1. Numerical analysis of
water disinfection plant
and its performance
Vittesh Bahuguna
Dr. Mita Ray
November 04, 2016

2. Problem Formulation
• Numerical analysis of an existing chlorine contact
tank of dimension 110 m x 60 m x 5 m with 11
channels (10 baffles) to analyze the disinfection
parameters such as hydraulic efficiency, chlorine
decay and THM formation.
• ANSYS Fluent is used to obtain residence time
distribution (RTD) and further RTD is utilized in
MATLAB to solve major chemistry.

3. Problem Formulation
• New tank configuration with different baffles (but
same outline dimension) is also considered and all
above disinfection parameters are analyzed and
compared.
Existing Trial

4. Literature Review
• It is evident that as the number of baffles increases,
plug flow is approached. However the benefit
derived from the addition of baffles diminishes with
the increasing number of baffles based on Morill
Index.
• Beyond 5 to 7 baffles very little efficiency is gained.*
*Van der Walt (2002), University of Johannesburg

5. Literature Review
• Recommendation of optimum number of channels
as five.*
• Hydraulic efficiency of a rectangular tank with baffles
positioned in the direction of longest tank dimension
is superior to baffles placed along the shortest
dimension. **
*Johnson et al. (1998)
**Van der Walt (2002), University of Johannesburg

6. Literature Review
• Poorly placed baffles do not lead to improved
hydraulic efficiency. An example of circular tank.
Bishop et al. (1993)
• Baffling classification by Kawamura (2000)

7. Literature Review
• Chemical disinfection and predictive model for
chorine decay [Brown et al., 2010] [Angeloudis et al.,
2014]
𝒅𝑪 𝑪𝒍
𝒅𝒕
= −𝒌 𝑭𝑹 𝑪 𝑭𝑹 𝑪 𝑪𝒍 − 𝒌 𝒃 𝑪 𝑪𝒍
Model inputs
kb = 2.77 × 10-4 s-1; kFR= 4 × 10-3 s-1; CFR=CFR0 × e-0.01t; CFR0=1 mg/L; CCl0=2 mg/L
CCl-Chlorine concentration (mg/L); CFR-Fast reactant concentration (mg /L)
CFR0-Fast reactant inlet concentration (mg/L); kFR-Chlorine decay rate due to
fast reactant (s-1) ; kb-Bulk decay rate (s-1); CCl0-Chlorine inlet concentration
(mg/L); CFR0-Fast reactant inlet concentration (mg/L)

8. Literature Review
• Continuous stirred tank reactor (CSTR) model
incorporated with chlorine decay
𝒅𝑪 𝑪𝒍
𝒅𝒕
= (𝑪 𝑪𝒍𝟎−𝑪 𝑪𝒍)/τ − 𝒌 𝑭𝑹 𝑪 𝑭𝑹 𝑪 𝑪𝒍 − 𝒌 𝒃 𝑪 𝑪𝒍
Model inputs
kb = 2.77 × 10-4 s-1; kFR= 4 × 10-3 s-1; CFR=CFR0 × e-0.01t; CFR0=1 mg/L; CCl0=2 mg/L
CCl-Chlorine concentration (mg/L); CFR-Fast reactant concentration (mg /L)
CFR0-Fast reactant inlet concentration (mg/L); kFR-Chlorine decay rate due to
fast reactant (s-1) ; kb-Bulk decay rate (s-1); CCl0-Chlorine inlet concentration
(mg/L); CFR0-Fast reactant inlet concentration (mg/L); τ-Theoretical detention

9. Literature Review
• Pathogen decay based on Hom model [Angeloudis
et al., 2016] [Greene et al., 2002] [Haas et al., 1995]
[Hom et al., 1975]
𝒅(
𝑵
𝑵 𝟎
)
𝒅𝒕
= −𝒎𝒌𝑪 𝑪𝒍
𝒏
𝒕 𝒎−𝟏(
𝑵
𝑵 𝟎
)
Model inputs
m=1.2; n=0.96; k=8.04 × 10-4 (mg CCl/L)-n (s)-m
where
N-microorganism population
n-coefficient of dilution
m-empirical Hom model constant
k-inactivation rate [mg CCl/L)-n (s)-m]

