Figure 1. Cruise ships can require up to 260,000 gallons of fresh water every day.
Figure 1. Cruise ships can require up to 260,000 gallons of fresh water every day.
Figure 2. Oil and gas platforms need dependable, consistent water supplies.
Figure 2. Oil and gas platforms need dependable, consistent water supplies.

Large commercial and military working marine vessels, sailboats and power cruisers, as well as offshore oil and gas platforms need a dependable and consistent supply of pure water on demand, whenever and wherever they are across the globe. Indeed, many boats will require huge amounts of fresh water for drinking, cooking, ice, showers, dishwashers and laundry every day, with some of the biggest cruise ships, for instance, requiring in excess of 260,000 gallons every day.

However, the need for such systems is not limited just to ships and offshore applications; there is also a requirement for potable water in regions with limited natural water resources, or in areas decimated by drought or other disasters. The solution, in many instances, lies with desalination technology. Indeed, fresh water produced by desalination has enabled the industrial and commercial development of some of the most water-scarce parts of the world that would have otherwise remained unproductive. Perhaps most importantly, this technology has also improved the health and welfare of many people by providing a supply of potable water, eliminating the risk of contamination and disease.

While there are a range of technologies available for desalination, reverse osmosis systems are typically the most effective and economical method of providing a constant supply of fresh water quickly and efficiently at the point of use. These systems generally provide a simple and efficient method of generating potable water of a consistent quality from a variety of saline or contaminated sources.

Indeed, so effective is reverse osmosis (RO) that it has become widely used not only for desalination in drinking water applications but also for high purity industrial processes, such as the manufacturing of electrical components, pharmaceuticals, chemicals, boiler feed water and medical applications, as well as for industrial wastewater recovery systems.

The principles of reverse osmosis

In essence, RO is a process that removes ions from water using membrane technology. It works by forcing pressurised water through a specialised semi-permeable or osmotic membrane to remove most of the impurities. The purified water, which is also known as permeate, continues to move through the system for further processing. Meanwhile, the water containing the rejected ionic salts, called the concentrate or retentate, is usually run to drain or reclaimed if there is an appropriate use for it. Since it was first applied commercially on seawater in the 1980s, this form of technology has generally been regarded as the most effective means of water purification.

Furthermore, by comparison with traditional thermal desalination technologies such as vacuum and flash distillation, RO typically uses less energy and requires less attention from maintenance crews. In the marine sector this has led to a reduction in overall costs for vessel operators, as well as freeing up space onboard as they typically have a far smaller footprint than competing systems.

For example, multistage flash distillation requires heat energy to boil the seawater before vaporisation and, as a result, is particularly energy intensive. Indeed, this method of desalination typically requires around 17 kWhr/m3 of water for heating and pumping power, whereas RO generally requires around only 5 kWhr/m3 of water.

Additionally, as multistage flash distillation systems are considerably larger than their RO equivalents, the construction costs are much higher and space requirements much greater. Furthermore, as piping, condensate and other associated components are needed for the steaming element of multistage flash distillation, maintenance requirements are also considerably greater than those of RO desalination.

As a result, RO often represents a far more cost effective means of desalination. This is even more true thanks to recent advances in technology, such as improved membrane filters, the use of low cost materials and modular systems, which also help to improve the reliability of RO systems. Ultimately, this method of desalination using modern systems means that the unit cost of water is often lower.

How onboard systems work

Initially, saltwater from the sea enters the desalination system and passes through an optional 100 micron strainer, designed to remove any large contaminants such as seaweed or fish, into a low pressure boost pump. This feed water is then boosted to approximately 1 bar through a sediment pre-filter. For example, the new Village Marine LT Series desalinators from Parker Hannifin feature a 5 micron pleated polypropylene sediment pre-filter which offers high dirt holding capacity and is designed to protect the high pressure side of the RO unit from feed water impurities. In some cases, a 20 micron filter precedes the 5 micron filter for greater service life, and even lower operational costs.

Once it has passed through the sediment pre-filter, the feed water moves onto a high pressure pump, which increases the pressure to approximately 57 bar. It is generally considered that the optimum operating pressure for a reverse osmosis desalination unit is between twice that of the osmotic pressure of seawater, typically 25 bar, and the maximum working pressure of the semi-permeable membranes, which is approximately 70 bar.

From the high pressure pump, the feed water is directed into the membrane pressure vessel where separation into two streams occurs: freshwater stream or permeate and rejection stream or retentate. At this stage, the freshwater stream may be analysed by a salinity monitor to ensure that the water meets the recommended safe limit for use as potable water, 500 PPM TDS (parts per million total dissolved solids), according to the World Health Organisation, before it is transferred into a storage tank. The LT series, for example, features a water quality monitor as standard, complete with automatic product diversion if the acceptable level is not maintained.

