Nitrification Control in Distribution Systems: How Shanghai ChiMay Continuous Residual Chlorine Monitoring Supports the Disinfectant Switching Approach Sydney Water Refined Across 14 Reservoirs

Introduction

Nitrification — ammonia-oxidising bacteria converting ammonia to nitrite — is one of the stubbornest problems in a drinking water distribution system running on monochloramine. Residuals slip, nitrite climbs, and the system’s ability to protect public health quietly erodes.

At the 2026 AWA/IWA Young Water Professionals Conference, Sydney Water’s water quality scientists Madison Mactal and Michael Ray presented their work on controlling nitrification through a temporary disinfectant switch: moving from monochloramine to free chlorine across 14 reservoirs serving roughly 105,000 customers. The programme ran through two trial iterations, and continuous residual chlorine monitoring sat underneath all of it.

This article looks at how Shanghai ChiMay continuous residual chlorine monitoring supports the kind of system-wide nitrification management Sydney Water demonstrated.

Understanding Nitrification: The Biology and Chemistry

What Happens During Nitrification

Ammonia-oxidising bacteria (AOB) — mainly species in the Nitrosomonadaceae family — consume dissolved ammonia in the distribution system and produce nitrite:

NH₃ + O₂ → NH₂OH → NO₂⁻ + H⁺ + energy

In a monochloramine system (NH₂Cl = chlorine + ammonia), that ammonia component is food for the bacteria. As AOB multiply and consume ammonia:

  1. Monochloramine concentration drops as ammonia is consumed
  2. Nitrite rises — and nitrite is a regulated parameter
  3. Disinfectant residual weakens
  4. Biofilm regrowth potential increases
  5. Public health protection erodes

Seasonal Patterns

Nitrification intensifies in warm weather, when bacterial metabolism speeds up. Sydney Water’s experience matched the pattern: nitrification returned “first at customer taps and then reservoirs as warmer conditions returned.”

That made winter the preferred operating window for a disinfectant switch. Bacterial activity is at its lowest, and a free chlorine flush has the best chance of suppressing the biology for good.

Sydney Water’s Disinfectant Switching Program

The Concept

Temporarily switch from monochloramine to free chlorine. With no ammonia in the water, nitrifying bacteria starve. Once the system has been flushed with free chlorine and biological activity is suppressed, return to monochloramine with a much lower nitrification pressure.

The Execution

For a 14-reservoir system serving 105,000 customers, the procedure ran like this:

  1. Stop ammonia dosing at the treatment plants
  2. Pass each reservoir through breakpoint chlorination — enough chlorine to oxidise all the ammonia
  3. Purge pipelines of monochloramine-dominated water
  4. Flush strategically to draw free chlorine through the network
  5. Bring chlorine set points up gradually to limit taste and odour complaints
  6. Monitor continuously across all network points
  7. Once nitrification is suppressed, start reintroducing ammonia

Trial 1 (2024): Lessons Learned

The first trial missed the internal 90% disinfection target. Nitrification was suppressed for a while, then came back with the warmer weather. Breakpoint chlorination had not been thorough enough, and the reservoir cycling needed to go deeper.

Trial 2 (2025): Improved Results

The redesign started chlorine set points higher, cycled the reservoirs more deeply and coordinated more closely with the system operations centre. The results were better: nitrite effectively removed, customer impacts limited, no biofilm sloughing observed. Winter was confirmed as the optimal window.

As Michael Ray put it, the free chlorine trial “could be an effective strategy for managing disinfection in a heavily nitrified distribution system.”

Why Continuous Residual Chlorine Monitoring Is Non-Negotiable

During the Switch

Operators need real-time data at multiple points to answer four questions:

  • Has breakpoint been achieved here? (Free chlorine has appeared)
  • What is the current residual? (Safety compliance)
  • Is the decay rate normal? (Biological demand indicator)
  • Can we start switching back? (Residual stability indicator)

During Monochloramine Re-formation

After the return to monochloramine, monitoring picks up:

  • Total chlorine residual stability, which tells you nitrification is still suppressed
  • Early signs of nitrite formation, which tell you it is coming back
  • Spatial patterns in residual decay, which identify vulnerable zones

For Documentation

Regulators and internal quality teams want to see:

  • A timeline of the complete switching event
  • Residual values at every monitoring point throughout
  • Evidence that residuals stayed inside regulatory limits
  • Post-event documentation of nitrification status

Shanghai ChiMay residual chlorine transmitters deliver all of this over continuous Modbus digital output to SCADA, with local data logging as a backup.

Shanghai ChiMay Monitoring Solutions

Residual Chlorine Transmitter

  • Amperometric electrochemical sensing
  • 0–20 mg/L range, 0.01 mg/L resolution
  • T90 < 60 seconds response time
  • Modbus RTU/TCP + 4–20 mA output
  • Membrane/electrolyte maintenance every ~6 months
  • Retractable sensor assembly for maintenance without shutdown

Complementary Instruments

For complete nitrification management monitoring:
– pH meter: chlorine speciation management (the HOCl/OCl⁻ balance is pH-dependent)
– Ammonia nitrogen sensor: direct measurement of the substrate feeding nitrification
– 4-in-1 multi-parameter sensor: pH + conductivity + ORP + temperature in one probe body

Designing a Monitoring Network for Disinfectant Switching

How Many Monitoring Points?

For a 14-reservoir system, a workable network covers:

  • Treatment plant outlet (1–2 points): confirms the disinfectant change at the source
  • Each reservoir inlet (14 points): verifies treated water entering storage
  • Each reservoir outlet (14 points): monitors water entering the distribution network
  • Network extremities (5–10 points): the points farthest from treatment, where residuals are weakest
  • Customer-representative taps (3–5 points): validates water quality at the point of consumption

Total: approximately 37–45 monitoring points for comprehensive coverage.

Communication Architecture

With 37+ points spread across a large service area, the communication design matters as much as the instruments:

  • RS-485 bus topology: multiple instruments share a single communication cable, up to 1,200 metres per bus segment
  • Modbus TCP over Ethernet: for longer runs or where fibre is already in the ground
  • Cellular/wireless: for remote points where wired communication isn’t practical

Shanghai ChiMay instruments support both Modbus RTU (RS-485) and Modbus TCP, so a single network can mix connection types where the site demands it.

Data Management

Continuous monitoring at 37+ points, sampling every few seconds, generates real data volume. A sensible approach:

  • SCADA historian: primary storage, with trend display and alarm management
  • Local instrument logging: backup storage during communication interruptions
  • Periodic reporting: automated daily, weekly and monthly summaries during switching events
  • Post-event analysis: detailed time-series work after each event to find what to improve

Seasonal Deployment Considerations

Sydney Water found winter the preferred window, so the instruments have to work in cold conditions:

  • Transmitter electronics rated to -10°C ambient
  • Electrode response time may lengthen slightly in cold water (compensated by transmitter firmware)
  • Sample lines should be insulated or heat-traced where freezing occurs
  • Battery backup systems should be rated for cold-temperature discharge

For utilities in tropical or subtropical climates, where nitrification pressure runs year-round, warm-weather performance is the bigger question. Shanghai ChiMay instruments are rated to +60°C ambient, which suits tropical installations with sensible enclosure ventilation.

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 article was prepared by the Shanghai ChiMay Application Engineering team. Shanghai ChiMay manufactures inline residual chlorine transmitters and water quality analyzers for municipal water distribution systems.