Sodium Thiosulfate Injection Systems: BWTS Hardware
Updated
A sodium thiosulfate injection system for a BWTS comprises a fresh-water make-up tank, a metering dosing pump, an injection point upstream of the overboard sample tap, and a calibrated TRO analyser closing a control loop that trims the dose. The hardware must be sized for the worst-case discharge flow and TRO, use compatible materials, and be protected against cold-climate crystallisation.
Request BWTS spare parts, annual calibration or ballast-water testing
Key takeaways
- A thiosulfate injection skid needs a fresh-water make-up tank, a metering dosing pump, an injection point and a calibrated TRO analyser with a control loop.
- Size the pump for the worst-case discharge flow and TRO, then convert to solution flow using the stock concentration.
- Place the injection point well upstream of the sample tap with good mixing so the analyser reads a true reacted residual.
- The closed-loop controller trims the dose to a small positive TRO setpoint below the approved discharge limit.
- Use thiosulfate-compatible materials, ensure full dissolution to avoid abrasive crystals, and calibrate the analyser on schedule.
- In cold climates trace-heat or insulate tanks and lines to prevent crystallisation below about 5 degrees Celsius.
What Are the Main Components?
A neutralisation skid has four functional parts: a make-up and day tank holding the thiosulfate stock solution, a metering dosing pump that injects it, an injection point in the discharge line, and a TRO analyser at an overboard sample point that measures the residual. A controller ties the analyser to the pump.
The make-up tank is where dry pentahydrate is dissolved in fresh water to a working concentration, commonly 100 to 300 g/L, well below the roughly 701 g/L solubility at 20 degrees Celsius. A separate day tank or a single tank with agitation ensures the pump always draws fully dissolved solution rather than abrasive undissolved crystals.
The dosing pump is typically a diaphragm or peristaltic metering pump sized to deliver the worst-case reagent flow with margin. Materials in contact with solution, including tank, pump heads, valves and pipework, must be compatible with thiosulfate, so appropriate plastics or stainless grades specified by the maker are used rather than reactive metals.
How Is the System Sized?
Sizing starts from the worst-case discharge: the maximum flow rate and the highest expected TRO. Using the rule of about 2.5 to 3.0 mg pentahydrate per mg of Cl2-equivalent TRO, calculate the peak reagent mass flow, then convert to a volumetric pump duty using the stock concentration.
For example, a 6,000 m3 discharge over four hours at 3.0 mg/L needs about 45 to 54 kg of pentahydrate, or roughly 11 to 14 kg per hour, which at 200 g/L stock is about 56 to 68 litres per hour of solution. The pump should be selected above this peak with turndown to handle low-TRO conditions without cycling.
The day tank is sized so it does not run dry within a discharge and can be replenished between operations, and the make-up tank is sized to the batch a ship prepares at once. Undersizing either forces awkward mid-discharge refills that risk interruption.
Where Should the Injection Point Be?
The injection point must be far enough upstream of the overboard sample tap that the thiosulfate has fully mixed and reacted before the analyser reads the residual. Thiosulfate reacts quickly, but if injection is too close to the sample point or the flow is poorly mixed, the analyser can read an unreacted spike and the controller can chase a false signal.
Turbulent, well-mixed flow at the injection point helps; a static mixer or a suitable length of pipe with adequate velocity ensures homogenisation. The sample point should draw representative flow, not a stagnant branch, so its location and tap design matter as much as the injection point.
Getting this geometry right is a common commissioning issue. Sea Clean provides independent, brand-agnostic support to review injection and sampling arrangements across BWTS makers so the control loop sees a true residual rather than a mixing artefact.
How Does the Control Loop Work?
The TRO analyser, using DPD colorimetric or amperometric measurement, sends the residual reading to a controller that modulates the dosing pump to hold the overboard TRO at a small positive setpoint below the approved discharge limit. This closed-loop control automatically compensates for changing TRO through the discharge.
The loop must be tuned to avoid hunting: too aggressive and it oscillates, too sluggish and it lags a rising TRO. A feed-forward element based on discharge flow, combined with feedback from the analyser, gives stable control across varying conditions, and the controller should log both TRO and reagent flow.
