Anaerobic Digesters Treating Food Waste Research Articles

Combined Pre-Treatment Technologies for Cleaning Biogas before Its Upgrading to Biomethane: An Italian Full-Scale Anaerobic Digester Case Study

Biogas produced by anaerobic digestion contains different types of contaminants, and it is preferable to eliminate those contaminants before biogas’ energetic valorization or upgrading to biomethane as they are harmful to human health and detrimental to combustion engines. This study presents the biogas cleanup system optimized by an Italian full-scale anaerobic digester treating food waste (FW) and represented by micro-oxygenation, chemical scrubber, cooling, and activated carbon sections. The cleaned biogas is upgraded to biomethane using a membrane-based upgrading unit and injected into the natural gas network for transport sector use. H2S and volatile organic compound (VOC) concentration in raw biogas was reduced from an annual average value of 1207 ppmv and 895 mg/Nm3, respectively, to below 0.1 mg/Nm3 in the final biomethane. In the summer, the H2S average content in raw biogas was 833 ppmv due to a greater presence of low-sulfur-containing vegetables in FW, while in the winter it was an average of 1581 ppmv due to a larger portion of protein-containing FW. On the other hand, raw biogas VOC content in the winter was an average of 1149 mg/Nm3, with respect to 661 mg/Nm3 in the summer, due to the greater consumption of citrus fruits containing high amount of terpene compounds. The concentration of other trace contaminants, such as HCl, NH3, and siloxanes, was lowered from 17, 36, and 0.6 mg/Nm3 in raw biogas, respectively, to below 0.1 mg/Nm3 in the final biomethane. All the considerations and evaluations underlying the technological and plant engineering choices together with the individuation of the best operating conditions are discussed.

Pulsed CO2 feeding promotes methane yield in anaerobic digester by diverting acidogenic pathways

CO2 injection or biogas recirculation can affect the process of anaerobic digestion, but there is a lack of systematic study on the injection strategy in the continuously operated reactors. In this study, the performance of a continuous stirred anaerobic digester treating food waste were evaluated under different injection rates (RC1-1.32 L/L/d v.s. RC2-0.66 L/L/d) and different CO2 injection modes (pulsed feeding v.s. continuous feeding). Under the pulsed CO2 feeding mode, the methane yields of RC1 and RC2 were increased by 19.3% and 27.1%, respectively as compared to the control reactor, correspondingly 34.5% and 59.0% of injected CO2 was reduced. When transferred to continuous feeding CO2 mode, only 12.4% increment of methane yield was found in RC1 as compared to RB. The intermediate metabolites analyses indicated that CO2 enrichment reinforced acetic acid and lactic acid production pathway and weakened the propionic acid pathway, which is favorable for elevating methane yield. Microbial community analysis revealed that the relative abundance of hydrogenotrophic methanogens increased accompanied with some acidogenic consortia and CO2 fixation associating consortia. This work provides a new approach to promote methane generation from organic wastes with sequestered more CO2 thus contributing to a circular economy.

Electro-polarization of the sludge with dynamic magnetic field enhanced the interspecies electron transfer in ZVI-added anaerobic digesters

Zero-valent iron (ZVI) has been widely reported to strengthen anaerobic digestion; however, ZVI passivation prevents its continuous function. In this study, a dynamic magnetic field (DMF) was imposed on a ZVI-added anaerobic digester treating food waste to promote microbial interspecies electron transfer to enhance methane production. Results showed that methane production with DMF was 15.3% higher than that without DMF. Meanwhile, the Fe(II) concentration from ZVI increased by 39.46% with DMF, but the electron equivalent was far less than the increased methane production even if all electrons from the ZVI were used for hydrogenotrophic methanogenesis. In agreement, the relative abundances of hydrogenotrophic methanogens were less changed after DMF. Notably, an induced electric current ranging from −0.017–0.021 μA was detected with DMF. With the DMF-induced current as electrical stimulation, the sludge conductivity and capacitance increased by 242% and 18%, respectively. In-situ electrochemical tests on the functional groups of the sludge showed that the protein-like substances were polarized by the induced electromotive force to enhance the proton-coupled electron transfer in the sludge. Together with the enriched electrotrophic methanogens, the enhanced microbial interspecies electron exchange was the main reason for the improved performance of anaerobic digestion with DMF.

