Wednesday, May 17, 2006

The Mechanisms of Chemical Removal by Reverse Osmosis

A number of people have expressed doubt regarding my earlier description of how reverse osmosis membranes work to remove other chemicals from water. Here's my description:

"Trying to squeeze larger chemicals such as human hormones through a reverse osmosis membrane is like trying to push a golf ball through a tea strainer. Reverse osmosis is so effective that some important ions such as carbonate need to be re-added to stabilise the over-purified water."

Two of the comments received were:

“NOT TRUE. I’m not quite sure where you have got your information from about RO membrane technology, perhaps it was Toowoomba City Councils Water Futures booklet. You will need to address this issue more scientifically if you wish to evoke any sort of intelligent conversation about the technology and the process which IMO is seriously flawed.”

and

“...The problem with your synopsis Stuart is that most of us now have researched this membrane technique a little more deeper then water recycling companies would have liked us to and the whole squeezing large things through small holes is a childish misconception of the whole process..."

My response to these comments is that they are partially correct and partially misinformed. The fact is that size-exclusion is a very important mechanism for rejecting the passage of large molecules through membranes. However, the correspondents are quite correct in identifying that size-exclusion is not the only mechanism. Other important mechanisms that assist in preventing molecules from passing through the membrane include hydrophobic adsorption and, in some cases, electrostatic repulsion.

Some of my colleagues at UNSW published an important paper on this topic in 2003:

Title: Removal of Natural Hormone Estrone from Aqueous Solutions using Nanofiltration and Reverse Osmosis.
Authors: AI Schaefer, LD Nghiem and TD Waite.
Source: Environmental Science and Technology, Vol 37 (2003), pages 182-188.

The abstract from that paper reads:

“The ability of a variety of nanofiltration and reverse osmosis membranes to retain the natural hormone estrone are examined here as a function of solution conditions. While size exclusion dominates retention with the tighter membranes, both size exclusion and adsorptive effects appear to be instrumental in maintaining high retention on nanofiltration membranes that otherwise exhibit relatively low ion retentions. These adsorptive effects may be driven by hydrogen bonding between estrone and the membrane. Electrostatic attraction appears to aid retention with an apparent slight decrease in retention at high NaCl concentrations. Deprotonation of estrone leads to a significant decrease in retention, most likely as a result of the effect of strong electrostatic repulsive forces decreasing the proximity of the negatively charged estrone to the negatively charged membrane surface and thus lowering the potential for adsorptive retention. This deprotonation effect is absent for tight RO membranes. The results reported here indicate that while open nanofiltration membranes may be effective in retaining estrone under some conditions, the extent of retention may be very susceptible to maintenance of adsorptive capacity at the membrane surface and depend on solution chemistry.”

To paraphrase the findings of that paper, it means that reverse osmosis membranes (which have very small pores…also known as “tighter membranes”) rely very much on size exclusion. Accordingly, I do maintain that my golf-ball in a tea strainer analogy is entirely appropriate.

However, if we consider membranes with larger pores (which is somewhat irrelevant in the case of Toowoomba) such as nanofiltration, we find that even in cases where we would not expect size-exclusion to sufficiently retain chemicals, we actually do get some removal. That removal is caused by adsorption of the chemical onto the membrane and is highly dependant on solution chemistry.

The same phenomena were recently confirmed in a study involving 52 different endocrine disrupting compounds (EDCs), pharmaceuticals and personal care products. This study looked only at nanofiltration and ultrafiltration. It did not include reverse osmosis (which has significantly smaller pores and thus significantly greater size-exclusion):

Title: Nanofiltration and Ultrafiltration of Endocrine Disrupting Compounds, Pharmaceuticals and Personal Care Products.
Authors: Y. Yoon, P. Westerhoff, S. Snyder, E. Wert.
Source: Journal of Membrane Science, vol 270 (2006), pages 88-100.

Abstract:

“Reports of endocrine disrupting compounds (EDCs) and pharmaceuticals and personal care products (PPCPs) have raised substantial concern among important potable drinking water and reclaimed wastewater quality issues. Our study investigates the removal of EDC/PPCPs of 52 compounds having different physico-chemical properties (e.g., size, hydrophobicity, and polarity) by nanofiltration (NF) and ultrafiltration (UF) membranes using a dead-end stirred-cell filtration system. EDC/PPCPs were applied to the membrane in one modelwater and three natural waters. Experiments were performed at environmentally relevant initial EDC/PPCP concentrations ranging typically from 2 to 250 ng/L. EDC/PPCP retention was quantified by liquid and gas chromatography with mass spectroscopy–mass spectroscopy. A general separation trend due to hydrophobic adsorption as a function of octanol–water partition coefficient was observed between the hydrophobic compounds and porous hydrophobic membrane during the membrane filtration in unequilibrium conditions. The results showed that the NF membrane retained many EDC/PPCPs due to both hydrophobic adsorption and size exclusion, while the UF membrane retained typically hydrophobic EDC/PPCPs due mainly to hydrophobic adsorption. However, the transport phenomenon associated with adsorption may depend on water chemistry conditions and membrane material.”

I realise that there is alot of technical jargon in this post, but the degree of detail seemed to be what was being requested. I will try to keep explanations simpler in the future!

I hope that this information will be helpful in providing a more detailed understanding of how membranes work to remove chemicals from water. I would be grateful to receive any comments.

Stuart

12 comments:

Annette said...

Brilliant! I think we can say that you called thier bluff.

Anonymous said...

it doesnt matter how your science studys prove anything.

WE DONT WANT TO DRINK SHIT!!

njta said...

