Impact of food industrial waste on anaerobic co-digestion of sewage sludge and pig manure

Study on the mechanism of action of methane production by co-fermentation of sludge and lignite

To improve the methanogenic efficiency of lignite anaerobic fermentation and explore innovative approaches to sludge utilization, a co-fermentation technique involving lignite and sludge was employed for converting biomass into biomethane. Volatile suspended solids were introduced as a native enrichment of the sludge and mixed with lignite for fermentation. The synergistic fermentation mechanism between sludge and lignite for biomethane production was analyzed through biochemical methane potential experiments, measurement of various parameters pre- and post-fermentation, observation of bacterial population changes during the peak of reaction, carbon migration assessment, and evaluation of rheological characteristics. The results showed that the addition of sludge in the anaerobic fermentation process improved the microorganisms' ability to degrade lignite and bolstered biomethane production. Notably, the maximum methane production recorded was 215.52 mL/g-volatile suspended solids, achieved at a sludge to coal ratio of 3:1, with a synergistic growth rate of 25.37%. Furthermore, the removal rates of total suspended solids, and total chemical oxygen demand exhibited an upward trend with an increasing percentage of sludge in the mixture. The relative abundance and activity of the methanogens population were found to increase with an appropriate ratio of sludge to lignite. This observation confirmed the migration of carbon between the solid-liquid-gas phases, promoting enhanced system affinity. Additionally, the changes in solid-liquid phase parameters before and after the reaction indicated that the addition of sludge improved the system's degradation capacity. The results of the study hold significant implications in realizing the resource utilization of sludge and lignite while contributing to environmental protection endeavors.

Determination of the optimal feed recipe of anaerobic digesters using a mathematical model and a genetic algorithm

Recently, numerous experimental studies have been undertaken to understand the interactions between different feedstocks in anaerobic digestion. They have unveiled the potential of blending substrates in the process. Nevertheless, these experiments are time-intensive, prompting the exploration of various optimization approaches. Notably, genetic algorithms have gained interest due to their population-based structures allowing them to efficiently yield multiple Pareto-optimal solutions in a single run. This study uses a simplified static anaerobic co-digestion model as the fitness function for a multi-objective optimization. The optimization aims to achieve a methane production set-point while reducing the output ammonia nitrogen and increasing the recipe' profitability. Thus, the study employs genetic algorithms to identify Pareto fronts and constraints confined the solution space within feasible boundaries. It also underscores the influence of economic considerations on the viable solution space. Ultimately, the optimal feed recipe not only ensures stable operations within the digester but also enhances associated profits.

A critical review on slaughterhouse waste management and framing sustainable practices in managing slaughterhouse waste in India

Global meat consumption is on a rise with around 253 million metric tons of meat produced globally in the year 2020. Because of the rise in population and change in food preferences, meat consumption trend is likely to continue. Meat production by animal slaughtering increases the slaughterhouse wastes in the form of both solid and liquid wastes. Although various technologies for slaughterhouse waste management are available in developed countries, the effective utilization of slaughterhouse waste management is still missing in developing countries like India. India plays an active role in the meat export business globally and stood 2nd in the world with a total export valuation of 2.89 billion US $ in the year 2020. In this context, this study presents a critical overview of the current technological advancements in the global slaughterhouse waste management including utilization of by-products and further, the prevailing slaughterhouse waste management of India is discussed. Finally, a sustainable slaughterhouse waste management strategy emphasizing circular economy and regulations improvements have been suggested for India to compete in this sector at global scale.

Stability of the Anaerobic Digestion Process during Switch from Parallel to Serial Operation—A Microbiome Study

Anaerobic digestion is a common procedure of treating sewage sludge at wastewater treatment plants. However, plants differ in terms of the number of reactors and, in case of several reactors, their operation mode. To confirm the flexibility of well adapted, full-scale anaerobic digestion plants, we monitored the physicochemical process conditions of two continuously stirred tank reactors over one hydraulic retention time before and after the operation mode was switched from parallel to serial operation. To investigate changes in the involved microbiota, we applied Illumina amplicon sequencing. The rapid change between operation modes did not affect the process performance. In both parallel and serial operation mode, we detected a highly diverse microbial community, in which Bacteroidetes, Firmicutes, Proteobacteria and Claocimonetes were high in relative abundance. While a prominent core microbiome was maintained in both configurations, changes in the involved microbiota were evident at a lower taxonomical level comparing both reactors and operation modes. The most prominent methanogenic Euryarchaeota detected were Methanosaeta and cand. Methanofastidiosum. Volatile fatty acids were degraded immediately in both reactors, suggesting that the second reactor could be used to produce methane on demand, by inserting easily degradable substrates.

