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ACME Industries Inc. discharges process wastewater to a stream. Their permit mandates that their discharged water contain less than 0.500 mg/L of...

1. ACME Industries Inc. discharges process wastewater to a stream. Their permit mandates that their discharged water contain less than 0.500 mg/L of DMCW. The wastewater leaving the plant has a DMCW concentration of 64.0 mg/L. To solve this problem, ACME decides to build effluent holding lagoons to allow their effluent to biodegrade before discharging to the stream. The process involves filling a lagoon; then when the DMCW concentration is below the discharge limit, the lagoon is emptied.
(a) Laboratory tests suggest that the half-life of DMCW in the lagoons will be about 7 days. How many days are required from the time the lagoon is filled until the time it may be emptied?
(b) If the lagoons are emptied after two weeks (14 days), what will be the concentration discharged to the stream? (assuming the half-life given in part (a))
(c) What is the first-order rate constant which corresponds to this half-life?

2. A biological treatment basin has a hydraulic residence time of 8.0 hours. The influent to the basin contains 250 mg/L organic matter (BOD). Tests have shown that the first order decay rate for this wastewater is 0.25 hour-1. The basin is rectangular, and we’re not sure if a PFR or a CSTR is a more appropriate model.

(a) What is the effluent BOD if the basin is modeled as a plug flow reactor (mg/L)?
(b) What is the effluent BOD if the basin is modeled as a CSTR (mg/L)?

For short periods, the system must be run with a hydraulic residence time of 45 minutes. What is the effluent BOD concentration in this case for a
(c) Plug flow reactor
(d) CSTR

Use a spreadsheet to plot the effluent concentration versus hydraulic residence time for times of 1 minute to 24 hours with
(e) Concentration plotted on a linear scale
(f) Concentration plotted on a log scale
(g) Make some observations on what you found

3. A sourdough bread starter is fed by adding 1 gram of sourdough microorganisms to 1 liter of water with sugar and potato starch. The starter has a growth rate constant of 1.39 hr-1, which means it doubles in mass about every 30 minutes.

a. how many grams of microbes do we have after 1 hr?
b. …………………….. 12 hr?
c. …………………….. 24 hr?
d. how much time does it take to reach 1 kg mass?
e. … to contain the mass of the earth (5.97 x 1024 kg)?
f. … to contain the mass of the universe (~ 1053 kg)?

This question was asked on Jan 08, 2013.

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