Reverse Osmosis (RO)

Reverse osmosis is a pressure-driven membrane process: feedwater is pushed at 8–18 bar (brackish) or 55–80 bar (seawater) against a thin-film composite spiral-wound membrane. The water permeates through; dissolved salts, metals, hardness and most small organics are rejected and concentrated into a brine stream. RO produces low-TDS permeate but is a removal technology — not a true disinfection step on its own.

RO rejects 95–99.7% of dissolved salts, heavy metals (lead, arsenic, cadmium), nitrate, fluoride, hardness (Ca²⁺, Mg²⁺), sulphate and most small organics. It is not a primary barrier against suspended solids — that is what pre-treatment is for — and it removes some volatile organics less effectively than activated carbon. Gases such as CO₂ pass through and are managed with degassing or pH correction where required.

Recovery typically runs 50–75% on brackish feedwater and 35–45% on seawater (using energy-recovery devices). Energy intensity is feed-dependent: modern brackish-water RO with efficient pumping runs 0.5–1.2 kWh per m³ permeate; high-salinity brackish 1.2–2.5 kWh/m³; seawater RO with ERDs 2.5–4.0 kWh/m³. The actual figure is calculated against feed TDS, recovery, temperature and array configuration during design.

DuPont FilmTec, Toray, LG Chem, Hydranautics and Lanxess — the established tier-one suppliers with published performance data, multi-year warranties and an active local presence. The specific element is chosen against the feed-water TDS, the target recovery, the fouling risk and the cleaning regime. We do not use unbranded or grey-market membranes.

At minimum: cartridge or media filtration to bring SDI₁₅ below 5; antiscalant dosing matched to the feed-water Langelier or Stiff & Davis index; chlorine removal (activated carbon or sodium metabisulphite) to protect the polyamide membrane; sometimes UF as a guaranteed physical barrier upstream. Inadequate pre-treatment is the single largest cause of premature membrane failure.

On a properly pre-treated brackish-water feed, RO elements typically deliver 3–5 years of service before the salt-rejection or normalised permeate flow drift outside warranted limits. Performance is tracked through normalised permeate flow, salt passage and differential pressure — three metrics our monitoring records continuously, so a failing membrane is flagged in time, not after the water quality drops.

Brine concentration depends on recovery: at 75% recovery the brine is roughly 4× the feed TDS. Disposal route is site-specific — sewer discharge with consent, evaporation pond, deep-well injection, blended discharge with treated effluent, or concentrate recovery (zero-liquid-discharge) for high-value sites. The route is part of the design pack and signed off against the relevant water-use authorisation.

RO permeate has very low TDS and is often slightly acidic. For potable distribution we typically re-mineralise (calcite/dolomite contactor or precision dosing of calcium hydroxide and CO₂) to lift hardness to 60–80 mg/L as CaCO₃ and stabilise pH around 7.5–8.0. This protects downstream piping from corrosion and meets the SANS 241 and WHO requirements for hardness, alkalinity and Langelier Saturation Index.

For potable applications in South Africa: SANS 241:2015 on the product water, the Water Services Act 108 of 1997 and the National Water Act 36 of 1998 for abstraction and discharge. Components are typically NSF/ANSI 58 or 61 certified (or equivalent). WHO Guidelines for Drinking-water Quality inform design margins. Specialised applications (food, pharma, healthcare) reference the relevant sector standard.

Continuous monitoring of pressure, flow, conductivity and recovery; routine cartridge changes (typically 4–8 weeks); periodic CIP (cleaning-in-place) cycles when normalised pressure drop or salt passage drift past trigger limits; antiscalant and pH dosing chemistry refill on schedule; SANAS-accredited monthly water sampling for compliance. All of this is included on the HidroVerse Care contract — not invoiced piecemeal.