Biogas Production Potential of Thermophilic Anaerobic Biodegradation of Organic Waste by a Microbial Consortium Identified with Metagenomics

Anaerobic digestion (AD) is a widespread biological process treating organic waste for green energy production. In this study, wheat straw and corn stalks without any harsh preliminary treatment were collected as a renewable source to be employed in a laboratory-scale digester to produce biogas/biomethane. Processes parameters of temperature, pH, total solids, volatile solid, concentration of volatile fatty acids (VFA), and cellulose concentration, were followed. The volume of biogas produced was measured. The impact of organic loading was stated, showing that the process at 55 °C tolerated a higher substrate load, up to 45 g/L. Further substrate increase did not lead to biogas accumulation increase, probably due to inhibition or mass transfer limitations. After a 12-day anaerobic digestion process, cumulative volumes of biogas yields were 4.78 L for 1 L of the bioreactor working volume with substrate loading 30 g/L of wheat straw, 7.39 L for 40 g/L and 8.22 L for 45 g/L. The degree of biodegradation was calculated to be 68.9%, 74% and 72%, respectively. A fast, effective process for biogas production was developed from native wheat straw, with the highest quantity of daily biogas production occurring between day 2 and day 5. Biomethane concentration in the biogas was 60%. An analysis of bacterial diversity by metagenomics revealed that more than one third of bacteria belonged to class Clostridia (32.9%), followed by Bacteroidia (21.5%), Betaproteobacteria (11.2%), Gammaproteobacteria (6.1%), and Alphaproteobacteria (5%). The most prominent genera among them were Proteiniphilum, Proteiniborus, and Pseudomonas. Archaeal share was 1.37% of the microflora in the thermophilic bioreactor, as the genera Methanocorpusculum, Methanobacterium, Methanomassiliicoccus, Methanoculleus, and Methanosarcina were the most abundant. A knowledge of the microbiome residing in the anaerobic digester can be further used for the development of more effective processes in conjunction with theidentified consortium.

Valorization of petroleum refinery oil sludges via anaerobic co-digestion with food waste and swine manure

Valorization of oil sludge has been gaining attention to improve the sustainability of the petroleum industry. This study aimed to assess the possibility of anaerobic co-digestion of oil scum and secondary sludge with food waste (or swine manure). Oil scum and secondary sludge were obtained from a wastewater treatment plant (WWTP) of a petrochemical plant. Physicochemical properties, hazardous materials, and microbial community were characterized and biochemical methane potential was performed by a simplex-lattice mixture design. More than 87% (wet wt.) of the oil scum consisted of total petroleum hydrocarbons (TPHs) (21,762 mg/L) that are difficult to be degraded by anaerobes. The secondary sludge showed low TPHs (5 mg/L) and a bacterial community similar to that of municipal WWTPs. The heavy metal (Cu, As, Cr, Ni, Mn, Zn, and V) concentrations in the oil scum and secondary sludge were similar (20-600 mg/L). The maximum methane potentials of the oil sludge and secondary sludges were 20 ± 2 and 56 ± 3 mL CH₄/g-volatile solid, respectively. The co-digestion with food waste or swine manure led to a synergy effect on methane production of the co-digestion substrate (10-40% increase compared to the calculated value; v/v) by balancing the C/N ratio. Due to the high TPH contents, oil scum is not appropriate for co-digestion. The co-digestion of secondary sludge with food waste and/or swine manure is recommended. It is necessary to consider whether the concentration of heavy metals is at a level that inhibits the anaerobic co-digestion depending on the operating conditions such as mixing ratios and solid contents.

Microbial electrochemical approaches of carbon dioxide utilization for biogas upgrading

Microbial electrochemical approach is an emerging technology for biogas upgrading through carbon dioxide (CO₂) reduction and biomethane (or value-added products) production. There is limited literature critically reviewing the latest scientific developments on the bioelectrochemical system (BES) based biogas upgrading technologies, including CO₂ reduction efficiency, methane (CH₄) yields, reactor operating conditions, and electrode materials tested in the BES reactor. This review analyzes the reported performance and identifies crucial parameters considered for future optimization, which is currently missing. Further, the performances of BES approach of biogas upgrading under various operating settings in particular fed-batch, continuous mode in connection to the microbial dynamics and cathode materials have been thoroughly scrutinized and discussed. Additionally, other versatile application options associated with BES based biogas upgrading, such as resource recovery, are presented. Three-dimensional electrode materials have shown superior performance in supplying the electrons for the reduction of CO₂ to CH₄. Most of the studies on the biogas upgrading process conclude hydrogen (H₂) mediated electron transfer mechanism in BES biogas upgrading.

Operational parameters influenced on biogas production in zeolite/anaerobic baffled reactor for compost leachate treatment

Nowadays, anaerobic processes are used for leachate treatment and biogas production that can be used as a source of renewable and eco-friendly energy. However, for optimal performance of the anaerobic system for gas production, an appropriate method must be used to reduce the inhibitors in the leachate. In this study an anaerobic baffled reactor (ABR) was used for investigating impact of OLR on biogas production and changes of alkalinity and pH. In order to decline inhibitors concentration on anaerobic microorganisms, zeolite was considered as a media and changes of biogas production was surveyed in different filling ratios. The highest produced biogas at the filling ratios of 10 %, 20 and 30 % were 0.6, 0.63 and 0.9 L/day, respectively and OLR increasing resulted in increase in produced biogas. The values of alkalinity and pH remained in the optimum range for methanogenic bacteria. In all three filling ratios, concentration of ammonia increased with increasing organic loading rate but it has not adverse effect on biogas production. Despite of high concentration of heavy metals, anaerobic baffled reactor with zeolite provided suitable condition for anaerobic microorganisms and biogas production.

