Determination of the inorganic pollutants in South African sewage sludges.
A total of 78 sludge samples originating from 69 sewage works of South Africa were used in this investigation. Seven of the sludge samples were liquid and the rest were solid. Moisture content, pH and mineral ion determination using ICP-OES were carried out. The methods used to digest the samples were EPA 3050B and 301OA for solid sludge and effluent (liquid sludge) respectively. The moisture content determination showed that fresh wet sludge was composed of water between 40 to 90 %. The minimum moisture content was found to be 2.70 % while the maximum was 88.50 % with a mean value of41 %. The pH results showed that the majority sludges produced had pH values between 4.8 - 6.5. The ICP-OES results which involved analysis and quantification of 22 mineral ions showed that the order ofabundance that was most common to the majority ofthe samples was P, Ca, Fe, AI, Mg, K, Zn, Na, Si, Mn, Cu, Cr, Ba, Pb, Sr, Se, B, Ni, Co, Mo and Cd. Even though phosphorus was the most abundant, 11 of the samples had calcium as the highest element. Looking at the heavy metals, zinc was the highest with cadmium being the least. The order of abundance in majority of the solid samples (64 in total) was Zn> Pb > Ni> Cd with the relative metal concentrations of Cu, Se, B, Cr, Co & Mo varying among the sewage works. The liquid samples also had phosphorus as one of the most abundant elements but was 102 times smaller comparing to solid sludges. In addition, most of the transition elements were found to be below the detection limit. Beryllium was exceptionally found to be below detection limit in all sludge samples. The results have pointed out that industrial effluent have 3 times the level of pollutants when compared to the domestic effluents. However, the methods of preparing sludge have no influence on the content or quantity of mineral ions. The results have been compared with 1989 data. The outcome shows that concentrations of the major nutrients namely calcium, magnesium and potassium remained relatively constant whereas phosphorus increased by more than 3 fold. On heavy metals, the data shows that the mean concentration level of Zn, Cu, Cr, Pb, Ni and Cd declined whereas that of Se, B and Mo showed an increase in 2002, all to a varying degree. The current results were also compared with the current maximum limits as stipulated in the permissible utilization and disposal of sewage sludge government guideline. The amount of Cu, Se, Pb and Zn were found to be above the limit in more than 90 % of the samples. There was no sewage works that met the required limits for all the elements of interest. When these results are compared with the intenational limits, all the elements fall within the acceptable range. It is therefore clear that the current South African guideline limit is too restrictive.