Reverse Osmosis — Frequently Asked Questions

Under good operating conditions with correct pretreatment, spiral-wound RO elements (the standard industrial type) have a service life of 5–7 years. In potable water applications with clean, well-pretreated feed, 7–10 years is achievable. Life is shortened by aggressive fouling (inadequate pretreatment), scaling (inadequate antiscalant), chlorine exposure (>0.1 mg/L free chlorine is damaging to polyamide membranes), and physical damage from water hammer or high differential pressure. Tracking normalised flux, salt rejection, and differential pressure monthly allows early identification of membrane degradation.

RO membranes are sensitive to fouling and scaling, so pretreatment is essential. Standard requirements: SDI (Silt Density Index) <3 at the RO inlet, achievable via UF membranes or multimedia filtration; turbidity <0.5 NTU; free chlorine <0.1 mg/L (dose sodium metabisulphite if chlorinated feed); pH 6.0–7.5 for standard polyamide membranes; scaling ions controlled by antiscalant dosing or softening (target Langelier Saturation Index <0 at the concentrate end). For detailed guidance see our RO protection strategies page.

Recovery (ratio of permeate to feed flow) depends on the feed water chemistry and target concentrate TDS. Typical values: brackish groundwater (500–5,000 mg/L TDS): 70–85% recovery; seawater (35,000 mg/L TDS): 35–50% recovery; industrial process water (low scaling potential): up to 90% with interstage chemical dosing. Higher recovery concentrates scaling ions proportionally — recovery is ultimately limited by the least soluble salt reaching its saturation point in the concentrate stream. Our membrane flux calculator can model recovery for your specific feed chemistry.

Specific energy consumption depends primarily on feed osmotic pressure (which is proportional to TDS). Approximate values: municipal water (100 mg/L TDS): 0.1–0.2 kWh/m³; brackish water (2,000 mg/L TDS): 0.3–0.8 kWh/m³; brackish water (8,000 mg/L TDS): 1.0–2.0 kWh/m³; seawater (35,000 mg/L TDS): 2.5–4.0 kWh/m³ with energy recovery. Energy recovery devices (pressure exchangers) are standard for seawater RO at this scale and reduce energy by 35–50%. See our energy recovery equipment.

Concentrate management is often the most challenging aspect of RO system design. Options depend on volume and composition: (1) Sewer disposal — often restricted by TDS limits in trade effluent consent; check with your water company; (2) Surface water discharge — requires dilution analysis and regulator approval; (3) Deep well injection — used in oil and gas; regulatory approval required; (4) Evaporation — for inland sites in arid climates; (5) Zero liquid discharge (ZLD) with evaporator + crystalliser — eliminates all liquid discharge. See our ZLD systems page.

Trigger a cleaning-in-place (CIP) cycle when any of these normalised parameters (corrected for temperature and pressure) change by more than 15% from baseline: (1) normalised permeate flow drops by 15%; (2) normalised salt passage increases by 15%; (3) normalised differential pressure across the first stage increases by 15%. Cleaning frequency in well-designed systems: 3–12 months depending on feed water quality. More frequent cleaning usually indicates a pretreatment problem rather than a membrane problem.

Standard polyamide RO membranes reject long-chain PFAS (PFOS, PFOA, PFNA) at >95–98%. Short-chain PFAS (PFBS, PFHxA) are rejected at lower rates, typically 70–90%, and may require additional treatment (GAC polishing) to meet the EU Drinking Water Directive limit of 0.10 µg/L total PFAS. Concentrate management is critical — PFAS concentrates 5–20× in the reject stream and requires either licensed disposal or electrochemical destruction. See our PFAS treatment guide.