In Bavaria, sewage sludge from municipal wastewater treatment is incinerated to a great extent. The large sewage sludge incineration plants in Munich, Neu-Ulm and Altenstadt handle around 30,000 t of sewage sludge ash per year, some of which are deposited and some of which are used as fertiliser in agriculture. The heavy metal contents of the sewage sludge ash considered here are below the limit values for fertilisers according to the German Fertiliser Ordinance (DüMV).
Featuring 8-13%, the P content in the ashes is high but their phosphate availability is rather low. The aim of the R-Rhenania project is to build and operate an AshDec plant with a capacity of 30,000 t per year which is able to process the Bavarian sewage sludge ashes to highly effective fertilisers with a view to their introduction to the regional market. AshDec is a thermochemical treatment method allowing for the ashes to be digested by means of sodium compounds at 850-900 C in a rotary kiln. Volatile heavy metals such as cadmium, lead and mercury can be separated via the gas phase.
The project comprises the planning, set up and operation of the plant. After its commissioning, the parameters will be optimised. A mass balance of the plant provides information about material and energy flows. The products of the plant are examined in detail in terms of their quality.
The P fertilising effect (as well as the availability and benefits of silicon and trace substances) of the recycled fertilisers will be documented by proven agronomic methods and extensive field trials. To assess their environmental impact, a life cycle assessment is carried out. The small scale examination of all German sewage sludge ashes will deliver a scheme for the transferability of the results to other regions in Germany. The introduction of recycled fertilisers in Bavaria is carried out in close cooperation with the most important stakeholders (farmers, ministries, approval authorities, research institutions).
The Kompetenzzentrum Wasser Berlin (KWB) examines the transferability of the process to other sites and is responsible for the environmental risk assessment of the fertiliser products as well as for the Life Cycle Assessment and the cost-benefit analysis of the process.