Isolation and characterization of Sphingomonadaceae from fouled membranes.

Membrane filtration systems are widely applied for the production of clean drinking water. However, the accumulation of particles on synthetic membranes leads to fouling. Biological fouling (i.e., biofouling) of reverse osmosis and nanofiltration membranes is difficult to control by existing cleaning procedures. Improved strategies are therefore needed. The bacterial diversity on fouled membranes has been studied, especially to identify bacteria with specialized functions and to develop targeted approaches against these microbes. Previous studies have shown that Sphingomonadaceae are initial membrane colonizers that remain dominant while the biofilm develops. Here, we characterized 21 Sphingomonadaceae isolates, obtained from six different fouled membranes, to determine which physiological traits could contribute to colonization of membrane surfaces. Their growth conditions ranged from temperatures between 8 and 42 oC, salinity between 0.0 and 5.0% w/v NaCl, pH from 4 and 10, and all isolates were able to metabolize a wide range of substrates. The results presented here show that Sphingomonadaceae membrane isolates share many features that are uncommon for other members of the Sphingomonadaceae family: all membrane isolates are motile and their tolerance for different temperatures, salt concentrations, and pH is high. Although relative abundance is an indicator of fitness for a whole group, for the Sphingomonadaceae it does not reveal the specific physiological traits that are required for membrane colonization. This study, therefore, adds to more fundamental insights in membrane biofouling.

Lachnotalea glycerini gen. nov., sp. nov., an anaerobe isolated from a nanofiltration unit treating anoxic groundwater.

A strictly anaerobic bacterium, strain DLD10T, was isolated from a biofilm that developed on a nanofiltration membrane treating anoxic groundwater using glycerol as substrate. Cells were straight to slightly curved rods 0.2-0.5 µm in diameter and 1-3 µm in length, non-motile and non-spore-forming. The optimum temperature and pH for growth were 30 °C and pH 7.0. Strain DLD10T was able to grow in the presence of 0.03-4.5 % (w/v) NaCl. Substrates utilized by strain DLD10T included glycerol and various carbohydrates (glucose, sucrose, fructose, mannose, arabinose, pectin, starch, xylan), which were mainly converted to ethanol, acetate, H2 and formate. Thiosulphate, sulphur and Fe(III) were used as electron acceptors, but sulphate, fumarate and nitrate were not. The predominant membrane fatty acids were C16 : 0, iso-C17 : 1 and C17 : 1ω8c. The DNA G+C content was 36.4 mol%. Strain DLD10T belongs to the family Lachnospiraceae and is distantly related to Clostridium populeti DSM 5832T, Hespellia porcina DSM 15481T and Robinsoniella peoriensis CCUG 48729T (93 % 16S rRNA gene sequence similarity). Physiological characteristics and phylogenetic analysis indicated that strain DLD10T is a representative of a novel species of a new genus, for which the name Lachnotalea glycerini gen. nov., sp. nov. is proposed. The type strain of Lachnotalea glycerini is DLD10T ( = DSM 28816T = JCM 30818T).

Effect of temperature shocks on membrane fouling in membrane bioreactors.

Temperature is known to influence the biological performance of conventional activated sludge systems. In membrane bioreactors (MBRs), temperature not only affects the bioconversion process but is also shown to have an effect on the membrane performance. Four phenomena are generally reported to explain the higher resistance for membrane filtration found at lower temperatures: (1) increased mixed liquor viscosity, reducing the shear stress generated by coarse bubbles, (2) intensified deflocculation, reducing biomass floc size and releasing EPS into the mixed liquor, (3) lower backtransport velocity and (4) reduced biodegradation of COD. Although the higher resistance at low temperatures has been reported in several papers, the relation with supernatant composition has not been investigated before. In this paper, the composition of the soluble fraction of the mixed liquor is related to membrane performance after exposing the sludge to temperature shocks. Flux step experiments were performed in an experimental system at 7, 15, and 25° Celsius with sludge that was continuously recirculated from a pilot-scale MBR. After correcting the permeate viscosity for temperature, higher membrane fouling rates were obtained for the lower temperature in combination with low fouling reversibility. The soluble fraction of the MBR mixed liquor was analysed for polysaccharides, proteins and submicron particle size distribution. At low temperature, a high polysaccharide concentration was found in the experimental system as compared to the MBR pilot. Upon decreasing the temperature of the mixed liquor, a shift was found in particle size towards smaller particles. These results show that the release of polysaccharides and/or submicron particles from sludge flocs could explain the increased membrane fouling at low temperatures.

