Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA)
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TOPIC: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA)

Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 29 Jun 2013 17:47 #4890

  • skdentel
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Hi everybody!

We've been doing a lot of work without posting anything, so here's an attempt to catch up. First let me introduce myself - I'm Dr. Steve Dentel, at the University of Delaware in the U.S. I've been working on water treatment and sludge treatment technologies for many years, and in my recent work with Engineers Without Borders, I've learned a lot about sanitation needs in Africa and elsewhere. So, with a great group of co-researchers, we've combined these topics along with some membrane technology to initiate our project, funded by a Gates Foundation Phase 1 grant (GCE Round 7). Our project started November 2011, runs until October 2013, and we've submitted a Phase 2 application.

We’re using “breathable membranes” to improve the performance and usability of basic latrines. Unlike most membranes used in environmental applications, these are hydrophobic in nature, which means they block the passage of liquid water and anything dissolved or suspended in the water, including pollutants and pathogens. However, they do allow water vapor to pass through. Because of this property, the membranes can be used as a latrine pit enclosure, allowing the sludge to dry while the heat from intrinsic biodegradation or from solar heat gradually expels water vapor, but preventing escape of particulate or dissolved constituents. This enables the protection of groundwater or floodwaters near the latrine pit, as well as the health of workers who often must empty filled pits under unsanitary conditions.

The breathable membrane is primarily intended to be used as an enclosure in waterless basic pit toilets worldwide, but its applications can be extended to uses in toilets with/without urine diversion, pumped latrine wastes, combined with other evolving toilet technologies, and commercial applications at larger scale.

Project title: Breathable Membrane Enclosures for Fecal Sludge Stabilization

The goal of this project is to test the use of breathable membranes to protect surrounding groundwater or floodwaters from contamination, while allowing fecal sludge to condense and stabilize. We have verified the concept and are now quantifying the drying rates under differing conditions. We’re also running experiments with “mini-latrines” to observe behavior under fairly realistic conditions.

Our tests started out with more idealized experiments, using wastewater sludge enclosed in “envelopes” of the membrane material, only 100 square cm in area. We found the that drying process is rapid, even with temperature differences of only 2 degrees. Seeing how fast the sludge dries in our experiments was almost unbelievable. Our most exciting results are that the membrane does not impair the drying very much, even while retaining contaminants, and that the membrane does not clog at all, so the drying rate is fairly constant and the membrane can easily be re-used. We’ve also found most of the breathable membranes to be quite strong, so they may allow withdrawal of the fecal sludge while it is contained in the membrane enclosure.

Our small-scale experiments are idealized, however. We have graduated to more complicated test systems to examine a wider range of conditions. For example, we have a dual-thermostatted setup to observe moisture movement from sludge on one side of the membrane into water on the other. This has been used to determine transfer rates with a variety of membrane types and temperature differences. We are now working with what we call “mini-latrines—about 1/10th scale—to observe drying in a 3-dimensional system that simulates field conditions.

Currently, we have intensive research in several directions as our Phase I project finishes up. We’ve developed a computer model of the drying process, but we want to add a heat transfer model to complete an overall process description. To do this, we’re measuring heating and cooling rates on both sides of the membrane. Ultimately, we’ll quantify the total process, so we can predict performance under a variety of conditions, such as different geometries, moisture levels in the waste, and soil conditions around the pit, and also determine how much solar heating might be needed to assure that drying happens faster than moisture is added to the pit. In other words, we want to engineer the system so it works optimally wherever it might be used.

Because only vapors can cross these breathable membranes, mainly water vapor is what moves through it. However, we know that other sludge constituents in fecal sludge can turn to vapor—especially ammonia and volatile organics. Our chemical transport model predicts these to be minor concerns, but we are conducting experiments to confirm this. Also, we will be measuring the strength of our membranes to see if they are can support the weight of fecal sludge if workers wanted to simply withdraw it as a container of the dried sludge.

Perhaps our greatest challenge has been developing our test methods. Even though our goal is a very simple sanitation system, it’s been surprisingly hard to set it up for carefully controlled measurements. Our process seems to be unprecedented, so we haven’t found any published experimental methods to use. So we’ve had to start from scratch with a lot of our tests.

