The wastewater treatment systems of a city in Turkey differ from each other depending on various factors such as population, economy, wastewater characteristics, topography, climatic conditions, urbanisation and cost of land. The construction and operation of the stabilisation ponds in all cities of Turkey are very easy due to the cheaper cost of the land and hot and mild climates. In this study, a computer model based on the Wehner-Wilhelm equation, developed by Thirumurthi, is used and for this application; first-order reaction rate (k(p)) and dimensionless dispersion coefficient (D) must be determined. Also, some parameters such as temperature, solar radiation, organic loading, altitude, turbidity. and hydraulic retention time are also required. The BOD, removal in the winter for 2m of pond depth was found 50%, and 52%, in the southern region (Sanliurfa), the northern region (Samsun), and the western region (Balikesir), respectively. No result could be given for the eastern region (Erzurum) in Winter, since wastewater in the pond was frozen. Generally, the BOD, removal was found to be 90-92% in the Summer for all provinces, and also 55-70% in the southern region, and 40-55% in the northern region in Winter during the 20 days of retention time when dimensionless dispersion coefficient and oxygenation factor are 0.2 and 1.5, respectively. The optimum effluent quality of the pond was obtained as 15-25 days of the retention time, 1-2 m of pond depth, and 0.2 dimensionless dispersion coefficient.