VEKTOR - Investigation on the Incineration Behaviour of Sewage Sludge with different Additives and Variation on the Process Conditions

Phosphorus (P) is an essential element for plants, animals and humans, which can be found naturally mostly in the form of phosphates in the earth's crust. These phosphate reserves are limited and unevenly distributed geographically, making many countries, including Germany, 100% dependent on its import. Consequently, the EU has classified phosphorus as one of the 30 critical raw materials with a high supply risk. As a measure to protect raw materials, the Sewage Sludge Ordinance (AbfKlärV) obliges operators of large sewage treatment plants in Germany to recover phosphorus from sewage sludge or sewage sludge ash (SSA) from 2029 or 2032 onwards in order to be re-incorporated in the circular economy. Currently, most conceptions are aimed at phosphorus recovery from mono-incineration ash in order to ensure disposal safety on the one hand and to generate high P recovery rates on the other.

Within the framework of the VEKTOR project, the combustion behaviour of sewage sludge is being investigated using various additives and process conditions. The overall objective is to produce an ash that is optimised in terms of phosphorus recovery in order to facilitate subsequent complex wet-chemical process steps.

So far, about 25 phosphorus recovery processes from SSA have been published. Despite the large number of processes, there is currently a lack of detailed knowledge on the influence of additives on ash quality and the effect on the digestion behaviour in the subsequent leaching process.

For this reason, in the VEKTOR project, representative sewage sludge or sewage sludge mixtures are to be added and incinerated in a laboratory and pilot plant scale by the project partner TEER. These ashes will be eluted in the ISA laboratory with different acids and under different process conditions, and examined for their changed digestion behaviour compared to untreated ashes. In addition to the heavy metal content, the P-solubility is also tested to fulfil the requirements of the Fertiliser Ordinance.

To explain the relationship between crystal structure and solution behaviour, the change in crystalline phosphate phases in the ashes or leaching residues will be analysed using X-Ray Diffraction (XRD) at the Institute for Building Research (IBAC).