Reverse osmosis (RO) membrane systems are widely used in the desalination of water. However, flux... more Reverse osmosis (RO) membrane systems are widely used in the desalination of water. However, flux decline due to fouling phenomena in RO remains a challenge. To minimize fouling, a reliable index is necessary to predict the fouling potential of the RO feed water. The ASTM introduced the silt density index (SDI) as a standard fouling index to measure the fouling potential due to colloidal and suspended particles. For decades, the SDI is worldwide accepted and applied. There are growing doubts about the predictive value of this parameter. In addition there are several deficiencies observed, affecting the accuracy and reproducibility e.g. no correction factor for temperature, nor for variations in membrane resistance, and no linear correlation with the concentration of colloidal/suspended particles. This paper gives an overview of our work on limitations, improvements and alternatives for the SDI. Firstly, the influence of the applied 0.45 lm test membrane on the SDI will be investigated. Variations in SDI values can be attributed to differences in properties of these membranes. In order to quantify the influence of pressure, temperature and membrane resistance on the SDI a mathematical relation was developed between the SDI and the MFI0.45, assuming cake filtration. In addition, also other fouling mechanisms were incorporated in the model using the wellknown blocking laws. Based on a cake filtration fouling mechanism and assuming 100% particle retention, the models were used to normalize the experimental SDI values for temperature, pressure and membrane resistance to the SDI + . By applying this normalization, the results of SDI tests carried out under different conditions and/or with different membranes can be compared easily as was proven experimentally in the lab and at a seawater desalination plant. Finally, an alternative filtration index will be introduced, the volume-based SDI_v. The SDI_v compares the initial flow rate to the flow rate after filtering a standard volume of feed water using MF membranes with an average pore size of 0.45 lm. Our experimental results show that SDI_v is independent of the membrane resistance. In that way, it eliminates most of the disadvantages of the SDI and has great potential to replace the SDI in the field.
Biolouling has detrimental effects on the feed channel pressure drop and the permeate flux in hig... more Biolouling has detrimental effects on the feed channel pressure drop and the permeate flux in high-pressure membrane processes such as NF and RO. Two-phase flow cleaning is a chemical-free technique that is able to remove such biotilms. This paper presents a study into the effects of the gas/liquid ratio, Feed spacer geometry, applied pressure and liquid velocity On the efficiency of two-phase flow cleaning in spiral-wound nanotiltration elements. A high-speed camera, optical coherence tomography and scanning electron microscopy were used to study biofouling and its removal. Our results show that two conditions must be met to ensure that a sufficiently high shear force is applied to biofilms on membrane and spacer surfaces. A good bubble distribution in the channel is the first requirement. While it is mainly the structure of the feed spacer that controls bubble flow and bubble size, a minimum gas/liquid ratio of 0.5 is necessary to achieve a good bubble distribution. The second con...
Worldwide, the application of a (gas/liquid) two-phase flow in membrane processes has received am... more Worldwide, the application of a (gas/liquid) two-phase flow in membrane processes has received ample scientific deliberation because of its potential to reduce concentration polarization and membrane fouling, and therefore enhance membrane flux. Gas/liquid flows are now used to promote turbulence and instabilities inside membrane modules in various membrane processes such as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, membrane distillation, electrodialysis, and membrane bio-reactors. This paper provides a comprehensive and critical literature review of the state of the art in this research area. A total of 205 scientific papers published in peer-reviewed journals from 1989 to 2013 were collected. The data in 195 of these papers (published up to 2011) were compiled and analyzed. These data were analyzed and normalized based on gas and liquid superficial velocities, gas/liquid ratio and feed types, trans-membrane pressure and membrane module type in order to mak...