Improving the Kinetics of H2-Fueled Biological Methanation with Quinone-Based Redox Mediators

The biomethanation process involves the conversion of CO2 into a valuable energy carrier (i.e., methane) by methanogenic archaea. Since it can be operated at mild conditions, it is more sustainable than traditional chemical approaches. Nevertheless, the efficacy of biomethanation is limited by the low kinetics of the microbiological reaction and the poor solubility of H2 in water. Herein, the effect of soluble (i.e., AQDS) and insoluble (i.e., biochar) quinone-based redox mediators on the kinetics of H2-fueled biological methanation in bench-scale microcosms was investigated. Microcosms were set up in 120 mL serum bottles and were initially inoculated with a methanogenic sludge deriving from a lab-scale anaerobic digester treating food waste. As a result, the kinetics of H2 consumption and CH4 generation were greatly increased (p < 0.05) in presence of AQDS as compared to the control, accounting for up to +160% and +125% in the last experimental cycle, respectively. These findings could be explained by a two-step mechanism, whereby microbes used H2 to quickly reduce AQDS into the highly soluble AH2QDS, which in turn served as a more efficient electron donor for methanogenesis. In contrast, the used biochar had apparently an adverse effect on the biomethanation process.

Enhancing anaerobic digestion of food waste with granular activated carbon immobilized with riboflavin

Previous studies have shown that anaerobic digestion of food waste can be enhanced by addition of conductive materials that stimulate direct interspecies electron transfer (DIET) between bacteria and methanogens. However, at extremely high organic loading rates (OLRs), volatile fatty acids (VFAs) still tend to accumulate even in the presence of conductive materials because of an imbalance between the formation of fermentation products and the rate of methanogenesis. In this study, granular activated carbon (GAC) immobilized with riboflavin (GAC-riboflavin) was added to an anaerobic digester treating food waste. The GAC-riboflavin reactor operated stably at OLRs as high as 11.5 kgCOD/ (m3·d) and kept VFA concentrations below 69.4 mM, COD removal efficiencies, methane production rates, and biogas methane concentrations were much higher in the GAC-riboflavin reactor than the GAC- and non-amended reactors. Transcripts associated with genes that code for proteins involved in DIET based metabolism were somewhat more highly expressed by Methanothrix in the GAC-riboflavin reactor. However, it is unlikely that riboflavin acted as an electron shuttle to stimulate DIET. Rather, it seemed to provide nutrients that enhanced the growth of microorganisms involved in the anaerobic digestion process, including those that are capable of DIET.

Startup of Demo-Scale Anaerobic Digestion Plant Treating Food Waste Leachate: Process Instability and Recovery.

A demo-scale (600 m3 working volume) anaerobic digester treating food waste leachate was monitored during its startup period. The operation strategy was adjusted twice (i.e., three distinct phases) during the operation to recover the process from instability. During the first phase, the organic loading rate (OLR) > 2.7 kg chemical oxygen demand (COD)/m3∙day corresponded to volatile fatty acid (VFA) accumulation along with a decreasing pH, resulting in the drop in biogas yield to 0.43 ± 0.9 m3/kg CODin. During phase 2, fast recovery of this process was aimed at using a sequencing batch operation. One batch cycle (5 to 2 days) consisted of the combined drawing and feeding step (5 h), the reacting step (91 to 17 h), and the settling step (24 h). The duration of the reacting step was determined for each cycle such that (1) the biogas production ceased before the cycle end and (2) the residual VFA concentration was < 1 g/L. In total, 11 cycles were operated with a gradual increase in biogas yield to 0.55 m3/kg CODin with the absence of any sign of system disturbance. After phase 2, the digester was fed at the designed OLR of 4.1 ± 0.3 kg COD/m3∙day. The biogas yield was elevated to 0.58 ± 0.2 m3/kg CODin during phase 3 with the residual VFA concentration maintained at 2.2 ± 0.6 g/L. Methanogen populations, as determined by real-time PCR, did not change significantly throughout the period. These results imply that the adaptation of this process to the OLR of ca. 4 kg COD/m3∙day was not due to the increase in methanogen population but due to the elevation of its activity. Overall, this study suggests that the sequencing batch operation with adjustable cycle duration can be one successful recovery strategy for biogas plants under system instability.

