Co-digestion of food and garden waste with mixed sludge from wastewater treatment in continuously stirred tank reactors

Impact of food waste addition in energy efficient municipal wastewater treatment by aerobic granular sludge process

Recently, one of the main purposes of wastewater treatment plants is to achieve a neutral or positive energy balance while meeting the discharge criteria. Aerobic granular sludge (AGS) technology is a promising technology that has low energy and footprint requirements as well as high treatment performance. The effect of co-treatment of municipal wastewater and food waste (FW) on the treatment performance, granule morphology, and settling behavior of the granules was investigated in the study. A biochemical methane potential (BMP) test was also performed to assess the methane potential of mono- and co-digestion of the excess sludge from the AGS process. The addition of FW into wastewater enhanced the nutrient treatment efficiency in the AGS process. BMP of the excess sludge from the AGS process fed with the mixture of wastewater and FW (195 ± 17 mL CH4/g VS) was slightly higher than BMP of excess sludge from the AGS process fed with solely wastewater (173 ± 16 mL CH4/g VS). The highest methane yield was observed for co-digestion of excess sludge from the AGS process and FW, which was 312 ± 8 mL CH4/g VS. Integration of FW as a co-substrate in the AGS process would potentially enhance energy recovery and the quality of effluent in municipal wastewater treatment.

Metagenomic absolute quantification of antibiotic resistance genes and virulence factor genes-carrying bacterial genomes in anaerobic digesters

Sewage treatment works have been considered as hotspots for the dissemination of antibiotic resistance genes (ARGs). Anaerobic digestion (AD) has emerged as a promising approach for controlling the spread of ARGs while destroying biomass in sludge. Evaluating the impact of AD on ARG removal relies on the absolute quantification of ARGs. In this study, we quantified the ARG concentrations in both full-scale and lab-scale AD systems using a cellular spike-ins based absolute quantification approach. Results demonstrated that AD effectively removed 68 ± 18 %, 55 ± 12 %, and 57 ± 19 % of total ARGs in semi-continuous AD digesters, with solid retention times of 15, 20, and 25 days, respectively. The removal efficiency of total ARGs increased as the AD process progressed in the batch digesters over 40 days. A significant negative correlation was observed between digestion time and the concentrations of certain ARG types, such as beta-lactam, sulfonamide, and tetracycline. However, certain potential pathogenic antibiotic resistant bacteria (PARB) and multi-resistant high-risk ARGs-carrying populations robustly persisted throughout the AD process, regardless of the operating conditions. This study highlighted the influence of the AD process and its operating parameters on ARG removal, and revealed the broad spectrum and persistence of PARB in AD systems. These findings provided critical insights for the management of microbial hazards.

Current Status and Prospects of Valorizing Organic Waste via Arrested Anaerobic Digestion: Production and Separation of Volatile Fatty Acids

Volatile fatty acids (VFA) are intermediary degradation products during anaerobic digestion (AD) that are subsequently converted to methanogenic substrates, such as hydrogen (H2), carbon dioxide (CO2), and acetic acid (CH3COOH). The final step of AD is the conversion of these methanogenic substrates into biogas, a mixture of methane (CH4) and CO2. In arrested AD (AAD), the methanogenic step is suppressed to inhibit VFA conversion to biogas, making VFA the main product of AAD, with CO2 and H2. VFA recovered from the AAD fermentation can be further converted to sustainable biofuels and bioproducts. Although this concept is known, commercialization of the AAD concept has been hindered by low VFA titers and productivity and lack of cost-effective separation methods for recovering VFA. This article reviews the different techniques used to rewire AD to AAD and the current state of the art of VFA production with AAD, emphasizing recent developments made for increasing the production and separation of VFA from complex organic materials. Finally, this paper discusses VFA production by AAD could play a pivotal role in producing sustainable jet fuels from agricultural biomass and wet organic waste materials.

Absolute quantification and genome-centric analyses elucidate the dynamics of microbial populations in anaerobic digesters

Anaerobic digestion (AD) relies on myriads of functions performed by complex microbial communities in customized settings, thus, a comprehensive investigation on the AD microbiome is central to the fine-tuned control. Most current AD microbiome studies are based on relative abundance, which hinders the interpretation of microbes' dynamics and inter-sample comparisons. Here, we developed an absolute quantification (AQ) approach that integrated cellular spike-ins with metagenomic sequencing to elucidate microbial community variations and population dynamics in four anaerobic digesters. Using this method, 253 microbes were defined as decaying populations with decay rates ranging from -0.05 to -5.85 d^-1, wherein, a population from Flavobacteriaceae family decayed at the highest rates of -3.87 to -5.85 d^-1 in four digesters. Meanwhile, 25 microbes demonstrated the growing trend in the AD processes with growth rates ranging from 0.11 to 1.77 d^-1, and genome-centric analysis assigned some of the populations to the functional niches of hydrolysis, short-chain fatty acids metabolism, and methane generation. Additionally, we observed that the specific activity of methanogens was lower in the prolonged digestion stage, and redundancy analysis revealed that the feedstock composition and the digestion duration were the two key parameters in governing the AD microbial compositions.