The application of ceramic membranes in water treatment is becoming increasing attractive because... more The application of ceramic membranes in water treatment is becoming increasing attractive because of their long life time and excellent chemical, mechanical and thermal stability. However, fouling of ceramic membranes, especially hydraulically irreversible fouling, is still a critical aspect affecting the operational cost and energy consumption in water treatment plants. In this study, four ceramic membranes with pore sizes or molecular weight cut-off (MWCO) of 0.20 lm, 0.14 lm, 300 kDa and 50 kDa were compared during natural surface water filtration with respect to hydraulically irreversible fouling index (HIFI), foulant composition and narrowing of pore size due to the irreversible fouling. Our results showed that the hydraulically irreversible fouling index (HIFI) was proportional to the membrane pore size (r 2 = 0.89) when the same feed water was filtrated. The UF membranes showed lower HIFI values than the MF membranes. Pore narrowing (internal fouling) was found to be a main fouling pattern of the hydraulically irreversible fouling. The internal fouling was caused by monolayer adsorption of foulants with different sizes that is dependent on the size of the membrane pore.
Water science and technology : a journal of the International Association on Water Pollution Research, 2003
A better understanding of microbiology and ecology of nitrifying bacteria in inner biofilms is an... more A better understanding of microbiology and ecology of nitrifying bacteria in inner biofilms is an important part of improving process performance and control. Microelectrodes and fluorescent in situ hybridization (FISH) in biofilm research have been used to investigate the spatial distributions of various microbial activities in biofilms and have led to new experimental findings as well as modifications of the homogeneous assumptions in the biofilm kinetic models. The objective of this study is to try the combination of two methods, both FISH and microelectrode measurements, and to provide reliable and in situ information on nitrifying bacterial activity in biofilms. The characteristics of biofilm developed on tygon slides were different according to the change of dissolved oxygen (DO). When the DO increased from 2 to 10 mg DO/L, the rate of the biofilm thickness increased and its dry density changed from 50-70 to 25-90 mg/cm3. Ammonia oxidizing bacteria were not uniformly distribut...
journal homepage: www.else vier.com/locate /wa tres w a t e r r e s e a r c h 4 8 ( 2 0 1 4 ) 4 9... more journal homepage: www.else vier.com/locate /wa tres w a t e r r e s e a r c h 4 8 ( 2 0 1 4 ) 4 9 8 e5 0 7 0043-1354/$ e see front matter ª
High loaded MBRs (HL-MBR) can concentrate sewage organic matter by aerobic bioflocculation for su... more High loaded MBRs (HL-MBR) can concentrate sewage organic matter by aerobic bioflocculation for subsequent anaerobic conversion to methane or volatile fatty acids. In the range of very short solid retention times (SRT), the effect of SRT on bioflocculation and EPS production in HL-MBR was investigated. This short SRT range was selected to find an optimum SRT maximising recovery of organics by aerobic bioflocculation and minimizing losses of organics by aerobic mineralization. Bioflocculation was studied in five HL-MBRs operated at SRTs of 0.125, 0.25, 0.5, 1 and 5 d. The extent of flocculation, defined as the fraction of suspended COD in the concentrate, increased from 59% at an SRT of 0.125 d to 98% at an SRT of 5 d. The loss of sewage organic matter by biological oxidation was 1, 2, 4, 11 and 32% at SRT of 0.125-5 d. An SRT of 0.5-1 d gave best combination of bioflocculation and organic matter recovery. Bound extracellular polymeric substances (EPS) concentrations, in particular EPS-protein concentrations, increased when the SRT was prolonged from 0.125 to 1 d. This suggests that these EPS-proteins govern the bioflocculation process. A redistribution took place from free (supernatant) EPS to bound (floc associated) EPS when the SRT was prolonged from 0.125 to 1 d, further supporting the fact that the EPS play a dominant role in the flocculation process. Membrane fouling was most severe at the shortest SRTs of 0.125 d. No positive correlation was detected between the concentration of free EPS and membrane fouling, but the concentration of submicron (45-450 nm) particles proved to be a good indicator for this fouling.
