Integrating granular activated carbon (GAC) to gravity-driven membrane (GDM) to improve its flux stabilization: Respective roles of adsorption and biodegradation by GAC.

As a low-maintenance and cost-effective process, gravity-driven membrane (GDM) filtration is a promising alternative for decentralized drinking water supply, while the low flux impedes its extensive application. In order to address such issue, an integrated process consisting of granular activated carbon (GAC) layer and GDM was developed. The performance of virgin (fresh GAC) or preloaded GAC (saturated GAC) was compared. Flux stabilization was observed both in the fresh and saturated GAC/GDM process during long-term filtration and their stable fluxes were both improved by approximately 50% relative to the GDM control. Moreover, integrating GAC with GDM contributed to efficient removals for dissolved organic compounds (DOC), assimilable organic carbon (AOC) and low molecular weight substances both in fresh and saturated GAC/GDM filtration. Compared to GDM control, coupling GAC to GDM could significantly reduce the concentrations of extracellular polymeric substances (EPS) and total cell counts (TCC) within the biofouling layer, and engineer highly heterogeneous structures of biofouling layer on the membrane surface. In the fresh GAC/GDM process, the improved flux obtained was mainly related to less coverage of biofouling layer and lower EPS concentrations due to efficient removals of membrane foulants by GAC adsorption. The achieved higher stable flux can be maintained during long-term filtration (after GAC saturation) owing to the combined effects of EPS reduction and formation of highly heterogeneous structures of biofouling layer in the saturated GAC/GDM system. Overall, the integrated GAC/GDM process can hopefully facilitate improvements both in the stabilized flux and permeate quality, with practical relevance for GDM applications in decentralized drinking water supply.

Enhanced virus filtration in hybrid membranes with MWCNT nanocomposite.

Membrane separation is proved to be a powerful tool for several applications such as wastewater treatment or the elimination of various microorganisms from drinking water. In this study, the efficiency of inorganic composite-based multi-walled carbon nanotube (MWCNT) hybrid membranes was investigated in the removal of MS2 bacteriophages from contaminated water. With this object, multi-walled carbon nanotubes were coated with copper(I) oxide, titanium(IV) oxide and iron(III) oxide nanoparticles, respectively, and their virus removal capability was tested in both batch and flow experiments. Considering the possible pH range of drinking water, the filtration tests were carried out at pH 5.0, 7.5 and 9.0 as well. The extent of MS2 removal strongly depended on the pH values for each composite, which can be due to electrostatic interactions between the membrane and the virus. The most efficient removal (greater than or equal to 99.99%) was obtained with the Cu2O-coated MWCNT membrane in the whole pH range. The fabricated nanocomposites were characterized by X-ray diffraction, specific surface area measurement, dynamic light scattering, zeta potential measurement, Raman spectroscopy, transmission electron microscopy and scanning electron microscopy. This study presents a simple route to design novel and effective nanocomposite-based hybrid membranes for virus removal.

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.

Copper-Coated Cellulose-Based Water Filters for Virus Retention.

Despite recent efforts in modernization of water treatment facilities, the problem of access to healthy drinking water for hundreds of millions of people has still not been solved. A water filter based on Cu-coated nanofibrillated cellulose with controlled porosity was prepared by the "paper-making" method. We have optimized the proper mass and ratio of functionalized and pure nanofibrillated cellulose for the preparation of the filter. MS2 bacteriophages were used to model human pathogenic virions. We tested our filter material in batch experiments and the fixed filters in flow experiments. The fabricated Cu-coated nanofibrillated cellulose filters were characterized by scanning electron microscopy, X-ray diffraction, specific surface area measurement (Brunauer-Emmett-Teller), dynamic light scattering, and inductively coupled plasma mass spectroscopy. Our measurements proved that the fixation of cellulose nanofibers plays a significant role in the degree of virus retention and it greatly enhances the efficiency of the filtration. By using these functionalized water filters, we were able to achieve a virus retention of at least 5 magnitudes (5Log) at three different pH values: 5.0, 7.5, and 9.

Assessment and review of organochlorine pesticide pollution in Kyrgyzstan.

