Anjo Sterringa recently approached SetSail about writing an article on watermakers. She wrote to us, “I am a cruiser fallen onto land, (Mallorca) where I now service and install watermakers. There is still a lot of mystery and different ideas about pre- and post-filtration of drinking water on board. I have seen a lot of different brands (Sea Recovery, Spectra, Aqua Base/Aquaset, HEM, Idromar, Aquafresh to name a few) in various conditions.” Having time on her hands while recovering from a broken leg, she proposed researching and writing an article for cruisers on watermaker filtration and maintenance.

How does a watermaker work? What does it have to do with plants? Or a pressure cooker?

Reverse Osmosis watermakers extract fresh water from salt (sea) water under pressure. When you think of plants: they extract water out of their surroundings through a semi-permeable membrane, having a lower solution inside the plant than outside. In other words: Osmosis is the process that describes the striving for an equilibrium between unequal liquid solutions which allows the liquid with the higher solution to pass through the membrane. To reverse this process, as is used in watermakers, you need high pressure. How much pressure? to give you an idea, in a pressure cooker you reach only 1bar/15 psi, and to make water through RO, the working pressure is around 60 bar (870 psi)!

The elements of the system

The heart of the system is a rolled-up semi-permeable material in a tube-the membrane in the pressure vessel-the photo shows a unit with three connected pressure vessels (the black tubes).

The necessary pressure is supplied by a high pressure pump (on the right side of the photo) and regulated by a valve (on the photo on top of the pressure vessels)

To assure the water flow and pressure coming into the high pressure pump, a booster pump is normally installed (the small pump on the left of the photo).

The most important result of the process is of course the fresh water (product water)-for this we return to the pressure vessel with the membrane inside. Salt water is pushed past the membrane under high pressure. Water molecules are allowed through the semi permeable membrane but salts and other solids are blocked and discharged. The fresh water leaves the tube through the middle.

The salt water flowing into the system should be as uncontaminated as possible. Pollution or particles present in the water can damage or clog the membranes. Normally there is an inlet strainer before the booster pump and at least two filters after the booster pump. (the two blue filters on the photo). Pre-filtration possibilites are particle filters and oil-water separators.

The process can be controlled and safe guarded by pressure switches, so that the unit will switch off when there is too much or too little pressure. The salinity of the product water can be monitored through a salinity sensor and be either discharged or sent to the tank.

Filtration general: Microbes and other animals

You can filter water to stop particles passing through (sand, weed, dust etc.) or to stop pollutants (bacteria, chemicals, viruses) coming through by using particle filters or absorption/reactive filtration. The “ultimate” filtration process (hyperfiltration) is Reverse Osmosis, as used in watermakers which will filter molecules and ions out of a solution, as small as 1/100 micron.

Particle filtration: Particle fitration will filter solids out of the water up to 1 micron in size. 1 micron (micro meter) is 0,000001 Meter. 10 micron is 0,01 millimeter. 1 micron is only visible under a microscope!

Particle filters are normally combined in a series of first the coarser filters (a strainer, 50 micron, 25 micron) and then the finer filters (5, 1 micron) The finer filters would clog immediately if they were the first in line. Particle filters do not prevent pollutants that are dissolved in the water (oil!) to pass through, nor will they stop all bacteria or any viruses-as they are mostly smaller than a micron.

Absorptive/reactive filtration: Absorptive filters contain a material (medium) that absorbs or reacts with a contaminant in water.

Examples are carbon filters which remove taste, odor and chlorine and organic chemicals (diesel, gasoline, dissolved oil) and oil-water separators that absorb oil.

The effectiveness of these filters depends on:

• the kind of contaminant: non soluble (hydrophobic) compounds are more likely to be absorbed to a solid (the filter medium)
• its attraction to the medium surface (e.g. activated carbon)
• the contact time between the contaminant and the filter material (subject to waterflow and size of the filter). The longer the contact time, the more effective. Some contaminants need a longer contact time than others.

Pre-filtration of the salt water (point of entry SW filtration)

As the filtration through the membranes is at hyper filtration level, you want the incoming salt water to be as free from contaminants as possible.

Pollution or particles present in the water can damage or clog the membranes.

Simple inlet strainer with service valve.

Inlet strainer: Normally there is an inlet strainer before the booster pump (the same kind of strainer as an engine cooling inlet has).

Sandfilter with valves for operation, backwash and bypass.

Sand filter: To protect the prefilters after the booster pump, and to prolong filter life, a sandfilter can be installed before the particle filters-the sand is backwashable and will take a lot of the particles out. Filter life can be tripled using a sand filter. You do need a lot of room for it though-the bigger it is, the more effective it is.

Particle filter: After the booster pump often two particle filters are installed: 25 and 5 micron.

Oil-water separator: When there is the risk of getting oil into the system, an oil-water separator (oil absorbing filter) can be added-either in the end of the line or in the beginning because it will also act as a 20 micron filter. These cartridges are quite expensive, which may be a reason to place them in the end of the line. The same type of cartridge can be used for the filtration of bilgewater.

