Sedimentation (water treatment)
Settling basins are ponds constructed for the purpose of removing entrained solids by sedimentation.
The limit sedimentation velocity of a particle is its theoretical descending speed in clear and still water.
Settleable solids are measured as the visible volume accumulated at the bottom of an Imhoff cone after water has settled for one hour.
This is feasible by widening the approach channel and lowering its floor to reduce flow velocity thus allowing sediment to settle out of suspension due to gravity.
The overflow rate is defined as:[citation needed] In many countries this value is named as surface loading in m3/h per m2.
There are recommendations on the overflow rates for each design that ideally take into account the change in particle size as the solids move through the operation: However, factors such as flow surges, wind shear, scour, and turbulence reduce the effectiveness of settling.
To compensate for these less than ideal conditions, it is recommended doubling the area calculated by the previous equation.
[citation needed] Settling basins and clarifiers can be designed as long rectangles (Figure 1.a), that are hydraulically more stable and easier to control for large volumes.
By sedimentation principles, the suitable treatment technologies should be chosen depending on the specific gravity, size and shear resistance of particles.
In general, if the concentration of the solutions is lower than 500 mg/L total suspended solids, sedimentation will be considered discrete.
[11] Concentrations of raceway effluent total suspended solids (TSS) in the west are usually less than 5 mg/L net.
So this says that particles can grow and develop a higher settling velocity if a greater depth with longer retention time.
However, the collision chance would be even greater if the same retention time were spread over a longer, shallower tank.
In fact, in order to avoid hydraulic short-circuiting, tanks usually are made 3–6 m deep with retention times of a few hours.
3 represents a typical batch-settling column tests on a suspension exhibiting zone-settling characteristics.
Sedimentation in potable water treatment generally follows a step of chemical coagulation and flocculation, which allows grouping particles together into flocs of a bigger size.
[16]: 5–9 Because of the large amount of reagent necessary to treat domestic wastewater, preliminary chemical coagulation and flocculation are generally not used, remaining suspended solids being reduced by following stages of the system.
