Effects of microcarrier pore characteristics on methanogenic fluidized bed performance

The cell retention capacities of three porous microcarriers with diversified pore characteristics and Ottawa silica sand were studied in methanogenic fluidized bed reactors with acetic acid as the sole substrate. Batch kinetic experiments on substrate utilization at different initial bulk-liquid substrate concentrations were also performed. The experimental data reveal that, under similar startup conditions, porous microcarriers are capable of reducing the startup times by more than 50% as compared to sand. Furthermore, under pseudo-steady-state conditions at an organic loading of 6 g total organic carbon (TOC)/1-day, porous microcarriers are capable of retaining three times more immobilized cells as compared to sand. More than 90% of total reactor cell mass is immobilized on porous microcarriers as opposed to 80% on sand. As a result, porous microcarriers are conducive for better proliferation of slow-growing methanogenic bacterial consortia. The experimental data clearly indicate that surface area, total pore volume and mean pore diameter should be used concomitantly to obtain better insight into the cell retention capacity of a given porous microcarrier. Batch kinetic data on substrate utilization reveal that mass transfer limitations are absent in methanogenic fluidized bed reactors at bulk-liquid TOC concentrations > 10 mg/l. The observed maximum substrate utilization rates, which are independent of initial bulk-liquid TOC concentrations ranging from 200 to 1000 mg/l, are low for porous microcarriers as compared to sand (0.5 vs 2.25 day −). These data confirm the results of the microscopic examinations performed which indicate that porous microcarriers attract Methanothrix type bacterial consortia whereas Ottawa silica sand attracts a mixture of Methanothrix and Methanosarcina.