Foaming mechanisms and control strategies during the anaerobic digestion of organic waste: A critical review

Foaming is a problem that affects the efficient and stable operation of the anaerobic digestion process. Characterizing foaming mechanisms and developing early warning and foaming control methods is thus critically important. This review summarizes the correlation of process parameters, state parameters, and microbial communities with foaming in anaerobic digesters; discusses the applicability of the above-mentioned multi-scale parameters and foaming potential evaluation methods for the prediction of foaming risk; and introduces the principles and practical applications of antifoaming and defoaming methods. Multiple causes of foaming in anaerobic digestion systems have been identified, but a generalizable foaming mechanism has yet to be described. Further study of the correlation between extracellular polymeric substances and soluble microbial products and foaming may provide new insights into foaming mechanisms. Monitoring the foaming potential (including the volume expansion potential) is an effective approach for estimating the risk of foaming. An in-situ monitoring system for determining the foaming potential in anaerobic digestion sites could provide an early warning of foaming risk. Antifoaming methods based on operating parameter management and process regulation help prevent foaming from the source, and biological defoaming methods are highly targeted and efficient, which is a promising research direction. Clarifying foaming mechanisms will aid the development of active antifoaming methods and efficient biological defoaming methods.

A GIS-Based Model to Assess the Potential of Wastewater Treatment Plants for Enhancing Bioenergy Production within the Context of the Water–Energy Nexus

The necessity of developing renewable energy sources has contributed to increasing interest in developing the anaerobic digestion for producing biomethane since it both provides green energy and reduces disposal treatment. In this regard, to assure efficient water utilization by finding alternative water sources, sewage sludge collected from the wastewater treatment plant (WWTP) was recently investigated because it could represent a suitable resource for producing biomethane within the context of a circular economy. Therefore, this study aims at improving the current knowledge on the feasibility of biomethane production from sewage sludge by optimizing the logistic-supplying phase. In this regard, a GIS-based model was developed and applied to the Emilia-Romagna region to consider the existing networks of WWTPs and biogas systems to valorize sewage sludge for bioenergy production and minimizing environmental impact. The results of the GIS analyses allowed to localize the highest productive territorial areas and highlighted where sewage sludges are abundantly located and could be better exploited within agricultural biogas plants. Finally, the achieved results could help plan suitable policy interventions that are centered on biomass supply and outputs diversification, governance, and social participation, since the regulatory framework could play a crucial role in planning the reuse of these wastes for developing a more sustainable biomethane sector in line with the green economy goals.

Anaerobic co-digestion of spent coconut copra with cow urine for enhanced biogas production

Laboratory-scale bioreactors were used to co-digest spent coconut copra (SCC) and cow urine (CU) as a co-substrate (SCC + CU) in a batch mode under thermophilic condition (45 ± 2°C) in order to enhance biogas production. The effect of CU pretreatment on the performance indicators (biogas and biomethane yields, total solids (TS), and volatile solids (VS) reduction, pH and volatile fatty acids (VFAs) concentrations) were also examined. This was compared with mono-digestion of SCC. The experiment was performed with different mixing ratios in reactors labelled as follows: A = 75 g SCC + 5 ml CU; B = 70 g SCC + 10 ml CU; C = 65 g SCC + 15 ml CU; and D (control) = 80 g SCC at a hydraulic retention time of 42 days. Co-digestion (SCC + CU) significantly improved anaerobic digestion (AD) performance resulting in a threefold and fivefold increase in biogas and biomethane production, respectively, with concomitant TS (44.9-57.7%) and VS (55.4-60.3%) removal efficiencies. But for mono-digestion (control experiment), all CU treated and co-digestion assays showed pH stability ranging between 6.6 and 7.4 and VFAs' concentrations ranging from 15-330 mgL^-1. By acting as a buffer, CU effectively enhanced the AD performance of SCC as demonstrated in this study.

Sugarcane Bagasse as a Co-Substrate with Oil-Refinery Biological Sludge for Biogas Production Using Batch Mesophilic Anaerobic Co-Digestion Technology: Effect of Carbon/Nitrogen Ratio

Man-made organic waste leads to the rapid proliferation of pollution around the globe. Effective bio-waste management can help to reduce the adverse effects of organic waste while contributing to the circular economy at the same time. The toxic oily-biological sludge generated from oil refineries’ wastewater treatment plants is a potential source for biogas energy recovery via anaerobic digestion. However, the oily-biological sludge’s carbon/nitrogen (C/N) ratio is lower than the ideal 20–30 ratio required by anaerobic digestion technology for biogas production. Sugarcane bagasse can be digested as a high C/N co-substrate while the oily-biological sludge acts as a substrate and inoculum to improve biogas production. In this study, the best C/N with co-substrate volatile solids (VS)/inoculum VS ratios for the co-digestion process of mixtures were determined empirically through batch experiments at temperatures of 35–37 °C, pH (6–8) and 60 rpm mixing. The raw materials were pre-treated mechanically and thermo-chemically to further enhance the digestibility. The best condition for the sugarcane bagasse delignification process was 1% (w/v) sodium hydroxide, 1:10 solid-liquid ratio, at 100 °C, and 150 rpm for 1 h. The results from a 33-day batch anaerobic digestion experiment indicate that the production of biogas and methane yield were concurrent with the increasing C/N and co-substrate VS/inoculum VS ratios. The total biogas yields from C/N 20.0 with co-substrate VS/inoculum VS 0.06 and C/N 30.0 with co-substrate VS/inoculum VS 0.18 ratios were 2777.0 and 9268.0 mL, respectively, including a methane yield of 980.0 and 3009.3 mL, respectively. The biogas and methane yield from C/N 30.0 were higher than the biogas and methane yields from C/N 20.0 by 70.04 and 67.44%, respectively. The highest biogas and methane yields corresponded with the highest C/N with co-substrate VS/inoculum VS ratios (30.0 and 0.18), being 200.6 mL/g VSremoved and 65.1 mL CH4/g VSremoved, respectively.