Impacts of food waste to sludge ratios on microbial dynamics and functional traits in thermophilic digesters

A self-stabilizing microbial community lays the foundation of the efficient biochemical reactions of the anaerobic digestion (AD) process. Despite extensive profiling of microbial community dynamics under varying operating parameters, the effects of food waste (FW) to feeding sewage sludge (FSS) ratios on the microbial assembly, functional traits, and syntrophic interspecies interactions in thermophilic microbial consortia remain poorly understood. Here, we investigated the long-term impacts of the FW: FSS ratio on the thermophilic AD microbiome using genome-centric metagenomics. Both the short reads (SRs) assembly, and the iterative hybrid assembly (IHA) of SRs and nanopore long reads (LRs) were used to reconstruct metagenome-assembled genomes (MAGs) and four microbial clusters were identified, demonstrating different microbial dynamics patterns in response to varying FW:FSS ratios. Cluster C1-C3 were comprised of full functional members with genetic potentials in fulfilling empirical AD biochemical reactions, wherein, syntrophic decarboxylating acetogens could interact with methanogens, and some microbes could be energized by the electron bifurcation mechanism to drive thermodynamics unfavorable reactions. We found the co-existence of both acetogenic and hydrogenotrophic methanogens in the AD microbiome, and they altered their trophic groups to scavenge the methanogenic substrates in ensuring the methane generation in digesters with different FW:FSS ratios. Another interesting observation was that two phylogenetically close Thermotogota species showed a possible strong competition on carbon source inferred by the nearly complete genetic overlap of their relevant pathways.

Environmental and Economic Aspects of Biomethane Production from Organic Waste in Russia

According to the International Energy Agency (IEA), only a tiny fraction of the full potential of energy from biomass is currently exploited in the world. Biogas is a good source of energy and heat, and a clean fuel. Converting it to biomethane creates a product that combines all the benefits of natural gas with zero greenhouse gas emissions. This is important given that the methane contained in biogas is a more potent greenhouse gas than carbon dioxide (CO2). The total amount of CO2 emission avoided due to the installation of biogas plants is around 3380 ton/year, as 1 m3 of biogas corresponds to 0.70 kg of CO2 saved. In Russia, despite the huge potential, the development of bioenergy is rather on the periphery, due to the abundance of cheap hydrocarbons and the lack of government support. Based on the data from an agro-industrial plant located in Central Russia, the authors of the article demonstrate that biogas technologies could be successfully used in Russia, provided that the Russian Government adopted Western-type measures of financial incentives.

Increasing 2 -Bio- (H2 and CH4) production from food waste by combining two-stage anaerobic digestion and electrodialysis for continuous volatile fatty acids removal

A novel approach of using two stage anaerobic digestion coupled with electrodialysis technology has been investigated. This approach was used to improving bio hydrogen and methane yields from food waste while simultaneously producing a green chemical feedstock. The first digester was used for hydrogen production and the second digester was used for methane production. The first digester was combined with continuous separation of volatile fatty acids using electrodialysis. The concentrations of carbohydrates, proteins and fats in the prepared food waste were 22.7%, 5.7% and 5.2% respectively. Continuous removal of volatile fatty acids during fermentation in the hydrogen digester not only increased hydrogen yields but also increased the production rate of volatile fatty acids. As a result of continuous VFA separation, hydrogen yields increased from 17.3 mL H₂/g VS _fermenter to 33.68 mL H₂/g VS _fermenter. Methane yields also increased from 28.94 mL CH₄/g VS _fermenter to 43.94 mL CH₄/g VS _fermenter. This represents a total increase in bio-energy yields of 77.1%. COD reduced by 73% after using two stage anaerobic digestion, however, this reduction increased to 86.7% after using electrodialysis technology for separation of volatile fatty acids. Electrodialysis technology coupled with anaerobic digestion improved substrate utilization, increased bioenergy yields and looks to be promising for treating complex wastes such as food waste.

Determination of the dewatered digestate amounts and methane yields from the co-digestion of biowaste as a basis for a cost-benefit analysis

Co-digestion is the simultaneous digestion of two or more substrates and a common practice at wastewater treatment plants (WWTPs). The amounts of methane and digested sludge produced are key parameters for evaluating the economic efficiency of co-digestion. However, the share of dewatered digestate produced from co-substrates is not known. Synergistic effects in co-digestion, i.e. a better biodegradability compared to the mono-digestion of each substrate, might reduce the amounts of digested sludge and increase methane yields. However, these effects might also influence the calculation of methane and digestate quantities from co-substrates. The main objective of this work was to provide a basis for the cost-benefit analysis of biowaste (BW) co-digestion at WWTPs for this data. Therefore, continuous and batch experiments with sewage sludge (SS) and BW co-digestion were conducted and evaluated for methane and digestate production, and possible synergistic effects. BW co-digestion led to an additional production of 0.35 t total solids (TS) of dewatered sludge per ton TS_added in continuous and 0.23 t TS of dewatered sludge per ton of TS_added in batch experiments. The methane yield from BW was 441 L/kg TS_added in continuous experiments and 482 L/kg TS_added batch test. No synergistic effects were observed in both batch and continuous co-digestion experiments. Batch tests were found to be suitable for a rough estimation of the co-digestion economic efficiency key parameters. Continuous experiments are recommended to obtain more robust data. A cost-benefit analysis found that electricity production from co-digestion can generate savings of 88-170 €/t TS_added compared to grid purchase.

