Introduction 7 operations within the factory are conducted so as, in this instance, to limit the freshwater demand. Actual examples of demand management, reuse and recycling (Table 1.5) demonstrate that the degree of sophistication of the solution to achieve a significant cost benefit depends upon the existing state of water management. It is apparent from Tables 1.2 and 1.3 that the quality of water demanded by industry varies considerably from one duty to another. There exist certain determinants, such as the Silt Density Index or SDI (Section 2.4.3), which are of critical importance for some applications but are meaningless in others. Moreover, it is generally the case that the volume of water demanded varies inversely with that of its purity across any one industrial sector. For example, according to the figures in Table 1.1, about 20 times as much water - making up around 90% of the total water demand of the plant - is used for cooling in power generation as that required for boiler feed, which demands a “high-purity’’ water. Such water can only be produced through a combination of adsorptive and membrane separation processes. Water for once-through, non-intrusive cooling, on the other hand, may not be required to meet any specification based on chemical and biological constituents, needing only to be below a certain temperature. For some industrial processes the quality of the discharged water does not substantially differ from that of the feedwater. Cooling towers, for example, concentrate the water as a result of the evaporative cooling process, but do not add significantly, if at all, to the chemical loading rate, in terms of mass flow rate of solutes, of the efllluent. For most industrial sectors, however, there is a significant pollutant load resulting from their activity. As already stated, the large temporal variation in effluent water quality can preclude water recovery and reuse in many cases due to the high cost of treatment to produce water of a reliable quality, particularly by the more established non-barrier technologies where treatment process performance varies with hydraulic and/or pollutant load. On the other hand, membrane processes, which can offer a highly selective barrier to the water being processed, are far more robust to changes in feedwater quality and can provide water of reliably high quality. 1.3 Membrane technology Membrane processes are designed to carry out physical or physicochemical separations. Although most membrane applications are water based, there also exist gas-liquid and gas-gas separation processes, although these are more recent developments and have not yet achieved widespread implementation. In terms of membrane sales, the most important application by far is hemodialysis, as carried out in kidney dialysis machines: almost half of all membrane sales are accounted for by this one application. The development of membrane-based bulk water and wastewater treatment processes, as defined in Table 1.6, is nonetheless significant, since they offer three clear advantages over conventional techniques: 'hble 1.5 Type Application Scenario Solution Cost benefit Audit Cellophane 750 m3/day of water unaccounted for Fix leak tlOO000p.a. saving[$l41580]* Audit Laboratory Mains water used for single-pass cooling, Fit cooler (€2000) [$2831]* 6 month payback Housekeeping Resin factory Housekeeping Plating plant Examples of water management, UK ._ production 12 m3/day Wash water contaminated by spillages from point sources Significant drag out of chemicals from Fit drip trays to point sources Improve drainage around plating bath Effluent COD reduced by 75%, €3000 p.a. saving [$4217]* Reduced rinsewater contamination, plating bath into rinse bath 0.4 m3/h mains water fed to three rinsing Use counter-current serial flow to all baths in parallel, 1.2 m3/h In total three baths €1000 p.a. saving [$1416]* 50% water saving, €1500 pa. [$2 Reuse Recycling Cadmium Zero discharge of cadmium imposed by Installation of deionisation plant to 12 monthpayback Plating plant plating regulator recover cadmium and water from effluent 241' * Based on rate of 1.4158.02/06/2002. lntroduction 9 Table 1.6 Membrane processes Process Usual objective Microfiltration (MF) Ultrafiltration (UP) Nanofiltration (NF) Reverse osmosis (RO) Electrodialysis (ED) and dialysis Removal of suspended solids, including microorganisms Removal of both large, dissolved solute molecules and suspended colloidal particles (Selective) removal of multivalent ions and certain charged or polar molecules Removal of inorganic ions (Selective) extraction of ions from water and/or concentration of these ions in the waste stream (Selective) extraction of molecular gas and/or volatile solutes Transfer of molecular gas into or out of water Pervaporation (PV) Gas transfer (GT) 1. Separation is achieved without requiring a phase change, and is therefore more energetically efficient than distillation. 