Membranes pretreatment

Microfiltration (MF) and ultrafiltration (UF) membranes can be used as forms of pretreatment for nanofiltration (ISIF) or reverse osmosis (RO) desalination processes. Membrane pretreatment reduces the amount of chemicals that are required and hence reduces the environmental impact of the final discharge. MF membranes can be used to filter particles with diameters of 0.1-10 pmm and typically remove bacteria, viruses, precipitates, coagulates and large colloidal particles. UF can remove particles with diameters as small as 0.002 pm, and... 

Membrane pretreatment can improve the feed water quality and reduce particulate matter to a greater degree than other pretreatment methods. This reduces the required cleaning frequency of the downstream membranes and the amount of cleaning chemicals that are required and subsequently disposed of (Pearce 2007). 

Although the capital cost of membrane pretreatment is usually higher than that of a conventional pretreatment, the additional cost of MF/UF is paid and also exceeded by reducing RO replacement and chemical cost for both dosing and RO cleaning [7]. 

Katoh et al. [3] prepared w/o emulsions composed of salt solution, polyglycerin polyricinolate (PGPR) at 2%wt and com oil. It has been proven that the disperse-phase flux was increased 100-fold using a hydrophilic membrane pretreated by immersion in the oil phase. This made the membrane emulsification system practical for large-scale production of a w/o emulsion in food application. 

Membrane pretreatment includes microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). Microfiltration and UF membrane processes can remove microbes and algae. However, the pores of MF and UF membranes are too large to remove the smaller, low-molecular weight organics that provide nutrients for microbes. As a result, MF and UF can remove microbes in the source water, but any microbes that are introduced downstream of these membranes will have nutrients to metabolize. Therefore, chlorination along with MF and UF is often recommended to minimize the potential for microbial fouling of RO membranes. The MF or UF membranes used should be chlorine resistant to tolerate chlorine treatment. It is suggested that chlorine be fed prior to the MF or UF membrane and then after the membrane (into the clearwell), with dechlorination just prior to the RO membranes. See Chapter 16.1 for additional discussion about MF and UF membranes for RO pretreatment. 

Fues, Jason and Jane Kucera, "High-Efficiency Filtration for Membrane Pretreatment," presented at the 68th International Water Conference, October, 2007, paper no. IWC-07-07. 

As fabricated, a Pd or its alloy membrane suffers from the relatively low catalytic activity of the attached catalyst due to the typical low surface area to volume ratio of the membrane geometry. The catalyst in a dense Pd-based membrane can be the Pd itself or its alloy or some other materials attached to the membrane. Pretreatments to the Pd-based membranes can help alleviate this problem. This and other membrane material and catalysis issues will be further covered in Chapter 9. 

Field evaluation of a hybrid membrane system consisting of an UF membrane pretreatment unit and an RO seawater unit was conducted by Glueckstem et al. [56]. For comparison a second pilot system consisting of conventional pretreatment and an RO unit was operated in parallel. The conventional pretreatment unit included in-line flocculation followed by media filtration. The smdy showed that UF provided a very reliable pretreatment for the RO system independent of the raw water quality flucmations. However, the cost of membrane pretreatment was higher than conventional pretreatment. This suggested that membrane pretreatment for RO desalting systems is only economic for sites that require extensive conventional pretreatment or where wide flucmations in the raw water quality are expected. 

Gronberg L, Shen Y, and Jbnsson JA. Ion chromatographic determination of organic acids in air with on-line liquid membrane pretreatment. J. Chromatogr. 1993 655 207-215. 

Manttari M, Pihlajamaki A, Kaipainen E, and Nystrbm M. Effect of temperature and membrane pretreatment by pressure on the filtration properties of nanofiltration membranes. Desalination 2002 145 81-86. 

In order to evaluate the steady-state water profile in the membrane of a PEFC under given operating conditions, the necessary membrane transport properties required thus include water uptake by the membrane as function of water activity and membrane pretreatment conditions, A(aw) (covered in Section 5.3.1) the diffusion coefficient of water in the membrane as a function of membrane water content, D ) the electroosmotic drag coefficient as a function of membrane water content, (A) and the membrane hydraulic permeability, A hy(i(A). Section 5.3.2 includes a discussion of water transport modes in ionomeric membranes. 

Fig. 28. Water uptake by a Nafion membrane immersed in liquid water at 28 °C as a function of the temperature of membrane pretreatment in a hot glycerol bath.

The ionic concentration to be treated is an overriding consideration governing the cost of plant of a given design and therefore for very high ion concentrations it is foreseeable that membrane pretreatments such as reverse osmosis and electrodialysis will continue to fulfil an important role as might a more widespread revival of continuous countercurrent ion exchange. 

In order to reduce fouling and protect the membrane, pretreatment must be performed for seawater. A pretreatment system removes suspended sobds and other potential fouling materials. Flocculation agents such as iron chloride or polyelectrolytes are added in order to remove suspended sotids. Chlorine is added to remove bacteria and algae. 

Chlorination of secondary effluent prior to membrane pretreatment may extend membrane run times between clean. Over 90 h of MF operation was achieved with prechlorinated secondary effluent compared to 42 h operation reported when secondary effluent was not chlorinated (72,73). Similar observations were reported with dosage of chloramine prior to microfiltration pretreatment (77). It was speculated that preoxidation due to chlorination altered the chemistry of extracellular polymeric substance (EPS) produced by the microorganisms in the secondary effluent. This could weaken the attachment of the EPS on the membrane and thus offset the detrimental effect on the membrane flux. However, care must be taken to verify compatibility of membrane with chlorination as some membranes are not tolerant to the aggressive action of chlorine. 

Although MF and UF membranes have been shown to be a viable option as feed pretreatment for RO, long-term operating and cost data are required to verily that membrane pretreatment is more cost effective than conventional pretreatment. 

Overall, studies on membrane pretreatment demonstrate the following advantages ... 

Figure 2. CEC inhibition of [ HJtamsulosin binding in HEK 293 cells expressing tti-AR subtypes. Membranes from transfected cells were incubated with 1 pM CEC for 10 min at 37°C. Membranes were washed and binding of a 90% saturating concentration of pHjtamsulosin determined. Values are expressed as a % of specific binding in membranes pretreated under identical conditions in the absence of CEC, and are the mean + S.E.M. of 5 experiments performed in duplicate.

It took a few years before the modules, the membranes, pretreatment and membrane cleaning procedures were sufficiently developed, but today the UF systems operate as steadily and reliable as other unit operations in this industry. 

Particles of different attributes, such as characteristic surface or charge, need to be investigated to be able to predict treatment performance. The impact of particle content on fouling of tight membranes is not clear in the literature, but it is generally assumed that p>articles play a major role in fouling. These effects need to be understood prior to optimising membrane pretreatment. 

Durham B. (1997), Membrane pretreatment of reverse osmosis - long term experience on difficult... 

Ghosh R, Cui ZF (1998) Fractionation of BSA and lysozyme using ultrafiltration effect of pH and membrane pretreatment. J Memb Sci 139(l) 17-28 Gianfreda L, Scarfi MR (1991) Enzyme stabilization state of the art. J Mol Cell Biochem... 

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