Simultaneous alkali supplementation and fouling mitigation in membrane bioreactors by on-line NaOH backwashing

In this study, we took advantage of the conventionally used physical backwashing and chemical cleaning methods to develop an on-line maintenance cleaning method using the NaOH solution (0.01 mol/L, pH around 12) as the backwashing agent in a membrane bioreactor (MBR) operated at two imposed fluxes (6.5 and 13 L/(m2·h) (LMH)). The results indicate that NaOH backwashing had little adverse effect on biological nutrient removal, and the pH in the MBR reactor remained at stable and acceptable levels (6.6–7.8) because of the alkali consumption by the nitrification process and the buffering role of the mixed liquor. Importantly, the NaOH backwashing significantly slowed the increased rates of total fouling (50% and 69% for 6.5 and 13 LMH, respectively) and irreversible fouling (40% and 50% for 6.5 and 13 LMH, respectively) compared with the pure water backwashing. Off-line backwashing tests indicate that NaOH backwashing enhanced the detachment of biopolymers from the surface of fouled membranes due to the co-occurrence of hydraulic actions and chemical alterations. The characterizations by scanning electron microscopy (SEM) indicate that both backwashing methods increased the size of membrane pores. Nevertheless, the Fourier transform-infrared spectrometry (FTIR) of membranes indicates that the long-term exposure to the NaOH solution did not change the chemical structure of membranes. This study suggests that on-line NaOH backwashing could not only maintain membrane permeability but also simultaneously supply alkali to bioreactors to ultimately facilitate the operation of MBRs.