Frequently asked questions
Why does Aquatec use duplex stainless steel 1.4462?
A fact that many of us do not realize is, that the austenitic stainless steels 1.4404 / 1.4571 / AISI 316 TI (also known as V4A) are not generally seawater -resistant! The materials have a so called PREN (Pitting Resistance Equivalent Number)-value, an index for pitting corrosion. This is a measured value for corrosion resistance. Steels with a PREN-value lower than 30 are not resistant to attack by seawater. 1.4571 has a PREN-value of 23,1 – 26,75. Certainly, a temperature factor has to be considered, the CPT (Current Procedural Terminology)-value, that gives information about the critical pitting temperature. 1,4571 has a CPT-value of 24 - 27,5 degree Celsius.
The austenitic-ferritic (Duplex) stainless steel 1.4462 has a PREN-value of 30,85 – 38,07 and a CPT-value of up to 34,5 degree Celsius.
As there is no pump manufacturer worldwide who produces pump heads for triplex plunger pumps in Duplex stainless steel, the Aquatec pump heads are made in-house production since 2008 in a complex way with 1.4462. Naturally, we also produce all parts inserted inside the pump head like e.g. valves and seal carrier assemblies with 1.4462. Due to the small difference in the electrical resistance between 1.4462 (0,79 Ω) and 1.4571 (0,75 Ω), it is safe to use connection fittings from 1.4571 (AISI 316Ti) – that can be purchased worldwide at a low price – on Duplex steel.
As Aquatec generally only uses best materials, the Aquatec pressure control valve with spring governor is also made from Duplex stainless steel.
Please find further technical details of all stainless steels on the website of "Deutsche Edelstahlwerke" www.dew-stahl.com.
Aquatec pressure vessels.
Our Pressure Vessels Made in Germany are produced from a world leader in the production of Fiberglas pressure vessels. The following the highest standard available for RO applications produced in accordance with the recommendations of the EU, KTW, DVGW and the Food & Drug Administration.
Choice of a desalination plant
In general, it can be said that when choosing a plant: The larger the plant, the lower the price per litre. As the running time is shorter for a large plant, the maintenance requirements are lower. This results in a reduction of expenses and a longer service life of the whole plant. The advantage of a small plant is that there is less current consumption. That allows a use without starting the machine or the generator. It is important to note that the required energy per litre of produced fresh water is similar, except for very little differences. In the often event of a cruising yacht with 2 persons, the size of approx. 30 to 60 litres per hour has proved its worth, also with regard to the price.
If the machine or a generator is used to charge a battery every day, the plant should cover the daily requirements within this running time. On large boats, of which the generator is running continuously, the plant should cover the daily requirements within 3 to 5 hours. Thereby, a good efficiency with reasonable maintenance costs is reached.
Systems operating from 12 or 24 volt on-board power source:
There are basically two types of system available on the market:
a) Systems where the required working pressure of up to 60 bar is generated by an industrial high-pressure pump. (e.g. Aquatec-Watermaker)
Easy management and proven technology with an optimum service life.
Smooth running with hardly any pulsation due to use of a three-piston pump.
Higher power consumption
b) Systems in which a specified input pressure is generated by a backing pump. An additional energy recovery system using either the retentate or brine (pressurized strongly saline waste water) increases the pressure to the required operating pressure.
Lower power consumption.
Generally has a higher price due to the sophisticated technology.
A special backing pump is required, often with inconvenient dimensions and sophisticated technology.
Pulsating system pressure usually requires additional pressure equalization.
Intricate control and switching valves in the energy recovery system lead to higher rinse water consumption due to the need to flush the system after each use.
High equipment maintenance costs.
The general disadvantage of 12/24 volt DC systems:
To keep power consumption within a reasonable range, pump power must be kept as low as possible. Because this results in a poor flow of seawater over the membrane, it always leads to a shorter membrane lifespan.
Systems operating from 230 or 400 volts alternating current.
Since power consumption is secondary in these systems, the necessary working pressure is generally produced directly through an industrial high-pressure three piston pump.
Long membrane service life.
Low-cost standard industrial motors with maximum service life.
Use of industrial quality feed pumps.
Electrical generator needed.
Here again, the system with the highest power consumption per liter of water produced achieves the longest membrane service life and also the best water quality.
