Reverse osmosis removes almost all dissolved salts, along with many other contaminants. That makes it the core of drinking water production from seawater and brackish sources across the Gulf. But water straight from the membranes is not finished. It needs post-treatment before it goes to storage and the taps. Two steps matter most: remineralisation and disinfection.
Why RO water needs minerals back
RO permeate is very low in calcium, magnesium and bicarbonate. Water like this tends to be slightly acidic and has little buffering capacity. In pipework it can dissolve metals and attack cement linings, which shortens the life of the network and can affect water quality at the tap. It also tastes flat.
Remineralisation adds back a controlled amount of hardness and alkalinity so that the water is stable in pipes and pleasant to drink.
Common remineralisation methods
Mineral contactors
The permeate flows through a bed of a natural mineral such as calcite or dolomite, often with carbon dioxide added to help it dissolve. The water picks up calcium, and with dolomite also magnesium, along with alkalinity. Several AFG hotel seawater plants on this site use dolomite remineralisation. The beds are topped up as the mineral dissolves.
Chemical dosing
Alternatively, chemicals are dosed into the permeate to reach the target hardness and alkalinity. Dosing gives fine control but needs chemical storage and careful handling.
Blending
In some brackish water plants, a small share of filtered raw water is blended with the permeate. This restores minerals but also brings back some of what RO removed, so it only works when the raw water quality allows it.
Disinfection
RO membranes block bacteria and viruses very effectively, but the water can be recontaminated in tanks and pipes afterwards, and no barrier is perfect. Disinfection is therefore part of every drinking water design.
UV-C disinfection passes the water through a reactor where ultraviolet light inactivates microorganisms. It adds no chemicals and leaves no taste. Because UV leaves no residual, a residual disinfectant is often added as well to protect the water in storage tanks and the distribution network, depending on the system and the applicable local requirements.
Where UV is placed
UV is most effective on clear water, so it usually sits after remineralisation filters and before storage or at the point of distribution. In buildings that store water for long periods, a second UV unit on the outlet of the tank is common.
Storage matters
Water that stands warm in tanks for long periods loses its disinfectant residual and can grow bacteria. Tanks should be sized to balance demand, not to store water for many days, and they should be easy to inspect and clean. This is especially relevant on islands and coastal sites such as Bahrain, where buildings often store water on site.
Taste and stability
Remineralised water should be checked not only for safety but also for how it behaves. Operators often track pH, alkalinity, hardness and an index of corrosion or scaling tendency to make sure the water neither attacks the pipes nor deposits scale. Taste matters too: guests and residents quickly notice flat or overly hard water.
Maintenance of post-treatment
Mineral beds need topping up, dosing pumps need calibration, and UV lamps lose output over time and are replaced at set intervals. The quartz sleeves around the lamps are cleaned so the light reaches the water. A simple maintenance plan keeps post-treatment doing its job, and a record of each task makes it easy to see when something starts to drift.
Designing to the requirements
Drinking water must meet the applicable local requirements, which set limits for many parameters and may specify disinfection practice. The post-treatment design, from the mineral dose to the disinfection method, should be agreed against those requirements for each project, and the water should be tested regularly once the plant runs.
AFG builds remineralisation and disinfection into its SWRO and RO plants where the water is for drinking. To discuss a drinking water project, send the source, capacity and use through the quote form.