Extending Hydraulics Modelling to Water Quality

George Kastl1, Ian Fisher2, Feng Shang1 and Michael Price1

1MWH 2Watervale Systems, PO Box 318, Potts Point NSW 1335, Australia

A NEW DECISION FRAMEWORK

Outline

• Acceptance of drinking water modelling• Drivers for drinking water quality modelling• Task Example• Modelled water quality parameters• Capability needed for drinking water quality

modelling• Tools required for modelling• State of various process models• Conclusion

Acceptance of drinking water modelling

• Hydraulic model for flow, pressure ...– Used for pipeline design– Pumping station– Provision of supply – Tank levels – Water age

• Water quality – Not routinely used – Academic interest (neural network, security)– Chlorine and THM

Drivers in drinking water quality modelling

• More stringent health regulations (DBP-THM, microbiological) & customers’ expectations

• Pressure on resources and use of lower quality sources

• Integration of water utilities and serving of larger geographical areas (longer residence time in the distribution system & multiple water sources)

• More complex operation of networks (balancing of water resources)

US Drivers for Water Quality Modelling

• Stage 2 DBP Rule ( THM <0.08mg/L, HAA <0.06mg/L, >95% of samples Cl>0.2mg/L)

Locational Running Annual Average (LRAA)– ISDE sampling to identify sample sites– Compliance required at all individual sample sites

• Total Coliform Rule Compliance– Measurable residuals in all TCR samples– Nitrification in chloraminated systems

• Contaminant Warning Systems– Potential overfeed of chemicals– Single source contamination (e.g., well supply)– Deliberate contamination

Task examples

• Existing DS, can it meet Cl (>95% >0.2mg/L) THM (max 0.2 mg/L)?

• What improvement can be achieved by a re-chlorination station(s)

• How to optimise operation of a DS (demand & temperature)• What would be chlorine and THM in new part of DS• A new WTP, what level of treatment guarantees the system

compliance?

0 0.2 0.6Increasing indicator failure

Desired level at tap for bacterial control

Increasing DBP & taste/odour problems

[Cl]

Disinfection Requirements

• Residual disinfectant declines with time

• Whether concentration stays within given limits (“envelope”) as time elapses depends on– water type (natural organic matter) – temperature– wall material and attached biofilm/particles

Why Water Quality modelling has low up take rate?

• Multidisciplinary – Chemical experiments– Chemical kinetics– Numerical analysis deriving parameters– Qualification of wall reaction– Network hydraulic model– Network water quality model

• Missing a good example (use of first order decay – not accurate)

9

Concept of bulk and wall reaction

distance (km)

Bulk Model

Measurement in system

Concentration

0

Bulk & wall reaction model

Reacted with bulk

Reacted with wall

Methods for Water Quality studies

1. Physical & online sampling and analysis, essential but costly and burdened by errors – only for existing systems.

2. Batch experiments and relating them via water age to network water quality

3. Batch experiments described by a simple Epanet water quality module and

4. Batch experiments described by chemical kinetics based MSX models

Batch experiments and relating them via water age to network water quality

Water Age

0 200 400 600 800 1000 12000

5

10

15

20

25

30

Plug flow

Back mixed

Run time [h]

Wa

ter

Ag

e

Chlorine concentration

0 100 200 300 400 500 600 700 800 900 10000

0.5

1

1.5

2

2.5

Plug flow

Back mixed

Run time [h]

Fre

e C

l [m

g/L

]

Chlorine decay description• Reaction scheme

– Cl + Fast → inerts + αTHM– Cl + Slow → inerts + αTHM– Cl → inerts + αTHM

• Rate equation:

Can be extended for multiple sources by having fast and slow components for each source

zSClSFClFCl kcckcckdt

dc

FClFF cckdt

dc SClS

S cckdt

dc

15

H2OMap InfoWater & MSX Multi-Species Extension

• Reaction rate in bulk• Reaction rate on surface• Equilibrium reactions

• Generic formulation of “any” kinetics scheme• Windows interface

Essentials for drinking water quality modelling

• Hydraulic & water quality software to project water quality processes into a distribution system, – MSX, originally by EPANET, available in H2OMap Water

• Quantitative description of processes of interest– Chlorine decay (bulk, walls & mixtures)– Chloramine decay (bulk, walls & mixtures)

• Method to derive model parameters

Quantitative description of processes of interest

• Accurate description of bulk reaction based on laboratory measurements including effects of:– Dose– Temperature– Re-chlorination

• Description of effect of wall (biofilm, sediment) based on field measurements

Status of chlorine and chloramine modelling

• Chlorine decay– reaction with DOC– modelled as 2 groups of organic compounds reacting with

chlorine– verified model used since 1994.