Optimizing the co-digestion supply chain of sewage sludge and food waste by the demand oriented biogas supplying mechanism

Co-digestion of sewage sludge with food waste is a beneficial pathway for sewage plants to enhance their biogas yield. This paper employs hybrid programming with system dynamics simulation to optimize such a co-digestion system from the perspective of demand-oriented biogas supply chain, thus to improve the efficiency of the biogas utilization. The optimum operational parameters of the co-digestion system are derived from the simulation model. It is demonstrated that the demand-oriented biogas supply mechanism can be effectively driven under market-oriented incentive policy. For better compensation of the external cost to assist the operations of the co-digestion supply chain, it is suggested that the substrate collection and transportation subsidy should be combined with the renewables portfolio standard to be implemented as the optimum incentives. The limitations of the study are discussed to lay the foundation for future improvements.

Thermophilic and mesophilic biogas production from PLA-based materials: Possibilities and limitations

Recently, the use of bio-based products, including biodegradable poly(lactic acid) (PLA), has increased, causing their rapid growth in municipal waste streams. The presence of PLA in biowaste may increase biogas production (BP). However, the rate of PLA biodegradation, which affects the time frame of anaerobic digestion, is a key parameter for an efficient process. In this study, detailed kinetics of BP from PLA were determined at 58 °C and 37 °C. At both temperatures, lag phases were observed: 40 days at 37 °C, and 10 days at 58 °C. After the lag phase BP proceeded in two phases, differed in process rate. At 58 °C, during the 1st phase (up to day 30), the rate of BP (r_B1,58) equaled about 35 L/(kg OM·d). At the end of this phase, the amount of biogas was 710 L/kg OM, which constituted 84% of the maximal BP (831-849 L/kg OM). In the 2nd phase (10 days), only 13% of maximal BP was produced (r_B2,58 of 16.1 L/(kg OM·d)). At 37 °C, maximal BP (obtained after 280 days) was 1.5-fold lower (558-570 L/kg OM) than at 58 °C. In the 1st phase (100 days), r_B1,37 was 1.4 L/(kg OM·d); at the end of this phase, BP constituted merely 14% of the maximal BP. A majority of biogas was produced in the 2nd phase (the next 180 days), and r_B2,37 doubled to 2.6 L/(kg OM·d)). At 58 °C, intensive biogas production took place when PLA pieces were still visible. At 37 °C, in contrast, biogas was mainly produced when the PLA pieces had been disintegrated. Although PLA anaerobically biodegrades and produces a high yield of biogas, the time frame of PLA digestion is much longer than that of biowaste and, in thermophilic conditions requires separate digesters. In mesophilic conditions, however, is unacceptable at technical scale.

Salinity enhances high optically active L-lactate production from co-fermentation of food waste and waste activated sludge: Unveiling the response of microbial community shift and functional profiling

Lactic acid (LA), a versatile platform molecule, can be fermented from organic wastes, such as food waste and waste activated sludge. In this study, an efficient approach using salt, a component of food waste as an additive, was proposed to increase LA production. The LA productivity was increased at 10 g NaCl/L and optical pure L-lactate was obtained at 30 g NaCl/L. The enhancement of LA was in accordance with the increased solubilization and the critical hydrolase activities under saline conditions. Moreover, high salinity (30-50 g NaCl/L) changed the common conversion of LA to volatile fatty acids. In addition, the key LA bacteria genera (Bacillus, Enterococcus, Lactobacillus) were selectively enriched under saline conditions. Strong correlations between salinity and functional genes for L-LA production were also observed. This study provides a practical way for the enrichment of L-LA with high optical activity from organic wastes.

Performance of a Full-Scale Biogas Plant Operation in Greece and Its Impact on the Circular Economy

Biogas plants have been started to expand recently in Greece and their positive contribution to the economy is evident. A typical case study is presented which focuses on the long-term monitoring (lasting for one year) of a 500 kW mesophilic biogas plant consisting of an one-stage digester. The main feedstock used was cow manure, supplemented occasionally with chicken manure, corn silage, wheat/ray silage, glycerine, cheese whey, molasses and olive mill wastewater. The mixture of the feedstocks was adjusted based on their availability, cost and biochemical methane potential. The organic loading rate (OLR) varied at 3.42 ± 0.23 kg COD m−3 day−1 (or 2.74 ± 0.18 kg VS m−3 day−1) and resulted in a stable performance in terms of specific biogas production rate (1.27 ± 0.12 m3 m−3 day−1), biogas yield (0.46 ± 0.05 m3 kg−1 VS, 55 ± 1.3% in methane) and electricity production rate (12687 ± 1140 kWh day−1). There were no problems of foaming, nor was there a need for trace metal addition. The digestate was used by the neighboring farmers who observed an improvement in their crop yield. The profit estimates per feedstock indicate that chicken manure is superior to the other feedstocks, while molasses, silages and glycerin result in less profit due to the long distance of the biogas plant from their production source. Finally, the greenhouse gas emissions due to the digestate storage in the open air seem to be minor (0.81% of the methane consumed).

