Effects of returning NF concentrate on the MBR-NF process treating antibiotic production wastewater. 
The optimization of the nanofiltration (NF) concentrate backflow ratio (R cb) and the influence of the NF concentrate on the performance of membrane bioreactor-nanofiltration (MBR-NF) process treating antibiotic production wastewater were investigated on a laboratory scale.  The R cb was optimized at 60 % based on the removal rates of chemical oxygen demand (COD) and NH4 (+)-N by MBR.  Data analyses indicated that salinity brought by NF concentrate is the major driver leading to the decrease of sludge activity, especially at a high R cb.  EPS analysis showed that electric conductivity (EC), proteins in soluble microbial products (SMP), and SMP brought by NF concentrate are the dominant factors causing the severe membrane fouling in MBR.  Furthermore, undegradable substances including fulvic acid-like and humic acid-like compounds accumulated in NF concentrate showed significant influence on fouling of NF.  MBR could well degrade small MW compounds in NF concentrate, which confirmed the enhancement of organic removal efficiency by recycling the NF concentrate to MBR.  The MBR-NF process showed a relatively stable performance at the R cb of 60 % (volume reduction factor (VRF) = 5), and the NF permeate could satisfy the water quality standard for fermentation process with a water recovery rate of 90.9 %.