Settling-Thickening Tanks

Settling-thickening tanks are used to achieve separation of the liquid and solid fractions of faecal sludge (FS). They were fi rst developed for primary wastewater treatment, and for clarifi cation following secondary wastewater treatment, and it is the same mechanism for solids-liquid separation as that employed in septic tanks. Settling-thickening tanks for FS treatment are rectangular tanks, where FS is discharged into an inlet at the top of one side and the supernatant exits through an outlet situated at the opposite side, while settled solids are retained at the bottom of the tank, and scum fl oats on the surface (Figure 6.1). During the retention time, the heavier particles settle out and thicken at the bottom of the tank as a result of gravitational forces. Lighter particles, such as fats, oils and grease, fl oat to the top of the tank. As solids are collected at the bottom of the tank, the liquid supernatant is discharged through the outlet. Quiescent hydraulic fl ows are required, as the designed rates of settling, thickening and fl otation will not occur with turbulent fl ows. Baffl es can be used to help avoid turbulence at the infl ow, and to separate the scum and thickened sludge layers from the supernatant.