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    Thread: RDF filter mesh flow rate/pressure drop

    1. #1
      ZephTheChef is offline Senior Member
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      RDF filter mesh flow rate/pressure drop

      I am thinking about DIY RDF stuff, and I was wondering if anyone has pressure drop/flow rate data on stainless filter mesh in the neighborhood of 50 micron. Like how much water it will flow @ x pressure drop per square foot. Alternately, those of you with RDFs, what is your flow rate and draw down clean vs dirty, dimensions of the drum and micron rating, and how often does the cleaning cycle typically run? Thanks in advance for any info or experiences. I don't have any specific details about my planned setup at this point, just trying to get an idea of what kind of capacity I could support for a given drum size...how much weight it will carry when dirty, etc.

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      I have also been looking at DIY RDF. Many of the RDF manufactures post screen micron size, drum size and recommended flow rates in the online literature. This is along with drum photos in many cases. This is what I'm going to base my estimates on.

      Here's an example for Synergy RDF:
      Attached Images Attached Images   
      Last edited by BWG; 03-17-2018 at 06:45 PM.

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      I have looked at a few of those kind of specs and made some inferences but haven't found any that specify at what head that flow rate is. The one you pictured specifies 395mm max water level and 420 drum size...meaning the waste tray level is 1" below the top of the drum. I'm just going to figure on total area though. So that flow rate I would assume is AT the max of 395mm, or 15.5", or half that average acting on the screen...7.75". With 5.875 square feet of surface area. So that's 7700/5.875/7.75 = 169 gallons per hour per square foot per inch of head for 70-micron screen...and it holds 7.69 gallons, so the weight on the drum would be 65 lbs at max flow, or dirty and full. I doubt flow is linear with pressure. The number I'm really interested in is at what flow rate are we only carrying a more free-flow amount, say more like 5" of water. If it is proportional to pressure differential then that's definitely something I can work based off of.

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      I don't understand your reference to pressure. These are all gravity flow devices. There would only be pressure below the water line on the inside of the drum determined by the drawdown of water on the outside of the drum. As it is flowing max flow, with max amount of debris, the drawdown would be max, and the pressure would be the miaximum,, on backwashing, the pressure would go down due to the lessening of the differential depth of the water on the inside vs. outside of the drum.
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      Quote Originally Posted by RichToyBox View Post
      I don't understand your reference to pressure. These are all gravity flow devices. There would only be pressure below the water line on the inside of the drum determined by the drawdown of water on the outside of the drum. As it is flowing max flow, with max amount of debris, the drawdown would be max, and the pressure would be the miaximum,, on backwashing, the pressure would go down due to the lessening of the differential depth of the water on the inside vs. outside of the drum.
      Rich,

      I don't know what the normspeak is when discussing pond filtration but in about any other kind of water filtration or situation where there's a restriction you talk in terms of pressure drop vs flow rate vs surface area. In this case, what's driving the water through the filter mesh is the pressure that is caused by the weight (gravity) of the water waiting its turn in line to get through the screen in the drum. If not enough pressure is present for the water to pass through at the same rate it's being added, water accumulates in the drum until such a pressure is achieved, or until it overflows. That happens at just under 1/2 psi per ft. Since we're talking about that being applied as a gradient across the surface area due to the curve of the filter being at different depths, I just threw a number of half the max pressure as an average...I didn't put much thought into how accurate that is for an arc as compared to a surface perpendicular to the water line, but it is what it is.

      My interest is really in finding out how much surface area I need to achieve a low enough requirement in terms of feed pressure, or drawdown, or however you want to look at it @ a certain flow rate to entertain a gravity-fed RDF and airlift return. I am probably just going to have to figure out how to rig up a test on the mesh that I've ordered and see for myself. Flow a known GPM into various sized filters area-wise and see where the water level sits in the pipe and crunch the numbers.

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      Quote Originally Posted by ZephTheChef View Post
      Rich,

      I don't know what the normspeak is when discussing pond filtration but in about any other kind of water filtration or situation where there's a restriction you talk in terms of pressure drop vs flow rate vs surface area. In this case, what's driving the water through the filter mesh is the pressure that is caused by the weight (gravity) of the water waiting its turn in line to get through the screen in the drum. If not enough pressure is present for the water to pass through at the same rate it's being added, water accumulates in the drum until such a pressure is achieved, or until it overflows. That happens at just under 1/2 psi per ft. Since we're talking about that being applied as a gradient across the surface area due to the curve of the filter being at different depths, I just threw a number of half the max pressure as an average...I didn't put much thought into how accurate that is for an arc as compared to a surface perpendicular to the water line, but it is what it is.