The current study describes the preliminary assessment and securing activities of the largest and most hazardous POPs-contaminated sites in Kyrgyzstan. In 2010, cattle died and population were found with high pesticide levels in blood, human milk, and placenta. In the first phase of the study, a historic assessment of the pesticide dumping at the landfill/dump sites have been conducted. In the second phase, soil analysis for organochlorine pesticides in the areas of the pesticide disposal sites, the former pesticides storehouses, agro-air strips, and the cotton-growing fields were conducted. By this assessment, a first overview of the types and sources of pollution and of the scale of the problem is compiled including information gaps. From major pesticides used, DDT, DDE, and HCH were measured in the highest concentrations. With the limited analytical capacity present, a reasonable risk assessment could be performed. This paper also reports on practical risk reduction measures that have been carried out recently at the two major pesticide disposal sites with support of a Dutch environmental engineering company, an international NGO (Green Cross Switzerland) and local authorities from the Suzak region within an UN project. Local population living near the sites of the former pesticide storehouses and agro-airstrips are advised not to cultivate vegetables and melons or to raise cattle on these areas. Instead, it is recommended to grow technical crops or plant trees. Further recommendations on monitoring and assessment is given including the suggestion to consider the findings in the National Implementation Plan of Kyrgyzstan.

Sensitive Detection of Competitive Molecular Adsorption by Surface-Enhanced Raman Spectroscopy.

Surface adsorption plays a critical role in a wide variety of fields from surface catalysis to molecular separation. Despite the importance, limited access to simultaneously sensitive and selective detection mechanisms has hampered the acquisition of comprehensive and versatile experimental data needed to understand the complex aspects of mixture adsorption, calling for a molecular detection method capable of obtaining the surface adsorption isotherms over a wide range of concentrations as well as distinguishing the competitive adsorption of different adsorbates. Here, we test surface-enhanced Raman spectroscopy (SERS) as an effective analysis tool of surface adsorption phenomena. Using a sensitive SERS substrate, we characterize the adsorption isotherms of chemical species of various binding energies. We obtained the isotherms for strongly binding species in a concentration range from subpicomolar to micromolar. A log-sigmoidal dependency of the SERS signals to the analyte concentration could be modeled by surface binding theories accurately using molecular dynamics simulations, thereby bringing out the potential capability of sensitive SERS for describing a single-compound adsorption process. SERS also enabled the determination of competitive adsorption isotherms from a multiple-compound solution for the first time. The successful demonstration of the sensitive and selective characterization of surface adsorption lends SERS adaptability to a cheap, facile, and effective solution for chemical analysis.

Three-dimensional excitation and emission matrix fluorescence (3DEEM) for quick and pseudo-quantitative determination of protein- and humic-like substances in full-scale membrane bioreactor (MBR).

The goal of this study is to help filling the research gaps linked to the on-line application of fluorescence spectroscopy in wastewater treatment and data processing tools suitable for rapid correction and extraction of data contained in three-dimensional fluorescence excitation-emission matrix (3DEEM) for real-time studies. 3DEEM was evaluated for direct quantification of Effluent Organic Matter (EfOM) fractions in full-scale MBR bulk supernatant and permeate samples. Principal Component Analysis (PCA) was used to investigate possible correlations between conventional Lowry and Dubois methods, Liquid Chromatography coupled to Organic Carbon and Organic Nitrogen Detection (LC-OCD-OND) and 3DEEM. 3DEEM data were analyzed using the volume of fluorescence (Φ) parameter from the Fluorescence Regional Integration (FRI) method. Two mathematical correlations were established between LC-OCD-OND and 3DEEM data to quantify protein-like and humic-like substances. These correlations were validated with supplementary data from the initial full-scale MBR, and were checked with samples from other systems (a second full-scale MBR, a full-scale conventional activated sludge (CAS) and a laboratory-scale MBR). While humic-like correlation showed satisfactory prediction for a second full-scale MBR and a CAS system, further studies are required for protein-like estimation in other systems. This new approach offers interesting perspectives for the on-line application of 3DEEM for EfOM quantification (protein-like and humic-like substances), fouling prediction and MBR process control.

Improved performance of gravity-driven membrane filtration for seawater pretreatment: Implications of membrane module configuration.

