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AbstractDuring recent decades, the anaerobic digestion process has been extensively studied and various methods for process enhancement have been explored. These methods include heat treatment, alkali addition, phase separation and membrane enhancement. In general, whilst technically feasible, the methods have not proved to be economically competitive.The pretreatment of sludges with high‐power ultrasound has been investigated as part of a twelve‐month research project funded by six UK water utilities. Cell lysis and particle‐size reduction (caused by ultrasonic cavitation) are thought to be the key parameters through which the digestion process is enhanced. A series of laboratory‐scale anaerobic digesters has been operated and significant increases in biogas yield have been noted following ultrasonication. Experiments have been completed with a variety of ultrasonic devices (of different geometries and construction materials) and sludge types (e.g. primary, secondary and co‐settled). Current experiments are investigating methods through which the technology can be successfully scaled‐up and applied on a full‐scale plant.

ABSTRACT This paper describes the enhancement of biogas production in laboratory‐scale anaerobic digesters (with batch and semi‐continuous operation) using iron‐enriched duckweed as a supplement to the digestion of feedstock. The relationship between the level of enhancement achieved and the retention period of the digesters was also investigated. The trials demonstrated that, in batch digesters, iron‐enriched duckweed significantly improved the rate of microbial succession. Batch digesters receiving no duckweed took 40 days to reach peak methane production compared to 15 days when duckweed was added. In the semi‐continuous digesters, an increase in gas production of about 44% was observed.