The effect of brewery spent grain application on biogas yields and kinetics in co-digestion with sewage sludge

The present study examines the effect of introducing dried brewery spent grain (BSG), known as the main solid by-product of the brewery industry on biogas yields and kinetics in co-digestion with sewage sludge (SS). The experiment was conducted in semi-continuous anaerobic reactors (supplied once a day) operating under mesophilic conditions (35°C) at different hydraulic retention times (HRT) of 18 and 20 d. In co-digestion runs, the BSG mass to the feed volume ratio was constant and maintained 1:10.The results indicated that the addition of BSG did not influence the biogas production, by comparison with SS mono-digestion (control run). At HRT of 18 d, in the co-digestion run, the average methane yield was 0.27 m³ kg/VS_added, while in the control run the higher value of 0.29 m³ kg/VS_addedwas observed. However, there was no difference in terms of statistical significance. At HRT of 20 d, the methane yield was 0.21 m³ kg/VS_added for both mono- and co-digestion runs. In the BSG presence, the decrease in kinetic constant values was observed. As compared to SS mono-digestion, reductions by 21 and 35% were found at HRT of 20 and 18 d, respectively. However, due to the supplementation of the feedstock with BSG rich in organic compounds, the significantly enhanced energy profits were achieved with the highest value of approx. 40% and related to the longer HRT of 20 d. Importantly, the mono- and co-digestion process proceeded in stable manner. Therefore, the anaerobic co-digestion of SS and BSG might be considered as a cost-effective solution that could contribute to the energy self-efficiency of wastewater treatment plants (WWTPs) and sustainable waste management for breweries.

Enhancement of bioenergy recovery from agricultural wastes through recycling of cellulosic alcoholic fermentation vinasse for anaerobic co-digestion

Cellulosic alcoholic fermentation generates large amounts of vinasse, which was utilized in the present work to enhance the anaerobic digestion of rice straw and swine manure at different total solid (TS) contents. Straw fermentation resulted in bioethanol and vinasse yields of 95.2 g and 857.7 mL, respectively, per kg dry straw. Vinasse-straw co-digestion showed the highest cumulative biogas yield of 633.4 L kg^-1 VS at 3% TS. Therefore, biogas productivity from co-digestion represented 92.1% higher than that of straw mono-digestion. Vinasse-manure co-digestion showed the highest cumulative biogas yield of 676.7 L kg^-1 VS at 3% TS at a shorter technical digestion time, which resulted in 53.9% higher biomethane productivity than the corresponding vinasse-straw. Consequently, vinasse co-digestion at all studied TS ratios enhanced the total gross energy output rate (GEOR_total) over mono-digestion. Amongst, vinasse-manure co-digestion at 3% TS showed 7.9% higher GEOR_total than the highest recorded value from vinasse-straw co-digestion.

Operational Parameters of Biogas Plants: A Review and Evaluation Study

The biogas production technology has improved over the last years for the aim of reducing the costs of the process, increasing the biogas yields, and minimizing the greenhouse gas emissions. To obtain a stable and efficient biogas production, there are several design considerations and operational parameters to be taken into account. Besides, adapting the process to unanticipated conditions can be achieved by adequate monitoring of various operational parameters. This paper reviews the research that has been conducted over the last years. This review paper summarizes the developments in biogas design and operation, while highlighting the main factors that affect the efficiency of the anaerobic digestion process. The study’s outcomes revealed that the optimum operational values of the main parameters may vary from one biogas plant to another. Additionally, the negative conditions that should be avoided while operating a biogas plant were identified.

Sewage enhanced bioelectrochemical degradation of petroleum hydrocarbons in soil environment through bioelectro-stimulation

•

Acetate and sewage were evaluated for enhanced hydrocarbons degradation in soil bioelectrochemical systems.

•

Sewage has superior function in improving in situ bioelectrochemical degradation.

•

Both acetate and sewage improved power density, substrate and sulfate removal.

•

Soil contaminated with produced water was remediated by more than 70 %.

The impact of readily biodegradable substrates (sewage and acetate) in bioelectroremediation of hydrocarbons (PW) was evaluated in a bench-scale soil-based hybrid bioelectrochemical system. Addition of bioelectro-stimulants evidenced efficient degradation than control operation. Acetate and sewage were exhibited power density of 1126 mW/m² and 1145 mW/m², respectively, which is almost 15 % higher than control (without stimulant, 974 mW/m²). Increased electrochemical activity was correlated well with total petroleum hydrocarbons (TPH) degradation through addition of acetate (TPH_R, 525 mg/L, 67.4 %) and sewage (TPH_R, 560 mg/L,71.8 %) compared to the control operation (TPH_R, 503 mg/L, 64.5 %). Similarly, chemical oxygen demand (COD) reduction was also enhanced from 69.0 % (control) to 72.1 % and 74.6 % with acetate and sewage, respectively. Sewage and acetate also showed a positive role in sulfates removal, which enhanced from 56.0 % (control) to 62.9 % (acetate) and 72.6 % (sewage). This study signifies the superior function of sewage as biostimulant compared to acetate for the bioelectroremediation of hydrocarbons in contaminated soils.