But we’ve now successfully quantified the drying process, and confirmed that the rates of water removal from a pit latrine lined with the breathable membrane should easily remove moisture at the likely loading rate from a population of several families.


Links and further readings:

My presentation at the FSM2 Conference in Durban, South Africa (October 2012):

Paper:
www.susana.org/docs_ccbk/susana_download/2-1624-dentel.pdf

Presentation:
www.susana.org/images/documents/07-cap-d...ity-delaware-usa.pdf

Video of my presentation:



Documents in SuSanA library:
susana.org/lang-en/library/library?view=...p;type=2&id=1810
_________________________________________________
Steven K. Dentel, Ph.D., P.E., DEE
Professor, Dept. Civil & Environmental Engineering
University of Delaware, Newark DE 19716 USA
Tel: 302-831-8120 Fax: 302-831-3640
ce.udel.edu/~dentel/
_________________________________________________
Last Edit: 27 Jan 2014 10:54 by muench.
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Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 30 Jun 2013 12:57 #4893

  • JKMakowka
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This is interesting research, do you have any estimates on the costs? Price per square meter of the membrane, or such?

I know of a research group in Germany at the Fraunhofer institute that use a similar membrane for improved solar distillation of salt water for decentralized applications, but I guess the overall parameters are really different.

Anyways... have you experimented with a "vapor drawing" material on the outside of the membrane? I could imagine that placing such a hygroscopic material of some sort around it would greatly increase the speed, and if it is compostable it could then in turn be used for water recycling in very dry areas.

An added bonus could be that such a material could maybe also capture the escaping NH4 gas and thus not only reduce the smell but also act as a save and slow release fertilizer on plant application.
Krischan Makowka
“You never change things by fighting the existing reality. To change something, build a new model that makes the existing model obsolete” - Buckminster Fuller
Last Edit: 30 Jun 2013 13:01 by JKMakowka.

Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 03 Jul 2013 16:01 #4919

  • skdentel
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Julius, thanks for the insightful questions.
Actually, my lab began in the area of membrane distillation as well, but found that the application to waste materials was much more unexplored and has interesting potential. The work became more exciting when we found that the membrane does not foul, and thus works for long periods and can be re-used.
This is really important because, as you point out, cost is an issue in these applications. We started with an expensive membrane, used for winter garments and camping equipment, but are now using a membrane used in building construction, less expensive and produced in large quantities. The retail cost at a local supply store here is still over $10 US per square meter, so a cost might be $50 unless we find a cheaper breathable membrane or a way to subsidize its cost. Still, I think this cost is low compared with other proposed technologies, especially if the membrane is usable for years. I might add that the membrane is quite strong and might allow waste removal directly in the membrane enclosure, which could be a savings.
You also asked about a vapor-drawing material. I think we'd be using an awful lot of hygroscopic material considering the amount of water involved...did you have a specific material in mind? We're still focused on the membrane process itself and trying to keep things simple, but have thought of a porous layer outside of the membrane to allow air passage to the surface for vapor removal. We believe the ammonia will not be significant as long as the waste pH remains neutral so ammonia is kept in the less volatile ammonium form.
_________________________________________________
Steven K. Dentel, Ph.D., P.E., DEE
Professor, Dept. Civil & Environmental Engineering
University of Delaware, Newark DE 19716 USA
Tel: 302-831-8120 Fax: 302-831-3640
ce.udel.edu/~dentel/
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Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 03 Jul 2013 16:22 #4920

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skdentel wrote:

You also asked about a vapor-drawing material. I think we'd be using an awful lot of hygroscopic material considering the amount of water involved...did you have a specific material in mind?


Well I though about dried mulch or something like that. After using it to protect the soil from drying out too much, the top-layer would even be totally dry again and could be reused. However that is probably only of interest in really water scarce areas with small scale drip-irrigation or such measures.

