The distribution of heavy heavy metals between the principal components of digested sewage sludge
Macnicol, R.D.; Beckett, P.H.T.
Water Research 23(2): 199-206
Sewage sludge is such a heterogeneous mixture of disparate components that it is difficult to discuss its chemistry except in terms of these separate components. The paper reviews procedures for separating sludge into its principal components. It is not possible to achieve complete separation without altering some of the fractions separated, so it presents procedures for a simple four-component division (particulate, biofloc, colloid, soluble) sufficient to provide a basis for further work, and a framework for computer modelling. Of the total copper, nickel and zinc in a typical digested sewage sludge from London: 85-95% were held in the biofloc (bacterial detritus); 5-15% on the mineral grains and organic fragments of the particulate fractions; and not more than 3% in soluble and colloidal organic matter. The high density separates of the particulate fraction comprised only 2% by weight of the whole sludge, but they contain the highest concentrations of the heavy metals, mainly as sulphide precipitates, in association with iron sulphides. Similar precipitates formed coatings on mineral grains and, finely disseminated, they appear to be the main forms of combination of copper, zinc, lead and cadmium in the biofloc. It is not clear whether the nickel in the biofloc is held by organic ligands or in mixed sulphide precipitates.