Ashes from the Mono-Incineration of Sewage Sludge (KARAT)

Phosphorus recycling will be an important issue in the future due to greater resource conservation. Phosphorus is considered a critical raw material; it is essential as a nutrient for plants and thus relevant to human life. In Germany, the amendment of the Sewage Sludge Ordinance (AbfKlärV) makes the recovery of phosphorus from sewage sludge or sewage sludge ash mandatory from 2029 or 2032. The recovery process is not specified, so that each sewage treatment plant or disposal company must intensively concern themselves with the sewage sludge or sewage sludge ash and P recovery in order to select and implement a suitable process.

In the KARAT project, different sewage sludge ashes are first examined with regard to their physicochemical parameters and elemental composition.

So far, about 25 processes for phosphorus recovery from sewage sludge ash have been described. These processes differ in their approach and are divided into wet chemical and thermal processes. Due to the large number of processes and the continuous development and new development of recovery technologies, the most promising processes are to be analysed in the KARAT project within the framework of a technology screening and evaluated by means of a matrix.

For some of the phosphorus recovery processes from ash, there is a lack of detailed knowledge about the chemical reactions taking place. Phosphorus is found in different mineral and amorphous phases in the ash matrix. However, the elemental and mineral phase compositions of sewage sludge ashes from different incineration plants differ significantly from each other and have a decisive influence on the solubility in wet-chemical phosphorus recovery. These ashes are eluted with different acids in order to show the solubility behaviour of main and accompanying elements as well as the limits and weaknesses of the redissolution in different elution solutions and with different acid concentrations. In addition, the time- and temperature-dependent redissolution will be investigated.

The project partner, the Institute for Building Materials Research (IBAC), will examine the crystalline phosphate phases of different ashes (X-Ray Diffraction in laboratory) in order to draw conclusions from the present phosphorus compound on the solubility behaviour.