# wastewater treatment

Question 1. Two parallel flocculation basins are to be used to treat water flow of 150m3/s. If the design detention time is 20minute, what is the volume of each tank? If the average velocity gradient in these two tanks is 124/s, calculate the velocity gradient is each basin if the gradient in second basin is half of the first one.

Question 2. Determine the volume of the aeration tank for the following operating conditions:

Influent BOD5 concentration after the primary is = 150mg/L

Wastewater flow rate = 10MGD

F/M ratio = 0.2/d

Mixed Liquor volatile suspended solid concentration = 2200mg/L

Question 3. Given below is the wastewater characteristics, determine the F/M ratio? (10 Points)

Influent BOD5 concentration = 84mg/L

Wastewater flow rate = 0.150m3/s

Volume of the aeration tanks = 970m3

Mixed Liquor volatile suspended solid concentration = 2000mg/L

Question 4. What is the terminal settling velocity of a particle with a specific gravity of 1.4 and a diameter of 0.010mm in 20oC water? Would this particle be completely removed in a settling basin with a width of 10.0m, depth of 3.0m, a length of 30.0m, and a flow rate of 7500m3/d? What is the smallest diameter particle of specific gravity 1.4 that would be removed in the sedimentation basin described above?

Question 5. Will grit particle with a radius of 0.04mm and a specific gravity of 2.65 be collected in a horizontal grit chamber that is 13.5m in length if the average grit-chamber flow is 0.15m3/s, the width of the chamber is 0.56m, and the horizontal velocity is 0.25m/s? The wastewater temperature is 22oC.

Question 6. Wastewater treatment plant flow rate is 20MGD. Chlorine dosage is 10mg/L. Determine chlorine requirement (lb/day)

Question 7. If a particle having a 0.0170-cm radius and density of 1.95g/cm3 is allowed to fall into quiescent water having a temperature of 4oC, what will be the terminal settling velocity? Assume the density of water = 1000kg/m3. Assume Stoke’s law applies?

Question 8. If the terminal settling velocity of a particle falling in quiescent water having a temperature of 15oC is 0.0950cm/s, what is its diameter? Assume a particle density of 2.05g/cm3 and density of water equal to 1000kg/m3.

µ@15oC = 1.139 mPa-s

ρ @15oC = 999.103kg/m3

Question 9. Determine the diameter of a single-stage rock media filter to reduce an applied BOD5 of 125mg/L to 25mg/L. Use a flow rate of 0.14m3/s, a recirculation ratio of 12.0 and a filter depth of 1.83m. Assume the NRC equations apply and that the wastewater temperature is 20oC?

Question 10. Bacterial kill rate typically follows Chick’s law. If the first-order kill rate for a certain weak disinfectant is 0.067/h. Determine the time it will take to reduce the bacterial population to half of its original concentration?

Question 11. A town discharges 17,360 m3/d of treated wastewater into the Creek. The Creek has a flow rate of 0.43m3/s and the DO of the creek is 6.5 mg/L and DO of the wastewater is 1.0 mg/L. Compute the DO?

Question 12. The reaction for a biologically degraded contaminant is first order. The half-life of the contaminant is 3 week. Determine the degradation rate?

Question 13. A sewage lagoon that has a surface area of 10 ha and a depth of 1 m is receiving 8640 m3/d of sewage containing 100 mg/L of biodegradable contaminant. At steady state, the effluent from the lagoon must not exceed 20 mg/L of biodegradable contaminant. Assuming the lagoon is well mixed and that there are no losses or gains of water in the lagoon other than the sewage input, what biodegradation reaction rate coefficient must be achieved for a first-order reaction

Question 14. A colony of bacteria exhibit exponential growth behavior. If the number of bacteria doubles in 3 hours, how long will it take for the size of the colony to triple?

Question 15. A sample of diluted wastewater (diluted 10 times) has a 5 – day BOD of 5 mg/L. If the rate constant is 0.1/d, determine the ultimate BOD of the original wastewater?

Question 16.

Question 17. The rate of reaction for an enzyme- catalyzed substrate in a batch reactor can be described by the following relationship:

where k = maximum reaction rate; mg/ L min C= substrate concentration, mg/ L; K = constant, mg/ L

Using this rate expression, derive an equation that can be used to predict the reduction of substrate concentration with time in a batch reactor. If k equals 40 mg/ L.min and K= 100 mg/ L, determine the time required to decrease the substrate concentration from 1000 to 100 mg/ L.

C

K

kC

r

c

+

=

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