Determination of inorganic pollutants and assessment of the current South African guidelines on permissible utilisation of sewage sludges

A total of 71 sludge samples originating from 61 sewage treatment works in South Africa were used in this investigation. Moisture, pH and total mineral ion content were determined. Moisture values were found to vary between 2.7% and 88.5% with the pH of the majority of the samples falling between 5.1 and 6.5. Mineral ion determinations showed that P was the most abundant in most of the sludges whereas, of the heavy metals, Zn had the highest concentrations and Cd the lowest. The current results (for 10 elements) were compared with the current South African maximum limits as stipulated in the permissible utilisation and disposal of sewage sludge for unrestricted use. The amounts of Cu, Se, Pb and Zn were found to be above the limit in more than 90% of the samples. No sewage works met the required limits for all the elements of interest. The results were also compared with the USA and EU limits, according to which all the elements were within the acceptable range and over 50% of the sludge samples met the required limits. These results suggest that the current South African limit is too conservative.