Chemical Injection BWTS: Chlorine Dioxide and Other Active Substances

How chemical injection differs from electrochlorination

Both chemical injection and electrochlorination disinfect with oxidant chemistry, but the source of the oxidant differs. Electrochlorination makes chlorine in situ from seawater chloride. Chemical injection introduces a biocide that is manufactured or generated separately and metered into the ballast stream, so its performance does not depend on the salinity of the intake water.

That salinity independence is the headline advantage. A vessel that frequently ballasts in fresh or brackish water, where electrolysis struggles, can dose the same chemical effectively regardless of intake salinity. JFE BallastAce, for example, uses a chemical injection approach with the active agent TG- ballast water dosing rather than relying on in-situ electrolysis across all conditions.

Chemical injection is always paired with a pre-filter, typically 40 to 50 microns, that removes large organisms and sediment before dosing. The chemical then inactivates the remaining organisms during the tank holding period, in the same way oxidant acts over time in an electrochlorination system.

Chlorine dioxide and other active substances

Chlorine dioxide is one of the active substances used in this category. It is a strong, selective oxidant that is usually generated on demand by mixing precursor chemicals, because it is unstable to store in bulk. Other systems dose stabilised liquid biocides or generate oxidant from concentrated salt solutions. In each case the agent is metered against the volume and quality of water being ballasted.

Because these are deliberately added biocides, each formulation and its disinfection by-products are assessed under the IMO procedure for approval of ballast water management systems that make use of active substances, known as procedure G9, before the system can be type-approved. The G9 evaluation sets the maximum allowable discharge concentration and reviews human and environmental safety of the chemistry.

The dosing target is chosen so that organisms across all D-2 size classes, plus the indicator microbes, are inactivated within the validated holding time. Dosing too little risks non-compliance; dosing too much increases the neutralisation burden and chemical consumption.

Dosing control, holding time and monitoring

A chemical injection system meters the active substance proportionally to the ballast flow rate, using a dosing pump controlled from the flow signal. Some systems also measure a residual oxidant or active-substance concentration to confirm the dose landed correctly. The control logic records the dose so the treatment can be verified later.

Disinfection then proceeds during the holding period in the tanks, which the type approval specifies as a minimum number of hours. As with electrochlorination, cold water can slow the kinetics and lengthen the required hold. Short ballast operations that do not allow the minimum hold can be a constraint for chemical and oxidant systems alike.

Residual concentration is monitored before discharge to confirm it is below the permitted limit. If it is too high, neutralisation is applied. The residual measurement is the compliance gate at the discharge end, so its sensor and reagent must be maintained.

Neutralisation and chemical handling

Oxidant-based chemical systems usually require neutralisation before discharge, commonly with sodium thiosulfate dosed in proportion to the measured residual. The neutralisation arrangement mirrors that of electrochlorination systems: a thiosulfate tank, a dosing pump and a feedback loop from the discharge residual analyser.

Chemical handling is the defining operational consideration. Crews must store, handle and dose biocide precursors or stabilised chemicals safely, following the safety data sheets and the procedures in the management plan. Precursor chemicals for chlorine dioxide in particular demand careful segregation and storage. Stock management matters too, because running out of either the biocide or the neutralising agent can force the vessel into contingency measures.

The consumable supply chain is therefore central to chemical injection systems: the active substance or its precursors, the neutralising chemical, and any residual-analyser reagents all need reliable resupply. Sea Clean AS supplies BWTS consumables and can advise on stock planning for chemical and oxidant platforms.

Where chemical injection fits best

Chemical injection is most attractive where intake salinity is low or highly variable, because it removes the salinity dependence that constrains electrochlorination. It also avoids the heavy UV power draw and UVT derating of UV systems, since disinfection is chemical rather than optical and is insensitive to water clarity beyond what the pre-filter handles.

The trade-offs are the holding-time requirement, the need to monitor and neutralise residual, and the logistics and safety of carrying biocide chemicals aboard. For some operators the simplicity of a sealed chemical supply is preferable to maintaining electrolytic cells; for others the chemical handling is a drawback.

As with every technology choice, the decision should follow the vessel's real trading pattern, ballast flow rate, available space and crew capability. A trade dominated by fresh-water ports is a classic case where chemical injection outperforms electrochlorination.

Frequently asked questions

How is chemical injection different from electrochlorination?

Both use oxidant chemistry, but electrochlorination makes chlorine on board from seawater chloride, whereas chemical injection meters in a separately produced biocide. The practical consequence is that chemical injection works in fresh and brackish water where electrolysis cannot generate enough chlorine. Both require a holding time, residual monitoring and neutralisation.

Why is chlorine dioxide generated on demand rather than stored?

Chlorine dioxide is unstable and hazardous to store in bulk, so systems generate it as needed by combining precursor chemicals immediately before dosing. This means the vessel carries the precursors rather than finished chlorine dioxide, and those precursors must be stored and handled according to their safety data sheets.

Do chemical injection systems need a holding time?

Yes. The biocide inactivates organisms over a contact period in the ballast tanks, and the type approval specifies a minimum holding time. Cold water slows the disinfection kinetics, so the required hold can be longer in cold conditions. Very short ballast operations may not allow the minimum hold.

What does G9 approval cover?

G9 is the IMO procedure for approving ballast water systems that use active substances. It evaluates the biocide and its disinfection by-products for human health and environmental safety and sets the maximum allowable discharge concentration the system must respect. G9 approval is required in addition to type approval under the BWMS Code.

Sources

Related articles

• JFE BallastAce Chemical Injection BWTS Overview
• Neutralisation Systems and Sodium Thiosulfate Dosing in BWTS
• TRO Monitoring in Ballast Water Systems: Measuring Total Residual Oxidant
• Electrochlorination BWTS Explained: Electrolysis, TRO Generation and Neutralisation
• Deoxygenation Ballast Water Treatment Systems: Mechanism and Trade-offs
• UV-Based Ballast Water Treatment Systems: Mechanism, UVT Dependence, Pros and Cons