Performance characteristics of bio-ultrafiltration on local surface waters

Abstract

Access to safe drinking water supply is still a major problem especially in remote rural areas of developing countries. These communities rely solely on untreated surface and ground waters for survival due to the lack of financial resources to provide access to piped water. The consumption of this water in turn makes them easily susceptible to water related diseases. Hence, there is a need for an interim solution while the government is still sourcing funds for the distribution of water to these communities. Membrane filtration is a promising technology for the treatment of surface water as it does not alter the taste or smell of the end product. The main limitation for the implementation of membrane technology in rural areas is still energy demand, fouling and the skills required for membrane cleaning. Biological ultrafiltration is an emerging technology that produces water of high quality in terms of turbidity, organics and bacteria removal. The technology has been evaluated using a gravity driven dead-end mode on European waters and it offered acceptable stabilisation of fluxes for extended periods without any chemical cleaning or backwashing. This is a promising technology which can be implemented to act as an interim solution for the treatment of surface water in remote rural areas prior to consumption. This study concerns the evaluation of a biological ultrafiltration membrane system on local three South African rivers, namely, Tugela River, Umbilo River and Umgeni River. A laboratory systems comprising of a feed tank and six membrane modules connected in parallel was set up to assess the performance of a bio-UF membrane on a range of surface waters. The performance was assessed on the system’s ability to produce stable fluxes from the three rivers, the system ability to produce water with acceptable quality in terms of SANS 241:2011 for turbidity, TOC, total coliforms and E-coli. The membranes were initial cleaned and the flux rates for ultra-pure water were determined for each membrane prior to being exposed to raw water. Raw water samples were collected from three rivers with varying turbidity, total coliforms and organics. The concentrations of these contaminants were tested prior to running the raw water through the system. Thereafter, permeate was collected with time and its quality was evaluated in terms of turbidity, TOC and coliforms. The impacts of algae on flux stabilisation were evaluated by allowing the bio-UF system to run for a minimum of 3 months with and without algae growth. The system was found to be able to produce water that is compliant with the SANS 241:2011 standard in terms of turbidity, total coliforms, E-coli and TOC concentration. The system was also found to be unable to produce stable fluxes for all three rivers. The observed responses were noted to be similar to normal dead-end response, however, a slow declining flux rates was observed for Umgeni River. The presence of algae during the operation was a bio-UF membrane system was noted to further decrease the rate of flux decline. There appears to be a correlation between the raw water quality and the rate of flux decline. A further investigation was carried out aimed at assessing the relationship between the concentration of bacterial counts, TOC and turbidity. From the obtained results, it was noted that feed water with low turbidity (≤ 5 NTU), high bacterial count (≥30 000) and high total organic carbon (≥70 mg/L) is able to reduce the rate of flux decline. Hence, it can be concluded that a dead-end gravity driven Bio-UF membrane system can be used for the treatment of surface water in remote where the most main contaminants are from natural organic matter, micro-organisms and turbidity. Furthermore, it is able to produce slower declining flux rates which will increase the filter run time. It is recommended that the impacts of algae, type of bacteria and organics that enable slow decline in flux rates during the operation of Bio-UF should be investigated in order to identify means of enhancing the flux rates. Microfiltration membranes are available on the local markets hence it is also recommended that the performance of Bio-UF should be evaluated in comparison to Bio-MF.