Test 21 - Self-Purification of Natural Streams

Question 1 ప్రశ్న 1

Which combination best describes natural stream self-purification after a biodegradable wastewater discharge?

A Only physical dilution, with no role for microorganisms or oxygen transfer. B Processes including dilution, sedimentation, biological oxidation and atmospheric reaeration, without guaranteeing complete safety. C Instantaneous elimination of organic contaminants once the effluent is mixed. D Conversion of all oxygen-demanding pollutants into dissolved oxygen by turbulence alone.

Question 2 ప్రశ్న 2

In the simple Streeter–Phelps model with natural-base constants, which pairing correctly identifies the instantaneous rates of deoxygenation and atmospheric reaeration?

A Deoxygenation = kr D and reaeration = kd L(t). B Deoxygenation = kd D and reaeration = kr L(t). C Deoxygenation = kd L(t) and reaeration = kr D. D Deoxygenation = kd L0 and reaeration = kr L0 at every downstream location.

Question 3 ప్రశ్న 3

At a given stream temperature and pressure, saturation DO is 9.2 mg/L and actual DO is 5.6 mg/L. What is the oxygen deficit?

A 3.6 mg/L B 14.8 mg/L C 0.61 mg/L D 5.6 mg/L

Question 4 ప్రశ్న 4

Which statement correctly describes the effect of increasing water temperature on stream dissolved-oxygen conditions, without assuming particular kinetic coefficients?

A Water always holds more saturated dissolved oxygen as temperature increases. B A temperature rise forces deoxygenation and reaeration constants to become equal. C An increase in temperature necessarily eliminates the oxygen sag. D Saturation DO generally decreases as temperature increases, while reaction-rate changes can also affect the sag.

Question 5 ప్రశ్న 5

At the critical point of a conventional dissolved-oxygen sag curve, which condition is correct?

A The oxygen deficit is zero and DO is at saturation. B The oxygen deficit is locally maximum and DO is locally minimum. C The remaining biodegradable oxygen demand is necessarily zero. D The rate of deoxygenation must be permanently zero downstream.

Question 6 ప్రశ్న 6

Starting immediately downstream of an appreciable organic-waste discharge and moving toward restored water quality, what is the conventional sequence of the four principal stream zones?

A Recovery, degradation, clear water, active decomposition. B Active decomposition, recovery, degradation, clear water. C Degradation, active decomposition, recovery, clear water. D Degradation, clear water, active decomposition, recovery.

Question 7 ప్రశ్న 7

Which observation is most characteristic of the conventional zone of active decomposition in a stream receiving heavy biodegradable pollution?

A Very low dissolved oxygen with intense organic decomposition and possible anaerobic conditions. B DO necessarily above saturation because photosynthesis exceeds all respiration. C No biodegradable matter and complete restoration of the upstream aquatic community. D Only unaffected river water with no change in microbial activity.

Question 8 ప్రశ్న 8

After the most oxygen-depleted reach, which condition most directly explains the increase in DO in the recovery portion of a simplified oxygen-sag model?

A The remaining BOD must increase faster than reaeration. B The oxygen deficit must become negative immediately. C Sedimentation ceases and atmospheric oxygen transfer becomes zero. D The reaeration rate exceeds the remaining deoxygenation rate, causing the deficit to decrease.

Question 9 ప్రశ్న 9

A stream-water mixture has initial ultimate carbonaceous BOD L0 = 20 mg/L and natural-base kd = 0.20 day^-1. After 3 days, what are the approximate BOD exerted and BOD remaining, respectively?

A 10.98 mg/L exerted and 9.02 mg/L remaining. B 9.02 mg/L exerted and 10.98 mg/L remaining. C 20.00 mg/L exerted and 0 mg/L remaining. D 4.00 mg/L exerted and 16.00 mg/L remaining.

Question 10 ప్రశ్న 10

Suppose a stream has no modeled deoxygenation load and only first-order reaeration acts, with D0 = 4 mg/L, kr = 0.40 day^-1 and constant saturation DO = 9 mg/L. What is DO after 2 days?

A 1.80 mg/L B 5.00 mg/L C 8.20 mg/L D 7.20 mg/L

Question 11 ప్రశ్న 11

For the classical model with L(t) = L0 exp(-kd t), deficit D(t), and constant natural-base kd and kr, what is the governing deficit equation?

A dD/dt = kd L0 exp(-kd t) - kr D B dD/dt = kd D + kr L0 exp(-kd t) C dD/dt = -kd L0 exp(-kd t) - kr D D dD/dt = kr D - kd L0 exp(-kd t)

Question 12 ప్రశ్న 12

At a particular time in a simplified stream model, kd = 0.25 day^-1, remaining ultimate BOD L = 12 mg/L, kr = 0.60 day^-1 and oxygen deficit D = 3 mg/L. What is the instantaneous trend?

A Deficit is decreasing at 1.2 mg/L per day. B Deficit is constant because the stream has a positive DO. C Deficit is increasing at 1.2 mg/L per day, so DO is falling if saturation is constant. D Deficit is increasing at 4.8 mg/L per day.

Question 13 ప్రశ్న 13

A river flows at 20 m^3/s with DO = 8 mg/L and receives 2 m^3/s of effluent with DO = 2 mg/L. Assuming immediate complete mixing, negligible initial reaction and saturation DO = 9 mg/L, what are mixed DO and initial deficit?

A 8.00 mg/L and 1.00 mg/L. B 7.45 mg/L and 1.55 mg/L. C 5.00 mg/L and 4.00 mg/L. D 7.45 mg/L and 2.00 mg/L.

