Summary
Fermentation of effluent sludge in digestion towers has already been implemented in many municipal sewage plants. However, the existing plant technology results in low reduction rates, incomplete reduction of the fermentable proportion and thus low gas yield. In addition, the digestion towers' structure can lead to degassing problems which, together with the incomplete reduction, cause foaming problems. Conversion of effluent sludge to biogas using this digestion process is neither economic nor ecologically sound.
The project's task was to integrate high-performance digestion based on the Schwarting-Uhde method as a first stage in the conventional sludge digestion of the Heidelberg waste water association. Besides ensuring stable operation, a balance was drawn for sludge digestion after integration of the high-performance digestion and this was compared to conventional base data. Further, general applicability and transfer to other municipalities was to be examined.
Results:
- The process features two stages. The first stage is usually mesophilic (37°C), the second stage is thermophilic (55°C). The temperature is constant and the agitator facility delivers a high mixing rate at simultaneously low shear forces.
- Sludge digestion does cause foaming problems that impair operation. The method facilitates a process characterised by high biogas production, high reduction rates and low operating costs.
- High-performance digestion is operated at a volumetric load of up to 10 kg oTS/m³*d. This is achieved through a holding time of 5 days and high oTS concentration in the intake.
- oTS reduction through high-performance digestion reaches approx. 48%. Stage 2 achieves approx. 20%. This results in a total reduction rate of approx. 60%.
- The results are very well-suited for application to other sewage plants with different effluent sludges.