2. Little or no accumulation takes place in the process which therefore operates continuously under steady-state condition without necessitating regeneration cycles, unlike adsorptive separation processes. 3. Little or no chemical addition is required, unlike conventional clarification which generally relies on the addition of chemical coagulants and flocculants. Membrane technology development as a whole began with the first high- performance reverse osmosis membrane produced by in the early 1960s (Loeb and Sourirajan, 19631, which led to the installation of large seawater desalination plant in arid regions of the world. Since that time, growth in the total quantity of membranes sold with reference to the water treatment capacity provided has increased exponentially with time, with growth in microfiltration and ultrafiltration technologies over the last decade of the twentieth century being particularly pronounced (Fig. 1.1). There has been a corresponding exponential decrease in membrane costs with installed membrane plant capacity, as reflected in data from an established hollow fibre microfiltration membrane (Fig. 1.2). The increasing stringency of water quality guidelines and standards being introduced for municipal water treatment, for example the European Union Urban Waste Water Treatment Directive (UWWTD) and the position taken by the UK Drinking Water Inspectorate regarding cryptosporidia, portend continued growth in this sector for membrane technology. Indeed, the EU wastewater treatment membrane market is predicted to increase by an average annual growth rate of 5.9% to $2 50 million in 2006 (RCC, 2002). Application of membrane processes within the industrial sector is also widespread and well established in many instances. Reverse osmosis and ultrafiltration, for example, are both widely used within the pharmaceutical industry to remove pyrogens in the provision of water for injection (WFI). These processes are also essential in providing ultrapure water in semiconductor fabrication plant, where ultrafiltration is used for removing colloidal material and reverse osmosis both for primary deionisation (followed by polishing using 10 Membranes for Industrial Wastewater Recovery and Re-use 10000 9 1000 6 3 100 a 0 m Q aI > m 3 c. - f 0 10 1 1991 1992 1993 1994 1995 1996 1997 1998 1999 Year Figure 1.1 Total membrane sales with respect to water treatment capacity (by kindpermission of Vivendi) 10000 - ~ 10 4 I00 1000 10000 100000 1 OOOO( Cumulative membrane area (m2) Figure I .2 Vivendi) Membrane costs 11s. installed membrane area (“Memcor” microfiltration plant) (courtesy of an ion-exchange process) and for polishing the rinse water. Cartridge microfilters are routinely used for the removal of extraneous suspended solids from boiler feedwaters. Although rather less well established, membranes are also used for the treatment of industrial effluents for discharge. Possibly the most recently developed and successfully implemented membrane process for wastewater treatment is the submerged membrane bioreactor, which is applied to both municipal and industrial effluents for the removal of dissolved organic material. Market penetration of this technology has followed much the same trend as that of the more established microfiltration process (Fig. 1.3). The use of membranes specifically for industrial wastewater reclamation and reuse has generally been constrained by the costs involved. Having said this, advances in membrane technology, and notably significant improvements in its Introduction 11 I00 No. of installations 1000 Figure I. 3 plunt) (jrom information provided by Kubota) Microfiltration membrane costs 17s. number of installations (“Kubotn” rnernbrane bioreactor efficiency and cost effectiveness, greatly increase the competitiveness of recycling over discharge. Existing examples of closed-loop water recovery and reuse, invariably based on membrane technology, suggest that payback periods as low as 18 months are achievable, the timescale obviously being very sensitive to supply and discharge costs. Given the continuing downward pressure on costs (Pigs. 1.2 and 1.3) and the increasingly severe stresses on freshwater supplies, it seems likely that membranes will be used ever more widely for water recycling within the industrial sector. References Raetens, D. (2002). Water pinch analysis: minimisation of water and wastewater in the process industry. Tn Lens, P., Hulshoff Pol, L., Wilderer, P. and Asano, T. (eds.) Water recycling and resource recovery in industry. IWA Publishing, London, pp. 203-2 51. BCC (2002). Advanced waste water treatment for global markets. Business Communications Company Inc., Norwalk, CT. Cologne, G. (1998). Legal aspects of water reclamation. In Asano, T. (ed.) Wastewater reclamation and reuse. Technomic, Lancaster, Basel, p. 