As a low energy and chemical free process, gravity-driven membrane (GDM) filtration has shown a potential for seawater pretreatment in our previous studies. In this study, a pilot submerged GDM reactor (effective volume of 720 L) was operated over 250 days and the permeate flux stabilized at 18.6 ± 1.4 L/m2h at a hydrostatic pressure of 40 mbar. This flux was higher than those in the lab-scale GDM reactor (16.3 ± 0.2 L/m2h; effective volume of 8.4 L) and in the filtration cell system (2.7 ± 0.6 L/m2h; feed side volume of 0.0046 L) when the same flat sheet membrane was used. Interestingly, when the filtration cell was submerged into the GDM reactor, the flux (17.2 L/m2h) was comparable to the submerged membrane module. Analysis of cake layer morphology and foulant properties indicated that a thicker but more porous cake layer with less accumulation of organic substances (biopolymers and humics) contributed to the improved permeate flux. This phenomenon was possibly associated with longer residence time of organic substances and sufficient space for the growth, predation, and movement of the eukaryotes in the GDM reactor. In addition, the permeate flux of the submerged hollow fibre membrane increased with decreasing packing density. It is thought that the movement of large-sized eukaryotes could be limited when the space between hollow fibres was reduced. In terms of pretreatment, the GDM systems effectively removed turbidity, viable cells, and transparent exopolymer particles from the feed seawater. Importantly, extending the reactor operation time produced a permeate with less assimilable organic carbon and biopolymers. Thus, the superior quality of the GDM permeate has the potential to alleviate subsequent reverse osmosis membrane fouling for seawater treatment.

Early testing of new sanitation technology for urban slums: The case of the Blue Diversion Toilet.

The toilets used most in informal urban settlements have detrimental consequences for the environment and human health due to the lack of proper collection and treatment of toilet waste. Concepts for safe, sustainable and affordable sanitation systems exist, but their feasibility and acceptance have to be investigated at an early stage of development, which is difficult due to the high costs of building working models. In this paper, we present an approach to estimate acceptance in a valid and representative form with only one working model, and apply it to test an innovative zero-emission toilet with recycling of wash water. Four basic principles were specified for investigation and nine hypotheses formulated to test the feasibility and acceptance of these principles: source separation of urine and feces with subsequent collection for resource recovery; provision of wash water in a separate cycle with on-site recovery through a membrane bioreactor; a convenient and attractive overall design; and a financially sustainable business plan. In Kampala (Uganda), in 2013, data was collected from 22 regular users, 308 one-time users and a representative sample of 1538 participants. Qualitative data was collected from the users, who evaluated their likes, perceived benefits, social norms and expected ease of use based on verbal and visual information. Most of the hypotheses were confirmed, indicating the feasibility and acceptance of the basic principles. Source separation and on-site water recovery were found to be feasible and accepted, provided users can be convinced that the emptying service and water recovery process work reliably. In the survey, the toilet was evaluated favorably and 51% of the participants agreed to be placed on a bogus waiting list. However, some design challenges were revealed, such as the size of the toilet, hiding feces from view and improving the separation of urine and water.

Optimization of gravity-driven membrane (GDM) filtration process for seawater pretreatment.

Seawater pretreatment by gravity-driven membrane (GDM) filtration at 40 mbar has been investigated. In this system, a beneficial biofilm develops on the membrane that helps to stabilize flux. The effects of membrane type, prefiltration and system configuration on stable flux, biofilm layer properties and dissolved carbon removal were studied. The results show that the use of flat sheet PVDF membranes with pore sizes of 0.22 and 0.45 µm in GDM filtration achieved higher stabilized permeate fluxes (7.3-8.4 L/m(2)h) than that of flat sheet PES 100 kD membranes and hollow fibre PVDF 0.1 µm membranes. Pore constriction and cake filtration were identified as major membrane fouling mechanisms, but their relative contributions varied with filtration time for the various membranes. Compared to raw seawater, prefiltering of seawater with meshes at sizes of 10, 100 and 1000 µm decreased the permeate flux, which was attributed to removal of beneficial eukaryotic populations. Optical coherence tomography (OCT) showed that the porosity of the biofouling layer was more significantly related with permeate flux development rather than its thickness and roughness. To increase the contact time between the biofilm and the dissolved organics, a hybrid biofilm-submerged GDM reactor was evaluated, which displayed significantly higher permeate fluxes than the submerged GDM reactor. Although integrating the biofilm reactor with the membrane system displayed better permeate quality than the GDM filtration cells, it could not effectively reduce dissolved organic substances in the seawater. This may be attributed to the decomposition/degradation of solid organic substances in the feed and carbon fixation by the biofilm. Further studies of the dynamic carbon balance are required.

