This study was focused on determining the optimal operational conditions of a small domestic wastewater treatment plant (SWP) with a flow rate of 1000 m(-3)d(-1) using oxygen uptake rate (OUR) data, microscopic analyses, and effluent toxicity testing with Daphnia magna in parallel to wastewater characterization. Chemical analyses of nine samples, taken from different points of the treatment plant over a nine months period, presented strong domestic wastewater characteristics. The plant performance for meeting discharge standards was not sufficient enough for TP parameter. Aeration capacity and sludge recycle ratio were adjusted to improve the performance of the system and to prevent any operational problems, such as bulking. Although the inflow was 100% toxic, effluent toxicity following treatment was decreased to 5%. Filamentous bacteria were in "excessive" amount. The dominant filamentous bacteria species were determined as Nostocoida limicola II. Reevaluation of the discharge limits with respect to nutrient removal, geographic location, and receiving water quality subject to treatment effluent discharges.