Mechanical separation of impurities in biowaste: Comparison of four different pretreatment systems

Impurities in biowaste, such as plastics, glass, metals and inert material, negatively impact the operation of anaerobic digestion plants and compost quality, and have to be removed prior to the anaerobic digestion process. Different mechanical pretreatments are available for this purpose. However, data on the removal efficiencies of pretreatment systems for different types of biowaste and for different kinds of impurities are still scarce. This study aims to determine the efficiencies for impurity removal of four biowaste pretreatment plants (BTPs) at real scale - (1) drum-screen + shredder + piston press; (2) shredder + piston press + screw press; (3) separation-mill; and (4) pulper + drum-screen. BTP 1 treats mixed food and garden wastes, while BTP 2, 3 and 4 treat mostly food waste. The efficiency of the pretreatment systems was influenced by the type of pretreated biowaste. The recovery of organics by the press was more efficient when pretreating food waste (BTP 2, 93%) than for treating mixed food and garden wastes (BTP 1, 77%). BTP 3 presented the highest recovery of biogas (up to 98%), but also the highest transfer of inert particles to the substrate. On the contrary, BTP 4 was the most efficient for the removal of inert particles; however, this system also presented 18% loss of biogas potential.

Stabilizing lactate production through repeated batch fermentation of food waste and waste activated sludge

Bio-valorization of organic waste streams, such as food waste and waste activated sludge, to lactic acid (LA) has recently drawn much attention. It offers an opportunity for resource recovery, alleviates environmental issues and potentially turns a profit. In this study, both stable and high LA yield (0.72 ± 0.15 g/g total chemical oxygen demand) and productivity rate (0.53 g/L•h) were obtained through repeated batch fermentation. Moreover, stable solubilization and increase in the critical hydrolase activities were achieved. Depletions of ammonia and phosphorus were correlated with the LA production. The relative abundance of the key LA bacteria genera (i.e., Alkaliphilus, Dysgonomonas, Enterococcus and Bifidobacterium) stabilized in the repeated batch reactor at a higher level (44.5 ± 2.53%) in comparison with the batch reactor (26.2 ± 4.74%). This work show a practical way for the sustainable valorization of organic wastes to LA by applying the repeated batch mode during biological treatment.

Anaerobic co-digestion of sewage sludge, food waste and yard waste: Synergistic enhancement on process stability and biogas production

Anaerobic co-digestion (co-AD) could be a more sustainable waste management solution by sharing the existed anaerobic digestion (AD) facilities and generating more biogas energy. In this study, a series of co-AD of different urban derived organic wastes (sewage sludge-SS, food waste-FW, yard waste-YW) were conducted in a semi-continuous mode, and the corresponding dynamic evolutions of microbial community structure were followed by using real-time quantitative polymerase chain reaction (qPCR). As for co-AD of two feedstocks, introduction of SS (25%, VS basis) in FW significantly improved the process stability and archaea/total microbe ratio (from 0.4% to 17.1%), which might be due to the regulating effect of abundant trace metals in SS; co-AD of SS (25%, VS basis) with YW improved the methane yield by 2.04 times than AD of YW only together with higher methane contents (57.4 ± 1.3% vs. 50.9 ± 2.2%); in co-AD of FW and YW, synergistic effects in terms of increased methane production (3.4-19.1%) were observed, which was correlated with more robust growth of both bacteria and archaea. As for co-AD of three feedstocks, high methane yields of 314.9 ± 17.1 mL/g VS were achieved with a reliable stability. These findings could provide some fundamental and technical information for the co-treatment of urban derived organic wastes in centralized AD facilities.

Enhancing methane production using anaerobic co-digestion of waste activated sludge with combined fruit waste and cheese whey

BACKGROUND

Recently, it has been indicated that anaerobic co-digestion of waste activated sludge with other waste streams at wastewater treatment plants is a promising strategy for enhancing methane production and materials recovery. The enhanced methane production can be used as a renewable source of energy in wastewater treatment plants. It can also reduce the amount of greenhouse gas emission in landfilling of the waste streams.

RESULTS

According to the results obtained in this study, anaerobic co-digestion of waste activated sludge with mixed fruit waste and cheese whey improves methane production and the quality of digested sludge in comparison to the anaerobic digestion of waste activated sludge individually. It was indicated that carbon/nitrogen ratio (C/N) in the mixture of waste activated sludge, fruit waste and cheese whey improved considerably, leading to better anaerobic organisms' activity during digestion. With assessing the activity of protease and cellulase, as the main enzymes hydrolyzing organic matter in anaerobic digestion, it was indicated that co-digestion of waste activated sludge with mixed fruit waste and cheese whey enhances the activity of these enzymes by 22 and 9% respectively. At the end of digestion, the amount of cumulative methane production significantly increased by 31% in the reactor with 85% waste activated sludge and 15% mixed fruit waste and cheese whey, compared to the reactor with 100% waste activated sludge. In addition, chemical oxygen demand (COD) and volatile solid (VS) in digested sludge was improved respectively by 9 and 7% when mixed fruit waste and cheese whey was used.

