Applying vermicomposting process for sewage sludge treatment is a low-cost method which has been gaining a growing number of supporters around the world. In Poland it is still rather unpopular method. The following publication presents the results of 12 years (2004–2015) of research and experiments on the use of red Californian earthworm in Wastewater Treatment Plant in Zambrów. Among others, the results of analyzing vermicompost quality were shown concerning selected micro and macro elements, fertilizing value and sanitary condition. Cumulative content of the examined heavy metals was highest in 2004 and reached over 1200 mg^-1 kg s.m., whereas the lowest content of 520 mg^-1 kg s.m. was observed in 2012. The study results indicate that in the entire 12-year research period vermicompost met the standards for organic-mineral fertilizers, which means it contained over 20% of organic substance in reference to dry mass and over 1% of total nitrogen and 0.5% of phosphorous in reference to phosphorus pentoxide (P₂O₅). During the entire research period no living eggs of intestinal parasites Ascaris sp., Trichuris sp., Toxocara sp. and Salmonella bacteria were observed in the vermicompost. The research concerning red Californian earthworm proved substantial mineralization and humification of sludge. The application of EM formula considerably quickened the process and the achieved modifications in the vermicompost structure and content.