Pathogen destruction in biogas plant vs ABR (Anaerobic Baffled Reactor)

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  • daneric
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Hygienic aspects of sewage sludge after AD

Hello,

I'd like to see if anybody have any advice on hygienizaion of faecal sludge (from toilets using less than one liter water per flushing) after 40 days retention time in batch AD. The location is Ngong, Kenya so the main issue is to get rid of parasite eggs. Since the AD process has probably metabolized large share of the organic compounds much heat will probably not be generated when composting. From what I've read these eggs can stay intact for 15 years. The intention is to make the sludge safe to use as bio-fertilizer. Anybody got advice or good ideas?

The idea is actually to use solar heaters to kill all pathogens before the AD process but seems we might not have enough funding for this. Read more about the project here: twowheelssouth.emulsionen.org/#post21

Regards,
//Dan-Eric
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  • Elisabeth
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Re: Hygienic aspects of sewage sludge after AD

Dear Dan-Eric,

In my role as moderator, I have moved your topic from the original location of "biogas systems" to the category "faecal sludge management", because your question does not really relate to biogas systems but to the downstream processing of the sludge. (hope that is OK by you)

Just for the benefit of all: With AD he means anaerobic digestion.

Did you see that I mentioned in another topic (same category as this one), the sludge pelletiser in South Africa (eThekwini)? This would be one option for hygienisation. It works on heat, not on composting. See the video link which I posted. (I don't have more details on it, but could obtain it via Chris Buckley for example)

The other point I want to raise is this: don't get too hung up about destroying each and every helminth egg to make your sludge perfectly safe for reuse. Think rather of the WHO Guidelines from 2006 which promote a multiple-barrier approach. A technology to reduce pathogens is just one barrier. Other possible barriers are the handling of the sludge, education, personal protective equipment, the type of crops which you plant (does not necessarily have to be edible crops - cotton is a great example), and so forth. If you don't know these guidelines yet, you can find them in the SuSanA library ( www.susana.org/lang-en/library?view=ccbktypeitem&type=2&id=1004 )

Remember, any technology may fail (especially in developing countries), so think from the start of multiple barriers, not just the treatment barrier.

I wish you lots of success for your project!

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Elisabeth
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  • Marijn Zandee
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Re: Pathogen destruction in biogas plant vs ABR (Anaerobic Baffled Reactor)

Dear all,

During a recent conversation a question came-up regarding pathogen destruction in biogas plants vs ABRs. In this case either was to be considered as primary treatment followed by a secondary step (probably a CW). Someone in the discussion claimed that in this set-up the biogas plant would offer a better pathogen destruction than the ABR.

I can only assume that this is because biogas plants may usually have longer retention times. As I don't have the time to dive into the subject in detail I would like to ask if any one here is familiar with this argument and or has an explanation to offer?

Thanks

Marijn
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Re: Pathogen destruction in biogas plant vs ABR

Dear Marijn,

I think your assumption is correct:

I can only assume that this is because biogas plants may usually have longer retention times.


Biogas plants have much much longer retention times than ABRs (many days compared to just hours or a few days).

Perhaps another reason could be that biogas plants can be operated at higher temperatures which would also lead to faster pathogen kill.

One publication that came to mind (because I was involved in creating it) is this Technology Review of GIZ on biogas plants:

Mang, H.-P., Li, Z. (2010). Technology review of biogas sanitation (draft) - Biogas sanitation for blackwater, brown water or for excreta and organic household waste treatment and reuse in developing countries. Deutsche Gesellschaft für Internationale Zusammenarbeit GmbH (GIZ)
www.susana.org/en/resources/library/details/877

There is a section in there on pathogen destruction, I copy a bit:

3.2.1 Incomplete pathogen removal

Human excreta are contaminated with all kinds of pathogens
and hence a reliable technology is necessary for their
inactivation. During anaerobic digestion an inactivation of
most animal and plant pathogens is obtained under
thermophillic conditions (>55°C for several days). Several
studies on wet fermentation report that also mesophilic and
lower temperature operation inactivates pathogens; further
findings indicate that reactors with retention times of at least
60 days at 20oC to 15 days and 35-55oC reduce significantly
any type of pathogens (Michael H. Gerardi 2005).