Very interesting to hear though that regular building material vapor membranes work for this purpose. This combined with a cheap ventilation option (solar chimney, wind driven fan?) would probably a quite efficient solution in relatively dry areas.
Krischan Makowka
“You never change things by fighting the existing reality. To change something, build a new model that makes the existing model obsolete” - Buckminster Fuller

Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 03 Jul 2013 21:29 #4921

  • skdentel
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Thanks again Julius! The solar chimney or wind-driven fan are exactly the low-cost approaches we're considering.
_________________________________________________
Steven K. Dentel, Ph.D., P.E., DEE
Professor, Dept. Civil & Environmental Engineering
University of Delaware, Newark DE 19716 USA
Tel: 302-831-8120 Fax: 302-831-3640
ce.udel.edu/~dentel/
_________________________________________________

Update: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 22 Nov 2013 05:39 #6499

  • skdentel
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We now have Phase 2 support for breathable membranes! * So here's an update on what we've done lately:
- we identified a membrane that's about 50% more efficient in allowing clean water through while excluding everything else in the sludge.
- we've run even more repeated tests, with greater volumes of sludge being completely dried in each test, and found the membrane performance to be pretty stable - that is, the flux rate through the membrane does not decrease with time. This goes along with what we've observed previously - the hydrophobic membrane is a "no-stick" surface so we don't have attachment to it, scaling, clogging, or other such problems.

These tests are continuing, but are main goal now is to identify at least one location where we can place our membranes in operating latrines. We think the best applications will be
(1) raised facilities where the outside of the membrane can be open to the air, which will allow moisture to escape (drying the fecal sludge) and air to penetrate (to allow slow composting. The breathable membrane is supported in a fabric that's really strong, so a hanging "sac" might also be a possibility.
(2) as a supplement to biodigesters, which can have issues from pathogen content in the liquid phase. Use of the breathable membrane could allow the water out while keeping the pathogens in. We have a couple configurations in mind.

These fabrics are commercially available and it is likely that they will be reuseable many times. All in all, it's a pretty simple technology, and sustainable in that respect.

Have any suggestions for collaborators? We're looking!


* Grant size for Phase 2 from BMGF: $ 250,000
_________________________________________________
Steven K. Dentel, Ph.D., P.E., DEE
Professor, Dept. Civil & Environmental Engineering
University of Delaware, Newark DE 19716 USA
Tel: 302-831-8120 Fax: 302-831-3640
ce.udel.edu/~dentel/
_________________________________________________
Last Edit: 24 Feb 2014 14:38 by muench.
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Re: Update: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 25 Nov 2013 06:57 #6527

Dear Sir,

WaterAid India may be interested to collaborate with you for piloting this technology in India. You can contact to Mr. Puneet Srivastava, Manager Technical at This e-mail address is being protected from spambots. You need JavaScript enabled to view it for details on the same.

Breathable membrange grant by Steve Dentel - Phase 2 extention 20 Jan 2014 10:51 #7056

  • arno
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Dear Steven,

This is a very interesting project, thanks for the update. Is it possible for you to attach the final report from Phase 1 and also the project brief for Phase 2? It would be very interesting to see all the reports in this link (so far, only your presentation from Oct. 2012 is available in the SuSanA library here: susana.org/lang-en/library/library?view=...p;type=2&id=1810).

You said that in Phase 2 you had identified a membrane that's about 50% more efficient in allowing clean water through while excluding everything else in the sludge. Could you please tell us more about this membrane, e.g. what makes it more efficient than the previous version? Who else is using this kind of membrane and for which applications?

Can you please post some photos of the membrane (in the lab and also in the field if you have started field tests already)?

And I always “stumble” across the term “breathable membrane”. Is it not really clear to me what it means although I gather it has something to do with the fact that water vapour goes through the membrane. But how would you explain to a layperson why it is called “breathable”? Breathing sounds to me like something is alive…

Has it got any similarities with the membranes used in the Nano-membrane toilet of Alison Parker (Cranfield Uni) or is this something completely different? I am just asking because the membranes used in the Cranfield project also lets only water vapour through:
Alison said (see post from 10 December 2013: forum.susana.org/forum/categories/105-pr...12&start=24#6662):
We are not expecting problems with fouling because the membrane is hydrophobic and doesn’t get wetted. Only the water vapour passes through. But we haven’t done long-term tests yet. However, this is not a novel membrane, it exists as an off the shelf membrane.


