A theoretical analysis of gravity dewatering of wastewater sludge is made, and an equation describing the drainage rate developed. The equation relates the depth of sludge with time, using the parameters of solids content, specific resistance, coefficient of compressibility, and dynamic viscosity and density of the filtrate. Extensive bench-scale tests substantiated the theory. The concept of media factor is introduced to account for the role of the supporting media. Potential applications of the equation developed are discussed. (Author)