Operation of passive membrane systems for drinking water treatment

Gravity-Driven Membrane Filtration with Passive Hydraulic Fouling Control for Drinking Water Treatment: Demonstration of Long-Term Performance at Full Scale

The present study evaluated the performance of a full-scale gravity-driven membrane filtration system with passive hydraulic fouling control (PGDMF) for drinking water treatment in a small community over a 3-year period. The PGDMF system consistently met the design flow and regulated water quality/performance parameters (i.e., total coliform, Escherichia coli, turbidity, and membrane integrity). The instantaneous temperature-corrected permeability (TCP) varied seasonally, being greater during the winter months. The overall TCP decreased slowly to ∼60% of the initial value by the end of 3 years, a TCP that is much greater than would have been expected without passive hydraulic fouling control. Although it was not possible to directly link the observed seasonal changes in TCP to potential seasonal changes in the biofilm microbiome, the analysis did suggest that the lower TCP during summer months was due to a greater microorganism richness in the feed and presence of filamentous, stalked, and biofilm-forming bacteria in the biofilm. Operation with higher trans-membrane pressure (i.e., ∼30 vs ∼20 mbar) and more frequent passive hydraulic fouling control (i.e., every 12 vs 24 h) enabled a greater flow to be sustained. The study demonstrated the long-term robustness and performance of GDMF with passive hydraulic fouling control for drinking water treatment.

New progress in the deep understanding of the biocake layer property: Combined effect of neglected protein secondary structure, morphology, and mechanism

The application of magnetic fields (MFs) and magnetic particles (MPs) in water treatment has attracted widespread attention due to their stability, strong biological compatibility, and less chemical consumption. This study introduced MPs and MFs to GDM and probed their effects on filtration performance. Predeposited large MPs (P-large) and batch-added little MPs (B-little) intervened biocake layer development, forming more open and porous structures, they also reduced biomass secretion, resulting in flux increases of 13 % in P-large and 40 % in B-little than P-little, respectively. Besides, MFs controlled MPs distribution on the biocake layer, resulting in forming of more rough and open structures. A relatively lower magnetic field of 20 mT facilitated biomass secretion, while a higher magnetic field of 50 mT decreased biomass. Furthermore, applying magnetic fields decreased the ratios of α-helix and β-sheet, and increased random coil percentage. Thus, applying magnetic field mediation would contribute to the flux improvements in I-20 and I-50 by 29 % and 32 % relative to I-0. Economic analysis suggested introducing MPs and MFs to GDM was economically feasible, synergy of MPs and MFs had more economic advantages on the community scale and MPs-assisted GDM had significant economic advantages on both community and household scales. Future works should focus on developing new technologies for the recycling of MPs and membranes. This study provided new insight into the protein secondary structures associated with GDM performance and would encourage new sustainable MFs and MPs-assisted GDM technological developments.

Gravity-driven membrane filtration with compact second-life modules daily backwashed: An alternative to conventional ultrafiltration for centralized facilities

Gravity-driven membrane (GDM) filtration is a strategic alternative to conventional ultrafiltration (UF) for the resilient production of drinking water via ultrafiltration when resources become scarce, given the low dependency on energy and chemicals, and longer membrane lifetime. Implementation at large scale requires the use of compact and low-cost membrane modules with high biopolymer removal capacity. We therefore evaluated (1) to what extent stable flux can be obtained with compact membrane modules, i.e., inside-out hollow fiber membranes, and frequent gravity-driven backwash, (2) whether we can reduce membrane expenses by effectively utilizing second-life UF modules, i.e., modules that have been discarded by treatment plant operators because they are no longer under warranty, (3) if biopolymer removal could be maintained when applying a frequent backwash and with second-life modules and (4) which GDM filtration scenarios are economically viable compared to conventional UF, when considering the influence of new or second-life modules, membrane lifetime, stable flux value and energy pricing. Our findings showed that it was possible to maintain stable fluxes around 10 L/m²/h with both new and second-life modules for 142 days, but a daily gravity-driven backwash was necessary and sufficient to compensate the continuous flux drop observed with compact modules. In addition, the backwash did not affect the biopolymer removal. Costs calculations revealed two significant findings: (1) using second-life modules made GDM filtration membrane investment less expensive than conventional UF, despite the higher module requirements for GDM filtration and (2) overall costs of GDM filtration with a gravity-driven backwash were unaffected by energy prices rise, while conventional UF costs rose significantly. The later increased the number of economically viable GDM filtration scenarios, including scenarios with new modules. In summary, we proposed an approach that could make GDM filtration in centralized facilities feasible and increase the range of UF operating conditions to better adapt to increasing environmental and societal constraints.

