Coating Ultrafiltration Membranes with Iron Nanoparticles to Enhance Operation During Periods of Severe Algal Blooms

The release of high concentrations of Transparent Exopolymer Particles (TEPs) by phytoplanktons during algal blooms (e.g. red tides) can cause severe impairment of ultrafiltration (UF) operation in seawater reverse osmosis (SWRO) pretreatment (Villacorte et al, 2010). TEP are negatively charged acidic polysaccharides and glycoproteins reaching a few hundred microns in length, and are transparent but can be visualized by staining with Alcian Blue. TEP can be measured in the particulate range >0.4um (Passow et al, 1995) and in the colloidal range >0.1um (Villacorte et al, 2009), but measurements of biopolymer concentrations using the liquid chromatography - organic carbon detection (LC-OCD) method can provide an indication of the concentration of TEP in source waters. TEPs are reportedly 2 - 4 orders of magnitude stickier than normal particles in seawater (Passow, 2002) and may cause operational problems by attaching to UF membrane surface or pores, resulting in both a higher rate of pressure increase and reduced permeability recovery.

Coagulation in different modes (full scale, inline, etc) is a strong tool in controlling irreversible fouling of UF membranes applied for seawater pretreatment (Schurer et al, 2010), with ferric chloride being the most commonly used coagulant in these applications.

This study investigates the feasibility of a novel mode of coagulant application, namely precoating, to enhance UF performance during algal blooms with high TEP concentrations. Precoating is performed by covering the membrane surface with a layer of nanoparticles of iron hydroxide (mineral hydroxy-oxides) at the start of each filtration cycle followed by direct filtration of feed water. The precoat layer is highly permeable and is easily removed by hydraulic backwashing. The precoat layer protects the membrane from irreversible fouling by preventing the highly surface-active TEPs to stick to the membrane surface and pores.

For comparison, inline coagulation is also studied by continuous dosing and inline mixing of ferric chloride into the feed stream of the UF membranes.

The efficiency of the different coagulation modes in minimizing non-backwashable fouling of UF membranes treating seawater with high TEP concentrations is quantified in terms of UF permeability recovery with hydraulic cleaning (i.e. backwashability).

Polyethersulfone UF hollow fibres (Seaguard, Pentair) with MWCO 150kDa are operated in dead-end mode at a flux of 50 - 100L/m²h. Algal bloom conditions are simulated by cultivating marine diatoms (Chaetoceros affinis) under controlled conditions in the laboratory, and feed solutions are prepared with cell counts and TEP concentrations comparable with real algal bloom conditions.

Preliminary results indicate that precoating significantly reduces irreversible fouling due to high TEP concentrations and stabilizes UF operation. To achieve stable UF operation at very low coagulant concentrations ≤0.5mgFe³⁺/L, particle size of the precoat solution is of crucial importance. For a range of particle sizes tested, precoating with d_p=700nm resulted in the most optimum UF performance. The resulting precoat layer thickness as measured by SEM imaging was in the order of 2-4μm in thickness (3-6 times nominal d_p).

Inline coagulation at low coagulant concentrations (0.5mgFe³⁺/L) was also successful in improving UF operation. However, UF operation could not be stabilized with continuous inline dosing of coagulant.

In terms of permeate quality, precoating and inline coagulation at approximately 0.5mgFe³⁺/L did not show enhanced biopolymer removal - biopolymer removal was approximately 60%, which is comparable with removal efficiency of UF membranes without coagulation. However, at higher coagulant concentrations (>5mgFe³⁺/L) biopolymer removal with inline coagulation increased to 80% while precoating removal efficiency remained at 60%.

In conclusion, to maintain UF operation during severe algal blooms with very low coagulant concentrations, precoating with nanoparticles of iron hydroxide is superior to the more commonly applied continuous inline coagulation.