Biological control of biofilms on membranes by metazoans.

Traditionally, chemical and physical methods have been used to control biofouling on membranes by inactivating and removing the biofouling layer. Alternatively, the permeability can be increased using biological methods while accepting the presence of the biofouling layer. We have investigated two different types of metazoans for this purpose, the oligochaete Aelosoma hemprichi and the nematode Plectus aquatilis. The addition of these grazing metazoans in biofilm-controlled membrane systems resulted in a flux increase of 50% in presence of the oligochaetes (Aelosoma hemprichi), and a flux increase of 119-164% in presence of the nematodes (Plectus aquatilis) in comparison to the control system operated without metazoans. The change in flux resulted from (1) a change in the biofilm structure, from a homogeneous, cake-like biofilm to a more heterogeneous, porous structure and (2) a significant reduction in the thickness of the basal layer. Pyrosequencing data showed that due to the addition of the predators, also the community composition of the biofilm in terms of protists and bacteria was strongly affected. The results have implications for a range of membrane processes, including ultrafiltration for potable water production, membrane bioreactors and reverse osmosis.

Gravity-driven membrane filtration as pretreatment for seawater reverse osmosis: linking biofouling layer morphology with flux stabilization.

In this study gravity-driven membrane (GDM) ultrafiltration is investigated for the pretreatment of seawater before reverse osmosis (RO). The impacts of temperature (21 ± 1 and 29 ± 1 °C) and hydrostatic pressure (40 and 100 mbar) on dynamic flux development and biofouling layer structure were studied. The data suggested pore constriction fouling was predominant at the early stage of filtration, during which the hydrostatic pressure and temperature had negligible effects on permeate flux. With extended filtration time, cake layer fouling played a major role, during which higher hydrostatic pressure and temperature improved permeate flux. The permeate flux stabilized in a range of 3.6 L/m(2) h (21 ± 1 °C, 40 mbar) to 7.3 L/m(2) h (29 ± 1 °C, 100 mbar) after slight fluctuations and remained constant for the duration of the experiments (almost 3 months). An increase in biofouling layer thickness and a variable biofouling layer structure were observed over time by optical coherence tomography and confocal laser scanning microscopy. The presence of eukaryotic organisms in the biofouling layer was observed by light microscopy and the microbial community structure of the biofouling layer was analyzed by sequences of 16S rRNA genes. The magnitude of permeate flux was associated with the combined effect of the biofouling layer thickness and structure. Changes in the biofouling layer structure were attributed to (1) the movement and predation behaviour of the eukaryotic organisms which increased the heterogeneous nature of the biofouling layer; (2) the bacterial debris generated by eukaryotic predation activity which reduced porosity; (3) significant shifts of the dominant bacterial species over time that may have influenced the biofouling layer structure. As expected, most of the particles and colloids in the feed seawater were removed by the GDM process, which led to a lower RO fouling potential. However, the dissolved organic carbon in the permeate was not be reduced, possibly because some microbial species (e.g. algae) could convert CO2 into organic substances. To further improve the removal efficiency of the organic carbon, combining carrier biofilm processes with a submerged GDM filtration system is proposed.

Activity of metazoa governs biofilm structure formation and enhances permeate flux during Gravity-Driven Membrane (GDM) filtration.

The impact of different feed waters in terms of eukaryotic populations and organic carbon content on the biofilm structure formation and permeate flux during Gravity-Driven Membrane (GDM) filtration was investigated in this study. GDM filtration was performed at ultra-low pressure (65 mbar) in dead-end mode without control of the biofilm formation. Different feed waters were tested (River water, pre-treated river water, lake water, and tap water) and varied with regard to their organic substrate content and their predator community. River water was manipulated either by chemically inhibiting all eukaryotes or by filtering out macrozoobenthos (metazoan organisms). The structure of the biofilm was characterized at the meso- and micro-scale using Optical Coherence Tomography (OCT) and Confocal Laser Scanning Microscopy (CLSM), respectively. Based on Total Organic Carbon (TOC) measurements, the river waters provided the highest potential for bacterial growth whereas tap water had the lowest. An increasing content in soluble and particulate organic substrate resulted in increasing biofilm accumulation on membrane surface. However, enhanced biofilm accumulation did not result in lower flux values and permeate flux was mainly influenced by the structure of the biofilm. Metazoan organisms (in particular nematodes and oligochaetes) built-up protective habitats, which resulted in the formation of open and spatially heterogeneous biofilms composed of biomass patches. In the absence of predation by metazoan organisms, a flat and compact biofilm developed. It is concluded that the activity of metazoan organisms in natural river water and its impact on biofilm structure balances the detrimental effect of a high biofilm accumulation, thus allowing for a broader application of GDM filtration. Finally, our results suggest that for surface waters with high particulate organic carbon (POC) content, the use of worms is suitable to enhance POC removal before ultrafiltration units.