Electronic biofouling and scale prevention system:This is a new development for a marine device which prevents scaling and bio fouling, so that fouling of membranes is reduced-it extends the membrane operating life and controls odors. Especially when the watermaker is not operated for a long time this may be a solution. This unit can be used in the pre filtration line for your watermaker and functions similar to the electrical-magnetic watersofteners (see later)

How it works: The water receives a positive charge which prevents the particles from approaching close enough to bind to each other or nearby surfaces. This prevents scale, sludge and biological fouling. Wetted surfaces also form a strong boundary layer, which prevents particles from attaching. Bacteria are unable to attach, absorb nutrition, or replicate into colonies. (based on information from the manufacturer, Zeta guard, www.spectrawatermakers.com )

Post-filtration of the fresh water (before you use the fresh water)

Product water from your watermaker is extremely pure, has very little salt content, is very soft and is slightly acid (low pH).

This is of course only valid for the moment the water leaves your watermaker. The piping and tanks the water passes through are bound to return bacteria to the water. As the soft water is also slightly acidic it tends to corrode your metal water tanks-you are lucky if you have a plastic water tank!

Pure water-tasteless, colorless, and odorless-is often called the universal solvent. It also contains (carbon dioxide to form) free carbonic acid gas, with even better solvent results.

To remedy this rather grim future for the pure water you just made there are several solutions, not necessarily all to be used together-it depends on your individual circumstances.

• Do you mix the product water with dock water or not?
• Do you use very much water (high turnover) or does the water stay in the tank for a long time?
• Do you have a metal or plastic tank?
• What do you use the product water for (drinking, showering etc.)?

1. Limestone filter/neutralizing filter (post filtration of product water)

The simplest way to raise the pH is to run the product water through a limestone or similar neutralizing filter, before it goes into the watertank. This filter contains a bed with a mixture of calcium and magnesium salts, which are the salts found in waters of pH greater than 7.0. This limestone gradually dissolves increasing the pH level of the water. So the water is “harder” again as well. The only maintenance for the cartridge is that it will need refilling as the limestone dissolves. As the water is re-mineralised this will also improve the taste.

“Hardness” is not the same as pH. While pH is a measure of acidity, hardness is a measure of how much calcium (and to a much smaller degree magnesium) is in the water. Nevertheless, hard water is likely to have a high pH, as it is also high in carbonates that buffer the pH toward the alkaline side. See later: hardness of dock water.

2. Treatment of water in the tank

The tank water can be treated with special desinfectants, often containing very low amounts of active chlorine. 20ml of Hadex, one of those products will treat 1000 liter of water. This will impede growth of bacteria, algae and other micro organisms in the water, and should be added at least once a month. For “shock treatment” (cleaning the tank) you can use the same product in a high dosage. (Fore more info on Hadex, see www.hatenboer-water.com ) There are also products which do not contain chlorine, but silver such as Micropur which kills germs in particle-free, clear water on the basis of silver ions. The water will therefore keep its original fresh taste. At the same time Micropur preserves the water; it will stay germ free for 6 months. (For info on Katadyn Micropur, see www.baproducts.com/micropur.htm )

3. Activated carbon filter (after the watertank, near point-of-use)

Activated carbon acts as a catalys-this is the point-of-use application. It will reduce (70-95%) chlorine and it will oxidise sulphides to sulphates, reducing bad taste and odour.

It will also absorb hydrophobic organic molecules (diesel, gasoline) which would be useful for a point-of-entry filter.

There are many different kinds of carbon filters, based on such different materials as petroleum coke, or coconut shells. Most carbon filters are combined with a built-in particle filter (between 20 micron, to 0,5 micron). Each different use has its “best” cartridge.The finest cartridges will also reduce Giardia Cysts.

The cartridge must be changed frequently as the flow does not reduce when the carbon becomes saturated. The reducing of the chlorine also makes the tank water, if filled with dock water, fit for being used in a fresh water flush system for the watermaker. Most automatic FWF systems have a dedicated carbon filter. This should be a cartridge with a high chlorine removing capacity.

4. UV light (after the watertank, near point-of-use)

UV is often the last device in a treatment train. UV radiation affects microorganisms by altering the DNA in the cells and impeding reproduction. UV treatment does not remove organisms from the water, it merely inactivates them. The effectiveness of this process is related to exposure time and lamp intensity.

There is no residual disinfection in the water to inactivate bacteria that may survive or may be introduced after the water passes by the light source, which means that the UV unit should be located as close as possible to the point-of-use since any part of the plumbing system could be contaminated with bacteria.

Used alone, UV radiation does not improve the taste, odor, or clarity of water-which is why it is often used after a carbon filter.

For unknown reasons, other than it being practical when shipping the unit, or lack of confidence in the quality of the membrane, watermaker manufacturers mount their post filtration (carbon filter, UV light) before the water tank, which as I have described above I think is not correct. This post filtration should be installed near the point-of-use.

