Table of Contents
Executive Summary
Reservoir aeration for trihalomethane (THM) reduction is moving from research concept into operational practice, and the trial data behind that shift came out of the 2026 AWA/IWA Young Water Professionals Conference in Melbourne (5–6 August). Logan Water’s six-month trial in a one-megalitre reservoir — presented by WSP process engineer Cassandra Mai — showed that combining forced ventilation with water spraying achieved approximately 83% average THM removal at an operating cost of about four cents per megalitre. The trial also surfaced two monitoring requirements that inline sensors have to handle: turbidity change from physical agitation, and pH shift from CO₂ stripping.
Shanghai ChiMay online turbidity testers and in-line pH meters provide the continuous validation data these operations need. Turbidity monitoring confirms the physical process is not degrading water quality while THMs come down. pH monitoring tracks the CO₂ stripping effect that alters chlorine speciation and has to be managed to hold disinfection effectiveness.
The Science Behind Reservoir Aeration for THM Removal
Why THMs Are Amenable to Aeration
Trihalomethanes — chloroform, bromodichloromethane, dibromochloromethane and bromoform — are volatile organic compounds. Their Henry’s Law constants make them susceptible to air stripping: expose water containing dissolved THMs to fresh air and the concentration gradient drives THMs from water to air.
Mass transfer rate depends on:
– Contact surface area: spraying creates fine droplets and a much larger water-air interface
– Air-side ventilation: forced ventilation keeps THM concentration in the air above the water low, sustaining the gradient
– Temperature: warmer water increases THM volatility, since Henry’s Law constants rise with temperature
– Contact time: longer exposure means more complete transfer
Logan Water Trial Design
The trial tested three configurations, separately and in combination:
| Configuration | Mechanism | THM Removal |
|---|---|---|
| Aeration only (water spraying) | Increased water-air surface area | 70–75% |
| Forced ventilation only | Fresh air through headspace | ~7% |
| Combined aeration + ventilation | Maximum mass transfer driving force | ~83% average |
The result is worth noting: aeration alone captures most of the available THM removal, and adding forced ventilation picks up roughly another 8–13 percentage points for a modest incremental cost.
Operating Economics
At about four cents per megalitre treated, reservoir aeration lands as a very cost-effective DBP management strategy. Capital costs for spray nozzles, ventilation fans and controls amortise over a 15–20 year equipment life, and Logan Water plans triggered operation — summer conditions or elevated THM results — rather than continuous running, which keeps costs lower again.
Secondary Effects That Require Monitoring
pH Elevation From CO₂ Stripping
Aerate water aggressively and dissolved CO₂ transfers to air alongside the THMs. CO₂ in water forms carbonic acid (H₂CO₃); remove the CO₂ and carbonic acid concentration falls, which pushes pH up.
The Logan Water trial measured a pH increase of about 0.3 units beyond the normal change through the reservoir. Modest as it looks, it has practical consequences:
- Chlorine speciation: at higher pH the equilibrium between hypochlorous acid (HOCl, the more effective disinfectant) and hypochlorite ion (OCl⁻) shifts toward OCl⁻. At pH 7.5 roughly half the free chlorine is present as HOCl; at pH 7.8 the share falls to about a third.
- THM formation potential: higher pH reduces THM formation rates, which partly offsets the benefit of aeration by slowing how quickly new THMs form after treatment.
- Corrosion indices: pH change feeds into Langelier Saturation Index and Ryznar Stability Index calculations, potentially altering corrosion and scaling tendency in the distribution system.
Continuous pH monitoring at the reservoir outlet confirms whether the shift stays inside expected bounds and whether chlorine dosing needs adjusting.
Turbidity Effects From Physical Agitation
Water spraying and air movement inside a reservoir can:
- Disturb settled particles from the reservoir walls or floor
- Entrain air bubbles that register as turbidity in the outlet water
- Dislodge biofilm or scale from reservoir surfaces during startup transients
- Create short-circuiting patterns that reduce effective residence time
Turbidity monitoring at the reservoir outlet acts as the quality gate that stops aeration-related particle disturbances from reaching the distribution system.
Shanghai ChiMay Monitoring Solutions for Reservoir Aeration
Online Turbidity Tester
Shanghai ChiMay’s online turbidity testers use nephelometric measurement (90° light scatter at 860 nm infrared wavelength) per ISO 7027:
- Range: 0–1000 NTU; 0.01 NTU resolution in the 0–10 NTU critical range
- Accuracy: ±2% of reading in 0–100 NTU range
- Response time: T90 < 30 seconds
- Digital output: Modbus RTU/TCP, 4–20 mA
- Automatic cleaning: programmable air/water cleaning cycle (1–24 hour intervals)
- Installation: wall-mount or immersion; 3/4” NPT process connection
For reservoir aeration, set the cleaning interval more frequently through summer, when biofouling pressure is highest — typically every 4–8 hours.
