Effects of Varying Sludge Quality on the Permeability of a Membrane Bioreactor

Abstract: This master thesis firstly includes a theory part describing, the conventional municipal wastewater treatment plant (WWTP) and especially the conventional activated sludge (CAS) process. As Stockholm municipality want to retrofit the current activated sludge system at Henriksdal into a membrane bioreactor (MBR), an extensive description of the MBR and its advantages and disadvantages are included. Fouling is considered a really important issue for the operation of an MBR since it reduces an MBR’s productivity over time. Therefore, description of the fouling mechanisms and the potential foulants is included as well as a description of the membrane cleaning procedures. Sludge composition is considered a very important parameter which contributes to membrane fouling and thus this master thesis aims to identify the effects of varying sludge quality on the membranes operation. Precipitation chemicals used for phosphorus chemical precipitation and especially ferrous sulphate which is examined in this master thesis are also affecting the sludge quality and the membranes operation. The report includes description of Henriksdal reningsverk and line 1 of the pilot MBR at Hammarby Sjöstadsverk where the experimental work was performed. The following chapter describes the experimental work performed in the laboratory including the determination of total suspended solids (TSS), volatile suspended solids (VSS), sludge volume index (SVI) and sludge’s filterability. The filterability was determined by performing the time to filter (TTF) method and the sludge filtration index (SFI) method. Furthermore, the samples were also examined in the optical microscope to determine their bulkiness and their filaments content. The iron content in the sludge was also measured from Eurofins Environment Testing Sweden AB. In the results section, the different parameters measured are illustrated in charts and they are compared to each other in order to define which factors contribute positively or negatively to the sludge’s filterability and thus affect the sludge quality and the membranes operation. The results indicate that SFI is a more reliable method for measuring filterability compared to TTF. Furthermore, the iron content in the sludge is proportional to the permeability as well as the filaments content observed during microscopy is proportional to the SFI or TTF. Finally, this master thesis includes recommendations for future research which basically include more analyses to identify the sludge biology and more samples taken for longer time periods.