Successive sewage sludge fertilization: Recycling for sustainable agriculture

Sewage sludge (SS) is widely used in agriculture in several countries around the world. However, the impact of successive applications of SS on soil and the risks of nutrient leaching are often neglected. In this study, corn was grown on a constructed wetland for four crop cycles (two years), in which the wetland was subjected to successive SS applications. The objective of this study was to evaluate how the successive applications of SS affect the availability and leaching of nutrients in the soil profile, after two years of cultivation. Experiments were performed using a randomized block design with repeated measurements in time, that is, soil was sampled in each harvest. Six treatments were tested: four fertilizations based on sewage sludge, resulting from biological and anaerobic treatment, calculated to provide 25 (SS25), 50 (SS50), 75 (SS75), and 100% (SS100), of the N required for corn production (140 kg ha^-1); a mineral fertilization (NPK) (140 kg ha^-1 N, 70 kg ha^-1 of P₂O₅ and 40 kg ha^-1 of K₂O) and a control (without fertilization). The results showed that four consecutive applications of SS100 for two years caused significant accumulation of nutrients and organic matter in the superficial layers of the soil. The electrical conductivity and the concentration of NO₃^- in the soil solution were higher than those permitted by Brazilian legislation. The adoption of domestic SS in Brazilian agriculture can be a viable alternative in the search for an environment-friendly and economically feasible method for SS disposal.

Co-composts of sewage sludge, farm manure and rock phosphate can substitute phosphorus fertilizers in rice-wheat cropping system

In the present study, various co-composts of sewage sludge (SS), farm manure (FM) and rock phosphate (RP) were prepared and their influence on phosphorus (P) uptake, soil P restoration and growth of rice crop and residual effect on wheat crop were investigated. The treatments comprised of T₁ (control, no amendment), T₂ (452 kg Nitrophos ha^-1, T₃ (724 kg SS₅₀:FM₅₀ ha^-1), T₄ (594 kg SS₁₀₀:FM₀ ha^-1), T₅ (728 kg SS₂₅:FM₂₅:RP₅₀ ha^-1), T₆ (726 kg SS_5O:FM₂₅:RP₂₅ ha^-1), T₇ (508 kg SS₇₅:FM₀:RP₂₅ ha^-1), and T₈ (546 kg SS₅₀:FM₀:RP₅₀ ha^-1). The post-experimental soil samples were analyzed for pH, EC, OM, Olsen's P. The plant samples (grains and straw of both crops) were analyzed for concentrations of P, and heavy metals. The P adsorption by post-wheat composts-amended soil was tested through Langmuir, and Freundlich adsorption isotherms. The investigated parameters (biomass, grain and straw yield, plant height and P concentrations in plant parts) were significantly increased in all composts as compared to the control treatment. The P uptake by the plants was higher in compost treatments as compared to the control and NP that shows long-term residual effect of applied composts. The maximum grain yield (1.63 Mg ha^-1) was obtained in T₅ followed by T₆ (1.52 Mg ha^-1). The P concentration in rice grains were recorded in the trend as T₈ (2.55%) > T₆ (2.24%) > T₄ (1.92%) = T₃ (1.88%) > T₇ (1.62%). It is evident that the combined application of FM (25%) and RP (50%) enhanced the effect of SS (25%) in terms of P bioavailability and yield parameters and can be effectively used as P fertilizer.

Methane production by co-digestion of poultry manure and lignocellulosic biomass: Kinetic and energy assessment

Six typical Brazilian lignocellulosic biomasses (rice straw, corn cob, peanut shell, sawdust, coffee husk and sugarcane bagasse) were evaluated for methane production by solid-state anaerobic co-digestion with poultry manure. The results showed the highest methane production was obtained with corn cob and poultry manure (126.02 Nm³ CH₄. ton residue^-1) using a food to inoculum ratio of 0.5, which lowered volatile fatty acids accumulation. In this condition, the thermal energy production (1.73 MJ.kg live chicken^-1) would be able to replace 53.2% of the energy with firewood in poultry farming. The high hemicellulose and low lignin content in corn cob seem to explain the biomethanation of such biomass, and this agrees with the microbial analysis which revealed the predominance of bacteria related to plant polysaccharides hydrolysis and carbohydrate conversion in the inoculum. The methane production was best modelled by Groot's multi-stage model, and the microbial adaptation to lignin might explain this.

Simultaneous Denitrification and Bio-Methanol Production for Sustainable Operation of Biogas Plants

This study was conducted to secure the sustainability of biogas plants for generating resources from food waste (FW) leachates, which are prohibited from marine dumping and have been obligated to be completely treated on land since 2013 in South Korea. The aim of this study is to reduce the nitrogen load of the treatment process while producing bio-methanol using digested FW leachate diverted into wastewater treatment plants. By using biogas in conditions where methylobacter (M. marinus 88.2%) with strong tolerance to highly chlorinated FW leachate dominated, 3.82 mM of methanol production and 56.1% of total nitrogen (TN) removal were possible. Therefore, the proposed method can contribute to improving the treatment efficiency by accommodating twice the current carried-in FW leachate amount based on TN or by significantly reducing the nitrogen load in the subsequent wastewater treatment process. Moreover, the produced methanol can be an effective alternative for carbon source supply for denitrification in the subsequent process.