Tip: Pay particular attention to the pump power indicated in the supplier’s data sheets, usually given as feed flow in liters/minute.
The properties of membranes in seawater desalination plants:
Some insight into the many complex interdependencies of a sea water desalination system.
In order to obtain maximum service life with highest quality product water from RO sea water membranes, the membrane surface must be flooded with sea water as much as possible (feed flow).
The energy needed for this stands in direct relation to the pump capacity. Especially in the case of 12/24 volt systems, this energy can rarely be reached economically, even with so-called energy recovery systems.
In the case of a small 12-volt system with one membrane, with minimally acceptable overflow and a power draw of approx. 250 Watt for approx. 30 L/ hr. product water, the service life of the membrane can reach approx. 800 operating hours.
By comparison, in the case of a 230 V system with a pump capacity of 13 L/ minute feed flow and two membranes (150 L/ hr. product water) often used in yachts, membranes can reach up to 4000 operating hours. However, due to the inductive load of the electric motor, a generator capacity of 4 kW (power consumption 2,300 W) is needed.
The higher the power capacity of water generated per liter, the longer the service life of the membranes. The quality of the product water rises accordingly.
How often does the plant have to be operated? Risk of frost damage?
Ideally every day. After fresh water flushing, the system may be unused up to 7 days. After that, it has to be rinsed with fresh water again for the next 7 days. The time required for fresh water flushing is approx. 1-2 minutes and about 7 litres of water are needed per membrane. If the system will not be used for longer than 7 days, it has to be preserved with a biocide. The required time is about 10 minutes. After that, the plant can be left unused up to 12 months. If there is the risk of frost, the pressure pipe with the internal membrane has to be dismounted and stored frost-proof. Required time for this approx. 5 minutes. Alternatively for frost protection, glycol in food quality can be added to the biocide at preservation.
Fresh water flushing after use of the watermaker?
Seawater gets filthy quickly due to the organisms it contains, especially if the surrounding temperatures are high. As a result, a layer occurs on the surface of the membrane, which hardens and interlocks the membrane. It is only possible to remove this organic fouling with chemical detergents. In case the system is used daily, it is no problem if it is not rinsed occasionally. But generally, the plant should be rinsed with chlorine-free fresh water after each operation. The fresh water intake should be carried out through a coarse filter. The coarse filter prevents the infiltration of suspended matters and chlorine that possibly entered the tank at the last landside water intake. Even small quantities of chlorine result in a total loss of the membrane.
Post-treatment of osmosis water.
A membrane that is technically in good condition produces drinking water of best quality and taste. This has been confirmed by countless users and by our own long experience.
One advantage of untreated osmosis water so that it is calcium-free which, among other things, reduces the consumption of detergents and soap.
The required drinking water should be drawn off directly at the membrane output, because only here can germ-free drinking water be obtained. This is a standard feature delivered with all Aquatec systems.
If water is drawn from the on-board drinking water tank, treatment with chlorine or other products may be needed to prevent germs. If chlorine is used, a carbon filter should be installed just before the faucet to remove the chlorine.
Recently some suppliers have started to offer post-treatment of product water by increasing the pH value, for example with calcium carbonate. The system proposed here, with a chemical volume fraction amounting to just under 1 L in all system sizes, and with no way to regulate it, will not change the pH in any measurable way, and in any case is not necessary for yachting.
If you should choose this type of “system”, the supplier should confirm the data related to the pH value to you in writing.
In order to understand the technical background, here is a short extract from our professional navigation handbook for operating a hardening system.
These systems are occasionally used in professional navigation when there is a requirement. Note especially the required filter volumes in order to judge how “useful” a volume of only 1 L can be.
Hardening filter operation:
Fill the hardening filter with approx. 80% -90% calcium carbonate. (JURAPERLE JW / Calcite or similar)
1) Open the cut-off valve, hardening filter inlet and cut-off valve, outlet hardening filter.
2) Adjust the pH value using the hardening filter bypass valve, according to the appropriate instructions
(typical: pH 6.5 – 8.5).
3) pH value too high: Open the hardening filter bypass valve.
4) pH value too low: Close the hardening filter bypass valve slightly.
5) Take samples at regular intervals at the sampling point on the hardening filter and re-adjust if needed.
25 kg of calcium carbonate are required per 100 L/hour product water.
