Insight into the role of biogenic manganese oxides-assisted gravity-driven membrane filtration systems toward emerging contaminants removal

Effective water purification technologies are required to remove emerging contaminants (ECs) and prevent their extensive occurrence in rural areas. In this work, coupling gravity-driven membrane (GDM) filtration with biogenic manganese oxides (BioMnOx) in the biofouling layer was utilized for treating water containing SMX. Comparisons between BioMnOx-GDM (with BioMnOx) and Control-GDM (without BioMnOx) indicated that BioMnOx could significantly promote the removal of DOC, NH₄⁺-N, and fluorescent pollutants due to its strong oxidating capacity and high biological activity. The formation of BioMnOx increased the abundance of SMX-degrading bacteria, enriched the metabolic pathway and mineralization rate of SMX, and effectively promoted the remove of SMX. More importantly, BioMnOx facilitated the removal of antibiotic resistance genes (ARGs) in the GDM, because it increased the link between microorganisms and reduced the concentration of SMX, thus reduced the expression of ARGs. LB-EPS played an important role in the membrane fouling. Compared with the Control-GDM, the concentration of LB-EPS in BioMnOx-GDM decreased, which was beneficial to alleviate membrane fouling. Although a thicker biofouling layer (1774.88 μm vs.775.54 μm) was formed in BioMnOx-GDM, the biofilm with higher porosity (64.93% vs. 41.24%) had a more positive effect on the flux. Overall, BioMnOx could improve the pollutant removal and stable flux level of the GDM system. BioMnOx-GDM effectively avoided the risks brought by ECs and ensured water safety in rural areas.

Toward emerging contaminants removal using acclimated activated sludge in the gravity-driven membrane filtration system

The occurrence of emerging contaminants is attracting widespread attention due to its potential threat to aquatic organisms and public health. Gravity-driven membrane (GDM) filtration can effectively eliminate water contamination with pathogenic microorganisms in rural areas while being challenged by various micro-pollutants (MPs). This study investigated the removal and transformation of sulfamethoxazole (SMX) in GDM for the first time, and pre-adding acclimated activated sludge was proposed to improve the removal of MPs. More rapid and higher SMX degradation was observed with pre-adding domestication sludge, and the system had better tolerance to SMX shocks. Besides, in the presence of domesticated sludge, more SMX metabolic pathways and better mineralization rates were obtained, which was related to more SMX-resistant bacteria and easier biodegradable carbon sources in the system. Pre-adding sludge also increased the richness and diversity of bacterial community, which provided higher removal efficiencies of conventional pollutants. Thus, the removal rates of DOC (14.7%), NH₄⁺-N (5.6%) and fluorescent substances were obviously improved compared with the control group. In this study, the crisis of MPs was tackled and the removal of conventional pollutants was enhanced by pre-adding domesticated sludge in GDM, which ensured the water quality in rural areas.

Regulated-biofilms enhance the permeate flux and quality of gravity-driven membrane (GDM) by in situ coagulation combined with activated alumina filtration

It is a critical challenge for drinking water production when treating algae-contaminated surface water. In this study, the impact of in situ coagulation (C), activated alumina filtration (AA) and their combination (CAA) on the performance of gravity-driven membrane (GDM) was systematically assessed during 105-day operation. The results indicated that pretreatments in particular CAA could effectively enhance GDM flux, and the stable fluxes were increased to 3.1, 4.9 and 8.3 L/(m2·h) (LMH) for CGDM, AA/GDM and CAA/GDM, respectively when compared to the control GDM (2.0 LMH). Coagulation was beneficial to formation of thick but loose biofouling layer, while AA filtration was effective to retain foulants including extracellular polymeric substances (EPS), organics, total nitrogen and total phosphorus. The CAA/GDM could mostly remove these foulants, and facilitate the proliferation of bacterial genera that could consume EPS, further alleviating membrane fouling. The difference in loosely bound EPS and tightly bound EPS of biofouling layer attributed to the difference of reversible fouling and irreversible fouling, respectively. Morphological observations, variation in functional groups or elements further confirmed the difference in biological layers in different GDM systems. The occurrence of specific bacterial genera involving the potential to degrade protein, chitin and other high molecular weight organics was responsible for contaminant removals.

Effects of predator movement patterns on the biofouling layer during gravity-driven membrane filtration in treating surface water

