Reverse osmosis (RO) type freshwater generators are continuing to grow in popularity as cruise shipowners and shipbuilding yards prioritise space saving and energy efficiency
A number of factors influencing the design and specification of freshwater generators for use on board cruise vessels are leading to a more widespread use of reverse osmosis (RO) based technology. Owners are looking for more compact equipment that is easy to install and maintain, and are also seeking to achieve big energy savings compared with earlier generation solutions. As a result, RO type generators continue to gain ground in the cruise market, both for newbuildings and as retrofits, at the expense of evaporator based alternatives.
“It is mainly due to the lower specific energy consumption of these plants, compared to that of evaporators, that the number of RO systems being installed on board is increasing, while the number of evaporators is on the decrease,” said Evac product manager, Georg Schumacher.
Similarly, Enwa Water Technology marketing manager Maria Lillieström agreed that owners in the cruise sector are increasingly coming to appreciate the benefits, and lower costs, associated with RO. “Reverse Osmosis systems operate with relatively low energy consumption to produce water and have a high recovery rate. Besides, RO units have a small footprint, and are easy to operate, service and maintain,”
One of the market leaders in supplying RO type freshwater generating systems to cruise ships, Evac has recently conducted a number of retrofit projects in which old evaporators have been replaced with RO plants. Some of these were carried out during drydocking; others while the vessel was at sea during cruises. “In addition to lowering energy consumption, the freshwater generating capacity of the new RO plant has typically been higher than that of the evaporators that were replaced, even though the space required has been smaller,” noted Mr Schumacher.
“We are seeing more and more owners abandoning evaporators for both potable and technical water” Maria Lillieström (Enwa)
The space saving and energy consumption benefits of RO systems are also leading to growing demand for this type of technology for use on smaller cruise ships, as well as vessels at the larger end of the market. To meet this demand, Evac has recently introduced the Rosys M reverse osmosis desalination plant that offers a daily freshwater generation capacity through a range of units, with 30m³, 60m³, 90m³, or 120m³ per day capacities. “These medium sized units are environmentally friendly, robust, easy to install and operate, quiet, and have an extremely small footprint,” adds Mr Schumacher. “The modular design of the Rosys M means that the plant can be custom-designed for the available space on board and made up of modules that best match the individual vessel’s requirements.”
As well as its strong performance in the retrofit market, Evac highlighted the fact that it has picked up some notable contracts for newbuild cruise ships. The company has been contracted to supply a package of equipment, including vacuum collection, waste water treatment, dry and wet waste treatment and freshwater generation systems, for two new ships for Star Cruises being built at the MV Werften shipyard in Germany. Scheduled for delivery in 2020 and 2021, these will each be fitted with four, double stage Evac Rosys reverse osmosis eCM type systems, each with capacity of 800m3 per day of seawater desalination.
The company is also working on several other new cruise ship projects, the largest of which require up to four double stage reverse osmosis desalination plants with capacities of between 600m³ and 1,050m³ per day.
For one recent newbuild cruise vessel project Evac delivered a total of four 600m³ per day RO units. Although all had the same performance, because of the limited available space each of the four had a different shape and their components were arranged in a different way.
Another trend highlighted by Evac is a growing requirement to install double stage RO plant to meet the particular requirements of the cruise lines for drinking water. This involves the water produced by the first RO plant in a series being further treated by a second RO plant, to remove nearly all salinity.
Also promoting RO-type freshwater generating systems is Enwa Water Technology. Ms Lillieström says: “We are seeing more and more owners abandoning evaporators for both potable and technical water. Many are also opting for energy recovery units to further reduce energy costs, by up to 60 per cent.” According to Enwa the return on investment for an energy recovery unit installed with an RO unit is around 12-18 months, depending on size.
Enwa has recently delivered, or has on order, a number of its RO type Water Maker units for cruise ships, with fresh water capacities ranging from 37m3 to 250m3 a day.
This July, Enwa delivered its latest design, a Water Maker SW75T Compact RO system, to a shipyard in Croatia for an expedition cruise vessel. “The benefit of this new system is that it has a very small footprint so the owner can utilise the available space better, while it is easy to operate and service and maintain,” says Ms Lillieström. “The vessel will be operating in Antarctica, among other places, so our system will be installed with a steam heat exchanger for the feed water to the RO units. This enables the seawater temperature to be below 0°C without freezing, despite its salinity.”
Despite increased market acceptance of the RO option, concerns have been raised that in some cases quality is being sacrificed to achieve lower costs. Daniel Shackleton, director of Salt Separation Services (SSS), says: “Unfortunately we are finding that the cruise ship RO plant market is almost being commoditised, with some shipowners and all newbuilding yards basing their purchasing decisions solely on price. While this may result in a lower capital cost, this is not always the case with regard to operational expenditure.”
Mr Shackleton suggests that in some cased the lower prices are being achieved through compromises in specification, including materials, quality and performance, leading to potential problems that may only be found too late, during sea trials or later. He says: “The prices currently being quoted for RO plant are ridiculously low and are effectively stifling development and innovation.”
SSS is working on a number of repeat cruise contracts for its RO-based freshwater generation technology.
As an alternative to RO technology, evaporator type systems are still widely used. Indeed, many owners are opting to have both evaporator and RO types on board, both as back-up and also to provide flexibility in accordance with the prevailing ocean water conditions.
One of the multi-effect evaporator based freshwater generation equipment orders that Alfa Laval received last year required a focus on achieving lower equipment weight both when the equipment is empty and in operation. “Thanks to the Alfa Laval multi-effect evaporators that were specified, the hull design could be made lighter and the cruise ship overall saved a significant amount of weight,” Alfa Laval global business manager Alex Jönsson observed.
The increased demand for expedition cruise ships designed for sailing in polar or colder waters gives the Alfa Laval multi-effect technology a competitive advantage, Mr Jönsson believes. Earlier this year, an Alfa Laval MEP-6-900 unit was successfully commissioned on board Norwegian Cruise Line’s Norwegian Joy and has reportedly been able to secure freshwater production when sailing in colder waters down to 0°C, or even lower.
Saga Cruises selects freshwater treatment
Rotterdam-based Hatenboer-Water has been commissioned to supply water treatment equipment for a new cruise vessel currently under construction at Meyer Werft in Germany. The company will supply United States Public Health (USPH) compliant equipment that conditions freshwater stores in tanks on board, after in-port bunkering or from seawater desalination, in a two-stage process: hardening through neutralising filters, followed by chlorine disinfection. Chlorine levels are constantly monitored by a remote analysis point within the water distribution system.
The Hatenboer-Water system on 55,900gt Spirit of Discovery, due for delivery in 2019, will treat technical and air conditioning drain water, as well as drinking water. The equipment supplied will comprise two rehardening/neutralising filters with CO2 gas dosing, as well as dosing skids and measuring and control devices.
While Meyer Werft will install the equipment, Hatenboer-Water will carry out commissioning at the yard and during sea trials.