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    Thread: Sand & Gravel Lesson Learned

    1. #1
      nwrunner's Avatar
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      Sand & Gravel Lesson Learned

      I've been a pimp for the sand and gravel filters as the most effective, low cost, DIY filters for quite a while, and frequently recommend them for people just getting into the hobby. I've never really had a bit of trouble with my two until this week (or actually I now realize for probably the last year in varying degrees)

      The quick version is that since I installed an RDF on the lower pond I've had to backwash the S&G less frequently as is evidenced by the stuff that goes to waste from the S&G's backwash. I also recalled that Birdman left his S&G's for a month when he first went to SoCal. Therefore I've been waiting 2, 3 + weeks between backwash of the filters. What I had no idea was happening was that as my filters were clogging up (and the time they remove ever smaller particles) the water was backing up through my backwash tube and gradually seeping out of my contraption of a air blow backwash manifold you can see. Therefore always cap that air intake tube during operations.

      This was a complete mystery to me over the last year because I meter the water on my autofill to regulate my water change. Gradually over a week, or two my autofill would double the water added. I assumed it was the RDF washing more because the pond was looking ever clearer (as the S&G's clogged and filtered ever finer). I also have a LOT of trees so I get large quantities of stuff at random intervals. I've tested every imaginable possibility of a leak because at times the water use was getting excessive. It was an absolute mystery to me as Norm can attest to. I would bounce every imaginable idea off of him and he politely listened and made suggestions. We could never trace the source of the problem.

      What became curious was that the water use seemed to always rise on Thursday and Friday. I even suggested cycles of the moon out of frustration. Again, this happened over a year so I knew it was as I neared the weekend that my water use would rise, and I had no explanation. I always backwash on Sundays so this explains the cycle.

      It wasn't until this week when I finally let the S&G's go over 3 weeks that the upper pond dropped a lot. I shut off the circulation between the two ponds and it continued to drop. I was absolutely sure I had a liner or BD leak and was crestfallen. I finally went to backwash the filters out of frustration and saw water pouring out of the bleed off valve in my manifold which I leave partially open as it's perfectly set for the bleeding off the right amount of air during backwash.

      If I was using my wet vac like I used to it wouldn't be a problem because the backwash tube simply flows back into the filter, but because of my manifold extending out of the barrel area and because it's covered and near the back of my filter area it was simply backing up into the air manifold and seeping out undetectable. I also have a river rock base under the raised barrels with a french drain so I couldn't see any water pooling nor even dampness.

      So, lesson learned. Backwash your S&G's weekly and have a way to cap your air tube leading into the bottom of the barrel if it extends beyond the edge of the barrel.

      Doh
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      BWG is offline Senior Member
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      Since the SG filters are also a good bio filter the build up might be just bio growth and related to what's available in the system after the RDF to go through the nitrogen cycle.

      Thanks for passing the info along. It was a good catch and great advice to pass along.

    3. #3
      koiman1950's Avatar
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      Quote Originally Posted by BWG View Post
      Since the SG filters are also a good bio filter the build up might be just bio growth and related to what's available in the system after the RDF to go through the nitrogen cycle.

      Thanks for passing the info along. It was a good catch and great advice to pass along.
      What makes you say this with such conviction? Compared to what other type of "bio" filters?
      Mike

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    4. #4
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      Mike, I saw a study on this a few years ago and I thought it was the Chicago Aquarium but I can't find it now. Here is a pseudo relevant link about bio build up on sand filters.

      https://en.wikipedia.org/wiki/Biosand_filter

      What I somewhat vaguely recall is that they were studying various types of bio media in giant moving bed type filters but also had a form of many large Sand filters included in the mix for mechanical. Well they couldn't get bio to grow well on the media after the upgraded filtration system was installed and they tried several methods including adding ammonia, but somehow the fish survived and the chemical levels were perfect. What they concluded to their surprise was that there were large colonies of bacteria growing in the S&G's and it was absorbing all of the ammonia from the few fish they had stocked. It was sort of a "we don't know how or why, but this seems to be occurring"

      Perhaps someone else could chime in who recalls the study, or could link to it, but it was one of those where I said " the more information we have about the nitrogen/bacteria cycle in the pond the less we actually know". I have no idea with limited air and no light how beneficial bacteria can grow in an S&G but I also believe it does.