The rejected water or retentate passes back to the sea; on some highly sophisticated systems this wastewater may be passed through an energy recovery system to optimise overall efficiency.

Building onboard systems

In essence, onboard desalination systems should be compact, lightweight, quiet and energy efficient, while offering easy and low maintenance operation over long service lives. In response to this, Parker has designed its latest watermakers to meet these requirements, featuring modular, semi-modular and framed models which produce more than 1 tonne of water per day. Standard units can produce up to 350 tonnes of fresh water daily. Furthermore, these latest units occupy a volume similar to that of a footstool and only weigh around 55 kg.

They have been designed to feature efficient, low energy pumps fitted on anti-vibration mounts for low noise levels and feature patented high pressure regulators that ensure optimum system pressure in all seawater conditions without the need for continuous operator adjustment. Ultimately, as they were developed to meet the high standards on naval submarines they offer high performance in all applications.

Latest technology developments

In recent years, RO technology has undergone considerable evolution in terms of functionality and performance. For example, the productivity of elements has been enhanced considerably, with the latest membranes capable of operating at an efficiency of up to 75% recovery, dependent on the feed water quality. The water quality being produced has also improved dramatically with the latest systems removing up to 98% of the minerals or salts. This includes silica and organic compounds and inorganic ions contained in a pre-treated water supply, as well as over 99% of bacteria, colloids, micro-organisms, endotoxins and macromolecules.

It is also important to remember that RO is a percentage removal process and the quality of the water will depend on both the characteristics of the raw water supply and the design of the desalination system. Accordingly, seawater desalination systems generally include a pre-treatment package that has been specifically developed to deal with the characteristics of the feed water. For example, the LT series features a sediment pre-filter that removes fine particulates down to 5 micron.

To date, however, many systems have been large, noisy in operation and consume relatively high levels of energy. To address these issues, the latest generation of purification systems incorporate compact, high efficiency designs of RO membranes, with low noise, low energy pumps and ancillary equipment. For example, the LT Series desalinators use highly efficient spiral wound membranes to remove the impurities, such as salt, minerals and organics from sea, brackish, fresh and tap water.

To ensure optimum integrity and performance, these membranes use the highest grade polyamide film composite materials. The LT RO watermakers also incorporate a product diversion valve, which diverts water overboard if the quality drops below acceptable standards, to guarantee safety still further.

The use of high performance spiral wound membranes, which offer excellent rejection rates and permeability, has also enabled the latest watermakers to feature lightweight, compact, semi-modular frame designs. In particular, the Parker desalinators have been developed to offer maximum flexibility in terms of installation options. For instance, they can be mounted above or below the water line, while boost pumps and pre-filters can be fitted elsewhere in a vessel to accommodate all sizes and configurations. Furthermore, the construction of the latest devices reduces maintenance requirements significantly. For example, the magnetic drive boost pump never requires seal replacement, which minimises maintenance and operating costs still further.

As the predominant component of seawater is sodium chloride, one of the keys to successful watermaker design is the use of materials that can resist the attack of chloride ions. As such, the latest watermakers have been developed to incorporate highly corrosion resistant components such as titanium or stainless steel pressure pump heads that can withstand the aggressive seawater environment.

Furthermore, the latest high performance systems feature protected electrical components, such as remote control systems and master control panels for motor starter and pump control that are classified as NEMA 4X to ensure that they are resistant to falling dirt, rain, sleet, snow, ice, dust, water and corrosion.

Just as importantly, the latest desalinators incorporate a number of features that make them extremely user friendly. For example, Parker LT Series units allow users to keep track of the freshwater production on an integrated flowmeter, while a digital water quality monitor displays the purity of the water produced. They are also supplied with a freshwater flush system complete with activated carbon filter, with the option for timed automatic flushes and adjustable interval durations for set and forget flushing.

In addition, a 316 stainless steel pressure regulator maintains optimum system pressure, irrespective of seawater condition, eliminating the need for continuous operator adjustment. The LT series also features a chemical cleaning and preservative (pickling) cartridge system for convenient clean in place (CIP) maintenance, making the task even more quick and simple.

Summary

As membrane and reverse osmosis technology continues to become more advanced, these systems offer a consistent and economically viable method of producing potable water through desalination where there is a lack of fresh water supplies, and with minimal environmental impact. Indeed, these systems overcome the paradox faced by many vessel operators and coastal communities who have access to an almost unlimited and reliable supply of saline water but have no way to use it. Furthermore, these recent developments ensure reliable functionality with minimal maintenance requirements and a low cost per unit of water, which will inevitably help support the bottom lines of many operators across the globe.