Biocarriers facilitated gravity-driven membrane filtration of domestic wastewater in cold climate: Combined effect of temperature and periodic cleaning

In this study, two lava stone biocarrier facilitated gravity-driven membrane (GDM) reactors were operated at ~8 °C and ~22 °C in parallel for treating primary wastewater effluent. Although the biocarrier reactor at 8 °C displayed less efficient removals of biodegradable organics than that at 22 °C, both GDM systems (without cleaning) showed comparable fouling resistance distribution patterns, accompanying with similar cake filtration constants and pore constriction constants by modelling simulation. Compared to the GDM at 8 °C, more foulants were accumulated on the GDM at 22 °C, but they presented similar soluble organics/inorganics contents and specific cake resistances. This indicated the cake layers at 22 °C may contain greater-sized foulants due to proliferation of both prokaryotes and eukaryotes, leading to a relatively less-porous nature. In the presence of periodic cleaning (at 50 °C), the cleaning effectiveness followed a sequence as ultrasonication-enhanced physical cleaning > two-phase flow cleaning > chemical-enhanced physical cleaning > physical cleaning, regardless of GDM operation temperature. However, significantly higher cake resistances were observed in the GDM system at 22 °C than those at 8 °C, because shear force tended to remove loosely-attached foulant layers and may compress the residual dense cake layer. The presence of periodic cleaning led to dissimilar dominant prokaryotic and eukaryotic communities in the cake layers as those without cleaning and in the lava stone biocarriers. Nevertheless, operation temperature did not influence GDM permeate quality, which met EU discharge standards.

Low maintenance gravity-driven membrane filtration using hollow fibers: Effect of reducing space for biofilm growth and control strategies on permeate flux

The implementation of centralized drinking water treatment systems necessitates lower operational costs and improved biopolymer removal during ultrafiltration (UF), which can be afforded by gravity-driven membrane (GDM) filtration. However, prior to implementing GDM filtration in centralized systems, biofilm growth in compacted membrane configurations, such as inside-out hollow fiber (HF), and its impact on permeate flux need to be investigated. To this end, we operated modules with distinct limits on available space for biofilm growth: (1) outside-in 1.5 mm 7-capillary HF (non-limited), (2) inside-out 1.5 mm 7-capillary HF (limited), and (3) inside-out 0.9 mm 7-capillary HF (very limited). Here, we observed that the lower the space available for biofilm growth, the lower the permeate flux. To improve GDM performance with inside-out HF, we applied daily shear stress to the biofilm surface with forward flush (FF) or combined relaxation and forward flush (R+FF). We showed that applying shear stress to the biofilm surface was insufficient for controlling flux loss due to low available space for biofilm growth. At the experimental endpoint, we backwashed with a stepwise transmembrane pressure (TMP) increase or a single TMP on all inside-out HF modules, which removed the biofilm from its base. Afterwards, higher fluxes were yielded. We also showed that all modules exhibited a gradual increase in biopolymer removal followed by stabilization between 70 and 90%. Additionally, control of biofilm growth with surface shear stress did not affect biopolymer removal. In summary, the implementation of inside-out HF with GDM filtration is challenged by low available space for biofilm growth, but may be remedied with a regular backwash to remove biofilm from its base. We showed that a wider range of GDM applications are available; making GDM potentially compatible with implementation in centralized systems, if space limitation is taken into consideration for operation optimization.

