Australian Biosolids: Characterization and Determination of Available Copper

Environmental Context. Land application of sewage-derived biosolids is both an inexpensive method to dispose of waste and a simple way to increase soil fertility and stability. However, biosolids often contain high concentrations of heavy metals, but not all of the metals are immediately available for uptake by the soil or other organisms. To determine if this toxicologic risk outweighs the benefits, the degree of ecologically available metal, rather than simply the entire metal content, must be known in both the as-disposed and worst conditions scenarios. Application of these principles requires regulatory bodies to amend their guidelines.

Abstract. Application of biosolids to agricultural land provides a low-cost disposal option with many potential benefits to soil. However, the practice may result in accumulations of potentially toxic heavy metals, and thus regulations are in place to limit the amount of metals applied to soil in this way. Current Australian regulations are not ideal because they are based on total metal concentrations in soils and biosolids, rather than the fraction that is ecologically available (the fraction accessible by organisms). Therefore more environmentally appropriate regulations, based on the available metal portion, need to be devised. However, before this is possible, more needs to be known about the characteristics of Australian biosolids, including the factors that influence the availability of biosolid metals. Copper is a metal of great concern because of its commonly high concentration in biosolids and because of its relatively high toxicity to certain groups of bacteria and fungi. Therefore an investigation was conducted to characterize the range of properties observed in Australian biosolids, and to determine the fraction of available metals and the factors that influence it (particularly in the case of copper). General properties such as pH, electrical conductivity, organic carbon, and total metal concentrations were measured. Availability of copper was specifically measured using isotopic exchange techniques and a Cu²⁺ ion-selective electrode. Results showed that total copper concentration and Cu²⁺ activity could be used to predict available copper. A new system of biosolid land-use regulation that incorporates the available metal fraction and a pH protection factor is proposed.