More Methane and Less Landfilling? The Case for Anaerobic Co-digestion of Yard Trimmings and Wastewater

With many municipalities having excess anaerobic digester capacity, anaerobic co-digestion (AcoD) provides an opportunity to improve biogas recovery while potentially reducing landfilling of organic municipal waste streams. In the United States, 12% of municipal solid wastes are yard trimmings (YT), which are often sent to landfills or composting for disposal (US EPA 2021a). However, YT landfilling and composting releases 475-1100 and 20-130 kg CO2 eq. per dry ton of fugitive methane (CH4) and nitrous oxide emissions per year, respectively (Brown and Chang 2014; Zhu-Baker et al. 2017). Utilizing yard trimmings as a co-digestion stream improves energy recovery, meets higher renewable energy credits (REU) credit standards than food waste and fats/oils/grease, and reduces climate impact due to fugitive emissions of CH4 and N2O. In this work, we developed nature-inspired bacterial enrichments for cellulose degradation and use them to pretreat common yard wastes to the Southwest and a stock chemical cellulose mixture. We evaluate the impacts of pretreatment using our inoculum and its improvement of CH4 production and landfilling offsets through sludge reduction as part of AcoD with thickened sludge. We cultivated lignocellulose-degrading batch inoculum over 10 generations based on feces obtained from Oryctolagus cuniculus domesticus, who primarily eat hays, grasses, and leafy matter with a digestive retention time of about 4 days. The inoculum were exposed to a variety of typical Phoenix yard waste & lantana and grass & or purchased cellulose. Each generation of inoculum was inoculated for 10-14 d, and then 1/10 of the total volume was added as seed for the next inoculum generation. An inoculum was used as the seed for fermenting reactors with 5-14 d hydraulic retention times (HRTs). Rehydrated plant matter was fed into the fermenters at 40 to 80 g DW/L each of lantana, grass, and cellulose. Fermented material was also fed to anaerobic co-digesters. Thickened sludge and seed anaerobic digester inoculum were provided by the City of Mesa, Arizona's Northwest Water Reclamation Plant (MNWWRP). To test pretreatment coupled with anaerobic digestion, AcoD reactors were fed the same amount of thickened sludge as the control plus an additional amount of material from the fermenters to maintain a 19.2-d HRT. We had two controls in the system: (1) only thickened sludge fed to control reactor was operated at a 25-d HRT to mimic MNWWRP; and (2) the same amount of thickened sludge was fed as a control plus 80 g DW/L undigested plant matter at a 19.2-d HRT. The reactors were allowed to operate at least 3 HRTs to achieve steady state prior to taking the data reported here. To ensure reactors would not sour, the pH in all reactors was kept at 7.5 ± 0.2 using NaOH for pH adjustment and CaCO3 to improve buffering capacity. We monitored pH, total and soluble COD (TCOD; SCOD), total and volatile solids (TS; VS), CH4, hydrogen gas, the volatile fatty acids (VFAs) of formate, acetate, propionate, butyrate, valerate, and caproate, and lignocellulose content based on an Ankom 2000 fiber analyzer. YT prefermentation provided increased solubilization of plant matter to VFAs and with negligible biogas production. For example, VS decreased an average 29% at a 14-d HRT with 80g DW/L loading of YT. At the same time, SCOD increased 20% leaving the reactor, with 6.3% being converted to VFAs. While the analysis is still in process, it appears that proteins in the leaf matter and some hemicellulose were hydrolyzed to soluble forms. Our initial results show that AcoD fed prefermented YT has higher CH4 production and better volatile solids reduction than the control or AcoD with untreated YT. As an example using the prefermented YT discussed above, Figure 1 shows that AcoD fed prefermented YT and thickened sludge averaged the highest CH4 production, 330 mg CH4/d, versus 270 mg CH4/d for the control and 160 mg CH4/d for AcoD with untreated YT. The AcoD with prefermented YT also achieved the highest sludge reduction, 52%, versus 40% with the control and 43% AcoD with untreated YT. At the same time, the AcoD with prefermented YT had a lower yield of CH4 per g VS reduced (610 mL CH4/g VS) versus the control (914 mL CH4/g VS). This lower yield is likely due to lignocellulose being highly recalcitrant. The encouraging results with AcoD fed prefermented YT could have far reaching impacts on the wastewater and AcoD community. Based on these results for 4000 m3 (~879,000 gallon) capacity anaerobic digester, AcoD fed prefermented YT produces 184,000 m3/yr more CH4 than the control and 805,000 kg/yr of YT diverted from landfilling. In addition, the gas produced from YT qualifies for D3 cellulosic RIN credits as part of the EPA's Renewable Fuel Standards program versus other co-digestion streams like food waste and fats/oils/grease (FOG), which qualify for D5 RIN credits. Based on September 20, 2021, data, D3 RINs were worth $1.13 more than D5 RINs (US EPA 2021b).