Final Activity Report Summary - BIOMEM (Interaction between biology and membranes in membrane bioreactors)
For more effective control of membrane fouling in membrane bioreactors (MBR), an understanding of the biological factors responsible for fouling and the microorganisms involved in biofilm development is important. The BIOMEM proposal aimed to:
- determine relevant biological characteristics of MBR mixed liquor;
- monitor and follow microorganisms responsible for biofilm fouling in MBRs;
- develop and test a method to evaluate the fouling potential of mixed liquors.
These were addressed through two parallel investigations, one at lab-scale and one at full-scale:
(1) Biological characteristics and fouling potential of mixed liquors from full-scale wastewater treatment systems: a comparison between a membrane bioreactor and a conventional process
The fouling-related biological properties of mixed liquor from a full-scale membrane bioreactor (MBR) for municipal wastewater treatment were monitored over the course of 13 months and compared with those of a parallel-running conventional activated sludge (CAS) system at the wastewater treatment plant of Schilde, Belgium. This plant is operated already for more than four years by Aquafin. Measurement of specific respiration and nitrification rates indicated lower activities per unit of biomass in the MBR. Despite the less active biomass, concentrations of soluble microbial products (SMP) and major constituents of extracellular polymeric substance (EPS) in the mixed liquor supernatant of the MBR were consistently higher than those of the CAS system. A novel, crossflow filtration-based method was used to directly evaluate the reversible and irreversible fouling potentials of the activated sludges. The fouling potential of the MBR mixed liquor as measured with this method, apparently increased in the last four months of the experiment, probably due to the elevated levels of dissolved organic matters present. Statistical analysis indeed indicated a significant correlation between dissolved organic carbon and EPS contents (in particular polysaccharides) and filtration resistance of the MBR mixed liquor. Our results indicated that the normalised CST (capillary suction time) value could be used as a potential indicator for membrane fouling potential of mixed liquor in full-scale wastewater treatment systems.
(2) Membrane bioreactors operated under different conditions: fouling propensities and microbial community analysis of biofilms
Two laboratory-scale submerged membrane bioreactors (MBR) were operated simultaneously at two different sludge retention times (SRT, 8 and 30 days) using real municipal wastewater. Both MBRs were first subjected to a low-flux (15 l/m2.h) running over the course of 19 weeks before switching to a high-flux (30 l/m2.h) running which lasted for three to four weeks. Fouling-related mixed liquor characteristics were monitored continuously. During the low-flux running, severe and reproducible membrane fouling was observed in the 8-day SRT MBR which had higher concentrations of SMP and EPS. No obvious membrane fouling was detected in the 30-day SRT MBR. Both MBRs experienced severe membrane fouling during the high-flux running. Biofilm samples were periodically retrieved from substantially fouled membrane modules, and cultivation-independent molecular approaches were used to elucidate the community composition and diversity of microorganisms responsible for biofilm formation. Denaturing gradient gel electrophoresis (DGGE) analysis and comparative rRNA sequencing revealed that diverse and distinct bacterial communities significantly differing from those in the planktonic biomass developed on the membrane surfaces. Relatively limited overlap existed between different biofilm communities. Although some phylotypes were phylogenetically associated with genera or taxa that were previously identified in membrane biofilms, numerous retrieved sequences displayed relatively low levels of similarity with any known 16S rRNA gene sequences and thus were derived from unknown taxa. These results suggest that the microbial communities responsible for membrane bio-fouling in MBRs are far more complex and variable than expected and thus could be challenging to control.
- determine relevant biological characteristics of MBR mixed liquor;
- monitor and follow microorganisms responsible for biofilm fouling in MBRs;
- develop and test a method to evaluate the fouling potential of mixed liquors.
These were addressed through two parallel investigations, one at lab-scale and one at full-scale:
(1) Biological characteristics and fouling potential of mixed liquors from full-scale wastewater treatment systems: a comparison between a membrane bioreactor and a conventional process
The fouling-related biological properties of mixed liquor from a full-scale membrane bioreactor (MBR) for municipal wastewater treatment were monitored over the course of 13 months and compared with those of a parallel-running conventional activated sludge (CAS) system at the wastewater treatment plant of Schilde, Belgium. This plant is operated already for more than four years by Aquafin. Measurement of specific respiration and nitrification rates indicated lower activities per unit of biomass in the MBR. Despite the less active biomass, concentrations of soluble microbial products (SMP) and major constituents of extracellular polymeric substance (EPS) in the mixed liquor supernatant of the MBR were consistently higher than those of the CAS system. A novel, crossflow filtration-based method was used to directly evaluate the reversible and irreversible fouling potentials of the activated sludges. The fouling potential of the MBR mixed liquor as measured with this method, apparently increased in the last four months of the experiment, probably due to the elevated levels of dissolved organic matters present. Statistical analysis indeed indicated a significant correlation between dissolved organic carbon and EPS contents (in particular polysaccharides) and filtration resistance of the MBR mixed liquor. Our results indicated that the normalised CST (capillary suction time) value could be used as a potential indicator for membrane fouling potential of mixed liquor in full-scale wastewater treatment systems.
(2) Membrane bioreactors operated under different conditions: fouling propensities and microbial community analysis of biofilms
Two laboratory-scale submerged membrane bioreactors (MBR) were operated simultaneously at two different sludge retention times (SRT, 8 and 30 days) using real municipal wastewater. Both MBRs were first subjected to a low-flux (15 l/m2.h) running over the course of 19 weeks before switching to a high-flux (30 l/m2.h) running which lasted for three to four weeks. Fouling-related mixed liquor characteristics were monitored continuously. During the low-flux running, severe and reproducible membrane fouling was observed in the 8-day SRT MBR which had higher concentrations of SMP and EPS. No obvious membrane fouling was detected in the 30-day SRT MBR. Both MBRs experienced severe membrane fouling during the high-flux running. Biofilm samples were periodically retrieved from substantially fouled membrane modules, and cultivation-independent molecular approaches were used to elucidate the community composition and diversity of microorganisms responsible for biofilm formation. Denaturing gradient gel electrophoresis (DGGE) analysis and comparative rRNA sequencing revealed that diverse and distinct bacterial communities significantly differing from those in the planktonic biomass developed on the membrane surfaces. Relatively limited overlap existed between different biofilm communities. Although some phylotypes were phylogenetically associated with genera or taxa that were previously identified in membrane biofilms, numerous retrieved sequences displayed relatively low levels of similarity with any known 16S rRNA gene sequences and thus were derived from unknown taxa. These results suggest that the microbial communities responsible for membrane bio-fouling in MBRs are far more complex and variable than expected and thus could be challenging to control.