RO Brine Concentrate (Reject) Storage & Tank Compatibility

Storing RO Brine Concentrate (Reject)? Start Here

RO brine concentrate — also called reject or reject brine — is the high-salinity stream left behind when reverse-osmosis membranes recover clean permeate from brackish, sea or process water. It is fundamentally water that has been concentrated, typically carrying 50,000-70,000 mg/L of total dissolved solids (roughly 1.5 times seawater), dominated by sodium chloride with magnesium and calcium chlorides, sulfate salts, residual hardness, and trace pretreatment and membrane-cleaning chemicals. The liquid is usually clear, near-neutral to mildly alkaline, and non-flammable.

It is generated everywhere RO is used: municipal and seawater desalination, power-plant and industrial water reuse, food and beverage processing, and oilfield/produced-water handling. The stream must be collected, stored and disposed of (deep-well injection, evaporation ponds, ocean outfall or zero-liquid-discharge crystallizers). Material selection matters because the concentrated chloride content is aggressively corrosive to ordinary metals, even though it is harmless to polyethylene.

Is Polyethylene Safe for RO Brine Concentrate?

Yes — polyethylene is an excellent, recommended choice. HDPE and crosslinked polyethylene (XLPE) are unaffected by aqueous salt solutions at any concentration, including saturated chloride and sulfate brines. Because RO concentrate is essentially a strong salt solution near neutral pH, it poses no chemical-attack risk to a polyethylene wall, unlike the pitting corrosion it inflicts on carbon and stainless steel.

Specify a tank with adequate wall thickness and the correct specific-gravity rating: RO brine is denser than water and grows denser as recovery increases, so size the tank for the actual brine density rather than for 1.0 SG. For highly concentrated or saturated brines, a higher-SG-rated polyethylene tank is appropriate. Use polyethylene or polypropylene fittings and EPDM gaskets, and provide for periodic cleanout of scale where hardness and sulfate are concentrated. Always confirm the controlling SDS — residual cleaning or pretreatment chemicals can shift the requirements.

Material compatibility at a glance

RO brine concentrate is a benign, non-flammable aqueous salt solution, so the dominant material-of-construction driver is chloride-induced corrosion of metals — not chemical attack on plastics. HDPE/XLPE, polypropylene and corrosion-barrier FRP store it indefinitely; bare carbon steel and even 316 stainless are at risk from pitting at elevated salinity. Account for higher hydrostatic load from above-water density and for scaling from concentrated hardness/sulfate.

Ratings: S suitable · C conditional / limited · U unsuitable. Verify against the cited resistance charts and your concentration/temperature before specifying.

The safety that actually matters

• High salinity / TDS: not suitable for discharge to sewer, soil or surface water without a permitted disposal route — treat as a regulated wastewater per local rules.
• Chloride corrosivity: aggressively pits bare steel and can attack 316 stainless — the primary handling-equipment hazard.
• Scaling and precipitation: concentrated hardness, sulfate and carbonate can foul pumps, valves and tank walls; plan for cleanout.
• Residual chemistry: traces of antiscalant, biocide or membrane-cleaning acids/bases may dominate the actual SDS hazard — review it before handling.
• Slip / housekeeping: spills leave a corrosive salt residue; rinse and contain.
• Density: heavier than water — account for added hydrostatic load and confirm the tank's specific-gravity rating.

Common questions

Can I store RO brine concentrate in a poly (HDPE/XLPE) tank?

Yes. Polyethylene is unaffected by aqueous salt solutions at any concentration, so HDPE and XLPE tanks are the standard, cost-effective choice for RO reject brine. Size the tank for the brine's actual specific gravity, which is greater than water and rises with recovery.

Why is bare steel a poor choice for RO concentrate?

The stream is rich in chloride, which drives pitting and crevice corrosion in carbon steel and even 316 stainless. Unprotected steel can corrode rapidly, so steel service requires linings, coatings, or a higher alloy such as duplex stainless.

What is the typical pH and hazard rating of RO brine?

RO concentrate is usually near neutral to mildly alkaline (representative pH about 6.5-8.5) and non-flammable, so a generic NFPA 704 of all zeros is representative. The real hazard is set by residual pretreatment/cleaning chemicals, so the controlling SDS governs.

Does the brine density affect tank selection?

Yes. RO concentrate is denser than water — commonly around 1.5 times seawater salinity and increasing with recovery. Always select a polyethylene tank with a specific-gravity rating that meets or exceeds the actual brine density to ensure the wall is not overstressed.