And have you found any places yet where you can test it in the real field? I am a bit confused: you were looking for a place where a pit latrine is raised above ground?? You find them in Kampala and Lusaka but usually not in very good shape… Here is a photo of a raised pit latrine in Lusaka:



Raised pit latrine in karst area in Lusaka by Sustainable sanitation, on Flickr

And here is a raised pit latrine from Kampala:


Typical Pit latrine in Kampala slums by Sustainable sanitation, on Flickr

Are these the types of toilets you are looking for to test your membranes? Raising a pit latrine is a desperate measure and usually shows that a pit latrine was not the right technology in the first place… But maybe just for the sake of testing the membranes it could make sense to apply your membrane to the raised "pit"?


Regards,
Arno and team
Arno Rosemarin PhD
Stockholm Environment Institute
Linnegatan 87D, Box 24218
10451 Stockholm, Sweden
arno.rosemarin@sei-international.org
Last Edit: 20 Jan 2014 11:00 by muench.

Re: Breathable membrange grant by Steve Dentel - Phase 2 extention 05 Feb 2014 05:56 #7245

  • skdentel
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Arno,
- Thanks for all the questions and comments. I'll start with some responses here, and will probably have more later. I'll attach our Phase I report to this posting.

- On the breathable membranes: technically, these are fabric laminates with the functional membrane sandwiched in the middle. So there can be differences in the membrane itself, and also in the fabric layers on either side. Plus, the membrane itself is a hydrophobic weave (often PTFE) but either the individual fibers or the entire membrane may be coated with an additional polymer. So there are numerous variables. We have had samples from Gore, DuPont, and GE of various types. Currently we are working with a DuPont product (1073B) plus some GE fabrics called eVent (R); all of these are significantly better than earlier products we tested. The DuPont product is not too exciting to look at - it's similar to the Tyvek(R) used for express mailing. I'll post a picture of one of the eVent products later that shows the three layers, because that's instructive.

- What does breathable mean? This type of material is used for tents and winter camping garments. When it rains, the liquid water will not go through it. But when you sweat, the vapor can escape because vapors go through the membrane (and that's aided by the warmer temperature on the interior side). The membrane is very hydrophobic, which is why the liquid water is repelled.

- We discovered that the hydrophobicity means you can put sludge right up against the fabric and it will not clog the membrane. That's because the sludge doesn't come in much contact with the membrane: just like the non-stick surface of a Teflon (R) pan, which is PTFE, the stuff "beads up" and separates from the membrane. After we dry sludge in contact with the membrane, it separates perfectly, leaving a clean membrane, for multiple cycles of use. This surprised us initially, and reviewers still don't believe it, so a lot of our work is still collecting lots of evidence to this point.

- We first envisioned using the fabric to line or enclose latrine pits. The lower vapor pressure, and perhaps lower temperature, on the outside of the membrane should drive water out of the pit. The fecal sludge dries, with the benefit being that nothing but water vapor escapes. This protects ground water or serves to protect the waste from floodwater incursion, but allows drying at other times.

- But the better use, we now believe, is for places where fecal sludge is collected above-ground. I'm not sure why you think of this as a desperate measure - it's simply a different approach, which can protect groundwater, allow easier access to the waste, and encourage air drying. We're collaborating with WaterAid India in urban areas where there's simply no open area for new pits, and latrines are actually being constructed on the roofs of homes (see below). In any configuration like this, a breathable fabric should enclose the waste, allowing only water vapor to escape (thus drying) and protecting from vermin. The fabric is actually strong enough that we may design enclosures that also facilitate emptying by pulling out an entire "bag" at once.

- But right now we're in a transition period. The BMGF wanted more proof of non-clogging, which we now have, and we are using our vapor flux rates for scale-up calculations. I expect to be in New Delhi next month as we plan our tests with WaterAid.

