Why Is Sodium Thiosulfate Required in Ballast Water Treatment?

Many marine ballast water treatment systems use electro-chlorination or other oxidizing technologies to control aquatic organisms and microorganisms in ballast water.

These systems generate or introduce oxidizing agents that provide disinfection during ballast water treatment.

However, residual oxidants remaining in the treated ballast water may require neutralization before discharge.

Sodium Thiosulfate Pentahydrate acts as a reducing agent, helping convert residual chlorine and suitable oxidants into less reactive compounds.

This process is known as ballast water dechlorination or neutralization.

The required neutralization procedure depends on the BWMS manufacturer's instructions, the oxidant concentration and the vessel's approved operating procedures.

Typical Applications

Sodium Thiosulfate Anhydrous can be used for:

• Ballast water dechlorination

• Residual chlorine neutralization

• Electrochlorination BWMS

• Sodium hypochlorite-treated ballast water

• Oxidizing biocide neutralization

• Ballast water discharge preparation

• Marine water-treatment systems

How Sodium Thiosulfate Works

Sodium thiosulfate functions as a reducing agent. It reacts with residual oxidizing species and converts them into less oxidizing chemical species.

In a chlorine-treated ballast water system, sodium thiosulfate can consume residual chlorine, helping reduce the Total Residual Oxidant (TRO) concentration before discharge.

The required dosage depends on:

• Initial TRO concentration

• Ballast water volume

• BWMS technology

• Water chemistry

• Required discharge residual

• Chemical concentration

• Reaction/contact time

• Dosing-system configuration

Therefore, dosing should be calculated based on the actual BWMS operating parameters rather than using a universal fixed dosage.

Illustrative Dosage Calculation

For complete oxidation of thiosulfate to sulfate by chlorine, the theoretical chemical demand is approximately:

0.875 mg of pure Sodium Thiosulfate Pentahydrate per 1 mg of chlorine expressed as Cl₂ equivalent.

For example, with 1,000 m³ of ballast water containing 2 mg/L residual oxidant expressed as Cl₂:

Important: This is an illustrative chemical stoichiometry calculation, not a recommended shipboard dosage. Thiosulfate may follow different reaction pathways, and actual demand can differ substantially. The approved BWMS neutralization chemical, dosing algorithm and TRO feedback controls must govern practical use.

Important Dosing Consideration

The required quantity of Sodium Thiosulfate should be determined from the actual TRO measurement and BWMS dosing requirements. Overdosing or underdosing can affect ballast-water treatment performance and discharge conditions.

For shipboard operation, dosing should therefore be controlled through the vessel's approved BWMS procedures, TRO monitoring system, and chemical-dosing equipment.

Sodium Thiosulfate for Ballast Water Neutralization

Many Ballast Water Management Systems (BWMS) use an oxidizing treatment process to control organisms in ballast water. Depending on the technology, treatment may generate or introduce free chlorine, hypochlorite, or other oxidizing residuals.

Before discharge, residual oxidant may need to be neutralized.

Sodium Thiosulfate Anhydrous can be dosed into the treated ballast water to reduce residual oxidizing compounds.

This makes it an important chemical for selected ballast-water neutralization systems.