How to Choose Ultrafiltration Membranes for Water Treatment

Pore size selection by contaminant target

The relationship between pore size and contaminant removal is non-linear. At 0.01 µm, virus removal exceeds 4-log (99.99%) but flux is limited to 50–70 L/m²·h. At 0.02 µm, virus removal drops to 3-log (99.9%) but flux increases to 70–100 L/m²·h — a 30–40% productivity gain for a marginal reduction in removal. At 0.04 µm, virus removal is minimal (1-log) but flux reaches 80–120 L/m²·h, suitable for applications where downstream disinfection handles pathogens. For drinking water without downstream UV, 0.02 µm is the recommended minimum. For RO pretreatment where virus removal is not required, 0.01–0.02 µm is specified for colloidal protection.

Flux rate: the economic balancing act

Design flux selection directly impacts both capital and operating costs. Higher flux reduces membrane area, lowering capital expenditure by $50–100 per m² of membrane area. However, higher flux increases fouling rate, requiring more frequent cleaning. Cleaning adds chemical costs ($0.02–0.05 per m³ permeate) and downtime (1–2% availability loss). The optimum flux balances these factors. For a 10,000 m³/day plant, increasing flux from 70 to 90 L/m²·h reduces membrane area by 22% (saving $150,000–200,000) but increases annual cleaning costs by $8,000–12,000. The net present value calculation typically favours higher flux when membrane costs exceed $100/m² and chemical costs are below $0.03/m³.

Flow configuration and installation

Flow direction and module type affect cleaning effectiveness and solids handling. Inside-out flow allows higher crossflow velocity (0.5–1.5 m/s) for membrane surface scouring, reducing fouling in pressurized systems. Outside-in flow is preferred for submerged systems where gravity provides feed, with air scouring (0.2–0.5 Nm³/h per m² membrane area) used for surface cleaning. Hollow fiber modules dominate the market with packing density of 300–500 m²/m³, compared to 100–200 m²/m³ for flat sheet. For large plants (>10,000 m³/day), hollow fiber provides significant space savings. For small systems (<1,000 m³/day), flat sheet offers easier inspection and maintenance.

Cleaning protocols by application

Cleaning strategy varies significantly by application. Drinking water systems typically require backwashing every 30–60 minutes with chemically enhanced backwash (CEB) every 1–3 days. Wastewater systems need more frequent cleaning — backwashing every 20–40 minutes and CEB every 1–2 days. Recovery cleaning with 200–500 ppm NaOCl is performed every 2–8 weeks for drinking water and 1–4 weeks for wastewater. PVDF membranes tolerate up to 1000 ppm NaOCl and pH 2–12, enabling aggressive cleaning that restores 95–100% of initial flux. PES membranes are limited to 200 ppm NaOCl and pH 3–10, typically achieving 85–95% flux recovery. The cleaning protocol should be specified by the manufacturer and validated through pilot testing.

Application parameter reference

Supplier documentation requirements