      My interest is really in finding out how much surface area I need to achieve a low enough requirement in terms of feed pressure, or drawdown, or however you want to look at it @ a certain flow rate to entertain a gravity-fed RDF and airlift return. I am probably just going to have to figure out how to rig up a test on the mesh that I've ordered and see for myself. Flow a known GPM into various sized filters area-wise and see where the water level sits in the pipe and crunch the numbers.
      I think most of this thread is over my head, but the highlighted text stuck out to me because at least my RDF this doesn't really happen that way.
      The water in the drum pretty much stays constant and slightly below pond level. The difference in water doesn't come from the drum filling
      up more but rather the clean water side dropping as the screen gets dirtier and the pump continues to pull the same amount of water and
      causes the level to drop until the sensors are triggered and the cleaning cycle starts, which causes the clean water side to rise and the cycle
      starts over again. But the water in the drum stays very stable.
      --Steve



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    7. #7
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      Well we're talking about the same thing happening and just looking at it differently. You're looking at it as dropping pressure on the clean side, and I'm looking at it as rising pressure in the drum, when in reality, it's both happening simultaneously. It's a feedback loop so they are always going to be in equilibrium. If the flow rate through the filter drops due to clogging, it requires a higher feed pressure to maintain the same flow rate. Meaning a higher water level in the drum relative to on the clean side. I'm too much of a gearhead to look at it any other way. In a supercharged engine boost pressure is a manifestation of the engine's flow rate. You make efficiency improvements elsewhere in the system and you get lower pressure for the same mass airflow. Same thing here, the pump flow is fixed so when the filter becomes more of a bottleneck the pressure differential across the restriction must rise to meet the volume flow requirements.

    8. #8
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      I suppose you're correct though. Gravity-fed the water level in the drum itself isn't really going to change relative to pond level...and the pond level isn't going to rise noticeably from pumping a few inches out of your reservoir so the inevitable place that reaction happens is by dropping the clean water. I might have had the wrong mindset about that having had a tiny garage pond where the level in the actual main pond can change extremely significantly when there's a restriction in the suction side. That's not going to happen on large ponds.

    9. #9
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      I have a 15" diameter by 12" length drum, and am flowing about 5000 gph through it. Max water level is 5" below top of drum. Draw down when clean is 1/2". I, too, looked at other manufactured units' drum sizes and max gph specs to design mine.
      I think I based mine on the Profidrum 45/40. It has a 17.7" x 15.7" drum and max flow is 6604gph. I have a 3000 gallon pond, so decided to derate my drum with a target of 4000 gph of flow. Plus, a 12"x48" sheet of 70 micron mesh on ebay is dirt cheap, and fits exactly my drum with minimal trimming.
      I had 50 micron mesh on it initially, but found that it clogged up with biofilm too easily. I switched to 70 micron 7 months ago, and the screen looks as clean as when new. Good luck with your build. You'll have many design challenges ahead, but if you can make it work, it will be a very gratifying project.

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      Drawdown on the dirty water side of the RDF will only be minor if the plumbing from the pond to the RDF is designed for near zero friction loss at the given flow. What you should be asking RDF users is can your RDF function properly at 2 to 3 inches of system airlift head pressure for a given flow rate. Obviously some can as there are airlift RDF examples posted.

      I would be asking RDF users what RDF model are you using? What is your estimated flow rate? What is your dirty water side VS clean side differential with a clean screen? Differential for cleaning can be whatever you desire. Once you know this data compare it to the manufacture drum size and screen hole micron size.
      Last edited by BWG; 03-18-2018 at 10:58 AM.

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    11. #11
      Zac Penn is offline Supporting Member
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      In my experience you shouldn't exceed 1 1/2" of drawdown on the clean side of the RDF. When you exceed that difference you can start to experience waste extrusion though the screen. Yes the dirty waste can actually get forced through the screen. This manifests itself as an extremely fine settlement in the clean section of the RDF. I have seen it a few times with the Pump Fed ZacoDraco systems when they first start up. Usually the pond water is quite dirty at the beginning and the water is being forced into the filter at a constant rate so at times the 1.5" water level difference is exceeded while the drum starts to rotate. Over time the pond gets so clean that this doesn't become an issue anymore.
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      Thank you guys for the thoughts and feedback. So basically what I'm gathering is that I'm worrying about it too much and they actually typically operate with very low pressure differential as long as it's set up to clean regularly. This year is probably going to be the same garage pond I ran last year but I want a better (easier to maintain) filtration solution, and to not be running my overkill, power hungry swimming pool pump all the time. So something like 1,000 gallons total, and a dozen or so very small koi that I can enjoy watching grow and move to a larger outdoor pond later. I am thinking about just the RDF, and airlift running full-time to remove most of the waste, and a pressure pump with finer filtration that I can run periodically for auxiliary cleaning if needed to clean up finer particles. I have something "different" in mind for bio...but do have a conventional backup plan if necessary. So basically on a pond that size at probably 2 turns per hour, 2000gph, I should be able to get away with a pretty small unit no problem. Where the issues might happen is if I do periodically double that flow rate or more to do a fine filtration cleaning cycle with the swimming pool pump.