• Chloramine decay– has slow chemical decay (reduction with organics and

auto-oxidation ) – potentially fast (within a day) due to microbiologically

facilitated decay (harder to model)– can be described and modelled.

Happy Valley Treated water

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

3

3.5

4

HV 15C 2 mg/L HV 15C 2mg/L

HV 15C 3 mg/L HV 15C 3 mg/L

HV 25C 3 mg/L HV 25C 3 mg/L

HV 25C 4mg/L HV 25C 4 mg/L

Time [h]

Fre

e C

l [m

g/L

]

Desalinated water

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

3

3.5

4

DB 15C 2 mg/L DB 15C 2mg/Ll

DB 15C 3 mg/L DB 15C 3 mg/L

DB 25C 3 mg/L DB 25C 3 mg/L

DB25C 4mg/L DB 25C 4 mg/L

Time [h]

Fre

e C

l [m

g/L

]

50 % Happy Valley + 50% desalinated

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

3

3.5

4

DB50%HV 15C 2 mg/L DB 50% HV 15C 2 mg/L

DB50%HV 15C 3 mg/L DB50% HV 15C 3mg/L

DB 50%HV 25C 3 mg/L DB50% HV 25C 3 mg/L

DB 50% HV 25C 4mg/L DB 50% 25C 4 mg/L

Time [h]

Fre

e C

l [m

g/L

]

Wall reaction “equivalent diameter” proportional to surface reaction rate

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

0 0.5 1 1.5 2

Chlorine concentration (mg/L)

Eq

uiv

ale

nt

dia

me

ter

[mm

] Cle cd

15.0

3.0

p

ebs d

drr

Measurements vs. Model Elanora

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.000.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Average measured Cl [mg/L]

Av

era

ge

mo

de

l Cl [

mg

/L]

Conclusion

• Use of water age is not adequate for water quality modelling

• Only MSX enables accurate water quality modelling • Chlorine decay and THM concentration can be

accurately modelled in distribution systems (including mixtures of water)

• Sampling and modelling provides “best available” insight into what is happening in a distribution system

• Chloramine decay modelling is developing (more complex due to microbiological decay)

75 % Happy Valley + 25% desalinated

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

3

3.5

4

DB75%HV 15C 2 mg/L DB 75% HV 15C 2 mg/L

DB75%HV 15C 3 mg/L DB75% HV 15C 3mg/L

DB 75%HV 25C 3 mg/L DB75% HV 25C 3 mg/L

DB 75% HV 25C 4mg/L DB 75% 25C 4 mg/L

Time [h]

Fre

e C

l [m

g/L

]

25 % Happy Valley + 75% desalinated

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

3

3.5

4

DB25%HV 15C 2 mg/L DB 25% HV 15C 2 mg/L

DB25%HV 15C 3 mg/L DB 25% HV 15C 3mg/L

DB 25%HV 25C 3 mg/L DB 25% HV 25C 3 mg/L

DB 25% HV 25C 4mg/L DB 25% 25C 4 mg/L

Time [h]

Fre

e C

l [m

g/L

]

Chloramine decay description …. continuation

• Chemical decay rate slow & well described• Biologically assisted decay characterised by Fm

0 20 40 60 80 100 120 1400

0.5

1

1.5

2

2.5

NH2Cl mg/L

NH2Cl mg/L

chemical decay only

Model Fm=20

Time [h]

NH

2C

l [m

g/L

]

Chloramine decay description

• Chemical Reaction scheme– NH2 Cl → NH3+inert

– NH2 Cl + C → NH3+inert

• Microbiological decay– NH3 + O2 + AOB → NO2 + xAOB

– 4NH2 Cl + 3H2O + CRB → 3NH3 + 4HCl + HNO3+ xCRB

• Mixing - just combining microbial concentration??

Examples of chlorine decay modelling

• Maximizing of delivery area in the desirable Cl range (0.2-0.6mg/L)

• Optimizing the dose with temperature and flow• Re-chlorination optimization• THM compliance • Forecast of Cl & THM profile for “planned” system

and WTP process