THE CONTAMINATION OF HERBAGE FOLLOWING THE APPLICATION OF SEWAGE SLUDGE TO PASTURE
Report No FR0078
A D BUTTIGIEG, D A KLESSA AND D A HALL
To assess the extent of herbage contamination when sewage sludge is spread on the surface of pasture.
Sludge solids adhering to herbage are likely to be ingested by grazing animals and this may result in elevated intakes of sludge-borne contaminants, particularly potentially toxic elements (PTE), which may pose a health risk to animals and man.
With thin (2% ds) sludge, rainfall and herbage growth effects reduced PTE levels to below safe limits within three weeks (EC mandatory no-grazing interval) but this took eight weeks following the application of thickened (12% ds) sludge.
Further experiments are required to identify fully the mechanisms which reduce herbage contamination in relation to sludge thickness and application rate, following which practical recommendations may be made to avoid exposing grazing animals to excessive levels of PTEs.
The weight of sludge metals adhering to leaf surfaces of a perennial ryegrass sward following the surface spreading of anaerobically digested sludge was dependent on rate and dry solid content of the sludge applied and sward height. Decreasing application rate, sward height and the dry solid content of the sludge all resulted in a decrease in adhering metals. The Adherence Index was seen to be similarly affected by the rate and dry solid content of sludge applied and sward height. In addition, the Adherence Index was also influenced by metal species, though this effect was only present where the sludge applied had been taken directly from the anaerobic digester (2% ds sludge) and not when the sludge was collected from storage lagoons where anaerobically digested sludge had been stored for several months (12% ds sludge).
Whereas for the 12% ds sludge the Adherence Index of all eight metals (Mn, Cd, Cu, Fe, Cr, Ni, Pb and Zn) was similar, application of 2% ds sludge led to Cr and Fe exhibiting a much higher Adherence Index than the other metals. In addition the Adherence Index of Cd, Cu, Mn, Ni, Pb and Zn from 2% ds sludge was similar to that recorded for the metals from the 12% ds sludge.
The dilution of the adhering metals was brought about as a result of both plant growth and rainfall. However, their effects were not clearly distinguishable and could not entirely explain the observed dilution of metals. Hence other factors, like wind blow, may have also contributed to the removal of sludge metal from the leaf surfaces. The affect of rainfall in bringing about metal dilution especially that of Cr, Fe and Ni was most evident when rain fell within one week of sludge application.
A pot experiment showed that drying the adhering sludge prior to the application of 'rain' resulted in no metal wash-off. The rate of dilution was not only influenced by the amount and timing of rainfall and the extent of plant growth but was also dependent on the dry solid content of the sludge. Adhering metals from the 12% ds sludge exhibited a higher dilution rate than the same metals from the 2% ds sludge.
The initial herbage metal contamination due to adhering sludge and the rate at which this was diluted determined the period over which the herbage was potentially unsafe for grazing. The EC Directive on the use of sludge in agriculture specifies a minimum 3-week no-grazing period following the application of sludge. In the field trial, where 2% ds sludge had been spread, herbage Cd, Cr, Cu, Ni, Pb, Mn and Zn concentrations returned to acceptable values (not considered to be zootoxic) within 3 weeks. However, in the case of Fe, herbage contamination remained above a concentration thought to be safe for 4-5 weeks. This period increased to 8 weeks where 12% ds sludge was applied.
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