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

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

Since I was clearly out to lunch when I wrote the original version of this post I've gone ahead and removed it so as not to further embarrass myself. I'm going to chalk up the inaccurate statement that I made in my first paragraph to sleep deprivation or temporary idiocy since I was obviously not in my right mind at the time of writing. For what its worth, I fully concur that the practice of land-filling sludge in the US is a dying practice. As for rates of land application in the E.U. I was relying on information that had been told to me which was something I obviously hadn't fact-checked myself. My apologies for muddying the conversational waters with woefully inaccurate information.

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

Back on topic, at least somewhat:

Here in Uganda we have been brainstorming for a while to find a sustainable way to deal with fecal sludge from emptied pit latrines (or potentially also UDDTs) in a simple, but relatively safe way that can be scaled up in a decentralized fashion.

One of the ideas was to use these kind of breathable membranes to come up with an more safe and less odor problematic alternative to sludge drying beds that could be used in (peri-)urban settings.

The concept I have in mind are long black sacks (not unlike the plastic bag bio-digesters) made out of these breathable membranes that are fed with freshly emptied sludge from one side, while the somewhat dried and thus storage, dehumidification and solar-heat treated sludge can be removed on the other side then.
Given a long enough storage period and sufficient heating from the sun (maybe supported by running a heat-exchanger coil underneath the bags that is feed by standard solar water heaters) the result should be quite safe to handle in (non-food?) agriculture I assume.
I also guess that the smelly ammonia that probably can escape through the membrane is less of a problem given that the pit-latrine contents are already a bit older mostly.

The question really is if it can be made to smell only a little and if the treatment is sufficient without needing a lot of space and expensive foil/membrane.

It would be also interesting to see if non-permeable foil might work sufficiently well too, although the drying properties are definitely a plus of the breathable membrane.

I also wonder how UV stable the material is and if it might be necessary to do the heating completely through external heat exchangers instead, which I would like to avoid as those add to the cost.

Any thoughts on this are appreciated.

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

The posting of Kris reminded me that I wanted to post some photos that I took of a sample of the breathable membrane that Steve Dentel brought with him to Delhi to the Toilet Fair in March.

Here is the photo, one can see the three layers of fabric (the photo was taken on a white table cloth). The teflon / PTFE / breathable membrane part is the thin white film in the middle - protected by the two other layers:



And two more photos attached below.

(and please let's not start that whole debate again whether this teflon/PTFE material is a problem for the environment, I think it has been discussed sufficiently above. If anyone wants to discuss it further in a more broader sense, please start a new thread on it. Here, let's rather focus on the research ideas, e.g. the ones that Kris posted above).

I saw that Steve Dentel's team got Phase 2 or follow-up funding from the BMGF - congratulations!!
Could you tell us more about it please, Steve?

I saw it here on Sanitation Updates:

sanitationupdates.wordpress.com/2014/04/...gs-for-pit-latrines/

A team at the University of Delaware has received US$ 250,000 in additional funding to continue its research on “breathable membrane” linings for pit latrines.

The breathable fabric helps to prevent groundwater pollution, while also protecting sanitation workers from exposure to pathogens. Heat from biodegradation of the feces or from the sun gradually expels water vapour, but prevents the escape of particulate or dissolved constituents.

Dentel is piloting the membrane technology in the slums of Kanpur, India, in collaboration with WaterAid. He wants to get them in place before the beginning of the rainy season in June. Since the membrane is reusable, the cost of using susch a sophisticated technology can be reduced.

At the same time, Dentel is working with UD engineering colleagues Daniel Cha and Paul Imhoff to apply the technology in wastewater treatment facilities in the USA and South Korea.


What exactly are the research objectives of this Phase 2? How much time do you have for it? What are these planned applications in the USA and South Korea?

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

@ Kai Mikkel Forlie,
You are incorrect that landfilling is the most common option in the U.S. - and I assume you are talking about wastewater sludges. The most common option is in fact application to soils (55%) with landfilling at 21% (Source: Dentel, S.K. Wastewater Sludge Global Overview: United States. In Wastewater Sludge: A Global Overview of the Current Status and Future Prospects, IWA Publishing,2011). And, of course, the U.S. as a country does not exemplify urban areas.