Stuart, I am very grateful that you are taking the time and energy to explain sensibly the science and technology behind the proposal for Toowoomba. You will be continually bombarded very ignorant and at times abusive comments, posted by people unable to admit that they don't know what they don't know, and too stubborn to learn.

Do not be discouraged. I would urge you to persist with posting clear and concise argument on the ability of this technology and the sensible place it will take in future water supply strategies throughout the country.

And just as a small trickle of water and soft breath of wind can wear away at the hardest of stone, so the continual trickle of knowledge and steady sensible argument will wear away at the dark ignorance that surrounds the topic of water recycling.

Keep it up and thank you.

Greg said...

To learn is one thing but to comprehend and believe is another story.

The recycled water industry is heading down the road of insolubility of the jargon, rejection of the simple, no pure middle ground and the masses will not give in to osmotic pressure.

Sometimes it is not what we believe or what is made out to be true but it is how we feel.

I would get into an intelligent scientific discussion about RO and NanoFiltration technology but not with someone who believes they need to convince the masses to drink recycled effluent. You would first have to convince me why!

Aslo I do not see no brilliance here annete and nor do I see any clear or concise argument here either njta.

All I see is an attempt to hide the technologies problems with scientific jargon!

Stuart Khan said...

Hello guys,

Thanks for the comments. They are very much appreciated.

Greg, I fully agree that it is inappropriate to bombard people with scientific jargon. That is exactly why I used the golf ball in a tea strainer analogy in the first place. It was yourself that then responded with the following:

“NOT TRUE. I’m not quite sure where you have got your information from about RO membrane technology, perhaps it was Toowoomba City Councils Water Futures booklet. You will need to address this issue more scientifically if you wish to evoke any sort of intelligent conversation about the technology and the process which IMO is seriously flawed.”

So that is precisely what I attempted to do. I felt that it was at your request. I quoted the original sources of the science and then did my best to paraphrase those sources in as plain language as I could. I apologise if I failed in that attempt…perhaps I will work on trying to find a simpler but equally accurate explanation.

I also agree that perceptions can be equally as important as an understanding of facts. This is a more difficult topic, but one that I would like to discuss in some more detail when I find time. Thanks for raising it.

Finally, I do not agree that I want to “convince the masses to drink recycled effluent”. I simply want to help the community understand that it can be a safe and realistic option. Of all of the sustainable water management options available, this one suffers most from misunderstanding, natural trepidation and, in some cases, intentionally spread hysteria.

Stuart

Greg said...

The Achilles heel of reverse osmosis (RO) membranes is biofouling. Bacteria which survive the disinfection process can attach to the RO membrane. As the membrane is organic, the bacteria may be able to metabolize the membrane material as a food source, depending upon the organism. As the bacteria metabolizes the membrane material, it chews a hole in the RO membrane. It also produces biofilm, which acts as a secondary membrane and becomes a part of the water purification process. If a biofilm-removing chemical, such as chlorine dioxide, is used, then leakage of cations and anions through the hole created by the bacteria can occur. This leakage has erroneously been attributed in the past to the oxidation of the membrane material by chlorine dioxide. Once the chlorine dioxide is removed from the membrane material, bacteria once again settle and produce biofilm, covering the hole. Leakage stops. Thus, trying to clean up a badly fouled RO membrane has not generally been very successful.

This is a major flaw, it relies on us trusting those operating the technology but who would trust TCC and governments? They have done nothing to protect the catchment areas of our dams and they have been feeding us sub-standard water for decades now so why would they be more prudent in the operations of the membrane technique and the more direct dangers this problem imposes. Remember this is not just bottled water we are talking about here, it is an entire dam. We can easily pull the infected bottled water of the shelf but we cannnot do that for Cooby Dam.

Feel free to check out the Chlourine Dioxide Hazard Rating

Anonymous said...

I personally beleve that anyone who beleves that toowoomba recycled water will be safe has been mislead. you are a sheep too beleve this is 100% safe. people should think about what other alternatives are there. not beleve rubbish thrown down our throats! for crying out loud! why should future generations be at risk because of a councils stupidity! this issue should of been addressed years ago. does toowoomba want too be know as the first place in the world too drink a 25% recycled effulant mix? people really need too do a bit of reasearch on the subject

Andy said...

Great blog. thank you for putting little sanity (i appreciate you arguing the technical points rather than just debating the "ick" factor) into this issue. water engineering is outside my field of engineering expertise, but know that there is enough controls, checks and measures that i'd be comfortable trusting the experts on it.

Keep it up.

AJ said...

Love the breakdown of the seperate processes. This was easier to read than the closer look at UV. I'll try UV again after a caffeine hit.
Stuart, is the management of bio-fouling as difficult as Greg suggests? By the way Greg, thanks for the explanation.

Stuart Khan said...

Thanks AJ,

I did actually respond to Greg's comment in a subsequent post on membrane biofouling. Let me know if you feel I haven't adequately answered the question.

Dr S Smith said...

There are a lot of pharmeceutical agents that pass through into sewage. These are rarely measured when analysing RO permeate. How do you know if these are not present in the recycled water? Also if the membrane which is after all a flimsy plastic material, has a very small leak, what happens? Membrane researches like you should not be so cock-eyed that they don't forsee problems with this technology in the future.

Anonymous said...

Some time ago, I questioned my Specialist MD (speciality is in human hormones and is NOT in the Toowoomba area) on the safety of recycled waste water. His comment to me was that even if the water was purified through the same process six times over, there would still be traces of hormones in the water. He said that to date, there is no filtration system that will remove ALL traces of hormones. He further added that we are going to see an explosion of various forms of cancer in (not so many) years to come if recycled sewerage water is used for human consumption. On that basis alone, I reject the use of recycled sewerage water for human consumption.

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