Water filters on a boat: How to keep drinking water clean right down to the tap

The water’s journey from the tank to the taps passes through a system comprising hoses, a pressure pump, a pressure-compensating tank, a boiler and fittings, which is rarely flushed through completely during normal on-board operation. Even if the tank remains free of bacteria thanks to a highly effective preservative, biofilms can form in these areas and find their way into the water being drawn off. Anyone who wishes to clean their tank with a chlorine-based product will also be familiar with a common effect: a slightly stale chlorine taste quickly develops, as is often the case on many boats. Some filter systems remove the chlorine from the water, thereby noticeably improving the taste. However, if the filter system is positioned directly downstream of the tank, it simultaneously negates the effect of the cleaning agents intended to clean the entire on-board system via the tank.

More on this topic:

• ​Pressurised water system: Running water on board – what you need to bear in mind
• ​Clean water in the tank: Which drinking water filters are best for boats?
• ​Workshop: How to disinfect the on-board drinking water tank in four steps
• ​Drinking water on board: Here are some ways to filter water properly

Mechanical pre-filter, activated carbon and hollow fibre

As with filtration upstream of the tank, there are also various stages of filtration systems downstream of the tank which affect the filtration performance. The first stage is a mechanical pre-filter, which traps coarse particles, rust flakes and deposits from the pipes. It protects the subsequent, finer stages from becoming blocked.

The second stage is handled by activated carbon, as already described, here in the form of a highly compressed block. Higher-end manufacturers also impregnate their elements with silver, which prevents bacterial growth in the damp filter bed whilst the water is left to stand. The third stage consists of a hollow-fibre membrane, which acts as a physical barrier against bacteria. Some manufacturers also offer complete solutions that combine several filter stages in series.

​The advantage is that, in theory, if you install a solution like this, you won’t need any further filter systems. However, care should be taken with regard to service life. Most systems need to be replaced after a certain period of time or once a certain flow rate has been reached. Many manufacturers now offer replacement filters, so you don’t have to buy a whole new system.

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UV-C disinfection as an alternative

UV-C disinfection systems are also effective. They offer an alternative to the membrane-based third filtration stage. The water passes through an irradiation chamber, where UV-C light from modern light-emitting diodes destroys the DNA of the microorganisms. LED-based systems require no warm-up time and, in principle, have no expiry date, unlike conventional filter systems. Their power consumption varies depending on the specifications. Thanks to integrated flow sensors, the irradiation can be automatically activated in some products and deactivated when the system is not in use. In standby mode, the power consumption of such systems is in the milliampere range.

Compared with membrane systems, UV-C disinfection has one key advantage: it does not cause any significant pressure drop in the pipework. Provided a pre-filter is fitted or the water does not contain any coarse contaminants, the filters can be installed regardless of the pump capacity.

The capacity the pump needs to deliver

Any filter stage installed downstream of the tank restricts the flow of water. With coarse pre-filters, this effect is barely noticeable. Dense hollow-fibre membranes and highly compacted activated carbon blocks, on the other hand, require sufficient pump pressure to function reliably. Simple submersible pumps, such as those commonly found on older yachts, are often not powerful enough for this. Anyone retrofitting a fine-mesh filter system should therefore check whether their own pump is up to the task.

Only the entire chain protects the system

Filling filters, tank preservation and post-tank filtration are not independent, individual measures, but rather links in a chain. A good filter when filling the tank protects the tank from external contaminants, but cannot prevent the water from starting to spoil whilst the boat is moored for weeks on end in a warm hull. An effective preservation system keeps the water in the tank stable, but has no effect on the pipes between the pump and the tap, where biofilms can build up. A high-performance post-tank filter protects the tap, but offers only limited protection against bacteria that have long since established themselves in the tank and from there colonise the entire on-board system. The level of effort that is reasonable depends on the sailing area and how the boat is used.

The area plays a crucial role

A weekend sailor on the Baltic Sea who uses their boat regularly and refuels at well-stocked marinas has different requirements to a recreational boater who sometimes leaves their boat moored for longer periods or who has to rely on water from unregulated sources. As a general rule, a fuel tank that has never been cleaned cannot be fully compensated for by any filter technology.

For European pleasure boaters who regularly stay overnight in well-equipped marinas, a mechanical coarse filter combined with an activated carbon stage should, in most cases, be sufficient. Anyone refuelling in the Mediterranean or in non-European ports, where the microbiological quality of the fuel pumps cannot be guaranteed beyond doubt, should consider incorporating a hollow-fibre membrane as a final safety measure. Ceramic filter elements, whose surface can be brushed with a simple sponge to regenerate them when the flow rate decreases, are the more durable option here.

A high-tech filter on the tap is of little use if the tank and pipes are neglected. Please let us know in the comments what combination of tank maintenance, filter and pump you use on board.