Anaerobic digestion of agricultural manure and biomass - Critical indicators of risk and knowledge gaps

Anaerobic digestion (AD) has been identified as a potential green technology to treat food and municipal waste, agricultural residues, including farmyard manure and slurry (FYM&S), to produce biogas. FYM&S and digestate can act as soil conditioners and provide valuable nutrients to plants; however, it may also contain harmful pathogens. This study looks at the critical indicators in determining the microbial inactivation potential of AD and the possible implications for human and environmental health of spreading the resulting digestate on agricultural land. In addition, available strategies for risk assessment in the context of EU and Irish legislation are assessed. Storage time and process parameters (including temperature, pH, organic loading rate, hydraulic retention time), feedstock recipe (carbon-nitrogen ratio) to the AD plant (both mesophilic and thermophilic) were all assessed to significantly influence pathogen inactivation. However, complete inactivation of all pathogens is unlikely. There are limited studies evaluating risks from FYM&S as a feedstock in AD and the spreading of resulting digestate. The lack of process standardisation and varying feedstocks between AD farms means risk must be evaluated on a case by case basis and calls for a more unified risk assessment methodology. In addition, there is a need for the enhancement of AD farm-based modelling techniques and datasets to help in advancing knowledge in this area.

Benefits and drawbacks of food and dairy waste co-digestion at a high organic loading rate: A Moosburg WWTP case study

Anaerobic co-digestion (AcoD) is a key technology in reframing organic waste as a viable energy source. A lack of documented experience on full-scale AcoD at wastewater treatment plants (WWTPs) has created a bottleneck in AcoD implementation, which is further tightened by the focus of existing AcoD studies being on low co-substrate loading (<50%) and the obtainable benefits. This study aims to fill this gap by investigating the drawbacks and benefits of high-ratio co-substrate dosing of food and dairy wastes at the Moosburg WWTP (Germany) from 2014 to 2017. The Moosburg WWTP co-digests sewage sludge, food waste, and dairy wastes at a 35:47:18 ratio by volatile solids (organic loading rate (OLR) of 3.0 kg_VS/(m³·day)). During the study period, this high co-substrate dosing increased the methane potential by 300 ± 50%. The corresponding high methane yield significantly increased the on-site electricity production, resulting in energy neutrality in 2014-2015. The corresponding economic gain from gate fees was 48,000 ± 5,000 € per year. The observed drawbacks included solids accumulation inside the digester (5 m³/month), high nitrogen backload (65% increase from co-substrate addition), reduced retention time (loss of 1.18 days/year from solids accumulation), and reduced dewaterability. The high nitrogen content in the centrate is treated by sequential batch reactors (SBRs), using lactose as the carbon source for denitrification. This study presents an alternative approach for determining gate fees based on the economic gains from inherent methane content, which identified waste milk, lactose and grease trap sludge as the most profitable co-substrates.

A critical review of volatile fatty acids produced from waste activated sludge: enhanced strategies and its applications

This paper reviews the recent achievements in the enhanced production of volatile fatty acids (VFAs) from waste activated sludge (WAS). The enhanced strategies are divided into two approaches. The first strategy focuses on the regulation of carbon-to-nitrogen (C/N) ratio by co-digestion of WAS with carbon-rich substrates, including municipal solid wastes (MSW), marine algae, agricultural residues, and animal manures. The other strategy is to enhance the solubilization and hydrolysis of WAS or inhibit the methanogenesis by applying various pretreatments, such as mechanical, chemical, enzymatic, and thermal pretreatment. Finally, the applications of WAS-derived VFAs are discussed. The future researches in enhancing VFAs production and wide application of the VFAs from both technical and economic perspectives are proposed.

Innovative operational strategies for biogas plant including temperature and stirring management

Finding the optimum operational conditions (mainly temperature and stirring) inside the fermenters is crucial in the field of anaerobic digestion. This study was conducted to contribute to the research area of anaerobic digestion process optimization and is an example for other biogas plants to improve efficiency. The research aimed at finding the optimum operational conditions in a large-scale biogas plant located in Lower Saxony, Germany, which started operation in 2011. The optimization activities were performed by operating the fermenters under different operational conditions: the temperature inside the fermenters ranged between 40°C and 43°C, while applying several stirring scenarios. These changes led to an increase in specific electricity yield of 11.7% and a decrease in internal energy consumption of 10.4%. The total internal energy consumption of the biogas plant was in the range of 6.3-7.2% (the average monthly internal energy consumption was 6.7% in 2013 and 6.0% in 2014) from their own production, and 28% of this energy consumption was used by the stirrers before optimization. Therefore, finding the optimum operational conditions leads to high energy harvesting and lower internal energy consumption.

