hw-18

CENE 599 Rural Water Wastewater Spring 2016

Homework #7

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Assigned: Wednesday 6 April Due: Wednesday 13 April in class Problem 1. Aerated Septic System Design. Use information from the Lecture 18 notes and the EPA manual that is cited in the notes to make some calculations on the aerated septic system that we studied in Lecture 17. Assume that the flow through the system is 500 gallons per day and the volume of the reactor is 500 gallons. Assume that the influent BOD is 200 mg/L and the effluent BOD is 30 mg/L. Also assume that the operating temperature is 22oC. Select the first order decay constant from the lecture notes.

a. Calculate the power [watts] necessary for diffused aeration to provide enough oxygen to treat the wastewater.

b. For the k value you used, combined with the design data above, calculate the predicted outflow BOD [mg/L]. Is it greater or less than the desired outflow value of 30 mg/L?

Problem 2. Aerobic Pond Design. Use the data below to design a partial mix aerated pond with two cells of equal volume. You can use Example 3-5 in the EPA Manual as a guide, but note that there are some mistakes in the calculations. The easiest way to solve this is using a spreadsheet. If you do, for full credit in grading, include example calculations so I can see how you made your calculations. I suggest these steps:

a. Design for winter conditions. b. Estimate the reaction rate k value using Equation 3-5 and a temperature higher than the winter

air temperature, but lower than the influent temperature. c. Calculate the total detention time using Equation 3-7. d. Calculate the volume of each reactor using the flow rate, number of cells, and detention time. e. Calculate the surface area of each cell using L:W = 3, and a depth of 4.0 m. For this problem,

assume the ponds have vertical walls. f. Check the pond temperature using the temperatures provided below, the surface area you just

calculated, and Equation 3-6. If this temperature is more than 10% different than your assumed temperature, then use the calculated temperature in Part b above and work your way back down to this point.

g. Calculate the effluent concentration from Cell 1 using the equation we derived on Lecture 18 Page 2 (see “markup” notes in BBLearn).

h. For the second cell, calculate the water temperature (Tw) and rate constant k at that temperature, then calculate the BOD. The volume, surface area, and HRT stay the same for all cells.

i. For the third cell, make the same calculations using the temperature of the influent water from Cell 2.

j. The concentration is slightly higher than 30 mg/L because of the decreasing k values as temperature decreased in the cells.

CENE 599 Rural Water Wastewater Spring 2016

Homework #7

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k. Prepare a summary table showing the volume, length, width, and depth of your ponds, and provide a sketch.

Data for Problems 2 and 3: Design flow rate = 1,200 m3/day Influent BOD5 = 250 mg/L Effluent BOD5 = 30 mg/L Reaction rate at 20 oC = 0.242 day-1 Influent temperature oC = 12 oC Summer air temp. oC = Ta = 27 oC Winter air temp. oC = Ta = 9 oC Temperature correction coefficient = 1.09

Problem 3 (graduate students). Continue with the design of the two-cell system:

a. Calculate the organic loading to each of the two cells [kg BOD/day]. b. Calculate the oxygen demand [kg O2/day] for each of the two cells using the data from Lecture

18, Page 11. c. Calculate the power requirements [kW] for diffused aeration equipment using the data from

Lecture 18, Page 11.