Watermaker, Frequently asked questions
In natural sciences and technology, osmosis is perceived as the directed migration of molecules through a semi permeable membrane. The chemical and physical structure of the membrane determines which molecules are able to pass and which are not. For this reason, it is called semi-permeable, which means as much as halfway or partial permeability.
If one mixes different liquids, like in this case water with different salinities, they aim to equalize their concentration. That way, the seawater would be thinned out and the drinking water would be enriched with salts. The effect would be a less concentrated, homogeneous dissolution. Pouring sea- and fresh water in equal amounts into a container where both liquids are separated by an adequate semi-permeable membrane, there would be one side with seawater that is highly loaded with salts, on the other side more or less “clean” water without or with little dissolved components.
The natural tendency of both liquids to equalize their salinity leads to the migration of water molecules from the fresh water side towards the seawater side. As a result, the volume of water on the fresh water side decreases while it increases on the seawater side. This process of osmosis takes place until the pressure on the seawater side is in accordance with the osmotic pressure. Then it stops. In this case, the osmotic pressure is around 30 bar.
The described process however is reversible by exposing the liquid on the seawater side to mechanical pressure. At a pressure of 30 bar, the osmotic process cannot take place or would rather be reversed. When pressure is increased beyond 30 bar, for instance 60 bar, water molecules from the seawater side migrate to the fresh water side. All other components of the seawater dissolution are not able to pass the membrane. As a result, the dissolution on the seawater side remains highly-concentrated while there is a gain of fresh water on the other side of the membrane. This process is referred to as reverse osmosis (R.O.)
The seawater is delivered to the pressure pipe’s entry side of a RO-system by a high-pressure pump. The osmotic membrane is located in the pressure pipe and is merely permeable for the vehicle “water “(solvent) and detain the solute substances. When the pressure difference is more than leveling out the osmotic head, the water molecules are able to pass the membrane that works like a filter, while the “unpurified” molecules are detained. In opposite to a classic membrane-filter, osmosis membranes do not have continuous pores. In fact, the ions and molecules are migrating through the membrane by diffusing through the membrane material. Inside of the membrane, the permeate-tube is located which transports the fresh water through one of the two end caps of the pressure pipe towards the fresh water tank. The excess seawater, now referred to as concentrate (brine), is drained off overboard through the outlet of the pressure pipe by a pressure control valve.
When deciding to go for a watermaker keep in mind:
1) the higher the capacity per liter of a plant, the lower the price per liter of product water.
2) less operating time of a larger plant extends the service life, reduces maintenance and costs.
The advantage of a small plant is that there is less current consumption. May that allows a use without starting the machine or the generator. 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 engine or a genset is used for daily power supply, the plant should cover the required daily demand within this running time. On boats where the generator runs continuously, a desalination plant should cover the daily demand within 3 to 12 hours of operation. This provides good economy at low cost.
12/24 volt DC or 230 volt AC: 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. The energy needed for this stands in direct relation to the pump capacity (feed flow of high pressure pump).
12/24 volt DC: with a pump capacity of about 3.3 liters/minute feedflow and a power consumption of about 500 watts, results in an output of about 55 liters/hr of product water, using one membrane. Due to the low feed flow, only one membrane size SW 2540 can be used. The lifetime of the membrane is here, due to the low flow over the membrane, less than 1000 hours of operation.
115/230/400/440 VAC: with a pump capacity of about 6 up to 13 liters/minute (depending on the capacity of the HP pump) and a power consumption of around 2400 watts, result in an output of up to 150 liters/hour of product water, using two membranes. The life span of a membrane amounts here, by the optimal overflow according to experience up to 4,000 operating hours. Due to the inductive load of the AC electric motor, a generator power of 4 kW is required.
Advantage12/24 Volt DC: No additional generator needed.
Disadvantage 12/24 Volt DC: Shortened life of the membranes due to low overflow. Abrasion of the carbon brushes in the DC motor. Seawater-suitable 12/24 volt feed pumps are less reliable than industrial 230 volt pumps
Advantage 115/230/400/440 Volt AC: Long membrane service life.
