Performance and membrane fouling of a step-fed submerged membrane sequencing batch reactor treating swine biogas digestion slurry

To identify the performance of step-fed submerged membrane sequencing batch reactor (SMSBR) treating swine biogas digestion slurry and to explore the correlation between microbial metabolites and membrane fouling within this novel reactor, a lab-scale step-fed SMSBR was operated under nitrogen loading rate of 0.026, 0.052 and 0.062 g NH4+-N (gVSS.d)(-1). Results show that the total removal efficiencies for NH4+-N, total nitrogen and chemical oxygen demand in the reactor (> 94%, > 89% and > 97%, respectively) were high during the whole experiment. However, the cycle removal efficiency of NH4+-N decreased significantly when the nitrogen loading rate was increased to 0.062 g NH4+-N (gVSS.d)(-1). The total removal efficiency of total phosphorus in the step-fed SMSBR was generally higher than 75%, though large fluctuations were observed during the experiments. In addition, the concentrations of microbial metabolites, i.e., soluble microbial products (SMP) and extracellular polymeric substances (EPS) from activated sludge increased as nitrogen loading rate increased, both showing quadratic equation correlations with viscosity of the mixed liquid in the step-fed SMSBR (both R-2 > 0.90). EPS content was higher than SMP content, while protein (PN) was detected as the main component in both SMP and EPS. EPS PN was found to be well correlated with transmembrane pressure, membrane flux and the total membrane fouling resistance. Furthermore, the three-dimensional excitation-emission matrix fluorescence spectroscopy results suggested the tryptophan-like protein as one of the main contributors to the membrane fouling. Overall, this study showed that the step-fed SMSBR could be used to treat swine digestion slurry at nitrogen loading rate of 0.052 g NH4+-N (gVSS.d)(-1), and the control strategy of membrane fouling should be developed based on reducing the tryptophan-like PN in EPS.