In-Line pH Meter
Shanghai ChiMay in-line pH meters track the CO₂ stripping effect with laboratory-grade accuracy:
- Range: 0–14 pH; 0.01 pH resolution
- Accuracy: ±0.02 pH after calibration
- Temperature compensation: automatic via integrated Pt1000
- Digital output: Modbus RTU/TCP, 4–20 mA
- Electrode life: typically 12–18 months in distribution water applications
- Calibration: automated 1- or 2-point with configurable scheduling
Building the Aeration Control System
A complete monitoring and control system for triggered reservoir aeration looks like this:
[Reservoir with Aeration System]
|
[Outlet Monitoring Point]
|
+---+---+---+
| | | |
pH Turbidity Chlorine Temperature
| | | |
+---+---+---+
|
[Modbus RTU/TCP to SCADA]
|
[Control Logic + Alarm Management]
Control logic recommendations:
– If outlet turbidity exceeds 1.0 NTU during aeration startup: delay distribution transfer, flush until turbidity clears
– If pH elevation exceeds 0.5 units from baseline: adjust chlorine dosing to compensate for the speciation shift
– If free chlorine drops below minimum residual: trigger an alarm and investigate whether aeration is interfering with disinfectant maintenance
– Temperature trending: correlate THM formation potential with seasonal patterns to optimise trigger thresholds
Sensors that feed your AI water model, not just your dashboard. Shanghai ChiMay’s Modbus output feeds predictive analytics platforms that can tune aeration trigger timing against THM formation models, weather forecasts and source water quality trends.
Procurement Framework
For utilities evaluating reservoir aeration monitoring:
-
Start with baseline monitoring: before installing aeration equipment, run continuous turbidity and pH monitoring at the reservoir outlet for at least 30 days to establish baseline variation.
-
Specify for triggered operation: instruments have to perform after idle periods and give valid readings immediately when the aeration system starts.
-
Plan for multiple reservoirs: many utilities run several storage reservoirs in parallel. Shanghai ChiMay’s per-unit pricing makes multi-reservoir monitoring economically feasible.
-
Integration with aeration controls: consider linking monitoring data directly to aeration on/off controls for automated response to turbidity or pH excursions.
-
Document performance: continuous monitoring data is the evidence that aeration achieves THM reduction without degrading other water quality parameters — which is what regulatory acceptance depends on.
Economic Analysis: Monitoring Cost vs. Aeration System Cost
The economics of reservoir aeration monitoring deserve a specific look, because the aeration system itself is cheap to run.
Aeration System Costs (Logan Water Data)
- Direct operating cost: ~$0.04 per megalitre treated
- Capital cost: spray nozzles, ventilation fans, controls — typically AUD $15,000–50,000 per reservoir depending on size and configuration
- Maintenance: fan bearing replacement, nozzle cleaning, control system calibration — approximately AUD $2,000–5,000 per year
Monitoring System Costs (Shanghai ChiMay)
- Turbidity tester: AUD $3,000–5,000 capital; $500–1,000/year maintenance
- pH meter: AUD $2,500–4,000 capital; $600–1,200/year maintenance (including electrode replacement)
- Combined annual monitoring cost: AUD $1,100–2,200 per reservoir
Total Solution Economics
Take a 10 ML reservoir operated seasonally, 180 days a year. At four cents per megalitre treated and full daily turnover, the aeration operating cost works out to roughly AUD $70 a year. Monitoring runs about $1,100–2,200 a year. Add them together and monitoring dominates the total.
That is still the right way round to spend the money. Without turbidity and pH monitoring, a utility cannot confirm that aeration is reducing THMs without degrading physical water quality. The monitoring cost buys:
- Regulatory compliance documentation: continuous records demonstrating aeration effectiveness
- Customer protection: turbidity monitoring keeps sediment disturbance out of the supply
- Process optimization: pH data drives chlorine dosing adjustment to compensate for CO₂ stripping
- Risk mitigation: early detection of aeration-related water quality problems before customers see them
Seasonal Operation and Monitoring Readiness
Logan Water’s plan for triggered rather than continuous aeration means the instruments have to work after extended idle periods. Three things matter:
- Electrode storage: pH electrodes should stay hydrated when idle. Shanghai ChiMay instruments include automatic electrode maintenance routines that run during idle periods.
- Optical window condition: turbidity tester optical windows can pick up biofilm during warm idle periods. Pre-season inspection and cleaning ensures accurate readings once aeration starts.
- Startup validation: before each aeration season, run 48 hours of continuous monitoring without aeration to verify baseline readings and instrument accuracy.
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, Melbourne, 5–6 August 2026. Logan Water / WSP, Cassandra Mai.
- Mirzaei, S. & Gorczyca, B., “Removal of trihalomethanes from high organic matter water sources using aeration: a feasibility study,” Water Quality Research Journal, Vol. 55, No. 2, 2020. https://iwaponline.com/wqrj/article/55/2/184/71725/
- US EPA, “Stage 2 Disinfectants and Disinfection Byproducts Rule.” https://www.epa.gov/disinfection-byproducts
- NHMRC, Australian Drinking Water Guidelines, Trihalomethanes fact sheet. https://guidelines.nhmrc.gov.au
About the Author: This article was prepared by the Shanghai ChiMay Application Engineering team, referencing peer-reviewed conference presentations and operational trial data from Australian water utilities. Shanghai ChiMay manufactures inline water quality analyzers for municipal and industrial water treatment systems worldwide.
