1. Abstract: Anaerobic co-digestion (AC) of organic fraction of municipal solid waste (OFMSW), with thickened waste activated sludge (TWAS) and primary sludge (PS) has the potential to enhance the biodegradation of solid waste, increase longevity of existing landfills and
lead to more sustainable development by improving waste to energy production. This study reports on mesophilic batch anaerobic biological methane potential (BMP) assays and semi-continously flow reactors carried out with different concentrations and combinations (ratios) of
OFMSW, TWAS (microwave (MW) pre-treated and untreated) and PS to assess digester stability, improved specific biodegradability and potentiality increased specific biogas production by the various substrate mixtures. Results indicated improvements in specific biogas production
with concomitant improvements in COD and volatile solid (VS) removal from co-digestion of OMSW, TWAS and PS.
Introduction and Objective: Anaerobic biological treatment of the OFMSW can be an prospective alternative to current disposal strategies. Figure 1 shows the basics of anaerobic digestion (AD) where organics are converted to long chain fatty acid then short
chain fatty acid and after awards degraded to CH4, CO2 and H2. AD of OFMSW or municipal sludge from waste water treatment plants (WWTP) by themselves as single substrates frequently faces problems such as imbalance in the C:N ratio, hydrolysis limiting conditions, buffering
capacity or nutrient shortages. Co-digestion of OFMSW and TWAS/PS has been proven an attractive alternative for the management of these two separate waste streams in Europe and America. Figure 2 illustrates the idea of counter balancing capacity of these two type of waste if they
are co-digested together. So, the main objective of this study was to compare and evaluate the benefits of mesophilic co-digestion of OFMSW, TWAS and PS using batch BMP assays and semi-continuous reactors.
Materials and Methods: OFMSW, TWAS and PS were used as substrates for AC. OFMSW simulated kitchen waste consisted of cooked rice (17.5 w%), cooked pasta (17.5 w%), carrot (11 wt%), apple (11 wt %), banana (11 wt %), cabbage (11 wt %), ground beef
(10 wt %), dog food (10 wt %) and biodegradable garbage bags (1 wt %) (Figure 3). TWAS (approx. 5% w/w TS), PS (approx. 4% w/w TS) and mesophilic anaerobic inoculum were obtained from Ottawa’s Robert O. Pickard Environmental Center. The VS/TS ratios of OFMSW, PS,
TWAS and anaerobic inoculum were 0.73±0.01, 0.94±0.01, 0.77±0.01 and 0.57±0.01 respectively.
500 mL Kimax glass bottles capped with butyl rubber stoppers were used to perform mesophilic BMP assays, shown in Figure 4. To investigate the effect of co-/tri- digestion, BMP assays were conducted at same initial specific load which was 3.5±0.08% gVS/assay to keep the VS or TS
concentration of TWAS unchanged during the entire experiment. Prior to feeding, OFMSW was diluted to approximately 5% by adding distilled water to match with the concentration of TWAS and PS. Mixing combinations are shown in Figure 5 and Figure 6.
Erlenmeyer flasks (1L) with a working volume of 600 mL sealed with two-hole rubber stoppers were used as reactors , shown in figure 7. Ports in the rubber stopper were used to collect biogas and to withdraw and add substrate. Reactors were fed daily in a semi-continuous mode, i.e.
by first withdrawing and then adding substrate. Biogas was collected in Tedlar bags (1L) and production was measured daily with a U-tube manometer. All reactors were maintained at 35C in a New Brunswick incubator at a rotational speed of 95 rpm. Figure 8 shows the schematic of
the conditions tested for AC for binary mix of TWAS:PS:50:50 and OFMSW:TWAS:PS:50:25:25 at three different organic loading rates( OLRs) and hydraulic retention time(HRTs).
Results: Although all the BMP assays tested were loaded at equal organic loading rate of 3.5% of gVS/assay biogas productions from all the combinations tested were found to be higher than the biogas produced by OFMSW and TWAS digested individually. A hypothetical term
called calculated expected biogas (CEB) production, calculated based on the weighted average of the biogas production from each of the individual single substrates and the portion of each waste in the binary mix. Biogas yield from low, medium and high organics mix was 48%, 76%
and 140% higher than their respective CEBs. Figure 9 shows biogas improvement results from OFMSW:TWAS at 50:50 combination. Figure 10 shows a hypothetical relation among biogas improvement and OFMSW:TWAS/PS combinations. Improvements in biogas production were
also observed from the trinary mix AC and figure 11 shows the percentages of improvement in biogas production from trinary mix AC over binary mix AC. Enhancement in biogas production from trinary mix of OFMSW,TWAS and PS over TWAS and PS mix ( Presented in Figure 12)
illustrates the benefits co-digestion of OFMSW with under loaded PS/TWAS digesters at MWWTPs can be accomplished via a time sliding diminishing supply contract between the local MWWTPs and suppliers of OFMSW ( More precisely described in Figure 13. Results from semi-
continuously flow reactor were not different than batch test illustrated in figure 14. A interactive comparison study (shown in Figure 15) draws the conclusion that effect of micro-wave pretreatment of TWAS in biogas enhancement was not that significant while % of OFMSW has a
second order interactive effect on improvement of biogas production.
Conclusion: Integrating conventional PS+TWAS anaerobic digestion at MWWTPs with co-digestion of OFMSW has a number of potentially positive outcomes for the AD plant: (1) increased stability of the AD process,
(2) increased biogas production for energy under Feed in Tariff (FIT) rates, (3) increased specific biogas yields, (4) alternative management plan for OFMSW, and (5) reduced use of landfill space.
Hydrolysis
rate limiting
Figure 1 Basics of AD Figure 2 Basic idea of anaerobic co-digestion (AC) is counter balancing
Figure 3 OFMSW mix which was used in the blend
Figure 4 BMP assays test for batch test of AC Figure 5 Experimental setup for binary mix AC Figure 6 Experimental setup for trinary mix AC
Figure 7 Semi-continuously flow reactor test of AC Figure 8 Schematics of different OLRs and HRTs for semi-continuously flow reactor
Figure 9 Cumulative biogas production from OFMSW:TWAS:50:50 and single
substrates
Figure 10 Hypothetical relation among
OFMSW,TWAS/PS and biogas
production
Figure 11 Comparison of specific methane yield from trinary
mix AC over binary mix AC Figure 12 Comparison of specific biogas yield from trinary mix
AC over binary mix of TWAS and PS
Figure 13 A time sliding diminishing supply contract between
the local MWWTPs and suppliers of OFMSW
Figure 14 Results shows improvement of biogas yield
from trinary mix than binary mix in semi-continous study
Figure 15 RSM results shows improvement of biogas
yield was only a second order interactive effect
Anaerobic Co-digestion of OFMSW and Bio-solids--- An Effective Approach Towards Integrated Solid Waste
Management
Efath Ara, M.A.Sc & Kevin Kennedy, Ph.D; University of Ottawa
Acknowledgments:The authors would like to thank Lafleche Environmental Inc. for financial support for this project.