DI resin, a chemical filtration agent. In some RO applications a dual stage RO process is used replacing the DI stage completely. It doesn't produce true DI purity but it gets pretty close. The problem is that RO units work best at around 200 psi. Home units are designed to run at far less pressure, usually 40-70 psi. At these low pressures the dual RO principle doesn't work that well.

Now osmosis is the process where a dilute solution flows through a semi-permeable membrane to an area of high dissolved solids concentration. If we place water in a U-tube with a semi-permeable membrane separating one side, with a brine solution, and the other side just plain water we see that the water levels on the salt side of the membrane rises higher up in the U-tube. This is because water permeates through the membrane into the brine solution. Over time the level in the brine side falls until it is almost level with the plain water side. That is because the salt side is coming into balance with the fresh water side and less plain water flows to it. This difference in elevation, when measured, is the osmotic pressure. In reverse osmosis we apply a force, pressure, on the salt side greater than the osmotic pressure. This reverses the flow so that the plain water will separate from the salt solution and flow toward the plain water side. The net result is that the water is purified through the process and the resulting contaminated water is flushed to waste. Again, the exact mechanism of why this happens is still cloaked in scientific mystery.

Now this membrane material is not visually porous and it is thought in one theory that the water diffuses between the molecules in the membrane itself. The common thin film composite membranes used in home systems are a polyamide supported on a polysulfone backbone material. These long chain polyamides have a pore structure between the molecules like a labyrinth and could explain the process with diffusion occurring in the intermolecular spaces.

Whatever the case, the process works and there are only a few things we can do to improve it.
  • Raise the temperature. The viscosity of water decreases as the temperature rises creating higher outputs.
  • Raise the pressure, either by adjusting the reject water valve or adding a booster pump
  • Optimize the wastewater flow rate

The last item is a little more confusing. There is no perfect balance because there are many variables. In general, if your source water is very hard you want a high wastewater flow to help sweep away the collected solids and prevent membrane fouling. With soft water you want a lower flow of wastewater to boost the pressure on the membrane and save on sewer bills. Things like pH and the type of ions dissolved in the water also come into play.

RO membranes can also remove a wide variety of organic compounds, something the DI stage does not do. In general most gasses pass through the RO membrane as do light molecular weight organics. If your water contains chloramines, the carbon filters removes the chlorine but releases the ammonia. This is not removed by the RO membrane but will be in the DI stage. If you use RO water only on your tank, be sure to check whether chloramines are used on your supply.

Well so endeth today's diatribe. It also pretty much ends the mechanical filtration primer so I hope you gearheads out there enjoyed it. I shall let Aschke do the part on Chemical Filtration as I have run completely out of activated carbon jokes.

For me I need to get out to the bars and gather votes for the" Canister Filter Party".


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Last edited by WaterKeeper; 11/17/2005 at 09:35 AM.
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Old 11/16/2005, 09:45 AM   #25
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Sorry gang about all the crazy characters that were in the above post. I didn't see them until today and have fixed the badly translated punctuation marks. I'm not sure why that happened as I often paste stuff from Word into posts here. Again, Sorry bout that.


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