Characteristics of soluble microbial products and extracellular polymeric substances in the membrane bioreactor for water reuse

In biological treatment processes, the effluent contains some portion of organic material originating from soluble microbial products (SMP) that is closely related with extracellular polymeric substances (EPS), although the biological activity is maximized. These SMP should be considered important target materials if additional processes are necessary to satisfy the required water quality for the intended reuse. In this study, the characteristic of SMP and EPS were investigated using a submerged membrane bioreactor, which is one of the most promising water reuse process. Most protein SMP in the reactor existed at a molecular weight (MW) above 10 kDa. Over 86% of the carbohydrate SMP contained in the permeate had a MW below 1 kDa. The protein and carbohydrate SMP in the permeate account for 83–91% of the COD. The MW of the protein and carbohydrate SMP in the reactor, when converted to COD showed a bimodal pattern (less than 1 kDa or greater than 10 kDa), but over 81% of those in the permeate existed below 1 kDa due to the membrane rejection. The relative hydrophobicity of the protein and carbohydrate SMP was affected by the influent characteristics, and the hydrophobicity was decreased after the membrane filtration. The total EPS concentrations increased with increases in the food to microorganism ratio in this study. The modified fouling index (MFI) was used to investigate the biofouling characteristics, in terms of soluble and suspended solid parts closely related with the SMP and EPS, respectively.

Three phases of the PSMBR were operated to analyze the characteristics of the SMP and EPS in the MBR, with the soluble and suspended solid parts measured using the fouling index. The following conclusions were drawn. The concentration of the carbohydrate SMP was higher than that of the protein. Most of the protein SMP existed above a MW of 10 kDa. Over 86% of the carbohydrate SMP contained in the permeate existed below a MW of 1 kDa. About 83–91% of the COD portion in the permeate was covered due to the carbohydrate and protein SMP. The MW fractions of the protein and carbohydrate SMP, converted to COD, in the reactor showed a bimodal pattern (less than 1 kDa or greater than 10 kDa), but over 81% of the protein and carbohydrate SMP in the permeate existed below a MW of 1 kDa, due to the membrane rejection. The relative hydrophobicity of the SMP was affected by the influent characteristics, and decreased after the membrane filtration. The EPS concentrations increased with increasing F/M ratio (0.13–0.29 gCOD/gVSS·d). It was assumed that the protein and carbohydrate from the influent and the SMP could be more adsorbed by the EPS matrix with higher F/M ratio conditions. Both the soluble and SS parts of the MFI increased with increasing F/M ratios. The soluble MFI increased with increases in the rejection of the COD concentrations. It was assumed that the SS MFI increased due to the higher EPS concentrations. The MFI during denitrification showed higher values than those during nitrification, possibly due to floc deterioration.