When membrane configuration is flat type and it is coupled to a photocatalytic reaction, the reactor can be named flat sheet membrane photocatalytic reactor. A re-circulation cell containing various types of flat sheet membranes which were able to retain the suspended catalyst and partially selective to the pollutant was studied by Molinari et al (2001).The membranes were: NTR7410 and NTR7450 (Nitto Denko); N30F and NF-PES-010 (Hoechst); MPCB0000R98 (SEPAREM). The measured permeate flux was in the range 5-30 l/h m2 at 4 bar and all membranes showed both a rejection and a capacity to adsorb the pollutant with a transitory phase varying from 80 to 400 min at 4 bar. This behaviour could be a benefit for the process because oscillations in the pollutant concentration were not transmitted in the permeate. Three factors: rejection, photocatalytic degradation and adsorption were able to maintain the 4-nitrophenol (4NP) concentration in the permeate at very low values. For the continuous system, the lowest 4NP concentration in the permeate was 6-7% (w/w) of the initial 4NP concentration (40 mg/l) after a transient period of 300 min. In another method, the catalyst, TiO2 P25 Degussa, was immobilized by physical deposition on a flat sheet polymeric membrane and 4-NP was used as a model molecule to evaluate the reactor performance (Molinari et al. 2000). Apreliminary investigation of the stability, under UV irradiation, of some eligible polymeric membranes was carried out by using scanning electron microscopy (SEM), optical microscopy (OM), determinations of water permeation flux (WPF) and total organic carbon (TOC). These tests showed that commercial membranes made of fluoride + PP (FS 50 PP-Dow), polysulphone + PP (GR 51 PP-TechSep) and polyacrylonitrile (PAN-TechSep) seemed to be quite stable to UV light over a 24 h period of irradiation. Immobilization of TiO2 onto membranes by ultrafiltrating TiO2 suspensions showed an optimal layer density slightly >2.04 mg TiO2 per cm2 of membrane surface area. Results obtained from membrane reactor studies indicated that the observed initial rate constants for the degradation of 4-NP were almost independent on the amount of TiO2 employed over the range 0.76-4.08 mg/cm2. A 50% weight degradation of 4-NP after 5 h of irradiation in the presence of air was obtained. An almost complete degradation of 4-NP, instead, was observed in the presence of TiO2 suspended in the solution and pure oxygen. The permeate deriving from the membrane photoreactor was clear and 4-NP concentration was approximately equal to that found in the retentate. The possibility of the continuous reuse of the photocatalyst and the continuous separation of products from the reaction medium gave some advantages over traditional approaches. A flat sheet membrane photocatalytic reactor operates, generally, with suspended solids (the catalyst) and air bubbling (oxygen feed) so, in the following, some cases of interest for operating such reactors, are reported. Selection of a membrane configuration is a crucial step in the design process and has a high impact on further plant operations. Despite increasing experience with full-scale applications, practical knowledge concerning the impact of different membrane configurations on process performance and operational costs is still lacking. Full scale performance data of a membrane bioreactor (MBR) comparing the use of flat sheet and hollow fiber membranes and analyses of the consequences on operation, performance and treatment efficiency have been reported by Krzeminski et al. (2012). Hollowfiber configurations, compared to the flat sheet, are designed for higher fluxes, operated at lower concentrations, cleaned more often and protected by stricter pre-treatment. Filterability of activated sludge from municipal MBRs was better than from industrial MBRs and did not depend on membrane configuration. The energy consumption depends more on the influent type than on the membrane configuration. An aerated flat sheet module focusing on the effect of bubble distribution on the spatial variation of surface mass transfer with intermittent slug bubbling was studied by Field et al. (2011). This mode of operation will delay the onset of the transmembrane pressure jump if biomass removal is more dependent upon maximum shear stress than mean shear stress. Two basic set-ups were considered: an orifice in the middle of the aeration tube, at the base, and two symmetrically placed orifices in the aeration tube. With relatively low air rates a single orifice generates a higher average enhancement than two orifices but the reverse was found at a relatively higher air rate. The enhancement in the centre area of the module was relatively higher than that of the edge regions when using a single orifice but more uniformity was achieved with two. Exploration of two major commercialized flat-sheet and hollow-fiber membranes in a submerged membrane fungi reactor fed with a synthetic textile wastewater revealed striking differences in the extent and mechanism of fouling between the two types, indicating a case-specific scope of choice between them for industrial wastewater treatment (Hai et al. 2005). Thehollow-fiber membrane exhibited fouling with a cake layer composed of fungi and starch, intensity being proportional to the operating flux (0.05-0.3 m/d). Conversely, the flat-sheet membrane suffered from immediate internal pore blocking beyond a critical flux of 0.2 m3 m-2 d-1. During the experiment with major constituents of the synthetic wastewater separately, while media containing only starch and only dye induced negligible fouling, flux-dependent pore blocking was