Anaerobic treatment of sewage sludge and grease trap sludge in continuous co-digestion

In recent years many researchers show a high interest in co-digestion, simultaneous anaerobic
decomposition of a homogenous mixture of at least two biodegradable waste. Anaerobic codigestion
is reported to offer several benefits over digestion of separate materials, such as increased
cost-efficiency, increased biodegradation of the treated materials, as well as increased biogas
production. Most often sewage sludge is digested alone while co-digestion with other substrates
could be beneficial.
In this study, the feasibility of co-digestion sewage sludge and grease trap waste (GTW) from meatprocessing
plant was investigated in lab-scale reactor experiment. The research was made on the
sewage sludge coming from municipal wastewater treatment plant and grease trap waste coming
from meat industry company. Anaerobic co-digestion was studied in semi-continuous experiment at
37oC. Feeding of reactors was performed once a day with hydraulic retention time (HRT) of 10 days.
The grease trap waste accounted for 2, 4, 6, 8 and 10 % of the mixture on the volatile solids basis.
The mixtures were analyzed for the following parameters: total solids, volatile solids, pH, volatile
fatty acids and long chain fatty acids (LCFAs). The control of digestion process was made every day
on the basis of the measurement of the biogas production. What is more, there was determined the
volatile solid removal as well the biogas yield in order to assess the efficiency of co-digestion
process.
It was found that co-digestion of sewage sludge and grease trap waste improved both biogas
production and methane content. Grease trap waste addition of 10% of feed VS increased the
biogas production by 16 % as well as methane concentration (72 % of biogas) compared to the
period when reactor was feed only with sewage sludge. Moreover, the addition of GTW to the
anaerobic digestion of sewage sludge increased organic matter removal.
Although, the significant variations in LCFAs reduction, the biogas production and methane yield
increased with higher addition of GTW.
The results of the present laboratory study revealed that the use of GTW as a co-substrate is
considered to be interesting option for sewage sludge digestion due to increased methane
production. However, the feed should be planned carefully with stepwise increase to the desired
feed ratio in order to acclimatize the bacteria and to prevent reactor overloading.