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Photosynthetically Induced Phosphate Precipitation

Discussion in 'Experimental Scrubber Concepts' started by Garf, Feb 12, 2013.

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  1. Garf

    Garf Member Trusted Member

    Had this flying around in my head for a few days now, and found a link that confirms my thoughts;


    It appears that at around a pH of 8.5 (needs to be tested), phosphate will precipitate out of solution, perhaps with some calcium. In normal tank conditions with a pH of higher than 8.5, the precipitate causes a nutrient sink somewhere in the tank. When the pH is then reduced, by bacterial production of Co2 for example, the phosphate is released once again, and the cycle continues.

    Be aware that the scrubber screen increases pH at the algae surface through Co2 depletion.

    The theory is if the water being processed by the screen has a pH slightly lower than the precipitation pH of phosphate (8.5 ish), the increase in pH on the algae due to photosynthesis will precipitate phosphate ONTO the algae cells for removal when harvesting the algae.

    It may be that some scrubbers require no fiddling with to facilitate this. I suggest MOST scrubbers could be improved with regards to phos removal.

    I have started to test this by allowing my sump to increase pH to 8.3ish. Of course this means removing my air flow to the screen. My phos level has been almost static for months now, so I should notice any difference easily.
  2. Turbo

    Turbo Does not really look like Johnny Carson Staff Member Site Owner Multiple Units! Customer

    So this is nothing new though, right? This was part of one of Adey's patents?
  3. Garf

    Garf Member Trusted Member

    Yes Floyd, strange we haven't seen a reference to it before on the scrubber site. I'm assuming nobody has either heard of it before (me !) or tested it or recognised its importance (maybe !) or tested it and it's got a fundamental flaw somewhere (hope not !).
  4. Garf

    Garf Member Trusted Member

    On a side note, during my searches on this I have found reference to techniques where if the pH were to increase to 9.7, all organics and phosphate bound to carbonate could flocculate into skimmable particles. I'm not suggesting that people wishing to strip all organics should increase their tank pH so drastically, because that would be rediculous but perhaps a recirculating skimmer fed with Kalk could improve skimmer efficiency for those willing to feed more. Gonna put this one on the back burner though.
  5. Garf

    Garf Member Trusted Member

    Been doing a bit more reading;

    So, 8.75 is the maximum but benefits increase from 8.3 to 8.75 (the algae cells then attain a pH of 9.9 which causes the precipitation)

    Running this as a precipitation filter alone, could increase the phos removed by 400% + any trapped particulates.
    Running the screen effluent through some sort of a particulate filter (horizontal scrubber for example) could increase phos removal by 1000%

    Now I'm trying to check if any other good stuff precipitates out at the same time
  6. Garf

    Garf Member Trusted Member

    This precipitation reaction can be hindered by numerous dissolved compounds and organics it seems. Would make sense as they also prevent spontaneous precipitation in saltwater.


    Im beginning to believe that the Alkalinity Depletion in scrubber tanks is nothing of the sort, but a rapid calcium carbonate precipitation (which includes phosphate). In my "Co2 Turbo" thread, it was assumed that the algae somehow consumed bicarbonate, and providing aeration would reduce this effect by supplying Co2. However, adding or subtracting Co2 from a system cannot increase or reduce alkalinity, it's a scientific fact! My new theory is that the aeration merely prevented screen pH from rising into the precipitation range, thus counteracting "Alkalinity Depletion" or as I now think of it "Calcium Carbonate Phosphate Precipitation".

  7. Garf

    Garf Member Trusted Member

    19 days complete with sparkly bits and fireworm; nitrate has got up a bit, phos has come down a bit, 470 gramme harvest (down);

    10 days, with more sparkly bits;

  8. Garf

    Garf Member Trusted Member

    A few results;

    1] phos is definately going down
    2] discolouration in the water has disappeared (I've always had a tint in the water, only visible under blues) so have until a few weeks ago, run a part time skimmer and a bit of carbon here and there.

    I was hoping to keep this next fact for a future testing but I feel it's too pertinent to this thread. Phosphate precipitates out at 9.9 pH (fact), dissolved organics flocculate into skimmable and trappable particles at pH 9.7 !!! (fact) So I don't think I'm over exaggerating when I say that running a scrubber with water around the 8.3 to 8.6 range (the rest of pH increase is at the photosynthetic surface) reduces phos levels and strangely dissolved organics also (the general accepted theory is that screens produce, not consume organics) maybe even more effectively than a skimmer, especially in slower running water, ie with thicker screens. Thus turning a scrubber into a green organic as well as an inorganic filter. This is a new concept so all thoughts are welcome.
  9. Garf

    Garf Member Trusted Member

    Similar reduction in phos levels this week, barely detectable on the Red Sea test I'm using. This would indicate that i can still run the screen aeration, and increase the "free food" supply for 80% of the time. Harvesting later, then back on with the screen aeration. If I don't get excessive frothing of the sump, this would indicate (to me) that the DOCs have been flocculated and trapped by the screen, or at least turned into particulates (critter food ?) instead of dissolved organics. Doing a 5% per month waterchange to minimise the risk of precipitating out any essential elements from the water column.

  10. Garf

    Garf Member Trusted Member

    Evidence that the increased pH actually binds organics, hence trace metal complexes etc to the precipitation;


    this confirms my observations on my own tank.

    however, if the concentration of organics in the tank is too high, the precipitation does not occur. In cases of rising phos levels, either skimming or activated carbon should be used to reduce organics, therefore allowing the precipitation to occur;

  11. crashmushroom

    crashmushroom Member Customer

    Hows tricks garf? Im following along with interest keep it coming ;-)

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