Hot, salty, and under scrutiny: the shipboard brine most ports don’t measure

On paper, distilling seawater into drinking water looks simple. At sea, it’s a different math problem: distillers (“marine evaporators”) can produce roughly 17 gallons of brine for every 1 gallon of freshwater, according to U.S. Navy/EPA documents (nepis.epa.gov). For a typical 12,000 gpd (gallons per day) freshwater plant, that’s ~136,000 gpd of brine (nepis.epa.gov).

Reverse osmosis (RO) plants run closer to ~4:1, a contrast that’s familiar to operators of sea‑water RO systems. But evaporators create an additional twist: the discharge is not just salty—it’s hot.

Evaporator brine volumes and chemistry

The distillate by‑product is essentially concentrated seawater. With ambient seawater at roughly 34–35 ppt (parts per thousand), a 17:1 brine ratio implies a salt level ~17× higher; at a 35 ppt feed, that back‑of‑the‑envelope puts the brine near ~600 ppt. The brine also carries any treatment chemicals or corrosion products from the plant. Studies of ship evaporator discharges report elevated nutrients (ammonia, nitrates, phosphorus) and trace metals (Cu, Fe, Zn, etc.)—often above ambient water‑quality criteria (nepis.epa.gov; researchgate.net). Where dosing is used, accurate chemical addition is typically handled by chemical dosing pumps.

Thermally, evaporator brine is warm: steam‑heated distillers typically discharge at 100–120 °F (≈40–50 °C) (nepis.epa.gov). Engine cooling seawater is cooler but moves in much larger volumes, a fact that matters for dilution.

Regulatory requirements and mixing zones

There is no international treaty that explicitly bans the discharge of pure salt from a desalination evaporator (brine is a seawater concentrate). Ships, however, must comply with general pollution‑prevention laws. Indonesia is party to MARPOL (International Convention for the Prevention of Pollution from Ships) and UNCLOS (United Nations Convention on the Law of the Sea), so any discharge “that causes harm” is effectively prohibited. In practice, brine falls under the broad mandate to avoid environmental harm. In the absence of a specific norm, mixing‑zone principles apply: at the edge of the mixing zone (the area around an outfall where higher concentrations are allowed), global guidelines suggest effluent should be ≤5% above ambient—implying ~20:1 dilution (researchgate.net).

Indonesian rules refer to general water quality standards (“baku mutu”), which typically set limits for pollutants (organics, toxics) rather than salinity per se. There is no known national standard capping salinity for discharged brine. Sectoral permits may impose best‑practice limits, and Indonesian shrimp‑farm discharge guidelines hold salinity ~28–30 ppt and require dilution if higher (id.scribd.com). The Ministry of Marine Affairs/Environment requires any outfall to meet ambient chemistry targets. Practically, a ship operator in Indonesia must show that brine discharge won’t elevate local salinity beyond natural variability (often presumed ±5% of normal saline).

Some maintenance operations are time‑and‑place sensitive. EPA guidance notes distiller citric‑cleaning cycles should be scheduled for open waters; spent citric acid solutions are normally held until the ship is >12 nm offshore, echoing the logic in sewage law (MARPOL Annex IV) for untreated discharges (nepis.epa.gov).

Environmental impact and monitoring metrics

Because of high salinity and temperature, brine can stress marine life. Even modest changes can impact osmoregulation in coastal species; dense, warm, saline plumes may sink or float depending on temperature, with the greatest effects near the source (researchgate.net). Without mitigation, a plume could affect eggs, larvae, or corals within tens of meters.

Operators rely on basics: brine flow rate (m³/day), salinity (often tracked as TDS—total dissolved solids), temperature, ambient currents, and the radius of influence of >5% salinity rise. One EPA analysis modeled U.S. Naval plants discharging ~2.47 billion gallons (9.35×10^9 L) of brine within 12 nm/year—mostly in port—illustrating the magnitude (nepis.epa.gov). Point‑source brine is usually highly diluted quickly, but near‑field verification matters: sample outfall TDS and temperature, and if co‑discharged, check the blended salinity at 3 nm, 10 nm, etc. Logs of freshwater generation and discharge hours help with Coast Guard and port‑state inspections. Supporting field gear such as sensors often sits within water‑treatment ancillaries.

