Brackish water reverse osmosis

Brackish groundwater is common across the interior of Southern Africa — inland aquifers routinely run 1 500 to 5 000 mg/L TDS, well beyond taste thresholds and irrigation limits for sensitive crops. Reverse osmosis is the established answer, and system economics hinge on three decisions made before any hardware is ordered: element type, array staging and pretreatment.

Element selection follows feed TDS and energy budget. Standard brackish elements (BW type) handle feeds to roughly 8 000 mg/L at 15.5 bar test conditions with stabilised rejection around 99.5%. Low-energy elements (LE type) run at materially lower pressure for a fractionally lower rejection — on power-constrained sites, that trade routinely wins. Never mix element types within a vessel: the downstream element sees what the upstream element passes, and mixed arrays flux-imbalance.

Staging determines recovery. A single-stage array recovers 50–60% of feed as permeate; a 2:1 two-stage array pushes system recovery to 70–75% by feeding stage-one concentrate to a second stage. Push recovery beyond what the water chemistry allows and sparingly soluble salts scale the tail elements — antiscalant dosing, set from the actual water analysis, is standard on any plant above hobby scale.

Pretreatment is where brackish RO plants live or die. Polyamide membranes have effectively zero chlorine tolerance (< 0.1 mg/L free chlorine) and foul rapidly above SDI 5. The minimum defensible train is 5 micron cartridge filtration immediately ahead of the high-pressure pump, with media filtration upstream where solids or iron are present, and carbon or bisulphite dosing where the feed carries chlorine.

DWT stocks DuPont FilmTec elements alongside cost-effective equivalents, membrane pressure vessels in 300 psi and 1 000 psi ratings, dosing pumps and the full pretreatment chain. Indent items are listed with lead-time bands — if it is not on the shelf, it is sourceable.