How Much Sodium Thiosulfate Per Ballast Discharge?
Updated
The amount of sodium thiosulfate per ballast discharge depends on the discharge volume and the measured TRO. Using the practical rule of about 2.5 to 3.0 mg pentahydrate per mg of Cl2-equivalent TRO, a discharge of 1,000 m3 at 2.0 mg/L TRO needs roughly 5 to 6 kg of pentahydrate; scale linearly with both volume and TRO, and always trim to the measured residual rather than a fixed assumption.
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Key takeaways
- Reagent mass equals discharge volume times measured TRO times about 2.5 to 3.0 mg pentahydrate per mg Cl2-equivalent TRO.
- A 1,000 m3 discharge at 2.0 mg/L TRO needs roughly 5 to 6 kg of sodium thiosulfate pentahydrate.
- Consumption scales linearly with both discharge volume and TRO, so worst-case sizing plus closed-loop trimming is the right approach.
- A calibrated TRO analyser trims the dose to a near-zero setpoint rather than a fixed amount, giving compliance and economy.
- Match packaging (25 kg bags, 50 kg drums, 1000 kg bulk bags) to consumption and rotate within the two to three year shelf life.
- Convert reagent mass to solution volume by dividing by stock concentration; higher strength stock keeps day-tank volume down.
What Is the Quick Estimate?
The estimate is a single multiplication: reagent mass equals discharge volume times measured TRO times the specific dose. In consistent units, 1,000 m3 is 1,000,000 litres, so at a TRO of 2.0 mg/L the Cl2-equivalent load is 2,000,000 mg, or 2.0 kg of chlorine to neutralise.
Applying the practical dose of 2.5 to 3.0 mg pentahydrate per mg Cl2 gives roughly 5 to 6 kg of sodium thiosulfate pentahydrate for that 1,000 m3 discharge at 2.0 mg/L. The figure already includes the generous excess folded into the specific dose, roughly 3x the theoretical stoichiometric demand of about 0.9 mg per mg TRO.
Everything scales linearly. Double the volume or double the TRO and the reagent mass doubles, so a 5,000 m3 discharge at 2.0 mg/L needs about 25 to 30 kg, and the same volume at 4.0 mg/L needs about 50 to 60 kg. The British spelling sodium thiosulphate refers to the same reagent in these calculations.
How Do You Do a Full Worked Example?
Take a ship deballasting 6,000 m3 over four hours with a measured overboard TRO averaging 3.0 mg/L. The chlorine load is 6,000,000 litres times 3.0 mg/L, which is 18,000,000 mg, or 18 kg of Cl2 to neutralise across the whole discharge.
At 2.5 to 3.0 mg pentahydrate per mg Cl2, that discharge needs about 45 to 54 kg of sodium thiosulfate pentahydrate. Spread over four hours, the mass flow is roughly 11 to 14 kg per hour, and dividing by a 200 g/L stock solution strength gives a pump delivery of about 56 to 68 litres per hour.
This shows why packaging matters: a single large discharge can consume most of a 50 kg drum, so a ship with frequent or large discharges is better served by 1000 kg bulk bags, while an occasional discharger manages on 25 kg bags. Sea Clean supplies all three formats from Norway.
Why Not Just Use a Fixed Amount?
A fixed dose ignores the fact that TRO varies within and between discharges depending on the initial oxidant dose, holding time, temperature and water chemistry. Warm water and long holding times can leave a lower residual, while short holds or high initial dosing leave more, so a fixed reagent amount either wastes chemical or under-treats.
The correct method is closed-loop: a calibrated TRO analyser at the overboard point trims the injection pump to hold the residual near a small positive setpoint below the approved limit. The estimate here sizes the pump and the reagent stock for the worst case, while the analyser decides the moment-to-moment dose.
Sizing for the worst case and trimming to the measurement gives both compliance and economy. It also produces a defensible record, because the logged reagent use tracks the logged TRO rather than a fixed number that an inspector cannot tie to the actual discharge.
How Do You Plan Reagent Resupply?
To plan resupply, estimate typical consumption per voyage from your usual discharge volumes and TRO, then set a minimum holding level and a reorder trigger with a margin for unusually high-TRO discharges. A ship that knows it discharges, say, 20,000 m3 per month at around 2 to 3 mg/L can predict roughly 100 to 180 kg of pentahydrate per month.
Match packaging to that consumption so stock turns over within the two to three year shelf life without ever running short at a discharge. Overstocking a bulk bag on a low-throughput ship risks expiry, while relying on a single 25 kg bag on a high-throughput ship risks a stockout mid-discharge.
Keep a simple inventory log tied to the ballast record so consumption is visible and reorder is timely. Sea Clean provides independent, brand-agnostic support to size resupply to a ship's real trading pattern rather than a generic figure.
What Factors Push Consumption Up?
Higher residual TRO is the biggest driver: it scales the reagent directly, so systems run at higher oxidant doses, or discharges after short holding times where less natural decay has occurred, consume more thiosulfate. High-bromide seawater that forms persistent bromamines can also keep measured TRO high.
Large discharge volumes obviously raise total consumption, and frequent short ballast operations can use more reagent per cubic metre than a few large ones if each carries a high residual. Poor mixing or a mis-sited sample point can cause the controller to over-inject chasing a false high reading.
Correct salt form matters too: dosing anhydrous salt on a pentahydrate rule overshoots, so what looks like high consumption may be a dosing error. Verifying analyser calibration and salt form often reveals that apparent overconsumption is really an instrumentation or procedure fault.
How Do You Convert Reagent to Solution Volume?
Once you know the mass needed, convert to solution volume by dividing by the stock concentration in grams per litre. For the 18 kg example, at 200 g/L that is 90 litres of stock solution for the discharge, and at 300 g/L it is 60 litres, so a higher stock strength keeps day-tank volume down.
The pentahydrate's high solubility of about 701 g/L at 20 degrees Celsius means even a concentrated 300 g/L stock stays well below saturation and will not crystallise unless the temperature drops sharply. Use fresh water and dissolve fully before the solution reaches the pump.
Set the pump delivery from the solution concentration and the discharge duration, then let the TRO analyser trim around that baseline. Recording both the litres of stock used and its concentration lets you back-calculate the reagent mass for the ballast record.
Frequently asked questions
How much sodium thiosulfate for 1,000 m3 of ballast?
About 5 to 6 kg of pentahydrate at a TRO of 2.0 mg/L, using the 2.5 to 3.0 mg per mg Cl2 rule. Scale linearly with both volume and measured TRO.
Should I dose a fixed amount per discharge?
No. TRO varies with holding time, temperature and water chemistry, so a fixed amount wastes reagent or under-treats. Size the pump and stock for the worst case and let a calibrated analyser trim the dose.
How do I plan how much reagent to carry?
Estimate consumption from your usual discharge volumes and TRO, add a margin for high-TRO discharges, then match packaging to that rate. Keep an inventory log and rotate stock within its two to three year shelf life.
Why is my reagent consumption higher than expected?
Common causes are higher residual TRO, short holding times, a drifted analyser causing over-injection, poor mixing, or dosing anhydrous salt on a pentahydrate rule. Check calibration and salt form first.
Sources
Related articles
- Sodium Thiosulfate Dosing Calculation for Ballast Water
- Total Residual Oxidant (TRO) Monitoring and Neutralisation
- Buying Sodium Thiosulfate for Ships: Supplier Checklist
- TRO Neutralization Before Ballast Discharge: Requirements
- Sodium Thiosulfate Pentahydrate vs Anhydrous: Marine Use
- What Is a Ballast Water Treatment System? Explained
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