| Due to the water shortage, wastewater recycling has been more and more popular recently. Grey water is defined as the urban wastewater that inclu des water from baths, showers, hand basins, washing machines; dishwashers and kitchen sinks, but excludes streams from toilets. Because of the low levels of contaminating pathogens and nitrogen compared with normal domestic sewage, reuse and recycle of grey water is receiving more and more attention. Membrane bioreactor(MBR) is a promising technology for grey water treatment due to the high effluent quality, detention of all the bacteria, small footprint and modular operation. However, membrane fouling, as well as complex operation, high energy consumption and high cost cause by the membrane fouling are the main obstacles for its widespread application. To address the problems above, gravity-driven membrane bioreactor system(GDMBR) which allowed operation at stable fluxes without any back wash, physical cleaning and chemical cleaning at pressures with 5-20 k Pa, was developed in this Ph. D thesis. We applied the GDMBR system to household grey water treatment to establish low energy-based technical system.First, the effect of dissolved oxygen concentration(high DO: 6.0-6.5 mg/L; low DO: 0.5-1.0 mg/L) on the organics and nutrient removal in GDMBR system was investigated. The results revealed that the removal efficiencies of COD and TOC in high DO condition were better than those in low DO condition. However, the removal efficiencies were nearly the same during the later stage of the operation, and quality of the effluent met the standard of water reuse. Additionally, the SMP concentrations(including biopolymers, humics and building block) in high DO condition were lower than those in low DO condition. The removal efficiencies of biopolymer increased with operation time in both conditions. The reason for this was the formation of bio-cake layer adhered on the membrane surface enhanced the removal of biopolymer, but not the humics and low molecular weight humics and acids. As for the nutrient removal, the ammonia and total nitrogen were removed efficiently in high DO condition, and the effluent quality met the standard of water reuse. However, GDMBR exhibited a low ammonia and total nitrogen in low DO conditions.Secondly, the effect of DO concentration on the permeate flux stability in GDMBR, as well as the corresponding membrane fouling mechanism were performed. The results showed that both of the permeate flux in two GDMBRs could keep stable after 40 days operation. The permeate flux and permeability were stable at 2 L/m2 h and 0.4 L/m2 hk Pa in high DO condition, respectively; The permeate flux and permeability were stable at 1 L/m2 h and 0.2 L/m2 hk Pa in low DO condition, respectively. The resistance analyses showed that the low DO mainly increased the hydraulic reversible resistance and cake layer resistance compared with high DO condition, and it also increased the hydraulic irreversible resistance and pore blocking resistance a little bit. The reason is the activity of suspended sludge was lower in low DO condition, which resulted in a lower biodegradation. Additionally, the sludge produced a higher SMP and accumulated to the bio-cake layer, leading to a higher EPS content(protein and polysaccharide) in the cake layer. In comparison with the high DO condition, the activity of the cake sludge in low DO condition reduced, the thickness of the cake layer increased and the surface roughness reduced, which indicated that the predation capacity might decrease, and the cake did not developed loosely and heterogeneously on the membrane surface. What’s more, low DO condition influenced the EPS distribution in the bio-cake. The proteins, polysaccharides and bacteria cells distributed homogeneously in the bio-cake layer in high DO condition, however, the bacteria cells were on the top part of the cake layer, and the proteins and polysaccharides located in the lower part of the cake layer which were near to the membrane surface. Therefore, this was the main reason causing severe membrane fouling in low DO condition.Finally, the effect of aeration shear stress on the permeate flux stability in GDMBR, as well as the corresponding membrane fouling mechanism were investigated. The results showed that the presence of aeration shear stress could retard the membrane fouling in the initial stage compared with the no shear condition; however, it caused more serious fouling in the following stage during the long-term operation. The permeate flux kept stable round 2 L/m2 h, whereas the flux did not stabilize and continues to decrease under shear conditions reaching a value of 0.5 L/m2 h after 120 days. The shear stress significantly increased the total filtration resistance, especially cake layer resistance and hydraulically reversible resistance; it also results in increased pore blocking resistance and hydraulically irreversible resistance, which was more persistent to hydraulic cleaning, compared to the system without shear stress. Therefore, creating the aeration shear stress on the membrane surface is not an effective way to retard membrane fouling in low-pressure GDMBR. OCT images revealed that although the shear stress decreased the thickness and surface roughness of bio-cake layer compared with no shear condition, it increased the density and made the cake layer more compact. The bio-cake layer strongly and heterogeneously attached on the membrane surface is the main reason causing severe fouling under shear condition. Additionally, the shear results in a selection of organisms which produce more EPS(proteins and polysaccharides) and are more resistant against the shear stress. Furthermore, the shear stress changed the distribution of the organisms in the cake layer compared with the control. The proteins located in the lower part of the cake layer at the membrane surface, and polysaccharides were in the middle position, as well as the bacterial cells were on the top of the cake layer. This is another important reason that the shear stress caused more serious fouling. |
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