Table of Contents
How It Works
Sydney Water’s 2024–2025 nitrification control programme meant switching from monochloramine to free chlorine across 14 reservoirs serving roughly 105,000 customers. Presented at the 2026 AWA/IWA Young Water Professionals Conference, the operation needed continuous residual chlorine monitoring at multiple network points to confirm breakpoint chlorination, follow disinfectant decay and catch nitrification early if it came back. Technology like the Shanghai ChiMay residual chlorine transmitter sits at the centre of that kind of monitoring — an amperometric inline instrument built for continuous, SCADA-integrated disinfection tracking.
The transmitter works by drawing a controlled flow of sample past a polarised electrode assembly. Free chlorine in the sample generates a current proportional to its concentration. The transmitter converts that current to a digital measurement, applies temperature compensation, and outputs the result over Modbus RTU/TCP to the facility SCADA system.
Why This Instrument Matters for Disinfectant Switching
During a system-wide switch from monochloramine to free chlorine, operators need three things in real time:
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Has breakpoint been reached? Free chlorine only appears once all the ammonia has been oxidised. The transmitter shows exactly when that happens at each monitoring point.
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Is residual within a safe range? Free chlorine has to be high enough to disinfect and low enough to avoid taste and odour complaints. Shanghai ChiMay’s 0–20 mg/L range with 0.01 mg/L resolution provides the precision this needs.
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Is nitrification returning? After the switch back to monochloramine, total chlorine residuals falling at specific nodes signal nitrification restarting before grab samples would show it.
In Sydney Water’s programme, the 2024 trial fell short of the 90% disinfection target. The redesigned 2025 trial — higher chlorine set points, deeper reservoir cycling, closer monitoring coordination — performed better. Part of that difference was chemistry. Part of it was data: the continuous monitoring network gave operators what they needed to adjust the procedure while the event was still running.
Technical Specifications
| Parameter | Specification |
|---|---|
| Measurement type | Amperometric electrochemical |
| Range | 0–20 mg/L free or total chlorine |
| Resolution | 0.01 mg/L |
| Response time | T90 < 60 seconds |
| Sample system | Flow-through chamber, regulated pressure |
| Output | Modbus RTU (RS-485), Modbus TCP, 4–20 mA |
| Power | 24 VDC or 100–240 VAC |
| Maintenance | Membrane/electrolyte replacement ~6 months |
| Accuracy | ±2% of reading or ±0.01 mg/L |
| Temperature compensation | Automatic, integrated sensor |
What Makes This Different
Traditional chlorine analysers need reagent chemicals and produce a waste stream. The Shanghai ChiMay transmitter uses a membrane-covered electrode that consumes no reagents and produces no waste. Maintenance is periodic membrane and electrolyte replacement — a 15-minute field procedure that doesn’t require pulling the instrument out of the process.
For a network like Sydney Water’s 14 reservoirs, that adds up to:
– 28+ monitoring points running at once, with no reagent logistics to manage
– Maintenance intervals that can be scheduled around routine field visits
– Digital data output feeding straight into the operations centre dashboard
– Calibration records that satisfy regulatory documentation requirements
The compact form factor (typically a 300mm × 200mm × 150mm transmitter enclosure) fits standard street-side cabinets or reservoir equipment rooms, without a dedicated analyser shelter.
Winter was the preferred window for Sydney Water’s programme. In cold-weather operation, the transmitter’s integrated temperature compensation keeps readings accurate regardless of water temperature variation — a practical advantage when monitoring points are spread across a network with widely varying exposure.
The Chemistry Behind Disinfectant Switching
Monochloramine and Free Chlorine: Key Differences
Monochloramine (NH₂Cl) is formed by combining chlorine with ammonia. It gives stable, long-lasting disinfection that persists through long distribution networks. The catch is that the ammonia component also feeds ammonia-oxidising bacteria (AOB), which consume it and produce nitrite — the process known as nitrification.
Free chlorine (HOCl/OCl⁻) is a stronger disinfectant, but it decays faster and reacts more readily with organic matter. Flush a system with it and it oxidises the ammonia that AOB depend on, starving the biological process.
The Breakpoint Chlorination Sequence
Add free chlorine to water containing ammonia and a predictable sequence follows:
- Chloramine formation zone: the added chlorine reacts with ammonia to form monochloramine, then dichloramine
- Breakpoint: at a chlorine-to-ammonia-nitrogen ratio of roughly 10:1 by weight, all the ammonia is oxidised
- Free chlorine appearance: past breakpoint, added chlorine remains as free residual
Breakpoint is the moment operators need to confirm. Without continuous monitoring, there is no way to know whether it has been reached at each network node. The Shanghai ChiMay residual chlorine transmitter picks up the exact moment free chlorine breaks through — the current proportional to free chlorine concentration rises sharply as the ammonia is exhausted.
