Sludge properties and their effects on membrane fouling in submerged anaerobic membrane bioreactors (SAnMBRs)

Two submerged anaerobic membrane bioreactors (SAnMBRs) (thermophilic vs. mesophilic) were operated for a period of 3.5 months with kraft evaporator condensate at a feed chemical oxygen demand of 10,000 mg/L. The results show that the filtration behavior of the two systems was significantly different. The filtration resistance in the thermophilic SAnMBR was about 5–10 times higher than that of the mesophilic system when operated under similar hydrodynamic conditions. Comparison of sludge properties and cake layer structure from the two systems was made to elucidate major factors governing the different filtration characteristics. There were more soluble microbial products (SMP) and biopolymer clusters (BPC) produced and a larger portion of fine flocs (<15 μm) in the thermophilic SAnMBR. Analysis of bound extracellular polymeric substances (EPS) showed that the thermophilic sludge had a higher protein/polysaccharide ratio in EPS, as compared to that in the mesophilic sludge. A series of analyses, including Fourier transform infrared (FTIR) spectroscopy, energy dispersive X-ray spectroscopy (EDX), confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and particle size analyzer showed that the cake layer formed in the thermophilic SAnMBR contained higher levels of both organic and inorganic foulants, smaller particle sizes, and especially, a denser and more compact sludge cake structure. These results indicate that floc size, SMP, BPC, bound EPS as well as cake layer structure are the major factors governing membrane fouling in SAnMBR systems.