This paper investigates the impact of the ionic environment on the charge of colloidal natural or... more This paper investigates the impact of the ionic environment on the charge of colloidal natural organic matter (NOM) and ultrafiltration (UF) membranes (charge screening effect) and the calcium adsorption/bridging on new and fouled membranes (calcium bridging effect) by measuring the zeta potentials of membranes and colloidal NOM. Fouling experiments were conducted with natural water to determine whether the reduction of the charge screening effect and/or calcium bridging effect by backwashing with demineralized water can explain the observed reduction in fouling. Results show that the charge of both membranes and NOM, as measured by the zeta potential, became more negative at a lower pH and a lower concentration of electrolytes, in particular, divalent electrolytes. In addition, calcium also adsorbed onto the membranes, and consequently bridged colloidal NOM and membranes via binding with functional groups. The charge screening effect could be eliminated by flushing NOM and membranes with demineralized water, since a cation-free environment was established. However, only a limited amount of the calcium bridging connection was removed with demineralized water backwashes, so the calcium bridging effect mostly could not be eliminated. As demineralized water backwash was found to be effective in fouling control, it can be concluded that the reduction of the charge screening is the dominant mechanism for this.
This paper presents a state of art review on the stability of supported liquid membranes (SLM). T... more This paper presents a state of art review on the stability of supported liquid membranes (SLM). The backgrounds of SLM instability phenomena are presented, and various mechanisms for explaining these phenomena are treated in detail. Several suggestions for stability improvement are discussed.* Present address: Agrotechnological Research Institute (ATO-DLO), Fatty Acid and Membrane Technology, P.O, Box 17, NL-6700 AA Wageningen, The
ABSTRACT The high total phosphorus content of raw domestic wastewater with its significant eutrop... more ABSTRACT The high total phosphorus content of raw domestic wastewater with its significant eutrophication potential offers an excellent possibility for phosphate recovery. Continuous recirculation of NF concentrate to an MBR and simultaneous phosphate recovery from the NF concentrate can be applied to produce reusable water, recovering phosphates, while at the same time decreasing the scaling potential of the recirculated NF concentrate, prolonging the retention times of slowly biodegradable soluble compounds (e.g. micropollutants) and recirculating multivalent cations to promote the bio-flocculation. Here we introduce an electrochemical system to recover phosphates. An electrochemical cell was divided into an anode and a cathode compartment separated by a cation exchange membrane. Precipitation of phosphates from nanofiltration concentrate was induced by locally increasing the pH at the cathode surface by water electrolysis and thereby creating supersaturated conditions at the cathode. 70-95% recovery of total phosphate was achieved at a pH of 8-10 near the cathode. Ion analysis, XRD and ATR-FTIR spectra indicated that the precipitate consisted of amorphous calcium phosphate (ACP) and minor proportions of amorphous calcium carbonate (ACC). The amount of ACC was dependent on the pH. Calcium phosphate scaling at the cathode surface did not occur due to H 2-gas formation preventing nucleation and growth at the cathode.
ABSTRACT Introduction The disposal of nanofiltration concentrates often causes environmental poll... more ABSTRACT Introduction The disposal of nanofiltration concentrates often causes environmental pollution of the ecosystem due to unsustainable removal pathways e.g. landfills or surface waters [Nederlof et al., 2005]. A more sustainable option is the recirculation of the concentrate to the secondary biological wastewater treatment stage like a membrane bioreactor (MBR). Rautenbach and Mellis (1994) were the first who have mentioned an NF concentrate recirculation to an MBR treating dumpsite leachate. A combined MBR-NF system with concentrate recirculation (Figure 1) offers high quality reusable water (NF permeate) [Asano et al., 2007] and compounds which potentially can be recovered (NF concentrate). Increased levels of multivalent ions such as Ca2+ or Mg2+ in the concentrate can enhance bioflocculation due to their presence in the recirculation stream [Urbain et al., 1993]. Furthermore, increased retention times of hardly biodegradable substances (e.g. micropollutants) might improve their degradation. Figure 1: MBR and NF pilot scale setup with concentrate recycle Disadvantages of the concentrate recycle, however, could be the accumulation of toxic compounds such as heavy metals or inert organic compounds like humic acids. Wastewater treatment parameters such as COD (chemical oxygen demand) and N removal are regulated by the urban wastewater treatment