Filter and membrane care are the principal maintenance issues with watermakers. In addition, most high-pressure pumps and some booster pumps have an oil-filled sump that needs checking from time to time. Then there is winterizing.
The addition of a monitoring device (such as a pressure gauge on the suction side of the main pump) that will warn when filters start to plug is recommended. More sophisticated watermakers also have an automatic shutdown circuit that will not allow the watermaker to run with plugged filters.
As with an engine raw-water strainer, you must clean the raw-water strainer in your watermaker – most have a removable basket you can rinse in seawater.
Additional filtration generally consists of some kind of removable pleated filter. Often you can clean these by careful flushing of the filter surface. Each hosing will yield approximately an additional 50 percent life expectancy (e.g., if the new filter runs 100 hours, after the first hosing it will go another 50 hours, and after the second another 25 hours). At the first sign of damage, discard the filter (some manufacturers recommend you clean no more than once, then discard).
Depending on how the watermaker is used, membrane care can be the major part of the maintenance. The extent of the work varies from unit to unit, principally as a function of the quality of the source water filtration, regularity of use, and the existence of an automatic flushing circuit.
Flushing a membrane If a membrane is left unused for more than a week, it will be susceptible to bacterial fouling. The best way to minimize watermaker maintenance is to use it often: in other words, do not fill the tanks then shut the watermaker down for two weeks; instead, top off the tanks at least every three days. If the unit will not be used for more than a week, either put it into storage mode (see below) or flush it with freshwater at least once a week.
Long-distance passagemakers confronted with an approaching storm ought to top off the tanks before the storm hits. This way there will be plenty of water onboard, and the system will not need to be run, flushed or maintained for several days.
Typically, the water used for flushing (somewhere between 5 and 7 gallons) is drawn from the boat’s water tanks. However, traces of chlorine will do permanent damage to membranes. For this reason, some installations include a separate tank into which product water is diverted and held for flushing purposes, while others include an activated charcoal filter between the water tanks and the flushing circuit. The charcoal filter removes any traces of chlorine. Note that once wetted, charcoal filters only last about six months.
Bare-bones systems have to be flushed manually, which is done by switching a couple of valves on the inlet and discharge sides of the watermaker, and turning it on. Sophisticated systems have automatic flushing controls powering solenoid-operated valves. The flush interval can be set by the user. This enables a watermaker to be left idle for some time without the need to pickle the membrane (so long as the unit does not run out of flushing water).
On some units, the flushing water also passes through the feed pump, while on others it does not, which leaves salt water standing in the pump. If the pump is made of plastic (e.g., the Shurflo pump used by Spectra and others), this is not a problem, but if it is made of stainless steel, it may lead to corrosion.
Pickling a membrane Special biocides are used for pickling a membrane. These are mixed with product water, then pumped through the system, leaving the membrane cylinder full of the biocide solution. When it comes time to put the unit back in service, the biocide is flushed out by running the unit in an unpressurized state (to achieve maximum flow through it, with no water production) for some time (generally 15 to 20 minutes) before restoring pressure and production. The initial product water will still be high in dissolved solids and must be discharged overboard via a bypass line (this is the case anytime a watermaker is first started). On cheaper units, this is done manually; on more sophisticated units, a salinity meter controls an automated bypass circuit.
Some watermakers (notably Spectra’s) include parts that are degraded by some pickling agents. It is essential to use only the pickling agent recommended by the manufacturer. During a relatively short-term lay-up, it is generally better to backflush the system every week than to pickle it.
Winterizing takes the form of pickling the membrane (it must never be left dry), draining the filters and the rest of the plumbing (filters should not be left wet), and in some cases, adding potable antifreeze (propylene glycol, such as is sold for winterizing RVs and caravans) to the high-pressure pump. (The low-energy pumps, in particular, can trap water that is hard to drain; this can crack the pump in a hard freeze.) Spectra also recommends pickling the membrane with propylene glycol, and in fact they suggest it for pickling in general because this seems to damage to the membranes less than other compounds.
Cleaning a membrane Over time, the membrane inevitably becomes fouled. In which case, a watermaker will exhibit higher than normal pressures and lower than normal product water flow rates. However, before assuming the membrane is to blame, check the feed salinity, feed temperature, operating pressures and filters. Also, check the operating voltage (DC or AC) at the pump, under load. DC systems, in particular, will be affected significantly by low voltage.
On average, membranes need cleaning once every two years (the average membrane life is five years). There are two types of membrane cleaner – alkali and acid. The alkali cleaner is most effective on biofouling and is generally used first (although Spectra recommends using the acid first). The acid cleaner is effective on mineral fouling (e.g., calcium), but it is generally only used if the alkali fails to restore product water flow rates.
To clean a membrane, first flush it with clean water (see above), then mix the alkali solution with clean water in a bucket (preferably hot water, because this will improve the performance of the cleaning agents). Depressurize the unit and divert the suction, product-water and discharge lines into a bucket. Run the unit for an hour or so to circulate the solution, allow it to rest for an hour or so, and run it again for about 15 minutes. After this, put the system back into service, unpressurized, and run it for 15 minutes or so to clear out the cleaning solution, then return it to regular service. Once again, discharge the initial product water overboard, either manually or automatically.
If performance is still poor, repeat the procedure with the acid cleaning solution. Cleaning membranes is hard on them, and something of a crap shoot. Only do it when other measures do not restore output or operating pressures.
When operated with properly filtered source water, traditional watermaker high-pressure pumps need valves and seals replaced approximately every 2,000 hours of use; manufacturers provide rebuild kits and instructions. Booster pumps and the feed pumps on low-energy systems need an overhaul at similar intervals. Those pumps with an oil-filled crankcase need an oil change every 500 hours.