Water Quality Management

RIVER WATER QUALITY MANAGEMENT

River Ganga in upper Stretch

River Ganga in Middle Stretch

River Yamuna in upper Stretch

River Yamuna in Middle Stretch

Addition of several drains into the river Yamuna

Water Quality Management in Rivers

Dissolved Oxygen Depletion

Dissolved Oxygen Sag Curve

Mass Balance Approach
• Originally developed by H.W. Streeter and E.B. Phelps in 1925 • Oxygen is depleted by BOD exertion

• Oxygen is gained through reaeration

Selecting System Boundaries

Initial Mixing
Qw = waste flow (m3/s) DOw = DO in waste (mg/L) Lw = BOD in waste (mg/L) Qr = river flow (m3/s) DOr = DO in river (mg/L) Lr = BOD in river (mg/L) Qmix = combined flow (m3/s) DO = mixed DO (mg/L) La = mixed BOD (mg/L)

Remember: We have to Use Ultimate BOD for DO predictions

1. Determine Initial Conditions
a. Initial dissolved oxygen concentration Qw DOw  Qr DOr DO  Qw  Qr b. Initial dissolved oxygen deficit

D  DOs  DO

where D = DO deficit (mg/L) DOs = saturation DO conc. (mg/L)

Qw DOw  Qr DOr Da  DOs  Qmix

1. Determine Initial Conditions
DOsat is a function of temperature. Values can be found in Table (Gilbert Masters) c. Initial ultimate BOD concentration

Qw Lw  Qr Lr La  Qw  Qr

2. Determine Reaeration Rate
a. O’Connor-Dobbins correlation 1/ 2 3.9u kr  3 / 2 h where kr = reaeration coefficient @ 20ºC (day-1) u = average stream velocity (m/s) h = average stream depth (m) b. Correct rate coefficient for stream temperature

kr  kr , 20

T  20

where Θ = 1.024

Determine the De-oxygenation Rate
a. rate of deoxygenation = kdLt where kd = deoxygenation rate coefficient (day-1) Lt = ultimate BOD remaining at time (of travel downstream) t b. If kd (stream) = k (BOD test)

Lt  L0e and  kd t

rate of deoxygentation  kd L0e kd t

3. Determine the De-oxygenation Rate
c. Correct for temperature

kr  kr , 20

T  20

where Θ = 1.135 (4-20ºC) or 1.056 (20-30ºC)

4. DO