Dissolved Oxygen Control in Desalination Product Water Storage Tanks: Application Notes from Shanghai ChiMay

Desalinated product water is essentially mineral-free — low alkalinity, low conductivity — and that clean chemistry has a downside. Stored in open or vented tanks, it readily dissolves atmospheric oxygen, especially in warm climates. And dissolved oxygen in a product water storage tank is not a water quality curiosity; it is a corrosion bill waiting to be paid.

The mechanism is textbook electrochemistry: dissolved oxygen acts as the cathodic reactant in the corrosion cell, consuming the electrons released by iron oxidation at the anode. Corrosion engineering research published in 2026 puts numbers on it — carbon steel immersed in desalinated water with DO above 4 mg/L corrodes at 0.25–0.40 mm/year; hold the water below 1 mg/L and the rate drops to 0.05–0.10 mm/year. That is a 3–4× difference in metal loss — the kind of gap that decides whether carbon steel pipework and tanks last their design life or fail early. As a rule of thumb, corrosion rates run roughly 30–50% higher once DO climbs above 4 mg/L compared to below 1 mg/L.

Measuring DO You Can Actually Control

Storage tank control needs sensors that stay accurate in the 0–8 mg/L range over long deployments with minimal attention. Two technologies sit on the shelf:

Electrochemical (galvanic or polarographic) sensors have a permeable membrane separating the sample from an internal electrolyte. Oxygen diffuses through, gets reduced at the cathode, and the current tracks DO concentration. Accuracy is fine. The catch is maintenance: membrane replacement every 3–6 months plus electrolyte refilling — a real burden at remote or unmanned storage facilities.

Optical fluorescence-based sensors are the current choice for this duty. A luminescent dye is immobilized on a solid-state sensor cap; an LED excites it, and the fluorescence decay time is inversely proportional to DO. No consumable membrane, no electrolyte — just an annual sensor cap replacement.

Shanghai ChiMay’s dissolved oxygen transmitter runs on fluorescence quenching technology with a solid-state sensor cap: 0–20 mg/L range, ±0.1 mg/L accuracy at the low end and ±2% of reading across the full range, with no membrane or electrolyte maintenance. For product water storage — where access may mean climbing a tank — that low-maintenance profile is the whole point.

Two Ways to Keep Oxygen Out

Nitrogen blanketing. An inert nitrogen layer sits above the water surface, physically blocking atmospheric oxygen from dissolving in. The trick is running the blanket on feedback, not on a schedule: the DO sensor raises nitrogen flow when oxygen levels climb, and eases off when they don’t. Deployment data from 2026 shows DO-feedback nitrogen systems holding product water below 1.0 mg/L while cutting nitrogen consumption 35–45% versus continuous-flow systems that purge regardless of actual DO. On a 5,000 m³ storage tank that saving lands around USD 8,000–15,000 per year in nitrogen costs alone.

Membrane degasification. A hydrophobic membrane contactor spreads the water over a large surface where a vacuum or sweep gas pulls dissolved gases — oxygen included — out of the stream. It can push DO below 0.5 mg/L, and it is typically applied at post-treatment, before water enters storage. The energy cost is modest: 0.02–0.05 kWh/m³, small change next to the 3–4 kWh/m³ the RO process itself consumes for seawater.

Fitting DO Control into Post-Treatment

Storage tank DO control does not operate in isolation — it interacts with everything downstream:

Re-mineralization. Product water is usually remineralized with calcium carbonate or limestone to stabilize chemistry and cut corrosivity. DO levels affect how efficiently those reactions run and how stable the final water chemistry stays.

Disinfection. Where chlorine is added for distribution, elevated DO can interact with chlorine residuals — shifting both disinfection effectiveness and disinfection by-product formation.

Corrosion inhibitor dosing. Plants dosing orthophosphate or silicate-based inhibitors get more from them at lower DO: keep DO down and required inhibitor dosage comes down with it.

ChiMay’s dissolved oxygen transmitter carries 4–20 mA analog and Modbus RTU outputs, so it ties into post-treatment PLC systems and coordinates DO control with remineralization, disinfection, and corrosion inhibition automatically.

Where to Put the Sensor in a Storage Tank

Placement determines whether the reading means anything:

• Measure mid-depth. Install at roughly 50–70% of tank depth to capture bulk water DO without surface turbulence or bottom sediment interference.
• One sensor per tank. Facilities with multiple tanks aggregate each tank’s reading on a central dashboard for comparison.
• Design for calibration access. The sensor mounting should allow withdrawal for calibration checks without draining the tank — non-negotiable for large municipal storage.

On that last point, ChiMay offers retractable sensor housings for the dissolved oxygen transmitter, so the unit can be calibrated and serviced in service — no tank draining, no interruption to stored product water.