      Zac, thanks for that info, it wasn't really something I was considering. I thought it would take a lot more pressure drop for that kind of thing to happen. I will definitely be going on the extremely large size for my pond volume, but I did get 50 micron screen. I have 4'x6' of it though so I could build up to say a 22" diameter x 48" long drum...so we're talking 4-5x the typical surface area of the smaller drums. The downside being I think it will be inefficient in terms of wastewater to clean such a large drum with lower relative accumulation of crud. It's also going to be a challenge structurally but I'm an auto mechanic and fabricator by day so I can certainly build just about whatever.

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      Quote Originally Posted by Huysy View Post
      I have a 15" diameter by 12" length drum, and am flowing about 5000 gph through it. Max water level is 5" below top of drum. Draw down when clean is 1/2". I, too, looked at other manufactured units' drum sizes and max gph specs to design mine.
      I think I based mine on the Profidrum 45/40. It has a 17.7" x 15.7" drum and max flow is 6604gph. I have a 3000 gallon pond, so decided to derate my drum with a target of 4000 gph of flow. Plus, a 12"x48" sheet of 70 micron mesh on ebay is dirt cheap, and fits exactly my drum with minimal trimming.
      I had 50 micron mesh on it initially, but found that it clogged up with biofilm too easily. I switched to 70 micron 7 months ago, and the screen looks as clean as when new. Good luck with your build. You'll have many design challenges ahead, but if you can make it work, it will be a very gratifying project.
      I have seen some RDFs that use a UV light inside to prevent biofilm/plaque. Is that something you've considered? I have already ordered 50 micron screen, but I bought a 4ft x 6 ft piece so I'm going to have a ridiculous amount of surface area for my planned flow rate of around only 2000gph. But something I was considering...granted, it is a tremendous "waste" of available surface area, was having a "dry" drum so I don't have to fuss with sealing which seems to be what a lot of people have the most trouble with. Meaning, I would have feed and waste pipes in the center, but they wouldn't be sealed. Water would only be utilizing 1/3 or so of my drum at a time. In which case the extra surface area becomes more necessary. It would greatly simplify some of the main challenges of an RDF build though.

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      Quote Originally Posted by ZephTheChef View Post
      I have seen some RDFs that use a UV light inside to prevent biofilm/plaque. Is that something you've considered? I have already ordered 50 micron screen, but I bought a 4ft x 6 ft piece so I'm going to have a ridiculous amount of surface area for my planned flow rate of around only 2000gph. But something I was considering...granted, it is a tremendous "waste" of available surface area, was having a "dry" drum so I don't have to fuss with sealing which seems to be what a lot of people have the most trouble with. Meaning, I would have feed and waste pipes in the center, but they wouldn't be sealed. Water would only be utilizing 1/3 or so of my drum at a time. In which case the extra surface area becomes more necessary. It would greatly simplify some of the main challenges of an RDF build though.
      Yes, I've seen the RDFs with UV light inside. I had considered modifying my RDF to have one, but haven't needed it after the change to 70 microns. To my eye, I do not notice any difference in the clarity of the water between 50 and 70 microns. I was also worried about the UV light degrading the plastic parts and the silicone seal.

      I am not understand your concept of a "dry" drum that is not sealed. 3/4 of the drum will be above pond water level? How would you gravity flow into it when the feed pipe is above pond water level?

      To me, the seal was one of the easier parts to build. Again, I copied what the manufacturers were doing. They've already figured out a way to make a good seal without much friction against the drum.

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      Quote Originally Posted by Huysy View Post
      Yes, I've seen the RDFs with UV light inside. I had considered modifying my RDF to have one, but haven't needed it after the change to 70 microns. To my eye, I do not notice any difference in the clarity of the water between 50 and 70 microns. I was also worried about the UV light degrading the plastic parts and the silicone seal.

      I am not understand your concept of a "dry" drum that is not sealed. 3/4 of the drum will be above pond water level? How would you gravity flow into it when the feed pipe is above pond water level?

