Matthias Kraume

Recent advances in membrane bioreactors (MBRs): Membrane fouling and membrane material

Membrane bioreactors (MBRs) have been actively employed for municipal and industrial wastewater treatments. So far, membrane fouling and the high cost of membranes are main obstacles for wider application of MBRs. Over the past few years, considerable investigations have been performed to understand MBR fouling in detail and to develop high-flux or low-cost membranes. This review attempted to address the recent and current developments in MBRs on the basis of reported literature in order to provide more detailed information about MBRs. In this paper, the fouling behaviour, fouling factors and fouling control strategies were discussed. Recent developments in membrane materials including low-cost filters, membrane modification and dynamic membranes were also reviewed. Lastly, the future trends in membrane fouling research and membrane material development in the coming years were addressed.

Filterability of activated sludge in membrane bioreactors

The filterability of activated sludge is an important factor for the economical operation of membrane bioreactors (MBR). In the literature mainly investigations on sludge dewaterability in respect to further disposal are published. In this study, a procedure for determining filterability in a crossflow test cell is introduced. Its features are: no increase in sludge concentration during batch trials, crossflow conditions, and little impact on the sludge structure. The activated sludge filterability is given as the ratio of permeate flux after 40 min of operation to clear water flux. Sludge samples of eight different MBR and one conventional wastewater treatment plant (wwtp) have been examined and compared. Contrary to the literature, no impact of suspended solids (SS), concentration, sludge viscosity or extractable extracellular polymer substances (EPS) concentration on the filterability was found. Instead, the composition of the liquid phase was found to effect most the filterability of activated sludge, a major influence being the concentration of suspended EPS: the higher the suspended EPS concentration, the lower the filtration index. Suspended EPS concentration increases with high mechanical stress in the MBR and high F/M ratios, if the treated wastewater contains considerable amounts of proteins or polysaccharides.

Performance of a bioreactor with submerged membranes for aerobic treatment of municipal waste water.

Aerobic treatment of municipal waste water in a membrane bioreactor was studied for 535 d. Apart from sampling, sludge was retained completely by a submerged hollow fibre membrane with a pore-size of 0.2 microm. The pilot plant comprised an anoxic zone to enable denitrification. The maximum liquid hold-up of the plant was 3.9 m3. In this study the reactor performance and the stability of the process and the membrane capacity were investigated. A stable flux of 181 m(-2)h(-1) could be realised with a mean transmembrane pressure difference of 0.3bar with air-bubbling and backflushing the membrane and cleaning it in place every two months for one or two hours. For about 140d, a flux of 271 m(-2)h(-1) was achieved, but cleaning became necessary more often. The hydraulic retention time (HRT) varied between 10.4 and 15.6h. Accordingly the volumetric loading rate was between 1.1 and 1.7kg CODm(-3)d(-1). No inoculum was used. The mixed liquor suspended solids (MLSS) concentration gradually increased to 18-20g MLSSl(-1). The feed to microorganism (F/M) ratio varied according to the operation conditions but decreased against a value of 0.07 kg COD kg(-1) MLSSd(-1). Treatment performance was very stable and on a high level. The COD was reduced by 95%. Nitrification was complete and up to 82% of the total nitrogen could be denitrified.

Microbiological aspects of a bioreactor with submerged membranes for aerobic treatment of municipal wastewater

An aerobic membrane bioreactor treating municipal wastewater at complete biomass retention was studied in respect of microbiological parameters over a period of 380 days. The results were compared to those obtained from a conventional activated sludge wastewater treatment plant (WWTP) treating the same wastewater. Microscopically, significant changes in the structure of the flocs and of the ratio between free suspended and aggregated cells could be observed. The presence of filamentous bacteria varied from almost not present to very high numbers. With the exception of short periods after changes in operating conditions, protozoa and metazoa were rarely present in the sludge community. The rate of oxygen consumption and the cell detectability by fluorescence in situ hybridizatio (FISH) with rRNA-targeted oligonucleotide probes were used to assess the physiological state of the bacterial cells Oxygen consumption rates of sludge samples obtained from both the conventional and membrane filtration plant wer determined without and after addition of different energy and carbon sources. In contrast to the conventional activate sludge, a pronounced increase in respiration activity upon the addition of organic substrates could be observed in th membrane filtration sludge. In situ probing with the Bacteria-specific probe EUB338 visualized 40-50% of all DAPI stainable bacteria in the membrane bioreactor, compared to 80% cells detectable by FISH in the conventional activate sludge. These results suggest that bacteria present in the highly concentrated biomass of the membrane reactor use the energy supplied for their maintenance metabolism and were not in a physiological state characteristic for growth This assumption could explain the zero net biomass production observed in the reactor.

Monitoring of transparent exopolymer particles (TEP) in a membrane bioreactor (MBR) and correlation with other fouling indicators

The occurrence of Transparent Exopolymer Particles (TEP), an acidic fraction of polysaccharides, was monitored for more than six months in the activated sludge of three MBR units, and the relationship between TEP and other fouling indicators was studied. These compounds consist mainly of exopolysaccharides of a sticky nature, a characteristic which makes them a group of interesting substances in processes like sedimentation, flocculation and membrane fouling. The relationship between capillary suction time (CST) and polysaccharides (PS) was linear for the three tested sludges, although the correlation with TEP concentrations was stronger. A slight linear correlation of both TEP and PS was found with the critical flux (CF) measured with a small filtration test cell, which was submerged in the membrane tank to assess the filterability performance of the sludge in situ. However, the correlation CF-PS was clearer. The relationship between TEP, polysaccharides and sludge filterability highlights the potential of this parameter for the monitoring of membrane systems.

