PRACTICALITIES BY ANDREW BRAY
Why Bother?
Watermakers, or to use their technical name, reverse osmosis desalinators are increasingly common on both cruising and racing yachts. In this article we examine the main options and delve into the practical side of using them.
If your water tankage always exceeds your needs, you’re not interested in boat speed and your marina provides good quality tap water free, then you’d have a hard time justifying a desalinator, which remains expensive bits of kit.
But if you cruise you’ve probably already had to ration water, install extra tanks, or plan to include periodic access to a tap. You may also have regretted filling with local water that proved to be hard or tainted, wondered about seagull droppings in the rainwater, or a dead cow upstream in that crystal brook.
You may have bemoaned sluggish progress burdened by full tanks, wished the expense of running your engine yielded more than a few ephemeral AmpHours in the batteries, or cursed the wasted time or strained muscles when ferrying jerrycans.
All reverse osmosis (RO) desalinators produce high quality drinking water from clean seawater. If such scenarios strike chords, then despite their cost and need for periodic DIY maintenance, desalinators may be well worth the investment. So what’s to know about them?
Reverse Osmosis
Osmosis is a natural process, in which water seeps through a semi-permeable membrane to dilute a salt solution on the other side. It is one of the factors that can make drinking seawater fatal – dehydrating your body to dilute saltwater in your stomach. However, if the pressure on the salty side is raised above this strong natural osmotic pressure, the flow of fresh water reverses – permeating out of the salty solution, through the membrane.
Membrane Cartridges: RO desalination became practical in the 1960s, with the development of cellulose acetate polymer membranes. Modern marine desalinators mostly use TF (Thin Film) composite membranes, assembled as a spiral within a cylindrical pressure vessel or cartridge. Seawater is supplied on the outside, and the fresh ‘permeate’ is collected in a tube passing down the centre. Larger systems have two or more cartridges, often running in series, with the slightly concentrated brine from the first unit becoming the input for the next. Pressure gauges monitor the pressure drop across the membranes.
The end-fed, spiral wound membrane structures support the tremendous pressure difference, creates turbulence and provide cross flow, counteracting the tendency for particles or larger molecules to block the membrane. Most of the seawater doesn’t go through the membrane, but flows across it, carrying the rejected materials away with it. This discarded water is saltier than ordinary seawater and is therefore termed ‘brine’.
On a boat, practical RO desalination systems need various ancillary components.
Pre-filtering: In addition to an ordinary sea strainer, all RO systems should have pre-filtering (tyThe high pressure pump may be damaged by cavitation or air, so the seawater inlet should be deeply immersed, and most systems have an inlet ‘boost’ pump. Some shut down automatically if the inlet pressure becomes too low.
Pressure Pumps: Brackish water desalinates at pressures between 17 and 27 bar (250 to 400 psi), and seawater anywhere between 54 and 80 bar (800 to 1180 psi). The operating pressure is adjustable, but boating RO systems ty Many higher capacity systems still use multi-stage piston pumps (related to those in some water blaster units), driven by an electric motor or belts from an engine. The pump will eventually clock up many hours and require servicing (usually including periodic changing of lubricating oil) or more serious mechanical repair, so pump durability and serviceability and parts availability are factors to be considered. Aluminium bronze pumps with 316 stainless steel valve gear and ceramic pistons are among the most durable. In recent years, quieter, smaller and more energy-efficient styles of pump have become popular, utilising a lower pressure conventional feed pump driving reciprocating hydraulic intensifier pistons, and incorporating some recovery of energy from the brine.
Product Flow Meter: A membrane cartridge is rated for a certain flow rate and a maximum pressure. Too low a flow encourages membrane fouling, but excessive flow makes it permanently ‘leaky’ to salts. A cup and a stopwatch can be used to measure output, but most systems incorporate a visual or even electronic flow meter. Flow is controlled by adjusting the pressure regulator valve. A clean membrane may produce its rated flow well below its maximum pressure, depending on water salinity and temperature, but as it fouls, the pressure required increases. Once the maximum has been reached, output declines, and cleaning is necessary.
Salinity Levels: Water containing
Tank Storage
A desalinator’s output is usually directed to one of the yacht’s general use water tanks, although with smaller systems there may be advantages in quarantining the RO water, keeping it aside for drinking.
Other feedwater sources: In some places, the local water supplies contain sufficient dissolved salts to taint the taste and prevent lathering with ordinary soaps. Provided the pressure is reduced to limit the flow, RO can be used to rectify both problems. RO will remove many pathogens but does not by itself, render polluted water safe to drink. Ultraviolet sterilisers are an option that should be considered, especially if the unit might be used in sewerage contaminated water ‘ even in a pristine anchorage your own boat is probably a source. Chlorinating water stored in tanks remains good practice. In addition to pre-filtering, booster pump, high pressure pump, the membrane, sterilizer, and pressure, flow, and salinity measuring, practical RO systems must facilitate maintenance.
Membrane Maintenance
Regular daily use helps to prevent biological growth but throughout their lives membranes will require three distinct maintenance operations ‘ flushing, preserving and cleaning.
Flushing: If the system will not be used for more than about three days the saltwater side should be flushed and left filled with fresh water, which shouldn’t take more than five minutes and 20 litres of water. Even with daily use, flushing will be recommended after so many hours or litres of operation. Some systems have automatic flushing cycles, but otherwise it’s important to back off the pressure.
Preserving: If the period of disuse may exceed about three weeks, a biocide should be added to the fresh water to prevent bacteria growth.
