Hello Cecilia,
to answer your question quickly. I do operate planted sludge drying beds (we call them "sludge mineralization beds") but I do not investigate currently (lack of resources). We will start a side investigation hoepfully in May of a comparison of sludge drying beds and sludge mineralization beds (also without much resources)

Christoph

Cristoph,

Apologies for a very delayed response.

In answer to your questions:

•TS content raw and after drying - we did not measure this accurately (lack of access to scales/ ovens etc) and ended up describing 'dryness' qualitatively (cue another discussion about FSM vocabulary..!)

•Did you do helminth eggs? - yes - no conclusive results though - unfortunately (and perhaps unsurprisingly) we did not see any significant reductions. I'm keen to see the impact of 'maturing' the desiccated sludge though - i.e. leaving it in a heap for a period of time.

•Did you measure temperature in the greenhouse and in the sludge? - yes, and we found they are a good proxy for each other: The sludge had a slower response time to outside temperatures, and retained its heat better at night (due to the specific heat capacity of high water content material versus air), and an interesting ‘step-up’ effect was noticed as the sludge dried out. This meant that the average sludge temperatures during the 10 day test were actually comparable to the average air temperatures inside the drying beds, although the peaks were around 5-10 degrees different. This suggests that air temperature is a useful proxy for sludge temperature in this context, although any maximal temperatures in the sludge are likely to be cooler than the corresponding air temperature.

•I was wondering about the comment that raking is not efficient for pathogens – I would expect better drying = less pathogens – could you comment a bit more on that (how many samples have you been able to do – is it more an observation/guess or is it proven data?) - key point here is that raking provided quicker drying, not better drying - at the end of the test period, both were the same dryness (by observation) but raking got there quicker. The resultant 'improvement' in temperatures attained in the raked bed was not significant enough for long enough to affect kill-rate for pathogens. I also note that Ewag found similar results in their recent work regarding the effect of raking on drying rate: www.eawag.ch/medien/publ/news/2014_na_32/index_EN
Sample size was essentially three beds and one test, hence it was more of a 'comment' than a 'result' (it wasn't the main focus of the tests).

•I was wondering as well that you wrote first drying than “cooking”. I would have expected first “cooking”, than drying? And cooking for me means hot  hot means no or much less pathogens. Why did you have though only 99% pathogen removal (which is 2 logs)- We needed to 'dry' before we 'cooked' as the temperatures rise much quicker once the moisture has been driven off (due to solar energy initially being used to evaporate water rather than raise temperatures), and we needed to keep the beds ventilated during the drying phase to allow the water vapour to escape. Once dry, then you can 'cook' and try and get the temperatures up by closing the beds etc. I think if we had been able to run the test a little longer, we may have exceeded 99% reductions.

•Did you try out how the sludge behaves if you try to come to really high drying (as to be able to be burned? As mentioned in the EAWAG video?) - no we didn't - we were seeing what temperatures could be achieved passively, with only the sun as the energy source, and using crude construction materials. Ambient daytime temperatures were around 35oC, and the highest air temperature achieved in the drying beds was 60oC. I think if we had dried the material for longer, we could have gotten it dry enough for burning, but our goal was for an agricultural product not an energy one (due to the local context, where energy was less of an issue than cheap and 'eco' soil conditioner).

Hope that helps!

Are you doing any work in this area at the moment?


Celia

Dear Celia,

Very interesting approach. I did some rough calcs:

600 – 900 l/batch with (assumption of 30 kg/m³ (3%)) is 18-27 kg/batch. Assuming 15 days for one cycle it would be a 100 – 150 kg TS/m²,a. As this would be close to normal drying beds the initial TS and final TS after 10 days would be interesting.
Could you provide some more details (or may be you did not have the chance to collect these data).

• TS content raw and after drying
• Did you do helminth eggs?
• Did you measure temperature in the greenhouse and in the sludge?
• I was wondering about the comment that raking is not efficient for pathogens – I would expect better drying = less pathogens – could you comment a bit more on that (how many samples have you been able to do – is it more an observation/guess or is it proven data?)
• I was wondering as well that you wrote first drying than “cooking”. I would have expected first “cooking”, than drying? And cooking for me means hot  hot means no or much less pathogens. Why did you have though only 99% pathogen removal (which is 2 logs)
• Did you try out how the sludge behaves if you try to come to really high drying (as to be able to be burned? As mentioned in the EAWAG video?)

Thank you

Christoph

Dennis,

The beds were sized to treat the contents of an 'average' pit latrine i.e. 600-900L (15-20cm depth of sludge on the 1.5 x 3m drying bed). The idea being that one pit's contents could be treated at a time, and it could be used sequentially by a group of households.

Layering wet sludge on top of dry would not be a good idea - suggest as with peppercorns, it's a batch process!


Glad our work is of interest,

Celia

I am glad Elisabeth linked to this from another post, else I may not have seen the attached doc.

Very useful information Celia - do you have specifications for bed sizes by expected volumes etc?

And in terms of processing, capital expenditure, set up etc, can you simply add daily FS layer upon layer or do you need to have multiple beds for different processing days?

(As a non FS example, for solar drying peppercorns in rural low resource locations, you pick today and keep that days product in one pile, then move it along a "conveyor system" - well another mat:) and then tomorrow's crop, you put in another pile for drying, so you don't get uneven drying and you can easily be aware of where each pile is at etc.

Does that matter with FS - if you keep piling the next days FS on top, do you just keep reintroducing pathogens?

Or are we talking some form of daily separation?

Folks,

I'm long overdue posting this on the forum, but I wanted to share our experience of designing and testing a simple solar sludge drying approach in Bangladesh. If you have any comments or queries on our work, we'd be really happy to discuss them.

The experimental work undertaken has proven the principle of simple solar sludge drying. It has shown that liquid waste can be dried to a consistency suitable for co-composting in a short period of time (<=10 days), even during the monsoon. We tested the design during the three main seasons (cool/ hot dry/ monsoon) and had good results in all conditions.

After drying the material is not 100% safe for public handling (although we had 90% reduction in key indicator pathogens), and needs to be composted to ensure risks are sensibly managed.

The various experiments conducted have shown that the performance of the solar dryers is strongly affected by various parameters, and much less affected by others. These are outlined below:

Things the performance is sensitive to:

• Ambient weather - the sludge will dry quicker, and higher temperatures will be achieved in the sludge, if the weather conditions are less humid, it is sunnier, and ambient temperatures are higher (particularly at night).
• Roof material - increased transparency increased the heat gain of the sludge, and allowed greater UV penetration, which is linked to improved pathogen reduction.
• Filter/ no filter - including a sand filter in the base of the drying beds is important for the performance of the drying bed, assuming the sludge being treated is as liquid as was experienced in the project.

Things the performance is robust to:

• Specifics of roof design - the detail of the frame shape, connection to the drying bed, and allowance for condensate drainage, are important in terms of its ability to be manufactured and used, but does not have a significant affect on the rate of sludge drying or levels of pathogen reduction.
• Raking/ not raking - Raking improves the drying rate, but seemed to negatively impact pathogen reduction rates. The differences over a 10 day test in terms of the final product were minimal however, suggesting that in terms of saving manpower, it may not be necessary to rake the sludge daily during drying, and turning it at the halfway point and on the penultimate day may be sufficient (or if resources are limited, it needn’t be raked at all).
• Ventilation strategy - The impact of the ventilation strategy was inconclusive, although the approach of ‘drying for 5 days, cooking for 5 days’ seems to make intuitive sense.

I've attached a two-page summary FYI.

Kind regards,

Celia