Pre-treatment

Pre-treatment is generally performed to prepare raw water (well, river, seawater, etc.) for entry into the main treatment system such as RO or MBR.

Main Categories of Pre-treatment

Pretreatment methods are usually divided into three main categories:

  1. Physical → Removal of suspended solids and turbidity

    • Screening: Removes leaves, plastics, and large debris.

    • Sand Filters: Reduces turbidity and protects downstream membranes.

    • Carbon Filters: Removes chlorine and organic matter.

    • Microfiltration (MF) / Ultrafiltration (UF): Polishes water, stabilizes SDI, and removes colloids or algae.

  2. Chemical → Scale and corrosion control

    • Antiscalant injection

    • pH adjustment chemicals

    • Chlorination or other disinfectants

  3. Biological → Reduction of organics and microorganisms

    • Aeration

    • Chlorination / Ozonation / UV

    • Biofiltration (BAC)

Physical Pre-treatment

1) Screening

  • Suitable for: Well, surface, or seawater to remove leaves, plastics, and large algae.

  • Notes: Bar spacing 10–50 mm (coarse) and mesh 0.5–5 mm (fine); automatic cleaning is recommended.

  • Common Errors: Ignoring pressure drop or maintenance → chain blockage along the line.

2) Grit / Sedimentation

  • Suitable for: Surface and river water containing sand or silt.

  • Notes: Surface settling rate ~1–2 m³/m²·h; lamella/tube settlers save space.

  • Common Errors: Failure to clean sludge → solids return to the line.

3) Sand / Multimedia Filters

  • Suitable for: Reducing turbidity and SDI before cartridge/UF/RO.

  • Notes: Multi-layer bed (anthracite/silica/garnet); flow 8–12 m/h; backwash with air/water; target NTU < 1.

  • Common Errors: Insufficient backwash, excessive velocity, incorrect media size.

4) Granular Activated Carbon (GAC)

  • Suitable for: Organic matter, taste/odor removal, free chlorine before polyamide membranes.

  • Notes: EBCT ~10–20 min; periodic backwash; disinfection to control microbial growth.

  • Common Errors: Relying solely on GAC for continuous chlorine removal without ORP monitoring; biological fouling.

5) Cartridge / Bag Filters

  • Suitable for: Final particle barrier before high-pressure RO pumps.

  • Notes: Multi-stage 20→10→5→1 μm (nominal or absolute as needed); adjust differential pressure switch for replacement.

  • Common Errors: Incorrect micron rating or low-capacity housing.

6) Hydrocyclone

  • Suitable for: Wells with sand or heavy particles (>50–75 μm).

  • Notes: No chemicals required; stable inlet pressure needed; periodic sludge discharge.

  • Common Errors: Not suitable for fine particle removal.

7) Microfiltration / Ultrafiltration (MF / UF)

  • Suitable for: SDI and turbidity stabilization for RO or removal of algae/colloids. Also common before MBR.

  • Notes: UF pore size 0.01–0.1 μm; typical flux 30–80 L/m²·h; periodic backwash (BW) and CEB/CIP; target SDI < 3 (often <2).

  • Common Errors: Failure to remove oxidants before sensitive UF; improper backwash/CEB design.

 

Chemical / Physicochemical Pre-treatment

1) Coagulation and Flocculation + Sedimentation / DAF

  • Suitable for: Surface waters with high turbidity, organics, or algae.

  • Notes: Coagulants (alum, ferric, PAC), flocculant aid (polyacrylamide); jar tests for optimal dose and pH; rapid mixing (high G, short), then slow mixing (low G, long); DAF excellent for algae/oil.

  • Common Errors: Excess cationic polymer near RO → risk of organic/biological fouling.

2) pH Adjustment

  • Suitable for: Scale/corrosion control and coagulation optimization.

  • Notes: Acid (H₂SO₄/HCl) to reduce LSI or prevent scaling; alkali (NaOH) for coagulation/carbonates; accurate and safe instrumentation required.

  • Common Errors: pH fluctuations due to weak control loop; injection at improper mixing point.

3) Oxidation / Disinfection (Chlorine, Ozone, UV) + Dechlorination

  • Suitable for: Microbial control and oxidation of Fe/Mn/sulfides.

  • Notes: Chlorine is effective and cheap but must be removed before RO (PA membranes are sensitive).

    • Dechlorination: Sodium bisulfite (SBS) or GAC; ORP monitoring (typically < +200 mV).

    • Ozone / UV: Ozone is strong oxidant (followed by BAC); UV inactivates microbes without residual.

  • Common Errors: Incorrect SBS dose → excess sulfite → microbial growth.

4) Antiscalant

  • Suitable for: Nearly all RO systems with medium/high recovery.

  • Notes: Phosphonate or carboxylate-based; typical dose 1–6 mg/L based on predictive software and analysis (CaSO₄, CaCO₃, Ba/SrSO₄, silica); injection point before cartridge filter to trap potential scaling.

  • Special Considerations: If producing acidic or alkaline antiscalant, select based on feedwater pH and dominant scale species (carbonate/sulfate/silica); use specialty grades for silica; avoid direct contact with cationic polyelectrolytes.

  • Common Errors: Using generic antiscalant for specific conditions (e.g., high silica) or changing recovery without adjusting dose.

5) Ion Exchange (IX) Softener

  • Suitable for: Very high hardness or high RO recovery.

  • Notes: Sodium cation resin; regular regeneration with NaCl; hardness leakage control.

  • Common Errors: Insufficient wash/regeneration → Ca/Mg passage → RO scaling.

6) Catalytic Iron/Manganese Removal

  • Suitable for: Wells with high Fe/Mn.

  • Notes: Oxidation (air/chlorine/KMnO₄) + catalytic filters (Greensand, MnO₂, Birm); proper pH and ORP required.

  • Common Errors: Insufficient oxidant or poor bed washing.

Biological Pre-treatment

1) Aeration / Stripping

  • Suitable for: Removing H₂S, CO₂, VOCs; increasing DO; reducing odors.

  • Notes: Air tower/spray/surface; followed by solids filtration.

  • Common Errors: Precipitate/biofilm formation if maintenance neglected.

2) Biofiltration (BAC)

  • Suitable for: Post-ozone or for reducing AOC/BDOC in surface waters; improves microbial stability of RO feed.

  • Notes: Similar to GAC but with controlled biofilm; regular backwash required.

  • Common Errors: Over-chlorination before BAC kills biomass.

3) MBR as RO Pre-treatment (Wastewater Reuse)

  • Suitable for: Industrial/municipal wastewater reuse.

  • Notes: MLSS 6–12 g/L; MF/UF membranes; very low TSS and SDI; followed by carbon/antiscalant/cartridge for RO feed.

  • Common Errors: Uncontrolled organic shock load; no scheduled CIP/CEB.

RO Feedwater Quality Targets (Quick Guide)

  • NTU ≤ 0.5–1

  • SDI ≤ 3 (better: ≤ 2)

  • Free chlorine: 0 (with proper ORP)

  • Fe / Mn: near zero

  • Oil / grease: very low (<0.1 mg/L preferred)

  • Stable pH with controlled LSI

Common Pre-treatment Mistakes

  • Incomplete chlorine removal before polyamide membranes

  • Fixed antiscalant dose ignoring seasonal water quality changes or increased recovery

  • Insufficient backwash of media and UF filters → increased ΔP, reduced CIP intervals

  • Relying on a single step instead of a multi-stage layout (Screening → Media → UF/Cartridge → RO)