The Technology Behind Watermakers
The Technology Behind Watermakers
Whether enabling an extended stay at a secluded location or a coastal voyage, the freedom and independence that a watermaker provides is indisputable – for some it’s indispensable.
Watermakers will be standard on the Riviera 78 Motor Yacht, with a choice between two brands and systems: Blue Water or Parker, fully or semi-automatic. Optional for all other models, a watermaker is a decision best made during the building of a Riviera motor yacht to enable the installation of wiring and skin fittings through the hull.
Watermakers also can be retrofitted, and units are available in compact or modular configurations for confined areas.
How they work – the reverse osmosis process
On-board watermakers use a high-pressure pump to force water through a semi-permeable membrane. This separates larger particles and unwanted molecules, such as salt and other minerals. While functional aspects vary from model to model, systems typically include a series of pre-filters and post-filters.
First a suction pump draws in seawater through a sea strainer, which eliminates large objects such as marine life and sea grass. The water passes through an optional media filter – filled with sand or glass beads – and also an optional plankton filter before it’s purified through two pre-filters (from 20 micron to 5 micron).
Cartridge sizes for pre-filters range from regular household-sized filters to commercial-sized filters, which are more than double the volume. Commercial filters are more likely to last a full year of use, while smaller standard filters may need to be replaced sooner if not used in particularly clean water.
At the heart of a watermaker is the tube-shaped semi-permeable membrane, roughly 75mm thick. A high-pressure pump exerts up to 900 pounds per square inch (psi) to force water through this membrane. Salt and other particles remain on the exterior of the tube, while fresh water is pushed into its centre and flows out. The process of reverse osmosis also strips out minerals and will alter the pH levels of the water. A pH neutralising filter balances pH levels and can also add minerals back into the water.
Fully automatic and semi-automatic watermakers include a control box that measures salinity before water is diverted to a holding tank via a carbon filter, an optional pH filter (standard on larger units) and an optional UV filter for the final removal of micro-organisms. The result is pure potable water.
{The Blue Water Express XT watermaker features fully automatic operation with capacities of up to 150 litres (40 gallons) per hour}
automatic and manual systems explained
This will largely come down to the size and use of a motor yacht. Fully automatic systems are the most hassle free. Their one-touch operation and continuous automated monitoring takes the thinking out of the equation: salinity is measured, and pump pressure is automatically regulated.
The larger units can be programmed to produce a certain volume of water or run for a pre-programmed time. Commonly fitted to a Riviera 64, 68, 72 Sports Motor Yacht and 78 Motor Yacht are the fully automatic Blue Water Legend 1850 or Parker Aqua Matic 1800, which produce around 7,000 litres per day. These larger desalination systems require two membranes, while commercial-sized pre-filters are recommended.
Semi-automatic systems may stop and start automatically, measure salinity and include optional tank float switches, but require manual flow-rate monitoring as a product water-flow rate that exceeds a membrane’s specifications will damage its materials. Float switches can automatically start when a holding tank is low and turn off when it is full, which is useful for extended stays offshore in clean water.
[While functional aspects vary from model to model, watermaking systems typically include a series of pre-filters and post-filters]
A Riviera 505 SUV or 5400 Sport Yacht, for example, may opt between a full or semi-automatic system capable of producing around 3,000 litres per day. Common fits are the Blue Water Express 950 (semi-auto) or Blue Water Legend 950 (fully-auto) or Parker HRO Seafari 900 (semi-auto).
Manual watermakers are designed to the same quality standards. However, they require a skipper to attend to salinity (by taste) before directing water flow to a holding tank, and to keep an eye on flow rate, especially if the marine environment and water quality is variable due to tidal movements. On the flip side, their lack of electronics reduces their overall investment and ongoing maintenance.
Depending on the model, a Rainman manual system produces between 70 and 140 litres per hour.
Care and maintenance
Ongoing maintenance of your watermaker is critical and the safest way to do this is to run it regularly. This keeps pumps lubricated, membranes moist and prevents seals from hardening.
The boating environment, specifically sedimentation in water, will have the biggest impact to a watermaker’s overall efficiency and maintenance demands. Sedimentation varies between inland waterways and open seas. It’s prudent to avoid using a watermaker in heavily sedimented water. Where possible in inland waterways, use a watermaker on an incoming tide when clean water is visible.
Inland waterways will also require more attention to pump pressure as salinity changes. Fundamentally, a watermaker should be set by flow rate, and not pressure.
The pressure required to make water will vary due to water temperature and salinity. In clean, offshore water it should require between 800psi and 900psi and should not exceed 900psi or damage to the membrane may occur. This can vary significantly in inland waters due to the amount of freshwater runoff. As low as 350psi to 400psi may achieve the specified water flow as it takes less pressure without the resistance of salt.
As finer mud sediments get stuck in a membrane, the product water flow will drop. Even if a system is run at a maximum of 900psi, the system will not be able to produce the specified water flow. Mud sediments cannot be flushed out.
Maintenance schedule
Daily: A media filter collects larger particles that will more easily build up and cause blockages – it should be back-flushed after four to five hours of use. Sand or glass beads inside a media filter should be replaced every five years.
Weekly: When not in regular use, the entire system should be back-flushed every seven days. This pushes stagnant water through and prevents the build-up of any bacterial or organic matter including in the membrane itself. The freshwater pump must be left on for this to happen. Automatic and semi-automatic units control this within their programs, but they must be connected to shore power. Smaller systems may suffice with house batteries maintained by solar panels; again, the fresh-water pump must be left on.
Monthly: A minimum monthly inspection and clearing of the sea strainer is recommended. It’s best to check more frequently when in high-weed environs. If sitting idle for longer periods, a system may be completely flushed out with a chemical membrane storage liquid that ensures no stagnant water remains – a process known as winterisation.
Annually: A yearly service will replace the pre-filters and carbon filters, and change the oil in the high-pressure pump. Hoses, clamps and fittings will also be inspected.
Biennially: Membranes should be removed and chemically cleaned every two years to maximise water quality and achieve maximum product water flow. Looked after correctly, membranes should last up to 10 years.