Anaerobic digestion of food waste coupled with biogas upgrading in an outdoors algal-bacterial photobioreactor at pilot scale

This work aimed at integrating the anaerobic digestion of food waste (FW) with photosynthetic biogas upgrading at pilot scale in order to obtain a high quality biomethane and a nutrient-laden algal biomass as the main byproducts from FW treatment. The performance of a 100 L anaerobic digester treating food waste integrated via raw biogas and digestate injection with a 1.2 m2 outdoors high-rate algal pond (HRAP) was evaluated. Biogas production in the digester averaged 790 ± 89 mL g VSin-1 (68 ± 8 L d-1) (35 °C, 1 bar) at a loading rate of 0.86 g VS L-1 d-1 and a steady state chemical oxygen demand removal efficiency of 83 ± 7%. The biogas produced (60% CH4 / 39% CO2) was upgraded in a 2.5 L absorption column interconnected with the HRAP via culture broth recirculation at a liquid to biogas ratio of 2, resulting in a maximum CO2 removal efficiency of 90% and a maximum CH4 content of 93.9%. The HRAP, supplied with the centrifuged liquid digestate supplemented with synthetic wastewater (5.0 ± 1.1 L d-1, Total nitrogen (TN) = 793 ± 110 mg N L-1, P-PO43- = 39 ± 19 mg P L-1), supported TN and total phosphorus maximum removal efficiencies of 100% in both cases. Pseudoanabaena sp. and Chlorella vulgaris were identified as the dominant species.

Substrate Characteristics Fluctuations in Full-Scale Anaerobic Digesters Treating Food Waste at Marginal Organic Loading Rates: A Case Study

The design of a full-scale bioprocess is typically based on parameters derived from smaller-scale experiments from a previous study. However, disagreements often occur at up-scaling of waste-to-energy processes due to the fluctuations of the substrate characteristics, etc. Therefore, once a commercial-scale waste digester has been built and operated, it is essential to test if the performance of the process agrees with its design value; during this process, fluctuations might occur in digesters operated at marginal organic loading rates. In this study, triplicate full-scale anaerobic digesters treating food waste were monitored for five months. The digesters, operated at the design feeding ratio, showed increasing volatile fatty acid (VFA) trends (per total alkalinity) due to a 30% higher chemical oxygen demand of the influent, than the design. The organic loading rate was adjusted on a daily basis until a stable performance was observed. Significant shifts of methanogen populations from Methanobacteriales to Methanomicrobiales and Methanosarcinales were observed during the stable operation period.

Valorization of poly-\u03b2-hydroxybutyrate (PHB)-based bioplastic waste in anaerobic digesters of food waste for bioenergy generation: reactor performance, microbial community analysis, and bioplastic biodegradation

This study aims to investigate the significance and biodegradation pathways of PHB-based bioplastic in anaerobic digesters treating food waste, where the reactor performance of changed methane generation, bioplastic biodegradation efficiency, and bioinformatic analysis of functional microbes were emphasized. The results showed that PHB-based plastic film could be partially biodegraded in the food waste digester, and a bioaugmentation use of Alcaligenes Faecalis (AF) and Bacillus Megaterium (BM) was beneficial to largely accelerate the degradation process through a beneficial shift of both the functional bacterial and archaeal species. Microbial community analysis indicated that the major bacterial species belonged to genera Candidatus_Cloacimonas, Rikenellaceae, and Defluviitoga, while the dominant methanogenic archaeal species belonged to genera Methanomassiliicoccus, Methanosarcina, and Methanosaeta. Bioplastic biodegradation analysis suggested that the optimal fractions of AF and BM for PHB-based plastic degradation were 50%AF and 75%BM, respectively, which deserves further optimization and scale-up validation. The finding of this study would contribute to the combined management of PHB-based bioplastic with food waste for clean energy recovery and a greener environment.

