NanoFiltration for Arsenic and Hardness Removal

Application of Nanofiltration in Arsenic Removal and Groundwater Hardness Reduction

Nanofiltration (NF), as one of the advanced membrane technologies, has gained a prominent position in the treatment of groundwater contaminated with arsenic and hardness-causing ions.

Compared to reverse osmosis (RO), NF offers significant advantages due to its lower operating pressure, reduced energy consumption, and selective contaminant removal.

Case Study: Arsenic and Hardness Removal in a Municipal Water Treatment Plant

In a pilot project, the use of polyamide NF90 membranes combined with microfiltration pretreatment produced highly promising results:

  • Arsenic reduction: from ~80 µg/L to <10 µg/L (meeting WHO guidelines).
  • Total hardness reduction: from 1500 mg/L to ~400 mg/L.
  • Energy consumption: as low as 0.9 kWh/m³.
  • Recovery rate: 82%.

These findings demonstrate that nanofiltration can simultaneously improve drinking water quality and lower operational costs.

Operating and Design Conditions

A practical test was conducted on groundwater with the following parameters:

  • Influent arsenic: 50–100 µg/L
  • Hardness: 1500 mg/L
  • pH: ~7.5

Treatment process:

  1. Microfiltration (0.1 µm) for suspended solids removal.
  2. Chlorine dosing (1 mg/L) to oxidize As(III) into As(V).
  3. NF90 membranes with pore size of 1–2 nm.
  4. Operating conditions: pressure 10–12 bar, temperature 22–25 °C, flowrate 1.2 L/min, recovery 82%.
  5. Regular chemical cleaning (acidic and alkaline) extended membrane life up to 4 years.

Performance Results

  • Arsenic removal: ~90% (final concentration 8 µg/L).
  • Hardness reduction: 65% (from 1500 → 400 mg/L).
  • TDS reduction: from 1500 → 400 mg/L.
  • Energy consumption: 60–70% lower than RO.

Comparison with Reverse Osmosis (RO)

Advantages of NF over RO:

  • Lower operating pressure (10–20 bar vs. 20–80 bar).
  • Selective removal of contaminants while retaining beneficial minerals (e.g., potassium).
  • Reduced energy demand and lower operating costs.
  • Higher drinking water quality (no remineralization required).

Design and Operational Insights

  • Optimum performance was achieved at ~10 bar and near-neutral pH (6–7).
  • As(V) removal >95%, while As(III) requires prior oxidation (chlorine, ozone, KMnO₄).
  • Pressures >12 bar increased energy use and fouling without significant removal improvement.
  • Shifting pH above 8 or below 6 reduced arsenic removal efficiency.

Practical Importance

Given the ongoing water crisis in Iran, the development and implementation of NF-based treatment and reuse systems can play a key role in providing safe and sustainable drinking water. Successful experiences in countries like Israel (e.g., Shafdan and Sorek projects) highlight how proper investment and management in this field can improve water quality while reducing national costs.

Reference and Experimental Results

[1] Siddique TA, Dutta NK, Choudhury NR. Nanofiltration for arsenic removal: Challenges, recent developments, and perspectives. Nanomaterials. 2020;10(7):1323.

NF Test Results for Arsenic Removal

Technical Notes Oxidation Required? As(III) Removal (%) As(V) Removal (%) Recovery (%) pH Feed Pressure (bar)
As(III) weakly removed – only As(V) efficiently rejected at neutral pH Yes 37 82 65 6 4
Higher pressure improves retention, especially As(V) Yes 44 90 70 6 6
Very good As(V) removal, As(III) still low Yes 48 95 77 6 8
Optimum point – high TMP, good recovery Yes 52 97 82 6 10
Removal stable for As(V), As(III) slightly better but fouling begins Yes 54 97.5 85 6 12
Higher pH → lower removal of both As(V) & As(III) Yes 46 94 80 8 10
Strongly alkaline → further drop due to deprotonation Yes 41 90 78 10 10
Acidic pH → lower removal, surface charge effect Yes 32 78 60 4 4
Acidic + moderate pressure → slightly better but still weak Yes 35 84 63 4 6

Key Technical Notes for NF in Arsenic Removal

  • As(V): removal up to 97–98% at 10–12 bar.
  • As(III): poorly removed (<55%), must be oxidized to As(V) beforehand.
  • Optimum removal and recovery at 10 bar, ~80–82% recovery, pH 6–7.
  • pH <6 or >8 reduces efficiency for both As(V) and As(III).
  • Higher pressures (>12 bar) → more fouling and energy demand, without significant benefit.
  • Design takeaway: oxidation pretreatment + optimum feed pressure + pH control are essential for reliable NF arsenic removal.