      Sorry I couldn't cite the study, but I'll keep looking.

    5. #5
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      Well, those filters are TOTALLY different, being downflow design, their flow rates are VERY SLOW in comparison, and the efficiency for our use in koi ponds would require constant cleaning to keep the small amount of bio bed sand clear of debris to allow it to keep from overflowing. It also uses a diffuser pipe like in a shower unit, to help diffuse/spread the water out evenly which is quite the opposite of the delivery system used in our typical upflow s/g filters which trap all the solids in the larger to smaller graduated size rock/gravel before it gets to the "sand" or grit bed. It is also completely SEALED! If you look at most of the DIY barrel filters used for this, they are roughly 22" in diameter. So, lets' use a 6" deep bed of chicken grit. That would equate to roughly 2.28cu ft of media. I haven't seen any true measurement of sq ft of surface area/cu ft of media, but this is quite the inefficient area for really relying on this unit to provide satisfactory bio for anything other than a smaller pond with a VERY LIGHT stocking density, unless other downstream bio filtration is included which will actually be performing the actual bio handling of the pond/fish. Also, consider that based on the porosity of the grit media, there is very little interspacial room other than the actual surface of the media to colonize, and thus, when weekly backwashes are performed as recommended by most EVERYONE using one of these, the loss of bio colony will be high enough to require a rebuilding of the bio colonies to get back up to speed/efficiency to handle the nitrogen cycle. Thus, this filter is truly not an effective bio converted by any means, just a water polisher, removing the finer particulates within a water column before returning the water to the pond. Any bio that it may perform would be just a plus factor to the more efficiently design bio filter, even if mats like Jap mat or Matala are used with much higher sq ft/ cu ft of area is provided.
      Mike

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    6. #6
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      I have failsafe openings on my S&G air blower pipes - simply tee's in the pipe right in or over the barrel so that if water rises in the pipe it just goes back into the barrel. I cap these when flushing. It's also a good visual to know that flushing is overdue.
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    7. #7
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      IV: Thou shall flush S/G's weekly...regardless of "dirty" we think they are.....
      Tim can always be reached at 850-380-7824 or timnye850@gmail.com

    8. #8
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      Quote Originally Posted by nwrunner View Post
      So, lesson learned. Backwash your S&G's weekly...
      Great post! Thanks for taking the time to explain the what and why's!
      --Steve



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      so basically the S&G filter does not have pre-filter. is this correct ? I am curious how long you can go without flushing if the S&G if it goes after the RDF.

    10. #10
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      Thanks for the cautionary tale!

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    11. #11
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      Quote Originally Posted by Appliance Guy View Post
      IV: Thou shall flush S/G's weekly...regardless of "dirty" we think they are.....
      I'll flush twice weekly if my overflow tells me to...
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    12. #12
      icu2's Avatar
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      Quote Originally Posted by vipertom1970 View Post
      so basically the S&G filter does not have pre-filter. is this correct ? I am curious how long you can go without flushing if the S&G if it goes after the RDF.
      1 week.

      I was thinking, if I understood correctly, that the RDF was before them.

      All I know is I put a 50 micron sock on the outlet of one of mine as an experiment and caught zero in the sock and have nothing
      but a skimmer basket and priming pot before it.
      I know the RDF's are suppose to filter even below that, but stuff will turn anaerobic in a heart beat deep in this filter...
      it's easy to clean and no reason to leave it unless you absolutely have to imo.
      --Steve



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    13. #13
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      Quote Originally Posted by icu2 View Post
      1 week.

      I was thinking, if I understood correctly, that the RDF was before them.

