Changes

For example, a relatively new watermaker technology uses a type of dehumidifier to condense water from the air. Manufacturers include companies such as Air Eater Proprietary Limited, Hyflux and Vapaire Water Purifier. But the capacity of such systems is limited, around 1 litre/hour (6.25 gal/day), in conditions of 45 to 60% humidity. These systems, while interesting, clearly do not meet the daily output requirements of a boat.

Reverse osmosis is the technology best suited to watermakers. Osmosis is the process whereby a solution passes through a semi-permeable membrane to seek equilibrium. Clear enough? For example, given a solution on one side of a membrane, and a lesser solution on the other, e.g., pure water, the denser solution will pass through the membrane into the less dense one, until both have the same density.

Reverse osmosis is the process of causing pure water to move out of the solution and through the membrane to the other side. To accomplish this, a high-pressure pump must be used, around 800 psi. The process is not very efficient, either. About 90% of the seawater and energy are wasted. In some designs, notably Spectra and Livol, this energy is partially recovered by applying it on the backside of the piston in the pump.

To prevent clogging the membranes, a pre-filtration system can remove particles of sand and organic matter down to 1 micron in size. Install a back-washed sand filter at the inlet. Use a series of filters (e.g., 50-25-5-1 micron) to remove other particles. If you put the finer filter first, it will clog immediately. Install an oil-water separator to remove oil. An absorption/reactive carbon filter will remove taste, odour, chlorine and organic chemicals.