I have long discouraged voyaging mariners from purchasing watermakers, because I felt the cost per gallon (when amortized over the life of a watermaker), the energy required to make water, and the maintenance involved was out of proportion to the benefits. However, over the past decade, all three things have changed:
- The cost of water in many parts of the world has gone up, sometimes considerably, making watermakers more cost effective.
- With some watermakers, the energy needed to make water has reduced dramatically.
- The maintenance associated with watermakers has declined considerably.
Given these changes, I am putting a watermaker on our next boat!
How they work
In essence, a watermaker is a very simple device. The core component is a membrane with a microscopic pore structure large enough to allow water molecules to pass through it, but small enough to block salt, minerals, bacteria, viruses, etc. A great deal of pressure is required to force water through such a membrane, generally on the order of 600 to 800 pounds per square inch (psi). Manufacturing a pump that can generate this kind of pressure is relatively simple, but it has taken space-age technology both to create membranes with the proper consistency in the pore structure and to assemble them to withstand the system’s pressure over time.
Given the microscopic pores in the membranes, fouling is an ever-present possibility. To prevent this, all watermakers have a prefiltration step for the source water (incoming seawater). Only a small percentage of the source water is converted to product water, with the rest (known as brine) used to flush the surface of the membrane and keep it clean. The brine is then discharged overboard.
Membranes
Worldwide, there are only a few membrane manufacturers – most membranes found in the marine marketplace are manufactured by Filmtec, a subsidiary of Dow Chemical. Membranes are designed to work at a certain flow rate and pressure. Significant deviations from these design parameters will result in lower-quality product water (higher levels of dissolved solids) and shortened membrane life (increased fouling).
With a traditional watermaker (but not the low-energy systems, which we’ll cover later), a number of things can affect the flow rate through a membrane and the pressure in the system, notably the salinity of the source water and its temperature. At any given pressure, the lower the salinity, the greater the percentage of the source water that will be converted to product water. If the pressure is not adjusted downward to lower the ratio of product water to brine, inadequate membrane flushing occurs, and membrane life is shortened. Warmer source water also increases the ratio of product water to brine, while lower temperatures decrease it. Again, pressure adjustments are needed to maintain an optimal ratio.
Aside from changes in salinity and temperature, any kind of fouling of the membrane will clearly impair its performance. Fouling may take the form of fine particles of silt, oil and chemicals – which make their way through the filtration system – or biofouling. Biofouling is especially likely if a watermaker is unused for more than a week.
Filtration
Effective filtration of the source water is a key component of any watermaker. The extent of the filtration process varies markedly from one watermaker to another. At its most basic, it may consist of a raw-water strainer and a single 20-micron filter with a replaceable pleated synthetic element. At its most complete, it is likely to include the raw-water strainer, maybe a plankton filter, two or more particle filters (with the final one having a mesh size as low as 5 microns) and an oil/water separator.
In most systems with more than minimal filtration, a booster pump draws the source water into the system and pumps it through the filters to the main pump. Without a booster pump, the main pump will be drawing in water under a vacuum, possibly sucking in air and cavitating. This can be quite damaging (tiny steam bubbles form and implode, eroding the internal pump surfaces). However, a booster pump adds to the energy load.
Better systems have a low-side pressure gauge, for detecting when the filters are beginning to plug, and a low-pressure shutdown circuit.
The higher the capacity of a unit, the better the filtration and protection circuits are likely to be and the more tolerant the unit will be of less-than-ideal source water. On the other hand, many of the smaller, energy-efficient watermakers have somewhat marginal filtration and little in the way of protective shutdown circuits. In these cases, it behooves the owner to refrain from using the unit in silty, contaminated or brackish water. In fact, many seasoned cruisers restrict watermaker use to the open ocean.