By the way, our first expected collaborator did not get Phase II support from the Gates Foundation, so our own Phase 2 project brief is obsolete. What I've posted here is a better update I think, and I'll post again as we make more progress.
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_________________________________________________
Steven K. Dentel, Ph.D., P.E., DEE
Professor, Dept. Civil & Environmental Engineering
University of Delaware, Newark DE 19716 USA
Tel: 302-831-8120 Fax: 302-831-3640
ce.udel.edu/~dentel/
_________________________________________________
Last Edit: 05 Feb 2014 14:01 by skdentel. Reason: add picture
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Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 13 Feb 2014 01:41 #7362

  • skdentel
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Hi all,

Followers of this topic may want to tune into the upcoming Webinar (number 6 in the series) - this one's titled “Productive Sanitation.” It has three presentations, all likely to be quite interesting.

My presentation, third in the Webinar, is titled Vapor-permeable membranes: Three potential uses in faecal sludge management for safe sanitation and resource recovery.

The Webinar is happening Tuesday Feb. 25th 2014, 16:30 - 17:15 CET.

CET is "Central European Time." To reliably convert to your local time, go to www.timeanddate.com/worldclock/converter.html .

More details are available here on the forum:
forum.susana.org/forum/categories/139-in...5-february-2014#7239 .

You won't have to download any software to attend. Simply go to this website: seint.adobeconnect.com/seiwebinar/.
However, you must obtain the password to enter the room. To obtain the password, e-mail Elisabeth von Muench: This e-mail address is being protected from spambots. You need JavaScript enabled to view it . Do this in advance!

I hope you'll be participating!
Steve
_________________________________________________
Steven K. Dentel, Ph.D., P.E., DEE
Professor, Dept. Civil & Environmental Engineering
University of Delaware, Newark DE 19716 USA
Tel: 302-831-8120 Fax: 302-831-3640
ce.udel.edu/~dentel/
_________________________________________________

Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 13 Feb 2014 10:43 #7365

  • JKMakowka
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This might indeed be a nice improvement for double vault UDDTs to allow better and faster drying. I also like the idea of using the bags for emptying, but rough handling by waste disposal service providers will be a problem as well as finding a feasible service chain for re-using the bags (maybe a something like a bottle deposit, e.g. you need to buy the bag but get a refund if you return it undamaged?).

Another interesting application would be to build the bag in a way that would allow physical turning over (similar to how you turn a compost pile) in a totally safe way for the person doing it e.g. no contact with the feces.

Hmm... in general have you looked into autothermophilic composting and how a gas-permeable membrane could be utilized in it? Such composting obviously needs a good oxygen supply at which which a comparted membrane enclosure could help.
Krischan Makowka
“You never change things by fighting the existing reality. To change something, build a new model that makes the existing model obsolete” - Buckminster Fuller
Last Edit: 13 Feb 2014 10:44 by JKMakowka.

Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 13 Feb 2014 15:00 #7370

  • skdentel
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Krischan,

Thanks for your thoughts!

The limiting factor with these membranes is how fast the gas can get through the membrane. We've measured this for water vapor using different fabrics and temperatures. It's fast enough for drying purposes (depending, of course, on how much liquid is added and the amount of fabric area). But we think the air supply for composting is more than we can expect under the conditions we foresee. We're measuring this in experiments currently.

There are "industrial" scale composting processes that use these types of breathable membrane fabrics already - you can learn about them, for example, by searching the terms "Gore" and "cover." These require mechanical aerators to speed up the process, and the fabric is only used on top of the compost windrow. The membrane is not for drying purposes, but to prevent condensation. Our approach is less hurried: we want a passive process that, in the time period of filling the enclosure, allows drying so the volume is filled with fecal solids rather than liquid.

In other words, I think composting may be helped by this breathable fabric, but it would still require some other means of air circulation (like the raised vent in some commercial home-scale waste composters). If this is being done, we're not sure how much assistance the breathable fabric provides. I feel like there's a lot of work that still needs to be done! (see more in the next comment....)
_________________________________________________
Steven K. Dentel, Ph.D., P.E., DEE
Professor, Dept. Civil & Environmental Engineering
University of Delaware, Newark DE 19716 USA
Tel: 302-831-8120 Fax: 302-831-3640
ce.udel.edu/~dentel/
_________________________________________________
Last Edit: 11 Mar 2014 10:37 by muench.
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