Neutralisation Systems and Sodium Thiosulfate Dosing in BWTS

Updated 2026-06-16

Oxidant-based ballast water systems must reduce total residual oxidant below the permitted limit before discharge, and the standard method is dosing sodium thiosulfate into the deballast line. The neutralisation system meters thiosulfate against the measured discharge TRO so the residual reaches a safe level at the ship's side. Reliable dosing depends on accurate discharge TRO measurement, adequate chemical stock and a well-maintained dosing loop.

Neutralisation Systems and Sodium Thiosulfate Dosing in BWTS

Key takeaways

Oxidant systems must cut discharge TRO below the G9 limit, typically low mg/L, before deballasting.
Sodium thiosulfate is the standard reducing agent: fast, safe to handle, inexpensive and widely available.
Dose scales with both discharge flow rate and measured residual TRO.
The dosing loop relies on an accurate, calibrated discharge TRO analyser as its feedback.
Under-dosing causes non-compliance; over-dosing wastes chemical and stock.
Long holds that decay TRO naturally reduce thiosulfate demand.
Carry adequate thiosulfate stock with reserve; running out can force contingency measures.

Why neutralisation is needed

In electrochlorination and chemical injection systems, oxidant remains in the ballast water throughout the holding period to keep inactivating organisms. By the time the water is discharged, the residual TRO may still exceed the maximum allowable discharge concentration set by the system's G9 active-substance approval, which is typically in the low mg/L range.

Discharging water above that limit would release oxidant into the receiving port, which is exactly what the approval is designed to prevent. Neutralisation chemically reduces the oxidant to harmless products before the water leaves the ship, closing the gap between the residual after holding and the permitted discharge concentration.

Not every cycle needs the same amount of neutralisation. If the residual has decayed naturally during a long hold to below the limit, little or no thiosulfate is required; if the water still carries substantial TRO, more is dosed. The system decides based on the real-time discharge TRO reading.

Sodium thiosulfate as the reducing agent

Sodium thiosulfate is the standard neutralising agent in BWTS because it rapidly reduces chlorine and related oxidants to chloride and other benign products, it is relatively safe to handle, and it is inexpensive and widely available. It is supplied as a solid or as a solution and made up to a dosing concentration aboard or bought ready-mixed.

The reaction consumes thiosulfate in proportion to the oxidant present, so the dose must scale with both the discharge flow rate and the measured TRO. The dosing pump rate is set by the control system from these two inputs so the residual at the ship's side falls below the limit.

Because thiosulfate is consumed every time oxidant-laden water is discharged, it is a recurring consumable whose usage depends on the trading pattern, the doses applied and the natural decay achieved during holding. Vessels with short holds and high residuals will use more than those with long ocean passages.

The neutralisation dosing loop

A neutralisation system comprises a sodium thiosulfate storage and mixing tank, a metering dosing pump, an injection point in the deballast line and a feedback signal from the discharge TRO analyser. During deballasting, the discharge analyser continuously measures residual oxidant and the controller adjusts thiosulfate dosing to keep the post-injection TRO below the limit.

The control logic must account for the lag between injection and the point where TRO is confirmed neutralised, ensuring the reading reflects the water actually being discharged. Reliable operation depends on the discharge TRO analyser being accurate and in calibration, because the entire dosing decision rests on its reading.

Mechanical reliability of the dosing pump, the injection nozzle and the supply line matters as well. A blocked injection point or a failed pump means oxidant water reaches the sea untreated, so these components are part of the compliance-critical equipment, not minor auxiliaries.

Under-dosing, over-dosing and stock planning

Under-dosing leaves residual TRO above the discharge limit, a direct non-compliance that sampling at the discharge point would reveal. Over-dosing wastes thiosulfate, increases cost and consumes stock faster than necessary, and adds reaction products to the discharge. The aim is to dose just enough to bring TRO reliably below the limit with a sensible margin.

Stock planning is a practical compliance issue. The vessel must carry enough sodium thiosulfate to neutralise the expected deballast volumes at the expected residual levels for the voyage, with a reserve. Running out mid-voyage means oxidant water cannot be safely discharged, which can force the ship into contingency measures and delay deballasting.

Estimating consumption requires knowing typical discharge TRO and volumes for the trade. Keeping a buffer stock and reordering before reserves run low avoids operational disruption. Sea Clean AS supplies sodium thiosulfate and neutralisation consumables for BWTS and can help estimate consumption for a given trading pattern.

Frequently asked questions

Why is sodium thiosulfate used for neutralisation?

It rapidly reduces chlorine and related oxidants to chloride and other harmless products, it is relatively safe for crews to handle, and it is cheap and readily available worldwide. These properties make it the standard neutralising agent across oxidant-based ballast water systems, both electrochlorination and chemical injection.

How much thiosulfate does a vessel need to carry?

Consumption depends on deballast volumes, the residual TRO at discharge and how much the oxidant decays during holding. Estimate the typical discharge TRO and volumes for your trade, calculate the thiosulfate needed and add a reserve. Vessels with short holds and high residuals use more than those with long ocean passages.

What happens if neutralisation under-doses?

Water is discharged with residual TRO above the permitted limit, which is a clear non-compliance and would be revealed by sampling at the discharge point. Under-dosing usually traces to an inaccurate discharge TRO reading, a failing dosing pump or a blocked injection point, so keep the analyser calibrated and the dosing loop maintained.

Can a long holding time reduce neutralisation needs?

Yes. Oxidant decays naturally during the holding period, so a long ocean passage may reduce residual TRO to below the discharge limit with little or no thiosulfate needed. Short coastal voyages give less decay time and therefore higher residuals and greater thiosulfate consumption at discharge.

Sources

Sea Clean BWTS service desk · All Insights