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)

Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 13 Feb 2014 15:16 #7372

  • skdentel
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Oh, and I didn't reply to all your questions.
- on the fabric strength: the membrane itself is not very strong, but the fabric is a laminate with supporting layer on each side of the membrane. You can choose from different weaves and thicknesses, so a very strong material is quite possible. They make boat sails out of it, for example.
- I have imagined emptying out the fabric enclosure on site, assuming it's dried enough to be non-objectionable, but your idea of an exchange might be better. That's a question for the people implementing this on-location, so we'll bring it up.
Thanks again for your contribution!
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/
_________________________________________________

Write-up from webinar 6 - Vapor-permeable membranes (Steve Dentel, University of Delaware, USA) 11 Mar 2014 10:51 #7738

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Dear all,

For the benefit of readers with slow internet connection (or unable to view Youtube videos), I am providing you here with a write-up of the presentation by Steve Dentel and the discussion on 25 February 2014 during webinar number 6 (*).

The topic of his presentation was:

Vapor-permeable membranes: Three potential uses in faecal sludge management for safe sanitation and resource recovery
(10 minutes)

By Steven K. Dentel, Department of Civil and Environmental Engineering, University of Delaware, Newark, USA

You can watch Steve's presentation here: (**)




Powerpoint slides from his presentation are available here:
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Some key points from his presentation according to my notes:
  • Only water vapour can go through the membrane; water can “dry” through the fabric (the fabric contains the membrane).
  • The membrane for drying can be used for treating/drying faecal sludge or other waste materials – in fact anything that contains water.
  • When the water vapour leaves the faecal sludge or other liquid, it leaves behind anything that was dissolved in the water; it is essentially a water purification process – called membrane distillation (this is how Steve’s research started out).
  • The membrane is made of a non-stick material like Teflon, PTFE.
  • While the vapour is going out of the faecal sludge (from left to right on slide 2), air is travelling in the opposite direction of the vapour (i.e. from the outside into the fabric and into the faecal sludge) and can therefore maintain aerobic conditions in the faecal sludge.
  • The faecal sludge doesn’t stick to the membrane, it doesn’t clog the membrane. People don’t believe this so Steve’s team keeps having to establich more evidence for this (i.e. that the fabric doesn’t clog up).
  • Steve’s team has dried materials on these fabrics five consecutive times, with only a 3% loss of flux per cycle (see slide number 4).
  • The membrane enclosure can be used many times – they are quite reusable which brings down the cost a lot, he said.
  • Steve showed three possible applications. The third application is the one they’re moving ahead with in the most direct way (trials planned with WaterAid in India). It’s about a latrine that is installed on the roof due to lack of space; there would be drum at ground level that is lined with the fabric that contains the membrane. There would be sand between the fabric and the inner side of the drum so that there is an air gap (so that air can go out and the material can dry). When the barrel is full with (dried) faecal sludge, the membrane sack would be removed, emptied and used again. The clean water that is infiltrating through the membrane (in the form of vapour) would infiltrate into the ground or could be used for some other purposes as it is totally clean.
  • These membrane fabrics would allow for more rapid drying, they allow air to enter, therefore we can recover water (amount is depending on how much urine is involved). Most important amount for recovery is however not the water but the material (dried faecal sludge) that can be composted and used as fertiliser.
  • Steve said: "We are looking for a very simple, low maintenance, low-cost technology – easy to apply (we want this to be a “viral technology”, something that is so simple and easy to use)."

We had a discussion on Steve’s presentation, which you can listen to in this video (discussion starts at 9:02 until 15:43):

youtu.be/dKE7OSz5VKM?t=9m0s

This is broadly what was discussed:

(1)
Hakan Jönsson asked: "Have you looked at the environmental impact of this type of hydrophobic substances if they get into the environment (they are difficult to degrade and bio-accumulate) - as this is a concern with these hydrophobic substances in Sweden?"

Steve’s answer: "This issue is quite unrelated to our research (might be more relevant during the production of these membranes?). The fabric would be used multiple times and could/should be sent back to the manufacturer where it could be recycled or properly disposed. These materials are widely used in camping wear, boat sails. We think that the small amount used in our applications would not cause more problems than it solves (i.e. less pollution from faecal matter)."