directive (UWWTD), which proposes certain discharge limits that have to be met to avoid excessive environmental pollution by the MBR permeate. The NF concentrate recirculation may cause toxicity and therefore inhibit the microbiological reactions responsible to meet these parameters. In addition, precipitation could occur inside the MBRs or on the nanofiltration membranes due to elevated concentration levels. This might cause more scaling in the MBR and NF system, and finally affect the feasibility of the NF concentrate recycle. Material and Methods This study investigates the impact of full recirculation of NF concentrate on the biological performance of a domestic MBR. Three MBRs (flat sheet Kubota microfiltration plates) were run with different recirculation ratios of NF concentrate produced from their permeates by a DOW NF270 membrane over a period of 230 days. The results of the biological MBR performance were analyzed and used to evaluate the biological feasibility of the complete recirculation of the tertiary NF concentrate. Results and Conclusions Results show the effect of the nanofiltration concentrate on the quality and composition of the MBR sludge as well as on regulated operational parameters like COD and ammonium removal. For instance, decreasing MLSS concentrations, and changes in VSS/MLSS ratio due to incorporation of inorganic material are a first indication that the recirculation might cause biological issues on the longer term. Influences of the recycle on the COD removal due to accumulation of non-biodegradable COD further underline this trend. Also differences in membrane performance, like the occurrence of membrane fouling in the MBR as well as in the NF step are observed. Concentrate recycle leads to more fouling and a reduced critical flux in the MBR. References Asano, T., Burton, F., Leverenz, H., Tsuchihashi, R., Tchobanoglous, G. (2007) Water Reuse: Issues, technologies, and applications, 1st Edition, Mc Graw Hill, New York Nederlof, M.M., van Paassen J.A.M. and Jong, R. (2005) Nanofiltration concentrate disposal: experiences in The Netherlands, Desalination 178(1-3 SPEC. ISS.), 303-312 Rautenbach, R., Mellis, R. (1994) Waste water treatment by combination of bioreactor and nanofiltration, Desalination 59(2), 171-188 Urbain, V., Block, C.J., Manem, J. (1993) Bioflocculation in activated sludge: An analytic approach, Water Res. 27(5), 829-838
A novel fabrication of composite mosaic membrane was studied in this paper. The fabrication conce... more A novel fabrication of composite mosaic membrane was studied in this paper. The fabrication concept was based on interfacial polymerization (IP) to coat a thin selective layer onto the surface of a micro-porous hollow-fiber membrane. In doing so, 2,5diaminobenzene sulfonic acid was used as one monomer of IP reaction , and a mixture of trimesoyl chloride (TMCl) and 4-(chloromethyl) benzoyl chloride was chosen as another monomer. Through the IP reaction a thin selective layer with the negative charged groups could be firstly formed on the polyethersulfone (PES) support membrane. Afterwards, trimethylamine solution was then introduced to modify the IP layer through a quaternization reaction. Thus the selective layer of this composite membrane contained both the negative charged and the positive charged groups, which performed the mosaic functionality. The characterizations of the composite mosaic membranes were carried out through permeation experiments using different inorganic salts and dyes as well as atomic force microscopy (AFM). The experimental results show that the membranes could permeate mono-and bi-valent inorganic salts, but reject larger organic molecules. Such a mosaic membrane is potentially useful for the separation of salts from water-soluble organics, especially in dye and textile industries.
Liquid and liquid/gas flows through spacer filled channels were studied using Particle Imaging Ve... more Liquid and liquid/gas flows through spacer filled channels were studied using Particle Imaging Velocimetry (PIV) to provide experimental support for velocity distributions obtained from Computational Fluid Dynamics studies available in the literature. It is shown that PIV is a suitable technique for measuring velocity profiles in spacer filled channels, although care has to be taken when interpreting the results. PIV measurements were carried out for an entire flow cell (5 cm × 15 cm), a smaller area of roughly 7 mm × 8 mm and for a single spacer cell (2.5 mm × 3 mm). The experimental results show that fluid flow is well distributed across the entire flow cell in the case of single-phase flow. The recordings of the single spacer cell showed that liquid flow is mainly parallel to the spacer filaments and therefore the direction of flow changes 90 • over the height of the channel. Introduction of air bubbles introduced strong local velocity gradients. The liquid velocity in two-phase flows is shown to be more unsteady than in the case of single-phase flow, which is advantageous when trying to prevent fouling or concentration polarization.