      All I know is I put a 50 micron sock on the outlet of one of mine as an experiment and caught zero in the sock and have nothing
      but a skimmer basket and priming pot before it.
      I know the RDF's are suppose to filter even below that, but stuff will turn anaerobic in a heart beat deep in this filter...
      it's easy to clean and no reason to leave it unless you absolutely have to imo.
      To be clear, Rob's S&G's are not after the RDF. Rob has two ponds, his upper pond (4400g) consists of BD & Skimmer feeding an Ultra Sieve followed by his pump and 2 MB's and 2 S&G's all in parallel. His lower pond (7200g) consists of the BD, Midwater & Skimmer feeding an RDF followed by a Flowfriend Pump that feeds a shower filter. He does, at his discretion, have the ability to commingle the ponds together through an interconnecting waterfall. So the RDF has only a minimal effect on the S&G's

      I do have a client with an S&G and an RDF and they have reported that they now only clean it every 2-3+ weeks and state the waste is mearly grey water, not dark green/brown as before the remodel.

      Regardless of the set-up I would suggest weekly cleaning though.


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    14. #14
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      Jeez stretch a few backwashes out two weeks and a guy gets killed here

      Norm has the setup exact, the big pond has rdf and double tray cermedia shower only and that's fine all on its own. I was only stretching out the backwashes when I have the two ponds connected and felt that the frequent cleaning of the AFIII and RDF would give me some lag time between needed backwashes. I was wrong for sure.

      Goldie, I absolutely love the canary in the mine overflow fitting idea to show when the backwash is needed. That will be going on my setup this weekend.

      The ponds are separated now after the upper pond partial draining incident, but thinking about this a bit more, I suppose I should be grateful the water didn't back up all the way and burn up the blower if I would have had all three valves closed.

    15. #15
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      My air manifold pipe is about a foot higher than the barrel, I've never had water coming out of it, but I will be paying more attention to it now. I also like GoldieGirl's mod, I think I will do that with my next one. Good info here, thanks!

      Cheryl

    16. #16
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      Quote Originally Posted by Norm Walsh View Post
      To be clear, Rob's S&G's are not after the RDF. Rob has two ponds, his upper pond (4400g) consists of BD & Skimmer feeding an Ultra Sieve followed by his pump and 2 MB's and 2 S&G's all in parallel. His lower pond (7200g) consists of the BD, Midwater & Skimmer feeding an RDF followed by a Flowfriend Pump that feeds a shower filter. He does, at his discretion, have the ability to commingle the ponds together through an interconnecting waterfall. So the RDF has only a minimal effect on the S&G's

      I do have a client with an S&G and an RDF and they have reported that they now only clean it every 2-3+ weeks and state the waste is mearly grey water, not dark green/brown as before the remodel.

      Regardless of the set-up I would suggest weekly cleaning though.
      Thanks Norm!
      I misunderstood the arrangement but I appreciate the info!

      Rob,
      I think if this is the worst mistake you make, you've got it made!
      I know I could write volumes...

      Thanks again for your write up... it was excellent information.
      --Steve



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      Similar lengthening of SG and bead filter cleaning cycles reported where people went from a settling chamber to a sieve.

      Why wouldn't the media in a SG filter be bio active? The usable bio surface area is probably greater than an equivalent volume of beads, ball, K1, mat, etc.... Its also not conceivable that the SG media is completely scrubbed of active bio growth after cleaning.

      You can find large sand filters in both the up flow and down flow designs. Not all of them are trickle type filters.

      Commercial pond builder types don't like SG filters because they are too easy to DIY and there is no money pushing them!

      The OP made it sound like cleaner water is going through the SG filter because of the RDF installation. Perhaps they can clear this up.

      Great idea from GoldieGirl on the bypass and filter cleaning indicator.
      Last edited by BWG; 07-31-2015 at 07:19 AM.

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      Quote Originally Posted by BWG View Post
      Why wouldn't the media in a SG filter be bio active? The usable bio surface area is probably greater than an equivalent volume of beads, ball, K1, mat, etc.... Its also not conceivable that the SG media is completely scrubbed of active bio growth after cleaning.
      When I was researching filter systems for my rather small pond (1300gal) I looked at that in a little detail. The specific surface area of sand is the largest of any of the materials studied, as I recall something like 2 or 3 times greater than K1 or ceramic media, and the amount of surface in the layer of sand in a S/G should provide more than enough bio. Usually S/G filters don't use sand but use a larger media which does not have as high a specific surface area. Still I figured there was more than enough bio for my size pond in a couple of S/G filters. So far so good. I worried about removing the biofilm during cleaning as well. I found some experiments done with upflow moving bed sand filters for the aquaculture industry. They found that the media expanded in size over time from the layers of biofilm building on the sand and they purposely scrubbed off some of the film to refresh the media. As I recall one of the features of K1 in a moving bed is that it is constantly scrubbing off some biofilm so the surface biofilm is refreshed. The thinking was that new growth is faster more hungry than old growth. I wouldn't expect there to be a problem with the weekly S/G cleaning.