Effect of total solids contents on the performance of anaerobic digester treating food waste and kinetics evaluation

The effects of total solids (TS) content on the performance of anaerobic digestion (AD) treating food waste (FW) and kinetic characteristics were investigated in mesophilic batch reactors. The results showed that FW could be digested normally in the AD system within the TS of 3%-30%. The volume methane yield of dry-AD increased by 5.2-10.6 times than that of wet-AD and the degradation time of unit substrate was shortened by 35%-71%. The system stability indicators of dry-AD, such as pH, VFA/TA, TAN and salt remain within the suppression threshold after digestion. dry-AD had obvious advantages. The kinetic analysis showed that ADM1 could be used to simulate the anaerobic digestion reactor of FW, and accurately simulated the methane production in different TS systems after calibrating. With the increase of TS, the hydrolysis and methane production showed a tendency to increase first and then decrease, but the decrease rate of hydrolysis was higher than that of methanogene. Increasing TS simultaneously weaken the hydrolysis rate and methanation rate, but the effect on the hydrolysis and acidification phase weaker. The ratio of the hydrolysis rate constant to the methanogenic Monod maximum specific absorption rate (khy/km_ac) was a new perspective to used to evaluate the balance between the hydrolysis stage and the methane production stage. The results show that the khy/km_ac ratio of the 15% TS and 20% TS experimental groups was close to 1. It can be speculated that the system TS between 15%-20% can be used as recommended value in anaerobic digestion engineering design to treat FW. Within this TS range, hydrolysis and methane production dynamics matching, the accumulation of inhibitors is also in a relatively moderate state in the threshold.

Dynamic behaviors of batch anaerobic systems of food waste for methane production under different organic loads, substrate to inoculum ratios and initial pH

This study characterized dynamic behaviors of batch anaerobic digesters treating food waste in terms of methane production, organics destruction and process stability under different organic loads (OLs), substrate to inoculum (S/I) ratios [on volatile solid (VS) basis] and initial pH. The results showed that OL, S/I ratio and initial pH significantly affected batch anaerobic process. Methane yield was proved to be inversely proportional to OL and S/I ratio. Digester with lowest OL (5g VS/L) obtained greatest methane yield (551.4mL/g VS), highest organics removal (94.1%) and good stability. Enhancing OL to 10g VS/L was recommended for satisfactory stability and higher volumetric methane productivity. When OL was designated as 10g VS/L, digester with low S/I ratio (1/2) achieved satisfactory methane yield (539.3mL/g VS), high organics removal (92.3%) and stable performance. When OL was relatively high (20g VS/L), adjusting initial pH to 7.5 contributed to stable performance via enhancing buffering capacity against volatile fatty acids (VFA) disturbance. Strong VFA inhibition occurred under high OL (40g VS/L) or great S/I ratio (2/1) or acidic initial pH (6.5). In this case, acetate was dominant VFA, followed by butyrate. However, when digester was stable, acetate was main VFA, followed by propionate. This study provided practical guidance on process configurations for batch digesters of food waste needed to achieve satisfactory performance and stability.

Microbial community structure in full scale anaerobic mono-and co-digesters treating food waste and animal waste

Five mesophilic full-scale anaerobic digesters treating food waste (FW-digester), animal waste (AW-digester), and co-substrate of food waste and animal waste (CO-digesters) were monitored identify bacterial and archaeal communities and to quantify the effect of substrate characteristics on them, and to identify ‘core’ microorganism. The substrate characteristics and microbial communities of the FW-digester, AW-digester, and CO-digesters were statistically different. Organic concentration and [Na+] were identified as major variations that effect microbial community. Methanogen community was more diverse in AW-digester than in FW-digester. Methanogen community in CO-digester was as diverse as in AW-digester, and the most dominant species was Methanoculleus bourgensis same as in FW-digester. Twenty-one bacterial genera and four methanogen species were found in all digesters as a consequence of their metabolic versatility to degrade organic and inhibitor compounds. The results implied that these core microorganisms may contribute to maintaining a stable microbial ecosystem.

