The project investigated the thermal disposal of sewage sludge in a reference rotating‑drum incinerator that incorporates a drying zone. The main technical goal was to redesign the internal inserts of the drying section to improve mixing, reduce fouling, and increase overall energy efficiency. Initial trials used conventional screw‑ and paddle‑type inserts that caused “back‑up” of material and poor mixing, leading to incomplete combustion and an undesirable ash composition. A trial‑and‑error approach was then applied to develop a honeycomb‑like geometry for the inserts. This new design produced a marked improvement in material circulation, reduced fouling, and achieved a more complete combustion of the organic fraction. The resulting process was able to lower the required energy input and the associated CO₂ emissions by approximately 12 % compared with the reference configuration without inserts.

To quantify the benefits, the team performed detailed process‑simulation studies. Heat and mass balances were calculated with dedicated rotating‑drum software for both configurations – with and without the new inserts. The simulations revealed that the modified geometry increased the residence time of the material in the drying zone, improved heat transfer, and reduced the need for auxiliary heating. The mass balance showed a higher dry‑matter content at the outlet, confirming the enhanced drying performance. The combined results of the heat and mass balances were used to evaluate the energy efficiency of the system. The comparison demonstrated that the new inserts not only improved the drying performance but also reduced the overall energy consumption, leading to the reported 12 % savings.

From a manufacturing perspective, the inserts were designed to be simple to produce and install. They can be fabricated without specialized facilities, and their dimensions allow for straightforward mounting and demounting. This modularity also enables retrofitting of existing drum incinerators and the replacement of individual components in case of damage. The additional investment required for the inserts was estimated at about €60 000, which represents roughly 4 % of the total cost of the reference drum incinerator. For new installations, the pay‑back period is expected to be short, whereas for existing plants the cost is higher because additional structural modifications may be necessary to accommodate the increased load on the drum wall. The project concluded that the low‑effort redesign offers a significant reduction in energy demand and environmental impact, and it recommends further research into integrating drying and combustion into a single‑stage process or coupling the thermal treatment with existing gas‑utilisation schemes.

The collaboration was led by ZADCON GmbH in Dessau, which coordinated the process‑technology work and overall project management. The Hochschule Anhalt in Köthen, specifically the Department of Applied Biosciences and Process Engineering, was responsible for investigating the motion dynamics of the innovative inserts. The project was funded by the Deutsche Bundesstiftung Umwelt (DBU) under grant number 38410/01. The regulatory context for the work was the 2017 revision of the German Sewage Sludge Regulation, which introduced stricter requirements for phosphorus recovery and ash handling, effective from 2029. The project ran from the regulation’s entry into force on 3 October 2017 through the subsequent years of design, simulation, and prototype testing, culminating in the final report that highlights both the technical achievements and the economic feasibility of the proposed insert design.