You are also incorrect about the situation in Europe. Even in the five countries with the largest sludge production, agricultural use is less than half (45%), and in the other countries that allow land application, it is typically used for only 10-15% of all sludges. Nor is there a "rampant" trend to increase these numbers over time(source: G. Mininni and S. Dentel, State of sewage sludge management and legislation on agricultural use in EU member states and in the United States" 1st Intl. IWA Conf. on Holistic Sludge Management,May 2013, Västerås, Sweden).

Your reference to a "blanket unwillingness" is also incorrect. I *have* done an in-depth literature search and, as I have indicated in my previous posts, although the membrane is refractory, there is no evidence of toxicity or environmental harm from its use. To repeat research of this nature when it has already been performed would not be an appropriate use of Gates Foundation funding.

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

Krischan,
Thanks for the question!
Your suggestion makes sense: the interior heat would increase the vapor pressure and drive off more vapor through the membrane. We also know that some air can penetrate from the exterior into the contents. However, this air entry will not be fast enough to maintain an aerobic interior, so the waste decomposition will be anaerobic and therefore generate organic sulfur and organic amine odorants. They're likely to penetrate the membrane, but we do not yet know the extent. There are also hydrophobic membranes that will not allow odor penetration, but this is at the expense of drying rate.
This is an interesting direction for further work. Because fecal wastes are predominantly water, the ability to release water content should be valuable in terms of volume reduction and waste stabilization.

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

skdentel wrote: Krischan,
Thanks for the question!
Your suggestion makes sense: the interior heat would increase the vapor pressure and drive off more vapor through the membrane. We also know that some air can penetrate from the exterior into the contents. However, this air entry will not be fast enough to maintain an aerobic interior, so the waste decomposition will be anaerobic and therefore generate organic sulfur and organic amine odorants. They're likely to penetrate the membrane, but we do not yet know the extent. There are also hydrophobic membranes that will not allow odor penetration, but this is at the expense of drying rate.
This is an interesting direction for further work. Because fecal wastes are predominantly water, the ability to release water content should be valuable in terms of volume reduction and waste stabilization.


Hmm, you are right, and potentially releasing quite a bit of methane into the atmosphere is probably not that wise either.
I am guessing that just increasing the surface to volume ration to keep in aerobic is probably not going to be cost efficient...

Unrelated to that:
I forgot to mention that the bottom should be probably water permeable to allow draining also, even though that would require some sort of treatment plant for the percolate.

If the drying/dewatering rate is too fast one might also run into trouble with the movement of the sludge through the "bag"... as always the devil lies in the details :-/

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

Krischan,
The breathable membrane fabric could be used on the bottom as well, and no treatment should be needed, because none of the sludge constituents get through the fabric. Only water vapor and other gases.
The fabric facing upward would need to be UV-resistant; some fabrics are available of this type.

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

Elisabeth, thanks for the encouragement to update!
Yes, the "breathable membrane" has been promoted to Phase 2. We are very excited about this. Of course we have a lot of work to do!

To tell the truth, reviewers have been pretty skeptical of our work. It is hard to believe that the membrane does not clog when fecal sludge is right up against it. It has been surprising for us too.

We started out using anaerobically digested wastewater sludge as a very reproducible substitute. We also collected sludge from a camping site outhouse. In neither case was there a decrease in drying rate as drying occurred (except, of course, when the moisture is almost gone). Drying to completion, shaking out the dried material, and rinsing, we then repeated the experiment with the used fabric, up to five times, with no loss in drying rate. So the fabric acts like a non-stick surface, perfectly, and there was no sign of clogging.

More recently we have been using fecal sludge which we have to obtain from student volunteers (I'll skip the anecdotes here!). And it turns out that true fecal sludge is a bit stickier than the other sludges we were using (other researchers using soy paste and other substitutes should be alerted to this!). yes, the rate of moisture penetration through the membrane fabric does slow as drying progresses.

We were pretty dismayed by this, but repeated tests have now shown that (1) the rate is still pretty fast for the initial drying phase, which is the most important, and (2) the drying behavior does not deteriorate when the fabric is rinsed and re-used - it's pretty much the same as in previous cycles. So the news is not so bad after all.

So now, our research will now go in two directions. The first is a scientific inquiry into what's different about fecal sludge, using a variety of sophisticated analyses, so we can try to decrease its importance.

The second is more practical. Our calculations show that the membrane enclosure should be very suitable for certain applications, and the most obvious is where the fecal waste is contained above ground level so there can be plenty of surface area for drying. So picture this: the toilet, with u-trap, mounted onto a surplus 200-L drum with perforated walls. The fabric is made into a cylindrical, water-tight "bag" that fits into the drum and seals onto the u-trap at the top. We'll need a spacer between the bag and the drum for air flow and to protect the fabric from any sharp edges. Practically speaking, steps and rails, a privacy barrier, and so forth are needed too.

