5. Conclusions

5.1    The evidence available at this point in time is regarded as insufficient to recommend any major revision of the Working Party Guidelines (55) with respect to metals and metalloids in sewage sludges.

5.2    It is considered that the value of the organic matter content of dried sewage sludges to agriculture was under-estimated in the Report of the Working Party (55).

5.3    Experience and analytical data on soils and crops from sites with a history of sewage and sewage sludge application, cannot be considered representative of operations bearing a much greater resemblance to normal agricultural practices and utilising much more efficient means of sludge distribution and lower rates of application.

5.4    The total soluble nitrogen content of sewage sludges would be a better indication of the nitrogen content readily available to crops, than is the total nitrogen content, although both are useful.

5.5    Experiments reported in the literature which involve the use of soluble inorganic salts to simulate the uptake of metals by crops from sewage sludge treated soils, are unlikely to yield results indicative of what actually occurs in such soils.

5.6    In the course of this investigation no serious detrimental effects arising from the use of sewage sludge on private land, on crop yields, animals, or waters were encountered (with the exception of one case, in which adverse effects on crop yield were observed, but for which no details were available). On water authority owned sites with a history of sewage and sewage sludge application, adverse effects have been observed.

5.7 Few analyses of the metallic content of sewage sludges have been made in the past and the soils to which they were applied were rarely analysed.

5.8    Sensitive, specific analytical methods for some of the metals of interest have not been available until recently. Therefore, data from some early investigations may be questionable.

5.9    There is a paucity of information relating to the possible adverse effects arising from the disposal of metal contaminated sewage sludges to agricultural land. Current research programmes are inadequate to resolve these problems in the time scale required.

5.10 Of the common metallic contaminants present in sewage sludge cadmium poses the most serious public health hazard.

5.11 The application of sewage sludge to land permits the recycling of useful nutrients and organic matter. While various fears exist regarding the long term implications of land disposal, the alternative methods of sludge disposal are not without limitations. It is considered that the disposal of sewage sludge to land in a manner consistent with Working Party Guidelines and good agricultural practice is an acceptable form of sewage sludge disposal.