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Two-phase anaerobic digestion (AD) of municipal wastewater separates the hydrolytic-acidogenic phase from the methanogenic phase, resulting in optimization of the respective microbial communities, increased stability, shorter residence times, and increased methane production rates compared to conventional single-phase treatment. Anaerobic co-digestion of fats, oils, and grease (FOG) with municipal wastewater also offers the potential for increased methane production rates; however, FOG can lead to system upsets and overall digester failure if not managed properly. Anaerobic co-digestion in a two-phase system could offer the potential of alleviating many of these upsets, including more efficient conversion of long chain fatty acids (LCFAs) and volatile fatty acids (VFAs) to acetate. This study employed six lab-scale semi-continuously fed reactors to further understand the growth dynamics involved in the two-phase and co-digestion processes by monitoring intermediates and products in the AD pathway and microbial community changes throughout the life of the reactors. Three reactors were operated without FOG: one as a single-phase reactor and the others as a two-phase system. Changes in solids residence time (SRT) were implemented in these reactors to study differences in performance under high and low organic loading rates (OLR). The final three reactors were operated using a 10% FOG loading rate by volume: one as a single-phase co-digesting reactor and the others as a two-phase co-digesting system. The two-phase system without FOG achieved 56.3+/-4.0% CODR at the lowest SRT, while the single-phase reactor achieved 43.6+/-10.0% CODR, indicating similar conversion independent of phased systems. Similar results were also evident with volatile solids (VS) destruction as 53.8+/-6.2% in the single-phase system and 64.4+/-10.4% in the two-phase system at the lowest SRT. Microbial communities remained consistent in the single and two-phase system without FOG, with high abundances of Dechloromonas, Methansaeta, and Methanobrevibacter during periods of high methane production, strongly indicating these organisms play important roles in conversion of influent organics to methane in a stable digester community. In the co-digesting reactors, buffering of feed for the single-phase reactor stimulated the growth of methanogenic bacteria and the subsequent balance with the acidogenic communities during startup but was not required once a stable community was established. This system maintained stable methane production, VS destruction, and carbonaceous oxygen demand reduction (CODR). Propionate, palmitic, and stearic acid conversion lagged during startup, but eventually degraded after Clostridium and Syntrophomonas genera became dominant members of the community, strongly indicating they play a key role in LCFA and VFA conversion. Methanesarcina was the dominant methanogenic genus in the single-phase co-digester, suggesting its importance for conversion when FOG is present. The two-phase co-digesting system never achieved stable methane production, VS destruction, or CODR. Palmitic and stearic acid did not degrade in the acidogenic phase and accumulated in the methanogenic reactor, stabilizing at a concentration of 5 mM, leading to a lag in methane production.

Abstract: In this study, the effect of conductive additives on co-digestion of fat, oil, and grease (FOG) and food waste (FW) was evaluated. Initially, the biochemical methane potential (BMP) test was conducted for the optimization of the mixing ratio of FW and FOG. The optimal methane production (800 L (kg VS)-1) was obtained from co-digestion of 70% FW + 30% FOG (w/w), which was 1.2 times and 12 times of that obtained from mono-digestion of FW and FOG, respectively. This optimal mixing ratio was used for subsequent fed-batch studies with the addition of two conductive additives, granular activated carbon (GAC) and magnetite. The addition of GAC significantly shortened the lag phase (from 7 to 3 d), reduced accumulation of various volatile fatty acids (VFAs), and enhanced methane production rate (50-80% increase) compared to the control and magnetite-amended bioreactor. Fourier transformation infrared (FTIR) analysis suggested that the degradation of lipids, protein and carbohydrates was the highest in GAC amended reactor, followed by magnetite and control reactors. GAC addition also enriched more abundant and diverse bacteria and methanogens than control. Magnetite addition also showed similar trends but to a lesser degree. The substantial enrichment of syntrophic LCFA β-oxidizing bacteria (e.g. Syntrophomonas) and methanogenic archaea in the GAC-amended bioreactor likely attributed to the superior methanogenesis kinetics in GAC amended bioreactor. Our findings suggest that the addition of GAC could provide a sustainable strategy to enrich kinetically efficient syntrophic microbiome to favor methanogenesis kinetics in the co-digestion of FW and FOG.

