Anaerobic fluidized bed digestion of primary and thickened waste activated sludges

• Activity of three attached anaerobic microbial groups was characterized.
• A new method for measurement of acetogenic and acidogenic bacteria was developed.
• Activity-based SRTs were 1.9–4.4 times longer than VSS-based estimates.
• VSS based detachment rates were 3.2–9.8 times higher than activity-based rates.

Two lab-scale anaerobic fluidized bed bioreactors (AnFBRs) treating primary sludge (PS) and thickened waste activated sludge (TWAS) were studied to explore biofilm microbial characterization. COD and VSS removal for PS showed 62% and 63%, respectively at an organic loading rate (OLR) of 18 kg COD m−3 d−1 and an hydraulic retention times (HRTs) of 2.2 days. Similarly, TWAS was treated at 56% COD removal and 50% VSS reduction efficiencies at an OLR of 12 kg COD m−3 d−1 and an HRT of 4 days. Furthermore, the specific bacterial community activity tests showed a significant difference between solids retention times (SRT) based on general VSS and retention times based on the activity of methanogenic, acidogenic, and acetogenic microbes. While SRT based on VSS measurements in the PS-AnFBR were 3.3 days, the activity-based retention times varied from 12.5 to 16.6 days. Similarly, in the TWAS-AnFBR, the SRT based on VSS measurements were 5.0 days, and the activity-based retention times ranged from 9.3 to 10.8 days. The BioWin model simulation predicted that conventional anaerobic digestion processes achieved lower VSS reduction efficiencies than the AnFBR process at the same SRT.