ENV 6015
Sedimentation
Problem Set
1. For what particle size range is the Stokes’ law assumption valid? Assume quiescent
conditions and use specific gravities of 1.1, 1.5 and 2.0.
2. Settling tests have been conducted on a wastewater containing 250 mg/l of non-
floccuable suspended solids. The test system is a graduated cylinder and the sampling
point is at a depth of 2 m. Using the given data, develop a curve of overflow rate
(m/min) as effluent suspended-solids (SS) concentration for an ideal process.
Time, min SS, mg/l
0 250
1 225
2 195
3 170
4 138
5 83
6 57
8 32
10 20
14 13
20 8
30 5
40 3
3. A settling test is run on a domestic sewage. Flocculant settling is assumed. Using the
data presented in the following table, develop a curve of effluent suspended-solids
concentration as a function of overflow rate (m/min).
Suspended Solids, mg/l
Time, min
Depth, m 0 10 20 30 40 50 60
0.3 250 198 152 97 75 52 31
0.6 250 225 180 149 109 80 70
0.9 250 230 200 163 138 110 82
1.2 250 235 206 175 158 129 107
1.5 250 240 210 187 167 137 120
1.8 250 243 214 192 170 158 130
