Enhanced Nitrate Removal And Fouling Behavior In A Denitrifying Membrane Bioreactor: Impacts Of Carbon Source And C/N Ratio

The impacts of the carbon source (sodium acetate, ethanol) and C/N ratio (1.0, 3.0, and 5.0) on nitrogen removal and membrane fouling behavior were investigated using a denitrifying membrane bioreactor (DNMBR) treating nitrate-rich wastewater. The results showed that the C/N ratio showed a more significant impact on the process performance than the carbon source. The nitrogen removal efficiency (NRE) was slightly higher when employing sodium acetate as the electron donor at different C/N ratios. The nitrogen removal improved with an increase in the C/N ratio due to the increased specific degradation rate of NO3--N (SDRNO3--N); the NRE increased from 24.43% and 13.23% at a C/N ratio of 1.0% to 97.79% and 96.46% at a C/N ratio of 5.0 for the sodium acetate and ethanol systems, respectively. The fouling rate accelerated due to decreased particle size and increased metabolic production, which resulted in the deterioration of sludge rheological properties and dewaterability with an increase in the C/N ratio. The membrane fouling rate with sodium acetate supporting the DNMBR was lower than that of the ethanol system at different C/N ratios, which was attributed to different fouling mechanisms. The high (>100 kDa) carbohydrate molecular weight fraction of soluble microbial production played an important role in membrane pore-blocking in the sodium acetate system. For the ethanol system, the high (>100 kDa) molecular weight protein fraction of extracellular polymeric substances and smaller-sized particles formed a denser cake layer. Fourier transform infrared analysis showed the carbohydrate-like and protein-like substances were the main membrane foulants throughout the entire operation.