Skip to content A watermaker is an essential piece of equipment for many sailors and motor yacht owners. It converts seawater into clean drinking water. To protect both the reverse osmosis membrane and the system’s pump, seawater must be thoroughly filtered beforehand. The sediment filter is the critical barrier against sand, silt, algae, plastic particles, and other debris.
In this article we will cover:
Seawater contains far more than just salt. Typical contaminants include:
Without an effective pre-filter, you risk:
1. Depth Filters
Depth filters trap particles not only on the surface but also within the filter structure. This allows for a high dirt-holding capacity. There are two main types:
Spinwound (string-wound)
Advantages: Generally very low pressure drop, good dirt-holding capacity.
Disadvantages: Lower filtration efficiency compared to other filter types.
Melt-blown
Advantages: Very uniform pore structure, high efficiency with fine sediments, no loose fibers.
Disadvantages: Pressure drop is higher than with wound or pleated filters.
2. Pleated Filters
Low pressure drop when clean.
Lower dirt-holding capacity compared to depth filters.
Washable and reusable, depending on the material.
Ideal for moderately turbid water.
3. Screen or mesh filters
First coarse stage for large particles.
Easy to rinse and clean.
In short: Yes!
Nominal vs. absolute filter rating
Filter fineness is often given in microns – but the key is the efficiency which greatly differs if it’s a nominal or absolute rating:
Nominal: This includes most commercially available filters. A nominal rating of, for example, 5 µm does not mean that no particles of 5 µm or larger will pass through. The efficiency between different nominal filters can vary widely, between about 70–95%. This means there are significant quality differences that are not immediately apparent.
Absolute: Costing several times more than nominal filters, absolute filters guarantee that 99.98% of particles of the rated size will be retained. Except for certain very specific pumps, this is usually not necessary for watermakers.
Pleated filters are especially suitable if you want to wash them out. It’s important that they are made entirely of plastic. This makes them significantly more expensive than cheaper versions that mostly contain cellulose or are entirely made from it. When washing them, you must work meticulously to avoid cross-contamination to the “clean” side of the filter.
Since there are several points to watch out for, and because filters cannot be washed indefinitely, we generally prefer depth filters.
Wound filters are recommended as a first stage in a two-stage filter system due to their low pressure drop, or for small systems with no or only a weak feed pump. In such cases, the lower filtration efficiency is usually sufficient to still provide satisfactory protection for the system.
For larger or more sensitive systems with energy recovery, melt-blown filters with high filtration efficiency should clearly be used to best protect the system.
The difference of about 6 mm between 9 ¾” and 10″ filters may seem small, but it has a major impact on proper sealing in the filter housing.
9 ¾ inch: Common in the marine sector, precisely fitting for many watermaker prefilter housings.
10 inch: Often used in domestic systems, but may fit too tightly or too loosely in some housings. Tolerances are also large – not every nominal 10″ cartridge fits in every 10″ housing.
Why this matters:
If the length isn’t right, the sealing lip cannot close properly. Water may bypass the filter and flow unfiltered into the membrane, or the housing may not close properly, allowing water to leak out.
Check regularly: Especially before long trips or after use in turbid water.
Replace in time: When flow rate drops or when pressure between filter and pump falls. In Ocean-Spring models, you can conveniently see this on the pressure gauge on the control panel. Otherwise a visual control of the filter will tell you whether to change or not – If it’s dark or even black on the outside, it’s time to Change!
Store spare filters on board: Especially in remote areas.
Don’t filter too finely: Use filters below 5 microns only when necessary to avoid stressing the pump.
Rinse after each use: Slows microbial growth and reduces the load on your membrane.
The sediment filter is the heart of prefiltration in any watermaker. Choosing between wound, melt-blown, or pleated filters – in the right length and with proper sealing – ensures reliable performance and a long service life for your system.
