EVE 290
Introduction to Environmental Engineering
Homework #23 (Exam #2 Review)
1. A zero-order reaction is known to have a temperature correction factor of 1.01. A batch reactor
was used in the laboratory (25° C) with a reaction time of 3 hrs to achieve a reduction in the
concentration of a synthetic organic compound from 3.0 to 0.1 mg/L. How long will the reaction
take under field conditions (12° C) to achieve the same treatment efficiency?
Ans: 3.41 hrs
2. An industrial wastewater was analyzed to determine the BOD5. For different dilutions were
completed, with initial and final dissolved oxygen concentrations as noted below. Determine
the BOD5 concentration.
Dilution factor
1.0
10.0
100.0
1000.0
Initial DO [mg/L]
9.2
9.2
9.2
9.2
Final DO [mg/L]
0.00
0.00
2.05
8.47
Ans: 722 mg/L
3. A biodegradeable industrial wastewater (petrochemical) has a BOD5 of 600 mg/L. If the BOD
progression follows first-order kinetics with a rate constant = 0.2 per day, determine the
ultimate demand.
Ans: 949 mg/L
4. A confined aquifer 20.0 m thick has two monitoring wells spaced 500 m apart along the
direction of groundwater flow. The difference in water level in the wells is 2.0 m and the
coefficient of permeability is 50 m/day. Estimate the rate of flow per meter of distance
perpendicular to the flow.
Ans: 4.0 m3/day per meter of width
5. A 50 cubic centimeter sample of dry soil from an aquifer weighs 100 g. When it is poured into a
graduated cylinder, it displaces 35 cm3 of water.
a. What is the porosity of the soil? Ans: 0.3
b. What is the average density of the actual solids contained in the soil? Ans: 2.86 g/cm3
6. A sample of water at pH 10.0 has 32.0 mg/L of CO32- and 56.0 mg/L of HCO3-. Find the alkalinity
as CaCO3.
Ans: 104.2 mg/L as CaCO3
7. The settling velocity of a particle is 0.002 m/s, and overflow rate of a settling tank is 0.08 m/s.
a. What percentage of the particles does the tank capture? Ans: 25%
b. If the particles are flocculated so that their settling rate is 0.05 m/s, what fraction of the
particles is captured? Ans: 100%
c. If the particles are not changed, and another settling tank is constructed to run in parallel
with the original, will all of the particles be captured? Ans: No