Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

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Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

Disclamer: This idea is purely speculative and has not been tested in any way.

I have been thinking for a while already how to solve the issues of typical methane leaking fixed dome biodigesters using low-cost means, and now I had an interesting idea in an related topic here: forum.susana.org/forum/categories/35-bio...5213-biogas-from-abr

The starting point was that capturing the methane in later stages of an ABR is not really possible, and how one could improve such a system. This is actually also related to earlier discussions about fixed dome biodigesters that leak a lot of methane due to faulty domes and simply due to the fact that the removed bioslurry is still containing dissolved methane and continues to produce methane to a limited extend.

Now recent scientific research has found (in lake bottoms, peatlands and wastewater) a special kind of bacteria that utilizes methane for it's metabolism under micro-aerobic conditions (basically anaerobic but with the presence of Nitrite and Nitrate). This is especially interesting as it also removes nitrogen, and thus has very interesting applications in low cost waste water treatment.
Here are some supplementing links:
www.nature.com/news/2010/100324/full/news.2010.146.html (news about the discovery)
edepot.wur.nl/246167 (experimental application of bacteria in MBR like system)
www.b-ware.eu/content/project/publicatie...icrobiology-2012.pdf (study of the natural habitat of these bacteria in nitrate enriched peatlands)

Especially the last link raised some interesting thought about using this as a simple DEWATS system, e.g. a "constructed peatland" akin to the idea of a constructed wetland.

Peat is already used as a very efficient post-septic tank filter in a intermittently loaded aerobic process (similar to vertical flow constructed wetlands or soil retention filters) see for example here:
www.extension.umn.edu/distribution/naturalresources/dd7669.html
www.envirolink.govt.nz/PageFiles/173/121...eatment-Gisborne.pdf
(Data shows a very good reduction of COD and nitrogen, as well as a good reduction in pathogens)

Now under these intentionally aerobic conditions the originally researched micro-aerobic bacteria will not thrive and the denitrification is likely due to other microorganisms or simple bio-integration due to organic growth in the filter.

However as the dutch peatland paper showed, naturally occurring peat is anaerobic in a depth of about 80cm and actually a methane producer. However if nitrate rich water is infiltrating in the peat a natural intermittent layer of special bacteria is formed that utilize the raising methane and the available nitrate (actually preferably nitrite, but also nitrate) such as that the peat is not emitting any methane (which is also a very interesting finding in regards to methane emitting north Siberian melting permafrost soils by the way).

Ok, so how could this be relatively cheaply utilized in a DEWATS (or actually biogas digestor as you will see below)?

Let me shortly outline my idea:

Lets start with a partially faecal sludge filled and plastic lined pond with a bottom sludge inlet(a bit like a horizontal flow constructed wetland). Now, on top of this layer of faecal sludge "floats" a 1m thick layer of peat (a net could probably be added to stabilize the peat from sinking into the sludge).
The result will likely be anaerobic conditions in the bottom layer and start of methane production (which will after a while disturb the peat-layer by upfloating however).
But because of the nitrate contents of the faecal sludge some of the methane utilizing bacterial will likely already form in the bottom area of the peat.

The next conceptual step would be to add liquid sludge to the bottom layer so that a liquid upflow filtration through the peatlayer starts. This should quite efficiently treat the waste-water and has two beneficial side effects:
1. The overall upwards mass-flow in the system makes it further unlikely that oxygen can diffuse down to the methane producing bottom layer.
2. The upflowing wastewater delivers a constant flow of dissolved nitrate and methane to the special bacterial in the peat-layer, thus providing pretty much perfect conditions for their growth and thus maybe a good denitrification and also dissolved methane removal.

To summarize up to this point: Thus a system should theoretically be running only on gravity, is very simple in construction and should likely result is a very high treatment efficiency e.g. high quality effluent waste-water.


This is all great to this point if low strength waste-water is used, but with higher organic load (or actually intentional addition of organic material) one might be able run this as a biodigestor too. The increased (and thus gas bubble forming) methane production could probably be captured by large upside down plastic funnels right at the bottom of the peat-layer, while the remaining dissolved methane continues upwards through the peat as before. The overall gas-tightness of such a system should be very high if care is taken that not too much pressure builds up. And the inherent 1m water head (in the peat-layer) on the anaerobic part additionally supplies the gas at an convenient 0.1 bar pressure I imagine.

The overall biogas production of such a system is probably a bit lower than a regular biodigestor, but the waste-water treatment should be much better, besides being very easy and probably cheap to construct compared to other DEWATS options except for a constructed wetland.

Ok, that is pretty much the idea... comments are welcome!
I place the concept in the public domain, and would be happy if someone would actually test it.

Krischan Makowka
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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

Dear Julius,
In a possible upcoming "Open Source" R&D project I will try to integrate your peat-ideas with mentioning/acknowledging proper the original source of idea (you). I will let you know directly. May I do it in German language?

