Ship Evaporators Make Pure Water — And a Hidden Potability Gap Shipowners Now Have to Close

Shipboard evaporators (distillers) deliver extremely pure water with very low TDS (total dissolved solids). Yet that purity is precisely the problem: the water is not automatically potable, and in practice it needs two downstream treatments — a final disinfection barrier and remineralization (“re‑hardening”) — before it can be declared safe.

The World Health Organization’s Guide to Ship Sanitation urges a multi‑barrier approach to water safety on vessels and notes that about 20% of historical shipborne disease outbreaks were attributed to contaminated water (WHO guide). Indonesian drinking‑water regulations (Permenkes 492/2010) require zero E. coli and total coliforms per 100 mL in finished water and set pH and TDS limits — pH 6.5–8.5; TDS ≤500 mg/L (regulatory zero coliform; pH/TDS limits). Evaporator output typically has pH below 6 and effectively zero minerals, so it must be adjusted and disinfected.

Add a practical complication: many ship evaporators use waste‑heat and operate below 80 °C. WHO flags that water produced below 80 °C needs disinfection before being potable (WHO evaporation guidance).

Microbiological safety: final disinfection

Even low‑TDS distillate can harbor pathogens unless temperature and contact time were lethal — a threshold many sub‑80 °C units won’t reach. A final chlorination step (e.g., sodium hypochlorite or similar) is therefore used as a last barrier, typically at the point the water enters storage. A 2005 survey of 950 potable‑water samples from 342 merchant ships found coliforms (E. coli or enterococci) in 8.6% of samples, highlighting residual fecal contamination risk onboard (International Maritime Health survey).

Guidance points to maintaining free chlorine residuals ≈0.5–1 mg/L at system entry and at least 0.2–0.5 mg/L throughout distribution (WHO chlorine residuals). WHO specifies an initial free chlorine level ≥0.5 mg/L after ≥30 minutes contact at pH < 8 for primary disinfection, with a minimum 0.2 mg/L free chlorine at the tap (WHO chlorine residuals). Chlorine dosing is typically metered with a dosing pump to hold that residual through tanks and lines.

Chlorine residuals and contact time

As a practical benchmark, 0.5 mg/L free chlorine with 30 minutes contact inactivates most bacteria and many viruses (same WHO reference: WHO chlorine residuals). Legionella control in cold water usually calls for a ≥0.5 mg/L residual as a preventive measure (WHO chlorine residuals). In small ship tanks, 30–60 minutes of contact time is readily achieved. Operators often use simple dose tables: to end up with ~0.5 mg/L free chlorine, an initial 2–3 mg/L dose is common, depending on demand.

After dosing, pH is checked because a high pH reduces chlorine efficacy (WHO pH effect). If contamination is detected (e.g., a positive coliform sample), shock chlorination raises free chlorine to much higher levels (tens of mg/L) for a few hours; WHO cites a “superdose” such as 50 mg/L for 4 hours in tanks if gross contamination is suspected (WHO superdose). In routine operation, holding a low‑level residual of 0.2–0.5 mg/L is usually sufficient. Indonesian law mandates no coliforms (hence disinfection is de facto required), though the regulation does not specify residual chlorine and generally follows WHO practice (regulatory zero coliform).

Alternate disinfection technologies (UV/ozone)

Some vessels use UV irradiation (ultraviolet) as the final step instead of chlorine. UV inactivates microbes but has no residual effect — any downstream contamination is uncontrolled (WHO on UV residual). As a result, even UV‑treated systems often rely on a small chlorine residual. Ozone or other non‑chlorine oxidants can be applied, but they likewise require careful handling and typically leave no lasting protection. Where UV is selected, operators deploy purpose‑built units such as ultraviolet systems within the potable train.

Remineralization for taste and corrosion control

Distilled water is nearly pure H₂O — good for removing salts, but bad for palatability and materials. Sensory studies show waters with <25 mg/L of major minerals (calcium, magnesium, etc.) often taste flat or slightly bitter, while waters exceeding ~500–600 mg/L TDS taste unpleasantly salty (MDPI taste review). A step change of ~180 mg/L TDS is generally detectable by consumers (ResearchGate taste threshold). Ships therefore add modest hardness — commonly bringing calcium and bicarbonate into the tens of mg/L as CaCO₃ equivalent — and in practice, raising total TDS by 100–300 mg/L makes the water taste livelier (still below the ≤500 mg/L national limit: pH/TDS limits). One guideline suggests combined Ca+Mg of ~60–120 ppm as optimal; communities with that level showed lower cardiovascular disease in epidemiological studies (MDPI hardness and health).

Chemically, demineralized water is “inherently acidic and therefore corrosive” to infrastructure — it can leach metals from tanks and piping (IntechOpen on corrosivity). The fix is reintroducing carbonate hardness. WHO notes the condensate is “free from any minerals and almost free from carbon dioxide,” so it “is necessary to add carbon dioxide” before re‑hardening (WHO rehardening note). In practice, operators add CO₂ and pass the water over a calcite (limestone) bed, or they dose lime (Ca(OH)₂) or calcium chloride (CaCl₂) with a neutralizing agent. The dosing can be controlled with a dosing pump and supported by simple contactors available as supporting equipment for water treatment.

Carbonate saturation and LSI targets

A common target is LSI (Langelier Saturation Index, a stability index indicating the tendency to dissolve or precipitate calcium carbonate) around zero (±0.5). Reviews note that most treatment systems aim for a zero or slight positive LSI, i.e., water just saturated with CaCO₃ (IntechOpen on LSI targets). In practice, post‑treatment pushes pH toward the upper end of the Indonesian standard — often 7.5–8.0 within the 6.5–8.5 requirement (pH/TDS limits) — with alkalinity raised into several tens of mg/L. One industry example targets alkalinity (as CaCO₃) of ~50–60 mg/L and calcium ~20 mg/L, with pH in the 7.5–8.5 range (aiming for LSI ≈ 0) (DrinTec remineralization example).

Remineralization also serves taste and policy: Indonesian rules specify potable water be “tidak berasa” (neutral taste) (pH/TDS limits). Anecdotally, mariners describe properly remineralized water as similar to spring water if a moderate calcium–magnesium balance is achieved. The net effect is less corrosive, more palatable water that aligns with both WHO guidance and national limits.

Regulatory anchors and system design

In short: final chlorination or equivalent plus remineralization is effectively mandated by ISO/WHO criteria and national standards — and by real‑world risk signals. Maintaining measurable chlorine residuals (e.g., ~0.5 → 0.2 mg/L from tank to tap) meets the “no E. coli/coliform” requirement (regulatory zero coliform) and WHO practice (WHO chlorine residuals), while carbonate re‑hardening to LSI ≈ 0 stabilizes pH and taste (IntechOpen on LSI targets; MDPI taste review). The 8.6% contamination finding at sea underlines why the last barrier matters (International Maritime Health survey), and why designing a simple post‑treatment train — contact time, residual verification, and carbonate stabilization — pays off in safety, acceptance, and system integrity.