1397. Gleick, P. H. (2000). The world’s water 2000-2001, the biennial report on freshwater resources. Island Press, Washington, USA. Jeffrey, P. (2002). Influence of technology scale and location on public attitudes to in-house water recycling in England and Wales. CTWEM Journal (in press). Loeb, S. and Sourirajan R. (1963). Seawater demineralisation by means of an osmotic membrane. Advances in Chemistry Series, 38,117. Mannapperema, J. D Yates, E. D. and Singh, R. P. (1993). Survey of water use in the California food processing industry. Proceedings of the Food Industry Environmental Conference, Los Angeles, CA. 12 Membranes for lndustrial Wastewater Recovery and Re-use Pauly, D. P. (2001). Kidney technology for whitewater treatment. Paper Technology, 42,29. Sakaji, R. H. and Funamizu, N. (1998). Micobial risk assessment and its role in the development of wastewater reclamation policy. In Asano, T. (ed.) Wastewater reclamation and reuse. Technomic, Lancaster, Basel, pp. 70 5-75 6. Water UK (2000). Waterfacts 2000. Water UK, London. Wesner, G. M. (1987). Historical review of Water Factory 21 Orange County Water District wastewater reclamation and groundwater recharge program. Report to OCWD. Chapter 2 Membrane technology Simon Judd School of Water Sciences, Cranfield University 14 Membranes for Industrial Wastewater Recovery and Re-use 2.1 The membrane 2.1.1 Membrane and membrane process definition There are a number of definitions of the word “membrane”, which can vary considerably in comprehensiveness and clarity. It is a very widely used term, and can mean anything from a cell wall to damp proofing material. Three definitions, arbitrarily chosen from pertinent technical literature from the last 20 years are provided below: “An intervening phase separating two phases and/or acting as an active or passive barrier to the transport of matter between phases” - the European Society of Membrane Science and Technology (now the European Membrane Society). “An interphase separating two homogenous phases and affecting the transport of different chemical components in a very specific way” - Prof. Heine Strathmann, former Head of the Department of Membrane Technology, University of Twente. “A material through which 01ie type of substance can pass more readily than others, thus presenting the basis of a separation process” - Prof. George Solt, former Director of the School of Water Sciences, Cranfield. For the purposes of this discussion of membrane technology for wastewater treatment, Solt’s definition can be considered adequate: it is that property of the membrane which permits the separation of components in and/or from water that is of key interest. For many processes the membrane acts to reject the pollutants, which may be suspended or dissolved, and allow the “purified” water through it. In some cases, however, the membrane may act so as to extract pollutants from the wastewater, or else transfer specific components (such as oxygen) into it. Currently employed extractive processes include electrodialysis (ED), dialysis, pervaporation (PV) and gas transfer (GT). In these cases the membrane is employed to allow selective permeation of specific components dissolved in the water. Of far more industrially importance, however, are the filtration processes of reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF). In these processes it is the bulk water that passes through the membrane under an applied pressure, leaving the pollutants in concentrated form on the unpermeated side of the membrane. If hemodialytic applications are ignored, then the pressure-driven filtration processes account for around 75% of the remaining membrane sales, almost all of these due to aqueous separation applications in municipal and industrial water treatment. 2.12 Membrane structure Although membrane materials vary vastly according to chemical composition and process type, the principal objectives in membrane manufacture are always the same. An ideal malerial will: Mfmhmne technologg 15 0 have reasonable mechanical strength, 0 maintain a high throughput, and 0 be selective for the desired permeate constituent. These last two parameters are mutually counteractive, since a high degree of selectivity is normally only achievable using a membrane having small pores and thus an inherently high hydraulic resistance (or low permeability). The permeability also increases with increasing density of pores, and the overall membrane resistance is directly proportional to its thickness (in accordance with Darcy’s law). Finally, selectivity will be compromised by a broad pore size distribution. An optimum physical structure for any membrane material is thus: 0 0 0 a high porosity. a thin layer of material, a narrow range of pore size, and Membrane materials can be categorised as either dense or porous, and by the mechanism by which separation is actually