The following is a transcript from the Office of Naval Research's online Tech Talk series. Dr. J. Paul Armistead, program officer in ONR's Naval Materials Division, answered questions from audiences on Facebook and Twitter on Oct. 6.
Armistead manages the Functional Polymeric and Organic Materials basic research program - which currently has interests in novel dielectric materials for high-density energy storage, organic photovoltaics and nontoxic antifouling coatings for ship hulls - as well as an applied research and development program in Advanced Shipboard Seawater Desalination.
Prior to joining ONR in 2000, Armistead worked in the Naval Research Laboratory Chemical Division, where he conducted research on composite interfaces, high-temperature composites and polymer crystallization kinetics. He received his bachelor and master degrees in chemical engineering from Virginia Polytechnic Institute and State University, or VA Tech, and a doctorate degree in materials science from Johns-Hopkins University.
Editors' Note: The following transcript includes questions submitted earlier on Facebook and Twitter, and e-mailed directly to ONR.
Event Transcript
Chris Harding: I remember learning a bit about reverse osmosis membranes in college as a chemical engineering student. From my understanding, fouling and "plasticizing" are two of the worse problems and each affects the voids differently. Is this correct?
Chris Harding: I may be using the wrong term, "plasticizing,” but it is the diffusion of a compound or elemental atom into the spaces between the polymers that, if a chemical reaction takes place, can cause a hardening of the membrane and reduce the overall effectiveness because of tears. What type of polymer systems are you using?
Patricia Nevins Kime: I believe ONR issued a 2010 CNR Challenge to solicit white papers on this topic. What kind of feedback have you received and can you discuss the most promising proposals?
ONR: Good morning! Thanks for joining us for ONR's fourth interactive “Tech Talk” with Dr. Paul Armistead. We will be taking questions on a first-come, first-served basis. Please post your questions below and remember to refresh your browser periodically to follow along in real time.
ONR: We'll take our first question from Chris Harding.
Dr. A: Chris, fouling can be a major problem with membranes. In reverse osmosis (RO) systems, usually there is a pretreatment stage or a pre-filtration stage to remove most foulants so that the RO membranes see fairly clean water and only have to remove the salt.
Dr. A: Most RO membranes are composites which may use a polysulfone support membrane coated with a thin, highly crosslinked aromatic polyamide separation layer. Plasticizing could be a problem if there are certain types of pollutants in the water, but generally, it’s not a problem.
ONR: Thanks Dr. A. We'll take the next question from Patricia Nevins Kime. Welcome, Patricia.
Dr. A: We have received a number of inquiries from people 1.) proposing improved membrane chemistries, or 2.) suggesting other means to minimize/mitigate fouling, therefore minimizing the need to use disinfectant.
Chris Harding: This question is not really appropriate for ROs used for drinking water but I am curious: Membranes are also susceptible to pH and heavy metals in sea water can enhance the degradative effects of chlorine. Assuming you are working on a means to utilize pH in pretreatment, has your program thought about using pH as a means to control "size exclusion" of specific polymers by causing some solution susceptible polymers to be repulsed by solution properties into a compact particle?
Chris Harding: This is assuming a "long polymer” is more likely to become entangled. Also, the use of solution effects may be useful in backwashing operations if a robust membrane can handle the pH ranges.
ONR: One example of a disinfectant would be chlorine, correct? Are there others and do they depend on the particular marine environment?
Dr. A: Yes, chlorine is one example. Many chemicals can be used. The Navy does not stock hazardous materials on ships, so we use chlorine because it can be electrochemically generated on site.
Dr. A: Currently, the Navy uses disposable cartridge filters to protect the reverse osmosis (RO) membranes. We are developing new desalination systems that will likely use back-flushable and cleanable microfiltration membranes to protect the RO membranes.
ONR: How long do RO membranes typically last on Navy ships?
Chris Harding: Ozone can be used as well and there are UV sources that can be used to reduce microbial load. Also, many microbial organisms are susceptible to salt, "water activity” and pH. I suppose much of the principles of low-acid canned foods could be extrapolated to disinfectant procedures.
Dr. A.: For Navy ships that are deployed in fairly clean, or deep blue waters, we can get 3-5 years use out of RO membranes. When deployed near shore or in warmer climates, the time period can be shorter.
ONR: We'll address Chris Harding's next question.
