BACKGROUND: In China, some biogas residue, which cannot be utilized by microbes in the anaerobic process, has been used as fertilizer. More has been deposited in biogas plants or on land around the plants. This has an effect on the environmental protection of the biogas plant, especially if the high lignin content in the biogas residue is not handled properly. RESULTS: In this study biogas residue has been used for the preparation of activated carbons by phosphoric acid activation. Textural characterization and feasibility of employing the prepared activated carbon to remove methylene blue (MB) from aqueous solution were investigated. The results show that the activated carbons have high surface area (1950 m(2) g(-1)) and pore volume (1.232 cm(3) g(-1)). Equilibrium data were best described by the Langmuir isotherm model, with a maximum monolayer adsorption capacity of 344.83 mg g(-1) at 25 degrees C. Among the kinetic models studied, the pseudo-first-order model was found to be the most applicable to describe the adsorption of MB. CONCLUSIONS: The adsorption performance of activated carbons prepared from biogas residues (BR-AC) was comparable with that of commercial material and other adsorbents reported in earlier studies and presents a high value added application for biogas residues. (C) 2010 Society of Chemical Industry