Dilution strategies and co‑disposal mechanics

Engineering analyses show that maximizing initial dilution is the first lever. Multiport diffusers, common on coastal outfalls, enhance near‑field mixing, and pre‑dilution—mixing brine with another effluent before release—can dramatically shrink the required mixing zone (intechopen.com). Simply co‑releasing brine and seawater without pre‑dilution increases the mixing challenge, but shipboard systems have a built‑in ally: engine cooling seawater.

Cooling flows are orders of magnitude larger than brine discharge. By combining the evaporator brine line with the engine cooling discharge, brine is pre‑diluted before sea entry. If cooling flow is 10–30× the distiller flow (typical for a big engine), initial brine concentration drops by that factor; a 10:1 pre‑mix yields a composite salinity much closer to ambient, moving toward the ~20:1 target consistent with the ≤5% guideline (researchgate.net). The dilution ratio at release becomes (Q_cooling+Q_brine)/Q_brine. Cross‑connecting the brine into a cooling‑water line with valves is a practical retrofit concept.

Compliance caveat: MARPOL Annex I prohibits oily water discharges, so the cooling stream must be free of oils/chemicals. Oil filters or separators should be upstream of any mixing. Provided the outlet remains “clean” (i.e., within oil or pollutant limits), studies find pre‑mixing with large‑volume streams lowers final concentrations and reduces diffuser demands (intechopen.com).

Once discharged, hull‑wake turbulence and ambient currents further dilute the plume. Regulations still require minimal excess salinity near sensitive receptors (coral, intakes, shore), so validating with buoys or sensors near the outlet and a few hundred meters away can confirm compliance.

Operator checklist and dilution targets

• Quantify brine output: Use [44] and [46] ratios to estimate daily brine flow. Smaller ships (12,000 gpd) produce ~100–200 m³/day of brine; carriers produce several thousand. Record production vs. marine time.
• Adhere to mixing guidelines: Aim for ≥20:1 dilution. If brine is discharged alone, ensure adequate distance and mixing (fast transit speed helps). Co‑disposal with cooling flows can raise dilution to meet or exceed this benchmark. Cite [39] for the 5%/20:1 criterion (researchgate.net).
• Monitor effluent quality: Measure salinity and temperature of output. In Indonesian waters, ensure discharges meet general port/regional water quality targets (e.g., salinity ≈ ambient). Maintain logs for compliance audits.
• Maintenance best practices: Dump acid‑cleaning brines offshore and only dilute brine in healthy mixing conditions; EPA notes spent citric acid solutions are normally held until the ship is >12 nm offshore (nepis.epa.gov). Avoid discharging high‑salinity brine in harbor or ecologically sensitive zones.
• Leverage co‑disposal: Where practicable, route evaporator discharge into a high‑flow cooling seawater line before ejection; pre‑mixing is data‑backed to lower effluent concentration (intechopen.com; researchgate.net). Evaluate ship‑specific flow rates to confirm dilution ratios (e.g., if cooling water is 10× brine, composite salinity is ~10% of raw brine).

Modeled outcomes and practical radius

Preliminary models (by analogy to coastal outfalls) show that pre‑mixing brine with a large‑volume flow cuts maximum salinity rise from hundreds of ppt above ambient to only a few ppt (researchgate.net; intechopen.com). Coastal plants in Singapore and Dubai routinely achieve ambient‑level salinity within ~50–100 m using multiport diffusers. On ships, similarly aggressive dilution means that, beyond a few hull‑lengths, brine ceases to be distinguishable chemically from seawater.

Bottom line for compliance

No statute forbids “salt” outright, but the standard is clear: keep the salinity increment small. The consistent target in guidance is a ≤5% rise at the mixing‑zone boundary—about 20:1 dilution (researchgate.net). By maximizing dilution—especially via co‑disposal with cooling water—timing maintenance for offshore, and verifying with simple monitoring, operators can meet Indonesian and international expectations while keeping the ocean’s chemistry essentially natural (researchgate.net; intechopen.com).