Better quality of product water. Low-cost standard industrial motors with maximum service life. Use of industrial quality feed pumps.
Disadvantage 115/230/400/440 Volt AC: Electrical generator needed. In inverter operation, due to the high energy consumption usually active charging technology needed.
Conclusion: The higher the energy requirement per liter of water produced, the longer the service life of the membranes and the higher the quality of the product water (pump capacity of the high-pressure pump in liters per minute = feed flow).
If a generator is available on board, it is highly recommended to use it for the operation of a watermaker. A 12/24 Volt DC watermaker can hardly reach, at least economically, the lifetime, reliability and quality of a 230/400Volt AC watermaker.
Hint: The water production changes according to the salinity, the temperature of the seawater and the age of the R.O. membrane. Capacity given at +/- 10% for standard seawater at 25° Celsius and 35 g/l salinity.
For the installation on yachts, the modular plant has clear advantages:
1. Lower price compared to a compact system.
2. The individual components can be installed where they are integrated best.
Ideally every day. After fresh water flushing, the system may be unused up to 7 days. A fresh water rinse can be repeated as desired approx. every 7 days. Flushing takes place (with Aquatec systems) via the fresh water pump on board and requires approx. 10 liters of water per membrane. If the system will not be used for longer than 7 days without flushing, 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 6 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. 10 minutes. Alternatively for frost protection, glycol in food quality can be added to the biocide at preservation.
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.
A three-piston high-pressure pump is not in a position to suck up the desired seawater by itself. In the case of small 12 or 24 volt systems when installed well below the waterline, the seawater is supplied without problems, but the necessary sediment pre-filter restricts the inflow even in new and clean condition. In addition, a feed pump simplifies the venting of the installation. This ensures an optimum service life of the pre-filters.
In systems with a high pump capacity of the high-pressure pump (e.g. 230 volts), trouble-free operation is not possible without a feed pump.
Aquatec generally supplies all plants with high head feed pumps. This reduces the costs for the sediment filters
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.
A sea water 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 order to understand the technical background, here is a short extract from our big ship commercial manual for operating a hardening system.These systems are occasionally used in commercial ships when there is a requirement. Here the necessary filter volume is to be considered particularly around a hardening to obtain.
25 kg of calcium carbonate are required per 100 L/hour product water.
The required minimum filter volume per 25 kg is 12.5 liter
Note: The filter must be refilled at a consumption of approx. 10 ... 15%.
Operation of hardening filter:
Fill the hardening filter with 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.
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 20 L of bottled or tap water in order to absorb enough minerals!!!
Minerals 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.
The body's daily mineral requirements
A normal person needs roughly 5 to 10 grammes of table salt per day to cover his or her average requirements. 1 litre of drinking water with a value of 200 mg sodium per litre corresponds to approx. 0.5 grammes of table salt. In order to supply the body's total salt requirement of 5 grammes per day with drinking water, it would be necessary to consume just 10 litres of drinking water every day!
A normal person needs roughly 1 gramme of calcium per day to cover his or her average requirements. It would be necessary to drink 20 litres of untreated Munich drinking water to cover the daily calcium requirement. This is just as impossible as it is with sodium.
Phosphate / Phosphor:
A normal person needs roughly 700 milligrammes of phosphate per day to cover his or her average requirements. According to Federal drinking water regulations, a maximum of 6.7 milligrammes of phosphate can be added to one litre of drinking water. It would be necessary to consume 104 litres of drinking water to the total daily requirement with drinking water alone! This would be much easier to achieve with 400 grammes of quark, 100 grammes of Emmentaler cheese or a litre of milk!
The body and supplying its requirements
When all's said and done, our bodies get their energy and mineral requirements and build up their power reserves from eating food, fruit and vegetables. Drinking 1 to 3 litres of water a day is definitely required, but not necessarily because of the minerals that it contains.
Water works in the body through what it takes with it, and not what it brings with it.
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.
This 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.
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. 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.
Of course, Aquatec's spring-regulated pressure control valve is also made of duplex stainless steel.