Membrane fouling silica

Reverse osmosis plants also are not immune from silica fouling, and where the raw water source naturally contains relatively high levels of silica, good pretreatment of the RO FW is a prerequisite. To reduce fouling of RO membranes by silica, pretreatment by acid adjustment, alum coagulation, and filtration usually is provided. 

To increase the solubility of calcium carbonate and reduce its potential for precipitation and hence membrane fouling, the pH of the feed water is lowered through the addition of acid (Greenlee et al. 2009). Sulphuric acid and hydrochloric acid are most commonly used. Acid treatment can also help to improve the coagulation of colloids and slightly increase the solubility of silica (Bergman 2007). 

Fig. 23. Visualization of membrane fouling by scanning electron microscopy (A) inorganic fouling dne to calcinm carbonate, calcium sulfate, silica, iron, barium and strontium sulfate (35,000x) (B) organic fonling dne to humic acid (35,000x) (C) flow channels in membrane fouled with biological growth.

Bowen et al. [40] used sulfonated poly(ether ether ketone) (SPEEK) as an additive in the polysulfone (PSf)/SPEEK/N-methyl-2-pyrrolidone (NMP) system. Membrane characterization was carried out using filtration studies and AEM. Membranes prepared from solutions in the region of polyelectrolyte behavior [41] showed more pronounced and systematic improvement of membrane permeability and salt rejection. A small decrease in pore size and surface roughness was also followed by an increase in SPEEK content. Compared with a - 28.5 mN m adhesion force of a 4 pm silica particle for a SPEEK free PSf membrane, a SPEEK modified membrane showed greatly reduced adhesion of - 0.75 mNm h This, together with the surface smoothening effect, leads to the reduction of membrane fouling when the surface is modified by the addition of SPEEK. 

Colloid fouling refers to membrane fouling with colloidal and suspended particles in the size range of a few nanometers to a few micrometers. Examples of common colloidal-sized fonlants inclnde clays, silica salts, and hydroxides of heavy metals (Potts et al. 1981). An increased concentration of the rejected ions at the front of the membrane snrface facilitates the aggregation of dissolved organic substances, for example, natural organic matter (NOM), into colloidal-sized particles (Hong and Elimelech 1997). 

PVDF-based ultrafiltration nanohybrid membranes prepared via an immersion-precipitation method using PHEMA-b/oc -PMMA-grafted silica nanoparticles as additives were shown to increase the pure water flux, improve the bovine serum albumin (BSA) rejection to a high level (>90%), and reduce membrane fouling at... 

Stephen Gray (chemical coagulation membrane distillation membrane fouling brine management solid/liquid separation processes sustainable water systems water treatment silica removal). Director, Institute for Sustainability and Innovation (ISI), Victoria University (VU), Melbourne, VIC... 

The type of membrane cleaning required depends on both the type and degree of fouling experienced, but typically it is either organic (bacterial slimes, natural organics, or process foulants and nutrients) or inorganic (silica, carbonate, sulfate, or phosphate deposits). 

Despite efforts to comply with the limitations on feed water quality, CEDI systems can still foul and scale with microbes, organics, iron and manganese, and calcium- and silica-based scales. This usually occurs due to upsets in the pretreatment system or a deficiency in the system design that result in excursion in feed water quality to the CEDI system. Organics, metals, hardness, and silica problems are usually found on the membranes and sometimes on the resin (as is the case with organics). Biofouling is typically found on the... 

Sahachaiyunta et al. [38] conducted dynamic tests to investigate the effect of silica fouling of reverse osmosis membranes in the presence of minute amounts of various inorganic cations such as iron, manganese, nickel, and barium, which are present in industrial and mineral processing wastewaters. Experimental results showed that the presence of iron greatly affected the scale structure on the membrane surface when compared to the other metal species. 

Sahachaiyunta P., Koo T., and Sheikholeslami R., Effect of several inorganic species on silica fouling in RO membranes. Desalination 144 2002 373-378. 

Bowen et al. [39] measured directly the adhesion (interaction) of cellobiose and cellulose with two polymeric UF membranes of similar MWCO, but of different materials. As probes, they used silica spheres (diameter 5-8 im) the surfaces of which were modified by static adsorption of cellobiose. They also used pure cellulose probes. Membrane ES 404 was made of poly(ether sulfone) alone, and EM 006 was made of a poly(ether sulfone)-polyacrylate blend, chosen specifically to increase the hydrophihc properties and decrease the fouling properties of the membrane. Study of ES 404 and EM 006 had shown that the interaction of cellobiose or of colloidal cellulose with the membranes was such that ES 404 always had the greater adhesion and greater fouling tendency. However, if the membrane was first fouled with cellobiose, the colloidal cellulose adhesion force was increased significantly, and the differences between the membranes diminished. Bowen et al. suggested that in the future, it would be possible to use the techniques developed to allow prior assessment of the fouUng propensity of process streams with different types of membranes. 

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