Australia is one of the major producers and exporter of agricultural products. Annually, Australian agriculture produces approximately 151 Tg CO2 equivalent emissions. The use of fossil fuels in crop cultivation, harvesting and transportation are considered as the primary source of these greenhouse gas (GHG) emissions. Moreover, agronomic management and crop residues left in the field also contribute to these GHG emissions. Alternative waste management practices include the use of crop residues and agro-wastes as feedstocks for bioenergy production. Anaerobic digestion is considered as sustainable environmental technology to convert industrial sugarcane residues to carbon dioxide (CO2) - neutral biogas. The biogas thus produced can be used to produce heat, electricity and upgrade to biomethane for vehicle use. The produced biomethane can replace the diesel consumption associated with GHG emission in cane transport. Sugarcane is one among the most cultivated crop in the world. Australia alone produced nearly 33.5 million tonnes of cane in 2018 (FAO 2018). These large production of sugarcane lead to an increase in crop residues and agro-wastes from the sugarcane industry. In this study, an investigation regarding the anaerobic co-digestion of crop residues and agro-wastes from sugarcane industry viz, sugarcane trash (SCT) or sugarcane bagasse (SCB) with chicken manure (CM) was investigated in a batch experiment at 37 °C. In spite of various researches conducted till date about co-digestion of lignocellulosic waste with manure, no research data was available regarding the effect of feed ratio on co-digestion of SCT/SCB with CM. This research gap was investigated in this study. In addition to this, steam explosion pre-treatment of SCT/SCB was included to investigate how the pre-treatment influence methane yield among different feed ratios of SCT/SCB with CM. At first, SCT and SCB were subjected to steam explosion pre-treatment (steam impregnation at 130 °C for 5 minutes followed by steam explosion). Later, two sets of biochemical methane potential (BMP) tests were conducted at an Inoculum to Substrate Ratio (ISR) of 2. Co-digestion of untreated and steam exploded SCT or SCB with CM was investigated at feed ratios of 75:25, 50:50 and 25:75 on volatile solids (VS) basis. Assays with 100% untreated and steam exploded SCT or SCB were also included. Chemical analysis revealed that the steam explosion improved the VS content in pre-treated biomass compared with untreated biomass. The increase in VS was 1.6% and 5.7% in SCT and SCB, respectively. On the other hand, a slight reduction in total solids (TS) of nearly 4% and 1% were observed in the case of SCT and SCB, respectively. BMP results showed that the steam explosion had a profound effect on the methane production rates and yields, especially for SCB than SCT. Methane (CH4) yields of 201.8 and 199 ml CH4/gVSadded were obtained during the mono-digestion of untreated SCT and SCB, respectively. The corresponding values for 100% steam-exploded SCT and SCB were 207.5 and 225.6 ml/gVSadded, respectively. In comparison to mono-digestion, the co-digestion of SCB or SCT with CM did not improve the methane yields. Nevertheless, pre-treatment improved the methane production rates and yields of pre-treated biomass than untreated biomass. Among the studied feed ratios, best methane yields of 206.5 ml/gVSadded were obtained when steam-exploded SCT was co-digested with CM at 75:25 ratio. However, methane yields decreased with an increase in the amount of CM added. SCB also showed a similar trend. The best methane yield of 199.5 ml/gVSadded was obtained when steam-exploded SCB was co-digested with CM at 75:25 ratio. Among the tested feed ratios, all co-digestion mixtures except for 75:25 and 50:50 ratios of untreated SCT to CM showed synergistic effects. The best synergistic effect of 18.57% was observed when untreated SCB was co-digested with CM at 25:75 ratio. Kinetic modelling results confirmed that the steam explosion pre-treatment improved the methane production rates and yields by increasing the hydrolysis rate constant values. However, a higher hydrolysis rate constant was noticed for SCT than SCB. The highest hydrolysis rate constant of 0.16 d-1 was achieved at feed ratios of 50:50 and 25:75 of pre-treated SCT:CM. Interestingly, more than 75% of methane in pre-treated assays was produced by Day 11. The study thus suggests that the steam explosion can improve the methane production rates, yields and productivity of SCT and SCB. However, the use of CM as co-substrate did not improve the methane yields when compared to the mono-digestion of SCT or SCB, but a positive synergism was evident in most of the co-digestion feed ratios.