evident for both the hollow-fiber (0.288 m3 m-2 d-1) and flat-sheet membranes (1.3 m3 m-2 d-1) for the mixture of starch and dye. Despite a remarkable 99% color and 97% TOC removal were achieved by both membranes, fouling with different modes and intensity for the two types under similar conditions and for the same type of membrane under different exposure conditions warrants development of suitable modules for such recalcitrant wastewater. The oxygen transfer across a micro-porous, flat-sheet membrane with and without a nitrifying membrane-aerated biofilm (MAB) was studied by Shanahan et al. (2006). Clean membrane oxygen transfer was quantified via average mass transfer coefficients and local fluxes calculated from profiles of dissolved oxygen (DO) along the membrane length. Stoichiometric calculations and DO profiles were used to characterize oxygen transfer with a nitrifying MAB. Comparison of the local fluxes with clean and biofilm coated membranes revealed that oxygen transfer was decreased in upstream sections of the membrane during cultivation of an MAB due to reduced advection and/or turbulence near the membrane surface. By contrast, oxygen transfer was increased in downstream sections of the membrane via bacterial respiration. Oxygen fluxes generally decreased in the downstream direction with biofilm coated membranes; however, variability in biofilm structure and membrane coverage altered this trend on several occasions. Furthermore, biofilm structures were observed to form via sloughing and settling of biomass at the membrane surface. Thus membrane configuration may have a significant influence on oxygen transfer rates in membrane reactors. References Molinari R, Grande C, Drioli E, Palmisano L, Schiavello M(2001) Photocatalytic membrane reactors for degradation of organic pollutants in water. CATALYSIS TODAY 67:273-279 Molinari R, Mungari M, Drioli E, Di Paola A, Loddo V, Palmisano L, Schiavello M(2000) Study on a photocatalytic membrane reactor for water purification. CATALYSIS TODAY 55:71-78 Krzeminski P, Gil JA, van Nieuwenhuijzen AF, van der Graaf JHJM, van Lier JB (2012) Flat sheet or hollow fibre - comparison of full-scale membrane bio-reactor configurations. DESALINATION AND WATER TREATMENT 42:100-106 Field RW, Zhang KS, Cui ZF, Hwang BK (2011) Flat sheet MBRs: analysis of TMP rise and surface mass transfer coefficient. DESALINATION AND WATER TREATMENT 35:82-91 Hai FI, Yamamoto K, Fukushi K(2005) Different fouling modes of submerged hollow-fiber and flat-sheet membranes induced by high strength wastewater with concurrent biofouling. DESALINATION 180:89-97 Shanahan JW, Semmens MJ(2006) Influence of a nitrifying biofilm on local oxygen fluxes across a micro-porous flat sheet membrane. J. MEMBR. SCI. 277:65-74
Flat sheet membrane photocatalytic reactor
MOLINARI, Raffaele
2013-01-01
Abstract
When membrane configuration is flat type and it is coupled to a photocatalytic reaction, the reactor can be named flat sheet membrane photocatalytic reactor. A re-circulation cell containing various types of flat sheet membranes which were able to retain the suspended catalyst and partially selective to the pollutant was studied by Molinari et al (2001).The membranes were: NTR7410 and NTR7450 (Nitto Denko); N30F and NF-PES-010 (Hoechst); MPCB0000R98 (SEPAREM). The measured permeate flux was in the range 5-30 l/h m2 at 4 bar and all membranes showed both a rejection and a capacity to adsorb the pollutant with a transitory phase varying from 80 to 400 min at 4 bar. This behaviour could be a benefit for the process because oscillations in the pollutant concentration were not transmitted in the permeate. Three factors: rejection, photocatalytic degradation and adsorption were able to maintain the 4-nitrophenol (4NP) concentration in the permeate at very low values. For the continuous system, the lowest 4NP concentration in the permeate was 6-7% (w/w) of the initial 4NP concentration (40 mg/l) after a transient period of 300 min. In another method, the catalyst, TiO2 P25 Degussa, was immobilized by physical deposition on a flat sheet polymeric membrane and 4-NP was used as a model molecule to evaluate the reactor performance (Molinari et al. 2000). Apreliminary investigation of the stability, under UV irradiation, of some eligible polymeric membranes was carried out by using scanning electron microscopy (SEM), optical microscopy (OM), determinations of water permeation flux (WPF) and total organic carbon (TOC). These tests showed that commercial membranes made of fluoride + PP (FS 50 PP-Dow), polysulphone + PP (GR 51 PP-TechSep) and polyacrylonitrile (PAN-TechSep) seemed to be quite stable to UV light over a 24 h period of irradiation. Immobilization of TiO2 onto membranes by ultrafiltrating TiO2 suspensions showed an optimal layer density slightly >2.04 mg TiO2 per cm2 of membrane surface area. Results obtained from membrane reactor studies indicated that the observed initial rate constants for the degradation of 4-NP were almost independent on the amount of TiO2 employed over the range 0.76-4.08 mg/cm2. A 50% weight degradation of 4-NP after 5 h of irradiation in the presence of air was obtained. An almost complete degradation of 4-NP, instead, was observed in the presence of TiO2 suspended in the solution and pure oxygen. The permeate deriving from the membrane photoreactor was clear and 4-NP concentration was approximately equal to that found in the retentate. The possibility of the continuous reuse of the photocatalyst and the continuous separation of products from the reaction medium gave some advantages over traditional approaches. A flat sheet membrane photocatalytic reactor