Correlation between system performance and bacterial composition under varied mixing intensity in thermophilic anaerobic digestion of food waste

This study examines the stability and efficiency of thermophilic anaerobic digesters treating food waste under various mixing velocities (50–160 rpm). The results showed that high velocities (120 and 160 rpm) were harmful to the digestion process with 18–30% reduction in methane generation and 1.8 to 3.8 times increase in volatile fatty acids (VFA) concentrations, compared to mild mixing (50 and 80 rpm). Also, the removal rate of soluble COD dropped from 75 to 85% (at 50–80 rpm) to 20–59% (at 120–160 rpm). Similarly, interrupted mixing caused adverse impacts and led to near-failure conditions with excessive VFA accumulation (15.6 g l−1), negative removal rate of soluble COD and low methane generation (132 ml gVS−1). The best efficiency and stability were achieved under mild mixing (50 and 80 rpm). In particular, the 50 rpm stirring speed resulted in the highest methane generation (573 ml gVS−1). High-throughput sequencing of 16S rRNA genes revealed that the digesters were dominated by one bacterial genus (Petrotoga; phylym Thermotogae) at all mixing velocities except at 0 rpm, where the community was dominated by one bacterial genus (Anaerobaculum; phylum Synergistetes). The Petrotoga genus seems to have played a major role in the degradation of organic matter.

Adaption of microbial community during the start-up stage of a thermophilic anaerobic digester treating food waste

A successful start-up enables acceleration of anaerobic digestion (AD) into steady state. The microbial community influences the AD performance during the start-up. To investigate how microbial communities changed during the start-up, microbial dynamics was analyzed via high-throughput sequencing in this study. The results confirmed that the AD was started up within 25d. Thermophilic methanogens and bacterial members functioning in hydrolysis, acidogenesis, and syntrophic oxidation became predominant during the start-up stage, reflecting a quick adaption of microorganisms to operating conditions. Such predominance also indicated the great contribution of these members to the fast start-up of AD. Redundancy analysis confirmed that the bacterial abundance significantly correlated with AD conditions. The stable ratio of hydrogenotrophic methanogens to aceticlastic methanogens is also important to maintain the stability of the AD process. This work will be helpful to understand the contribution of microbial community to the start-up of AD.

Dynamics of microbial community in a mesophilic anaerobic digester treating food waste: Relationship between community structure and process stability

Organic loading rate (OLR) disturbances were introduced into a mesophilic anaerobic digester treating food waste (FW) to induce stable and deteriorative phases. The microbial community of each phase was investigated using 454-pyrosequencing. Results show that the relative abundance of acid-producing bacteria and syntrophic volatile fatty acid (VFA) oxidizers increased dramatically at deteriorative phase, while the dominant methanogens did not shift from acetoclastic to hydrogenotrophic groups. The mismatching between bacteria and methanogens may partially be responsible for the process deterioration. Moreover, the succession of predominant hydrogenotrophic methanogens reduced the consumption efficiency of hydrogen; meanwhile, the dominant Methanosaeta with low acetate degradation rate, and the increase of inhibitors concentrations further decreased its activity, which may be the other causes for the process failure. These results improve the understanding of the microbial mechanisms of process instability, and provide theoretical basis for the efficient and stable operation of anaerobic digester treating FW.

Performance optimization and validation of ADM1 simulations under anaerobic thermophilic conditions

In this study, two experimental sets of data each involving two thermophilic anaerobic digesters treating food waste, were simulated using the Anaerobic Digestion Model No. 1 (ADM1). A sensitivity analysis was conducted, using both data sets of one digester, for parameter optimization based on five measured performance indicators: methane generation, pH, acetate, total COD, ammonia, and an equally weighted combination of the five indicators. The simulation results revealed that while optimization with respect to methane alone, a commonly adopted approach, succeeded in simulating methane experimental results, it predicted other intermediary outputs less accurately. On the other hand, the multi-objective optimization has the advantage of providing better results than methane optimization despite not capturing the intermediary output. The results from the parameter optimization were validated upon their independent application on the data sets of the second digester.

Comparative study of methanogens in one- and two-stage anaerobic digester treating food waste

