Fat, oil, and grease residues, food particles, solids and some kitchen wastewaters
are collected in grease traps which are separate from the municipal wastewater stream.
Grease traps are emptied periodically and grease trap waste (GTW) is hauled for
treatment. This dissertation focuses on anaerobic co-digestion of un-dewatered (raw)
GTW with municipal wastewater treatment sludge (MWS) at wastewater treatment
plants. In particular, this research focuses on the biochemical methane potential of undewatered
GTW as well as the stability and performance of anaerobic co-digestion of
MWS and un-dewatered GTW.
A set of modified biochemical methane potential tests was performed to
determine the methane potential of un-dewatered GTW under mesophilic conditions (35
0C). Methane potential of un-dewatered GTW in this study was 606 mL CH4/g VSadded
which is less than previously reported methane potentials of 845 – 1050 mL CH4/g
VSadded for concentrated/dewatered GTW. However, the methane potential of undewatered
GTW (606 mL CH4/g VSadded) was more than two times greater than the 223
mL CH4/g VSadded reported for MWS digestion alone.
A comprehensive study was performed to determine the stability and performance
of anaerobic co-digestion of MWS with un-dewatered GTW as a function of increasing
vii
GTW feed ratios. The performance of two semi-continuously fed anaerobic digesters at
35 0C was evaluated as a function of increasing GTW feed ratios. Anaerobic co-digestion
of MWS with un-dewatered GTW at a 46% GTW feed ratio (on a volatile solids basis)
resulted in a 67% increase in methane production and a 26% increase in volatile solids
reduction compared to anaerobic digestion of MWS alone. On the other hand, anaerobic
co-digestion of un-dewatered GTW resulted in a higher inhibition threshold (46% on VS
basis) than that of dewatered GTW. These results indicate that using un-dewatered GTW
instead of dewatered GTW can reduce the inhibition risk of anaerobic co-digestion of
MWS and GTW.
Recovery of the anaerobic digesters following upset conditions was also evaluated
and semi-continuous feed of digester effluent into upset digesters yielded of the biogas
production level of the undisrupted digestion. Finally, a mathematical model was used to
describe the relationship between methane potential and GTW feed ratio on a VS basis.
The results of this research can be used to predict methane production and identify
suitable GTW feeding ratios for successful co-digestion of un-dewatered GTW and
MWS