Question 14 ప్రశ్న 14

A river carrying 12 m^3/s has ultimate carbonaceous BOD of 3 mg/L and receives an effluent of 3 m^3/s having ultimate BOD of 45 mg/L. With instantaneous mixing and no immediate reaction, what is the mixed ultimate BOD?

A 11.4 mg/L B 24.0 mg/L C 48.0 mg/L D 9.0 mg/L

Question 15 ప్రశ్న 15

Which expression correctly represents the classical Streeter–Phelps oxygen deficit D(t), if kd and kr are distinct natural-base day^-1 constants, L0 is mixed initial ultimate carbonaceous BOD, and D0 is the initial deficit?

A D(t) = L0 exp(-kr t) + D0 exp(-kd t) B D(t) = kd L0/(kr + kd) [exp(-kd t) + exp(-kr t)] C D(t) = D0 + (kd + kr)L0 t D D(t) = kd L0/(kr - kd) [exp(-kd t) - exp(-kr t)] + D0 exp(-kr t)

Question 16 ప్రశ్న 16

Use D(t) = [kd L0/(kr-kd)](exp(-kd t)-exp(-kr t)) + D0 exp(-kr t). For kd = 0.20 day^-1, kr = 0.50 day^-1, L0 = 12 mg/L, D0 = 1 mg/L and t = 2 days, what is the approximate deficit?

A 1.84 mg/L B 4.80 mg/L C 2.79 mg/L D 0.37 mg/L

Question 17 ప్రశ్న 17

A stream follows the classical model with D0 = 0, kd = 0.20 day^-1, kr = 0.60 day^-1, L0 = 10 mg/L and saturation DO = 9 mg/L. What is DO after 2 days?

A Approximately 7.15 mg/L B Approximately 1.85 mg/L C Approximately 9.00 mg/L D Approximately 5.45 mg/L

Question 18 ప్రశ్న 18

Under the classical Streeter–Phelps model with D0 = 0, kd = 0.20 day^-1 and kr = 0.60 day^-1, what is the time to an interior maximum deficit?

A Approximately 1.10 days B Approximately 5.49 days C Approximately 0.92 day D Approximately 2.75 days

Question 19 ప్రశ్న 19

For D0 = 0, L0 = 12 mg/L, kd = 0.20 day^-1, kr = 0.60 day^-1 and constant saturation DO = 9 mg/L, critical time is 2.7465 days. What are the approximate critical deficit and minimum DO?

A Critical deficit 6.69 mg/L and minimum DO 2.31 mg/L. B Critical deficit 2.31 mg/L and minimum DO 6.69 mg/L. C Critical deficit 4.00 mg/L and minimum DO 5.00 mg/L. D Critical deficit 0 mg/L and minimum DO 9 mg/L.

Question 20 ప్రశ్న 20

The modeled critical time from a wastewater outfall is 2.7465 days, and stream velocity is constant at 0.25 m/s. What is the downstream critical distance, assuming plug-flow travel?

A 14.83 km B 237.30 km C 2.75 km D 59.3 km

Question 21 ప్రశ్న 21

Use the classical critical-time expression tc = ln[(kr/kd){1 - D0(kr-kd)/(kd L0)}]/(kr-kd). For kd = 0.20, kr = 0.60 day^-1, L0 = 12 mg/L and D0 = 1 mg/L, what is tc when an interior maximum exists?

A Approximately 2.29 days B Approximately 2.75 days C Approximately 0.92 day D Approximately 4.58 days

Question 22 ప్రశ్న 22

At the wastewater mixing point, kd = 0.20 day^-1, L0 = 10 mg/L, kr = 0.50 day^-1 and D0 = 5 mg/L. Under the classical model with no additional loads, which conclusion best describes the immediate downstream oxygen-deficit trend?

A The deficit must initially rise because L0 is positive. B The deficit initially falls at 0.5 mg/L per day, so DO initially rises if saturation remains constant. C The deficit is stationary because kr is greater than kd. D The deficit initially rises at 4.5 mg/L per day.

Question 23 ప్రశ్న 23

Two otherwise identical modeled stream reaches have the same kd, L0, D0, temperature and travel time, but Reach X has a larger positive reaeration constant kr than Reach Y. In the classical model, which conclusion is justified?

A Reach X must have a greater oxygen deficit because its atmospheric exchange is larger. B The result is independent of reaeration because D0 is fixed. C Reach X generally has a lower modeled oxygen deficit after positive travel time due to faster oxygen replenishment. D Reach X necessarily has a greater biochemical deoxygenation rate kd L(t).

Question 24 ప్రశ్న 24

A river has constant velocity 0.50 m/s and saturation DO = 9 mg/L. At 43.2 km downstream, use the classical model with kd = 0.20 day^-1, kr = 0.60 day^-1, L0 = 12 mg/L and D0 = 1 mg/L; what DO is predicted?

A 7.45 mg/L B 2.17 mg/L C 5.33 mg/L D 6.83 mg/L

Question 25 ప్రశ్న 25

A treatment improvement lowers the initial mixed ultimate carbonaceous BOD L0 from 12 to 8 mg/L while leaving D0 = 1 mg/L, kd = 0.20 day^-1, kr = 0.60 day^-1 and saturation DO unchanged. In the classical model, how much higher is DO after 2 days?

A Approximately 0.74 mg/L higher B Approximately 4.00 mg/L higher C Approximately 0.37 mg/L higher D Approximately 2.00 mg/L higher

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