RO reject & desalination brine ponds

Reverse osmosis (RO) plants split their feed water into fresh water and a reject stream, also called brine or concentrate, that carries the salts removed from the feed. Where a plant has no sea outfall or sewer connection, the reject is often sent to a lined pond to evaporate, or to be stored before disposal.

Where RO reject ponds are used

Brackish groundwater RO plants serving towns, farms and factories
Industrial water treatment plants
Zero liquid discharge (ZLD) schemes, where an evaporation pond can be the final step

Single liner or double liner with leak detection

Single HDPE liner on a prepared subgrade, where the design and the reject water allow
Double liner with a leak detection layer between the two geomembranes, for higher-risk sites or where the specification or permit requires it
Protection geotextile where the subgrade could damage the liner

Designed for Saudi heat, UV and salt

RO reject ponds are shallow and exposed, so the liner sits in full sun for its whole life. Specify HDPE to GRI-GM13, which requires 2–3% carbon black and a UV resistance test for every formulation.

Salt itself is not the problem. Manufacturers' chemical-resistance tables rate HDPE as little or not affected by salt water and by sodium, calcium and magnesium chloride. The practical risks are the daily expansion and contraction of the sheet, wind uplift when the pond is low, and damage when salt and sludge are removed.

Testing and QA/QC

HDPE to GRI-GM13 with manufacturer certificates
Fusion (hot wedge) welding for field seams; extrusion welding for pipe inlets, corners and details
Vacuum box and spark testing; peel and shear tests on trial welds and seam samples
Test and repair records handed over with the as-built data

Evaporation ponds

Frequently asked questions

Do RO reject ponds need a double liner?

Not always. A single HDPE liner on a well-prepared subgrade is common where the design allows. A double liner with a leak detection layer between the two geomembranes is used where the groundwater needs a higher level of protection, or where the project specification or permit calls for it. The design engineer decides; we install and test either system.

How is the size of an RO reject evaporation pond decided?

The designer sizes the pond from the daily reject volume and the local net evaporation rate (evaporation minus rainfall), with freeboard and enough storage for the cooler months when less water evaporates. The liner quantity, and so the lining cost, follows from that area and the slope lengths.