10. Literature Review
• Formation of by-products model previously used in
the study for 2D geometries in accordance with
[Brown et al., 2011] [Zhang et al., 2000] [Amy et al.,
1987] [Singer et al., 1994]
𝑻𝑻𝑯𝑴 = 𝟎. 𝟎𝟎𝟑𝟎𝟔[(𝑻𝑶𝑪)(𝑼𝑽 𝟐𝟓𝟒)] 𝟎.𝟒𝟒
𝑪 𝑪𝒍
𝟎.𝟒𝟎𝟗
𝑻 𝒆
𝟎.𝟔𝟔𝟓
(𝒑𝑯 − 𝟐. 𝟔) 𝟎.𝟕𝟏𝟓
(𝑩𝒓 + 𝟏) 𝟎.𝟎𝟑𝟔
𝒕 𝟎.𝟐𝟔𝟓
Model input
TOC=4 mg/L; UV254=0.06 cm-1; Te=25 °C; Br=0.036 mg/L; pH=7
where
TTHM-total trihalomethanes (µg/L); TOC-total organic carbon; Te-Temperature (°C)
Br-bromide ion concentration; t-contact time (hours);
UV254-ultraviolet at 254 nm (cm-1)

11. Literature Review
• For understanding the implication of humic acid on
the formation of THM, following model is considered
where only humic acid is considered as dissolved
organic compound
𝑻𝑯𝑴 = 𝟖. 𝟐 × 𝟏𝟎−𝟒
(𝒑𝑯 − 𝟐. 𝟖) 𝑻𝑶𝑪 𝑪 𝑪𝒍
𝟎.𝟐𝟓
× 𝒕 𝟎.𝟑𝟔

12. Literature Review
• Another model used for prediction of THM formation
by [Boyella et al., 2004] has also been considered
𝑻𝑻𝑯𝑴 = 𝟎. 𝟎𝟎𝟏𝑪 𝒄𝒍
𝟑.𝟏𝟒
𝒑𝑯 𝟏.𝟓𝟔(𝑻𝑶𝑪) 𝟎.𝟔𝟗× 𝒕 𝟎.𝟏𝟕𝟓

13. Results
Table 1: Simulated parameters
Number
of baffles
Flow
rate,
m3/s TDT, s t10 t90
Baffling
Factor
(BF) t10/t90
Morill
Index
10 3 10833 7600 14800 0.7015 0.5135 1.9473
6 3 10833 7900 14500 0.7292 0.5448 1.8354

14. Results
Figure 7: Velocity vector for stream function at 20000 seconds for (a) 10 baffles
existing configuration and (b) 6 baffles trial configuration
(a) (b)

15. Results
Figure 1: RTD curves
τ=10833 s

16. Results
Figure 2: Chlorine decay
τ=10833 s

17. Results
Figure 3: THM formation, Boyella’s model
τ=10833 s

18. Results
Figure 4: THM formation, Singer’s model
τ=10833 s

19. Results
Figure 5: THM formation, Samuel D Faust
τ=10833 s

20. Results
Table 2: Predicted THM formation
S.No. Model Prediction of Total THM (max)
formation, µg/L
Existing design Proposed design
1. Boyella et al. 76.3683 76.8233
2. Singer et al. 55.2294 57.074
3. Samuel D Faust 22.539 23.2133

21. Results
Figure 6: Pathogen culture
τ=10833 s

22. Conclusion
• It seems that 5 baffles with the suggested orientation
provides better result w.r.t to baffling factor and
hydraulic efficiency (Morill Index)
• In the existing plant, where number of baffles
considered as 10 with different orientation,
performance could have been better if it were in
compliance with the above finding.

23. Conclusion
• Improvement in pathogen deactivation is observed in
trial design when compared with existing.
• THM formation using different models are predicted
for both existing and proposed design. THM formation
is slightly more for trial design due to more chlorine
residence within the tank.