Planning a Chlorine Decay Test

Brian CarolanCwMAG Autumn Conference 2015

Introduction

• The various stages in planning a chlorine decay test will be discussed

• This will be done in the context of a fictitious case study

Planning a Chlorine Decay Test

Network

Service Res & WBS

ServiceRes

DMA1

DMA2

DMA3

N

• 3 DMAs, two supplies

• DMA 2 & 3 are gravity fed

• DMA 1 is pumped

• Low flows throughout the system

• Flushing required to maintain water quality

Planning a Chlorine Decay Test

Network Water Quality

Service Res & WBS

ServiceRes

N

• Chlorine residuals are lower than desired upon entry to the DMAs from the two sources

• Critical to water company that no water quality failures occur in this area

• Chlorine decay model required, to examine if water quality issues are within the network

Planning a Chlorine Decay Test

DMA1

DMA2

DMA3

Overview of Chlorination

Planning a Chlorine Decay Test

Toxic by-products possible

1.0 mg/l

Effective Disinfection

0.2mg/lIneffective Disinfection

• Chlorine decay occurs within the distribution system due to chemical reactions with– Materials in water (Bulk decay)– Materials along pipe surfaces (Wall decay)

WHO Guidelines

• Chlorine is added to water as a disinfectant to:– Inhibit bacterial growth– Prevent algal growth

Chlorine Decay Theory

1st Order Bulk Decay Equation (kb)

Arrhenius Equation

[Cl2] = chlorine concentration (mg/l)[Cl2] 0 = initial chlorine concentration (mg/l)t = time (days)A = Arrhenius pre-exponential factor (takes same units as kb)n = Order NumberEa = Activation energy (J/mol)kw = wall coefficientkf = mass transfer coefficientR = Gas constant (8.3144621 J/mol.K)T = Temperature (K)

Where:

Wall Decay Equation (kw)

Planning a Chlorine Decay Test

Chlorine Modelling

• Modelling is dependent on the accuracy of the following inputs to the model:– Initial chlorine concentration– Organic matter soluble in water (its concentration & nature)– Hydrodynamic conditions– Temperature

Planning a Chlorine Decay Test

• 1.9 – 5.0 fold increase in decay coefficient when temperature increases from 10 to 20oC (L.Monteiro et al.2015)

Chlorine Monitoring SoftwareInfoWorks

Maximum Velocity Age of Water

Planning a Chlorine Decay Test

Location Selection• Hydraulic modelling identified

locations with issues such as:

– Age of water

– Low velocities

• Monitoring also required at:– Sites

– Outlying locations (for good coverage)Service Res & WBS

ServiceRes

N

Planning a Chlorine Decay Test

DMA1

DMA2

DMA3

Chlorine Monitoring Equipment

Planning a Chlorine Decay Test

Hardware

Chloroclam chlorine monitor, courtesy of Evoqua Water TechnologiesATi chlorine monitor

Conclusions

Calibrated Hydraulic

Model

Location Selection for Monitoring

Points

Investigative Site Visit

Installation of Chlorine Monitors

Calibrate Chlorine Decay Model

Planning a Chlorine Decay Test

Thank you

Questions Welcome

Planning a Chlorine Decay Test 12