Drinking seawater

Watermakers have put a dent in the ancient mariner’s lament: “water, water everywhere, nor any drop to drink.” Of course, the magic of watermakers comes with some important maintenance requirements that can’t be ignored. While watermakers have become easier to operate and maintain than they were just a few years ago, these machines have not reached hands-off status yet.

Small craft reverse osmosis (RO) watermakers can be broken into two categories based upon their fresh water output capacity. One school of thought is to harness lots of power when the engine is running and use a hefty electrical or mechanical drive system with enough flow volume and an RO membrane array that is large enough to make copious amounts of fresh water quickly. This get-the-job-done-in-a-hurry approach can even coincide with battery charging, shortening overall engine run time.

The alternative is a smaller-capacity 12-VDC system that has modest current demands and modest freshwater output. The strategy behind this system is based upon a longer duty cycle, enabled by the energy stored in a large battery bank. The latter unit is smaller in physical size and often comes with less complex controls, smaller RO membrane and fewer monitoring features. Both systems can provide years of high-quality fresh water, but the operational procedures and regular maintenance routine must be carefully embraced. Professional yacht captains and engineers have a love-hate relationship with refrigeration compressors and marine sanitation systems, but these pale in comparison to their can’t-live-with-it-can’t-live-without-it feelings toward RO systems.

On the one hand, watermakers are a wonderful technology and have become de rigueur aboard a wide range of vessels. But before diving into the ABCs of a regular maintenance routine and common failure scenarios, a couple of user-oriented comments are in order.

First of all, reverse-osmosis produced water needs some special handling for one significant reason: the potential for membrane failure. Although the failure of an RO membrane is unlikely, if it does occur it can contaminate the water being made as well as what is already in a tank. And if this is the only source of water aboard, big problems can result. For this reason alone it makes sense to treat the RO water supply like a separate bank account, and avoid the all-eggs-in-one-basket scenario. Providing a secondary tank for watermaker product makes sense, as does installing a total dissolved solids (TDS) meter that monitors water quality.

This extra tank approach also encourages a crew to keep enough already tested water in reserve so that even if the RO system fails on the first day out, there is enough drinking and cooking water for the duration of the passage. This may seem like a lot of extra concern, but it’s all too easy to start seeing RO-produced water in the same reliability context as a water main on land. However, being ready to do without the luxury of the extra water supply harkens back to good seamanship and the art of passage planning.

How they work
In the reverse-osmosis process, salt water is pressurized to about 800 psi and interfaces with a semipermeable membrane (Osmonics, Dow Filmtec, CSM or others). The seawater defies the law of osmosis, moving the solvent (water) from high-solute (salt) concentration to low-solute concentration on the freshwater side of the membrane through pores that are .0005 microns in diameter.

In normal osmosis, the flow trends in the opposite direction due to a natural pressure caused by water’s tendency to balance solute content. Normally osmosis involves diffusion, a flow through a semipermeable membrane that goes from lower to higher solute concentration. In other words, osmosis is an ongoing effort to balance solute levels on each side of a semipermeable membrane. Fresh water ends up on one side of the barrier, and solute comprised of salt, inorganic and organic compounds, bacteria and even viruses build up on the other.

When it comes to reverse osmosis, pressure on the high-solute side is raised enough to overcome natural osmotic pressure (it takes 400 psi of pressure to overcome the osmotic pressure of seawater, which is 35,000 ppm salt). In this process fresh water is forced to flow upstream in a fluid, dynamic context. The three big challenges in the RO process lay in: 1. creating sustainable pressure free from hydraulic shock and overfeeding and poor cross flow; 2. providing uniform semipermeable media; and 3. devising a means of cleaning salt brine, bacteria and other contaminants from the fragile, thin-film spiraling surface.

Today’s compressor technology is reliable and cost effective, but high pressure and the chloride ion associated with brine takes a toll on metal and moving parts. One of the first pieces of information to file away about a particular system is the manufacturer-suggested life span of a particular pump and how long it should run prior to rebuilding the seals and O-rings.

The membrane element itself has no moving parts, but must go through regular cleansing flushes, remain moist and be treated chemically to reduce the effects of suspended and precipitated solids, scale, oxides, oil and grease as well as biological contaminants. It’s no wonder that membrane maintenance is often the Achilles heel of the system. Those who really count on the water they create often go to the extreme of carrying a wide array of spare parts, or even doubling-up with two stand-alone systems.

When it comes to watermakers, the forgiveness factor associated with deferred maintenance is all but nonexistent. The first line of defense lies in the prefiltering of seawater, and an inferior system or improper service can spell trouble. Every installation needs a strainer and set of filters that can strip out debris and suspended particles. The strainer handles sizable flotsam and prolongs the life of the filter(s), which are defined by each manufacturer. Silt can be highly abrasive and microscopic in size, that’s why a double set of filters, plumbed in series (20 micron primary — five micron secondary) is worth the effort and expense.

In cases where the RO pump is mounted well above the waterline, a low-pressure delivery pump is needed to lift the water to the high-pressure pump. A partially clogged strainer and/or filter can cause a reduction in the inlet flow to the RO pump and cause a decrease in efficiency.

