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Estimate the concentration of microplastic in sea water — Parallax Forums

Estimate the concentration of microplastic in sea water

zuanazzizuanazzi Posts: 10
in General Discussion
Hello everyone

It is my fisrt time here, so be gentle =)

I'm building a system to estimate the concentration of microplastic in a tank full of sea water. The concentration of plastic will increase over time, as the water is filtered and return plastic free to the sea. When a concentration is reached, the plastic will be drained from the tank.

I am still in the comceptual phase, before making a new sensor for that, I'd like to know if there are products available to use or to at least help validate some hypothesis.

There are a few parameters I am considering to estimate the concentration of plastic in salt water: thrust, capacitancy and voltage drop. Please few free to sugegst any other as well as what I can use to do so.

Thanks
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Comments

  • evanhevanh Posts: 15,916
    I'm gonna guess as the plastic concentration increases it'll start clogging the water return filter. You'll get a pressure differential forming across that filter which will be easy to measure.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    Could you pass a known volume of seawater through a microfilter and weigh the precipitate? Or boil away the water, weigh what's left, and subtract the salt content, given the known salinity?

    -Phil
  • potatoheadpotatohead Posts: 10,261
    What about acoustic means?

  • lardomlardom Posts: 1,659
    edited 2017-02-20 14:07
    I found this in a Google search. I have gone back to my hometown and seen a pronounced 'bathtub ring' accompanied by an odor in the local bay where I grew up.

  • tonyp12tonyp12 Posts: 1,951
    It looks to me the hardest part is to make sure you only filter out plastics, how do you plan to not measure other natural sediments?
  • kwinnkwinn Posts: 8,697
    You may want to take a look at instruments and methods that use light scattering/reflection to measure particle concentrations.
  • zuanazzizuanazzi Posts: 10
    evanh wrote: »
    I'm gonna guess as the plastic concentration increases it'll start clogging the water return filter. You'll get a pressure differential forming across that filter which will be easy to measure.

    Genius! I have not thought about it. I don't have details about the filtering system, but it can indeed be the easiest way.

    Thanks =)
  • zuanazzizuanazzi Posts: 10
    Phil Pilgrim (PhiPi) wrote: »
    Could you pass a known volume of seawater through a microfilter and weigh the precipitate? Or boil away the water, weigh what's left, and subtract the salt content, given the known salinity?

    -Phil

    We can do that in lab, but the system will be monted in open ocean in a bouncing plataform with limited power supply (solar powered). That makes difficult to use weigh as a parameter (the extra acceleration from the waves will add too much noise) and to boil water would be a not optimal use of energy.
  • zuanazzizuanazzi Posts: 10
    potatohead wrote: »
    What about acoustic means?

    could you detail more what you mean? Please
  • zuanazzizuanazzi Posts: 10
    tonyp12 wrote: »
    It looks to me the hardest part is to make sure you only filter out plastics, how do you plan to not measure other natural sediments?

    The filtering happens in oppen sea, so far other sedments than bioplancton were not registered as relevant. Does that answers your question?
  • zuanazzizuanazzi Posts: 10
    kwinn wrote: »
    You may want to take a look at instruments and methods that use light scattering/reflection to measure particle concentrations.

    A friend recommended me something similar, the problem of using light is that the plastics have different shapes and colors (including transparent), I have not tried anything alike so far, but I'm afraid that the results will not be really accurate as the plastic composition changes over time.
  • Tracy AllenTracy Allen Posts: 6,664
    Maybe conductivity, differential between unfiltered sea water and the water in the tank. Although, the plastic concentration might have to be pretty high before it has much effect. Don't know.

    You are right about the difficulties of a nephelometer to measure turbidity. While it is a go-to instrument for that purpose, it has to be calibrated for each use case. Standard calibrations against formazin are next to useless for conversion to mass units or for help in distinguishing, say, red mud from fine sand from plankton.

    I like the differential pressure idea. How big are these microplastics, anyway? Like 10s of micrometers or what?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2017-02-20 18:59
    Since suspended microplastics are still solid, they will have a different acoustic index than that of seawater. That means that they will disperse sonic waves of the right wavelength. If you pick a sonic or ultrasonic frequency whose wavelength in seawater is smaller than the millimeter scale of the microplastics, but larger than the scale of plankton or suspended sediment, I think you should be able to estimate the concentration of microplastics by measuring the sonic dispersion.