CONCLUSIONS

This study revealed that mixed fruit waste and cheese whey is potentially applicable to anaerobic digestion of waste activated sludge, as fruit waste and cheese whey have high C/N ratio that enhance low C/N in waste activated sludge and provide a better diet for anaerobic organisms. This is of significant importance because not only could higher amount of renewable energy be generated from the enhanced methane production in wastewater treatment plants, but also capital costs of the companies whose waste streams are being transported to wastewater treatments plants could be reduced considerably.

Modeling the anaerobic co-digestion of solid waste: From batch to semi-continuous simulation

The main purpose of this study was to validate the use of a simple model for forecasting methane production in co-digestion reactors run semi-continuously using substrate data acquired in batch mode. Firstly, seven solid substrates were characterized individually in successive batches to assess their Biochemical Methane Potential (BMP) and kinetic parameters. Afterwards, eight mixtures of two, three or five substrates were processed in semi-continuous mode at an organic loading rate of 1 g VS L^-1 d^-1. The experimental methane production was always greater than that calculated from the BMP of each substrate. This result suggested that, endogenous activity needs to be taken into consideration in order to predict total methane production accurately. Near equivalence between experimental and modeled methane production was found after integration in the model of the endogenous activity. The results confirmed the possibility for use of substrate batch data (BMP and kinetics) to predict methane production in semi-continuous operations.

Influence of two types of sludge on the biogas production of assorted waste streams and the significance of beef cattle waste and liquid cheese whey in the organic fraction of municipal solid waste

The aim of this study was to assess the biogas production generated by the anaerobic co-digestion of two co-substrates-liquid cheese whey (LCW) and beef cattle waste (BCW)-mixed with the organic fraction of municipal solid waste (OFMSW) and inoculated with either granular or suspended sludge. At the end of co-digestion, a high biogas yield was observed for the granular sludge mixture of OFMSW and BCW, which provides support for beef cattle waste as a promising substrate for biogas production. The mixture of OFMSW and LCW resulted in an enhancement of biogas production compared to OFMSW alone; however, the characteristics of LCW led to instability during the process. The key finding was that the type of sludge used influences the biogas production of the mixture. For the two sludges tested, the reactors containing granular sludge produced more biogas than those with suspended sludge. Reactors inoculated with a granular sludge produced 70% more biogas with the mixture of OFMSW and BCW compared to those with the suspended sludge. The OFMSW and LCW mixture with granular sludge produced 16% more biogas than with the suspended sludge.

Thermophilic Anaerobic Digestion: Enhanced and Sustainable Methane Production from Co-Digestion of Food and Lignocellulosic Wastes

This article aims to study the codigestion of food waste (FW) and three different lignocellulosic wastes (LW) (Corn stover (CS), Prairie cordgrass (PCG), and Unbleached paper (UBP)) for thermophilic anaerobic digestion to overcome the limitations of digesting food waste alone (volatile fatty acids accumulation and low C:N ratio). Using an enriched thermophilic methanogenic consortium, all the food and lignocellulosic waste mixtures showed positive synergistic effects of codigestion. After 30 days of incubation at 60 °C (100 rpm), the highest methane yield of 305.45 L·kg−1 volatile solids (VS) was achieved with a combination of FW-PCG-CS followed by 279.31 L·kg−1 VS with a mixture of FW-PCG. The corresponding volatile solids reduction for these two co-digestion mixtures was 68% and 58%, respectively. This study demonstrated a reduced hydraulic retention time for methane production using FW and LW.

Synergistic effects of anaerobic co-digestion of whey, manure and fish ensilage

Biogas production potential of the three feedstocks fish ensilage, manure and whey was evaluated using biochemical methane potential (BMP) tests. Since anaerobic digestion of single substrates may be inefficient due to imbalances in the carbon-nitrogen ratio, degree of biodegradability and/or due to lack of nutrients needed by the microbial community, co-digestion of these substrates was also assessed, revealing synergistic effects and a particularly good effect of combining manure with fish ensilage. In this latter case, methane yields were up to 84% higher than the weighted average of the methane yields obtained with the individual substrates. The type of substrate was the dominating cause of variation in methane production rates and yields.

Production of lactic acid from thermal pretreated food waste through the fermentation of waste activated sludge: Effects of substrate and thermal pretreatment temperature

Valorization of organic-rich waste stream to lactic acid by the mixed microbial consortium has attracted tremendous research interests in recent years. In this study, thermal pretreatment was involved in co-fermentation of food waste (FW) and waste activated sludge (WAS) to enhance lactic acid production. First, sole FW was observed as the most suitable substrate employing thermal pretreatment for the generation of lactic acid. The fermentation time for reaching the maximal plateau was significantly shortened at a corresponding thermal pretreatment temperature. The mechanism study found that the enhancement of lactic acid yield was in accordance with the acceleration of solubilization and hydrolysis. Furthermore, the physicochemical characteristics of fermentative substrate and surface morphology of the fermentation mixture varied with the pretreatment temperatures. Further investigations of microbial community structure also revealed that the proportions of key microorganisms such as Bacillus and Lactobacillus were changed by the thermal pretreatment.

