How to prevent Legionella in marine water systems

In recent years, sustainable water management onboard vessels has become a central concern in the nautical industry, for shipowners Legionnaires , captains , shipyards , and charter companies alike . Beyond guest comfort and operational efficiency, the health and safety of ship water systems has become a priority, especially given the risks associated with the proliferation of bacteria such as Legionella pneumophila . This microorganism, responsible for ‘ disease , can develop in poorly maintained water systems, posing a serious health risk to crew and passengers.

This article analyzes the causes, risks, and technological solutions for preventing Legionella in marine water systems , offering practical and up-to-date guidance for those working in the yachting , marine services design industries , and onboard water system . The goal is to provide value through a technical yet accessible approach, in line with international regulations and best marine sustainability practices .

Types of water on board: differences and critical issues

Before addressing the topic of legionella , it is essential to understand the different types of water present on board a vessel and the related management implications.

Fresh water: the primary resource for human consumption

Fresh water is used for drinking, cooking, washing, and showering. On board, it is usually produced by desalination plants that convert seawater into potable water, or stored in tanks after refueling in port.

However, even initially potable water can become a vehicle for contamination if not managed properly. Temperatures between 25°C and 45°C, stagnation, and the presence of biofilms favor the growth of bacteria, including Legionella . Critical points include taps, showers, storage tanks, and poorly insulated pipes.

Greywater: an often underestimated risk

Graywater comes from sinks, showers, kitchen sinks, and washing machines. It doesn’t contain fecal matter, but it’s rich in soap, detergents, oils, and organic residues. If not treated promptly, it can degrade, create odors, and encourage microbial growth.

On many yachts, grey water is piped directly into the sea or into storage tanks, but in protected areas (e.g., MARPOL zones or marine reserves), its discharge is prohibited. Inefficient management can cause blockages, pipe corrosion, and cross-contamination with fresh water.

Wastewater: regulated management and environmental impact

Black water is sanitary effluent, containing fecal matter and urine. On board, it is collected in dedicated tanks (black water tanks) and treated using disinfection systems or disposed of in port.

If not treated properly, sewage can contaminate freshwater through malfunctioning valves or poorly designed systems. Furthermore, illegal discharge into the sea violates international regulations and damages the marine ecosystem.

International regulations and safety standards

Water management on board is regulated by a set of international standards that aim to ensure health, safety and sustainability .

MARPOL Annex IV: Rules for the Discharge of Waste Water

Annex IV of the MARPOL (International Convention for the Prevention of Pollution from Ships) treaty establishes the conditions for the discharge of sewage. In seas designated as “special areas,” discharge is prohibited unless it occurs through an approved treatment system and within a minimum distance from the coast.

Grey water is also increasingly regulated, especially in sensitive areas such as the Greek islands, the Balearics or the waters off California.

ISO 24444: On-board water treatment systems

ISO 24444 provides guidelines for the design, installation, and maintenance of marine water systems , with particular attention to preventing microbial contamination. It includes recommendations for piping materials, storage temperatures, inspection frequency, and disinfection procedures.

WHO and CDC guidelines for Legionella

The World Health Organization (WHO) and the Centers for Disease Control (CDC) indicate that Legionella grows in stagnant aqueous environments with temperatures between 20°C and 50°C. They recommend:

Keep hot water above 60°C in boilers and water heaters.
Avoid stagnation with continuous circulation.
Rinse showers and faucets periodically.
Clean and disinfect tanks at least twice a year.

For boats, these recommendations are difficult to apply consistently, especially during periods of inactivity or in hot climates.

Innovative technologies for sustainable water management

Fortunately, in recent years technological solutions have been developed that can ensure safety, efficiency and sustainability in naval water systems .

High-efficiency, low-energy watermakers

Modern reverse osmosis watermakers offer superior quality soft water, with 30-40% lower energy consumption than previous models. Some systems integrate UV filters and activated carbon post-treatment to eliminate bacteria and odors.

A well-sized watermaker reduces the need for resupply in port, limiting exposure to potentially contaminated water sources.

