Bacterial biofilms are ubiquitous in nature and industrial settings. In this study, a biofilm consortium was enriched in a continuous-flow-cell system using humic acid as the sole carbon and energy source. The degradation of the humic acids by the consortium under two supplementary carbon sources was investigated by ultraviolet (LTV) absorbency, Fourier transform infrared (FTIR) spectroscopy, and electrospray mass spectrometry (ES-MS). The morphological characteristics of the biofilm consortium and the isolated cultures from the biofilm were observed under an epifluorescence microscope. The metabolic diversity of the selected cultures from the degradative consortium, based on substrate usage pattern, was examined using Biolog EcoPlates. Microscopic analysis revealed that the biofilm was formed by various morphotypes of bacteria, fungi, and yeasts, as well as amoebas. The substrate usage profiles of the bacteria confirmed that, in addition to yeasts and fungi, two groups of bacterial consortia were developed in the biofilm to degrade the humic acids. The degradation of humic acid in the biofilm was mostly carried out in a secondary or a cometabolic path. Addition of the readily digestible external carbon source enhanced the growth of the biofilm consortium. The FTIR and ES-MS spectra confirmed the changes in chemical structure of the humic acid by the biofilm community.