Background

The proportion of biogas in the mix of renewable energies is still remarkably high. The process of anaerobic digestion (AD) provides the basis of biogas production but often leads to excessive foaming. Identifying the reasons for foaming is difficult for biogas plant operators because many factors may play a role. It is therefore difficult for laboratory research to give answers to this specific problem, as the consistency of the digestate itself plays a crucial part in the foam formation process. Hence, careful investigation of foaming in full-scale biogas plants is important in order to identify the main causes and to develop strategies for the prevention of foaming.

Methods

Fifteen operators of biogas plants treating biogenic waste have been reviewed in order to estimate the frequency of foaming events in full-scale biogas plants. Samples from foaming digestates were subsequently analyzed. Seven foaming periods in five biogas plants were investigated closely in order to ascertain the causes of foaming events.

Results

It was noted that 80% of surveyed biogas plants have had excessive foam formation during the AD process. The foam of two wastewater-treating biogas plants contained filamentous microorganisms. An abrupt temperature increase and the use of grain products and yeasts were identified to be the reason for foaming in four cases. It was, however, not possible to identify the real causes of the foaming event in two cases.

Conclusions

Foam formation is a common phenomenon in waste-processing biogas plants. It is important to identify the reasons for foaming because this knowledge helps biogas plant operators to prevent foam formation in the future.