Brackish Water RO: Design Rules for Stable < 100 μS/cm (1000 LPH)

Executive summary

Brackish water RO design targets <100 μS/cm at 1000 LPH by combining reliable feed characterization, pretreatment to SDI ≤3, realistic 55–70% recovery, and trend-based instrumentation.To deliver < 100 μS/cm consistently at 1000 LPH, anchor the design on: reliable feed characterization, pretreatment to SDI ≤ 3, realistic recovery (55–70%), and tight instrumentation with trend-based control. This article converts those principles into field-ready rules and checklists. A configured reference is available here once you’re scoping: 1000 LPH RO Water Treatment Solution

1) Brackish water RO design: a practical framework

• Product quality: < 100 μS/cm typical for brackish feeds up to ~3000 ppm TDS with correct pretreatment and set-points.
• Duty: confirm hours/day and days/month; this sets pump sizing and consumables cadence.
• Regulatory & hygiene: SS304 baseline; choose SS316L when chloride and hygiene exposure are high.
• Chemistry: dechlorination before polyamide membranes; antiscalant and occasional pH control sized to recovery and LSI/S&DSI.
• Start-up expectations: trend from Day-1; enforce interlocks to protect membranes (low pressure, high conductivity, ΔP).

2) Characterize the feed (no guessing)

Collect a 2–4 week snapshot across shifts or seasons: TDS, SDI, turbidity, pH, temperature, free chlorine, Fe/Mn, organics (if available).

• Temperature: colder water → higher viscosity → higher pressure for same flux; plan winter set-points.
• SDI: your most important fouling KPI before RO. Target SDI ≤ 3 at the RO inlet.

Consistent SDI control is the backbone of brackish water RO design.

3) Pretreatment rules to hit SDI ≤ 3

• MMF/AF sized for variable loads; backwash program matched to dirt load.
• UF when SDI > 3 or turbidity spikes: stabilizes flux, lowers cartridge consumption, extends membrane life.
• Dechlorination (A/C or SMBS) ahead of RO if any free chlorine is present.
• Cartridge 5 μm as the final barrier; size for duty and ΔP monitoring.
• Verification: log SDI and turbidity after each pretreatment stage during commissioning.

4) Core RO design window (1000 LPH)

• Membranes: typically 2 × 4040 polyamide; check element datasheets vs TDS/temperature.
• Recovery: 55–70% depending on scaling potential; match antiscalant to recovery.
• Pressure: usually 8–16 bar for brackish feeds; verify pump curve with winter temperature.
• Controls: continuous conductivity on feed/permeate; ΔP across RO; flow on feed/permeate/reject.
• Set-point discipline: avoid aggressive recovery on poor pretreatment; optimize on trend data, not on single-point readings.

Set-points should be tuned on trend data—this is central to brackish water RO design.

5) Instrumentation, trending and interlocks

Trend Cond-permeate, ΔP, normalized flux, temperature.

• CIP triggers: ΔP +15–20%, conductivity drift, or normalized flux loss.
• Interlocks: low feed pressure, high conductivity reject, tank high level, emergency stop.
• PLC/HMI: log and export 30–90 days; alarms require acknowledgment and root-cause notes.

6) Commissioning & SOP (1000 LPH)

1. Flush pretreatment to clear fines and carbon dust.
2. Ramp the HP pump with VFD; confirm ΔP and conductivity baselines at nominal recovery.
3. Validate SDI ≤ 3 at the RO inlet; only then proceed to acceptance testing.
4. Record a 7-day trend pack (CSV) to establish your “healthy” envelope.

7) O&M cadence (typical)

• Cartridge filters: weekly–monthly.
• UF backwash/CIP: per vendor, adjusted to turbidity seasonality.
• RO CIP: every 3–6 months on stable pretreatment; follow ΔP/conductivity triggers.
• Membrane life: ~2–3 years with correct pretreatment and operating window.

Over 2–3 years, disciplined brackish water RO design lowers CIP frequency and membrane spend.

For background on water-quality principles, see the
WHO Guidelines for Drinking-water Quality.

8) Troubleshooting quick table

• Permeate conductivity rising: check dechlorination, recovery too high, element damage, temperature shifts.
• ΔP increasing: fouling; verify pretreatment, SDI trending, cartridge loading; schedule CIP.
• Flux dropping suddenly: air ingress, pump/VFD issue, feed blockage.
• Cartridge life short: insufficient pretreatment or turbidity spikes; consider UF.

9) RFQ checklist (copy/paste)

• Latest TDS/SDI/temperature series (2–4 weeks)
• Target permeate conductivity and recovery
• Pretreatment scope (MMF/AF, UF if SDI>3, dechlorination, 5 μm cartridge)
• Materials (SS304/SS316L), utilities, noise
• Instrumentation (pressures, flows, conductivity), PLC/HMI and I/O list
• GA/Layout: footprint, nozzles, access, service clearance
• OPEX sheet assumptions (kWh/m³, chemicals, filter cadence, CIP)