operates, generally, with suspended solids (the catalyst) and air bubbling (oxygen feed) so, in the following, some cases of interest for operating such reactors, are reported. Selection of a membrane configuration is a crucial step in the design process and has a high impact on further plant operations. Despite increasing experience with full-scale applications, practical knowledge concerning the impact of different membrane configurations on process performance and operational costs is still lacking. Full scale performance data of a membrane bioreactor (MBR) comparing the use of flat sheet and hollow fiber membranes and analyses of the consequences on operation, performance and treatment efficiency have been reported by Krzeminski et al. (2012). Hollowfiber configurations, compared to the flat sheet, are designed for higher fluxes, operated at lower concentrations, cleaned more often and protected by stricter pre-treatment. Filterability of activated sludge from municipal MBRs was better than from industrial MBRs and did not depend on membrane configuration. The energy consumption depends more on the influent type than on the membrane configuration. An aerated flat sheet module focusing on the effect of bubble distribution on the spatial variation of surface mass transfer with intermittent slug bubbling was studied by Field et al. (2011). This mode of operation will delay the onset of the transmembrane pressure jump if biomass removal is more dependent upon maximum shear stress than mean shear stress. Two basic set-ups were considered: an orifice in the middle of the aeration tube, at the base, and two symmetrically placed orifices in the aeration tube. With relatively low air rates a single orifice generates a higher average enhancement than two orifices but the reverse was found at a relatively higher air rate. The enhancement in the centre area of the module was relatively higher than that of the edge regions when using a single orifice but more uniformity was achieved with two. Exploration of two major commercialized flat-sheet and hollow-fiber membranes in a submerged membrane fungi reactor fed with a synthetic textile wastewater revealed striking differences in the extent and mechanism of fouling between the two types, indicating a case-specific scope of choice between them for industrial wastewater treatment (Hai et al. 2005). Thehollow-fiber membrane exhibited fouling with a cake layer composed of fungi and starch, intensity being proportional to the operating flux (0.05-0.3 m/d). Conversely, the flat-sheet membrane suffered from immediate internal pore blocking beyond a critical flux of 0.2 m3 m-2 d-1. During the experiment with major constituents of the synthetic wastewater separately, while media containing only starch and only dye induced negligible fouling, flux-dependent pore blocking was evident for both the hollow-fiber (0.288 m3 m-2 d-1) and flat-sheet membranes (1.3 m3 m-2 d-1) for the mixture of starch and dye. Despite a remarkable 99% color and 97% TOC removal were achieved by both membranes, fouling with different modes and intensity for the two types under similar conditions and for the same type of membrane under different exposure conditions warrants development of suitable modules for such recalcitrant wastewater. The oxygen transfer across a micro-porous, flat-sheet membrane with and without a nitrifying membrane-aerated biofilm (MAB) was studied by Shanahan et al. (2006). Clean membrane oxygen transfer was quantified via average mass transfer coefficients and local fluxes calculated from profiles of dissolved oxygen (DO) along the membrane length. Stoichiometric calculations and DO profiles were used to characterize oxygen transfer with a nitrifying MAB. Comparison of the local fluxes with clean and biofilm coated membranes revealed that oxygen transfer was decreased in upstream sections of the membrane during cultivation of an MAB due to reduced advection and/or turbulence near the membrane surface. By contrast, oxygen transfer was increased in downstream sections of the membrane via bacterial respiration. Oxygen fluxes generally decreased in the downstream direction with biofilm coated membranes; however, variability in biofilm structure and membrane coverage altered this trend on several occasions. Furthermore, biofilm structures were observed to form via sloughing and settling of biomass at the membrane surface. Thus membrane configuration may have a significant influence on oxygen transfer rates in membrane reactors. References Molinari R, Grande C, Drioli E, Palmisano L, Schiavello M(2001) Photocatalytic membrane reactors for degradation of organic pollutants in water. CATALYSIS TODAY 67:273-279 Molinari R, Mungari M, Drioli E, Di Paola A, Loddo V, Palmisano L, Schiavello M(2000) Study on a photocatalytic membrane reactor for water purification. CATALYSIS TODAY 55:71-78 Krzeminski P, Gil JA, van Nieuwenhuijzen AF, van der Graaf JHJM, van Lier JB (2012) Flat sheet or hollow fibre - comparison of full-scale membrane bio-reactor configurations. DESALINATION AND WATER TREATMENT 42:100-106 Field RW, Zhang KS, Cui ZF, Hwang BK (2011) Flat sheet MBRs: analysis of TMP rise and surface mass transfer coefficient. DESALINATION AND WATER TREATMENT 35:82-91 Hai FI, Yamamoto K, Fukushi K(2005) Different fouling modes of submerged hollow-fiber and flat-sheet membranes induced by high strength wastewater with concurrent biofouling. DESALINATION 180:89-97 Shanahan JW, Semmens MJ(2006) Influence of a nitrifying biofilm on local oxygen fluxes across a micro-porous flat sheet membrane. J. MEMBR. SCI. 277:65-74I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