In a possible much earlier "try and error" during this summer I will try to recovery an eutrophic natural swinging pool, using "your" interesting CP system to reduce nutrient level of water.

On the full scale use of peat as you nicely described I am still hesitating, because of worries about by when will be the "special bacterias" inside peat are used off by the nutrient rich sewage and by when the system needs a replacement of peat. In other words, how long lasting might be the desired process by passing through permanent liquid sludge by up-flow "filtration"?

Julius wrote:

"The starting point was that capturing the methane in later stages of an ABR is not really possible, and how one could improve such a system. This is actually also related to earlier discussions about fixed dome biodigesters that leak a lot of methane due to faulty domes ..."


I am disagreeing with you on the "impossible" gas-tightening of separated ABR-compartments made by plastics. I am actual on the way to do gas-tightening on all my little ABRs made out of standard PE-tanks (each camber is a PE-tank by it self), to modify the HT-pipework by little changes (I just adding some 110 mm sealing cups on top of T-fittings) to stop mixing of gases between the cambers in order to check on the different gases levels camber by camber, a bit similar to the Malaysian ABR-study. Just to find out by my self what I am actual doing... ;-)

On the prefab ABRs by BORDA I agree with you, on this it is not any more possible as above the ABR-cambers are connected to each other by one open space and the lids of the covers are not gas-tight.

All the Best
Detlef

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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

AquaVerde wrote: Dear Julius,
In a possible upcoming "Open Source" R&D project I will try to integrate your peat-ideas with mentioning/acknowledging proper the original source of idea (you). I will let you know directly. May I do it in German language?

In a possible much earlier "try and error" during this summer I will try to recovery an eutrophic natural swinging pool, using "your" interesting CP system to reduce nutrient level of water.

On the full scale use of peat as you nicely described I am still hesitating, because of worries about by when will be the "special bacterias" inside peat are used off by the nutrient reach sewage and by when the system needs a replacement of peat. In other words, how long lasting might be the desired process by passing through permanent liquid sludge by up-flow "filtration"?


Ok, cool. Looking forward to what you come up with. I claim no "possession" of the idea though... feel free to alter it in any way. And yes German is fine, but why not keep us all informed via this forum?.

The durability of such a system is of course a question that would need to be tested. The aerobic peat filters claim a typical lifetime of 10 years though. I would assume if load per square meter isn't too high, the idea presented above should last at least as long.
I am basing this assumption on the fact and the solids can settle at the bottom (and can be pumped out for the other side of the pond from time to time, like with a septic tank), and thus the filter will mostly have to deal with the dissolved substances. These however are probably easily metabolized by this living filter, but maybe one has to shave off newly grown peat from time to time ;) (actually peatland plants growing on the top are fine too, but one has to be careful with their roots. Reet/papyrus or rice has actually been shown to channel methane out of anaerobic soil areas and also aerate these to some extend through their roots and both is probably undesirable for this filter).
Additionally such an upflow arrangement with an open-top might be somewhat back-flushable for cleaning, if one can allow clean rainwater to accumulate on top and then remove sludge from the bottom so that the water flow is reversed for a limited amount of time.

Not sure though just how well those two factors (bottom sedimentation and back-flush) will extend the lifetime.

Edit: And these bacteria species are actually growing in the peat naturally and have also be found in waste-water, so there is no way they can be "used up". In fact under the described conditions I am hoping to create near perfect living conditions for them.
According to the above linked paper one might have to prevent the pH from getting too low though.

About the ABR gas tightness... yes I should have stated that less broadly. I am usually looking at it from the point of the typical brick/cement structures found and promoted in developing countries. But you are right, such plastic tanks become more and more common here too.

P.S.: one could probably find a way to create artificial peat from anaerobically composting fibrous plant material, so the mining of natural peat is not strictly necessary I guess. But that is not something you can do quickly.

Krischan Makowka
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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

creating artificial peat for large scale use:
I am not sure, maybe made out of a kind of "terra preta"...???
Hallo dear "terra preta" experts, could this be a possible alternative to peat and its possible "use" in CPs?

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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

Hi Julius,

JKMakowka wrote: P.S.: A sustainable peat mining for such uses might actually improve the protection of peat areas which are otherwise in threat of draining for agricultural production in many areas of the world and drained peatlands are one of the largest non fossile-fuel sources of CO2 in the athmosphere.


Isn't that a lot of whishful thinking? Protecting peat resources by introducing an additional demand for the use of peat? What is "sustainable peat mining" anyway? Does this exist at all? I feel that if you go into developing technologies based on the use of peat, you should right from the outset look how you can substitute or at least minimise the use of peat.

Then for the technology: If the main aim is to mitigate methane emissions to the atmosphere, wouldn't be the logical way to maximise biogas production (e.g. by fully mixed reactors, UASB, etc.) and use the gas to to substitute the use of fossile fuels, rather than seeking at tightening ABRs or developing a complicated peat-treatment technology (which, I admit, I've not yet fully understood)?

