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# Lecture 4

Lecture 4. Introduction to Environmental Engineering January 20, 2000. Engineering Dimensions and Units. Being Technically Sound! Fundamental Dimensions force (F) mass (M) length (L) time (T). Engineering Dimensions and Units. Derived Dimensions

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## Lecture 4

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1. Lecture 4 Introduction to Environmental Engineering January 20, 2000

2. Engineering Dimensions and Units • Being Technically Sound! • Fundamental Dimensions • force (F) • mass (M) • length (L) • time (T)

3. Engineering Dimensions and Units • Derived Dimensions • calculated dimensions produced by combining fundamentals • velocity (L/T) • mass balance (M/T) • Dimensions do not describe the quantity • What? Not HOW Much?

4. Engineering Dimensions and Units • Units and their values (SI, American, cgs) • 5 lbs = 2.27 kg • 20 ft = 62.6 m • 12 L = 3.17 gal • 30 °C = 86 °F • SI units are the best because they increase and decrease by powers of ten

5. Engineering Dimensions and Units • Density • CA= concentration of A, MA = mass of material A, • VA = volume of material A, VB = Volume of material B

6. Engineering Dimensions and Units • Ex. 150g of NaCl are added to 50 m3 of water. The volume of the NaCl = 0.001 m3. What is the concentration of the mixture after the NaCl has completely dissolved (in mg/L)?

7. Engineering Dimensions and Units • Notice in the previous example that the volume of the salt was quite small in comparison to the volume of water. • Concentration as a percentage (usually by mass) • A= percent of material A, • MA, MB = mass of materials A and B respectively

8. Engineering Dimensions and Units • Ex.The sand and biological material in an expanded bed reactor has a concentration of 40,000 mg/L, given a density of 1 g/cm3, what is the percent mass of the material?

9. Engineering Dimensions and Units • Air Pollution Dimensions • generally expressed as mass of pollutant per volume of air at standard temperature and pressure • occasionally expressed as ppm or part per million, in which case • one volume of a pollutant per 1 x 106 volumes of air • conversion from mass/volume (mg/m3) requires knowledge of the molecular weight

10. Engineering Dimensions and Units • at standard temperature (0 ° C) and pressure (1 atm) one mole of gas occupies 22.4 L of volume

11. Engineering Dimensions and Units • Flow Rate and Residence Time • either gravimetric or volumetric • kg/s or m3/s • they are not independent of each other but related by density of the substance • [mass] = [density] x [volume] • QM =  QV

12. Engineering Dimensions and Units • Ex. The BOD5 of the influent to a WWTP is 500 g/L with the flowrate of 100 m3/s. What are the gravimetric and volumetric flowrates of BOD5? • Gravimetric = 100 m3/s x 500 g/L x 1000 L/m3 = • = 5 x 107 g/s • Volumetric = ?, need to know the percent of the 100 m3/s that is BOD5

13. Engineering Dimensions and Units • Residence Time •  = V/Q, the amount of time for a particle to pass through a reactor • What is the residence time of BOD5 in the previous example for a reactor of 100 m3? •  =100 m3/100 m3/s = 1 s • it takes 1 second for the water to pass through the reactor

14. Homework #2 • Chapter 2: 2, 5, 9, 12, 16, 18

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