achieved (Table 2.1). Separation by dense membranes relies to some extent on physicochemical interactions between the permeating components and the membrane material, and relate to separation processes having the highest selectivity (Fig. 2.1 ). Porous membranes, on the other hand, achieve separation mechanically by size exclusion (i.e. sieving), where the rejected material may be either dissolved or suspended depending on its size relative to that of the pore. Since some membranes exhibit properties that can be associated with more than one process type, the boundaries between the adjacent membrane processes in Fig. 2.1 are somewhat nebulous. For example, IUPAC (1985) state that the upper and lower boundary limits for mesopores, as are characteristic of a UF membrane, are 2 and 50 nm. According to Kesting (1989), howcvcr, these boundaries are at 1 and 20 nm. respectively. Whilst microfiltration membranes are assigned a characteristic pore size, the exact value of which is dependent on the method of measurement, ultrafiltration membranes are mostly rated on the basis of the size of the smallest molecule the membrane can be expected to reject. This is routinely expressed as the molecular weight cut-off (MWCO) in daltons (i.e. grams per mole). As the precise relationship between MWCO and pore size is obviously dependent on the physical and chemical nature of the solute molecule, precise cross-referencing is impossible. The actual pore size of nanofiltration and reverse osmosis membranes is of little practical consequence, since there are other mechanisms more dominant than simple sieving that determine membrane performance. The purification performance of these membranes can only be rated according to their actual demonstrated permselectivity, i.e. the extent of the rejection of key contaminants by the membrane, under some defined set of conditions. Nanofiltration membranes, which have a charge rejection component, are generally designed to be selective for multivalent rather than univalent ions. Reverse osmosis membranes are designed to reject all species other than water, 16 Membranes for Industrial Wastewater Recovery and Re-use Table 2.1 Dense and porous membranes for water treatment (adapted from Stephenson et aJ., 2000) Dense Porous Membrane separation processes Reverse osmosis (RO) Ultrafiltration (UF) Separation achieved by virtue of differing solubility and diffusion rates of water (solvent) and solutes in water. Electrodialgsis (ED) Microfiltration (MF) Separation achieved by virtue of differing ionic size, charge and charge density of solute ions, using ion-exchange membranes. Pervaporation (PV) Gas transfer (GT) Same mechanism as RO but with the (volatile) solute partially vaporised in the membrane by partially vacuumating the permeate. Nanofiltration (NF) Formerly called leaky reverse osmosis. Separation achieved through combination ofcharge rejection, solubility-diffusion and sieving through micropores ( < 2 nm). Membrane materials Limited to polymeric materials. Separation by sieving through mesopores (2-50 nm)a. Separation of suspended solids from water by sieving through macropores ( > 50 nm)a. Gas transferred under a partial pressure gradient into or out of water in molecular form. Both polymeric and inorganic materials available. a IUPAC (1985). although they are unable to offer a significant barrier to dissolved gases and certain low-molecular-weight organic molecules. Membranes may also be categorised according to the material composition, which is either organic (polymeric) or inorganic (ceramic or metallic), or on the basis of their physical structure, i.e. their morphology. The membrane morphology is dependent on the exact nature of the material and/or the way in which it is processed. In general, however, membranes employed in pressure- driven processes tend to be anisotropic: they have symmetry in a single direction, and hence are often referred to as asymmetric, such that their pore size varies with membrane depth (Fig. 2.2). This arises out of the requirement for a thin permselective layer (or skin) to minimise the hydraulic resistance, the porous support providing minimal resistance and acting purely to provide the necessary mechanical strength. For integral (i.e. single, non-composite material) asymmetric polymeric membranes, such as the one shown in Fig. 2.2a, the skin is normally 2-5 pm in thickness. Flat sheet reverse osmosis membranes have an additional ultrathin active layer, less than 0.5 pm in thickness, attached to the anisotropic substrate to produce a thin-film composite (TFC) membrane (Fig. 2.3). This ultrathin layer provides the required permselectivity, rejecting all charged species and permitting only the passage of water and small organic molecules. On the other hand, ion exchange membranes, which are also dense by definition, are essentially homogeneous. These membranes comprise a three-dimensional array of fixed ionogenic sites (i.e. functional groups capable of dissociating to form charged species) which facilitate transport of either cations or anions [...]