Smallest particle removed by rapid sand filtration

Published on by in Technology

Hi all,

What is the smallest particle a municipal rapid sand filtration can retain either alone or as a floc, 1,2,5 ...10 µ? (assuming no problems with carry over, channeling, clogging....etc. ...

Much appreciated

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10 Answers

  1. Dear Carlos,

    As has been mentioned by others, sand filters can get down to around 10 micron (depending on electrochemistry and other factors). Multimedia filters can achieve better quality and a longer runtime because they use different media grades and solids can be stored throughout the filter. FilterClear is a patented multimedia filtration process using four media layers for optimum performance. It typically achieves filtration to 1 micron without the need for chemical dosing.

  2. Dear Carlos, we have conducted verification of sand and our AFM filtration media performance independently by IFTS in France and in our own laboratory. We therefore have definitive data on sand filter and AFM performance.  Filtration performance depends upon, water chemistry, filtration velocity, filter design, sand quality.  There are many factors involved. Also sand becomes biofouled after a few weeks, the biofouling leads to sand coagulation and transient channelling of unfiltered water directly through the filter bed,  So giving a performance for a sand filer is not straightforward.  

    A very high quality sand filter will remove approximately 90% of particles down to 10 um, when you use new sand.   Mature sand, can actually work better, but then at set intervals due to bio-dynamic instability it will dump solids back into the water. This may only happen, once a week of once a day for a few minutes,  but it is sufficient to block membranes or cause a disease incident.

    Our AFM media  from www.DrydenAqua.com gives a verified performance 10 times better than sand and will remove better than 99% of particles down to 10 um and over 98% at 1 um.  The second advantage is that AFM does not suffer from bio-coagulation and as such it will not allow channelling and passage of unfiltered water through the filter bed.

    If you would like more information and  formal reports from an accredited laboratory, then let me know.  Howard@drydenaqua.com

  3. Gold Coast Desalination Plant (125MLD Australia) has gravity dual media filter pretreatment that aims for less than 25um and SDI less than 3.5. Normally operates much better. Had no real problem with membrane fouling when I was there.  :)

  4. Pressure sand filters should be able to remove up to 10 microns at its best. You should take in mind that multimedia and other media suppliers guarantee better filtration results. 

    Pay attention to the flux rate of your filtration system and don't go for high flux if you need better results for your application.

  5. Carlos. Best place to check is Metcalfe & Eddy design manual. Why the question? Is this for re-use of wastewater? Is there a standard you have to meet?

    1 Comment

    1. Dear Peter. Thank you very much for your attention to this question.The objective is to explore which is the opinion of experts on the topic in order to design an experiment to determine if rapid sand filtration preceded by good coagulation -  floculation - sedimentation stages is capable of retaining small particles (<0.5µ) and challenge this vs the DBP's rule that aims to remove DOC which is <0.45µ particles. If the current filtration technique used around the world even when using enhanced coagulation is not capable to remove <0.5µ particles then we can conclude that this technique dated back to 1900 is obsolete and that is time to look for another options for TTHM control.

      2 Comment replies

      1. DBP's stands for Disinfection Byproducts Rule, aim to control de formationof TTHM formed when chlorine reacts with organic material. Dissolved organic mater is <0.45 there the importance of being able to remove <0.45 particles

  6. Hi dear Carlos. As far as I know the smallest sand media size, which is near the size of aquarium sand, can remove particles up to 20 micron. After that you can use the micro filteration systems for particles up to 5 and 1 micron. Please note that 5 and 1 micron PP cartridges are different in weight. The 1 micron should be more dense and heavier as well. If you needed more clear water then you should go for ultra filteration moduls (UF).

    Cheers...

    1 Comment

  7. In general the spec is 5 micron.  I would use this to base calculations.  Thanks, Sean

    2 Comments

    1. True,  However I ran into a problem about 30 years ago at a power plant with colloidal silica that formed a sludge layer and just would not be removed by filtration and carried over into the backwash.    It took several months to document the problem and then finally figure out that the problem was colloidal. Just keep looking, and asking questions and forget the high pressure sales guys with easy answers,  who may not even be chemists. 

    2. Dear Sean: Thank you very much for the follow up on this question. That range was my gut feelings. Please read my comments  for Peter

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    2 Comments

    1. Dear Frank: Thank you very much for the follow up on this question. Please read my comments  for Peter. Will take a look on your product.

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  9. If we assume that the filtration system is designed properly, then looking at the performance of the media itself for removing TSS, Turbidity, and SDI. Commercial sand media at a feed of 31 mg/l removes 23 mg/l and for SDI feed of 0.40 removes 0.38. We have a product called High-Efficiency Media with the same above criteria removes <5 mg/l of TSS and SDI of 0.18. Therefore, the media plays an important rule in how much you can remove.

    2 Comments

    2. Dear Ken: Thank you very much for the follow up on this question. Please read my comments  for Peter.

      1 Comment reply

      1. The numbers I gave you were tested without polymer. I used a commercial sand filter after a clarifier which used polymer for removing heavy metals like Cu. In that project, Cu was tested as ND. However, we did not test for SDI or TSS.  Therefore, you must test your theory before concluding that it does not work. 

  10. Well that depends on bed depth, media size (d10) and uniformity coefficient. A 24" bed of 0.5 mm ES sand running at 10 gpm/ft2 takes out around 35% of 3-5 micron particles depending on water TDS, pH, DOC and a bunch of other factors that influence particle filterability.

    2 Comments

    1. Usually it it 5-10 micron, depending on media size and depth. Particles smaller than 5 micron can only be removed by the granular media filter if they are coagulated and flocculated into larger particles. Membrane filters (MF/UF) can remove much smaller particles (<0.1 micron) without the need for coagulation.

      1 Comment reply

      1. Dear Joe. Thank you very much for your attention to this question.The objective is to explore which is the opinion of experts on the topic in order to design an experiment to determine if rapid sand filtration preceded by good coagulation - floculation - sedimentation stages is capable of retaining small particles (<0.5µ) and challenge this vs the DBP's rule that aims to remove DOC which is <0.45µ particles. If the current filtration technique used around the world even when using enhanced coagulation is not capable to remove <0.5µ particles then we can conclude that this technique dated back to 1900 is obsolete and that is time to look for another options for TTHM control.