Many studies reveal also that under fully mixed mesophilic
conditions, pathogens are not completely inactivated.
Therefore recommendations on the use of the not posttreated
slurry should limit irrigation only to fruit trees, and
exclude spray irrigation to vegetables. Effluent water could be
post-treated with UV desinfection by natural sunlight in
shallow polishing ponds. Post-composting of sludge may be
required for a one year period. If the effluent is directly
worked into the soil as soil conditionner no further restriction
applies.

Two main factors regulating the inactivation of pathogens
have been identified, namely the temperature and the concentration
of free ammonia as a function of the time of treatment/
exposure.




When it comes to helminth eggs (my most favourite topic), then I think they are pretty much just settled out but not destroyed in biogas plants. I could not imagine which mechanism should destroy them in a biogas plant given that they are so hardy that they can survive in the digetive tract of humans? (about helminths: en.wikipedia.org/wiki/Helminths - another page that I have been working on).

So when the sludge from a biogas digester is used for reuse activities you still need to keep those helminth eggs in mind and put treatment steps in place (or other safety precautions during reuse).

There is also this presentation from Heinz-Peter Mang about reuse of digestate in China which is very interesting:

forum.susana.org/forum/categories/174-sa...fertilizer-bioslurry

Perhaps my little input here will prompt others to share their experiences (or questions) on this important aspect of pathogen destruction in biogas sanitation systems.

And how about yourself, Marijn: As you work on a biogas programme in Nepal, what are your experiences with pathogen destruction in biogas plants, and reuse activities? Or is your work not dealing with human faecal matter?

Regards,
Elisabeth
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  • joeturner
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Re: Pathogen destruction in biogas plant vs ABR

Just in case anyone finds this and is not familiar with the jargon:

Anaerobic baffled reactor



Biogas reactor



from Akvopedia
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Re: Pathogen destruction in biogas plant vs ABR

Elisabeth, I wonder if the effluent from an ABR will contain helminths. Would they not settle out with the sludge?
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Re: Pathogen destruction in biogas plant vs ABR

Thanks for adding these schematics, Joe.

And yes, you are right: When I was talking about helminth eggs, I was thinking of the sludge. As these helminth eggs are relatively heavy, they would settle out in the sludge.

All wastewater treatment processes produce an effluent and a sludge - the latter is often forgotten about which is not good.

So you are right with regards to the effluent from the ABR, the helminth eggs should not be a concern, but rather the other pathogens (bacteria, viruses, protozoa) need to be looked at.

How much solids-liquid separation would we expect in a biogas reactor? Probably not so much. Would the helminth eggs nevertheless settle to the bottom of the reactor and into the thicker part of the sludge that stays in the reactor for a very very long time (even "indefinite"?)? - Help! Where are the biogas digester experts on this forum?

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Elisabeth
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Re: Pathogen destruction in biogas plant vs ABR (Anaerobic Baffled Reactor)

Again, apologies if this is an ignorant question, but is the ABR wetter than the biogas reactor? Am I wrong in thinking that unseparated blackwater enters the ABR whereas it is just faeces (maybe plus other solid materials?) in the biogas reactor? [as a sidenote, in the UK, maize grain is used as a feedstock in biogas reactors because on their own the foodwaste and faeces do not provide enough of the nutrients that the microbes need. Which seems a bit daft to a lot of people]

If so, then this might also come down to how well the helminth survive in a wetter environment in the one than in the other. Also, presumably, there is more of a build up of methane in the biogas than in the ABR, so the helminth survival might (or might not) be affected by that.
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  • JKMakowka
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Re: Pathogen destruction in biogas plant vs ABR

muench wrote: How much solids-liquid separation would we expect in a biogas reactor? Probably not so much. Would the helminth eggs nevertheless settle to the bottom of the reactor and into the thicker part of the sludge that stays in the reactor for a very very long time (even "indefinite"?)?


This depends in the design, but usually there is no seperation as the digesters are either actively mixed / plug-flow designs, or at the very least try to minimize "dead space" where solids could settle.

