1. Water Quality Management in Rivers

2. Dissolved Oxygen Depletion

3. Biochemical Oxygen Demand Measurement
Take sample of waste; dilute with oxygen saturated water; add nutrients and microorganisms (seed)
Measure dissolved oxygen (DO) levels over 5 days
Temperature 20° C
In dark (prevents algae from growing)
Final DO concentration must be > 2 mg/L
Need at least 2 mg/L change in DO over 5 days

4. Example
A BOD test was conducted in the laboratory using wastewater being dumped into a Lake. The samples are prepared by adding 3.00 mL of wastewater to the mL BOD bottles. The bottles are filled to capacity with seeded dilution water.

5. Example : Raw Data

6. Example : Calculations
What is the BOD5 of the sample?
Plot the BOD with respect to time.

7. Example : Time – Concentration Plot

8. Modeling BOD as a First-order Reaction
Organic matter oxidized
Organic matter remaining
Suppose we imagine a flask with some biodegradable organic waste in it. As bacteria oxidize the waste, the amount of organic material remaining decreases (black line). If the organic material is completely degradable eventually all of it will disappear. Another way to describe to organic matter in the flask is to say the converse: that the amount of organic matter oxidized already increases with time (red line). As the amount of organic matter remaining decreases, the amount of oxygen used to oxidize the waste has increased, but that which is remaining in the bottle decreases. What we often look at is OXYGEN DEMAND - the amount of oxygen needed to degrade a waste - the oxygen used follows the red line, the amount of oxygen demand remaining with time follows the black line - since there is less organic matter remaining there is less oxygen demand. We need to describe these relationships mathematically.

9. Modeling BOD Reactions
Assume rate of decomposition of organic waste is proportional to the waste that is left in the flask.

10. Lo BOD exerted BODt Lt L remaining Lo- Lt
Lo is the ultimate carbonaceous oxygen demand - amount of oxygen required by microorganisms to oxidize the carbonaceous portion of the waste to simple CO2 and water.
Ultimate carbonaceous oxygen demand is the sum of the amount of oxygen already consumed by the waste in the first t days (BODt) plus the amount of oxygen remaining to be consumed.

11. Ultimate Biochemical Oxygen Demand
Lt = amount of O2 demand left in sample at time, t
Lo = amount of O2 demand left initially (at time 0, no DO demand has been exerted, so BOD = 0)
At any time, Lo = BODt + Lt (that is the amount of DO demand used up and the amount of DO that could be used up eventually)
Assuming that DO depletion is first order
BODt = Lo(1 - e-kt)
Thus, Lo = BODt + Lt
BODt = Lo(1-e-kt)

12. Example
If the BOD5 of a waste is 102 mg/L and the BOD20 (corresponds to the ultimate BOD) is 158 mg/L, what is k?
BODt = Lo(1 - e-kt)
BOD5 = 102 mg/L
BOD20 = 158 mg/L  Lo
t = 5
102 mg/L = 158 mg/L(1 - e-k(5 days))

13. Example (cont)

14. Biological Oxygen Demand: Temperature Dependence
Temperature dependence of biochemical oxygen demand
As temperature increases, metabolism increases, utilization of DO also increases
kt = k20T-20
 = if T is between oC
 = if T is between oC

15. Example
The BOD rate constant, k, was determined empirically to be 0.20 days-1 at 20 oC.
What is k if the temperature of the water increases to 25 oC?
What is k if the temperature of the water decreases to 10 oC?
Use  = since T is between oC
k25 = (0.20)(1.056) k25 = 0.26 days-1
Use  = since T is between oC
k25 = (0.20)(1.135) k25 = days-1

16. Example

17. Nitrogenous Oxygen Demand
So far we have dealt only with carbonaceous demand (demand to oxidize carbon compounds)
Many other compounds, such as proteins, consume oxygen
Mechanism of reactions are different

18. Nitrogenous Oxygen Demand
Nitrification (2 step process)
2 NH3 + 3O2  2 NO2- + 2H+ + 2H2O
2 NO2- + O2  2 NO3-
Overall reaction:
NH3 + 2O2  NO3- + H+ + H2O
Theoretical NBOD =
When living things die or excrete waste products, the nitrogen that is tied up as complex organic chemicals is converted to ammonia by bacteria and fungi.
Aerobic environments:
Bacteria Nitrosomonas: converts ammonia to nitrite
Nitrobacter converts nitrite to nitrate

19. Nitrogenous Oxygen Demand

20. Nitrogenous oxygen demand
Untreated domestic wastewater
ultimate-CBOD = mg/L
ultimate-NBOD = mg/L
Total Kjeldahl Nitrogen (TKN) = total concentration of organic and ammonia nitrogen in wastewater: mg/L as N
Ultimate NBOD  4.57 x TKN

21. Other Measures of Oxygen Demand

22. Chemical Oxygen Demand
Chemical oxygen demand - similar to BOD but is determined by using a strong oxidizing agent to break down chemical (rather than bacteria)
Still determines the equivalent amount of oxygen that would be consumed
Value usually about 1.25 times BOD

23. Water Quality Management in Rivers

24. Dissolved Oxygen Depletion

25. Dissolved Oxygen Sag Curve

26. Mass Balance Approach
Originally developed by H.W. Streeter and E.B. Phelps in 1925
River described as “plug-flow reactor”
Mass balance is simplified by selection of system boundaries
Oxygen is depleted by BOD exertion
Oxygen is gained through re-aeration

27. Steps in Developing the DO Sag Curve
Determine the initial conditions
Determine the re-aeration rate from stream geometry
Determine the de-oxygenation rate from BOD test and stream geometry
Calculate the DO deficit as a function of time
Calculate the time and deficit at the critical point

28. Selecting System Boundaries

29. 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)

30. Determine Initial Conditions
Initial dissolved oxygen concentration
Initial dissolved oxygen deficit
where D = DO deficit (mg/L)
DOs = saturation DO conc. (mg/L)

31. 1. Determine Initial Conditions
DOsat is a function of temperature. Values can be found in Table.
Initial ultimate BOD concentration

32. 2. Determine Re-aeration Rate
O’Connor-Dobbins correlation
where kr = reaeration 20ºC (day-1)
u = average stream velocity (m/s)
h = average stream depth (m)
Correct rate coefficient for stream temperature
where Θ = 1.024

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

34. 3. Determine the De-oxygenation Rate
However, k = kd only for deep, slow moving streams. For others,
where η = bed activity coefficient (0.1 – 0.6)
Correct for temperature
where Θ = (4-20ºC) or (20-30ºC)

35. 4. DO as function of time
Mass balance on moving element
Solution is

36. 5. Calculate Critical time and DO