Analyser calibration is the loop's foundation. A drifted or fouled sensor makes the whole loop dose wrongly, so calibration against a known standard on the maker's interval, plus checks of DPD reagent age and optics, are essential maintenance, not optional.
What Maintenance Does the Hardware Need?
Routine maintenance covers the dosing pump (check valves, diaphragms or tubing, and calibration of delivered volume), the analyser (calibration, reagent replenishment for DPD units, and optics cleaning), and the tanks (cleaning to prevent buildup and microbial growth in standing solution).
Undissolved crystals are abrasive and can wear pump seals and block valves, so ensuring full dissolution and occasionally flushing lines with fresh water protects the pump. Standing solution can degrade and support growth, so batch make-up to near-term use and do not leave large volumes for long periods.
In cold climates, trace-heat or insulate the day tank and dosing lines to prevent crystallisation below about 5 degrees Celsius. Sea Clean, based in Norway, routinely advises on winterising thiosulfate injection systems for North Sea operation.
How Does It Integrate With the BWTS?
On an electrochlorination system the neutralisation skid is the discharge-side counterpart to the oxidant generation at uptake, and it must be interlocked so discharge cannot proceed with the analyser in fault or the reagent tank empty. On a chemical-injection system the oxidant and neutraliser skids coexist and must be arranged so a leak of one cannot reach the other.
The neutralisation loop should tie into the ship's ballast control and alarm system so that high-TRO alarms halt or throttle discharge and are recorded. This integration is what turns the hardware into a compliant, auditable process rather than a standalone pump.
Because the exact integration is defined by the type-approved system, always follow the maker's documentation. Sea Clean offers independent, brand-agnostic support to commission, maintain and document these injection systems, and is an authorized agent for Headway Technology; for vessels in Norway and the North Sea region we can typically attend within 0 to 24 hours.
Frequently asked questions
What does a sodium thiosulfate dosing system include?
A fresh-water make-up and day tank, a metering dosing pump, an injection point in the discharge line, and a calibrated TRO analyser feeding a control loop. The controller trims the pump to hold the overboard residual below the approved limit.
Where should the thiosulfate injection point be located?
Far enough upstream of the overboard sample tap, with good mixing, that the reagent has fully reacted before the analyser reads the residual. Otherwise the analyser can read an unreacted spike and the controller chases a false signal.
How is the dosing pump sized?
From the worst-case discharge flow and highest expected TRO, using about 2.5 to 3.0 mg pentahydrate per mg Cl2, converted to solution flow via the stock concentration. Select above peak duty with turndown for low-TRO conditions.
What maintenance does a thiosulfate skid need?
Pump check valves and calibration, analyser calibration and reagent replenishment, tank cleaning, and line flushing to clear crystals. In cold climates protect tanks and lines from crystallisation.
Sources
Related articles
- Sodium Thiosulfate and Electrochlorination BWTS Chemistry
- Sodium Thiosulfate Dosing Calculation for Ballast Water
- Total Residual Oxidant (TRO) Monitoring and Neutralisation
- Electrochlorination BWTS Explained: Electrolysis, TRO Generation and Neutralisation
- Sodium Thiosulfate vs Sodium Bisulfite for TRO Neutralization
- What Is a Ballast Water Treatment System? Explained
Request spares, calibration or ballast-water testing
Include BWMS brand/model, vessel/IMO number, port/terminal, date/ETA and scope or part numbers. These links prepare an email draft; you must send it yourself.
- Request a parts quote — Sensors, TRO reagents, calibration kits and replacement parts. Confirm compatibility, availability and delivery for your installed system.
- Request a calibration booking — Annual instrument calibration with a calibration certificate for the work completed. OEM instructions, drift or applicable rules may require earlier work.
- Request testing coordination — Arrange sampling and testing with an appropriately qualified party for your IMO D-2, commissioning or other stated purpose.
Authorised Headway agent. Other-brand enquiries are independently reviewed; no OEM authorisation is claimed. Sea Clean is not an accredited laboratory or approving authority. Calibration does not demonstrate biological D-2 compliance. Attendance, testing scope and parts delivery are confirmed per enquiry.