Impact of the restraint of biofilm volume and thickness on the performance and microbial composition in anaerobic moving-bed biofilm reactors (AnMBBRs)

The effect of limiting biofilm volume using carriers with maximum thickness control on the performance and microbial community composition was investigated in anaerobic moving-bed biofilm reactors (AnMBBRs). Three parallel, continuous AnMBBRs (4 L) were operated (288 d, 24/36 °C) to treat the soluble fraction of a wastewater from a food-manufacturing facility. Two different biofilm carriers controlling maximum biofilm thickness at 200 and 1000 μm (AnoxK™Z-200 and Z-1000) were used in the three reactors, targeting the same projected surface area for biofilm growth (410 m2/reactor): R200 (Z-200), R1000 (Z-1000) and RMix (both Z-200 and Z-1000). The composition of the bacterial (16S rRNA V1-V3) and methanogenic (mcrA gene) communities was analysed using amplicon sequencing. Under relatively constant organic loads (3.6-4.2 gSCOD/Ld, 34-39 gSCOD/m2d, hydraulic retention time=7.7-8 h), R200 presented lower soluble chemical oxygen demand (SCOD) removals (51-60%) than the other reactors (R100 = 91-93%, RMix = 86-91%), likely due to a limited amount of active protected biofilm volume. Lower SCOD removals were associated with higher concentrations of volatile fatty acids in the effluent (R200 =400-800 mgCOD/L; R1000 and RMix<200 mgCOD/L) and lower methane production (R200 =0.66; R1000 =0.72; RMix=0.73 gCH4-COD/gSCODremoved). The Z-200 carriers contained a different fermentative/acidogenic bacterial community abundant in representatives of the families Christensenellaceae, Anaerovoracaceae and Synergistaceae, and led to less amount of biofilm biomass growth, albeit more active than that of carriers allowing for thicker biofilms (Z-1000). In contrast, methanogenic populations were less sensitive to biofilm thickness restraint by these carriers. This study shows for the first time that limiting biofilm volume in MBBR carriers can impact the performance and bacterial community in AnMBBR biofilms.