The importance of fluid dynamics for MBR fouling mitigation

The importance of the multiphase fluid dynamics for fouling mitigation in MBR systems has been widely acknowledged with air sparging having been applied commercially for about 20 years. However, the effects of air scouring are still not fully understood since the transient orthogonal and parallel flows as well as turbulent eddies created by bubbling generate complex hydrodynamic flow fields in the vicinity of a membrane. There is no generally valid model that describes the relationship between fouling rate and fluid dynamics. So, a reliable and universally applicable model to optimize membrane module and tank geometries, air scouring and filtration cycles is still pending. In addition to providing a discussion on the importance of multiphase fluid dynamics for fouling control, this review aims at developing guidelines to choose appropriate experimental and numerical methods for fluid dynamics investigations in MBR systems.

Occurrence, source, and fate of dissolved organic matter (DOM) in a pilot-scale membrane bioreactor.

In this study, the fate of carbohydrates, proteins, and humic substances in feedwater, sludge supernatant, and permeate of a pilot-scale membrane bioreactor (MBR) was investigated. Over 10 months, carbohydrates were observed to have a lower bioelimination degree (45%) and higher rejection degree (79%) than those of proteins (81% and 44%, respectively), which led to a high carbohydrate/protein ratio of dissolved organic matter (DOM) in sludge supernatant. The batch tests showed that DOM derived from feedwater and bound extracellular polymeric substances (EPS) was eliminated by activated sludge via biosorption and biodegradation. The proteins in bound EPS and feedwater were also found to have much higher biosorption potential (27% and 31%, respectively) than humic substances (11% and 17%, respectively) and carbohydrates (16% and 14%, respectively), indicating that proteins had a high affinity with sludge flocs. The results also showed that carbohydrates and humic substances in bound EPS were more difficult to be eliminated by activated sludge. In addition, the batch tests confirmed that feedwater was mainly composed of readily biodegradable matter, and bound EPS was mainly composed of slowly biodegradable matter. Size exclusion chromatography with continuous organic carbon and UV(254) detection (LC-OCD) showed that large-size substances (i.e., carbohydrates and macromolecular proteins) in sludge supernatant might originate from bound EPS, which can be rejected by membranes. The LC-OCD analysis, together with the results obtained from batch tests, suggested bound EPS might be the most important source of DOM in the sludge suspension.

Characterization of the size-fractionated biomacromolecules: Tracking their role and fate in a membrane bioreactor

This article presents a study aimed at the fractionation and characterization of what is thought to be one of the most complex organic mixtures produced by activated sludge: biomacromolecules (BMM). Photometric quantification combined with excitation-emission matrix (EEM) fluorescence spectroscopy and nuclear magnetic resonance (NMR) measurements were used to characterize BMM in a membrane bioreactor (MBR) from a chemical perspective. Overall, the BMM in sludge supernatant were mainly present in three fractions: colloidal BMM (BMMc, >0.45 μm), biopolymeric BMM (BMMb, 0.45 μm-100 kDa) and low molecular weight (MW) fraction (<5 kDa). The analysis of fluorescence regional integration (FRI) showed that the organics in membrane permeate and those in the low-MW fraction of sludge supernatant were of similar chemical composition. The characterization by NMR suggested that the BMMc fraction had similar carbon content of proteins and polysaccharides. In contrast, the BMMb and the low-MW BMM were proved to be carbonaceous and aromatics, respectively. Moreover, because of the high MW and gelling property, polysaccharides were found to have a high potential to accumulate on the membranes. In addition, the lipids present in the BMMb of the sludge supernatant were demonstrated to be another important foulant due to their large size. Our results also indicated that aromatic proteins had a higher fouling propensity than tryptophan proteins though they were of similar size nature. This work could be useful for better understanding of the chemical nature of BMMs in MBRs.

Automated drop detection using image analysis for online particle size monitoring in multiphase systems

Abstract Image analysis has become a powerful tool for the work with particulate systems, occurring in chemical engineering. A major challenge is still the excessive manual work load which comes with such applications. Additionally manual quantification also generates bias by different observers, as shown in this study. Therefore a full automation of those systems is desirable. A MATLAB ® based image recognition algorithm has been implemented to automatically count and measure particles in multiphase systems. A given image series is pre-filtered to minimize misleading information. The subsequent particle recognition consists of three steps: pattern recognition by correlating the pre-filtered images with search patterns, pre-selection of plausible drops and the classification of these plausible drops by examining corresponding edges individually. The software employs a normalized cross correlation procedure algorithm. The program has reached hit rates of 95% with an error quotient under 1% and a detection rate of 250 particles per minute depending on the system.

Impacts of membrane flux enhancers on activated sludge respiration and nutrient removal in MBRs

This paper presents the findings of experimental investigations regarding the influence of 13 different flux enhancing chemicals (FeCl3, polyaluminium chloride, 2 chitosans, 5 synthetic polymers, 2 starches and 2 activated carbons) on respirometric characteristics and nitrification/denitrification performance of membrane bioreactor (MBR) mixed liquor. Flux enhancing chemicals are a promising method to reduce the detrimental effects of fouling phenomena via the modification of mixed liquor characteristics. However, potentially inhibiting effects of these chemicals on mixed liquor biological activity triggered the biokinetic studies (in jar tests) conducted in this work. The tested polyaluminium chloride (PACl) strongly impacted on nitrification (-16%) and denitrification rate (-43%). The biodegradable nature of chitosan was striking in endogenous and exogenous tests. Considering the relatively high costs of this chemical, an application for wastewater treatment does thus not seem to be advisable. Also, addition of one of the tested activated carbons strongly impacted on the oxygen uptake rate (-28%), nitrification (-90%) and denitrification rate (-43%), due to a decrease of pH. Results show that the changes in kLa values were mostly not significant, however, a decrease of 13% in oxygen transfer was found for sludge treated with PACl.