Reverse osmosis (RO) membrane systems are widely used in the desalination of water. However, flux... more Reverse osmosis (RO) membrane systems are widely used in the desalination of water. However, flux decline due to fouling phenomena in RO remains a challenge. To minimize fouling, a reliable index is necessary to predict the fouling potential of the RO feed water. The ASTM introduced the silt density index (SDI) as a standard fouling index to measure the fouling potential due to colloidal and suspended particles. For decades, the SDI is worldwide accepted and applied. There are growing doubts about the predictive value of this parameter. In addition there are several deficiencies observed, affecting the accuracy and reproducibility e.g. no correction factor for temperature, nor for variations in membrane resistance, and no linear correlation with the concentration of colloidal/suspended particles. This paper gives an overview of our work on limitations, improvements and alternatives for the SDI. Firstly, the influence of the applied 0.45 lm test membrane on the SDI will be investigated. Variations in SDI values can be attributed to differences in properties of these membranes. In order to quantify the influence of pressure, temperature and membrane resistance on the SDI a mathematical relation was developed between the SDI and the MFI0.45, assuming cake filtration. In addition, also other fouling mechanisms were incorporated in the model using the wellknown blocking laws. Based on a cake filtration fouling mechanism and assuming 100% particle retention, the models were used to normalize the experimental SDI values for temperature, pressure and membrane resistance to the SDI + . By applying this normalization, the results of SDI tests carried out under different conditions and/or with different membranes can be compared easily as was proven experimentally in the lab and at a seawater desalination plant. Finally, an alternative filtration index will be introduced, the volume-based SDI_v. The SDI_v compares the initial flow rate to the flow rate after filtering a standard volume of feed water using MF membranes with an average pore size of 0.45 lm. Our experimental results show that SDI_v is independent of the membrane resistance. In that way, it eliminates most of the disadvantages of the SDI and has great potential to replace the SDI in the field.
Biolouling has detrimental effects on the feed channel pressure drop and the permeate flux in hig... more Biolouling has detrimental effects on the feed channel pressure drop and the permeate flux in high-pressure membrane processes such as NF and RO. Two-phase flow cleaning is a chemical-free technique that is able to remove such biotilms. This paper presents a study into the effects of the gas/liquid ratio, Feed spacer geometry, applied pressure and liquid velocity On the efficiency of two-phase flow cleaning in spiral-wound nanotiltration elements. A high-speed camera, optical coherence tomography and scanning electron microscopy were used to study biofouling and its removal. Our results show that two conditions must be met to ensure that a sufficiently high shear force is applied to biofilms on membrane and spacer surfaces. A good bubble distribution in the channel is the first requirement. While it is mainly the structure of the feed spacer that controls bubble flow and bubble size, a minimum gas/liquid ratio of 0.5 is necessary to achieve a good bubble distribution. The second con...
Worldwide, the application of a (gas/liquid) two-phase flow in membrane processes has received am... more Worldwide, the application of a (gas/liquid) two-phase flow in membrane processes has received ample scientific deliberation because of its potential to reduce concentration polarization and membrane fouling, and therefore enhance membrane flux. Gas/liquid flows are now used to promote turbulence and instabilities inside membrane modules in various membrane processes such as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, membrane distillation, electrodialysis, and membrane bio-reactors. This paper provides a comprehensive and critical literature review of the state of the art in this research area. A total of 205 scientific papers published in peer-reviewed journals from 1989 to 2013 were collected. The data in 195 of these papers (published up to 2011) were compiled and analyzed. These data were analyzed and normalized based on gas and liquid superficial velocities, gas/liquid ratio and feed types, trans-membrane pressure and membrane module type in order to mak...