... PLAEIDE UF/MF module (Fig 2. 7) and electrodialysis stacks (Fig 2. 8) The latter are all based on a plate-and-frame design since the process relies 26 Membranesfor btdustrial Wastewater Recovery and Re-use Figurc 2. 6 Rochem stackeddisc RO module detailingfluidpath Figure 2. 7 Rhodia Orelis PLABIDB MF membrane module on transport of ions through the membranes under the action of an electric field and this configuration... spacers also serve the important task of turbulence promotion, and in the permeate channels A number of membrane elements, up to seven for very large plant, can make up a single module by 28 Membranesfor Industrial Wastewater Recovery and Re-use Pressurized F ' ~ Figure 2. 9 Spiral-wound module, schematic showingflow streams ( b y kingpermission oflonics) linking the retentate streams in series and placing... 0-14 pH, 70°C 35%H*S04 ,25 %H3P04 15%NaOH, 60°C pH < 1 pH 1- 12 - - PVDF/Cellul PI PEEK PAEK/PEI Membrane technology 2 3 which is no more than 14%porous Unfortunately, the membrane thickness of 60 pm makes the hydraulic resistance of the Anopore membranes relatively high and, more crucially, they can currently only be produced in small batches as disc filters for use in the laboratory 2. 7.4 Membrane configurations... limitations (Table 2. 5) The configurations are based on either a planar or cylindrical geometry and comprise: 0 0 0 0 0 Pleated filter cartridge Plate-and-frame Spiral wound Tubular Hollowfibre Of the flat plate geometries the lowest cost is the pleated filter cartridge (Fig 2. 4), used exclusively in microfiltration and designed as a relatively inexpensive 24 Membranesfor lndustrial Wastewater Recovery and... operate out-to-in, such that the skin is formed on the outer membrane surface Capillary fibres modules, which are slightly larger filaments, are operated in-to-out with the skin therefore formed on Membrane technology 29 Figure 2 IO Tubular UF modules: (a) polymeric (PCI Membranes) , and (b) ceramic (Kerasep", by Rhodia Orelis) 0-ri sec Feed eate tube Figure 2. 1 1 Hollowfibre module the internal surface... alternative materials or designs that are less complicated in 30 Membranes for Industrial Wastewater Recovery and Re-use manufacture Hence, some companies have started producing cassettes alongside their pressure-sealed module products One noteworthy recently development concerns ceramic membranes As already stated, the onus is on suppliers of these membranes to reduce costs to make them economically competitive... organic solvents Only PTFE and PVDF can be considered highly stable in this regard The main limitation of the more robust polymeric materials, in terms of resistance to Membranes for lndustrial Wastewater Recovery and Re-use 20 Table 2. 3 Commercially available phase inversion polymeric membrane materials Polymera Advantages CA Chlorine resistant Inexpensive More fouling resistant than PA PA PAN PSU,... removal of natural organic matter (NOM) from upland surface waters for potable water production using nanofiltration membranes NOM contains a number of different fractions of which humic acids are one Humic acids, which contain phenolic and carboxylate functional groups, tend to be negatively charged in 22 Membranes jor Zndustrial Wastewater Recoverg and Re-use neutral solutions, such that a highly... manufacture Membrane manufacture ostensibly concerns the production of a permeable material at a reasonable cost The membrane cost is dependent on not only the 18 Membranes for Industrial Wastewater Recovery and Re-use Backing Cloth 80 - 100 pm Figure 2. 3 Composite ROmembrane structure ( F T 3 0 , Film-Tec) raw material but also on the ease with which pores of the desired size or size distribution can be... of available membrane materials employed in water and wastewater treatment is very broad, and they vary rather more widely in chemical composition than in bulk morphology Membranes can be produced by stretching, sub-atomic particle bombardment combined with etching and, in the case of ceramic materials, sintering (Table 2. 2) These membranes are formed by the pressing and sintering of fine powders onto . other than water, 16 Membranes for Industrial Wastewater Recovery and Re-use Table 2. 1 Dense and porous membranes for water treatment (adapted from Stephenson et aJ., 20 00) Dense Porous. Judd School of Water Sciences, Cranfield University 14 Membranes for Industrial Wastewater Recovery and Re-use 2. 1 The membrane 2. 1.1 Membrane and membrane process definition There are. California food processing industry. Proceedings of the Food Industry Environmental Conference, Los Angeles, CA. 12 Membranes for lndustrial Wastewater Recovery and Re-use Pauly, D. P. (20 01).