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

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

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 email address is being protected from spambots. You need JavaScript enabled to view it. for details on the same.

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

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

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

Thanks again Julius! The solar chimney or wind-driven fan are exactly the low-cost approaches we're considering.

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

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.

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

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.

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

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.

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

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):



Documents in SuSanA library:
susana.org/lang-en/library/library?view=...eitem&type=2&id=1810

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Note by moderators: This post was made by a former user with the login name skdentel who is no longer a member of this discussion forum.

Edit on 25 March 2015: sadly, Steve Dentel passed away earlier this year (see here ).

His details were:
Steven K. Dentel, Ph.D., P.E., DEE
Professor, Department of Civil and Environmental Engineering
University of Delaware, Newark DE 19716 USA

We now list him as a "former member".

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