The application of ceramic membranes in water treatment is becoming increasing attractive because... more The application of ceramic membranes in water treatment is becoming increasing attractive because of their long life time and excellent chemical, mechanical and thermal stability. However, fouling of ceramic membranes, especially hydraulically irreversible fouling, is still a critical aspect affecting the operational cost and energy consumption in water treatment plants. In this study, four ceramic membranes with pore sizes or molecular weight cut-off (MWCO) of 0.20 lm, 0.14 lm, 300 kDa and 50 kDa were compared during natural surface water filtration with respect to hydraulically irreversible fouling index (HIFI), foulant composition and narrowing of pore size due to the irreversible fouling. Our results showed that the hydraulically irreversible fouling index (HIFI) was proportional to the membrane pore size (r 2 = 0.89) when the same feed water was filtrated. The UF membranes showed lower HIFI values than the MF membranes. Pore narrowing (internal fouling) was found to be a main fouling pattern of the hydraulically irreversible fouling. The internal fouling was caused by monolayer adsorption of foulants with different sizes that is dependent on the size of the membrane pore.
Water science and technology : a journal of the International Association on Water Pollution Research, 2003
A better understanding of microbiology and ecology of nitrifying bacteria in inner biofilms is an... more A better understanding of microbiology and ecology of nitrifying bacteria in inner biofilms is an important part of improving process performance and control. Microelectrodes and fluorescent in situ hybridization (FISH) in biofilm research have been used to investigate the spatial distributions of various microbial activities in biofilms and have led to new experimental findings as well as modifications of the homogeneous assumptions in the biofilm kinetic models. The objective of this study is to try the combination of two methods, both FISH and microelectrode measurements, and to provide reliable and in situ information on nitrifying bacterial activity in biofilms. The characteristics of biofilm developed on tygon slides were different according to the change of dissolved oxygen (DO). When the DO increased from 2 to 10 mg DO/L, the rate of the biofilm thickness increased and its dry density changed from 50-70 to 25-90 mg/cm3. Ammonia oxidizing bacteria were not uniformly distribut...
journal homepage: www.else vier.com/locate /wa tres w a t e r r e s e a r c h 4 8 ( 2 0 1 4 ) 4 9... more journal homepage: www.else vier.com/locate /wa tres w a t e r r e s e a r c h 4 8 ( 2 0 1 4 ) 4 9 8 e5 0 7 0043-1354/$ e see front matter ª
High loaded MBRs (HL-MBR) can concentrate sewage organic matter by aerobic bioflocculation for su... more High loaded MBRs (HL-MBR) can concentrate sewage organic matter by aerobic bioflocculation for subsequent anaerobic conversion to methane or volatile fatty acids. In the range of very short solid retention times (SRT), the effect of SRT on bioflocculation and EPS production in HL-MBR was investigated. This short SRT range was selected to find an optimum SRT maximising recovery of organics by aerobic bioflocculation and minimizing losses of organics by aerobic mineralization. Bioflocculation was studied in five HL-MBRs operated at SRTs of 0.125, 0.25, 0.5, 1 and 5 d. The extent of flocculation, defined as the fraction of suspended COD in the concentrate, increased from 59% at an SRT of 0.125 d to 98% at an SRT of 5 d. The loss of sewage organic matter by biological oxidation was 1, 2, 4, 11 and 32% at SRT of 0.125-5 d. An SRT of 0.5-1 d gave best combination of bioflocculation and organic matter recovery. Bound extracellular polymeric substances (EPS) concentrations, in particular EPS-protein concentrations, increased when the SRT was prolonged from 0.125 to 1 d. This suggests that these EPS-proteins govern the bioflocculation process. A redistribution took place from free (supernatant) EPS to bound (floc associated) EPS when the SRT was prolonged from 0.125 to 1 d, further supporting the fact that the EPS play a dominant role in the flocculation process. Membrane fouling was most severe at the shortest SRTs of 0.125 d. No positive correlation was detected between the concentration of free EPS and membrane fouling, but the concentration of submicron (45-450 nm) particles proved to be a good indicator for this fouling.