Research has demonstrated the abundant treatment and economic opportunities presented by co-digestion of high strength wastewater, but these wastes also have the potential to cause digester failure. Indeed, based on what is known about the disruption of digesters, if a municipality is considering co-digestion, it needs to know the characteristics of high strength wastewaters that have a predictable potential for disruption. The Metropolitan Council-Environmental Services (MCES) is one such municipality interested in pursuing co-digestion. MCES is interested in expanding current digester use to include co-digestion of high strength waste streams to reduce their energy and sludge disposal costs. The objective of this research was to develop a protocol for MCES to be used in evaluating the feasibility of co-digesting municipal wastewater sludge with other high strength wastes. The protocol involves anaerobic biochemical methane potential (BMP) assays inoculated by source anaerobic digesters and amended with PBS, sludge (TPS/TWAS mixture), waste, or waste combined with sludge. Co-digestion experiments examined well-characterized synthetic wastes (starch, gelatin, tween 80, and cysteine) and wastes of interest to MCES with unknown impacts (dimethylformamide, dairy processing waste, propylene glycol, ethylene glycol, thickened primary sludge, thickened waste activated sludge, and scum collected from the primary clarifier at an MCES wastewater treatment facility). These wastes were characterized as degradable, non-degradable, toxic or degradable after an adaptation period. Gelatin, tween 80, dairy processing waste, ethylene glycol, and propylene glycol were degradable, while dimethylformamide was non-degradable. Cysteine, a sulfur-rich waste, was found to be toxic at concentrations greater than 200 mg/L. Scum, ethylene glycol, and propylene glycol required an adaptation period before being degraded. Additionally, the microbial analysis via qPCR indicated that the abundance of Bacteria and methanogens in the co-digestion experiments were not statistically different from the sludge-only experiments, even when different types of wastes were tested (i.e., degradable, non-degradable, toxic, etc.). The developed method and subsequent guidelines will be useful for MCES and other wastewater treatment facilities when making informed decisions about whether or not to accept potential high strength wastes for co-digestion with sludge.

Two different sludge pretreatments were investigated in an attempt to improve the management and performance of processes for the co-digestion of biosolids with fats, oils, and grease (FOG). The mechanisms of long chain fatty acids (LCFA) degradation in thermal hydrolysis pretreatment and the influence of pH on LCFA degradation in two-phase co-digestion systems were studied. LCFA thermal hydrolysis was investigated at different temperatures (90-250 °C) and reaction times (30 minutes and 8 hours). Approximately 1% of saturated fatty acids were degraded to shorter chain fatty acids at 140 and 160 °C (8-hr thermal hydrolysis). Only 1% or less of unsaturated fatty acids were degraded from 90 to 160 °C (8-hr thermal hydrolysis). Little degradation (< 1%) of both saturated and unsaturated LCFAs was observed at a 30-min reaction time. Both groups of LCFAs were stable up to 250 °C (30-min hydrolysis). The use of chemical-thermal treatments was also investigated. Only unsaturated LCFAs, C18:1 and C18:2, were degraded when thermally hydrolyzed with hydrogen peroxide coupled with activated carbon or copper sulfate. Semi-continuous, acid-phase digesters (APDs) under different pH conditions were studied in order to understand the effects of pH on FOG degradation. Increases in soluble chemical oxygen demand (SCOD) were observed in all APDs. However, the APDs with pH adjustment appeared to perform better than the controls in terms of solubilizing organic compounds. Approximately 38% and 29% of total COD (TCOD) was solubilized, and maximum volatile fatty acid (VFA) concentrations of 10,700 and 7,500 mg/L TCOD were achieved at pH 6 and 7, respectively; It is useful to note that the feed sludge had a VFA concentration of 2,700 mg/L COD. Higher pH (6.0-7.0) showed less accumulation of LCFA materials and more soluble LCFAs in the APDs. This indicates that the lower pH in the APDs was most likely the cause of precipitation and accumulation of LCFAs due to saturation of unsaturated LCFAs.