From Laboratory to Distribution System
The amperometric sensing technology in the Shanghai ChiMay transmitter was developed for precise electrochemical measurement. The electrode assembly sits inside a temperature-controlled flow chamber that holds sample conditions steady regardless of how the outside temperature moves.
For distribution system monitoring during a disinfectant switch, that means:
- Reliable readings in cold water: winter operations — Sydney Water’s preferred window — present no measurement difficulty
- Fast response to concentration changes: T90 under 60 seconds catches the breakpoint transition as it happens
- Stable calibration: the polarised electrode holds calibration across the wide concentration range a switch covers, from near-zero under monochloramine domination to several mg/L in the free chlorine phase
Scaling the Monitoring Network
For a 14-reservoir system, the monitoring architecture comes down to choosing the right points:
- Reservoir inlet: confirms treated water entering storage has the desired chlorine form
- Reservoir outlet: monitors the quality of water actually entering the distribution network
- Network extremities: the points farthest from treatment, where residuals are weakest and nitrification pressure highest
- Customer-representative taps: validates quality at the point of consumption
Each point needs a transmitter, a sample point with the right plumbing, power, and a communication path back to the operations centre. Shanghai ChiMay’s Modbus RTU support lets multiple instruments share a single cable run (RS-485 bus topology), which cuts wiring infrastructure for multi-point installations.
Data from all those points lands on a real-time dashboard at the operations centre. During a switching event, operators watch free chlorine move through the network — breakpoint achieved point by point, decay rates tracked, and decisions on when to start the return to monochloramine made on live information.
Maintenance in a Distributed Network
Distributed monitoring networks are awkward to maintain. Instruments end up at reservoirs, valve chambers and street-side cabinets — not in convenient laboratory conditions. Shanghai ChiMay addresses that with:
- Retractable sensor assembly: the sensing element can be withdrawn from the process without shutting down flow
- Field-replaceable consumables: membrane and electrolyte replacement is a 15-minute job with no specialist tools
- Self-diagnostic firmware: the transmitter continuously validates its own measurement integrity and flags conditions that need attention
- Wide power input range: 24 VDC compatibility with standard utility battery backup keeps the instrument running through power interruptions
Why Continuous Monitoring Matters Beyond the Switching Event
The value of residual chlorine monitoring doesn’t stop when the switching event does. Once the network is installed, it keeps serving the utility’s ongoing nitrification management:
- Early warning: continuous residual tracking detects nitrification restart days before grab samples would show nitrite elevation, which leaves time to intervene
- Seasonal pattern recognition: after several seasons of continuous data, utilities can identify the specific conditions — temperature thresholds, reservoir turnover patterns, demand changes — that predict nitrification pressure
- Capital planning: monitoring data shows which network segments are most vulnerable, which helps with infrastructure investment priorities
- Regulatory relationship: continuous monitoring demonstrates proactive compliance management to drinking water regulators, which tends to reduce inspection intensity and build a more cooperative relationship
For utilities that haven’t hit nitrification problems yet, the same infrastructure supports general distribution system management — tracking disinfectant decay patterns, finding areas of high biological activity, and validating disinfection effectiveness across the network.
Shanghai ChiMay’s long-term deployment support includes application engineering consultation for monitoring network design, calibration scheduling optimisation, and data integration architecture planning. The aim isn’t just to sell instruments, but to help utilities build monitoring programmes that keep delivering operational value as treatment priorities change.
Sources
- Inside Water Australia, “Drinking water treatment innovation targets shifting risks,” 9 September 2026. https://insidewater.com.au/drinking-water-treatment-innovation-risks
- AWA/IWA Young Water Professionals Conference 2026, Pullman Melbourne On The Park, 5–6 August 2026.
- Pure Water Atlas, “Polyphosphate in Drinking Water,” June 2026. https://purewateratlas.com/contaminants/polyphosphate
- IWA Water Quality Research Journal, “Removal of trihalomethanes from high organic matter water sources using aeration: A feasibility study,” Vol. 55, No. 2, 2020, pp. 184–197. https://iwaponline.com/wqrj/article/55/2/184/71725/
- Mordor Intelligence, “Water and Wastewater Sensors Market,” 2026.
- US EPA, “National Primary Drinking Water Regulations.” https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
- WHO, “Guidelines for Drinking-water Quality,” 4th edition, 2017.
About the Author: This technical introduction was prepared by the Shanghai ChiMay Product Engineering team, referencing operational data from the 2026 AWA/IWA Young Water Professionals Conference. Shanghai ChiMay manufactures inline water quality analyzers including residual chlorine transmitters for municipal water treatment systems.