This paper investigates the impact of the ionic environment on the charge of colloidal natural or... more This paper investigates the impact of the ionic environment on the charge of colloidal natural organic matter (NOM) and ultrafiltration (UF) membranes (charge screening effect) and the calcium adsorption/bridging on new and fouled membranes (calcium bridging effect) by measuring the zeta potentials of membranes and colloidal NOM. Fouling experiments were conducted with natural water to determine whether the reduction of the charge screening effect and/or calcium bridging effect by backwashing with demineralized water can explain the observed reduction in fouling. Results show that the charge of both membranes and NOM, as measured by the zeta potential, became more negative at a lower pH and a lower concentration of electrolytes, in particular, divalent electrolytes. In addition, calcium also adsorbed onto the membranes, and consequently bridged colloidal NOM and membranes via binding with functional groups. The charge screening effect could be eliminated by flushing NOM and membranes with demineralized water, since a cation-free environment was established. However, only a limited amount of the calcium bridging connection was removed with demineralized water backwashes, so the calcium bridging effect mostly could not be eliminated. As demineralized water backwash was found to be effective in fouling control, it can be concluded that the reduction of the charge screening is the dominant mechanism for this.
This paper presents a state of art review on the stability of supported liquid membranes (SLM). T... more This paper presents a state of art review on the stability of supported liquid membranes (SLM). The backgrounds of SLM instability phenomena are presented, and various mechanisms for explaining these phenomena are treated in detail. Several suggestions for stability improvement are discussed.* Present address: Agrotechnological Research Institute (ATO-DLO), Fatty Acid and Membrane Technology, P.O, Box 17, NL-6700 AA Wageningen, The
ABSTRACT The high total phosphorus content of raw domestic wastewater with its significant eutrop... more ABSTRACT The high total phosphorus content of raw domestic wastewater with its significant eutrophication potential offers an excellent possibility for phosphate recovery. Continuous recirculation of NF concentrate to an MBR and simultaneous phosphate recovery from the NF concentrate can be applied to produce reusable water, recovering phosphates, while at the same time decreasing the scaling potential of the recirculated NF concentrate, prolonging the retention times of slowly biodegradable soluble compounds (e.g. micropollutants) and recirculating multivalent cations to promote the bio-flocculation. Here we introduce an electrochemical system to recover phosphates. An electrochemical cell was divided into an anode and a cathode compartment separated by a cation exchange membrane. Precipitation of phosphates from nanofiltration concentrate was induced by locally increasing the pH at the cathode surface by water electrolysis and thereby creating supersaturated conditions at the cathode. 70-95% recovery of total phosphate was achieved at a pH of 8-10 near the cathode. Ion analysis, XRD and ATR-FTIR spectra indicated that the precipitate consisted of amorphous calcium phosphate (ACP) and minor proportions of amorphous calcium carbonate (ACC). The amount of ACC was dependent on the pH. Calcium phosphate scaling at the cathode surface did not occur due to H 2-gas formation preventing nucleation and growth at the cathode.
ABSTRACT Introduction The disposal of nanofiltration concentrates often causes environmental poll... more ABSTRACT Introduction The disposal of nanofiltration concentrates often causes environmental pollution of the ecosystem due to unsustainable removal pathways e.g. landfills or surface waters [Nederlof et al., 2005]. A more sustainable option is the recirculation of the concentrate to the secondary biological wastewater treatment stage like a membrane bioreactor (MBR). Rautenbach and Mellis (1994) were the first who have mentioned an NF concentrate recirculation to an MBR treating dumpsite leachate. A combined MBR-NF system with concentrate recirculation (Figure 1) offers high quality reusable water (NF permeate) [Asano et al., 2007] and compounds which potentially can be recovered (NF concentrate). Increased levels of multivalent ions such as Ca2+ or Mg2+ in the concentrate can enhance bioflocculation due to their presence in the recirculation stream [Urbain et al., 1993]. Furthermore, increased retention times of hardly biodegradable substances (e.g. micropollutants) might improve their degradation. Figure 1: MBR and NF pilot scale setup with concentrate recycle Disadvantages of the concentrate recycle, however, could be the accumulation of toxic compounds such as heavy metals or inert organic compounds like humic acids. Wastewater treatment parameters such as COD (chemical oxygen demand) and N removal are regulated by the urban wastewater treatment directive (UWWTD), which proposes certain