A quick retrospective on some of our results:
Ambient daytime temperatures were around 35oC, and the highest air temperature achieved in the drying beds was 60oC.
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.
(remember the temperature achieved by the sludge is important because if it can be held at 55oC for four hours, a meaningful (destructive) impact should be had on pathogens within the sludge.)
As it was, while air temperature in the beds did get over 55oC for 4 hours during the final days of the test, the sludge only reached this for around an hour.
Given the interesting step-up effect as the sludge dries, it would be useful to repeat the test for a longer duration to assess whether these temperatures can be achieved in the sludge during days 10-15 (for example).
There were minimal differences between the temperature gain of the polycarbonate roof and the polythene roof. This suggests that the roof design is robust to changes in shape and size, and to a certain extent material - so long as the transparency is maintained.
Pathogens:
A 90% reduction was achieved across all three beds tested during this phase of work for the three core indicators (faecal coliforms, e.coli, and faecal streptococci).
Slightly better reductions were found in the unraked bed (a 99% reduction in faecal coliforms and e.coli) suggesting that the level of disruption of the sludge could affect the kill-rate. However, the raked beds dried out marginally faster, and a little more completely.
This suggests a trade off between the speed of drying and thoroughness of pathogen reduction. Given the increased manpower required for regular raking, if time is not critical, then an unraked approach would seem viable for FSM in this context.
