The goal of this diploma thesis is to evaluate the solubility of iron and phosphate after a post-treatment of digested sludge. In the course of chemical phosphorus elimination at municipal sewage treatment plants, iron is added to the wastewater. When recovering phosphate fertilizers from sewage sludge or its ashes, iron has to be removed from the phosphorus through a complex separation process since iron can narrow the plant availability of phosphorus in the soil. Moreover, iron can inhibit the extraction of phosphorus during the phosphorus recovery process from sludge ashes. The aerobic post-treatment of digested sludge has already been implemented on a large scale at a municipal sewage treatment plant in Austria, with the aim to remove nitrogen through nitritation and denitritation. In the course of this research work it was examined whether a selective dissolution of iron during nitritation can be achieved through the post-aeration of digested sludge. A selective dissolution of iron would be advantageous to separate dissolved iron from the digested sludge and recycle it as part of the sludge liquor back into the aeration tank. For this purpose, lab-scale experiments with the aeration of digested sludge were carried out to determine the influence of nitritation and the resulting pH-value decrease on the solubility of iron and phosphate. Furthermore, acidification down to pH-value 2 of aerated digested sludge, anaerobic reference sludge as well as of sludge ash was carried out to evaluate the solubility in strongly acidic environments as applied by the wet-chemical P-extraction processes. Digested sludge of two different municipal sewage treatment plants both with a high total iron content were used for the lab-scale experiments. The post-aeration of digested sludge was performed in batch mode in 3L Plexiglas cylinders for a duration of 15 - 20 days. The photometric method using 1,10 phenanthroline chloride approved to be suitable for the analysis of the concentration of dissolved iron in membrane filtered samples of digested sludge in the range of 0,03-4 mg Fe/L. By the acidification of anaerobic digested sludge samples, both iron and phosphate went in solution. On the contrary, oxidizing conditions in the aerobically treated digested sludge led to a significant reduction of the solubility of iron and phosphate. The comparison with the theoretical solubility product of different iron compounds confirmed that iron was mainly present as ferrous hydroxide in aerobically treated digested sludge. The pH-value decrease achieved by nitritation only (pH-value 6) even led to a reduction of the dissolution of iron. Therefore, a selective dissolution of iron in the digested sludge and a subsequent separation via sludge dewatering could not be achieved through post-aeration. When decreasing the pH-value to 2, a selective dissolution of phosphorus was obtained in the aerated sludge, however it was limited. In acidified sludge ashes of anaerobic digested sludge a high solubility of phosphate was found, whereas the present iron compounds showed very low solubility even in strongly acidic environments. Therefore, in view of the selective phosphate dissolution in wet-chemical extraction processes, treatment of sludge ashes rather than digested sludge seems to be the more suitable option. A selective dissolution of phosphate could be also achieved by acidifying the ashes of aerated digested sludge samples. The comparison with the results of the anaerobic sludge ashes reveals that the post-aeration of digested sludge led to a limitation of the phosphorus solubility in the sludge ashes. Hence the aerobic post-treatment of digested sludge prior to the wet-chemical extraction of phosphorus from digested sludge or sludge ashes showed to be not advantageous for the phosphorus recovery process.