Metal Speciation and Availability in Biosolids As Affected by Chemical Amendments During the Anaerobic Digestion of Wastewater.

Biosolids are unavoidable by-products of municipal wastewater treatment. Irrespective of the wastewater catchment area, biosolids always contain some level of metal contamination and this characteristic frequently limits their beneficial reuse in agriculture. Although regulatory guidelines specify maximum permissible limits for biosolids amended soils in terms of total metal concentrations, it is well known that a proportion of that metal will generally not be labile. In order to further reduce the risks associated with the use of biosolids in agriculture, cost effective ways to ‘immobilise’ metal contaminants within the biosolids matrix are sought.

This study used x-ray absorption spectroscopy (XAS) and isotopic dilution to investigate Cu and Zn speciation and lability in biosolids when three different chemical amendments were added during the anaerobic digestion process. Sewage sludge from a large-scale Australian wastewater treatment plant was anaerobically digested in benchtop bioreactors together with 5 different levels of ferric chloride, aluminium sulphate or water treatment residual (i.e. alum sludge). Control treatments were also included, and the variables monitored included pH, Eh, temperature, biogas volume and composition. Following digestion, the solid fraction was separated by centrifugation and dried to produce ‘fresh biosolids’, whilst subsamples subjected to a 3-month regime of wetting and drying to simulate composting/stockpiling produced ‘aged biosolids’. Total copper/zinc was measured by aqua regia digestion and ICP-OES, whilst copper/zinc lability (i.e. potential bioavailability) was investigated using isotopic dilution techniques (Cu by stable isotope dilution with ⁶⁵Cu; and Zn by radioisotopic dilution with ⁶⁵Zn). Selected samples were also analysed by XAS so that associations between metal lability and speciation could be established. Both fresh and aged samples were analysed as previous experiments have shown important changes in biosolids speciation to occur post-production. The potential for these amendments to act as biosolids metal fixation agents and the implications for related regulatory controls will be discussed.