Filtration of dock water (point of entry FW filtration)

Hardness of the water:“Hard” water contains a high solution of calcium and magnesium salts. When you heat hard water, (above 60 degrees C), a non-soluble deposit is formed of calcium carbonate (CaCO3) and magnesium-hydroxyde (Mg(OH)2). This deposit accumulates especially in the hot water piping and can clog the pipes. It will also cause white stains when cleaning the decks.

Hardness of the water is generally expressed in ppm or mg/l CACO3. Soft water has less than 125 ppm, hard water has greater than 200ppm CACO3 content.

Water softeners:Soft water, on the other side, does not contain these salts, and is generally more acid than hard water, and will corrode metal piping and tanks faster. To soften hard water there are several filter solutions.

Electromagnetic water conditioner:The most harmless one is an electrical device that ionizes the calcium and magnesium carbonates so that they do not form a deposit. In fact the water is still “hard”, but there is no deposit. The opinions about this solution are divided.

An example of a household unit is the “water-imp” (their web site is http://www.waterimp.co.uk/waterimps.html )

A marine unit which prevents scaling and bio fouling which can be used in the pre filtration line for your watermaker is the Zeta guard, found at www.spectrawatermakers.com . (We discussed this in the section on prefiltration watermakers above.)

Phosphates filter water conditioner:

The filter cartridges containing “salts” (phosphates) have the same objective and will prevent scaling and corrosion (as the water is still “hard”) The dissolved water softener helps to prevent the crystallization of calcium carbonate in water (up to around 65 degrees C).

Specific proportions of sodium phosphates and calcium salts are combined at high temperature; the resulting molten product is cooled and then shattered into glasslike balls.The phosphates will very slowly dissolve in the water passing through the filter.

Mechanical water softener (ion exchange process):

To really change hard water into soft water, the process involves the exchange from Calcium and Magnesium salts for embedded Na (sodium) salts. The regeneration of the filter bed is easy: just add salt.

The treated water is softer but also saltier which may present worse problems than you started with if the salt concentration becomes higher than 150mg/liter.

The ion exchange softening process adds sodium at the rate of about 4,6 mg/liter for each 10ppm of hardness removed. For example, if the water hardness is reduced with 330 ppm this maximum amount of sodium is already reached.

Not recommended for drinking water or watering your on-board plants, but a solution for wash downs.

A marine dockwater filter, based on this principle and suitable for cleaning your boat, is the Boji filter found at www.bojifilters.com .

Iron stains: High concentrations of iron or manganese sediments create reddish-brown or black stains on laundry, causing certain people to be very unpopular on board.

The culprit may be the metal watertank or metal piping on board, especially when used with watermaker product water-which is soft and slightly acid.

When oxygen content and pH are low, water containing dissolved iron or manganese appears colorless. When exposed to air, the dissolved iron or manganese reacts with oxygen and is converted to a colored, solid material that settles out of the water.

This process is called oxidation. Iron changes to a reddish-brown. Manganese forms a black residue. Combined iron and manganese are often present in both dissolved and oxidized forms

A simple solution: As a single filter, an activated carbon filter removes limited quantities of rust. A TS-GAC carbon filter cartridges consist of carbon combined with 20% phosphate crystals that reduce rust and scaling.

If that is not enough:

A multistage treatment operation may be necessary. For example, the water supply could be aerated to oxidize the bulk of iron and manganese, then chlorinated to oxidize residual iron and kill iron bacteria, followed by activated carbon (and particle) filtration to remove excess chlorine and iron and manganese particles.

(source: Water treatment notes, Cornell Cooperative Extension, New York State College of Human Ecology)

The ideal dock water filter

The ideal dock water filter is a combination of filters that suits your needs, and is not for every boat or situation the same:

• It will contain a particle filter or even two (25 and/or 5 micron).
• It may contain a carbon filter, depending on whether you want to filter out the chlorine and possibly other contaminants.
• It may contain a salts (phosphates) filter to reduce scaling.

How to put a dockwater filter together:An example: Take one, two or three filterbowls. Mount the ports together with M/M fittings. Mount a gardenhose connector on the beginning and the end. You can add a bracket on top if you have a place to hang the filters (e.g. clip them on the fenderholders which are empty when in port). That’s all!

Filtration of bilge water (post-filtration)

A very small amount of oil or diesel causes a very large surface contamination when pumped overboard. Bad bilge water may have 100-200 mg/liter oil contamination.

For these relatively low concentrations there are good oil absorbing filter cartridges on the market that remove dissolved and dispersed hydrocarbons from water . The Clerify removes up to 95 percent of the hydrocarbons. A small 10″ standard size cartridge (the clear filter housing on the photo) has a capacity of 360gm.

A “Big Blue” 10″ cartridge (the middle filter cartridge) will hold 1290gm. (6000-7000 liter of oily bilgewater.)

The filter you need depends on the flow required by your bilgepump – a standard 10″ with ports handles 28 l/min.

The largest 20″ with 1″ ports will handle 189 l/min (on the right on the photo).

These filters can be bought as ready-made filter systems (e.g. wavestream, www.wavestream.co.uk ) or by buying the (standard) filter bowls and elements to customize your system.