(2)
Hakan Jönsson asked: "But would this hydrophobic material not get into the fertiliser?"

Steve’s answer: "No, it’s like a large bag; the bas is emptied out when full; the dried material could be composted and used as fertilizer."

(3)
I asked: "You mentioned “multiple use”, how often can be bag be used?"

Steve’s answer: "In the graph on slide 4 I showed an example with 5 times of use – it lost 3% of the permeability only; therefore could be used almost indefinitely, e.g. if it takes three months to fill the bag – it could be used for many years."

(4)
Richard (Chip) Fisher: "At what conditions do you run your drying tests? (temperatures on either side of membrane, relative humidity, etc.). Do you have to supply heat to the waste-side of the membrane?"

Steve’s answer: (I wasn’t able to write it all down correctly, listen to it here if you can: youtu.be/dKE7OSz5VKM?t=13m46s)
In brief, his answer explained the effects of different water vapour pressures (lower temperature = lower water pressure); water vapour would move from the faecal sludge to the cooler outer side; have done many tests at room temperature, elevated temperature, temperature gradients… best way to use the membrane is to have faecal sludge on one side and air on the other side, rather than relying on the small temperature difference that you would find e.g. in a water-logged soil for a pit latrine (therefore, application 3 in his presentation is more promising than application 1).


I hope you found this write-up useful (particularly if Youtube is banned in your country). Please don’t hesitate to put any follow-up questions, comments or clarifications into this thread. Thanks again to Steve Dentel for giving this presentation at this webinar!

Regards,
Elisabeth


(*) More information about these webinars is available in this thread here:
forum.susana.org/forum/categories/139-in...n-now-available#7519

A Playlist with all the videos from our webinars so far is available here:
www.youtube.com/playlist?list=PL0gMdVBup...ymOPomtqL_XYT5YtLTSK

(**) If you have any problems with displaying the videos, please take a look at this FAQ for a possible solution:
forum.susana.org/forum/categories/135-qu...--but-it-doesnt-work
Dr. Elisabeth von Muench
Independent consultant
Frankfurt, Germany
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Twitter: @EvMuench
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Last Edit: 11 Mar 2014 18:16 by muench.

Re: Write-up from webinar 6 - Vapor-permeable membranes (Steve Dentel, University of Delaware, USA) 11 Mar 2014 19:53 #7744

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Steve, I am still not convinced.
My reasons, PTFE is a material like Gore-tex. This type of material is used e.g. in composting under semi-permeable cover, which normally has a roof made up of a Goretex like material. This is really good for decreasing the odour and emissions from the compost. There is a semi-permeable compost like this this in Sala. After about 10 years, the semi-permeable roof was almost see-through. This was in spite of nothing but air, rain and sun. The PTFE probably bit by bit had dripped onto the compost. I believe that the wear of the bags will be much larger.

As far as I know, PTFE is non-biodegradable and it, or at least similar perfluorinated chemicals, are bioaccumulating. Their levels in blood samples are, as far as I know, increasing. Even if the wear is small, it gives non-degradable substances ending up in arable soil. But of course the use of Gore-tex in shoes and clothes should also be a big concern then.

Re: Write-up from webinar 6 - Vapor-permeable membranes (Steve Dentel, University of Delaware, USA) 11 Mar 2014 20:41 #7746

  • skdentel
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Hakan, I am trying to understand your opinion but it is difficult. PTFE, firstly, is different from the lower molecular weight, bioaccumulative PFOA chemicals that I believe you are referring to. PFOAs are used in the manufacture of PTFE and other fluorinated polymers, but you are indicating that PTFE degrades into these or similar chemicals. I have not found any evidence of that in the scientific literature, with the exception of some studies on PTFE degradation in cookware at quite high temperatures. Our applications are not going to be much above natural temperatures.

You also indicate that PTFE is nonbiodegradable, which doesn't seem consistent with your concern about its degradation products. I do know that Gore-Tex and other such fabrics include a thin membrane of the hydrophobic material, but outer layers of other materials. These outer layers can be treated to be UV-resistant, but over time they are probably photo-degraded in any case. This is probably what you observe at Sala.
We do not envision any applications like the Gore-Tex composting method, which exposes the fabric to direct sunlight. So even the other materials are unlikely to photodegrade at any appreciable rate.

