Alkali-treated cellulose carrier enhancing denitrification in membrane bioreactor

The performance of hybrid MBRs and the improvement of denitrification with the addition of alkali-treated cellulose as microbial carrier and carbon source were investigated in this study. Three kinds of natural celluloses were evaluated by batch experiments. Straw was selected to construct a hybrid MBR. Results showed that the effect of alkali treatment on the accessibility of carbon source was obvious, the COD release capacity of the three carriers increased from 136–178 mg g−1 to 250–323 mg g−1, and the denitrification potential increased from 112.04–157.94 mg g−1 to 233.82–250.69 mg g−1 as compared with the untreated groups. All MBRs achieved high COD and NH4+-N removal irrespective of rice straw addition or not. However, the denitrification rate was enhanced effectively by adding straw. TN removal in the straw group and alkali-treated straw group increased to 49.83% and 63.46% respectively as compared with the control group of 28.80%. Nitrate saturation constant (KNO) of the control group, straw group and alkali-treated straw group MBRs were 1.82, 0.43, 0.21 mg NO3−-N L−1, respectively. The KNO of alkali-treated straw group MBR was the lowest one, which is the closest to the single-stage denitrified KNO. Addition of cellulose carrier can improve simultaneous nitrification and denitrification (SND) due to the presence of dissolved oxygen gradient in hybrid MBRs. More significantly, alkaline pretreatment increased the cellulose solid carbon source bioavailability and enhanced the function of microbial carrier thereby accelerating SND effectively.