UV disinfection and ozone treatment systems

is UV light disinfection an effective, chemical-free technology for eliminating bacteria, viruses, and protozoa, including Legionella . UV systems are installed inline, immediately after the freshwater tank or in the recirculation loop.

Ozonation , on the other hand, introduces ozone ( O₃) into the water, destroying microorganisms and oxidizing organic compounds. It is particularly useful for treating gray and black water, reducing odors and bacterial load before discharge.

Real-time monitoring with IoT sensors

The new IoT monitoring systems allow you to monitor in real time:

Water temperature in tanks and circuits.
Pressure and flow.
Presence of biofilm or microbial contamination.
Residual chlorine level or conductivity.

This data can be viewed by commanders and technicians via dedicated apps, enabling preventative interventions before problems arise.

Innovative materials for pipes and tanks

The use of pipes made of antibacterial materials (e.g., copper-silver or polymers with antimicrobial additives) reduces the formation of biofilm, the primary breeding ground for Legionella . Tanks made of AISI 316L stainless steel or food-grade composite are preferable to those made of non-certified plastic to avoid the release of unwanted substances.

Case studies: refits, charters, and large yachts

Refitting a 40-meter yacht: from risk to water safety

A 40-meter yacht, idle for six months, reported cases of fever among guests after the first cruise of the season. Microbiological tests revealed the presence of Legionella in the hot water tanks and showers.

The shipyard has performed a complete refit of the water system :

Replacing plastic tanks with stainless steel tanks.
Installing an inline UV system.
Integration of a recirculation circuit with pump and thermostat.
Chemical and thermal cleaning of pipes (thermal sanitization at 70°C).

After the intervention, tests confirmed the absence of pathogenic bacteria. The system is now monitored monthly via IoT sensors.

Charter Companies: Safety Protocols for Multiple Fleets

A charter company with 15 yachts in the Mediterranean has implemented a standardized water management protocol:

Quarterly microbiological control.
Emptying and cleaning tanks between seasons.
Use of desalination plants with UV post-filtering.
Crew training on shower disinfection procedures.

Thanks to this approach, the company has reduced complaints related to odors and skin problems by 90%, improving its reputation and customer repeat business.

Large yacht (80+ meters): integration with intelligent onboard systems

On megayachts, the complexity of the systems requires integrated solutions. One example is the adoption of water management systems that coordinate: centralized

Production and storage of fresh water.
Grey and black water treatment.
UV disinfection and temperature control.
Automatic alarms in case of anomalies.

These systems not only prevent legionella, but also optimize consumption, reduce environmental impact, and facilitate regulatory compliance.

Benefits for owners, crew and guests

Adopting a sustainable and technologically advanced approach to onboard water management brings tangible benefits for all stakeholders.

For shipowners: safety, value and compliance

Reduced legal risk : Preventing diseases like Legionnaires’ disease avoids fines, compensation claims, and reputational damage.
Higher resale value : A modern, certified plumbing system is an attractive asset for buyers.
Regulatory compliance : facilitates entry into ports and protected areas with stringent environmental regulations.

For Captains and Crew: Operation and Well-Being

Greater system reliability : fewer breakdowns, fewer emergency interventions.
Healthier work environment : reduction of odors, mold and infectious diseases.
Training and professionalism : adopting clear protocols improves crew competence.

For guests: comfort and safety

High quality drinking water : without unpleasant tastes or odors.
Sanitized showers and bathrooms : reduced risk of skin or respiratory infections.
Complete luxury experience : comfort is not only aesthetic, but also sanitary.

Sustainable water management is a strategic choice

Preventing Legionella in marine water systems is not simply a matter of maintenance, but a strategic commitment to safety , sustainability and operational excellence .

In a sector like yachting , where every detail counts, neglecting water quality puts the health, image, and value of the vessel at risk. Conversely, investing in eco-friendly technologies , intelligent monitoring , and crew training transforms a technical system into added value .

For shipowners , captains , shipyards , and charter companies , sustainable water management is no longer an option: it’s a responsibility. And, above all, an opportunity to stand out in a market increasingly attentive to quality , safety , and respect for the marine environment .