discharge limits that have to be met to avoid excessive environmental pollution by the MBR permeate. The NF concentrate recirculation may cause toxicity and therefore inhibit the microbiological reactions responsible to meet these parameters. In addition, precipitation could occur inside the MBRs or on the nanofiltration membranes due to elevated concentration levels. This might cause more scaling in the MBR and NF system, and finally affect the feasibility of the NF concentrate recycle. Material and Methods This study investigates the impact of full recirculation of NF concentrate on the biological performance of a domestic MBR. Three MBRs (flat sheet Kubota microfiltration plates) were run with different recirculation ratios of NF concentrate produced from their permeates by a DOW NF270 membrane over a period of 230 days. The results of the biological MBR performance were analyzed and used to evaluate the biological feasibility of the complete recirculation of the tertiary NF concentrate. Results and Conclusions Results show the effect of the nanofiltration concentrate on the quality and composition of the MBR sludge as well as on regulated operational parameters like COD and ammonium removal. For instance, decreasing MLSS concentrations, and changes in VSS/MLSS ratio due to incorporation of inorganic material are a first indication that the recirculation might cause biological issues on the longer term. Influences of the recycle on the COD removal due to accumulation of non-biodegradable COD further underline this trend. Also differences in membrane performance, like the occurrence of membrane fouling in the MBR as well as in the NF step are observed. Concentrate recycle leads to more fouling and a reduced critical flux in the MBR. References Asano, T., Burton, F., Leverenz, H., Tsuchihashi, R., Tchobanoglous, G. (2007) Water Reuse: Issues, technologies, and applications, 1st Edition, Mc Graw Hill, New York Nederlof, M.M., van Paassen J.A.M. and Jong, R. (2005) Nanofiltration concentrate disposal: experiences in The Netherlands, Desalination 178(1-3 SPEC. ISS.), 303-312 Rautenbach, R., Mellis, R. (1994) Waste water treatment by combination of bioreactor and nanofiltration, Desalination 59(2), 171-188 Urbain, V., Block, C.J., Manem, J. (1993) Bioflocculation in activated sludge: An analytic approach, Water Res. 27(5), 829-838
A novel fabrication of composite mosaic membrane was studied in this paper. The fabrication conce... more A novel fabrication of composite mosaic membrane was studied in this paper. The fabrication concept was based on interfacial polymerization (IP) to coat a thin selective layer onto the surface of a micro-porous hollow-fiber membrane. In doing so, 2,5diaminobenzene sulfonic acid was used as one monomer of IP reaction , and a mixture of trimesoyl chloride (TMCl) and 4-(chloromethyl) benzoyl chloride was chosen as another monomer. Through the IP reaction a thin selective layer with the negative charged groups could be firstly formed on the polyethersulfone (PES) support membrane. Afterwards, trimethylamine solution was then introduced to modify the IP layer through a quaternization reaction. Thus the selective layer of this composite membrane contained both the negative charged and the positive charged groups, which performed the mosaic functionality. The characterizations of the composite mosaic membranes were carried out through permeation experiments using different inorganic salts and dyes as well as atomic force microscopy (AFM). The experimental results show that the membranes could permeate mono-and bi-valent inorganic salts, but reject larger organic molecules. Such a mosaic membrane is potentially useful for the separation of salts from water-soluble organics, especially in dye and textile industries.
Liquid and liquid/gas flows through spacer filled channels were studied using Particle Imaging Ve... more Liquid and liquid/gas flows through spacer filled channels were studied using Particle Imaging Velocimetry (PIV) to provide experimental support for velocity distributions obtained from Computational Fluid Dynamics studies available in the literature. It is shown that PIV is a suitable technique for measuring velocity profiles in spacer filled channels, although care has to be taken when interpreting the results. PIV measurements were carried out for an entire flow cell (5 cm × 15 cm), a smaller area of roughly 7 mm × 8 mm and for a single spacer cell (2.5 mm × 3 mm). The experimental results show that fluid flow is well distributed across the entire flow cell in the case of single-phase flow. The recordings of the single spacer cell showed that liquid flow is mainly parallel to the spacer filaments and therefore the direction of flow changes 90 • over the height of the channel. Introduction of air bubbles introduced strong local velocity gradients. The liquid velocity in two-phase flows is shown to be more unsteady than in the case of single-phase flow, which is advantageous when trying to prevent fouling or concentration polarization.
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Papers by Antoine Kemperman