    This could be done by positioning a narrow-beam sending transducer across from an array of receiving transducers. By measuring the relative response of the transducers at an angle from the beam, compared to the one straight ahead, you might be able to estimate the concentration of the microplastics, independently of the smaller stuff that's in suspension. The additional advantage of this is that it could be done in situ, without having to draw samples into a vessel for analysis.

    Going even further, if you are able to sweep the frequency of the sending transducer, you might also be able to estimate the size distribution of the suspended microplastics.

    -Phil
  • PropGuy2PropGuy2 Posts: 360
    In private industry & military contract I once worked on, I have tried all the above (filtering, light scattering and all the rest) Sea water is full of "stuff" and it comes in all sizes (micro to Minnows LOL) Your regular methods probably will not work. The best solution (used by the military for the nasty stuff) is a Fast Fourier Transform (FFT) It is able to distinguish many organic and in-organic particles, it is very highly selective. The down side is that you have to have a calibration sample of the contaminate you are looking for. A savvy engineer can get down to Parts per Billion this way.
  • potatoheadpotatohead Posts: 10,261
    What about acoustic means?
    zuanazzi wrote: »
    potatohead wrote: »
    What about acoustic means?

    could you detail more what you mean? Please

    Was just a thought hit me when I read your project. Pure water will have resonances and a frequency profile. Can be known for a body of water, and it's container. Given some acoustic spectrum input, there should be a consistent response.

    Does that response change with the microplastics in there? Is it differentiated from the salt / minerals in sea water?

    Explore a variety of frequency / noise / pulse profiles.

    Maybe suck up a quantity into the known area, allow it to rest, apply the acoustic energy in various ways, map response to indicate things other than water being in there.





  • PropGuy2PropGuy2 Posts: 360
    A hint - You have to find something / a process that is naturally resonate (or not) to the particles you are trying to detect. Just about any particle (atom, molecule, dust & dirt, organism) has a natural surface (surface, as on the surface of a atom, molecule. dirt, organism...) resonance that can be used to measure what ever you want to measure. In practice this is ultraviolet, visible and infrared light - a spectrum of light that you can use to detect unique (light) patterns given off by contaminants, using FFT.
  • potatoheadpotatohead Posts: 10,261
    Ah, then the frequencies need to be higher. Was just a thought. :D

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    PropGuy2 wrote:
    The best solution (used by the military for the nasty stuff) is a Fast Fourier Transform (FFT) It is able to distinguish many organic and in-organic particles, it is very highly selective.

    FFT is just a mathematical technique used to analyze temporal or spatial data in the frequency domain. So what data are you saying it should be applied to? And what physical means do you propose using to acquire that data?

    -Phil

  • Heater.Heater. Posts: 21,230
    My question exactly. Fourier Transform of what data?

  • zuanazzizuanazzi Posts: 10
    edited 2017-02-22 17:21
    Tracy Allen wrote: »
    Maybe conductivity, differential between unfiltered sea water and the water in the tank. Although, the plastic concentration might have to be pretty high before it has much effect. Don't know.

    You are right about the difficulties of a nephelometer to measure turbidity. While it is a go-to instrument for that purpose, it has to be calibrated for each use case. Standard calibrations against formazin are next to useless for conversion to mass units or for help in distinguishing, say, red mud from fine sand from plankton.

    I like the differential pressure idea. How big are these microplastics, anyway? Like 10s of micrometers or what?

    It is called microplastic, but it ranges from tenths of milimeters up to tens of mililiters! I like the differential pressure idea too.

    I just made an experiment on the conductivity. There is a clear relationship between plastic concentration and resistance in ohms. That would be inexpensive and rather accurate way of taking it.

    Thanks for your help =)
  • zuanazzizuanazzi Posts: 10
    Phil Pilgrim (PhiPi) wrote: »
    Since suspended microplastics are still solid, they will have a different acoustic index than that of seawater. That means that they will disperse sonic waves of the right wavelength. If you pick a sonic or ultrasonic frequency whose wavelength in seawater is smaller than the millimeter scale of the microplastics, but larger than the scale of plankton or suspended sediment, I think you should be able to estimate the concentration of microplastics by measuring the sonic dispersion.

