Water Quality L14-16

Question 1

Rate of re-aeration

Answer 1

d(DO)/dt = K_r(DO_sat - DO)

K_r = re-aeration constant

DO_sat = saturated dissolved oxygen concentration

DO = actual dissolved oxygen concentration in the water

Question 2

Greater difference between DO_sat - DO in d(DO)/dt = K_r(DO_sat - DO) means

Answer 2

Faster re-aeration

Question 3

Oxygen deficit

Answer 3

D = DO_sat - DO

Question 4

Travel time, t_t can also be represented by

Answer 4

x/u

Question 5

What occurs at the lowest steady State O2 concentration?

Answer 5

The critical point

Question 6

How can the critical point be found mathematically?

Answer 6

Differentiating deficit equation, setting it equal to 0, and solving for travel time.

Question 7

For what should the critical point be determined for?

Answer 7

The worst case scenario

Question 8

How are flowrates and temperatures characterized in the summer?

Answer 8

Lowest flows and highest temps

Question 9

What occurs in the summer regarding dilution?

Answer 9

Less dilation

Question 10

What occurs in the summer regarding O2 saturation?

Answer 10

Lower O2 saturation

Question 11

What occurs in the summer regarding velocity?

Answer 11

Lower velocities (lower kr)

Question 12

What occurs in the summer regarding the rate coefficient?

Answer 12

Higher kd

Question 13

How can engineers manage a critical point that is too low?

Answer 13

Adjust Do (oxygen deficit after mixing)
Adjust L0,mix (ultimate BOD in water after mixing)

Question 14

Describe a DO vs. distance graph for clean water in words

Answer 14

The DOsat is at 9 mg/L, and 9 - the y intercept is the initial deficit. The critical point is the lowest point on the curve, where the lowest DO occurs.

Question 15

Describe the oxygen sag curve.

Answer 15

You start off in clean zone where there isn’t a lot of waste causing consumption of dissolved oxygen, so the BOD is low. All of sport fish and flies indicative of healthy water are present. In the decomposition zone, there is a pipe putting waste in the river and there is a spike in BOD that causes dissolved oxygen to go down until it reaches a minimum point in the septic zone. Few fish are in the decomposition zone, and nome are present in the septic zone.eventually the river enters the recovery zone where the DO recovers through Henry’s law because the BOD is getting consumed until you get back to a clean level.

Question 16

Two competing reactions occur in the Streeter-Phelps model:

Answer 16

Re-aeration and deoxygenatinon

Question 17

Aquatic plants may produce oxygen, because they carry out the process of photosynthesis. (True/false)

Answer 17

True

Question 18

The ______ is the point in a river where the dissolved oxygen concentration is lowest.

Answer 18

Critical point

Question 19

A large oxygen deficit in a stream is desirable or undesirable?

Answer 19

Undesirable

Question 20

Only carbonaceous compounds can cause biochemical oxygen demand (true/false)

Answer 20

False

Question 21

_____ and _______ are two main nutrients that should be removed from wastewaters before discharge

Answer 21

Nitrogen and phosphorus

Question 22

Why is phosphorus problematic?

Answer 22

It causes the growth of algae- which cause oxygen demand when they die

Question 23

As a means of reducing phosphorus inputs to the environment, some detergents/fertilizers no longer contain phosphate

Answer 23

Detergents

Question 24

The lowest do concentration in a stream is often observed in the summer/winter

Answer 24

Summer

Question 25

Where do electrons go in oxidation problems?

Answer 25

The left side of the equation

Question 26

Where do electrons go in reduction problems?

Answer 26

The right side of the equation