Food loss and waste management in Turkey

Food waste can be an environmental and economic problem if not managed properly but it can meet various demands of a country if it is considered as a resource. The purpose of this report is to review the existing state of the field in Turkey and identify the potential of food waste as a resource. Food loss and waste (FLW) was examined throughout the food supply chain (FSC) and quantified using the FAO model. Edible FLW was estimated to be approximately 26milliontons/year. The amount of biodegradable waste was estimated based on waste statistics and research conducted on household food waste in Turkey. The total amount of biodegradable waste was found to be approximately 20milliontons/year, where more than 8.6milliontons/year of this waste is FLW from distribution and consumption in the FSC. Options for the end-of-life management of biodegradable wastes are also discussed in this review article.

In-situ biogas upgrading process: Modeling and simulations aspects

Biogas upgrading processes by in-situ hydrogen (H₂) injection are still challenging and could benefit from a mathematical model to predict system performance. Therefore, a previous model on anaerobic digestion was updated and expanded to include the effect of H₂ injection into the liquid phase of a fermenter with the aim of modeling and simulating these processes. This was done by including hydrogenotrophic methanogen kinetics for H₂ consumption and inhibition effect on the acetogenic steps. Special attention was paid to gas to liquid transfer of H₂. The final model was successfully validated considering a set of Case Studies. Biogas composition and H₂ utilization were correctly predicted, with overall deviation below 10% compared to experimental measurements. Parameter sensitivity analysis revealed that the model is highly sensitive to the H₂ injection rate and mass transfer coefficient. The model developed is an effective tool for predicting process performance in scenarios with biogas upgrading.

Biogas Digester Hydraulic Retention Time Affects Oxygen Consumption Patterns and Greenhouse Gas Emissions after Application of Digestate to Soil

Knowledge about environmental impacts associated with the application of anaerobic digestion residue to agricultural land is of interest owing to the rapid proliferation of biogas plants worldwide. However, virtually no information exists concerning how soil-emitted NO is affected by the feedstock hydraulic retention time (HRT) in the biogas digester. Here, the O planar optode technique was used to visualize soil O dynamics following the surface application of digestates of the codigestion of pig slurry and agro-industrial waste. We also used NO isotopomer analysis of soil-emitted NO to determine the NO production pathways, i.e., nitrification or denitrification. Two-dimensional images of soil O indicated that anoxic and hypoxic conditions developed at 2.0- and 1.5-cm soil depth for soil amended with the digestate produced with 15-d (PO15) and 30-d (PO30) retention time, respectively. Total NO emissions were significantly lower for PO15 than PO30 due to the greater expansion of the anoxic zone, which enhanced NO reduction via complete denitrification. However, cumulative CO emissions were not significantly different between PO15 and PO30 for the entire incubation period. During incubation, NO emissions came from both nitrification and denitrification in amended soils. Increasing the HRT of the biogas digester appears to induce significant NO emissions, but it is unlikely to affect the NO production pathways after application to soil.

Effects of loading rate and temperature on anaerobic co-digestion of food waste and waste activated sludge in a high frequency feeding system, looking in particular at stability and efficiency

A continuously stirred tank reactor (CSTR) with a high feeding frequency (HFF) of once every 15min was employed in order to ease the loading shock frequently occurred in digester with a low feeding frequency. The effects of the organic loading rate (OLR) and temperature on the co-digestion of food waste and waste activated sludge was evaluated in a 302-day long-term experiment. Due to the high hydrolysis rate, the maximum CH₄ yield in a thermophilic reactor was 407mL CH₄/gVS_added, a value that was significantly higher than the 350mL CH₄/gVS_added that occurred in a mesophilic reactor. Although the alkalinity declined when HRT was shorted than 10d, caused by the decrease of conversion ratio from protein to ammonium, the increase of specific methanogenic activity helped HFF system to achieve stable performance at an OLR of 11.2 (HRT 7.5d) and 30.2gVS/L/d (HRT 3d) under mesophilic and thermophilic conditions, respectively.

A new approach for concurrently improving performance of South Korean food waste valorization and renewable energy recovery via dry anaerobic digestion under mesophilic and thermophilic conditions

Dry semicontinuous anaerobic digestion (AD) of South Korean food waste (FW) under four solid loading rates (SLRs) (2.30-9.21kg total solids (TS)/m³day) and at a fixed TS content was compared between two digesters, one each under mesophilic and thermophilic conditions. Biogas production and organic matter reduction in both digesters followed similar trends, increasing with rising SLR. Inhibitor (intermediate products of the anaerobic fermentation process) effects on the digesters' performance were not observed under the studied conditions. In all cases tested, the digesters' best performance was achieved at the SLR of 9.21kg TS/m³day, with 74.02% and 80.98% reduction of volatile solids (VS), 0.87 and 0.90m³ biogas/kg VS_removed, and 0.65 (65% CH₄) and 0.73 (60.02% CH₄) m³ biogas/kg VS_fed, under mesophilic and thermophilic conditions, respectively. Thermophilic dry AD is recommended for FW treatment in South Korea because it is more efficient and has higher energy recovery potential when compared to mesophilic dry AD.

