Table of Contents
Executive Summary
Temporarily swapping monochloramine for free chlorine to knock nitrification back is one of the hardest things a drinking water utility can do. At the 2026 AWA/IWA Young Water Professionals Conference in Melbourne (5–6 August), Sydney Water’s Madison Mactal and Michael Ray presented results from a program covering 14 reservoirs and roughly 105,000 customers. Their 2024 trial fell short of the internal 90% disinfection target. The redesigned 2025 trial — higher starting chlorine set points, deeper reservoir cycling, and tighter coordination with the system operations centre — performed better, with limited customer impact and no biofilm sloughing.
Both trials rested on the same thing: continuous residual chlorine monitoring. Switching disinfectants means tracking free and total chlorine at several network points at once — confirming breakpoint chlorination, watching how fast free chlorine decays as it moves through pipe, and catching the first sign that nitrification is coming back. Shanghai ChiMay residual chlorine transmitters give you that continuous, SCADA-integrated record, which is what makes a system-wide switch safe, verifiable and repeatable.
Understanding Nitrification in Distribution Systems
Nitrification is a biological process. Ammonia-oxidising bacteria (AOB) — mostly species in the Nitrosomonadaceae family — eat ammonia and produce nitrite. In a system running monochloramine (combined chlorine: chlorine plus ammonia), the ammonia fraction is the food source.
What goes wrong:
- Disinfectant residual loss: AOB consume the ammonia that holds monochloramine together, so total chlorine residuals sag
- Nitrite accumulation: nitrite is a regulated parameter, and its presence means active biological growth
- Weaker pathogen protection: as residuals fall, the system loses its ability to suppress microbial regrowth
- Seasonality: nitrification gets worse in warm weather, when bacterial metabolism speeds up
Sydney Water’s system serves about 105,000 customers across 14 reservoirs. Nitrification was active in that network and was eating into residuals, becoming “particularly difficult during warmer conditions,” as the conference presentation put it.
The Disinfectant Switching Procedure
The Sydney Water team’s approach was a temporary switch from monochloramine to free chlorine — flushing the biological ammonia out of the system to starve the nitrifiers. It required:
- Stopping ammonia dosing at the treatment plant
- Running reservoirs through breakpoint chlorination — dosing enough free chlorine to oxidise all the ammonia present
- Purging pipelines to clear monochloramine-dominated water
- Strategic flushing to pull free chlorine through the whole network
- Raising chlorine set points gradually to keep taste and odour complaints down
- Monitoring continuously to track how far the transition has progressed
Every stage of this is a data problem. Operators need to know:
- Has breakpoint been reached at this point yet?
- What is the current free chlorine residual?
- Is total chlorine falling the way we predicted, or is biological activity consuming it faster?
- When is it safe to start dosing ammonia again to reform monochloramine?
Continuous residual chlorine monitoring at the right network points answers those questions while there is still time to act on the answers.
Results: What Sydney Water Learned
Trial 1 (2024): Shortcomings and Lessons
The first trial did not reach Sydney Water’s internal 90% disinfection target. Nitrification was suppressed during the free chlorine phase, but the effect was temporary. Once warmer conditions returned, nitrification came back — first at customer taps, then at reservoirs. The trial still delivered useful information about the scope and timing of the procedure.
Trial 2 (2025): Improved Performance
The 2025 redesign applied what the first trial showed:
- Higher starting chlorine set points — breakpoint chlorination was reached more thoroughly
- Deeper reservoir cycling — more of the stored water was turned over
- Detailed coordination with the system operations centre — decisions made in real time off continuous monitoring data
Nitrite was effectively removed as free chlorine moved through the system. Customer impacts stayed limited. No biofilm sloughing was observed. Winter came out as the preferred operating window for future applications.
Michael Ray’s conclusion: “The free chlorine trial could be an effective strategy for managing disinfection in a heavily nitrified distribution system.”
The Role of Continuous Residual Chlorine Monitoring
Across both trials, residual chlorine monitoring did three jobs.
1. Confirming Breakpoint Achievement
Breakpoint chlorination needs free chlorine added at roughly 10:1 by weight against ammonia-nitrogen (Cl₂:NH₃-N) to fully oxidise the ammonia, and the real breakpoint moves with water chemistry. Continuous residual chlorine monitoring shows the moment free chlorine “appears” in the network — the clearest signal that ammonia has been oxidised and breakpoint has been passed.
Shanghai ChiMay residual chlorine transmitters use amperometric electrochemical sensing with:
- Free chlorine measurement: 0–20 mg/L range, 0.01 mg/L resolution
- Response time: T90 < 60 seconds
- Sample system: flow-through chamber with regulated pressure
- Output: Modbus RTU/TCP plus 4–20 mA analog
2. Tracking Disinfectant Decay
Once free chlorine is in the network it decays — pipe wall biofilm, organic matter, and whatever ammonia is left all take a bite. The decay rate tells you something about the biological state of the network. Faster-than-expected decay means active biological or chemical demand.