Unfortunately, the cycle of strainer and filter service will vary greatly depending upon where a vessel is moored, the silt content of the water, the impact of biological factors such as a plankton boom, and the amount of water being made. Water temperature and salinity also effect output efficiency. Visual inspections of the strainer chamber and regular readings of flow volume are good indicators of filter condition, and periodically back-flushing the filter(s) and cleaning the strainer insures more efficient operation and a greater lifespan of all components.

Another critical input that is essential to all units is a consistent, appropriately sized energy source (either electrical, mechanical or hydraulic). Rotary energy can be taken directly from a flywheel pulley and delivered, via a pair of heavy-duty V-belts, to a multistage pump such as a Cat 241. An electro-magnetic clutch on the drive pulley of the pump engages the system, and as long as the ratio between the two pulleys and the engine rpm is correct, the belt tension and bracket alignment are set up appropriately, and the electric clutch is engaged, the pump will spin. Modern diesel engines tend to operate at a higher and wider rpm range, and matching cruising rpm with the most efficient pump rpm is an important concern.

This direct energy transfer from the diesel engine to the pump is efficient, but it encumbers the main propulsion engine with added brackets and drive belts, and vibration can fatigue pump fittings more quickly, upping the chance of a high-pressure line failure. Releasing an 800-psi saltwater spray in an engine room can be more than a mood dampener, raising the importance of preventative maintenance and regular component inspections.

A preferred option, even for those with high-capacity output in mind, is a remote, away-from-the-engine installation in which the compressor is driven by an electrical motor. Higher horsepower units can also be run hydraulically but this again raises the question of system complexity. Aboard larger vessels, a generator is usually part of the program, and a 120/240-VAC drive system for an RO unit makes sense. Smaller 12- and 24-VDC systems can also be remotely mounted away from engine heat and vibration, either in a stern locker or anywhere that available space can be found. Spectra, Sea Recovery, HRO and Katadyn are popular brands offering smaller capacity 12-VDC units.

Some use Clark pumps with shuttle valves, known as an energy-recovery pump design. It’s a connected piston technology that uses pressure from the discharge side of the system to augment pumping. These are small-output, longer-duty cycle units that have a good track record, especially among those sailing aboard smaller vessels. Village Marine, Sea Recovery and others also offer tried and proven single- or multiple-plunger pump units. Higher capacity units built by HRO and SCI are examples of automated systems targeting larger vessels. All of these designs have one thing in common: a spiraled semipermeable RO membrane that requires scrupulous ongoing maintenance.

Cleaning the membrane
Back flushing is the next step on the path to watermaker longevity, and with smaller units, manual operation is the name of the game. The process uses a freshwater bypass that sends salt-free water through the system forcing brine, bacteria and other residue from the membrane. These particles are too small for the filter(s) to catch and build up on the saltwater side of the membrane. Proprietary membranes like Dow’s ROSA are rugged and durable if regularly back-flushed, but can quickly foul if the process is omitted. This Filmtec material, like its competitors, is also highly susceptible to damage from oil, chorine and other chemicals, another reason that in-port watermaking is seldom a wise idea.

Many installations even incorporate a charcoal filter in the back flush line so that chlorine inadvertently put into a tank from a shoreside source will not reach the membrane. More sophisticated units with flow meters, TDS sensors, relays and solenoid-operated valves flush automatically, or with a simple flip of a switch. Top-of-the-line units provide auto diversion for flushing filters and membrane cleaning, plus high and low pressure shut off as well as fail-safe TDS monitoring. All this automation requires exacting installation and rigorous adherence to the maintenance cycle. Manually operated systems require some valve turning and timing but have less complex equipment to maintain.

In a pickle
If the unit is to be left unused for a week or more, a pickling solution should be introduced into the system, providing a biocide to reduce the likelihood of microbial growth. After about 1,500 to 2,000 hours of use, the pump needs to be disassembled and seals and O-rings replaced. More often than not, calcium and other minerals will be found, and should be removed using the manufacture’s acid-base cleaning chemicals. Never attempt to use an abrasive cleaner or emery cloth to do this job. Moving parts, high pressure and salt water are an unholy combination and a close look for signs of leaks will pay off in the long run.

After a couple of years of operation the membrane usually needs a major cleaning that involves the use of an acidic or base solution (in some case both) to rid the unit of accumulated scale and bacterial growth. The process involves disconnecting inlet and outlet lines and setting up the system to cycle the cleaning solution to and from a bucket, a process that involves an hour of unpressurized recycling. A well-maintained thin-film RO membrane has an average lifespan of about five years and replacement is a straightforward swap out process.

A watermaker can be like a stranger that becomes a best friend. Its value during a cruise to the Bahamas or passage to an even more arid part of the Caribbean is more than incidental. And with daily use and proper filtration of the supply water, a well-installed system will yield plenty of water. As the system ages, its efficiency and reliability can be revived through carefully planned refits. The technology may not be deemed essential, but once a crew becomes accustomed to regular showers during a lengthy passage, opinions may change.    

Ralph Naranjo is a freelance writer and photographer living in Annapolis, Md.

By Ocean Navigator