Temporal trends of total and potentially available element concentrations in sewage biosolids: a comparison of biosolid surveys conducted 18 years apart

The application of biosolids to agricultural land is becoming increasingly popular because of the many benefits they can offer in terms of improved soil fertility and productivity, and also because land application offers an economically viable disposal option. However, the heavy metal and nutrient content of biosolids are potential causes for concern due to the possibility of their accumulation in soils to levels that may adversely affect the environment. Therefore, total concentrations of elements in biosolids and the fraction potentially available to organisms must be known in order to accurately gauge the risks and to classify the biosolids. In 1983, deVries (deVries, MPC. Investigations on twenty Australian sewage sludges-their evaluation by means of chemical analysis. Fert Res 1983;4:75-87.) conducted a survey to determine the total and available contents of elements in biosolids collected from various treatment plants around Australia. We conducted a similar survey on biosolids collected in 2001, and thus were able to identify temporal trends in biosolid characteristics. The 2001 survey included biosolids from several treatment plants sampled in 1983, thus some direct comparisons were possible, and in four cases, it was possible to examine temporal trends in biosolid characteristics within individual treatment plants. Total element concentrations were determined by aqua regia digestion, and available element concentrations were determined by 0.01 M CaCl2 extraction. Results showed total concentrations of Cu, Mn, Ni, Na, and Ca changed little over the period examined, however the 2001 median values of Cd, Mg, Pb, and Zn were all reduced by 60% or greater. Median total concentration of K was more than 50% higher, and P 120% higher, in the 2001 survey. Changes in the availability of elements surveyed in 1983 and 2001 were varied for different elements and for different biosolids. Extractable element concentrations (viewed as percentages of total concentrations) of Cu, Cd, and Ni all fell by 50-72% between 1983 and 2001, while K extractability fell by 35%.