The dimensioning includes finding the required surface area and the volume of the thickener. The surface area can be calculated based on the solid loading rate [kg/(m²*d)] and the volume based on a maximum sludge retention time.

Table 3 shows achievable solid concentrations and solid loadings for various sludges. The daily solid load divided by the solids loading rate gives the required surface area.

Now the maximum hydraulic overflow rate at peak flow conditions has to be checked. Recommended maximum hydraulic overflow rates range from 0.6 to 1.5 m/h for primary sludges, 0.15 to 0.35 m/h for secondary sludges and 0.25 to 1.0 m/h for combined primary and secondary sludge. High hydraulic loadings can cause excessive solids carryover. Conversely, low hydraulic loading can cause septic conditions and odours and floating sludge can result. In the second case dilution water can help maintaining the optimal hydraulic loading. The height of the sedimentation zone should be generally 1 m.

To compute the volume of the compaction zone, the solid loading at thickened conditions has to be multiplied by the maximum retention time. For central Europe 36h are common, at warmer climates this value should be reduced. The solids concentration in the sludge blanket attenuates with increasing height, therefore the values for the concentration of thickened solids have to be reduced by the multiplication factor 0.75. On the bottom of the thickener 0.3 m should be added for the scrapers.

Table 3: Concentrations of different sludges and solids loadings

Thickening can be achieved by mechanical devices, too. Examples are rotary-drums and gravity-belt thickeners. Solid contents up to 9% can be achieved. Centrifuges are suitable for both, thickening and dewatering. Disadvantages are the need for polymers, electrical energy and relatively high expenditures for operation personnel.