Environmental Engineering Problems

 

Fall 2014 CEE 2502 Physical Chemical Principles of Environmental Engineering Homework No. 1

1.

What is the settling velocity of a particle having a diameter of 10-5 cm and a density of 2.0 g/cm3 in air at 27 oC and 2 atm of pressure? Assume that air is dry and has 21% O2  and 79% N2 by volume.

2.

A particle has a settling velocity of 1x10 -7 and 1x10-4 cm/sec in water and air at 20 oC and 1 atmosphere, atmosphere, respectively. Compute the particle particle diameter and its density. The density and viscosity of water at 20 oC are 0.99823 g/cm3  and 1.0087 cp, respectively. The viscosity of air at 1 atm and 20 oC is 0.000178 g/cm-sec. g/cm-sec. Assume that air is dry and has 21% O2 and 79% N2 by volume.

3.

Fluidized bed biological reactors are often used for denitrification and the treatment of toxic wastes. In such reactors, granular granular activated carbon (GAC) particles can be us used ed as a support media for the growth of microbial biofilm and an d are kept in suspension (fluidized)  by an upflow fluid velocity. As the particles become coated with a bacterial bact erial films, their settling characteristics characteristics are altered. Assuming that a bacterial bacterial film with a specific density of 1.04 grows on a GAC particle (specific density = 1.4) to a thickness kD where D is the diameter of a GAC particle, develop a general mathematical relationship for the settling velocity in terms terms of k and D. Plot the variation of settling velocity wit with h bacterial film thickness for a GAC particle particle with a diameter of 0.077 cm and for 0.0 < k < 1.0. Explain the effect of bacterial growth on the settling properties of GAC particles.

4.

In an ideal settling tank, a particle is completely removed if its terminal settling velocity is greater or equal to the depth/detention time ratio. a. Calculate the diameter of the smallest smallest particle with with a specific specific gravity 1.6 that would  be removed rectangulat basin that is 45m long, 6m wide, and 2m deep with a retention timeinofa 30 min.  b. To improve removal efficiency, a consultant has proposed to install plates 6m wide and 2m high inclined at 60 o to the horizontal and placed 10 cm apart (perpendicular distance between the plates) along the whole length of the tank. Find the new area available for settling and hence, compute the diameter of the smallest particle that will be removed. Comment on the results.