Investigations on twenty Australian sewage sludges ? their evaluation by means of chemical analysis

Abstract

To evaluate twenty sludges from different places in Australia for disposal on arable land, these sludges were extracted with (a) HNO3 to determine the ‘total’ contents of some heavy metals and other elements; (b) a solution containing DTPA to determine contents of ‘available’ heavy metals; and (c) with a CaCl2-solution to determine contents of water-soluble heavy metals and some other elements. Measurements of the pH of the sludges, their electrolytic conductivity, NaHCO3-extractable P, water-extractable Cl, and contents of ash and organic carbon were also made. The HNO3-extractable Cd, Cu, Mn, Ni, Pb, and Zn in the sludges were compared with those in sludges from three other countries. High correlations between HNO3-extractable and DTPA-extractable metals were found in the Australian sludges; therefore, the HNO3 is equally good as the DTPA as an extractant of heavy metals in sewage sludges. The correlations between HNO3- and CaCl2-extractable Cd, Cu, Mn, and Zn were not significant. ‘Young’ and ‘aged’ sludges from the same treatment plants consistently differed in their Na and Cl contents, which were lower in the ‘aged’ sludges. A wheat experiment treated with high application rates of a saline and metalliferous sludge showed that the balance of nutrients and the amount of soluble salts in a sewage sludge are of greater direct importance to the growth of plants than the heavy metal content of that sludge.