Reactor performance and economic evaluation of anaerobic co-digestion of dairy manure with corn stover and tomato residues under liquid, hemi-solid, and solid state conditions

Anaerobic co-digestion of tomato residues, dairy manure, and corn stover at ratios of 20:48:32, 40:36:24, and 60:24:16 (volatile solid basis) were compared for liquid anaerobic digestion (L-AD), hemi-solid state AD (HSS-AD), and solid state AD (SS-AD) systems. The highest methane yield (353.5 L/kg-VS_added) and volumetric methane productivity (24.5 m³_methane/m³_{reactor volume}) were both obtained with 20% tomato residues addition under L-AD and HSS-AD conditions, respectively. Total solid and feedstock mixing ratio affected the degradation of protein and lipids during AD, but not cellulose and hemicellulose. Economic analysis results indicated that capital and labor costs have the dominant effect on total investment. SS-AD of tomato residues, dairy manure, and corn stover at ratios of 20:48:32 (VS basis) has the highest net present value (2.6 million US$) and shortest payback period (10.1 year), which indicated SS-AD was financially attractive under analysis conditions.

Ambient-temperature co-digestion of low-solids municipal and industrial waste mixtures: Insights from molecular analyses

The performance of ambient temperature anaerobic co-digestion was investigated for mixtures of six substrates: canned tomato and salsa waste, portable toilet waste, septic tank waste, winery waste, beer and cider waste, and fats, oils, and grease (FOG). Laboratory semi-continuous reactor studies and molecular biological analyses revealed that beer/winery, and tomato/FOG/winery/beer mixtures resulted in the best performance in terms of biogas production (515 and 371 mL CH4/g VS, respectively) and methanogenic populations. A portable toilet/septage mixture resulted in the overall poorest performance and inhibition of microbial activity was evident. Average methane content was ~70% for all mixtures tested. The findings of this study reveal that healthy methanogen populations were present, further supporting the feasibility of biogas production via the novel feedstock mixtures in ambient temperature lagoons. Implications: Disposal of septic tank waste and other high chemical oxygen demand (COD) 10 industrial food processing waste at a small wastewater treatment plant is uncommon, because it can upset the treatment process and requires additional power for treatment. Ambient-temperature covered lagoon digesters can be an alternative low-cost technology for co-digestion of these recalcitrant waste streams while generating bioenergy. The results of this study demonstrated that there is potential for implementation of unheated covered lagoon digester systems 15 for conversion of liquid wastes for production of renewable biomethane while eliminating the need to treat these wastes at a wastewater treatment plant.

Fungal bioaerosols in biomethanization facilities

Biomethanization is a new technology used for green-waste valorization where organic waste is biodegraded by microbial communities under anaerobic conditions. The main product of this type of anaerobic digestion is a biogas used as an energy source. Moving and handling organic waste may lead to the emission of high concentrations of bioaerosols. High exposure levels are associated with adverse health effects amongst green environment workers. Fungal spores are suspected to play a role in many respiratory illnesses. There is a paucity of information related to the detailed fungal diversity in biomethanization facilities. The aim of this study was to provide an in-depth description of fungal bioaerosols in biomethanization work environments using a next-generation sequencing approach combined with real-time polymerase chain reaction (PCR). Two biomethanization facilities treating different wastes were visited during the sampling campaign (n = 16). Quantification of Penicillium/Aspergillus and Aspergillus fumigatus revealed a greater exposure risk during summer for both facilities visited. Concentrations of Penicillium and Aspergillus were similar in all work areas in both biomethanization facilities. Taxonomy analyses showed that the type of waste treated affects the fungal diversity of aerosols emitted. Although eight classes were evenly distributed in all samples, Eurotiomycetes were more dominant in the first facility and Agaricomycetes were dominant in the second one. A large diversity profile was observed in bioaerosols from both facilities showing the presence of pathogenic fungi. The following fungi detected are known allergens and/or are opportunistic pathogens: Aspergillus, Malassezia, Emericella, Fusarium, Acremonium, and Candida. Daily exposure to these fungi may put workers at risk. The information from this study can be used as a reference for minimizing occupational exposure in future biomethanization facilities. Implications: Biomethanization is a new technology used for green-waste valorization where organic waste is biodegraded by microbial communities. Effective waste management is increasingly recognized as a strategic approach for achieving newly created regulations concerning the disposal of organic residues; therefore, an expansion of facilities is expected. Workers' exposure to diverse fungal communities is certain, as fungi are ubiquitous and necessary in organic matter decomposition. Monitoring this occupational exposure is important in order to prevent workers' health problems.

Fuzzy multi-objective optimization case study based on an anaerobic co-digestion process of food waste leachate and piggery wastewater

This paper presents the development and evaluation of fuzzy multi-objective optimization for decision-making that includes the process optimization of anaerobic digestion (AD) process. The operating cost criteria which is a fundamental research gap in previous AD analysis was integrated for the case study in this research. In this study, the mixing ratio of food waste leachate (FWL) and piggery wastewater (PWW), calcium carbonate (CaCO₃) and sodium chloride (NaCl) concentrations were optimized to enhance methane production while minimizing operating cost. The results indicated a maximum of 63.3% satisfaction for both methane production and operating cost under the following optimal conditions: mixing ratio (FWL: PWW) - 1.4, CaCO₃ - 2970.5 mg/L and NaCl - 2.7 g/L. In multi-objective optimization, the specific methane yield (SMY) was 239.0 mL CH₄/g VS_added, while 41.2% volatile solids reduction (VSR) was obtained at an operating cost of 56.9 US$/ton. In comparison with the previous optimization study that utilized the response surface methodology, the SMY, VSR and operating cost of the AD process were 310 mL/g, 54% and 83.2 US$/ton, respectively. The results from multi-objective fuzzy optimization proves to show the potential application of this technique for practical decision-making in the process optimization of AD process.

