The study aims at assessing in long-term trials a gravity-driven ultrafiltration pilot plant designed for a capacity of 5 m3/d. The unit was operated in South Africa with Ogunjini surface water and was run with restricted chemical intervention or maintenance (no backflush, no aeration, no crossflow and no chemical). Under South African environmental conditions and with direct filtration of the river water and only one manual drainage of the membrane reactor every weekday, the unit could fulfil the design specification in terms of water production (5 m3/d) as long as the turbidity of the raw water remained in a reasonable level (up to 160 NTU), with a filtration flux typically 4 to 6 L/h.m² (corrected at 20°C). This value was in the same range as the lab results and was consistent with the first phase results (around 5-7 L/h.m² after biosand filtration). However, the flux dropped significantly to a range of 2 to 4 L/h.m² after a rain event resulting in a turbidity peak over several days up to > 600 NTU. This demonstrated that for variable raw water types with expected turbidity peaks above 100 NTU, a pre-treatment would be required for the system (biosand filter or other). The performance of microbiological tests confirmed the integrity of the membrane and the ability of the system to achieve advanced disinfection.

The study aims at assessing in long-term trials a gravity-driven ultrafiltration pilot plant designed for a capacity of 5 m3/d. The unit was operated in South Africa with Ogunjini surface water and was run with restricted chemical intervention or maintenance (no backflush, no aeration, no crossflow and no chemical). Under South African environmental conditions and with direct filtration of the river water and only one manual drainage of the membrane reactor every weekday, the unit could fulfil the design specification in terms of water production (5 m3/d) as long as the turbidity of the raw water remained in a reasonable level (up to 160 NTU), with a filtration flux typically 4 to 6 L/h.m² (corrected at 20°C). This value was in the same range as the lab results and was consistent with the first phase results (around 5-7 L/h.m² after biosand filtration). However, the flux dropped significantly to a range of 2 to 4 L/h.m² after a rain event resulting in a turbidity peak over several days up to > 600 NTU. This demonstrated that for variable raw water types with expected turbidity peaks above 100 NTU, a pre-treatment would be required for the system (biosand filter or other). The performance of microbiological tests confirmed the integrity of the membrane and the ability of the system to achieve complete disinfection.

Access to microbiologically and chemically safe water is limited not only in developing countries, but also in transition countries and even in remote areas of developed countries. For these cases, decentralized water supply concepts such as point-of-use (POU), point-of-entry (POE) or small-scale system (SSS) technologies can be promising alternatives to centralized treatment concepts. Membrane-based treatment systems have gained importance for drinking water treatment in the developed countries. In principle, application of membrane technology is attractive also for the transition and developing countries, because it provides absolute barriers for control of hygienic hazards (Ultrafiltration (UF)) and because the modular construction enables implementation on each possible scale size. However membrane technology is still not affordable for the poorest part of the world population. The sustainable application of POU membrane system presumes that system should be operated without or with limited addition of chemicals, with limited possibility of regular backflushing and with low pressure, presumably hydrostatic. On the other hand, while the water needs for drinking and cooking for a family of four people constitute approx. 20 l/day, operation of POU UF system under low flux conditions is possible. One of the most important limitations for application of ultrafiltration in simple household devices, is membrane fouling. In order to overcome the reasons of the limited application of UF in POU systems, the better understanding of the UF process in these specific conditions and specially membrane fouling is needed. Recent studies have shown that dissolved or colloidal polysaccharides and proteins and their interactions with the membrane and between macromolecules might have more severe impact. During long term dead-end filtration, accumulation of the macromolecules on the membrane surface and increase of their concentration is severe. The interactions between those macromolecules in the conditions of high concentrations in the boundary layer affect the structure of the layer and its permeability. However, in most of the studies, only the foulant-membrane interactions are considered like relevant for reversibility of fouling. The foulant-foulant interactions in the boundary layer have been studied only superficially. Therefore, we systematically investigated the impact of polysaccharide and solution properties on UF membrane fouling in conditions of low flux and limited backflushing, under constant TMP conditions (hydrostatic pressure of 120 mbar - 150 mbar. Our experimental results lead us to the following conclusions: Regarding the initial stage of flux decline (0-80 ml permeate/cm2) the polysaccharide structure, and particularly availability of carboxyl groups, has a major impact on the membrane fouling, while the molecular weights of polysaccharides does not play a significant role (in the studied range of Mw 5-250 kDa). Such solution conditions as presence of metal ions and ionic strength are also detrimental for the fouling, while both metal ions and ionic strength have impact on the gel structure and properties, generally stabilizing it, and increasing the possibility of water trapping by hydrogen bonding, which leads to the higher permeability. However, independently of the initial solution conditions, after approx. 80 ml has been filtered through 1 cm2 of the membrane, flux becomes stable on the level of approx. 10 L/(hm2) over the whole period of operation (several weeks in some cases). We suppose that the gel layer formed by polysaccharides play a role of a “second” membrane on the surface of the PES UF membrane, keeping remaining permeability on the certain level, determined by the water retention properties of the gel structure. Regarding practical application, the obtained results open a new direction for the ultrafiltration in specific conditions of household systems. The long term ultrafiltration should be studied on natural waters to prove the flux stabilization phenomenon. This phenomenon may give a possibility to produce up to 10 L/h of water from 1 m2 of the membrane applying only 120 mbar of hydrostatic pressure (1.2 m water level difference) without backflushing or crossflow, which may simplify the design and maintenance of the system and significantly reduce its costs. Next activities in Techneau project will include the further evaluation of the long term ultrafiltration on natural waters; characterization of the impact of biofouling on the flux decline; and evaluation of the operational parameters of the Point-of-use system, based on the proposed above concept to treat at least 20 L/day.

Access to microbiologically and chemically safe water is limited not only in developing countries, but also in transition Countries and even in remote areas of some developed countries. For these cases, point-of-use (POU) technologies can be promising alternatives to centralized treatment concepts. Membrane-based treatment systems have gained importance for drinking water treatment in the developed countries and can be considered as the dominant technology for new applications at present. Due to the high retention of pathogens and the possibility of downscaling (modular construction) membrane technology seems to be attractive also for application as POU system in developing and transition countries. However, no scientific publications on such systems are available and application is limited. Therefore we conducted an extensive literature and state-of-the art review to evaluate relevance, current use and the research and development needs of membrane-based POU systems in developing and transition countries. POU technologies are widely being used to produce safe and high quality drinking water in rural areas of industrialized countries, where access to centralized supply is not available, or for additional treatment of tap water. However, the cost level of POU systems applied in industrialized countries is in general not acceptable in other cases. Therefore simple low cost systems were developed and applied in developing and transition countries. In a range of case studies, described in literature, these systems show themselves as an appropriate short term solution, but often fail to provide improved access to necessary amounts of safe water. Economical growth of developing and transition countries leads to increasing public concern, affordability and requires long term sustainable solutions of the drinking water problem. Membrane-based POU/POE systems are especially attractive for application in developing and transition countries while they can provide high removal of bacteria, protozoa and viruses, have modular design and can be operated with a range of different energy sources, including mechanical and hydrodynamic energy. But, for their application in developing and transition areas, the cost level is in general not acceptable. Furthermore, the source water quality is often very low and can differ regionally as well as seasonally, and the POU/POE systems should be able to treat this kind of waters. Another critical factor in transition and especially in developing countries is the maintenance and control. Not only the level of education of the local population may be insufficient, but also structural financial means for maintenance and control may be lacking.