Intermittent operation of ultra-low pressure ultrafiltration for decentralized drinking water treatment.

River water was treated by ultrafiltration at a relatively low transmembrane pressure (40 mbar). As observed before, flux stabilization occurred after several days of operation although no back-flushing or cross flow was applied. Interruptions in flux were applied by temporary offset of the transmembrane pressure. After restoration of the transmembrane pressure, the initial flux was higher than the stable flux level, and the flux recovery depended on the standstill time. Furthermore, if a short cross flow was applied after standstill, the flux was restored to an even higher level. In all cases, the flux decreased again during operation to reach finally the same stable level as before standstill. In order to evaluate the influence of intermittent operation as practiced for water treatment on a household level, daily interruptions of flux were applied. An optimum of total daily water production rate was obtained at 21 h of operation and 3 h of standstill per day. A model was developed which can describe the impact of intermittent operation on the flux depending on the duration of the standstill and operating periods. This enables the prediction of production capacity of the system operated intermittently. The flux increase during standstill could be explained by a relaxation and expansion of the biofouling layer, while the higher flux after forward-flushing was caused by this layer being partially sloughed off. Household water treatment with the process presented here will generally be operated on a discontinuous basis. The results show that such operation schemes do not compromise the permeability of the system, but actually lead to higher fluxes after standstill.

Predation influences the structure of biofilm developed on ultrafiltration membranes.

This study investigates the impact of predation by eukaryotes on the development of specific biofilm structures in gravity-driven dead-end ultrafiltration systems. Filtration systems were operated under ultra-low pressure conditions (65 mbar) without the control of biofilm formation. Three different levels of predation were evaluated: (1) inhibition of eukaryotic organisms, (2) addition of cultured protozoa (Tetrahymena pyriformis), and (3) no modification of microbial community as a control. The system performance was evaluated based on permeate flux and structures of the biofilm. It was found that predation had a significant influence on both the total amount and also the structure of the biofilm. An open and heterogeneous structure developed in systems with predation whereas a flat, compact, and thick structure that homogeneously covered the membrane surface developed in absence of predation. Permeate flux was correlated with the structure of the biofilm with increased fluxes for smaller membrane coverage. Permeate fluxes in the presence or absence of the predators was 10 and 5 L m(-2) h(-1), respectively. It was concluded that eukaryotic predation is a key factor influencing the performance of gravity-driven ultrafiltration systems.

Nanofiltration and nanostructured membranes--should they be considered nanotechnology or not?

Nanofiltration is frequently associated with nanotechnology - obviously because of its name. However, the term "nano" in nanofiltration refers - according to the definition of the International Union of Pure and Applied Chemistry (IUPAC) - to the size of the particles rejected and not to a nanostructure as defined by the International Organisation of Standardisation (ISO) in the membrane. Evidently, the approach to standardisation of materials differs significantly between membrane technology and nanotechnology which leads to considerable confusion and inconsistent use of the terminology. There are membranes that can be unambiguously attributed to both membrane technology and nanotechnology such as those that are functionalized with nanoparticles, while the classification of hitherto considered to be conventional membranes as nanostructured material is questionable. A driving force behind the efforts to define nanomaterials is not least the urgent need for the regulation of the use of nanomaterials. Since risk estimation is the basis for nanotechnology legislation, the risk associated with nanomaterials should also be reflected in the underlying standards and definitions. This paper discusses the impacts of the recent attempts to define nanomaterials on membrane terminology in the light of risk estimations and the need for regulation.

Characterisation of aquatic humic and non-humic matter with size-exclusion chromatography--organic carbon detection--organic nitrogen detection (LC-OCD-OND).