    This could be done by positioning a narrow-beam sending transducer across from an array of receiving transducers. By measuring the relative response of the transducers at an angle from the beam, compared to the one straight ahead, you might be able to estimate the concentration of the microplastics, independently of the smaller stuff that's in suspension. The additional advantage of this is that it could be done in situ, without having to draw samples into a vessel for analysis.

    Going even further, if you are able to sweep the frequency of the sending transducer, you might also be able to estimate the size distribution of the suspended microplastics.

    -Phil

    WOW! thanks!

    I'll definelty keep this idea, if that woks it would be really interesting to have this data to see if the filtering is being really effective or if it is overdimentioned.
  • zuanazzizuanazzi Posts: 10
    PropGuy2 wrote: »
    In private industry & military contract I once worked on, I have tried all the above (filtering, light scattering and all the rest) Sea water is full of "stuff" and it comes in all sizes (micro to Minnows LOL) Your regular methods probably will not work. The best solution (used by the military for the nasty stuff) is a Fast Fourier Transform (FFT) It is able to distinguish many organic and in-organic particles, it is very highly selective. The down side is that you have to have a calibration sample of the contaminate you are looking for. A savvy engineer can get down to Parts per Billion this way.

    A FFT on what exactly?
  • T ChapT Chap Posts: 4,223
    edited 2017-02-23 14:22
    Does microplastic have any ablity to be charged electrically while submerged, then move another charged plate through the water to collect all the plastic only particles. Not sure what you would do after they are collected on a plate. Would a thin film of plastic have any opacity that can be detected/measured/viewed with a camera? maybe the measurement is done on land with a photograph of the collected plastic on a plate. Then the plate is cleaned by a method for the next test


  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    It's saltwater, remember. A charged plate would get discharged instantly.

    -Phil
  • Heater.Heater. Posts: 21,230
    edited 2017-02-23 16:33
    I have no idea about plastics in sea water but...

    The famous Forrest M. Mims is partly famous for making photometers out of LEDs for measuring atmospheric turbidity. And using them successfully for many years.

    http://www.instesre.org/papers/Snowmass/MimsSnowmass.htm

    Perhaps that helps measure gunk in sea water.
  • T ChapT Chap Posts: 4,223
    Frequency of the test needs to be considered if the sensor is more of a filter then can the filter be cleaned and reused or do multiple filters need to be used.
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2017-02-23 19:31
    That is promising that you got a measurable response with conductivity. I hesitated to mention conductivity, because a lot goes on between the electrodes and seawater that makes it difficult to achieve a stablity and longevity. Electrodes have to be made of a material highly resistant to corrosion, and the current through the electrodes has to be balanced, AC. A good thing in this present case is that the measurement could be differential. It could compare pure seawater as a reference with polluted seawater as the variable, and it doesn't need absolute stability. The best instrument for conductivity in seawater is a toroidal transformer comprised of side by side toroids wound on hi-µ cores. Non-contact, nothing to corrode. The conductive path through the seawater links the cores and thus coupling is proportional to conductivity.
  • zuanazzizuanazzi Posts: 10
    Following up!

    Thanks for all the help, it gave me quite some ideas for further developments.

    However, we've got pretty good results with an ohm-meter. The resistance rises as the plastic concentration does =) For now it has results that are good enough for further study.

  • ercoerco Posts: 20,256
    I'm late to this party and I see my top Forumista picks have already weighed in. I was even going to mention eco-friendly Forrest Mims, already mentioned here. The man is a genius and a gentleman, he replied VERY quickly to an email I sent him (saved for posterity in my Hero file). He is well worth emailing. I won't post his addy here but it is easily found on his site at http://www.forrestmims.org/home.html
  • MikeDYurMikeDYur Posts: 2,176
    edited 2017-02-25 01:20
    erco wrote: »
    I was even going to mention eco-friendly Forrest Mims, already mentioned here. The man is a genius and a gentleman

    He was well published when I was young, and I knew his later research was directed towards our environment. A true inventor, experimenter and scientist, from the ground up.

    And some of his work, is a good way to see forum software in action.

    EDIT: Though it doesn't animate like I thought.
    192_Frost_castle_15_sec_interval_12252004.gif
    192 x 144 - 2M
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