Biological predation has a significant effect on biofouling layers in gravity-driven membrane (GDM) filtration systems. However, the detailed process of predatory activities is still not well known. This study explored the effects of predator movement patterns on the biofouling layer at different temperatures and the factors affecting the stable flux level. The results indicated that Demospongiae, Spirotrichea and Saccharomycetes were the main species, with the body contracting or rotating in one position at 5 °C, and Litostomatea accounted for 55.1% at 10 °C. The weak agility of these species resulted in a less porous biofouling layer with a high extracellular polymeric substance (EPS) concentration, which was responsible for the low permeate flux and the time to reach flux stability. Bdelloidea was dominant at 20 and 30 °C, and the more heterogeneous biofouling layer with a lower EPS concentration was related to their intense creeping and swimming movements and their ability to create current in the water. The grazing of spongy flocs by predators affected the GDM system performance, and a high stable flux was obtained with large and loose flocs. In addition, the diversity of the eukaryotic community decreased after the flux stabilized due to the particular predominance of Bdelloidea at high temperatures, corresponding to a high stable flux. Pollutant removal was less affected by eukaryotes, and decreased ammonia nitrogen removal rates were related to the lower activity of nitrifying bacteria. Moreover, the reliable linear correlation between the temperature and the stable flux implied that the stable flux could be well predicted in the GDM system. The findings are beneficial for developing new strategies for regulating flocs and the biofouling layer to improve the performance of GDM systems.

Integrating granular activated carbon (GAC) to gravity-driven membrane (GDM) to improve its flux stabilization: Respective roles of adsorption and biodegradation by GAC

As a low-maintenance and cost-effective process, gravity-driven membrane (GDM) filtration is a promising alternative for decentralized drinking water supply, while the low flux impedes its extensive application. In order to address such issue, an integrated process consisting of granular activated carbon (GAC) layer and GDM was developed. The performance of virgin (fresh GAC) or preloaded GAC (saturated GAC) was compared. Flux stabilization was observed both in the fresh and saturated GAC/GDM process during long-term filtration and their stable fluxes were both improved by approximately 50% relative to the GDM control. Moreover, integrating GAC with GDM contributed to efficient removals for dissolved organic compounds (DOC), assimilable organic carbon (AOC) and low molecular weight substances both in fresh and saturated GAC/GDM filtration. Compared to GDM control, coupling GAC to GDM could significantly reduce the concentrations of extracellular polymeric substances (EPS) and total cell counts (TCC) within the biofouling layer, and engineer highly heterogeneous structures of biofouling layer on the membrane surface. In the fresh GAC/GDM process, the improved flux obtained was mainly related to less coverage of biofouling layer and lower EPS concentrations due to efficient removals of membrane foulants by GAC adsorption. The achieved higher stable flux can be maintained during long-term filtration (after GAC saturation) owing to the combined effects of EPS reduction and formation of highly heterogeneous structures of biofouling layer in the saturated GAC/GDM system. Overall, the integrated GAC/GDM process can hopefully facilitate improvements both in the stabilized flux and permeate quality, with practical relevance for GDM applications in decentralized drinking water supply.

A combination of membrane relaxation and shear stress significantly improve the flux of gravity-driven membrane system

Gravity-driven membrane (GDM) filtration system is a promising process for decentralized drinking water treatment. During the operation, membrane relaxation and shear stress could be simply achieved by intermittent filtration and water disturbance (created by occasionally shaking membrane model or stirring water in membrane tank), respectively. To better understand the impact of membrane relaxation and shear stress on the biofouling layer and stable flux in GDM system, action of daily 60-min intermission, daily flushing (cross-flow velocity = 10 cm s^-1, 1 min), and the combination of the two (flushed right after the 60-min intermission) were compared. The results showed that membrane relaxation and shear stress lonely was ineffective in improving the stable flux, while their combination enhanced the stable flux by 70%. A more open and spatially heterogeneous biofouling layer with a low extracellular polymeric substance (EPS) content and a high microbial activity was formed under the combination of membrane relaxation and shear stress. In-situ optical coherence tomography (OCT) observation revealed that, during intermission, the absence of pushing force by water flow induced a reversible expansion of biofouling layer, and the biofouling layer restored to its initial state soon after resuming filtration. Shear stress caused abrasion and erosion on the biofouling surface, but it exerted little effect on the interior of biofouling layer. Under the combination, however, both the surface and interior of biofouling layer were disturbed because of 1) the water vortexes caused by rough biofouling layer surface, and 2) the porous structure after 60-min intermission. This disturbance, in turn, helped the biofouling layer maintain its roughness and porosity, thereby improving the stable flux of GDM system.

Effects of water temperature and light intensity on the performance of gravity-driven membrane system

The selection of favorable environmental conditions for gravity-driven membrane (GDM) systems is crucial to their widespread application. In this study, GDM systems operated under different light intensities (illuminance levels of 0, 200, and 3000 Lux) and water temperatures (10, 20, and 30 °C) were investigated for their performance and fouling layer characteristics. The results showed that indoor light (200 Lux) had limited effects on the performance of the GDM system. However, full daylight (3000 Lux) led to algal growth; these algae increased fouling resistance and deteriorated permeate water by releasing algogenic organic matter, although they could also enhance the heterogeneity of the biofouling layer by increasing the microbial activity. Water temperature rarely influenced the total organic matter removal. The fouling layers had different thicknesses and heterogeneity, but the same level of EPS; therefore, the hydraulic resistances of these fouling layer were almost the same at different water temperatures. These findings suggest that GDM system could be operated at low water temperature and indoor light conditions, and that strong light should be avoided during the operation of GDM systems.