The anaerobic digestion (AD) process is generally classified into two categories based on the total solids (TS) content of the mixture in the digester. TS contents of 15% or greater are classified as solid-state anaerobic digestion (SS-AD) while TS contents of 15% or lower are classified as liquid AD. SS-AD has many advantages such as smaller reactor volume, less moving parts due to no need of agitation, acceptance of a wide range of feedstocks, and less energy demand for heating the material in the digester. Lignocellulosic biomass is a suitable feedstock for SS-AD as it is the most abundant biomass source in the world and is generally a drier material. However, there are no reports on the comparison of SS-AD and L-AD systems in terms of methane yield and volumetric productivity. Yard waste is prevalent year round and is widely available at low cost; thus, yard waste presents itself as an ideal candidate for SS-AD. However, the methane yield of yard waste is generally lower compared to other feedstocks such as crop residues, energy crops, municipal solid waste, fats, oils, and greases (FOG). Therefore, increasing the overall methane yield of the SS-AD via co-digestion of food wastes presents itself as a viable option. Food waste is also prevalent year round, may garner tipping fees, and achieves much higher methane yields than most other substrates. Mesophilic batch anaerobic digestion (AD) of eight lignocellulosic feedstocks including switchgrass, corn stover, wheat straw, yard waste, leaves, waste paper, maple, and pine were carried out under liquid and solid-state anaerobic digestion (SS-AD) for 30 days and evaluated for methane yield and volumetric productivity. Results showed no significant difference in methane yield between the systems except for waste paper and pine. Volumetric productivity of the feedstocks except for waste paper was 70% to 88% greater in the SS-AD system compared with the liquid system. Solid-state mesophilic batch co-digestion was carried out for 30 days using yard waste and food waste to improve overall methane production and volumetric productivity. Results showed increased methane yields and volumetric productivities with increasing the percentage of food waste up to 10% and 20% of the feedstock for feedstock/effluent (F/E) ratio of 2 and 1, respectively. The highest methane yield of 122 L/kg volatile solids (VS) was obtained at F/E ratio of 1 and 20% food waste (based on dry VS). The highest volumetric productivity of 8.59 Lmethane/Lwork was obtained at F/E ratio of 2 and 10% food waste. The reactors with F/E ratio of 3 and 10% and 20% food waste and with F/E ratio of 2 and 20% food waste failed. Volatile solids (VS) destruction of 42.6% was maximized at F/E ratio of 2 and 10% food waste. The results obtained from this study showed that lignocellulosic biomass can achieve comparable methane yields and increased volumetric productivities in SS-AD. In addition, co-digestion of food waste with yard waste demonstrated that overall methane yields and volumetric productivities could be enhanced over SS-AD of yard waste alone.

Lignocellulosic biomass feedstocks, in particular yard waste, are highly desired for anaerobic digestion as they are widely available and tipping fees are commonly associated with disposal. The high concentration of lignin presents a major challenge for utilizing yard wastes as a feedstock for anaerobic digestion. Therefore, additional steps are necessary to increase the biodegradability of yard waste. The initial focus of this study was to test two potential methods for increasing the methane production of yard waste: pretreatment and co-digestion. Sodium hydroxide (NaOH) pretreatment was studied at NaOH concentrations of 3% and 5%. It was found that 3% NaOH pretreatment had no significant improvement on methane production while 5% NaOH pretreatment had a significant negative effect on methane production. The second method studied was co-digestion of yard waste and food waste. Co-digestion at a ratio of 75% food waste and 25% yard waste showed a four-fold increase in methane yield compared to digestion of 50% food waste and 50% yard waste and a two-fold increase in methane production compared to digestion of 100% food waste. Anaerobic digestion of food waste containing protein and other sulfur containing substances produces hydrogen sulfide in biogas. Hydrogen sulfide (H2S) in biogas causes corrosion of metal components such as regulators, gas meters, valves, and mountings. Combustion of biogas containing H2S produces poisonous sulfur dioxide (SO2). The SO2 also dissolves in engine oil causing the oil to become acidic and lose its ability to lubricate. One method for H2S removal involves using hydrated iron oxides supported on a media comprised of wood chips or wood shavings. The iron oxide reacts with the H2S in the gas to form iron sulfide and water. This method is called an iron sponge. The second focus of this study was to test hydrogen sulfide removal using the iron sponge method. The first goal was to determine the optimum operating parameters of an iron sponge system. Using ground yard waste, ideal operating parameters for the iron sponge were determined to be 0.1875” particle size and 15%-25% moisture content. Moisture was found to be a critical factor in the operation of the H2S scrubber system. Optimum moisture content varies with particle size and the materials’ ability to absorb water. The iron sponge needs water to properly facilitate H2S conversion to Fe2S3, but high moisture can be problematic. The second and primary goal of this research was to develop and test iron sponge materials using alternative media to the commercial material, SulfaMaster¿¿¿¿¿¿¿, and to identify those with equal or superior performance. An iron sponge using digested yard waste from an anaerobic digester as a media material at 25% moisture was determined to be an ideal alternative to the SulfaMaster¿¿¿¿¿¿¿ material removing 90% of the hydrogen sulfide in biogas at initial levels near 1000ppm H2S. The digested yard waste had similar performance to SulfaMaster¿¿¿¿¿¿¿ and is an inexpensive substitute. Ground yard waste at 0.1875” particle size performed similarly but results were more erratic. Spent tobacco was not an effective medium for the iron sponge.