Rapid biochemical methane potential prediction of urban organic waste with near-infrared reflectance spectroscopy

The anaerobic digestibility of various biomass feedstocks in biogas plants is determined with biochemical methane potential (BMP) assays. However, experimental BMP analysis is time-consuming, costly and challenging to optimise stock management and feeding to achieve improved biogas production. The aim of the present study is to develop a fast and reliable model based on near-infrared reflectance spectroscopy (NIRS) for the BMP prediction of urban organic waste (UOW). The model comprised 87 UOW samples. Additionally, 88 plant biomass samples were included, to develop a combined model predicting BMP. The coefficient of determination (R²) and root mean square error in prediction (RMSE_P) of the UOW model were 0.88 and 44 mL CH₄/g VS, while the combined model was 0.89 and 50 mL CH₄/g VS. Improved model performance was obtained for the two individual models compared to the combined version. The BMP prediction with NIRS was satisfactory and moderately successful.

Microbial population dynamics in urban organic waste anaerobic co-digestion with mixed sludge during a change in feedstock composition and different hydraulic retention times

Microbial communities play an essential role in the biochemical pathways of anaerobic digestion processes. The correlations between microorganisms' relative abundance and anaerobic digestion process parameters were investigated, by considering the effect of different feedstock compositions and hydraulic retention times (HRTs). Shifts in microbial diversity and changes in microbial community richness were observed by changing feedstock composition from mono-digestion of mixed sludge to co-digestion of food waste, grass clippings and garden waste with mixed sludge at HRT of 30, 20, 15 and 10 days. Syntrophic acetate oxidation along with hydrogenotrophic methanogenesis, mediated by Methanothermobacter, was found to be the most prevalent methane formation pathway, with the only exception of 10 days' HRT, in which Methanosarcina was the most dominant archaea. Significantly, the degradation of complex organic polymers was found to be the most active process, performed by members of S1 (Thermotogales), Thermonema and Lactobacillus in a reactor fed with a high share of food waste. Conversely, Thermacetogenium, Anaerobaculum, Ruminococcaceae, Porphyromonadaceae and the lignocellulosic-degrading Clostridium were the significantly more abundant bacteria in the reactor fed with an increased share of lignocellulosic biomass in the form of grass clippings and garden waste. Finally, microbes belonging to Coprothermobacter, Syntrophomonas and Clostridium were correlated significantly with the specific methane yield obtained in both reactors.

A systematic methodology to extend the applicability of a bioconversion model for the simulation of various co-digestion scenarios

Detailed simulation of anaerobic digestion (AD) requires complex mathematical models and the optimization of numerous model parameters. By performing a systematic methodology and identifying parameters with the highest impact on process variables in a well-established AD model, its applicability was extended to various co-digestion scenarios. More specifically, the application of the step-by-step methodology led to the estimation of a general and reduced set of parameters, for the simulation of scenarios where either manure or wastewater were co-digested with different organic substrates. Validation of the general parameter set involved the simulation of laboratory-scale data from three continuous co-digestion experiments, treating mixtures of different organic residues either at thermophilic or mesophilic conditions. Evaluation of the results showed that simulations using the general parameter set fitted experimental data quite well, indicating that it offers a reliable reference point for future simulations of anaerobic co-digestion scenarios.

Assessment of methane production from shredder waste in landfills: The influence of temperature, moisture and metals

In this study, methane (CH₄) production rates from shredder waste (SW) were determined by incubation of waste samples over a period of 230days under different operating conditions, and first-order decay kinetic constants (k-values) were calculated. SW and sterilized SW were incubated under different temperatures (20-25°C, 37°C, and 55°C), moisture contents (35% and 75% w/w) and amounts of inoculum (5% and 30% of the samples wet weight). The biochemical methane potential (BMP) from different types of SW (fresh, old and sieved) was determined and compared. The ability of metals (iron, aluminum, zinc, and copper) contained in SW to provide electrons for methanogens resulting in gas compositions with high CH₄ contents and very low CO₂ contents was investigated. The BMP of SW was 1.5-6.2kg CH₄/ton waste. The highest BMP was observed in fresh SW samples, while the lowest was observed in sieved samples (fine fraction of SW). Abiotic production of CH₄ was not observed in laboratory incubations. The biotic experiments showed that when the moisture content was 35% w/w and the temperature was 20-25°C, CH₄ production was extremely low. Increasing the temperature from 20-25°C to 37°C resulted in significantly higher CH₄ production while increasing the temperature from 37°C to 55°C resulted in higher CH₄ production, but to a lower extent. Increasing the moisture and inoculum content also increased CH₄ production. The k-values were 0.033-0.075yr^-1 at room temperature, 0.220-0.429yr^-1 at 37°C and 0.235-0.488yr^-1 at 55°C, indicating that higher temperatures resulted in higher k-values. It was observed that H₂ can be produced by biocorrosion of iron, aluminum, and zinc and it was shown that produced H₂ can be utilized by hydrogenotrophic methanogens to convert CO₂ to CH₄. Addition of iron and copper to SW resulted in inhibition of CH₄ production, while addition of aluminum and zinc resulted in higher CH₄ production. This suggested that aluminum and zinc contribute to high CH₄ production from SW by providing H₂ for hydrogenotrophic methanogens. Gas compositions with higher CH₄ and lower CO₂ observed in landfilled SW are thus most likely due to the consumption of existing CO₂ in the produced biogas and the produced H₂ by biocorrosion of aluminum and zinc by methanogens.

