Keith, I was actually going to post the same thing as Christoph, but he was faster.

The suggestion I would make to you is this: If you want to bring up the botulism issue again in a new thread, please be so kind to include in your post a link to the earlier discussion (you are right, Dan-Eric might have missed it, which is why you should have pointed it out; otherwise we would start from scratch unnecessarily).

You could simply add in a future similar case a sentence such as this "As we have discussed here on the forum in the past (see: xxx (URL of the post)), I am concerned about xx and invite you to take a look at that thread to find out more about my concerns."

Thank you.
The same goes of course for any other user, it is always helpful to provide a link to an earlier discussion if it exists (if you forgot where it was, use the search field above to find it again).
Thanks!


Regards,
Elisabeth

Thanks for directing Dan-Eric to the concerns posted in that thread, Cristoph. It's quite presumptuous of you to think he would have found that thread otherwise, so I see no problem in reposting some information from an older thread.

Keith, don´t you remember there was a lenthly discussion in this forum about this aspect (here) ? - You took part in it.

Let us not repeat the discussion here please.

Christoph

Dan-Eric, have you researched pathogenic clostridia in biogas plant waste? My general understanding is clostridium spores are selected out in AD and may cause chronic botulism in the environment including in livestock and humans. I wonder if adding lactic acid bacteria to the sludge would balance it just as they appear to do in the mammalian intestine.

Detection of pathogenic clostridia in biogas plant wastes, 2014
link.springer.com/article/10.1007%2Fs12223-014-0334-2

In conclusion, BGWs could present a biohazard risk of clostridia for humans and animals.

[Start of Page 2 of the discussion]

Marijn Zandee wrote: 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?

I guess you are referring to the dung mixed into the systems? This is an interesting topic indeed, but as long as the overall hygienic conditions in regards to animals living very close to humans (especially chicken, which are all around you in rural Nepal except maybe your bedroom ) I don't think it makes a huge difference as long as general precautions are taken in the application of the sludge.

I also read a study a while ago, where they compared E.coli strains between chicken and children living close to / on these chicken-farms (I think the study was done in the rural US) and where surprised that there was very little overlap, e.g. different strains seem to "keep" to their preferred host intestines. But yeah, probably not that transferable to a situation like in Nepal and those E.colis are not actually pathogens for the most part.

Anyways, I think household hygiene in general and keeping the animals in a separated enclosure is probably vastly more effective than trying to improve the safety of slurry application further. Further studies/investigations on that topic would be interesting indeed though.

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


[End of Page 1 of the discussion]

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

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

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.

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.

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?

Regards,
Elisabeth

Elisabeth, I wonder if the effluent from an ABR will contain helminths. Would they not settle out with the sludge?