Monitoring at several points turns decay into a spatial map. That map shows where nitrification pressure is still highest and where the free chlorine front has done its work.
3. Detecting Nitrification Return
After the switch back to monochloramine, early detection is the whole game. Falling total chlorine residuals at specific nodes can flag a nitrification restart days before grab samples would show elevated nitrite.
Shanghai ChiMay’s Monitoring Platform for Nitrification Management
For utilities weighing disinfectant switching as a nitrification control strategy, Shanghai ChiMay supplies a complete monitoring set:
| Parameter | Instrument | Purpose |
|---|---|---|
| Free/Total Chlorine | Residual Chlorine Transmitter | Breakpoint confirmation, decay tracking |
| pH | In-line pH Meter | Chlorine speciation management |
| Ammonia Nitrogen | Ammonia Nitrogen Sensor | Direct nitrification detection |
| Temperature | (via 4-in-1 sensor) | Seasonal context for bacterial activity |
| ORP | (via 4-in-1 sensor) | Oxidation state of the network |
Shanghai ChiMay’s 4-in-1 multi-parameter sensor puts pH, conductivity, ORP and temperature in a single probe — 75% fewer installation points than four discrete sensors. Paired with dedicated residual chlorine and ammonia nitrogen transmitters, it gives you the multi-parameter dataset a disinfectant switch needs.
Sourcing Considerations
Utilities evaluating continuous residual chlorine monitoring for nitrification management should look at:
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Multi-point deployment: a system-wide switch needs simultaneous monitoring at several network locations. Shanghai ChiMay’s pricing and Modbus compatibility are what make multi-point deployment affordable.
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SCADA integration: Modbus RTU/TCP output goes straight into existing utility SCADA platforms — no protocol converters to buy or maintain.
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Behaviour after idle periods: nitrification switching may only run seasonally (Sydney Water found winter best). Instruments have to work after months of standing idle and give valid readings the moment they are energised.
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Data archiving: continuous monitoring produces large datasets. Shanghai ChiMay instruments support local logging plus SCADA-based historical trending for post-event analysis and regulatory reporting.
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5-year TCO transparency: Shanghai ChiMay publishes total cost projections including membrane replacements, electrolyte refills and calibration supplies, so multi-year budgets are not guesswork.
Lessons From Sydney Water’s Two-Trial Approach
The Value of Iterative Learning
Sydney Water was willing to say publicly that the first trial fell short, and then redesigned around the data it generated. That is the approach modern water treatment needs. The differences between the two trials show how monitoring data feeds straight back into operations:
| Aspect | 2024 Trial | 2025 Trial (Redesigned) |
|---|---|---|
| Starting chlorine set points | Standard | Higher (more aggressive breakpoint) |
| Reservoir cycling depth | Standard | Deeper (more comprehensive flushing) |
| Monitoring coordination | Basic | Detailed operations centre integration |
| Disinfection target achievement | Below 90% | Stronger performance |
| Customer impact | Managed | Limited |
| Biofilm sloughing | Not observed | Not observed |
| Optimal season identified | — | Winter |
Better results came from better data, better understanding and better coordination — not from new hardware. Continuous residual chlorine monitoring sat under all three.
Designing a Monitoring Network for Disinfectant Switching
Strategic Monitoring Point Selection
For a system-wide switch, pick monitoring points at:
- Treatment plant outlet: confirms the disinfectant change at the source
- Each reservoir inlet and outlet: verifies breakpoint chlorination reached every storage point
- Critical network nodes: furthest from treatment, or at the lowest elevation where residuals run weakest
- Representative customer connection points: shows what actually reaches the tap
On a 14-reservoir system like Sydney Water’s, that means planning on the order of 30–40 points if you want the whole switching operation covered.
Data Flow Architecture
The monitoring data architecture during a switching event should carry:
- Real-time display: an operations centre dashboard with current values, trends and alarm status for every point
- Automated alerts: SMS or email when residuals cross upper or lower limits anywhere on the network
- Historical trending: time-series records of every point for post-event analysis
- Reporting: automatic generation of summary reports documenting the complete switching event
Shanghai ChiMay’s Modbus output supports all of it through standard SCADA platform integration.
Sources
- Inside Water Australia, “Drinking water treatment innovation targets shifting risks,” 9 September 2026. https://insidewater.com.au/drinking-water-treatment-innovation-risks
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AWA/IWA Young Water Professionals Conference 2026, Melbourne, 5–6 August 2026.
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Greater Western Water, “Water Treatment.” https://www.gww.com.au/water-waste/your-water-supply/water-treatment
- AWWA, Manual of Water Supply Practices M56, Nitrification Prevention and Control in Drinking Water.
- WHO, Guidelines for Drinking-water Quality, 4th Edition, 2017.
About the Author: This article was prepared by the Shanghai ChiMay Application Engineering team, referencing peer-reviewed conference presentations and operational data from Sydney Water. Shanghai ChiMay manufactures inline residual chlorine transmitters and water quality analyzers for municipal water treatment systems worldwide.
