Activated sludge processes contains various groups of microorganisms with different metabolic properties, which are responsible for contaminants removal. Therefore, it is important to elucidate the general structure and functional properties of biomass in activated sludge processes. For this purpose, a full-scale domestic biological wastewater treatment plant in Tunceli (Turkey), Tunceli WWTP, was monitored to observe seasonal variations in process performance and biomass properties for a year. It was observed that nitrifying bacteria developed abundantly in the rainy and cool spring season as they were suppressed in summer because their large losses took placed due to an environment containing high alkalinity values. In September, aerobic heterotrophic, nitrify, denitrify, and anaerobic activities increased. It can be said that mature biomass contained young and active levels in an environment in which the sludge volume index (SVI) value increased to 196 mL/g. As a result of the improvement in the structural and functional properties of biomass, the nitrogen removal e ciency reached to 99%.Throughout whole study, the structural improvement observed in biomass re ected in its removal activity. The amount of biomass and removal activity decreased with the abundance of organic matter in the in uent at the period in which biomass was closer to being categorized in the aged sludge class. The results showed that as the lowest mixed liquid volatile suspended solids (MLVSS) value of the year was 400 mg/L in November, MLVSS value reached the highest amount (1,400 mg/L) in December which aerobic heterotrophic activity accelerated with a decrease in organic matter level.