Synergism of co-digestion of food wastes with municipal wastewater treatment biosolids

Five semi-continuous flow anaerobic digesters treating a mixture of food waste (FW) and municipal biosolids (primary sludge and thickened wasted activated sludge) at an solids retention time (SRT) of 20 days and different blend ratios i.e. 0, 10%, 20%, 40% by volume with the fifth digester treating only biosolids at the same COD/N ratio as the 40% FW digester were operated to investigate co-digestion performance. Sixty days of steady-state operation at organic loading rates (OLR) of 2.2-3.85kgCOD/m³/d showed that COD removals were higher for the three co-digesters than for the two municipal biosolids digesters i.e. 61-69% versus 47-52%. Specific methane production per influent CODs were 1.3-1.8 folds higher in co-digestion than mono-digestion. The first-order COD degradation kinetic constants for co-digestion were more than double the mono-digestion. Additional methane production through synergism accounted for a minimum of 18-20% of the overall methane production. The estimated non-biodegradable fraction of the FW particulate COD was 7.3%. However, the co-digesters discharged 1.23-1.64 times higher soluble nitrogen than the control.

Anaerobic co-digestion of food waste and chemically enhanced primary-treated sludge under mesophilic and thermophilic conditions

Anaerobic co-digestion of food waste with primary sewage sludge is beneficial for urban centers, while the optimized conditions reported in the literature are not locally suitable for Hong Kong. Therefore, the present study was aimed to develop an optimized mixing ratio of food waste to chemically enhanced primary-treated sewer sludge (CEPT) for co-digestion using batch tests under mesophilic (37°C) and thermophilic (55°C) conditions. The mixing ratios of 1:1, 1:2, 1:3, 2:1 and 3:1 (v v(-1)) of food waste to CEPT sludge was tested under the following conditions: temperature - 35°C and 55°C; pH - not regulated; agitation - 150 rpm and time - 20 days. The thermophilic incubations led a good hydrolysis rate and 2-12-fold higher enzyme activities than in mesophilic incubations for different mixing ratios. While the acidogenesis were found retarded that leading to 'sour and stuck' digestion for all mixing ratio of food waste to CEPT sludge from thermophilic incubations. The measured zeta potential was most favourable (-5 to -16.8 mV) for methane production under thermophilic incubations; however the CH4 recovery was less than that in mesophilic incubations. The results suggested that the quick hydrolysis and subsequent acid accumulation under thermophilic incubation lead to inhibited methanogenesis at the early stage than in mesophilic systems. It is concluded that buffer addition is therefore required for any mixing ratio of food waste to CEPT sludge for improved CH4 recovery for both mesophilic and thermophilic operations.

Optimising the anaerobic co-digestion of urban organic waste using dynamic bioconversion mathematical modelling

Mathematical anaerobic bioconversion models are often used as a convenient way to simulate the conversion of organic materials to biogas. The aim of the study was to apply a mathematical model for simulating the anaerobic co-digestion of various types of urban organic waste, in order to develop strategies for controlling and optimising the co-digestion process. The model parameters were maintained in the same way as the original dynamic bioconversion model, albeit with minor adjustments, to simulate the co-digestion of food and garden waste with mixed sludge from a wastewater treatment plant in a continuously stirred tank reactor. The model's outputs were validated with experimental results obtained in thermophilic conditions, with mixed sludge as a single substrate and urban organic waste as a co-substrate at hydraulic retention times of 30, 20, 15 and 10 days. The predicted performance parameter (methane productivity and yield) and operational parameter (concentration of ammonia and volatile fatty acid) values were reasonable and displayed good correlation and accuracy. The model was later applied to identify optimal scenarios for an urban organic waste co-digestion process. The simulation scenario analysis demonstrated that increasing the amount of mixed sludge in the co-substrate had a marginal effect on the reactor performance. In contrast, increasing the amount of food waste and garden waste resulted in improved performance.

Characteristics of the organic fraction of municipal solid waste and methane production: A review

Anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) is a viable alternative for waste stabilization and energy recovery. Biogas production mainly depends on the type and amount of organic macromolecules. Based on results from different authors analysing OFMSW from different cities, this paper presents the importance of knowing the OFMSW composition to understand how anaerobic digestion can be used to produce methane. This analysis describes and discusses physical, chemical and bromatological characteristics of OFMSW reported by several authors from different countries and cities and their relationship to methane production. The main conclusion is that the differences are country and not city dependant. Cultural habits and OFMSW management systems do not allow a generalisation but the individual analysis for specific cities allow understanding the general characteristics for a better methane production. Not only are the OFMSW characteristics important but also the conditions under which the methane production tests were performed.