This study aimed to evaluate operation technology and functioning of facilities for the treatment of municipal sewage in a small wastewater treatment plant operating based on the trickling biofilters technology, in the winter (February) and summer (June-July) periods. Additional analyses were conducted to determine the effectiveness of contaminants removal in particular facilities of the technological system tested. Study results indicate that the sewage treatment plant operating based on the trickling biofilters technology ensures effective removal of organic compounds, total nitrogen, total phosphorus, and total suspended solids. In the summer period, the effectiveness of contaminants removal reached 92%, 52.9%, 97.9%, and 99.9%, respectively, whereas in the winter season it was lower an reached 69.5%, 12.4, 93.0, and 95.0%, respectively. Treatment effectiveness achieved in this study was referred to results of investigations conducted in the same sewage treatment plant 24 years ago. Results obtained may provide a valuable guideline to designers and operators of small sewage treatment plants.