All classical models for thickening assume one-dimensional continuity. However, the free-settling domain in continuous thickeners is not one-dimensional. Therefore, a two-dimensional model is investigated. The two-dimensional model gives the same values for thickener area demands as the one-dimensional model, but the relationships between batch and steady-state thickening are not the same. Free-settling Kynch characteristics can arise in the continuous operation that do not arise in batch tests. Therefore, design procedures that rely on Kynch theory, such as that of Talmage and Fitch, are not completely valid.

A method is developed, based on extrapolating the Kynch or free-settling segment of a batch settling, curve, that yields an improved prediction for thickener area demand.