Cleaning: Fouling does progressively blind the membrane, requiring higher pressures to maintain output, and some manufacturers recommend cleaning every 1,000 operating hours or annually. Cleaning may also become necessary if bacteria growing on the membrane create a noticeable taint. Procedures vary with the membrane type, what type of fouling is suspected, and on the system’s plumbing arrangements. The process may take four hours or more, and usually involves circulating two different chemical solutions (one acidic, and the other alkaline) at low pressure.
If cleaning doesn’t adequately restore flow, and there is no other cause (such as a faulty pump, or blocked filters) membrane cartridges can be replaced, or returned to the manufacturer for more thorough cleaning. Cleaning frequency can be reduced by good input filtering, and not using dirty feedwater.
Flushing, preserving, and cleaning will probably be done using tanked water. This may contain chlorine added to prevent bacteria growth or already present in tap water. Chlorine will damage the membrane so the line leading from the tank to the flushing inlet should contain a carbon filter, which can also provide a taint-free galley supply.
General Maintenance
Spare filter elements, O-rings, seals, cleaning and preserving chemicals, and if appropriate, Vee-belts should be on board. The water pressures are high enough to be dangerous. Water and AC power systems can be a fatal combination.
Efficiency: The energy used in pressurising the water is considerable, but in small high pressure pump systems most is lost when the brine returns to normal atmospheric pressure. The overall efficiency then largely depends on the system’s ‘recovery rate’ – the fresh water output as a percentage of saltwater input. But as the recovery rate increases, so does the brine salinity and the pressure needed. Many single cartridge RO systems work on about 15 per cent recovery, which minimises the pressure required, and maximises self-cleaning tendencies, but has the potential to waste lots of energy if not deliberately recovered.
Energy efficiency may not be a major concern when moderately small systems are powered by an engine’s PTO or a genset, but it is when systems are powered manually or driven by DC motors from storage batteries. Hydraulic intensifiers incorporating energy recovery features have brought outputs above 0.28 litres per Watt Hour (>3.3 litres per Amp hour at 12 Volts).
Selecting a desalinator involves estimating daily water needs, considering how long it might run each day (to determine output rate required), what space is available and what power source.
Size Requirements
The watermaking capacity measured in litres or gallons per hour – don’t confuse Imperial Gallon (4.5 litres) with US gallon (3.8 litres) – is primarily determined by the surface area of the membrane, although output falls rapidly with water temperature. Heat exchangers can be used to heat the feedwater, but this introduces further complications and the possibility of overheating the membrane.
Larger Advantages: If sufficient power is available there is much to be said for installing a larger unit, because membrane and pump running hours will be minimised, extending the overall life and maintenance intervals. A higher capacity also makes it easier to build up a reserve prior to entering a port where the system should probably not be used. Higher capacity may also be indicated if the aim is to produce the daily quota while an engine is run for other domestic purposes, such as eutectic refrigeration, battery charging, water heating etc. For example, a 100 litre/hour unit leaves a 20 per cent reserve if the aim is to produce 80 litres each day while the engine is run for an hour.
Power Source: The decision about whether the unit should be mechanically driven, or powered by DC or AC motors may be affected by limitations on these power sources, especially when gensets or PTOs would need to power other equipment such as refrigeration compressors simultaneously. Although AC motors are smaller, cheaper and more compact than DC equivalents, they are likely dependent on one power source, since few inverters are large enough to start a high pressure pump even if it has a ‘soft start’ motor. Desalinators shouldn’t be fed port seawater, so the ability to use shore power may be no advantage – except perhaps if treating ‘hard’ local water.
DC systems on the other hand are relatively flexible because AC power from a genset or portable generator can also be used via a battery charger, and the batteries can be recharged by solar panels, engines or even wind or water turbines. In many areas, the daily output of a modest 60 Watt horizontal solar panel can provide enough power (via battery storage) to produce around 80 litres per day from a modern energy-efficient watermaker.
Energy-efficient intensifier/recovery DC pumps do tend to be at the smaller end of the scale (~
Package versus Distributed RO Systems
Components can be bought as modular kits for customised installation or as packaged systems, with all the main components mounted within one frame. Modularisation allows the pumps to be installed in the sound insulated engine space, wet items such as filters near the bilge, the cartridge (which must not be heated above 49C) in a locker, and just the flow meter and pressure regulator in a convenient place.
However, packages incorporating sophisticated automatic controls for flushing, and monitoring of system pressures, flows, and water quality (including sterilisers) are now reasonably compact and probably have lower installation costs. Space may be the deciding factor on smaller yachts. Larger yachts with higher consumptions are also more likely to have space for a package system and a genset to power it from AC.
Conclusion
With appropriate maintenance, desalinators are durable, and if initial and running costs are amortised over say a decade, the cost per litre of water produced may be just a fraction of a cent. On the other hand, it can be much higher if the usage is low, and generally a desalinator is better justified on other grounds.
Portable manually powered RO systems are potential lifesavers in a liferaft, but a vessel should always carry enough reserve water distributed among several containers or tanks to be more than sure the crew won’t go thirsty if the main desalinator or its power source fails in mid-ocean. Freshwater for showers, dishwashing and laundry on the other hand is not essential, so large reserves for such purposes don’t make much sense once a desalinator is on board.
Resources
Many firms offer RO packages, components and kits, including in Australia:
www.purifiersaustralia.com.au (Selmar desalinators)
www.southernseasmarine.com (Spectra desalinators)
www.seairland.com.au (Village Marine Tec desalinators)
