Level of tolerable risk in a reuse context?

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  • joeturner
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Re: What is Terra Preta Sanitation (TPS) all about? Hype or ingenious?

Florian wrote: Well, I think there is at least as much evidence supporting the currently used recommendations.


I'd be interested to hear some. I don't think I've seen anything which suggests the storage recommendations are robust.

joeturner wrote: This is why storage is so popular as safety measure. Much more easy to control in practice.


Easier to control or easier to show you've done it? I think composting is far more robust than storage.

joeturner wrote: It's all about weighting relative risks and beneftis. Putting requirements too high will make reuse impossible. If you want reuse happen, then you need to put forward saftey measures that are approriate and feasible, but not extreme and prohibitive.


It depends what the focus should be. I've made it clear that I think the focus is wrong and that spreading untreated or part treated sludge is an infection risk. I don't think we can [simply] talk about risk when we're talking about users and workers of systems who do not understand it, and whom are frequently given partial or bad information. I don't think the safety measures are 'extreme and prohibitive', I think they are proportional and necessary.

A good example are our favorite Ascaris eggs. Very hard to kill all of them, treatments that want to assure 100% of Ascaris elimination will always be complicated or expensive. On the other side, Ascaris infections are far from being deadly. They become problematic only if they are extreme and widespread.
So WHO thinks the benefit of reuse (e.g. of large scale wastewater irrigation) outweights the remaining small disease burden caused by 1 Ascaris egg per g soil or L water.


I don't think the issue is only the actual infection risk of Ascaris, but the notion that Ascaris is a relatively easy pathogen to measure (which itself is problematic, given the discussions we have had here about methods) and that if one can kill Ascaris to safe levels, you have a good chance of also destorying other more difficult pathogens.

It might not be the best standard, but I'd be interested to hear a rationale for any other standard. Not having a standard is a silly solution, in my opinion.

Now, if you would require 0 Ascaris eggs in water used for irrigation, treating wastewater to that level would not be possible at reasonable costs. Thus irrigation with wastewater would need to be prohibited. It may happen then that farmers continue to irrigate illegally with raw wastwater. The final health impact of extremely strict guidelines can therefore be worse than of more pragmatic ones.


First, I don't think anyone is arguing for a 0 Ascaris ova standard, and personally I'm not making any statements about irrigation waters because I don't know anything about that side of things. But I think it is a reasonable standard for feces reuse, which we know contain a range of pathogens, some of which are difficult to isolate and measure.

Second, I think it is reasonable to expect new systems to be better than the old ones and to meet the highest standards. In fact, there is a case to be made for having more stringent standards than those which would be accepted in Western countries, given that the systems are largely unmonitored, dispersed and may well be less effective than the ideal.

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  • Florian
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Re: What is Terra Preta Sanitation (TPS) all about? Hype or ingenious?

joeturner wrote: I think there is a lot of evidence to suggest that 2 year storage time is too little - and that secondary treatment is necessary.

Well, I think there is at least as much evidence supporting the currently used recommendations.

joeturner wrote: However, I have my doubts how often co-composting systems are working correctly, hence I would guess that is makes sense to assume that they're often not properly sterlising the feces. If they're not being batch tested, I'm not sure how you ever could know how sterile the end product is.

This is why storage is so popular as safety measure. Much more easy to control in practice.

joeturner wrote: I agree that is not the same as saying there should be a 10 year storage (or limit to land use), however I'm not sure that is bad advice to allow a margin for safety.

It's all about weighting relative risks and beneftis. Putting requirements too high will make reuse impossible. If you want reuse happen, then you need to put forward saftey measures that are approriate and feasible, but not extreme and prohibitive.

A good example are our favorite Ascaris eggs. Very hard to kill all of them, treatments that want to assure 100% of Ascaris elimination will always be complicated or expensive. On the other side, Ascaris infections are far from being deadly. They become problematic only if they are extreme and widespread.
So WHO thinks the benefit of reuse (e.g. of large scale wastewater irrigation) outweights the remaining small disease burden caused by 1 Ascaris egg per g soil or L water.

Now, if you would require 0 Ascaris eggs in water used for irrigation, treating wastewater to that level would not be possible at reasonable costs. Thus irrigation with wastewater would need to be prohibited. It may happen then that farmers continue to irrigate illegally with raw wastwater. The final health impact of extremely strict guidelines can therefore be worse than of more pragmatic ones.

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Re: What is Terra Preta Sanitation (TPS) all about? Hype or ingenious?

Florian wrote: I understand that this 10 years time brought forward by Ralf is not something that all (or even many)experts agree on.


