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INTRODUCTION
As land availability decreases and the volumes of sludge increase, sustainable thermal methods of sludge processing gain more significance. In Germany, the municipal water boards face further restrictions in the sludge disposal routes because, according to an upcoming regulation, they will be required to recover phosphate from the sludge.
At present, sludge disposal varies between different German states; however, on average approximately 50% undergoes thermal disposal (Weihman et al. 2013). This situation has many drawbacks for wastewater treatment plant (WWTP) operators, such as dependence on energy companies and sensitivity to gate fee changes, high transportation cost and loss of valuable materials such as phosphate.
Decentralised thermal treatment can provide a cost-effective and innovative solution whilst responding to environmental, regulatory and public pressures. The EloDry-Pro® process facilitates utilisation of the thermal energy and chemical resources in sewage sludge.
The Linz-Unkel WWTP (design capacity: 30,000 PE) is adjacent to the Rhine, located ca. 20 km south of Cologne, and is operated by the Linz-Unkel Joint Waste Management Association (Figure 1). The Linz-Unkel WWTP is the first full scale plant in Germany using the EloDry-Pro® process for sewage sludge utilisation. The technology uses low temperature drying and a process of staged combustion to reduce dried sewage sludge to around 40% of its volume, with simultaneous full conversion to a high grade fertiliser material containing a high percentage of plant available phosphate. Additionally, in the process, removal of harmful substances (e.g. mercury, micro-plastics, and hormones) takes place.
The EloDry-Pro® installation at Linz-Unkel WWTP was commissioned in September 2015. Even though PYREG® technology has been used in the past for utilisation of biomass and organic waste materials, the Linz-Unkel project is the first where the Pyreg® 500 module is combined with a matching EloDry® low temperature belt dryer (by ELIQUO Stulz) in one innovative sewage sludge utilisation plant called EloDry-Pro® (Figure 2).
SLUDGE LINE OVERVIEW
The sludge line at Linz-Unkel WWTP processes 770 tonnes DS of sludge annually, of which approximately 23% is imported. Shortly before implementation of EloDry-Pro®, in 2013 and 2014, the sludge line at Linz-Unkel underwent a major upgrade:
* An old anaerobic tank was changed to a primary settlement tank.
* A two-step compact anaerobic mesophilic...