Efficient integrated module of gravity driven membrane filtration, solar aeration and GAC adsorption for pretreatment of shale gas wastewater

Low-cost and efficient treatment processes are urgently needed to manage highly decentralized shale gas wastewater, which seriously threatens the environment if not properly treated. We propose a simple integrated pretreatment process for on-site treatment, whereby gravity driven membrane filtration is combined with granular activated carbon (GAC) adsorption and solar aeration. The rationale of exploitment of sustainable solar energy is that most shale gas production areas are decentralized and located in desert/rural areas characterized by relatively scarce transportation and power facilities but also by abundant sunshine. In this study, GAC and aeration significantly increased the stable flux (170%) and improved effluent quality. Specifically, the dissolved organic carbon removal rate of the integrated system was 44.9%. The high stable flux was attributed to a reduction of extracellular polymeric substances accumulated on the membrane, as well as to the more porous and heterogeneous biofilm formed by eukaryotes with stronger active predation behavior. The prevailing strains, Gammaproteobacteria (35.5%) and Alphaproteobacteria (56.5%), played an important active role in organic carbon removal. The integrated system has great potential as pretreatment for shale gas wastewater due to its low energy consumption, low operational costs, high productivity, and effluent quality.

Numerical Modelling Assisted Design of a Compact Ultrafiltration (UF) Flat Sheet Membrane Module

The increasing adoption of ultra-low pressure (ULP) membrane systems for drinking water treatment in small rural communities is currently hindered by a limited number of studies on module design. Detailed knowledge on both intrinsic membrane transport properties and fluid hydrodynamics within the module is essential in understanding ULP performance prediction, mass transfer analysis for scaling-up between lab-scale and industrial scale research. In comparison to hollow fiber membranes, flat sheet membranes present certain advantages such as simple manufacture, sheet replacement for cleaning, moderate packing density and low to moderate energy usage. In the present case study, a numerical model using computational fluid dynamics (CFD) of a novel custom flat sheet membrane module has been designed in 3D to predict fluid flow conditions. The permeate flux through the membrane decreased with an increase in spacer curviness from 2.81 L/m²h for no (0%) curviness to 2.73 L/m²h for full (100%) curviness. A parametric analysis on configuration variables was carried out to determine the optimum design variables and no significant influence of spacer inflow or outflow thickness on the fluid flow were observed. The numerical model provides the necessary information on the role of geometrical and operating parameters for fabricating a module prototype where access to technical expertise is limited.

A combination of membrane relaxation and shear stress significantly improve the flux of gravity-driven membrane system

Gravity-driven membrane (GDM) filtration system is a promising process for decentralized drinking water treatment. During the operation, membrane relaxation and shear stress could be simply achieved by intermittent filtration and water disturbance (created by occasionally shaking membrane model or stirring water in membrane tank), respectively. To better understand the impact of membrane relaxation and shear stress on the biofouling layer and stable flux in GDM system, action of daily 60-min intermission, daily flushing (cross-flow velocity = 10 cm s^-1, 1 min), and the combination of the two (flushed right after the 60-min intermission) were compared. The results showed that membrane relaxation and shear stress lonely was ineffective in improving the stable flux, while their combination enhanced the stable flux by 70%. A more open and spatially heterogeneous biofouling layer with a low extracellular polymeric substance (EPS) content and a high microbial activity was formed under the combination of membrane relaxation and shear stress. In-situ optical coherence tomography (OCT) observation revealed that, during intermission, the absence of pushing force by water flow induced a reversible expansion of biofouling layer, and the biofouling layer restored to its initial state soon after resuming filtration. Shear stress caused abrasion and erosion on the biofouling surface, but it exerted little effect on the interior of biofouling layer. Under the combination, however, both the surface and interior of biofouling layer were disturbed because of 1) the water vortexes caused by rough biofouling layer surface, and 2) the porous structure after 60-min intermission. This disturbance, in turn, helped the biofouling layer maintain its roughness and porosity, thereby improving the stable flux of GDM system.