As I stated previously, these fabrics will be reusable and, if it is determined that they pose an environmental risk (which I do not believe), they could be returned for a deposit when a new one is purchased.

So I do not believe there is a significant environmental risk from the use of these membranes. Even if there were, it must be compared to the environmental benefits the fabric's use should offer, and to the massive use of similar plastics in less beneficial applications. But I should repeat, I can find no convincing evidence that PTFE degrades to the types of chemicals you are referring to.
_________________________________________________
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: Write-up from webinar 6 - Vapor-permeable membranes (Steve Dentel, University of Delaware, USA) 12 Mar 2014 08:56 #7751

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Dear Steve,
I worry both about the PTFE, which is not biodegradable and about the PFOAs, which exist both both in the production and when the material is thermally degraded. We do know that the material will wear. I mentioned the example of the compost with semipermeable membrane in Sala and I think we all have had to change our frying pans due to the non-stick layer, of PTFE, being worn out. And this is in spite of us using wooden and plastic tools, as we know that the surface is sensitive to wear. Faecal sludge will contain some sand and other hard particles, which will cause wear and remove PTFE particle from the surface. Some of these small partilces in the soil will be exposed to sunlight. I do not know if this means that they will degrade in the UV-light. But the fact that we are spreading a slow or non-degradable man made substance in nature without knowing how it is removed or accumulated goes, as I see it, against the precaution principle. So it worries me.

In the 50-ies and 60ies we spread lots of cadmium, also a non-degradable substance, as a pollution in chemical fertilizer in Sweden. Now several percent of the old age Swedish population have kidneys that leaks protein. Due to other soils in many other european countries, they still spread fertilizers with lots of cadmium, around 180 mg Cd/kg P and are not yet worried. The fertilizers in Sweden are around 3-10 mg Cd/kg P and to protect health we want strict restrictions on cadmium on all of EU. However, due to the economic interestes involved, the other countries do not want to put restrictions on cadmium.

So even though the precaustion principle means we should wait with implementation until we now how large the risks are, I know that this is not how the world works at present.
Best wishes,
Håkan

Re: Write-up from webinar 6 - Vapor-permeable membranes (Steve Dentel, University of Delaware, USA) 12 Mar 2014 11:08 #7753

  • arno
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Steve/Håkan
Common sense says we shouldn't be putting non-degradable compounds into the soil especially if it is part of a potential food-sanitation-fertiliser-food value chain. For communities to get a license to use PTFE teflon bags, EIAs (Environmental Impact Assessments) would be necessary. And stakeholders will use the common sense or precautionary principle. The question is whether we can make these bags out of other materials that can sooner or later breakdown in the environment. eg Green Coat www.polymateltd.com/files/6_GreenCoat.pdf

Bentonite is commonly used to contain buried solid waste and prevent it from contaminating the ground water. If the purpose is dewatering of sewage in a pit latrine, there may be alternatives available - the first one that comes to mind is to reduce the volume of water in the flush water. The Sulabh silos in India dewater through porous brickwork and then seep into bentonite in areas where groundwater is vulnerable. Dry toilets with shallow pits that compost using biologically active soil should be considered as an alternative (so called Fossa alterna) in vulnerable soil-ground water zones.

Regards
--Arno
Arno Rosemarin
Stockholm Environment Institute
Arno Rosemarin PhD
Stockholm Environment Institute
Linnegatan 87D, Box 24218
10451 Stockholm, Sweden
arno.rosemarin@sei-international.org
Last Edit: 12 Mar 2014 13:08 by muench.

Re: Write-up from webinar 6 - Vapor-permeable membranes (Steve Dentel, University of Delaware, USA) 12 Mar 2014 13:57 #7758

  • skdentel
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Arno/Håkan,

This is not the place to argue about the precautionary principle, but I think this is an example of taking it to extremes. As I noted, the external surfaces of the fabrics are not made of Teflon or other hydrophobic substances, so abrasion is not a likely issue. Nonetheless, I cannot prove it will not happen under some conditions that you may come up with. That any abrasion products themselves might be harmful is also not established.