Size-exclusion chromatography in combination with organic carbon detection (SEC-OCD) is an established method to separate the pool of NOM into major fractions of different sizes and chemical functions and to quantify these on the basis of organic carbon. One specific approach, also known as LC-OCD-OND, is based on the Gräntzel thin-film UV-reactor. This approach is described with recent improvements in fraction assignation (humic substances, biopolymers, building blocks, low molecular weight organic acids and neutrals, hydrophobic organic carbon), the coupling of a novel organic nitrogen detector (OND), and an improved diagram for the characterisation of aquatic humic substances (HS-diagram). The diagram replaces the operational distinction between humic and fulvic acids by a continuum ranging from aquagenic fulvic acids to pedogenic humic acids.

Decision support in urban water management based on generic scenarios: the example of NoMix technology.

Urine source separation (NoMix technology) followed by processing the concentrated nutrient solution has the potential to become a cost-efficient alternative to conventional end-of-pipe nutrient elimination. A choice of processing technologies can only be made for specific scenarios, and there is currently no methodology for analyzing generic situations. In setting up a formalized decision-support methodology (based on STEEPLED analysis), we discuss how to create such generic scenarios, how to couple them with process engineering objectives, how to define the technology requirements, and finally how to produce a realistic subset of technology alternatives. The methodology is tested in five real scenarios. We also touch on the criteria for a final choice of technology taking into account large uncertainties about the performance of real technologies. We conclude that technology development is one of the most important requirements for implementing urine source separation in practice. There is an urgent need to develop cost-efficient processing technologies that satisfy the requirements of stakeholders.

Stabilization of flux during dead-end ultra-low pressure ultrafiltration.

Gravity driven ultrafiltration was operated in dead-end mode without any flushing or cleaning. In contrary to general expectations, the flux value stabilized after about one week of operation and remained constant during an extended period of time (several months). Different surface water types and diluted wastewater were used as feed water and, depending on the feed water composition, stable flux values were in the range of 4-10 L h(-1) m(-2). When sodium azide was added to the feed water to diminish the biological activity, no stabilization of flux occurred, indicating that biological processes play an important role in the flux stabilization process. Confocal laser scanning microscopy revealed the presence of a biofouling layer, of which the structure changed over time, leading to relatively heterogeneous structures. It is assumed that the stabilization of flux is related to the development of heterogeneous structures in the fouling layer, due to biological processes in the layer. The phenomenon of flux stabilization opens interesting possibilities for application, for instance in simple and low-cost ultrafiltration systems for decentralized drinking water treatment in developing and transition countries, independent of energy supply, chemicals, or complex process control.

Decentralized systems for potable water and the potential of membrane technology.

Decentralized drinking-water systems are an important element in the process of reaching the Millennium Development Goals, as centralized systems are often deficient or non-existent in developing and transition countries (DC and TC). Most water-quality problems are due to hygiene factors and pathogens. A range of decentralized systems is available to counter these problems, including thermal and/or UV methods, physical removal and chemical treatment. This review focuses on decentralized systems that treat the potable water (drinking and cooking) of a single household (point-of-use systems) or a community (small-scale systems). For application in DC and TC, important boundary conditions for decentralized systems include low costs, ease of use, sustainability, low maintenance and independence of utilities (energy sources). Although some low-cost systems are available, their application is limited by time-consuming daily operation and maintenance. Other systems are too expensive for the poor populations of DC and TC and in most cases do not fulfill the system requirements described above. Point-of-use systems based on membranes are commercially available and are designed to operate on tap pressure or gravity. Membrane systems are attractive since they provide an absolute barrier for pathogens and remove turbidity, thus increasing the palatability of the water. The costs of membrane have decreased rapidly during the last decades and therefore membrane systems have also become within reach for application in low-cost applications in DC and TC. Some membrane systems rely on gravity as a driving force, thereby avoiding the use of pumps and electricity. On the basis of the present literature data, no small-scale systems could be identified which meet all the requirements for successful implementation. Furthermore, in the available literature the performance of highly fouling water types has not been reported. For such cases, more extensive studies are required and a need for suitable pre-treatment was identified. It can be concluded that there are good prospects for decentralized systems based on membranes, but that a need exists for research and development of systems with low costs and low maintenance, specifically designed for DC and TC.