Low-energy pumps
All the water passing through a watermaker has to be raised to the operating pressure (600 to 900 psi), but the brine is discharged overboard at atmospheric pressure. The pump needed to generate the high operating pressure is the principal energy consumer in the system (in some cases, the only energy consumer). The higher the ratio of product water to source water, the more efficient the process in terms of energy use, but the greater the risk of membrane fouling. Typically, around 10 percent of the source water is turned into product water, with the other 90 percent discharged overboard, although this ratio may be increased to as much as 30 percent product water and 70 percent brine.
In a traditional watermaker, the 70 to 90 percent of source water discharged as brine represents a substantial energy loss, since the water is raised to pressure and then dumped overboard. A few manufacturers (notably Spectra, HRO and Sea Recovery) have patented processes that capture a considerable amount of the energy contained in the brine, and as a result reduce the energy required per gallon of product water. In the case of Spectra (all its watermakers) HRO (Seafari Escape) and Sea Recovery (Ultra Whisper), the energy requirement is as little as 1/3 that of a traditional watermaker.
The technology that makes Spectra, HRO and Sea Recovery watermakers so efficient is very similar. All systems use a relatively low-pressure feed pump (around 100 psi) to pump water into one of two opposed cylinders, each of which contains a piston connected by a common rod. The water entering the first cylinder moves both pistons. Water in the second cylinder is driven into the membrane housing and from there to the backside of the piston in the second cylinder. When the pistons reach the limit of their travel, the pump output is directed to the second cylinder, driving both pistons back the other way, with the discharge from the membrane housing now switched to the backside of the piston of the first cylinder.
Because of the rod connecting the two cylinders, there is not as much volume on the backsides of the pistons as on the driven sides. This loss of volume is equal to +/-10 percent of the volume of the cylinders fed by the pump (it varies from system to system, from as low as 7.5 percent to as high as 20 percent). Water is incompressible, so the “extra” +/-10 percent that comes out of the cylinders must go somewhere: it is forced through the membrane to become product water. The pressure needed to do this is generated by the fact that the driving surface of the pistons is +/-10 times as great as the surface area of the rod. The net result is that, allowing for some inefficiencies, 100 psi of inlet pressure is geared up to around 800 psi of discharge pressure.
The device that makes all of this possible is a shuttle valve. The shuttle valve takes the brine coming out of the membrane housing (at this point in the process, the brine is at full system pressure) and feeds it to the backside of one or the other piston in a manner that recaptures much of the energy used to pressurize the water in the first place. Only after this is the brine discharged overboard at atmospheric pressure. This is some clever technology that really works, especially for an energy-conscious boat. A spin-off benefit is that it is extremely quiet, in contrast to conventional watermakers.
There are also towed watermakers, which use the boat’s speed through the water to spin a propeller that drives the watermaker pump. These require no energy from the boat’s systems. I have to say, I am skeptical. I find it hard to believe the necessary filtration can be achieved with such a minimalist device.
Low-energy systems
Given the primary goal of minimizing energy consumption with the low-energy watermakers, all of them dispense with the booster pump on the downstream side of the filters (a booster pump would eat up much of the saved energy). The feed pump becomes the only pump in the system. As such, it must either draw or push the source water through whatever filters are installed. To some extent, this limits the degree to which the source water can be filtered: there may be just a strainer and one 20-micron filter (although Spectra filters to 5 microns). The owner of such a system needs to be especially careful not to run it in visibly fouled or contaminated water.
Command and control
Given that the ratio of product water to source water in a low-energy system is determined by the ratio of the volume of the piston rod to the volume of the cylinders, the ratio cannot be varied to adjust for such things as changes in salinity and temperature. To some extent, this can be seen as an advantage, as there is not the loss of product water at lower temperatures that occurs with most traditional watermakers. Added to this, the units are very simple to operate, requiring little or no user interaction. However, to some extent it, is also a disadvantage, in as much as the system cannot be fine-tuned to secure the maximum life expectancy from the membrane.
This article is excerpted from the third edition of Nigel Calder’s Boatowner’s Mechanical and Electrical Manual, which has 200 new pages and is published by International Marine.
