How to train your digester - Using step and pulse feeding of grease waste to increase community resistance and methane yield by up to 336

摘要

The disposal of grease interceptor waste (GIW) from food service establishments remains a major challenge due to its high fat, oil, and grease (FOG) content Co-digesting GIW with sewage sludge in locally available digesters provides a cost-effective disposal alternative while simultaneously increasing biogas production to offset onsite energy costs. Despite this tremendous potential, implementation of GIW co-digestion is limited due to the potential inhibition of methanogenesis because of GIW overloading. When introducing lipid-rich materials that are high in chemical oxygen demand (COD) and FOG concentrations, accumulation of major intermediates (e.g. long chain fatty acids, propionate, and acetate) can cause drops in pH, adverse shifts in microbial populations, inhibition of bacterial growth, and consequently, process failure. In this study, we explored two ways of achieving greater resistance to GIW inhibition: step loading (gradual increases in loading rate) and pulse feeding (short periods of overloading), through which beneficial microbial shifts by active adaptation were developed. We conducted step and pulse feeding experiments at mesophilic conditions (37°C) and a solids retention time of 20 days. Mixtures of GIW and thickened waste activated sludge (TWAS) at different organic loading rates were treated in two lab scale semi-continuous 8 L digesters (one control and one treated). Step and pulse bioreactor studies showed positive effects on the development of microbial adaptation and reduction in inhibition of methanogenesis. Methane yield was significantly increased through the use of step feed, achieving the highest methane yield ever reported for GIW co-digestion (0.785 L CH_4/g VS added, representing a 336% increase over the baseline). We applied ecology principles and demonstrated that high load pulses increased digester resistance to GIW overloading and allowed methane yield up to 0.748 L CH_4/g VS added. To better understand the microbial population dynamics in response to high strength GIW co-digestion, we conducted a 16S metagenomics study on forty genomic DNA samples collected throughout both experiments. Next-generation (Illumina) sequencing of the 16S rRNA gene of bacteria and archaea and subsequent bio informatics analyses were performed to determine the key microbtal populations that limit the anaerobic pathway to methane under high GIW loading conditions. The preliminary results indicated major archaeal composition shifts occurred among Methanosaeta spp., Methanospirillum spp., Methanolinea spp., and Methanoculleus spp. with addition of 70% (w/w)GlW. The significant increases in Methanosaeta spp. (acetate-degrading methanogens) and Methanospirillum spp. (hydrogen-degrading methanogens) after GIW treatment suggests that these two groups of microorganisms very likely played important roles in increasing methane production and reducing GIW inhibition, specifically the accumulation of acetate and hydrogen during higher GIW loading rate. Additionally, bacterial populations at the phylum level showed increases in relative abundance of Bacteroidetes, Firmicutes, Proteobacteria, and Synergistetes. More in-depth bioinformatic analyses are ongoing to reveal key microbial population dynamics throughout both bioreactor experiments. These results are expected to give the first insights into the high-yield yet resistant mtcrobial communities developed at GIW loading rates previously thought inhibitory. This knowledge will greatly benefit future full-scale applications in optimizing digester performance and resistance.