Biological waste materials are mixed with water and subjected to intense ultrasonic irradiation (14), before being supplied to an anaerobic digester (16), so that a biogas is generated which contains methane. The biogas is supplied to a catalytic reformer unit (20) to form a synthesis gas; steam may also be supplied, and the proportion of steam to methane is adjustable so that the synthesis gas may be rich in hydrogen or alternatively rich in carbon monoxide. Adjusting the proportion of steam to biogas enables the output of the process to be adjusted according to market conditions. If the synthesis gas is rich in hydrogen, it may be supplied to a fuel cell (26) to generate electricity, while if it is rich in carbon monoxide, it may be used to generate liquid hydrocarbons in a Fischer-Tropsch synthesis reactor (32).