Sorry for the sceptical comments, I hope you can take it as constructive critism ;)

Best, Florian

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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

No, skeptical comments are very much what is needed right now :)

Regarding the peat mining, I can understand your concern, but as far as I can tell most peatlands actually have the problem that they are of no value unless drained. Thus introducing a way those can be used without large-scale draining might be beneficial overall. But yeah, that's probably too much positive thinking...

About the proposed technology... I guess it is rather a way to simplify the construction of a waste-water biodigestor (as it needs no gas dome at all) with the added benefit that very little methane should be leaking into the athmosphere and the reduction of nitrates in the effluent.

My greatest concern is actually that the overall load capability and specific permeability of the upflow peat-filter might be very low, thus requiring a large surface area. That in turn would drive up costs and also make capturing the excess bio-gas probably not possible/very difficult.

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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

Dear Julius,

I just discussed the possible nutrition reduction option of a swimming pool via CP with my college.

Existing is a wwtp (ABR, CW + CW for hygienisation) to feed a swimming pool. Sound maybe crazy to you, but it is functioning well. Still P & N is too high as you can see on the picture: http://www.aqua-verde.de/page4.php?view=preview&category=9&image=71
We will start it very soon, using 3 old PE-tanks 2 x 0,7 x 1,4. We will start with one module and will test P & N before and after. If successful 3 modules will be used in total. We will let you know the practical P & N outcomes.

Do you have access to more detailed background information on peat filter additional to the 2002 www.extension.umn.edu/distribution/naturalresources/dd7669.html ?

All the Best
Detlef

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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

Ok, but that will be an aerobic peat-filter, right? How does that pool get so contaminated with N & P in the first place?

Otherwise, sorry no, I just stumbled on these yesterday (+ the other link above: www.envirolink.govt.nz/PageFiles/173/121...eatment-Gisborne.pdf which mentions some other interesting papers in their references).
Before I was mainly looking into anaerobic, stratified organic mud-layers for pond lining (so called "gleying") and other similar things, but ultimately some of the details from that might somehow be applicable to the idea proposed above too.

The aerobic version of a peat-filter I can't say too much about, but it seems to me that since they are using dried (and thus dead) peat, it is probably not too much different from any other organic matrix filter, like for example the coconut husk filter presented not too long ago on this forum.

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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

Dear Julius & Florian and Others,

On the use of peat for small scale I would not have a problem, on large scale I am hesitating like Florian too.

Therefor, how about using mainly Coconut fiber residues instead of peat? Maybe to get the important denitrifying methanotrophic bacteria, ‘Candidatus Methylomirabilis oxyfera’ just to start the process by putting a layer of peat ("soil" from the DIY-store with 95% peat) on top of Coconut fiber "filter".

Any thoughts on substituting peat are welcome!

All the Best
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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

AquaVerde wrote: Maybe to get the important denitrifying methanotrophic bacteria, ‘Candidatus Methylomirabilis oxyfera’ just to start the process by putting a layer of peat ("soil" from the DIY-store with 95% peat) on top of Coconut fiber "filter".

Yes, I was thinking about something like this when saying "substituting" above.

If I've I got it right, it's the specific tpye of bacteria you want to have (that grow naturally on peat) but it's not the peat itself you need. So if you maintain the conditions the bacteria need to thrive, they would grow on other support material as well. If you don't manage to maintain these conditions, they would also die off on peat... Lots of "ifs" here, I know ;)

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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

Yes, some peat have to be used for inoculating Coconut fiber. The papers say, the bio-experts found this special bacterium in peat.
IFs are the starting point for our thinking exercises :)

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Re: Idea: Theoretical concept of a "upflow micro-aerobic peat-filter digester"

Sure, some substitute would be great, but as all the papers describe difficulties growing the special microbial community (it's probably more than just that special one) outside of its natural habitat, it might not be that easily done.

What I could imagine to work is to first fill the pond with cut reed or papyrus (with green leaf material to have a better NPK-carbon ratio) and run it for a year or two in intermittent aerobic mode, so that the fibrous plant material has some time to start decomposing. Then you would add relatively nutrient rich and decomposed organic material (mixture from humus and manure?) to the bottom and let it sit for a while for the anaerobic bacterial community to start (during that time one could just run it as a open septic tank already, to avoid downtime of the system?).
But who knows, what will work best. When using peat directly chances are for sure higher to get something working to test the general concept in praxis.

Edit: basically one has to think about how peat is created naturally, and that is by decomposing fibrous plant material in an water-logged area, so that anaerobic (and acidic) conditions are created. The problem likely is that it is a rather slow process.

But maybe another "cover" material that lets water seep through, but inhibits the oxygen-diffusion downwards somewhat can be found?

Edit2: Maybe first heat-composting the plant-material for some time, can yield a fibrous humus like substance that can be turned into artificial peat more quickly?

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