Sedimentation overflow rate

Flocculation is accelerated and higher overflow rates are achieved by external or internal recirculation of settled soflds into the feed which leads to the collection of fine particles by interception. Addition of conditioned fine sand to the feed induces separation by differential sedimentation, and sometimes increases overflow rates to 6—8 m/h. 

Suspended Solids Removal. Depending on the concentration and characteristics of the suspended soflds, they can be removed by filtration, flotation, or sedimentation. Coarse soflds are removed by screening. Settleable suspended soflds are removed in a clarifier, which may be circular or rectangular. The efficiency of soflds removal is a function of the overflow rate (m /-d (gal/ft -d) as shown in Figure 5. 

The surface overflow rate (SOR) for piimaiy sedimentation is normally held close to 40.74 mV(m day) [1000 gal/(ft day)] for average flow rates, depending upon the solids characteristics. Lowering the SOR below 40.74 mv(m day) does not produce improved effluent... 

Depth tends to be determined from the retention time and the surface overflow rate. As surface overflow rates were reduced, the depth of sedimentation tanks was reduced to keep retention time from being excessive. It was recognized that depth was a valid design parameter and was more critical in some systems than retention time. As mixed-liquor suspended-solids (MESS) concentrations increase, the depth should also be increased. Minimum sedimentation-tank depths for variable operations should be 3.0 m (10 ft) with depths to 4.5 m (15 ft) if 3000 mg/L MESS concentrations are to be maintained under variable hydraulic conditions. With MESS concentrations above 4000 mg/L, the depth of the sedimentation tank should be increased to 6.0 m (20 ft). The key is to keep a definite freeboard over the settled-sludge blanket so that variable hydraulic flows do not lift the solids over the effluent weir. 

Example 5.9 To meet effluent limits, it has been determined that the design of a primary sedimentation basin must remove 65% of the influent suspended solids. The average influent flow is 6,000 mVd the peak daily flow is 13,000 mVd and the minimum daily flow is two-thirds of the average flow. It was previously determined that 65% removal corresponds to an overflow rate of 28 m/d. Design a circular basin to meet the effluent requirement at any cost. 

Figure 20. Overflow rate in sedimentation tank (-) (1 m/hr) predictions of plant performance—comparison with standard system (-) (2...

Figure 21. Overflow rate in sedimentation tank effects on particle size distribution function...

Continuous Settling Dilute Systems. When apphed to continuous sedimentation the long tube test is used to model the settling of particles in an elemental vertical volume of liquid moving throu the length of the clarifier. The corresponding overflow rate Vc, of a continuous ideal basin is given by H/ta, where H is the sanq)le point depth. 

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