Enhancement in pervaporative performance of PDMS membrane for separation of styrene from wastewater by hybridizing with reduced graphene oxide

The removal of styrene from wastewater by pervaporation was investigated by using composite PDMS membranes filled with reduced graphene oxide on PES support layers. Graphene oxide was synthesized through modified Hummers' method and then chemically reduced. The filler was characterized by TEM, SEM, XRD, and AFM. The top layers with different PDMS molecular weights were cast on the PES supports, which were prepared by phase inversion method. The characterizations of prepared membranes were investigated by SEM, AFM, contact angle measurement, TGA, and DSC. It was observed that presence of the filler in the polymeric matrix controls the swelling of the membrane and enhances its solubility parameter in favor of styrene. Moreover, it significantly improves the thermal stability of the membranes. The mechanism of separation in the process was found to be affected mainly by enhancing in the membrane's solubility rather than in its diffusivity. The pervaporative performance of prepared membranes showed their great affinity toward styrene so that the separation factor of the optimum membrane (M2/S) was increased about 250% (600.4 in comparison to 241.4 for the unfilled membrane) while its total flux was decreased from 772.5 g m^-2.h^-1for the unfilled membrane to 321.9 g m^-2.h^-1. Increasing the molecular weight of PDMS lowered the optimal rGO content due to the complexity of the diffusion path and occupation of free volume by longer polymer chains. Accordingly, a lower total flux (124.7 g m^-2.h^-1 for high MW compared to 718.0 g m^-2.h^-1 for low MW) and higher separation factor (822.5 for high MW compared to 230.8 for low MW) were yielded for the same filler content (0.1 wt% rGO).

The hydrothermal solution for self-sustaining drinking water purification at point of use

Decentralized drinking water purification complements water supply in areas with unreliable or absent infrastructure. The exacerbating consequences of climate change in form of droughts and floods force remote households to tap various water sources. Hence, household-based processes must be versatile to cope with e.g. contaminated ground water and turbid surface waters. Purification at household level must be self-sustaining in order to enable independence from continuous supply of power and consumables. In this study, we design a process accordingly and we prove its technical feasibility on pilot scale. The two-step process utilizes gravity-driven ultrafiltration and activated carbon adsorption to purify water, whereas the process regeneration is accomplished by combining Temperature Enhanced Backwash and Temperature Swing Adsorption to clean the membrane and adsorber, respectively. We obtained stable operation over >40 days with a sustained flowrate of ∼5 Lh^-1 and consistent product quality (turbidity ≤0.2 NTU) for all relevant water matrices: synthetic ground water, river water and even secondary effluent. We achieved a high removal of the spiked model micropollutant amitrole, environmental endocrine disruptors and bulk dissolved organics of ∼93%, >65% and ∼69%, respectively, at the optimal water recovery for river water of ∼80%. In-situ regeneration promises long-term, self-sufficient operation without exhaustion.

Gravity-driven membrane filtration for water and wastewater treatment: A review

Gravity-driven membrane (GDM) filtration has been investigated for almost 10 years. The technology is characterized not only by relatively lower transmembrane pressures which can be achieved by gravity (extremely low energy consumption), but also by the phenomenon of flux stabilization: A biofilm is allowed to form on the membrane and a stabilization of flux occurs which is related to biological processes within the biofilm layer on the membrane. This enables stable operation during a year or longer without any cleaning or flushing. Initially, the technology was developed mainly for household drinking water treatment, but in the meantime, the research and application has expanded to the treatment of greywater, rainwater, and wastewater as well as the pretreatment of seawater for desalination. This review covers the field from the rather fundamental research on biofilm morphology and microbial community analysis to the impact of feedwater composition, process parameters and organic removal performance. Not only household applications, but also for community-scale treatment and full-scale applications are discussed. In addition, the application potential is highlighted in comparison to conventional ultrafiltration. Finally, an overall assessment is illustrated and the research and development needs are identified.