The material allows water penetration but no penetration of any water-borne contaminants, and the water transfer is rapid enough that the pit surface can release all the water contributed by a family's feces and urine. These two combined benefits are significant, and they are not attained with bentonite or porous brickwork.

Certainly we can imagine cases where an entire fabric enclosure, with its hydrophobic membrane, is not returned for deposit and ends up buried, where its nondegradable membrane will remain indefinitely. This possibility, which I see as exemplifying your concerns, illustrates issues inherent in the precautionary principle: it asks that a hypothesis be proven under an infinite number of conditions, and it does not consider societal or environmental benefits relative to these (even unlikely) possible harms.

I am looking forward to your replies, but I am not sure we will reach mutual agreement on these questions.
_________________________________________________
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: 12 Mar 2014 16:21 by skdentel. Reason: Didn't get irst names right on line 1

Re: Write-up from webinar 6 - Vapor-permeable membranes (Steve Dentel, University of Delaware, USA) 12 Mar 2014 15:03 #7760

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Dear Steve,
I just wanted to point out that this is an issue that ought to be watched out for, especially as Steve has not said that the PTFE is not on the surface of the bag meterial.

I do realize that this is an interesting technology with many benefits, and these might well outweigh the potential risks. None the less, one should be aware also of potential risks. This was my whole point.
Best wishes,
Håkan
Last Edit: 16 Mar 2014 14:27 by muench.

Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 16 Mar 2014 01:18 #7823

  • KaiMikkel
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Allow me to lend moral support to Håkan and Arno. Their concerns are quite valid. I am a big supporter of the precautionary principle and feel that its general absence from the majority of work on sanitation (and most everything else) that is taking place in North America is a big problem. The "Tyvek" conversation that we've been having is a great example of this. Instead of having determined at the front any inherent dangers, a well-intentioned researcher is now in the position of having to defend his work without the benefit of any proof one way or another regarding the safety of allowing these materials to come into contact with organics. It would have been nice had his research focused on the safety of these materials in advance of hundreds of thousands of dollars in research funding being expended examining their use in a new, innovative and unforeseen (by the materials' inventors) way. This "cart before the horse" methodology sets up the unfortunate dynamic that I've witnessed in this thread whereby people working on the same issue are pitted against each other, a situation that stifles communication and breeds animosity.

A perfect sanitation corollary exists in the production and land application of toxic sewage sludge in the Global North. We all know that sewage sludge contains a plethora of industrial toxics but because its yet to be proven beyond a doubt that dumping this material on land (including agricultural land and, increasingly, on our gardens, yards and school grounds) is dangerous, we persist in doing so. Many of us are just waiting for the other shoe to drop, an event that will likely occur too late to clean up the mess that we've created. Of course, strict adherence to the precautionary principle would preclude these actions. But, at the same time, Europe is quite guilty of land applying sludge too, proving that how the precautionary principle is applied has its limitations.

Anyway, I see this as a lesson in performing due diligence and always adhering to the precautionary principle. Our work is simply too important to be so easily undercut.

Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 16 Mar 2014 04:07 #7827

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Dear Steve,

I think the concept of your research is very interesting and would like to ask you a few more questions.

1.) You switched from using the "bags" as a pit liner to having them suspended above ground. Is this purely because that works better for your Indian research partner? In other words, in a rural setting, where digging pits is cheap and feasible could the technology still be used as your team originally envisioned?

2.) In your original post you mention this is a concept for waterless toilets. Yet now you work in India where people are washers. Can the technology cope with the washing water or are people encouraged to shift to wiping?

3.) Could you post some pictures and a description and analyses of the final dried product? As there is no additive (such as ash) I have no idea of what to expect from the dried material. Further, you state in your presentation that the dried material can be an input in agriculture. I am not sure I see how this technology would kill all pathogens in the fecal waste without secondary treatment. Do you have a vision for this?

Kind regards

Marijn Zandee
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Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 18 Mar 2014 13:54 #7872

  • skdentel
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Kai Mikkel,

If sewage sludge is a "perfect sanitation corollary" then it also points out issues with the precautionary principle. If these sludges are not land applied, the only alternative for urban areas is incineration. This has not been proven perfectly safe, and it not likely to be, given the dioxin and Hg stack emissions and the continued presence of heavy metals in the ash, but it is practiced in the European countries where land application has been ruled out. Essentially, there is NO solution to this problem that is completely consistent with the precautionary principle. If Hakan wants us to throw away our Teflon frying pans, should we also shut down the big wastewater treatment plants that generate these sludges?

