Most pleasure boats present their crews with a kind of paradox. Water is rationed on board while we float on 123 billion trillion liters of seawater... Another paradox, less pleasant, is that we bring dozens of water bottles to quench our thirst even though the tanks contain hundreds of liters and sometimes there’s even a watermaker on board. What are the solutions for producing and treating seawater and tank water to make it drinkable?
That’s the subject of this article.
Freshwater can be supplied from a marina water connection and stored in onboard tanks, as is the case on most pleasure boats, in a tank.
However, this solution has several drawbacks, because it forces boats to stop at ports to replenish water, and in Northern Europe, ports don’t provide water in winter due to frozen pipelines.
To avoid this constraint, a well-equipped cruising yacht should ideally be able to produce its own freshwater.
To do this, different levels of water treatment and filtration are available.
Sanitary clean water: This is the water used for showers, toilets, and washing machines. Tank water falls into this category.
Drinking water: Suitable for human consumption, this water requires specific treatment.
Throughout the history of sailing, freshwater was stored for months in water containers, tanks, or barrels before being served to sailors.
The reason is that microorganisms can multiply in freshwater tanks, even shielded from light, as some microorganisms can still thrive.
In the age of sail, dysentery was common, and no one really cared...
It's also worth mentioning that the Royal Navy's daily ration of rum (a pint in the 17th century, then half a pint in the 18th century, mixed with water, sugar, and lemon) likely helped eliminate some germs... This is actually where the expression "hot toddy" originated.
A watermaker uses high pressure to force seawater through a reverse osmosis membrane under high pressure (60 bars) through a membrane with very fine pores that block salt crystals, bacteria, algae, and microorganisms.
The freshwater produced is then transferred to the boat's storage tanks. However, these tanks are not always clean, and as a result, this precious freshwater, energy-intensive to produce, is usually not consumed... For that, it must be filtered and sterilized.
A seawater desalination filtration system consists of several components:
Strainer
A strainer is a filter with a mesh insert (0.5 to 3 mm spacing) that filters out the largest particles.
It should be installed right before the pump to protect it from damage. The mesh filter can be easily cleaned and reused.
Filter
Filters are categorized by pore size and the type of filter itself. Pore sizes range from 0.050mm to 0.0001mm. Since water must be pressurized to pass through the filters, they are installed after the water pump.
Activated Carbon Filters
Activated carbon is a special type of filter with very fine pores (0.15 to 0.3 mm) and chemical properties that improve water taste while filtering chlorine, hydrocarbons, and bacteria.
Reverse osmosis membranes are composed of a material so dense that high pressure is required to "push" water through the membrane at the molecular level, thereby filtering out bacteria, viruses, and chemicals.
There are two types.
These membranes work with the pressure of a standard water pump (around 3 bars) and purify freshwater to make it drinkable. Contaminants accumulate on the membrane surface, which clogs over time, causing the flow rate to gradually decrease.
The volume of water that can be filtered depends heavily on the quality of the original water and the size of the filter.
These membranes are designed to produce drinking water from saltwater by filtering everything, including sodium and chloride ions that carry the salt in seawater.
This filtration process uses high-pressure pumps operating at nearly 60 bars.
Unlike freshwater membranes, suspended particles in the water that are filtered out do not remain in the membrane but pass through it and are flushed out. This reduces clogging of the membranes and extends their lifespan.
Desalination systems can produce large amounts of water but require significant electrical power due to the power of the pump needed to operate.
Reverse osmosis membranes are sensitive to contaminated seawater, and its use is not recommended in ports or marinas where occasional oil spills or other contaminants can be expected.
UV sterilization
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Ultraviolet (UV) light is an effective way to eliminate bacteria, algae, and other living microorganisms.
Nowadays, low-energy consumption LEDs are used for this purpose.
Operation of the Purification Chain
The typical process for producing drinking water from seawater:
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A simpler option is available for boats that refill their tanks with shore-supplied freshwater.
The tanks are made of plastic, which gives the water an unpleasant taste. Water can stagnate, and bacterial growth can render it unfit for consumption.
For these reasons, and to reduce reliance on bottled water and the associated plastic waste, a simple system for purifying already potable tank water can be installed.
To do this, only two components from the system described above are needed: the activated carbon filter and the UV lamp, installed under a tap. Ideally, and unlike the standard factory-installed configuration on some Lagoon models, a dedicated draw-off point should be used.
