Question

Design a circular primary sedimentation basin for a town with a population of 45000 people. Assume the wastewater flow is 400 L/person/d.

a) Design the tank for a nominal detention time of 2 hrs at an average overflow rate of 36 m3/ m2/d.

b) Determine the tank depth and diameter given the tanks can be constructed in diameter increments of 1 m and depth increments of 0.5 m.

c) Repeat the design considering 2 tanks.

d) Prepare a summary table reporting the weir loading as m3/ m2/d, and the % deviation from the nominal detention time and overflow rate for each case.

e) Comment on how the basin design may be modified to accommodate sludge thickening in addition to particulate removal.

Answer 1

Page No. Date : | A Compl uline Exam. of El t 500 x 4 " 0 18x100 ml day. 1871 hr = 500 x 2.4+H 18 x 103 (H = am] the tank - 3 m. Depth of

Sludge Thickening
Sludge thickening is a process in which the solids concentration is increased and the total sludge volume is correspondingly decreased, but the sludge still behaves like a liquid instead of a solid. Thickening commonly produces sludge solids concentrations in the 3% to 5% range, whereas the point at which sludge begins to have the properties of a solid is between 15% and 20% solids. Thickening also implies that the process is gravitational, using the difference between particle and fluid densities to achieve the compaction of solids.
The advantages of sludge thickening in reducing the volume of sludge to be handled are substantial. With reference to Figure 9-3, a sludge with 1% solids thickened to 5% results in an 80% volume reduction (since 5% = 1/20). A concentration of 20% solids, which might be achieved by mechanical dewatering (discussed in the next section), results in a 95% reduction in volume, with resulting savings in treatment, handling, and disposal costs.