So the precautionary principle tells us many things we can't do, because they can't be proven completely safe. Unfortunately, the precautionary principle does not offer any solutions unless we stop everything we are doing. If you think there's another alternative to the sludge problem besides land application and incineration, I can identify environmental risks associated with it which, according to the precautionary principle, would rule it out.

I am not against the precautionary principle if it is used as guidance. But taken to absolutism, it may even be harmful. Back to the current topic, we have 600,000 child deaths per year from diarrheal causes, but you would have me do additional research on the environmental fate of PTFE instead of direct sanitation research. I have done my due diligence in searching the wide range of prior research on PTFE and, as I said previously, I find no convincing evidence that it will cause ANY environmental harm in the applications I propose. But you propose that I should spend all the funds entrusted to me by the Gates Foundation on further such research, which would again find no environmental harm.

You are free to choose your own priorities, but I can state from experience that there is not an infinite pool of research money. Given this fact, and the finite amount of funding that I have, I will continue to work on directly solving sanitation problems--problems that lead to clear environmental harms, including human deaths--and I am working on solving them using what I know to be a process with very minimal (or zero) environmental risk.
_________________________________________________
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: 18 Mar 2014 14:46 by skdentel. Reason: clearer language in final paragraph

Re: Breathable membrane enclosures for fecal sludge stabilization (University of Delaware, USA) 18 Mar 2014 14:40 #7873

  • skdentel
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Marijn,

Thanks for the very pertinent questions!

1) You are correct that we originally proposed a simple liner that would be placed directly in a latrine pit. This will prevent any contaminants or pathogens from escaping, and will allow water to be gradually released through the fabric enclosure (which contains the membrane).

The rate at which the water can be removed has to exceed the rate of water addition with feces and urine. When we examined the membrane performance in our research, we had to divide the drying process into two scenarios. Essentially, the membrane lets water out of the fecal sludge if either(a) the temperature of the sludge is higher than that of water outside the enclosure, or (b) there is air outside the membrane that is at less than 100% humidity.

In either case, the thermodynamic reason is the same, but the practical result for drying is that for (a), the sludge must be pretty warm. We've done a heat balance and found that the solar energy above an outhouse might provide enough heat, but only on sunny days, and it complicates what we want to be a very simple system.

Solution (b) is faster than (a) and it will work with simple pit liners IF the soil around the pit is fairly porous and also unsaturated. This might be the case in very dry climates. But in more common situations, we need a way for the water vapor to escape once it has gone through the membrane. I think a practical approach is to combine the membrane fabric with a porous material on its exterior; when this lines the pit, the vapor should go through the membrane fabric and then travel upward and out in the porous layer. The porous layer can be connected to the air vent above the outhouse to increase air flow. We have not tested this approach yet.

What we anticipate in India is a bit similar. The main difference is limited space for dug pits in the crowded urban spaces, so the enclosure is above ground and more open to the air - probably not directly, but within a perforated drum, for example. This means that the outside of the membrane fabric always contains air rather than water, so the drying will be faster. Of course we need to figure out how fast this happens in the rainy season when the humidity is high.

2) The rooftop latrines are unlikely to have running water directly available, so water needs to be carried up. This will mean the water used will not be excessive, so the amount of water sent to the enclosure can be handled. Our tests will determine whether this is the case; if it is not, we will need to increase the amount of membrane area by redesigning or enlarging the enclosure.

3) The result is simply dried sludge as you would have if you air dried it. There is a picture in my paper and presentation, linked previously in this thread. Although we have measured 99% decrease in fecal coliform levels in the dried sludge, this does not mean land application is safe; we need to look at parasite survival such as Ascaris, which will undoubtedly be more difficult. I think the answer is that this process will make the sludge a bit safer, but not completely safe. I would not recommend its use in agriculture unless we find drying conditions that really produce a sanitized dry product.

Thanks for your interest, and I hope my answers are helpful.
_________________________________________________
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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