Effects of water temperature and light intensity on the performance of gravity-driven membrane system

The selection of favorable environmental conditions for gravity-driven membrane (GDM) systems is crucial to their widespread application. In this study, GDM systems operated under different light intensities (illuminance levels of 0, 200, and 3000 Lux) and water temperatures (10, 20, and 30 °C) were investigated for their performance and fouling layer characteristics. The results showed that indoor light (200 Lux) had limited effects on the performance of the GDM system. However, full daylight (3000 Lux) led to algal growth; these algae increased fouling resistance and deteriorated permeate water by releasing algogenic organic matter, although they could also enhance the heterogeneity of the biofouling layer by increasing the microbial activity. Water temperature rarely influenced the total organic matter removal. The fouling layers had different thicknesses and heterogeneity, but the same level of EPS; therefore, the hydraulic resistances of these fouling layer were almost the same at different water temperatures. These findings suggest that GDM system could be operated at low water temperature and indoor light conditions, and that strong light should be avoided during the operation of GDM systems.

Ultra-low pressure membrane-based bio-purification process for decentralized drinking water supply: Improved permeability and removal performance

Ultra-low pressure gravity-driven membrane (GDM) filtration has been proposed as a cost-efficiency alternative for the decentralized drinking water supply in terms of its simple operation and low energy consumptions, whereas its undesirable removals of dissolved organic compounds (DOC) and relatively low flux impede its widespread application. In order to improve its filtration performance, filter media (granular activated carbon (GAC), zeolite and bio-ceramsite) was directly coated on the membrane surface to engineer an integrated GDM system. The coating filter layer and bio-cake layer on the membrane surface could engineer a highly porous "multifunctional double layer" structure, which facilitated improvements of stabilized flux by 30%-120% relative to GDM control. Besides, coating filter media to GDM can efficiently combine the complementary performance between filter coat and GDM filtration, and thus the removals of COD_Mn were improved to 21%, 30% and 70% in bio-ceramsite, zeolite and GAC coated systems. Furthermore, the integrated GDM systems conferred much higher potentials in resisting the shock load of contaminants (e.g. organics, ammonia, iron and manganese) compared to GDM control. In addition, a low-aeration cleaning in presence of filter media scouring could efficiently improve the flux recovery from 35% to 50-94%, while the membrane integrity test indicated that such filter media scouring would not damage the membrane surface. Overall, these findings can hopefully spark improvements of both permeability and permeate quality in GDM filtration and bring relevant benefits to the applications of GDM technologies for decentralized drinking water supply.

Computational Fluid Dynamics (CFD) Simulation of Cross-flow Mode Operation of Membrane for Downstream Processing

BACKGROUND

Membrane filtration process produced good quality of permeate flux due to which it is used in different industries like dairy, pharmaceutical, sugar, starch and sweetener industry, bioseparation, purification of biomedical materials, and downstream polishing etc. The cross-flow mode of operation has also been used to improve the quality of the Rubber Industrial effluent of Tripura, India.

METHOD

The Computational Fluid Dynamics (CFD) simulation of the cross-flow membrane is done by using ANSYS Fluent 6.3. The meshing of the geometry of the membrane is done by Gambit 2.4.6 and a grid size of 100674, the number of faces is 151651 and number of nodes being 50978 has been selected for the simulation purpose from the grid independence test. We have revised and included all patents in the manuscripts related to the membrane filtration unit.

RESULTS

Single phase Pressure-Velocity coupled Simple Algorithm and laminar model is used for the simulation of the developed model and Fluent 6.3 used for the prediction of pressure, pressure drop, flow phenomena, wall shear stress and shear strain rate inside the module is studied for cross flow membrane.

CONCLUSION

From the study, it has been found that CFD simulated results hold good agreement with the experimental values.