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RO Desalination Plants

RO Desalination Plants

Reverse-osmosis desalination behind a full pre-treatment train, at any capacity.

Reverse osmosis (also called hyperfiltration) is the process by which solvent molecules move out of the more concentrated solution when a pressure higher than the osmotic pressure is applied to that solution.

In practice, reverse osmosis is achieved through a membrane that retains the solute on one side, blocking its passage, and extracts the solvent on the other.

The phenomenon is not spontaneous: it requires equivalent mechanical work to cancel out the effect of the osmotic pressure.

This is the most refined water technology available, because it is not simply a physical barrier whose pore size determines which molecules pass.

Instead it exploits the differing chemical affinity of each species for the membrane, which lets hydrophilic (water-like) molecules through — chemically speaking, substances similar to water such as short-chain alcohols.

At plant level the method applies the cross-flow filtration principle alongside other membrane separation technologies such as microfiltration, ultrafiltration and nanofiltration.

Reverse osmosis is used in water treatment to desalinate water and to remove traces of phosphates, calcium, heavy metals, pesticides, radioactive materials and virtually every other contaminant.

Zero liquid discharge systems have appeared in recent years, in which the reverse-osmosis section raises the concentration of the chemical species in the waste stream up to or beyond their solubility limits (supersaturated solutions).

Thin-film composite membranes (TFC or TFM) are used in the reverse-osmosis process.

These semi-permeable membranes are manufactured primarily for water purification and desalination systems, and are even used in chemical applications such as batteries and fuel cells.

TFC is essentially a molecular sieve built as a film of two or more laminated materials.

Osmosis membranes are generally polyamide, a material selected for its permeability to water and impermeability to many dissolved impurities, including salt ions and other small molecules that cannot otherwise be filtered.

Another example of a semi-permeable membrane is the one used in kidney dialysis.

Treatment Stages

1

Stage One

Chlorine dosing. Chlorine is injected by a dosing pump at a rate matched to the flow, oxidising ferrous iron into ferric iron and manganese into manganese dioxide.
2

Stage Two

The sand filter removes turbidity — suspended solids such as clay and silt, colloidal matter such as fine silica, metal oxides such as rust, and organic waste. It also retains ferric iron and manganese dioxide up to 3 ppm.
3

Stage Three

The carbon filter removes free chlorine, organics such as pesticides, unpleasant odours and larger suspended matter, ensuring no chlorine residue reaches the consumer.
4

Stage Four

Birm media removes iron up to 10 ppm along with manganese and arsenic, using the dissolved oxygen in the feed water to oxidise those metals and the oxidised particles accumulated on the bed.

Applications

Used in areas suffering from iron, manganese, turbidity, suspended solids and bacterial or biological contamination.
Used in areas supplied from Nile canals.
Used across industrial and residential communities.