Page 241 - Materials Chemistry, Second Edition

CAT3525_C07.qxd 1/29/2005 9:57 AM Page 212
212 Waste Management Practices: Municipal, Hazardous, and Industrial
5. To avoid carryover of glass fines in an air classifier, what device or unit operation may
wastes first be passed through?
6. What are the advantages to using a shredder for MSW processing in a MRF? What are
the disadvantages?
7. Plastic wastes can be sorted optically or mechanically using ‘color screening’(true/false).
8. What is the ideal trommel rotation speed?
9. List and discuss some of the major safety hazards associated with shredding MSW.
10. All other factors being equal, are there any practical advantages to horizontal hammer-
mills over vertical models? Consider efficiency of shredding, energy requirements, noise
production, and jamming by rigid articles.
11. Using mechanical separation equipment for separation mixed MSW, it is not possible to
achieve 100% separation efficiency. Explain and provide an example.
12. During air classification, the recovery of organics is complicated by what factors?
13. What factors influence the effectiveness of magnetic separation?
14. Calculate the waste thickness for a conveyor belt measuring 1.0 m in width with an aver-
age belt speed of 20 m/min. The waste loading rate is 25.5 MT/h and the average density
3
of the waste on the belt is 105 kg/m .
15. A trommel screen measuring 9 m long and 3 m in diameter is rotating at 2.5 r/min. The
feed rate is 12 MT of raw MSW per hour. Calculate the critical speed. At the above speed
would the waste input be cascading, cataracting, or centrifuging?
16. If the trommel screen in Question 15 were to be adjusted to a steeper angle, how would
the separation efficiency and critical speed be affected?
17. A magnetic separator is employed at a MRF for ferrous recovery from MSW. The feed
rate to the separator is 1255 kg/h. The feed contains 4.2% ferrous. A total of 40 kg is col-
lected in the product stream and 32 kg is actually ferrous. Calculate the recovery, purity,
and efficiency of the unit operation.
18. List the key components of the receiving area of an MSW processing facility.
19. In terms of separation of potentially recyclable components, what is a detrimental effect
of shredding raw MSW to a fine particle size?
20. Contact your local waste management authority and determine the fraction of recovered
materials for recycling collected via drop-off centers vs. curbside pickup programs or
MRFs.
21. Diagram a sequence of unit operations for a mechanized waste separation system. The
input waste includes paper products, food waste, glass, plastic (PETE, HDPE, and oth-
ers), metals (ferrous and nonferrous), and hazardous wastes. List all the equipment that
would be required for adequate separation. Include at least one shredder.
22. Hazardous materials can be removed from the commingled wastes arriving at a MRF via
(a) magnetic separation, (b) manually, (c) air knife, (d) froth flotation.
23. Waste paper should be removed: (a) using an air classifier, (b) after ferrous removal, (c)
after shredding the waste stream, (d) after trommel screening, (e) all of the above.
24. Suppose your community has decided to develop a waste management program with a
goal of substantially greater waste recycling and materials recovery. The three major
alternatives are source separation, a dirty MRF, and a clean MRF. Discuss the key factors
for consideration by the community in order to make an educated decision about the opti-
mum alternative. Consider issues such as short- and long-term economics, public accept-
ance, environment and aesthetics, and convenience to local citizens.