      To me, the seal was one of the easier parts to build. Again, I copied what the manufacturers were doing. They've already figured out a way to make a good seal without much friction against the drum.
      It's difficult to describe what I was envisioning. But the more I've thought about it, I'm realizing it's not a great idea for a lot of reasons. I know a lot of the drums are open on one end and sealed around the circumference of that face. I was planning on having a through pipe on mine that's blocked half way, feed on one side and the waste goes out the other. So my drum will be sealed to the feed and waste pipes instead of to the clean water reservoir. In other words the drum will be suspended on, and rotate around, my feed and waste pipe assembly. So I'm dealing with a 4" diameter seal instead of a 15-20" one. Not sure what I'll be doing for a drive mechanism. Probably an automotive window or windshield wiper motor. Haven't decided if I'll use a pump or just city water and a 12V solenoid valve for the sprayers.

      I sort of like the idea of hitting it with lightly chlorinated water to help prevent bacterial plaque so I may just use tap water. I do worry about the chlorine, but I don't think any significant amount of it is going to end up in the pond water itself. I mean even if it cleaned once an hour for 15 seconds at full hose flow let's say that's 5 gallons...and suppose 10% of it gets misted around or attached to the mesh and doesn't end up in the waste tray then I'm adding a half gallon of chlorinated water to 1000 gallons of pond water every cleaning cycle @ 4ppm chlorine = .002ppm. I shouldn't think that would stick around any amount of time at all with all sorts of fun stuff in the pond water to react with/oxidize, particularly when spraying it at literally the dirtiest part of the pond. Perhaps I'll build an enclosure for the spray bar as well to contain any overspray/splash and reduce the amount that might make it into the pond water. I'm also thinking with 24 square foot of mesh area and my flow rate, cleaning cycles are realistically going to be more like a couple times a day if I use a float switch. I may just put it on a fixed interval/scheduled cleaning instead.

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      Quote Originally Posted by coolwon View Post
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      Here's a good video that shows you the type of seal that I am using. It's in German, but helped me a great deal while I was building mine.

      https://www.youtube.com/watch?v=a-yZIK-9__8&t=261s

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      Quote Originally Posted by ZephTheChef View Post
      It's difficult to describe what I was envisioning. But the more I've thought about it, I'm realizing it's not a great idea for a lot of reasons. I know a lot of the drums are open on one end and sealed around the circumference of that face. I was planning on having a through pipe on mine that's blocked half way, feed on one side and the waste goes out the other. So my drum will be sealed to the feed and waste pipes instead of to the clean water reservoir. In other words the drum will be suspended on, and rotate around, my feed and waste pipe assembly. So I'm dealing with a 4" diameter seal instead of a 15-20" one. Not sure what I'll be doing for a drive mechanism. Probably an automotive window or windshield wiper motor. Haven't decided if I'll use a pump or just city water and a 12V solenoid valve for the sprayers.

      I sort of like the idea of hitting it with lightly chlorinated water to help prevent bacterial plaque so I may just use tap water. I do worry about the chlorine, but I don't think any significant amount of it is going to end up in the pond water itself. I mean even if it cleaned once an hour for 15 seconds at full hose flow let's say that's 5 gallons...and suppose 10% of it gets misted around or attached to the mesh and doesn't end up in the waste tray then I'm adding a half gallon of chlorinated water to 1000 gallons of pond water every cleaning cycle @ 4ppm chlorine = .002ppm. I shouldn't think that would stick around any amount of time at all with all sorts of fun stuff in the pond water to react with/oxidize, particularly when spraying it at literally the dirtiest part of the pond. Perhaps I'll build an enclosure for the spray bar as well to contain any overspray/splash and reduce the amount that might make it into the pond water. I'm also thinking with 24 square foot of mesh area and my flow rate, cleaning cycles are realistically going to be more like a couple times a day if I use a float switch. I may just put it on a fixed interval/scheduled cleaning instead.
      One of the biggest advantages of an RDF is getting rid of the waste often and not letting it dissolve or convert. Having an extra large drum and putting a very long time between clean cycles and waste removal isn't an advantage.

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      Quote Originally Posted by Huysy View Post
      Here's a good video that shows you the type of seal that I am using. It's in German, but helped me a great deal while I was building mine.

      https://www.youtube.com/watch?v=a-yZIK-9__8&t=261s

      I see the drum being eased into position and forced the last bit

      I don't see the shaft seal or drum seal.

      Which seal would you be using?

      Is my video cutting in late after the seals are displayed?

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