Comparison of Different Culturing Methods for Enumeration of E. coli in Thermophilically Digested Biosolids

Pathogen removal in sludge treatment is an important step. Several Class A processes that utilize thermal treatment to achieve pathogen reduction have observed large increases in indicator bacteria, namely fecal coliform or E. coli, after centrifuge dewatering. In many cases, the densities before dewatering were below the detection limit. One hypothesis to explain this observation is that the bacteria were poorly enumerated after the thermophilic processes, but some were still viable and were able to be recovered after dewatering. Bacteria can be non-culturable based on type of media used and methods used for enumeration. To investigate the non-culturable hypothesisE. coli was enumerated using EPA Method 1680 and other culturing methods using microplates for investigation of non-culturable E. coli. Effect of hydraulic retention time (HRT) on pathogen removal was investigated for two enhanced digestion processes in addition to a conventional mesophilic anaerobic digestion (MAD) process. The enhanced digestion processes were dual digestion process: Autothermal Thermophilic Aerobic Digestion (ATAD) followed by MAD and acid/gas phase digestion process: Acid Phase Digestion (APD) followed by MAD. This study suggests that EPA Method 1680 could be underestimating E. coli density and a need to develop new culturing methods that are able to enumerate injured microorganisms. ATAD is more effective in pathogen removal when compared to APD. Class A Biosolids requirements was achieved with E. coli density below 3 log10 for only 3.5 d HRT for ATAD-MAD process, with enumeration conducted with all the culturing methods investigated for this study. The better performance of ATAD can be linked to O2 input for this process since both APD and ATAD processes were operated at the same temperature and HRTs. Additional research is necessary for comparing the effect of O2 input and HRT on pathogen removal for ATAD.