I agree with this - although there is a very dramatic difference between the usual advice on reuse from many ecosan (and dry toilet) systems - which usually suggests (usually unspecified) reuse after storage for 3 months to 2 years - and Ralf's suggestion that the material should be treated as hazardous for much longer and that it may even have an effect on soil in which it is applied.

I think there is a lot of evidence to suggest that 2 year storage time is too little - and that secondary treatment is necessary. A properly monitored aerobic co-composted sludge is not so much of a hazard providing it is not directly consumed, in my opinion. However, I have my doubts how often co-composting systems are working correctly, hence I would guess that is makes sense to assume that they're often not properly sterlising the feces. If they're not being batch tested, I'm not sure how you ever could know how sterile the end product is.

I agree that is not the same as saying there should be a 10 year storage (or limit to land use), however I'm not sure that is bad advice to allow a margin for safety.

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Re: What is Terra Preta Sanitation (TPS) all about? Hype or ingenious?

I agree, a system with 10 years storage time (or use restriction) would hardly be feasible in practice on a larger scale. Perhaps for wine or whiskey, but rather not for feaces, fertilzer and soil. If 10 years storage would really be necessary, then treatment or disposal would be more feasible ways. However, I understand that this 10 years time brought forward by Ralf is not something that all (or even many)experts agree on.

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Re: What is Terra Preta Sanitation (TPS) all about? Hype or ingenious?

I was imagining that storage was impractical - how much of the compost would remain after 10 years?

I think the material needs to be used on permanent non-food crops, ideally dug into the ground, and I think Ralf is saying that if you use the material on land (say forestry), you shouldn't then reuse it for food crops for 10 years.

Of course, the problem here is that to transport it and to dig it into the ground requires handling, which is a major hazard.

Unfortunately I think this means that our systems are broken. Either we need systems which are better at sterilising the feces properly, or we need to develop much more integrated sludge-agricultural systems whereby the feces can be safely disposed of.

Again, I think maybe we need to change the terms of the conversation. Urine is microbially much safer than feces and contains most of the useful plant nutrients. So we need to develop a model of reuse of urine and safe disposal of feces.

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Re: What is Terra Preta Sanitation (TPS) all about? Hype or ingenious?

Florian, I suppose the question I'm asking with all these technologies is how you (or anyone else) is assessing whether the complexity is worth it for the results. How much better would TPS have to be than what you currently use to be worth developing systems to include lacto-fermentation and charcoal production?

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Re: What is Terra Preta Sanitation (TPS) all about? Hype or ingenious?

joeturner wrote: For the sake of argument, if the only difference between the communities had been the presence of VIP latrines, would you say that was a success or failure?


Hi Joe, I rather tend towards considering the results as indicating success. After all the study proves a real impact of WASH interventions (reduction by half of diarrhoa incidence). If the result is ok in asolute terms, or if it rather indicates that there is still a lot to improve, I do not know.

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Re: What is Terra Preta Sanitation (TPS) all about? Hype or ingenious?

Florian, that is fair comment, although the comparison is between communities that do not have any WASH interventions and those that have VIP plus other interventions.

For the sake of argument, if the only difference between the communities had been the presence of VIP latrines, would you say that was a success or failure?

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Re: What is Terra Preta Sanitation (TPS) all about? Hype or ingenious?

joeturner wrote: I'm really interested to know whether anyone thinks this is a success. On the one hand, the number of infections have reduced, but on the other, 0.7 infections per person per year seems to me to still be an unacceptable rate. And if that is true, what is an acceptable rate for these systems?


Hi Joe,
I think you are quoting results from different studies that are not directly related. Eg. the

89.5% of the urine diversion latrines contained microbial pathogens

were about parasites only, whereas the infection rates were about diarrhea cases.

I agree that 0.7 infections per person still sounds like too much. But if that result now points to a problem of the UDDTs or if the infections are caused somewhere else (water supply, personal hygiene, food hygiene etc.) can't be said from the quoted results alone.
Best, Florian

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Level of tolerable risk in a reuse context?

Elisabeth, I'm glad you have pointed to David Hawksworth's work, which is both interesting and scary. In his MSc thesis the UDD latrines in eThekwini were studied.

Individuals who had access to multiple WASH interventions had a diarrhoea rate of 1.9 per 1000 person days (0.71 per person per year) compared with 3.3 per thousand person days (1.23 per person per year) for those who had no access to those interventions.

David’s research showed that when inspected, 89.5% of the urine diversion latrines contained microbial pathogens.

I'm really interested to know whether anyone thinks this is a success. On the one hand, the number of infections have reduced, but on the other, 0.7 infections per person per year seems to me to still be an unacceptable rate. And if that is true, what is an acceptable rate for these systems?

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