It would be possible oft course to design a hybrid ABR that tries to optimize pathogen reduction and also produce Biogas to some extend.
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Re: Pathogen destruction in biogas plant vs ABR (Anaerobic Baffled Reactor)

Actually those are two different concepts.

ABR is aimed to Wastewater treatment a) desctruction of organic material by anaerobic digestion (and by that produces biogas) b) separation of liquid fase and settable material.

The biogas plant is (as the name says) aimed to biogas production (and therefore reduction of organic material) but as Krishan wrote the aim is not to separate the liquid fase and the settable material, as typically the complete content should be put to agricultural use.

So for a normal biogas plant I would say – as there is no separation of sludge and water the efficiency for pathogens should be less than in an ABR. But as I said two different functions!

Yours Christoph
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  • MichaelCarr
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Re: Pathogen destruction in biogas plant vs ABR

Hi there all: I still have my L plates on but trying to catch up. I see two potential ways for further destruction of helminth eggs in fecal sludge - 1) aerobic thermophillic composting, mixed with other biomass. 2) laying sludge out in drying beds. How long does it take for UV light to positively kill all pathogens when laid out like this?
From what I know, the HRT - Hydrolic Retention Time in a BGD is 40 days.
Farmers here in Cambodia are instructed to mix 1 for 1 - ie; one gallon of cow poo for one gallon of water. They dry the ensuing sludge in beds, but I haven't been able to see any empiric info on safe UV drying times.

Due to the one on one poop v water situation - adding a flush toilet to the system - which farmers in the provinces aspire to - creates a problem of too much water entering the BGD and breaking down the system. Anybody any ideas on how to overcome this? Is there some kind of intervention that already exists?

best regards
Mike
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  • Marijn Zandee
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Re: Pathogen destruction in biogas plant vs ABR (Anaerobic Baffled Reactor)

Dear Elisabeth,

Thanks for giving my post a second life.

Regarding the household scale biogas in Nepal. Many plants that are being build these days have toilets attached. And I must say that I do wonder sometimes about the potential for spreading pathogens that this poses.

If we really consider this, I think once more we will come down to the discussion whether sanitation systems in the developing world should go for 100% pathogen destruction, or whether a longer (say 5-10 years) view of incremental change should be taken. I think the WHO guidelines, and the multi-barrier concept take an interesting middle ground here.

If we look at the situation in rural Nepal, where the sludge of (toilet attached) biogas plants is used, I think the following helps us achieving a safe situation:

1.- Biogas plants are normally sized for a 60 day retention time. In practice most are too big, which means retention times are usually bigger than needed.
2.- Sludge is normally not used directly on the fields, it is normally either dried or co-composed for some time. Which adds extra time before soil application
3.- Carrots and radish are the only two vegetables in Nepal that I can think of now that are regularly eaten raw and grow inside the soil. Others are usually cooked.
4.- Most people in rural Nepal don't grow their produce for the market, but for consumption in their own household.

Some things that work against achieving pathogen destruction:
1.- Composting of sludge is in very small piles, so temperatures are low
2.- Process temperatures inside the digester are also relatively low

One further comment, I think it would also be interesting to consider in the debate what the influence of pathogens that can be transferred between animals and humans is. Especially if some studies exist concerning the relative transmission rates of human-human and animal-human pathogen transfer in rural communities?

Further:

With relation to Micheal Carr's post:

Due to the one on one poop v water situation - adding a flush toilet to the system - which farmers in the provinces aspire to - creates a problem of too much water entering the BGD and breaking down the system. Anybody any ideas on how to overcome this? Is there some kind of intervention that already exists?


This definitely can be an issue. Only very low water use flushing solutions can be used in combination with a biogas plant. In rural Nepal, where people use a small pitcher for anal washing and flushing, this seems to work quite well. I think one good thing to do would be to visit a number of toilet attached biogas plants and check if the consistency of the slurry is good. If it is consistently too thin, the biogas programmes maybe should consider adapting the user trainings to add less water with the dung.

regards

Marijn


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