A numerical model is presented for simulating the flow and the settling performance of primary clarifiers. The model consists of 2 parts: a flow model providing the velocity and turbulent viscosity/diffusivity field and a suspended-sediment transport model for determining the particle concentration field. The model is an improvement over simple algebraic relations and previous numerical models because, with the aid of the k-e turbulence model employed, it allows a more realistic prediction of the velocity and turbulent diffusivity distribution, and also because it accounts for differential settling by solving concentration equations for individual fractions of different particle sizes. The model is applied to calculate the flow field, the flow through curve and the suspended solids concentration field in the primary settling tank of the City of Sarnia, Ontario, Canada for various overflow rates and settling velocity curves. In spite of the geometric simplifications employed in the model the concentration profiles and removal efficiencies are in very good agreement with measurements, while the agreement about the flow field and the flow-through-curve is more qualitative. The calculations were carried out both on a personal and a Siemens 7881 main frame computer, and the ratio of computing times was found to be 1:150.