This is the working plan for our first generation model. It's obviously intended to be simple and low cost, while letting the fecal sludge lose water but keep everything else contained for later composting or other use. We're talking with apparel manufacturers about fabricating the bags to be water-tight, and we will be testing them with Wateraid in Kanpur and elsewhere. WaterAid suggests two drums, on roller wheels, so one can continue drying when the other is being used. Sounds good, although it doubles the cost.

While we learn how these units perform, we'll be working on designs for pit latrines and other systems. These are more challenging because we need ways for air to circulate around the fabric exterior to carry away the moisture. We've got some ideas on how this can be done. Phase 2 will be exciting!

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

You are planning to use a flush toilet with that (implied by the u-trap)? Wouldn't the bag fill up with water way too quickly?

Otherwise: anaerobic septic sludge really isn't the best substitute for fresh feces. My guess is that the higher oil and fat content of fresh feces is what is causing you trouble.

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

This is not a conventional "flush" toilet. In India, a small amount of water is used for rinsing instead of paper, and this amount is included in our calculation of the required drying rate. So the "u-tube" being planned by WaterAid fill not have a large hold-up volume. Its purpose, of course, is odor control, so it is desirable but not essential.
The membrane we used is also defined as oleophobic, so I don't believe oil is penetrating into the membrane. There is some research suggesting that nonpolar functional groups on proteins are the culprits.

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

Hello everyone. My name is Shray Saxena and I have been working on this project for the past 2 years under the advisement of Dr. Steven Dentel, Dr. Paul Imhoff and Dr. Daniel Cha.
To summarize, in this project we use a breathable membrane for the dewatering of fecal sludge and containment of pathogens as the sludge dries due to the partial pressure difference between the inside and outside the membrane bags. This partial pressure difference is created due to the temperature difference or the humidity difference (wet sludge is 100% humid).
Recently, we started field trials of these membrane bags in Kanpur, India where we provided two toilets to 2 households in urban slum areas. These bags line the inside of a perforated 55-Gallon drum which will act as the storage for fecal matter from active toilet users. This perforated drum is housed on the roof of the house in a honeycombed wall chamber surrounding it. The toilet seat is housed in the superstructure above the drum and the connection is a funnel shaped sheet metal design. The toilet seat used is a locally made squatting type seat based on the SATO pan design. This particular design contains a counter-weighted trapdoor through which any solid or liquid passes through due to its weight. It has been shown that this SATO pan design reduces the amount of flush water used in a toilet. It should also be noted that all connections between the toilet seat and the drum is air tight and no odors are expected to escape back into the superstructure.
In addition, the drum is placed on a wheeled platform which makes the replacement of a fully sludge filled bag easy to maneuver. We also provide a replacement drum and membrane so that the toilet does not go out of commission once the active drum is full. This supplementary drum also provides a good amount of waiting period for the fully sludge filled drum before the replaced drum fills up, hence more drying time for the full drum. Each drum also has a seep hole in the bottom connected to a pipe which runs through the nearest drain, so that in case of leakages the users’ house is not contaminated.
Since, this is a pilot research study, we have limited the number of uses to 4 per day which includes 1 liter/use of wash water. We are regularly monitoring these toilets on a weekly basis through our collaborators WaterAid India and Shramik Bharti.
Currently, we see that one 55 Gallon drum gets full in approximately 50 days of regular usage under the winter conditions of Kanpur. We hope that in the summer months (June-July), the filling rate and drying rate will reach an equilibrium.
Please find attached two presentations:
1) Fecal Sludge Management 3 conference
2) Graduate Seminar at University of Delaware. Spring 2015.

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

Thanks, very interesting presentations.

Did I understand the "wetted surface area" slide correctly, that after a while a dry crust forms close to the membrane, that slows the drying of the more interior fecal sludge? And ideas how the sludge could be stirred before that happens?

I am also a bit dubious about the conclusions based on the "current progress" slide... volumes without measuring the actual water content can't really measure the effectiveness of the membrane because you only estimate what went in. And even if your assumption is correct, the actual reduction in water content seems not all that high, i.e. at the current size of the drum you would have to empty it every month without the membrane and maybe every 1.5 months with?

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