Characterisation of sewage sludge metals for classification purposes using the potentially leachable metal fraction

Abstract

The South African sludge guidelines were revised in 1997 with considerable amendments to the heavy metal loading and usage restrictions. The industry continued using the aqua regia extraction method to characterise and classify sewage sludge. This meant that a Class D sludge (most stable class and sellable product) could not be generated at any South African plant. This caused the industry to seek alternative nonbeneficial disposal methods. It was then realised that the metal limits were in fact the TCLP (Toxicity Characteristic Leaching Procedure) extractable limit. This method was developed to determine the potential of a waste to contaminate ground and/or surface water. The agriculturists, however, use the NH4EDTA extractable fraction to establish metal concentration in soils. This paper specifically aims to establish the feasibility of using the available metals extraction methods such as NH4EDTA and TCLP to determine the fate and therefore the risk of sludge-borne metals (Cd, Pb, Zn, Cu). The results show that the TCLP extraction method could potentially underestimate the impact of sewage sludge-borne metals when amended to agricultural soils. The extraction efficiency of the NH4EDTA and TCLP methods relative to the aqua regia extraction method was presented for Cd, Pb, Zn and Cu. The NH4EDTA extraction method seems to be an alternative to the aqua regia method, especially since the NH4EDTA measures the fraction of the metal that may become available in the environment. It is recommended that the aqua regia and NH4EDTA be used to classify and characterise the metal content of South African sludges.