The required minimum filter volume per 25 kg is 12.5 L
Note: The filter must be 10 to 15% full when in use.
The issue of MINERALS in trinking water.
The following paragraphs are intended to provide only a brief insight into a very emotional topic.
The water in an osmosis system still contains minerals, and they are the ones that are also bioavailable! In this way it is different from distilled water, which if used exclusively, can remove minerals from the body.
The issue of MINERALS is really quite separate from issue of drinking water. In order to drink water for the purpose of absorbing minerals, you would have to drink approximately 25 L of bottled or tap water in order to absorb as many useful minerals as by eating one carrot for example!!!
The minerals in a carrot (and in all vegetable and animal foodstuffs) are organic and therefore can be absorbed almost 100% by the body cells, whereas water contains inorganic minerals, only a few percent of which can be absorbed by the body. These remaining, extremely fine particles of minerals (colloidal organic type minerals) are the kind that also remain in osmosis water.
Therefore it is not precisely correct to claim that you need mineral-rich drinking water to supply the body's mineral requirements. In fact the mineral content in common mineral waters is much lower than in food. It would be totally impossible to cover requirements for the various minerals just by drinking mineral water.
According to the German Nutrition Society, the human body requires a daily intake of 500-100 mg of calcium and approximately 200-400 mg of magnesium.
For example, in 1 L of mineral water there are approximately 80 mg of calcium and 26 mg of magnesium. You would have to drink about 6-15 liters of mineral water in order to cover your daily requirements for these minerals.
Water works in the body through what it takes with it, and not what it brings with it.
Why a modular desalination plant?
For the installation on yachts, the modular plant has clear advantages:
1. The price is lower.
2. The individual components can be installed where they are integrated best.
Can I run the high-pressure pump with the main drive? Apply the existing hydraulic system?
AQUATEC high pressure pump installed on a Volvo D2
Generally, this is possible by means of a magnetic clutch. The high pressure pump should be operated at a predetermined speed in order to remain in the range of the membrane. Unintended pressure fluctuations by a change in engine speed is prevented by the specially developed Aquatec pressure control valve in a wide range. As per our experience, an electrical operation is usually more cost-efficient, since an electric motor is in most cases cheaper than a magnetic clutch with additional belt pulleys and a possibly very complex installation of the high-pressure pump with the main drive.
DThis option is suitable though in case that the alternator doesn’t provide enough energy to charge also the batteries on operation of the plant. However, make sure in advance if the installation of a larger or a second alternator would be the better choice. For larger watermakers from approx. 100 litres per hour onwards and if there is no alternator available, the direct drive is often the only solution. But it has to be checked with the engine producer if the additional power take-off at the intended position is allowed.
In case there is a hydraulic pump driven by the main engine available, e.g. for the anchor winch or the bow thruster, this can also drive a hydraulic motor which is flange-mounted on the high-pressure pump. This option is a very good possibility to provide the requested power for a watermaker in each size.
For one of this applications we offer all our plants whitout electrical motor, you only need to buy external a magnetic clutch, pully and a belt or a hydraulics motor.
Application of a feed pump?
A three-piston high-pressure (HP) pump is not in a position to suck up the desired seawater by itself. By installing it well below the waterline, the seawater does indeed reach the pump easily but the necessary sediment pre-filter prevents inflow, even when it is in clean, new condition. In addition, a feed pump simplifies the venting of the installation. But the most important reason for inserting a feed pump lies in its increasing the input pressure at the intake of the HP pump. Our competitors occasionally use a 20-micron sediment filter. But this is not acceptable, according to the manufacturers of the membranes. Here, 5-micron filters are necessary.
A high pre-pressure also permits the sediment filter to last longer. It follows that the selected feed pump should boast as high a discharge head as possible because this will reduce costs for sediment filters. Since operation without a feed pump makes no sense, AQUATEC Watermakers are usually delivered with a feed pump. AQUATEC uses only pumps with a high discharge head because the higher the discharge head, the higher the pre-pressure.
Please note: In order to achieve maximum lifetimes for the necessary sediment filters in all AC installations, a 20-micron filter should always be inserted ahead of the 5-micron filter. For 12/24 volt installations, a single 5-micron filter is entirely adequate for economical operation because of the HP pumps’ lower delivery rate.