      At one point I remember someone on koiphen running a side by side comparison of moving bed, shower and a couple of other bio filters. While there were differences they all seemed to work. Our ponds have a biofilm on everything, even the pipes. I can't imagine that there isn't biofilm on the "sand" in a S/G filter. The question is if there is enough. For me, it seems to be working well.

      I wouldn't mind seeing the S/G bio discussion taken to a new thread if we could keep it friendly and throw some data or research into the thread.

      Bill

    19. #19
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      Well, we're not dealing with "sand" but rather mostly granite type grit which, by itself reduces the volume of habitable area for bio. Is there bio going on, sure. ANY surface within a pond environment will have bio after becoming mature, but how much is enough. I mentioned this in another thread but let's use this example again. A normal 55gal barrel used in this exercise is pretty much standard at a size of 22" in diameter. So, if you add 6" of grit as your top layer, doing the math this equates to around 1.3 cu ft of media. I won't take into consideration any of the other gravel/rock used as it's just not a feasible study. If sand is only 2-3 times greater in sq ft/cu ft than K1 or ceramic media, and the grit is, say, 10 times the size of sand, then I think it's obvious to say that it's not close to the K1 or ceramic media. You'd also need to quantify "which" ceramic media is being compared as nothing come close the the MP2C/Cermedia used by Zac and Norm in their shower units at nearly 100,000sq ft/cu ft.

      As far a bio colony goes, and it's ability to "hang on" for the ride when a filter is cleaned, you have to look at the surface area of the particle and how it's effected by the "scrubbing/agitation" action of the cleaning process. There will be a higher loss effect if the only surface for bacterial colonization is the outside or shell of an object. As such, sand and grit have no porosity, therefore, very little "holding" power. So, yes, the bio colony will be highly effected, but not totally eliminated when cleaning an s/g filter with a blower, but I wouldn't say it would be "up to snuff" as it was before the cleaning cycle. Two of these filters on a 1300 gal pond, with a 4000gph total turnover through the two, with a decent to low stocking density should provide enough mechanical/bio to handle the load. But, as has been stated exhaustively, each pond is different!

      BWG - your remark about commercial pond builders couldn't be further from the truth. Not all commercial pond builders have a store front or charge higher prices for filtration units that are commercially made. We install what we KNOW works based on the desire of what the customer is trying to achieve and scope of the project and the design of the "system" being installed. In many cases, these s/g units are hard to install in inconspicuous locations and the flow rates are such that, on large/larger ponds don't make them feasible. And, again, we're not talking about commercial grade "sand filters" here as they're just not feasible for a "pond" environment unless pre-filtered to trap sediment/particulates to less than 100 microns or finer.

      Regradless of what is installed in front of an s/g or pressurized filter, cleaning frequency should not go beyond at least once/week.
      Last edited by koiman1950; 07-31-2015 at 06:13 PM.
      Mike

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      Try looking at grit that has been manufactured by crushing and sifting under the microscope. Then take bead filter beads and do the same. Then think about holding power.

      It doesn't take scientific analysis to quickly surmise that a cubic foot of grit or course sand is going to be a lot greater surface area than a product with a lot of air between the surfaces and pieces such as K1.

      As for ceramic media there is absolutely no credible information that micron sized fissures only seen with a high powered microscope have any bio value. The square foot surface area equivalency is nothing more than advertising hype and not a reliable comparison between media types. Nothing but a over priced and hyped product with a good profit margin. 100,000 square feet of usable bio growing area in a cubic foot? I'll ask the Tooth Fairy and Santa on that one.

      There are many old school koi ponds still running with large diameter SG filters.
      Last edited by BWG; 07-31-2015 at 06:37 PM.

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