Anaerobic digestion processes have been evaluated in terms of process performance, hygienisation efficiency, and dewaterability. Mesophilic digestion with pasteurisation showed greater removal of viral indicators, and the inclusion of a heat-exchanger can led to positive heat balances.
Researchers at the University of Barcelona in Spain have evaluated the anaerobic digestion of raw sewage sludge in terms of process efficiency and sludge hygienisation.
Four different scenarios were analysed: mesophilic anaerobic digestion, thermophilic anaerobic digestion, and mesophilic anaerobic digestion followed by hygienisation treatment at 60°C or 80°C.
Digester performance (organic matter removal, process stability, and biogas yield) and hygienisation efficiency (reduction of E. coli, somatic coliphages, and F-specific RNA phages) were the main factors examined.
In addition, a preliminary economic feasibility study of each option was carried out through an energy balance (heat and electricity).
The results show that both thermophilic anaerobic digestion and mesophilic anaerobic digestion followed by a hygienisation step were able to produce an effluent sludge that fulfills American and European legislation for land application.
However, higher removal efficiencies of indicators were obtained when a hygienisation post-treatment was present.
In respect of the energy balance, it should be noted that all scenarios have a significant energy surplus.
In particular, positive heat balances will be obtained for thermophilic anaerobic digestion and for mesophilic anaerobic digestion followed by 60°C hygienisation post-treatment, if an additional fresh-sludge/digested-sludge heat-exchanger is installed for energy recovery.