AbstractChanges in methanogenic archaea were investigated in pilot-scale experiments during one- and two-stage mesophilic anaerobic digestion (AD) of food waste. Methane yields were 379.7±75.3 liters of methane per kg of volatile solids [L-CH4(kg-VS)−1] added to the system, during one-stage operation, and 446±922 L-CH4(kg-VS)−1added during two-stage operation. Populations of methanogenic archaea were monitored quantitatively by targeting the functional gene for methyl-coenzyme-M reductase (mcrA) using real-time quantitative polymerase chain reaction techniques. During one-stage operation, meanmcrAgene concentrations were 2.48×109±2.7×109copies ml−1. Two-stage operation yielded meanmcrAgene concentrations of 9.85×108±8.2×108copies ml−1in the fermentation and 1.76×1010±8.5×109copies ml−1in the methanogenesis reactors, respectively. Diversity of archaea in the methanogenic reactors was investigated by denaturing gradient gel electrophoresis targeting the V3 region of 16S rRNA of archaea. The Shannon index (H) was 2.98 for one-stage operation and 7.29 for two-stage operation, suggesting greater archaeal diversity in the two-stage AD. The fivefold increase in methanogenic archaea populations during the two-stage operation, as indicated bymcrAgene concentration, corresponded to an increase in methane production rates. While the diversity may also be related to the stability of the microbial bioprocesses and improved methane production rates, the correlation between diversity and production rates should be studied further.

Performance and kinetic evaluation of a semi-continuously fed anaerobic digester treating food waste: Effect of trace elements on the digester recovery and stability

This study investigated the effects of trace elements (TEs: Fe, Co, and Ni) on the anaerobic digestion of food waste in laboratory scale semi-continuously fed anaerobic digesters. The digesters were operated for approximately 460d at organic loading rates (OLRs) ranging from 1.0 to 5.0gVSL−1d−1 at 37°C. Results showed that methane production was severely inhibited when volatile fatty acids (VFAs) concentration was 30000mgL−1 at OLR of 4.0gVSL−1d−1 without the addition of TEs. However, the inhibition was gradually decreased and the methane production increased from 0.13 up to 0.44Lg−1VSadded when the TEs was added. Fe was proved to be the essential element in maintaining the stability of the digester. The conversion coefficient of substrate into biogas (YS/G) was 1.00 and 1.26gVSremovedL−1 biogas with and without TEs addition, respectively. The estimated biomass yield with feeding TEs (0.06gVSSg−1CODremoved) was higher three times than that without TEs (0.02gVSSg−1CODremoved).

Performance and kinetic evaluation of semi-continuously fed anaerobic digesters treating food waste: Role of trace elements

This study investigated the effects of trace elements (TEs) on the anaerobic digestion (AD) of food waste (FW) in laboratory-scale semi-continuously fed anaerobic digesters. The duration of digesters operation was approximately 400days. Organic loading rates (OLRs) ranged from 1.0 to 5.5gVSL−1d−1 at 37°C. Results showed that methane production of the digester was severely inhibited at a volatile fatty acid (VFA) concentration of 30,000mgL−1 at OLR of 4.0gVSL−1d−1 in the absence of TEs. Contrary to the failed digesters, a stable performance was achieved in the TEs added digesters. High methane yield (approximately 465.4mLCH4g−1VSadded) was obtained, and no significant accumulation of VFA was observed in the TEs added digesters at OLR of 1.0–5.0gVSL−1d−1 and HRT of 40days. These findings strongly indicated that the addition of TEs has an important impact on the operation stability of AD of FW.

Pretreatment of cottage cheese to enhance biogas production.

This study evaluated the possibility of pretreating selected solid fraction of an anaerobic digester treating food waste to lower the hydraulic retention time and increase the methane production. The study investigated the effect of different pretreatments (thermal, chemical, thermochemical and enzymatic) for enhanced methane production from cottage cheese. The most effective pretreatments were thermal and enzymatic. Highest solubilisation of COD was observed in thermal pretreatment, followed by thermochemical. In single enzyme systems, lipase at low concentration gave significantly higher methane yield than for the experiments without enzyme additions. The highest lipase dosages decreased methane yield from cottage cheese. However, in case of protease enzyme an increase in concentration of the enzyme showed higher methane yield. In the case of mixed enzyme systems, pretreatment at 1 : 2 ratio of lipase : protease showed higher methane production in comparison with 1 : 1 and 2 : 1 ratios. Methane production potentials for different pretreatments were as follows: thermal 357 mL/g VS, chemical 293 mL/g VS, and thermochemical 441 mL/g VS. The average methane yield from single enzyme systems was 335 mL/g VS for lipase and 328 mL/g VS for protease. Methane potentials for mixed enzyme ratios were 330, 360, and 339 mL/g VS for 1 : 1, 1 : 2, and 2 : 1 lipase : protease, respectively.