[Tim Evans]

Tim Evans  asked the Naked Scientists:
   
"Chemicals in sewage sludge leads to physiological changes" (Naked Scientists radio programme and podcast 9/9/12)
 
I'm afraid that you have picked on a poorly designed experiment.  Sewage sludge was used to fertilise pasture, which was grazed by sheep and this was compared with sheep grazing pasture given mineral fertiliser.  

BUT the designers matched the nitrogen on the basis that the N in the sludge would be 100% available, which it is not.  Thus the SS pasture received much less plant available N than the mineral-N pasture.  The sward composition would have changed but the researchers didn't check that.  

Low N (and probably more clover) has been reported by others to result in the physiological changes observed.  The researchers say "chemicals in the sludge" but they don't actually measure any to prove causal relationship.  

It is especially sad because the institutions had a good agricultural pedigree.  This work was just poorly designed.
 
Regards
 
Tim
 
Dr Tim Evans
TIM EVANS ENVIRONMENT

What do you think?

[Stewart Rhind]

   The aim of the experiment was to provide independent research into the performance of sheep grazing pastures fertilised with sewage sludge. Dr Evans criticises the experimental design but does not explain why he considers the design inappropriate to address the question posed. 
   At no time have we claimed to have demonstrated a causal relationship between chemicals in sludge and the observed effects on the structure or function of parts of the brain, gonads, thyroid, adrenal or bones; what we have shown is an association between the application of sludge to pasture and perturbations of the physiology of exposed sheep.
   Contrary to Dr Evans’ statement, however, we have reported concentrations of selected chemicals in sludge, soil and animal tissues as a search of the literature will show.  On the basis of many reports in the scientific literature which do demonstrate causal relationships between individual chemicals and such physiological responses, we have postulated that exposure of animals to low levels of mixtures of endocrine disrupting compounds, including those present in sludge, may be responsible for the changes observed in our experimental animals.
   In none of the 15 peer-reviewed scientific publications and multiple experiments on which the podcast was based was the issue of N availability ever addressed; it is accepted that the patterns of nutrient delivery would have differed greatly in the two treatments.   Dr Evans states that “low N (and probably more clover) has been reported by others to result in the physiological changes observed”.  However, the observed effects cannot be attributed to long terms changes in the clover content of the pasture since effects of sludge application were observed within 3 months of application at a time of year (mid-September) when pasture growth in central Scotland had already almost ceased and so there was no possibility of effects being mediated through a change in the pasture clover content.
   At this time, it is not possible to state, categorically, that either the low N or other chemicals in sludge are responsible for the observed changes in animal physiology. The important message is that the changes were observed in sheep exposed to sewage sludge treated pastures. Such changes may have adverse implications for the health and reproductive success of these animals.

Dr Stewart Rhind
Research Scientist,
The James Hutton Institute.

[chris]

Thank you, Stewart, for taking the time to reply.