A critical evaluation has been made of the literature regarding the sources of heavy metals in sludges from municipal wastewater treatment plants. Residential loadings of heavy metals as a percentage of total metal loads are highly variable with respect to both the particular element under consideration and the geographic area. Only rarely is the percentage contribution of any metal attributable to residential sources greater than one-half the total. The disagreement between studies seems to indicate that the available information concerning the residential loading estimates may be biased due to the inclusion of unsurveyed industrial discharges. The sludge content of heavy metals is frequently correlated with industrial density, but the many confounding variables make a general statement regarding this relationship impossible. Diffuse sources such as laundries, street runoff and small family-owned operations may contribute to the discharges from a supposedly residential area.

Bibliography: p. 51-54. Photocopy.

A critical evaluation has been made of the literature regarding the sources of heavy metals in sludges from municipal wastewater treatment plants. Residential loadings of heavy metals as a percentage of total metal loads are highly variable with respect to both the particular element under consideration and the geographic area. Only rarely is the percentage contribution of any metal attributable to residential sources greater than on-half the total. The disagreement between studies seems to indicate that the available information concerning the residential loading estimates may be biased to the inclusion of unsurveyed industrial discharges. The sludge content of heavy metals is frequently correlated with industrial density, but the many confounding variables make a general statement regarding this relationship impossibe. Diffuse sources such as laundries, street runoff and small family-owned operations may contribute to the discharges from a supposedly residential area. Varying proportions ofthese minor and essentially uncontrollable sources presumably account for the high variability in the available data. The contribution of urban runoff to metal loads of municipal treatment plants is not large relative to other sources at present, because peak flows are usually bypassed. Local studies will have to be made to identify the controllable sources of heavy metals whenever their concentrations in sludge destined for agricultural purposes exceeds acceptable limites[sic] because each community has its own unique mixture of industrial and residential sources.