As concern continues to increase over "greenhouse" gases, such as methane and carbon dioxide, many sewage treatment plants have collected and utilized their biogas mixture as a fuel for producing energy. In the case study of sewage treatment plant, biogas has been used as fuel for the gas boilers for the digesters. However, during the several months, unknown white powder is deposited inside the boiler after combustion. It caused severe problems and raised the maintenance cost. We analyzed the deposited mineral and biogas by ICP-MS and GC-MS. As a result, the main deposited component was oxidized silicon, and the biogas sample contained octamethylcyclotetrasiloxane (D4, 20 ppm). Accordingly, a removal process for D4 from the biogas fuel was investigated for the smooth operation of wastewater treatment plants. Through an experiment of the adsorption and detection, it was found that alumina exhibited a more adsorptivity than other materials. The analytical methodology of D4 in the biogas is established in the study.