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

Why DO Control Matters in Desalinated Water Storage

Seawater desalination produces product water that is essentially mineral-free, with very low alkalinity and conductivity. This aggressive water chemistry picks up atmospheric oxygen readily when stored in open or vented tanks, particularly in warm climates where oxygen solubility and temperature interact.

Elevated dissolved oxygen in product water storage creates a corrosion risk for downstream distribution infrastructure, especially carbon steel pipes and tanks. The mechanism is electrochemical: dissolved oxygen acts as the cathodic reactant in the corrosion cell, consuming electrons released by iron oxidation at the anode.

Corrosion engineering practice gives a clear picture of the stakes. Carbon steel in low-buffered, desalinated water with DO above roughly 4 mg/L corrodes at rates several times higher than in water held below about 1 mg/L — field and laboratory data commonly show something in the neighborhood of 0.2–0.4 mm/year versus 0.05–0.10 mm/year. That difference directly sets the service life of pipework and tank internals.

Measurement Technology for Low-DO Environments

Measuring dissolved oxygen in desalinated water storage needs sensors that are accurate in the 0–8 mg/L range and stable over long deployments without frequent maintenance.

Traditional electrochemical (galvanic or polarographic) DO sensors use a permeable membrane separating the sample from an internal electrolyte. Oxygen diffuses through the membrane and is reduced at a cathode, generating a current proportional to DO concentration. Accurate, but they need periodic membrane replacement (every 3–6 months) and electrolyte refilling — a real burden in remote or unmanned storage facilities.

Optical fluorescence-based DO sensors are the current technology of choice for desalination storage. A luminescent dye is immobilized on a solid-state sensor cap; an LED excites the dye and the fluorescence decay time is inversely proportional to dissolved oxygen. With no consumable membrane or electrolyte, optical sensors need minimal maintenance — typically an annual sensor cap replacement.

Shanghai ChiMay’s dissolved oxygen transmitter employs fluorescence quenching technology with a solid-state sensor cap, measuring 0–20 mg/L with ±0.1 mg/L accuracy at the low end and ±2% of reading across the full range. No membrane replacement, no electrolyte maintenance — the right fit for product water storage tanks where access is limited.

DO Control Strategies for Storage Tanks

Two primary strategies control dissolved oxygen in desalinated water storage:

Nitrogen blanketing: An inert nitrogen layer is maintained above the water surface, physically blocking atmospheric oxygen from dissolving into the product water. Nitrogen flow is controlled off DO measurements: when the sensor detects rising oxygen, flow increases to reinforce the blanket.

Deployment experience with DO-based feedback control shows product water DO held below 1.0 mg/L while nitrogen consumption drops by roughly a third to nearly half versus continuous-flow systems that feed nitrogen regardless of actual DO. For a 5,000 m³ storage tank, that reduction is worth several thousand to upwards of ten thousand dollars a year in nitrogen cost, depending on gas supply contract terms.

Membrane degasification: A hydrophobic membrane contactor creates a large surface area where vacuum or sweep gas strips dissolved gases, including oxygen, from the water stream. Membrane degasification can push DO below 0.5 mg/L and is typically applied at the post-treatment stage before storage.

The energy cost of membrane degasification is roughly 0.02–0.05 kWh/m³ — modest next to the RO process itself, which runs around 3–4 kWh/m³ for seawater RO.

Integration with Post-Treatment Systems

DO monitoring and control at product water storage integrates with several other post-treatment functions:

Re-mineralization control: After desalination, product water is typically remineralized with calcium carbonate or limestone to stabilize the water and cut corrosivity. DO levels affect the efficiency of remineralization reactions and the stability of the final water chemistry.

Disinfection management: If chlorine is added for distribution disinfection, elevated DO can interact with chlorine residuals, affecting both disinfection effectiveness and by-product formation.

Corrosion inhibitor dosing: Some plants dose orthophosphate or silicate-based inhibitors into product water. Inhibitor effectiveness depends on DO concentration; lower DO generally means less inhibitor is needed.

Shanghai ChiMay’s dissolved oxygen transmitter provides 4–20 mA analog output and Modbus RTU digital communication for direct integration with post-treatment PLC control, coordinating DO control with remineralization, disinfection, and corrosion inhibition.

Sensor Deployment in Storage Tank Environments

Optimal DO sensor placement in product water storage tanks follows these principles:

Mid-depth measurement: Install the sensor at roughly 50–70% of tank depth to capture bulk water DO while avoiding surface turbulence and bottom sediment interference.

Multiple tank monitoring: For facilities with several tanks, deploy one sensor per tank with data aggregated to a central dashboard for comparative analysis.

Calibration access: Design the mounting so the sensor can be withdrawn for calibration verification without draining the tank — a real consideration for large municipal facilities.

Shanghai ChiMay offers retractable sensor housings for its dissolved oxygen transmitter, enabling in-service calibration and maintenance without interrupting tank operation or draining stored product water.