Structural and Microbial Dynamics Analyses of MABR Biofilms

Membrane aerated biofilm reactor (MABR) technology is currently garnering wide acceptance as a wastewater treatment technology due to process and operational advantages that have accrued during many years of investigation, but despite these achievements, the challenge of biofilm thickness control still persists. This work was therefore designed to expand the current knowledge of MABR operations, particularly with respect to biofilm thickness management and reactor performance stability. Biofilm thickness was controlled with intermittent washing of the membrane bundle based on a bulk dissolved oxygen (DO) concentration set-point of $0.2\mathrm{mg}/\mathrm{L}$. This nonaggressive membrane cleaning mechanism subdued biofilm sloughing, but maintained biofilm erosion, which supported the development of a multifunctional biofilm with the thickness and microbial activity adequate to stabilize reactor performance over 185 days. Applied in the treatment of municipal wastewater, the MABR demonstrated average organic carbon and ammonia nitrogen (${\mathrm{NH}}_4^{+}─\mathrm{N}$) removal efficiencies of $92\%\pm 2\%$ and $100\%\pm 7.8\%$, respectively. The total inorganic nitrogen removal reached $84\%\pm 5\%$ at mean surface loading rates of $10\%\pm 0.7\mathrm{g}\mathrm{COD}/{\mathrm{m}}^2/\mathrm{d}$ and $0.93\pm 0.07\mathrm{g}\mathrm{N}/{\mathrm{m}}^2/\mathrm{d}$ within a hydraulic retention time of 2.5 h and with minimal reactor down time. The average biofilm density determined at the end of the study was $17.6\mathrm{g}/\mathrm{L}$, while the biofilm thickness determined to be 0.49 mm. A 16S rRNA analysis of the MABR microbial population at each stage indicated that a microbial community with sufficient biodiversity and relative abundance for stable reactor performance was sustained. Demonstrating that intermittent membrane cleaning with water effectively stabilized the MABR biofilm thickness, reduced process upsets, and maintained high performance at high substrate loading.

The current drive for energy neutrality, drastic reduction in energy costs, and increasingly stringent wastewater discharge rules has led to the quest for the development of energy efficient and effective wastewater treatment technologies. MABR is one of these developing technologies. Compared to traditional biological wastewater treatment methods, MABRs offer improved performance with regards to the removal of pollutants from wastewater and energy efficiency. MABR is a biofilm-based technology, whose success depends on the biofilm thickness. Moreover, a biofilm with adequate thickness is required to maintain the diversity and abundance of the microbial community required for high and stable performance over extended operational periods. But, the effective control of biofilm thickness remains a lingering challenge in the application of MABR technology. In practice, biofilm thickness control is commonly accomplished by promoting biofilm detachment using air bubbles. However, several studies have reported performance upsets and performance lag that can sometimes last up to several months following each biofilm detachment episode due to substantial biofilm loss. This study recommends a biofilm thickness management protocol that involves gentle detachment of loosely bound biofilms through nonaggressive cleaning and cleaning episodes initiated when a set operating condition is attained.