The hybrid anaerobic solid-liquid (HASL) system was developed to be used in industrial-scale operations to minimize the amount of food waste for disposal in Singapore. Thermal pre-treatment of food waste at 70 degrees C for 2 h (experiment E1) or at 150 degrees C for 1 h (experiment E2) facilitated the hydrolytic and acidogenic processes in the acidogenic reactor and methanogenesis in the methanogenic reactor in the HASL system. The highest dissolved chemical oxygen demands in the effluents from the acidogenic reactors were 17575, 19980 and 24 235 mg dm(-3) in the control with food waste without thermal pretreatment and experiments E1 and E2, respectively. The maximum concentrations of methanogens in the methanogenic reactor were 2.3 x 10(7), 3.8 x 10(7), 4.3 x 10(7) cells cm(-3) for the control and experiments E1 and E2, respectively. However, the performances of the methanogenic phase in terms of specific activity of methanogens did not differ significantly for the control and experiments E1 and E2. Use of thermally pretreated food waste halved the time to produce the same quantity of methane in comparison with anaerobic digestion of fresh food waste. The fluorescent measurements of co-enzyme F-420 and oligonucleotide probe Arc915 specifically bound (hybridized) with 16S rRNA were used for monitoring of methanogens during anaerobic digestion of food waste. There was a linear correlation between these parameters and the concentration of methanogens in the effluent from the methanogenic reactor. (c) 2005 Society of Chemical Industry