CEE 370 Spring 1997

Exam #2

April 15, 1997

Closed book, 2 sheets of notes allowed.

Please answer all of questions #1 through #5 and one of either #6 or #7. Show all work. Be neat, and box-in your answer.

1. BOD calculations (20 points)
2. The Northampton wastewater treatment plant discharges 4.5 MGD of treated wastewater with a BOD5 of 7.5 mg/L. What is the loading in lb of ultimate BOD per day (i.e., lb-BODu/d). Assume a BOD rate constant (k) of 0.18 d-1.

Solution to #1

First calculate the ultimate BOD in the effluent

then multiply this by the flow with the appropriate conversion factor.

3. Gas Transfer (20 points)
4. You have designed a batch diffused aeration system for transferring oxygen into a groundwater. The tank holds 5000 gal, and it has a gas transfer coefficient (KLa) of 0.12 min-1. If the raw groundwater has a DO of 1.5 mg/L and the treated water must have a DO of 7.5 mg/L how may gallons per day can be treated by this system? Assume that the saturation value (Cs) is 10.2 mg/L. Ignore the time required to fill and drain the tank.

Solution to #2

First calculate the time required to process one batch of 5000 gal.

Now calculate the number of batches per day and the total volume treated:

# batches = 24hrs(60min)/9.75min = 147.7

Volume treated = 147.7 batches/day * 5000 gal/batch

= 738,000 gal/day

= 0.74 MGD

5. Process Sizing (15 points)
6. A conventional water treatment plant is to be constructed to process 185,000 m3/d. Pilot-plant analysis indicates that an overflow rate of 5 m/hr will be acceptable in the gravity settling tanks. Assuming a surface configuration of approximately 12x30 m, how many settling tanks will be required? Allow one unit out of service for cleaning.

Solution to #3

Thus, a minimum of 5 settling tanks are needed, and allowing 1 to be continually out of service for backwash, means that 6 tanks are required.

7. Chemical Dosing (10 points)
8. How many mg/L of hypochlorous acid are required to oxidize 0.8 mg/L ferrous iron?

Solution to #4

First balance the equation:

Next calculate the mass requirements

Since most forms of chlorine are expressed as Cl2, the more accepted answer would be:

1. Define total hardness and carbonate hardness (3 points)
2. What is the meaning of "CT" when discussing disinfection? (3 points)
3. Describe conventional drinking water treatment. Identify 2 treatment processes that are not considered "conventional" (4 points)

Total hardness: sum of calcium and magnesium concentration in a water

Carbonate hardness: portion of the total hardness that is matched by the anions, carbonate and bicarbonate

The product of concentration (C ) times time (t) that gives a certain degree of kill of a given organisms with a specific disinfectant.

Includes rapid mix, coagulation, flocculation, settling, filtration.

Non-conventional processes may include, GAC adsorption, biological filtration, flotation, membrane processes, intermediate ozonation.

1. Gravity Settling (25 points)
2. A settling tank has a width of 20 ft, a depth of 10 ft and a length of 60 ft. It receives 0.60 ft3/s of flow. What fraction of all 0.10 mm diameter alum floc particles will be removed at 5oC (assume 1.04 g/mL floc density)?

Solution to 6:

3. Intercontinental Paper and the Mill River (25 points)

The Intercontinental Paper Co. is discharging its wastewater directly into the Mill River. The discharge flow is 3.8 ft3/s (cfs) the discharge D.O. is 8.5 mg/L and the discharge ultimate BOD (BODu) is 35 mg/L. They obtain half of this water from an intake 0.5 miles upstream of the wastewater outfall, and half from groundwater via a nearby well field. On average, the Mill River water upstream of the IPC outfall has an ultimate BOD (BODu) of 2.5 mg/L and a D.O. of 8.5 mg/L. If the Mill River has a flow of 12 cfs upstream of the IPC intake, and if the state permits a minimum DO of 7.5 mg/L in the Mill River, will the state have to further restrict the BOD in IPC's wastewater (i.e., is the stream out of compliance)? Calculate the minimum stream D.O. expected as a result of the IPC discharge.

Additional assumptions: BOD deoxygenation rate: k1 = 0.23 d-1

reaeration rate constant (k2) of 0.82 d-1

D.O saturation concentration = 9.5 mg/L

River flow velocity = 0.5 ft/s

Solution to #7

First the downstream flow is:

The solve for the in-stream BODu concentration at the point of discharge using a mass balance:

Now determine the critical travel time:

Yes, the stream will be out of compliance.

Appendix

Some physical constants of Water:

 Temp., oC Density, kg/m3 Viscosity, N-s/m2 Kinematic Viscosity, m2/s 0 999.8 1.781x10-3 1.785x10-6 5 1000.0 1.518 x10-3 1.519x10-6 10 999.7 1.307 x10-3 1.306 x10-6 15 999.1 1.139 x10-3 1.139 x10-6 20 998.2 1.002 x10-3 1.003 x10-6 25 997.0 0.890 x10-3 0.893 x10-6 30 995.7 0.798 x10-3 0.800 x10-6 35 994.0 0.725 x10-3 0.729 x10-6 40 992.2 0.653 x10-3 0.658 x10-6

Selected Chemical Constants

 Element Symbol Atomic # Atomic Wt. Valence Electronegativity Aluminum Al 13 26.98 3 1.47 Boron B 5 10.81 3 2.01 Calcium Ca 20 40.08 2 1.04 Carbon C 6 12.01 2,4 2.50 Cerium Ce 58 140.12 3,4 1.06 Chlorine Cl 17 35.453 1 Holmiuum Ho 67 164.93 3 1.10 Hydrogen H 1 1.01 1 2.20 Magnesium Mg 12 24.31 2 1.23 Manganese Mn 25 54.94 2,3,4,6,7 1.60 Osmium Os 76 190.2 2,3,4,8 1.52 Oxygen O 8 16.00 2 3.50 Potassium K 19 39.10 1 0.91 Sodium Na 11 22.99 1 1.01 Sulfur S 16 32.06 2,4,6 2.44

Useful conversion factors

1 ft = 0.305 m