Feasibility of bioflocculation in a high-loaded membrane bioreactor for improved energy recovery from sewage.

The feasibility of a high-loaded membrane bioreactor to improve methane recovery from sewage was investigated. Although the process needs further optimization, it already is feasible to recover at least 35% of the sewage COD. Important aspects for further research are the occurrence of membrane fouling, and the optimum process conditions for bioflocculation, i.e. the proper SRT/HRT ratio, dissolved oxygen concentration and shear and overall energy production and consumption under optimised conditions.

Low dose powdered activated carbon addition at high sludge retention times to reduce fouling in membrane bioreactors.

The addition of a low concentration of PAC (0.5gL(-1) of sludge, i.e. a dose of 4mgL(-1) of wastewater), in combination with a relatively long SRT (50 days), to improve membrane filtration performance was investigated in two pilot-scale MBRs treating real municipal wastewater. Continuous filterability tests at high flux showed the possibility to run for 18h at 72Lm(-2)h(-1) and 180h at 50Lm(-2)h(-1), while significant fouling occurred without PAC. In addition, measurements of the critical flux showed an increase of 10% for this strategy. Low dosage and high retention time makes it feasible and cost effective. Further advantages with regard to permeate quality and possible micropollutants removal are currently under investigation.

Biological black water treatment combined with membrane separation.

Separate treatment of black (toilet) water offers the possibility to recover energy and nutrients. In this study three combinations of biological treatment and membrane filtration were compared for their biological and membrane performance and nutrient conservation: a UASB followed by effluent membrane filtration, an anaerobic MBR and an aerobic MBR. Methane production in the anaerobic systems was lower than expected. Sludge production was highest in the aerobic MBR, followed by the anaerobic MBR and the UASB-membrane system. The level of nutrient conservation in the effluent was high in all three treatment systems, which is beneficial for their recovery from the effluent. Membrane treatment guaranteed an effluent which is free of suspended and colloidal matter. However, the concentration of soluble COD in the effluent still was relatively high and this may seriously hamper subsequent nutrient recovery by physical-chemical processes. The membrane filtration behaviour of the three systems was very different, and seemed to be dominated by the concentration of colloidals in the membrane feed. In general, membrane fouling was the lowest in the aerobic MBR, followed by the membranes used for UASB effluent filtration and the anaerobic MBR.

Nutrient removal by NF and RO membranes in a decentralized sanitation system.

Decentralized treatment of domestic wastewater offers the possibility of water and nutrient reuse. In a decentralized sanitation system the household wastewater streams are separated in a large diluted stream (gray water) and a small and concentrated stream (black water) containing important nutrients like ammonium and phosphate. Nanofiltration (NF) and reverse osmosis (RO) membranes might be used to recover the nutrients from anaerobically treated black water. The permeate might be used in a water reuse scheme. In case of water reuse the produced permeate should meet guidelines for potable water or meet new guidelines which might be applied in the future for intermediate quality of water, for example toilet flushwater; when this is not possible the permeate should meet guidelines for discharge. The most stringent guidelines apply for ammonium and phosphate. The focus of this paper is to test commercially available NF and RO membranes to remove nutrients from anaerobically treated black water in order to meet the Dutch guidelines. A large number of commercial tubular, capillary and flat sheet NF and RO membranes was tested on laboratory scale on their performance to meet the Dutch guidelines for ammonium and phosphate. The ammonium and phosphate concentrations used were based on the effluent composition of anaerobically treated black water. Ammonium and phosphate rejection were both measured in synthetic single salt and multi-ion mixtures and in anaerobic effluent. The rejection for ammonium (30-95%) is neither sufficient for discharge nor potable water use. The rejection of phosphate (74-99%) is in most cases almost sufficient to meet the standards for potable water.