A comparison of the dewaterability of a range of water treatment plant sludges has been completed through computation of dewatering performance indicators for a diaphragm filter press. Real parameter data, obtained from the characterisation of alum and ferric sludges, generated under precisely controlled conditions, was used for input to a phenomenological model. Comparisons of dewaterability based on throughput curves largely agree with previous analysis of the underlying parameter data. The difference in approach provides a quantification of benefit. Greater throughputs and output concentrations are predicted at the lowest coagulant doses and at pH ∼ 6. Typical industrial cloth resistances consistently reduce throughput by a factor of 3-7, but the assessment of relative benefit is shown to be insensitive to this parameter. Quantitative agreement of the predictions with observed performance can be attained. Finally, the twin effects of solids loading and dewaterability are assessed together, showing that each has a significant influence on the required filter surface area. This quantification shows that high coagulant doses adversely affect both of these aspects, leading to filter area requirements larger than might otherwise be expected.