This study was designed to evaluate the present status of Silivri small-scale wastewater treatment plant considering the effluent quality to meet the receiving water standards in Turkey. The basic focus was to rehabilitate the system in terms of microbiological dynamics which may cause basic operational problems in treatment plants. Two procedures were applied in the plant. Increasing oxygen concentration in the areation basins could improve the COD efficiency up to 89%, which was 81% before, and decrease SVI. Increasing the sludge return ratio proved higher removal efficiency of COD up to 92% in parallel to a significant improvement in both MLVSS and SVI. Both procedures removed filamentous bacteria to the acceptable level. OUR profiles of basin 1 showed a healthy sludge character confirming the efficiency increase.