Lab-scale studies of the conditioning of an alum sludge by an anionic polymer showed that the excess polymer can lead to clogging of the filter medium, thereby increasing its resistance. Tests showed that medium resistance (R-m) behaved as R-m = alpha e(beta v) in which alpha and beta are coefficients and V is the volume of filtrate. Standard dewatering theory for the Specific Resistance to Filtration was modified to take account of this behavior. The modified theory shows that failure to take proper account of the characteristics of the medium resistance can distort the estimate of the cake resistance and can provide a misleading view of the optimum polymer dosage. The study also indicated that a simple test based on the moisture content of the sludge sediment can be used as a means of gauging an optimum polymer dosage.