Chris Harding: What is that in comparison to a commercial filter? In other words, how good is the Navy's process when compared to civilian and life of filter?
Patricia Nevins Kime: I am so definitely not a scientist. What are these membranes made from?
Dr. A: These are good questions. In the past, ONR has funded academic research programs to develop improved microfiltration or ultrafiltration membranes for shipboard wastewater purification. Some academic researchers have developed membranes in which the pores are lined with molecules that do expand and contract with pH changes, changing the effective pore size.
Dr. A: This underscores the value we place on working with academia and others (e.g., industry, government labs and small businesses) to advance capabilities for the U.S. Navy.
Chris Harding: @Pore Size: That is interesting, but I was not referring to changing the "void volume.” Rather, I was referring to affecting the "physical" structure of a polymer that may be in solution. As I mentioned, it may not be related to "drinking water" filtration but could be used for increasing the efficiency of back flushing and reducing fouling. For example: "--------" polymer to a "@" polymer
Dr. A: Chris, ozone can be used as an oxidant in water, but there are some concerns about disinfection reaction byproducts. Also, ozone does not last long and does not effectively leave a residual oxidant capability in the water.
ONR: We'll take the next question from Patricia.
Chris Harding: I am not the doctor, but this is from my Perry's Chemical Engineering Handbook:
- Cellulose Acetate is often used if I remember correctly. Chlorine does not affect it as much.
- Armomatic polyamides: not much resistance to chlorine
- Cross-linked aromatic polyamides: some chlorine resistance.
Dr. A: Patricia, reverse osmosis membranes are typically made with a more conventional polymer as a support membrane. These support membranes may have pores of hundreds of angstroms (where an angstrom is roughly the size of the space between atoms).
Dr. A: On top of the support membrane is a dense layer that is typically comprised of highly crosslinked aromatic polyamides where the polyamide links are similar to that in common nylon. This layer is thin, roughly 1000 angstroms thick, and has an effective pore size of about 10 angstroms. This allows water to diffuse through but keeps the hydrated salts from passing through.
ONR: We'll take the next question from Chris Harding on naval systems vs. commercial products.
Dr. A: Navy systems are built for expeditionary use. They are very ruggedized and optimized for compactness and reliability. The actual functional membranes in naval systems are commercial products or variants of commercial products.
Chris Harding: @Dr. A: So you have outstanding pre-treatment processes. I have not worked with RO but I would assume most industries do not have the lifetime. If true, put a notch on the belt for our soldiers!
Dr. A: When it makes sense, the Navy tries to use commercially available components to help control total ownership costs -- which include initial acquisition cost, routine maintenance and repair -- across a vessel’s entire life cycle.
ONR: FYI, there's more on the Navy's efforts to control Total Ownership Costs in the Science & Technology Strategic Plan at http://www.onr.navy.mil/about-onr/science-technology-strategic-plan/total-ownership-cost
Dr. A: Chris, in general, pH changes can affect the confirmations of polymer chains and aid in removing fouling from surfaces of membranes. On a ship, we don’t necessarily want to stock chemicals that will allow us to shift the pH of the water.
Dr. A: Chris, the problem with cellulose acetate is that it's susceptible to biodegradation. All of these membrane chemistries have their strengths and weaknesses, and that defines which applications they are used for.
ONR: Thanks for all of your questions. Any last thoughts or comments, Dr. Armistead?
Dr. A: We look forward to receiving interesting proposals on this topic at the 2010 ONR Naval Science & Technology Partnership Conference, Nov. 8-10.
ONR: Thanks Dr. Armistead. Ultimately, our future naval forces will definitely benefit from the ideas generated in academia and industry.
Chris Harding: Hence, the reason that civilian underwater RO systems have to be replaced more often. From my information, cellulose acetate is used in these filtration systems. Thanks for the reminder! :I am sure "back-flushing" may be a problem for "household" underwater sinks.
ONR: Thanks to everyone for participating. Join us next week at 11 a.m., Oct. 13, for a Q&A on Compressive Sensing for Urban Warfare.
Chris Harding: There must be more people than I out there. Mrs. Kime had a great question!
Chris Harding: Personally, I believe discussions like these are so valuable. In industry, an important part of the process is "brain storming" ideas and scientific and non-scientific persons have so much to offer. Another example: Star Trek! It is the creative thought processes of so many literary persons that allow scientist and engineers to move forward with "new" and creative ideas!