Reverse Osmosis for Waste Water Treatment: TDS vs COD reduction

[plantprowler]

In conventional industrial waste water treayment setups I've seen Reverse Osmosis (RO) units being fed water from Biological treatment systems to reduce the very last bits of dissolved solids. Say, to bring down TDS from 1000 ppm to 50 ppm.

My question is, do RO systems work at reducing residual COD (Chemical Oxygen Demand) as well? Typyically in the systems I handle the COD is in the 5000 ppm range.

Can an RO system reduce this as well to the desired specs, say 100 ppm? If so, then I may be able to take the Bio Treatment system out.

The culprits for COD are small organic molecules like Ethylbenzene, Toluene etc. Nothing toxic or corrosive.

 

[bimr]

Reverse osmosis membranes do not remove well organic contaminants (formaldehyde, methanol, acetonitrile, methyl ethyl ketone, etc).

The removal rates of organic compounds using membranes are variable and difficult to predict, through rejection models are being created such as one by the Water Research Foundation. It's hard to treat all organics the same; organic solute rejection will vary as a function of the solute molecular weight for different RO membranes.

You would have to do a pilot study to determine what is possible.

The other problem is that RO does not actually remove anything, RO just concentrates the contaminants into a smaller reject stream.

 

[plantprowler]

Thanks @bimr.

Not removing but merely concentrating is OK. So long as the permeate is "clean" I can always recycle the reject with high COD to a Biological Treatment tank but with much smaller capacity. Or recycle it to my Multi Effect Evaporator where part of my organics seem to adhere to the solids removed thereby establishing some sort of equilibrium COD and providing a sink for the organics.

One question: On an Angstrom scale a typical salt ion like Na+ is smaller than (say) Ethyblbenzene. So how come the membranes can hold the Na+ / Cl- back but let the organics through?

 

[bimr]

The following parameters are generally affect solute rejection: molecular weight (MW), molecular size (length and width), acid disassociation constant, hydrophobicity/hydrophilicity, and diffusion coefficient. Key membrane properties affecting rejection that were identified include molecular weight cut-off, pore size, surface charge hydrophobicity/hydrophilicity, and surface morphology. In addition, feed water composition, such as pH, ionic strength, hardness, and the presence of organic matter, was also identified as having an influence on solute rejection.

http://ro.uow.edu.au/cgi/viewcontent.cgi?article=2437&context=smhpapers

I would expect that the amount of organics is too high for treatment with RO. It should be noted that organics have been known to foul RO systems.

https://static1.squarespace.com/sta...9717016697/Petroleum+Wastewater+Treatment.pdf

The TOC rejection of the UF process is determined at 34%, which is highly dependent on the molecular weight of the organics. Low molecular weight organics could pass the UF unit and foul the RO membrane surface, causing serious organic fouling.

http://www.tandfonline.com/doi/abs/10.5004/dwt.2011.1137?needAccess=true&

 

[ashtree]

I have attached a DOW document on the estimated rejection of some of the solutions you refer to.
As bimr has alluded to ro rejection is much more complicated than molecular size alone, but molecular weight is still a useful guide where no other information is available.


Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[plantprowler]

Thanks @ashtree! That's a great document.