Application of dynamic membranes in anaerobic membranes in anaerobic membrane bioreactor systems

Anaerobic membrane bioreactors (AnMBRs) physically ensure biomass retention by the application of a membrane filtration process. With growing application experiences from aerobic membrane bioreactors (MBRs), the combination of membrane and anaerobic processes has received much attention and become more attractive and feasible, due to advantages provided by the combination with regard to developments for energy-efficient wastewater treatment. The major drawbacks of MBR technology are related with membrane costs, especially for the full-scale applications, fouling and low flux. Dynamic membrane (DM) technology may be a promising approach to resolve the drawbacks encountered in MBR processes. One of the most important potential benefits of DMs is that the membrane itself may be no longer necessary, because solids rejection is accomplished by the secondary membrane layer that can be formed and re-formed as a self-forming DM in situ. Different kinds of materials such as mesh, woven or nonwoven fabric instead of microfiltration and ultrafiltration membranes can be used as the support layer for creating DM. In this way, the replacement of the membrane by a low cost filter material is possible. By decreasing membrane cost and generating energy, dynamic AnMBRs (AnDMBRs) would be attractive for waste(water) treatment. The main aim of this study was to investigate the applicability of DM technology for the treatment of concentrated wastewaters in AnMBRs. Moreover, this thesis provides additional information and understanding of DM technology, including assessment of DM formation and filtration characteristics under different conditions. Submerged and external membrane module configurations were used in order to determine the effect of the configuration on removal efficiency and DM filterability. Synthetic concentrated wastewater with an average COD concentration of 20 g/L was used as the substrate. Determination of an optimal support material and investigations about its structure were achieved by testing various types of support materials including monofilament, multifilament and staple yarn types. Besides, different operating conditions were tested at low fluxes under mesophilic conditions to determine the optimal operation conditions enabling the optimal removal efficiency and permeate quality. Moreover, cost estimation in terms of support material acquisition was also presented. The results show that support material properties were critical for the formation of an effective dynamic membrane (cake) layer over the filter surface. The critical fluxes obtained with the staple and monofilament filter cloths were higher than those obtained with multifilament material. The results indicate that staple filter cloth was more suitable for depth 8 filtration, whereas mono-monofilament filter was more suitable for surface (cake) filtration. Thus, mono-monofilament filter was considered more appropriate for DM technology. The results presented in this thesis show that the DM filtration concept can turn one of the most important disadvantages of MBRs, membrane fouling, into an advantage. Polypropylene mono-monofilament filter cloth was used to form a dynamic membrane (cake) layer and to provide filtration by this self-forming layer as an alternative to microfiltration or ultrafiltration membranes. The AnDMBR achieved over 99% organic matter removal and particulate matter retention. Moreover, over 60% soluble COD removal and over 50% VFA removal were obtained by the DM layer. Considering the results of this research, it was shown that a stable operation with AnDMBRs could be possible for a long period. Sludge retention time (SRT) was found an important factor in AnDMBRs that had a significant effect on soluble microbial products (SMP) and extracellular polymeric substances (EPS) production, protein/carbohydrate ratio, particle size of the sludge, DM layer formation and bulk sludge filterability. Bound EPS is mainly composed of cell surface materials, including proteins, polysaccharides, lipids, nucleic acids and humic acids. EPS keeps the sludge flocs together on the membrane surface by surrounding them. EPS had a significant positive effect on particle flocculation and thus, particle size distribution in the bulk sludge. Prolonged SRT resulted in lower EPS concentrations in the bulk sludge compared to short SRTs. A combination of backwashing and biogas sparging enabled the control of DM layer thickness, which is of great importance to obtain a stable operation and high quality permeate. A combined effect of biomass activity and physical retention capacity through the cake layer might be responsible for the removal of organic matter and retention of particulate matter by the DM layer. Pyrosequencing analyses showed that diversity and richness of the microbial communities including bacteria and archaea in the DM layer were high and microbial population composition in the DM layer was different compared to the bulk sludge in the AnDMBR. Following the DM layer morphological analyses results, the DM layer was formed by both organic and inorganic materials, such as sludge particles, SMP, EPS, Ca, N, P, and Mg precipitates. Moreover, a partial gel layer formation under the cake layer was detected. Accumulation of SMP and bound EPS in the DM layer in high amounts led the formation of a dense cake layer and effective retention. Accumulation of organic matters is also related with operating conditions such as SRT. This research also showed that although slightly better permeate quality in terms of COD concentration was obtained by submerged AnDMBR, high COD removal efficiencies were achieved in both submerged and external AnDMBR configurations. Comparison of the effects of membrane configuration on treatment and filterability performance showed that more time was needed in the external AnDMBR in order to form an effective DM layer enabling a stable removal efficiency and low soluble COD concentration in the permeate. Therefore, submerged AnDMBR configuration appears more suitable when a short start-up period is 9 necessary. Higher methane production rate and methane yield were obtained in the submerged configuration compared to the external configuration reflecting the negative effect of sludge recirculation in the external DM configuration. Conversely, sludge recirculation in the external configuration was more effective in decreasing DM thickness, thus transmembrane pressure, than the bottom biogas sparging in the submerged configuration. Considering the tested different gas sparging velocities (GSVs), over 99% organic removal was obtained with the external AnDMBR configuration for high strength wastewater treatment irrespective of the GSV, although total filtration resistance increased with decreasing GSV. Total filtration resistance mainly consisted of the resistance by the DM layer that provided effective and stable treatment. Following the organic loading rate study, the AnDMBR achieved high COD removal efficiency at 3.6 kg COD/m3.d. In conclusion, following the results obtained in this study, DM technology achieved a stable and high quality permeate. Thus, AnDMBRs can be used as a reliable and satisfactory treatment technology for treatment of high strength wastewaters. Low capital costs of support material and energy generation can make AnDMBRs feasible for those situations in which a high flux is not necessary, such as sludge and slurry treatment or highly concentrated industrial wastewater treatment. However, research on AnDMBRs is still very limited. Longterm applicability and reliability of the DM applications need further research, focusing on cake layer control methods to allow satisfactory DM layer formation as well as on the effect of sludge properties on DM filtration characteristics for large-scale applications.