Co-digestion of food waste and sewage sludge for methane production: Current status and perspective

Food waste (FW) is a valuable resource which requires sustainable management avenues to reduce the hazardous environmental impacts and add-value for better economy. Anaerobic digestion (AD) is still reliable, cost-effective technology for waste management. Conventional AD was originally designed for sewer sludge digestion, is not effective for FW due to mainly high organics and volatile fatty acid (VFA) accumulation, hence better technical aptitudes and biochemical inputs are required for optimal biogas production. Besides, to overcome these challenges, FW co-digestion with complementary organic waste e.g. sewage sludge (SS) mixed which complement each other for better process design. The main aim of this article is to summarize the recent updates and review different holistic approaches for efficient anaerobic co-digestion (AcoD) of FW and SS to provide a comprehensive review on the topic. Moreover, to demonstrate the status and perspectives of AcoD at present scenario for Hong Kong and rest of the world.

Effects of lipid concentration on thermophilic anaerobic co-digestion of food waste and grease waste in a siphon-driven self-agitated anaerobic reactor

To investigate the influence of lipid concentration (of total solids, w/w) on anaerobic treatment of food waste under thermophilic condition, a siphon-driven self-agitated anaerobic reactor was operated for 220 days. The average lipid concentration was changed from 12.8% to 59.3% (w/w) step by step. The gas production rate increased from 1.97 to 2.31 L/L/d with lipid concentration increased from 12.8% to 19.7% (w/w), whereas decreased sharply to 0.78 L/L/d when the concentration further increased to 59.3% (w/w). The COD recovery from output at different lipid concentration was analyzed in this study. With the concentration increased from 12.8% to 59.3% (w/w), the percentage of COD recovered as methane gas decreased from 80.9% to 35.4%, while the percentage of COD remained in the effluent was also decreased significantly from 15.5% to 2.60%. The lipid concentration under 40% (w/w) was recommended in the co-digestion of food waste and grease trap waste.

Effect of the mycotoxin aflatoxin B1 on a semi-continuous anaerobic digestion process

Cereals are primary crops and are the most important raw material for feed and food production. Increasing aflatoxin B1 (AFB1) contamination of corn is an emerging issue, and disposal procedures for AFB1-contaminated corn are not currently defined. Recovery of contaminated corn through anaerobic digestion may represent a suitable strategy for its valorisation; however, only a few studies concerning the effect of AFB1 on anaerobic processes can be found. Thus, the purpose of the present work was to evaluate the effect of the mycotoxin AFB1 on a semi-continuous anaerobic digestion (AD) process. Semi-continuous trials were carried out, and the biomethane production from ABF1-contaminated feedstocks (25, 50, and 100 µg kg^-1 AFB1 wet weight) was compared to that from non-contaminated feedstock. Moreover, the feasibility of the agronomic re-use of the digestate, and the fate of AFB1 during AD was assessed. No adverse effect of 25 µg kg^-1 AFB1 contamination of feedstock on biomethane yield was observed. In contrast, 100 µg kg^-1 AFB1 in the feedstock resulted in inhibition of the process due to the accumulation of organic acids, and to the decrease of the pH in the digestate (from 8.1 to 5.4). The continuous addition of AFB1-contaminated feedstock led to accumulation of the mycotoxin in the digestates. Consequently, a composting process should always precede the agricultural re-use of digestates in order to remove AFB1 and the residual phytotoxicity.

Organic acid production from potato starch waste fermentation by rumen microbial communities from Dutch and Thai dairy cows

BACKGROUND

Exploring different microbial sources for biotechnological production of organic acids is important. Dutch and Thai cow rumen samples were used as inocula to produce organic acid from starch waste in anaerobic reactors. Organic acid production profiles were determined and microbial communities were compared using 16S ribosomal ribonucleic acid gene amplicon pyrosequencing.

RESULTS

In both reactors, lactate was the main initial product and was associated with growth of Streptococcus spp. (86% average relative abundance). Subsequently, lactate served as a substrate for secondary fermentations. In the reactor inoculated with rumen fluid from the Dutch cow, the relative abundance of Bacillus and Streptococcus increased from the start, and lactate, acetate, formate and ethanol were produced. From day 1.33 to 2, lactate and acetate were degraded, resulting in butyrate production. Butyrate production coincided with a decrease in relative abundance of Streptococcus spp. and increased relative abundances of bacteria of other groups, including Parabacteroides, Sporanaerobacter, Helicobacteraceae, Peptostreptococcaceae and Porphyromonadaceae. In the reactor with the Thai cow inoculum, Streptococcus spp. also increased from the start. When lactate was consumed, acetate, propionate and butyrate were produced (day 3-4). After day 3, bacteria belonging to five dominant groups, Bacteroides, Pseudoramibacter_Eubacterium, Dysgonomonas, Enterobacteriaceae and Porphyromonadaceae, were detected and these showed significant positive correlations with acetate, propionate and butyrate levels.

CONCLUSIONS

The complexity of rumen microorganisms with high adaptation capacity makes rumen fluid a suitable source to convert organic waste into valuable products without the addition of hydrolytic enzymes. Starch waste is a source for organic acid production, especially lactate.

A critical review on the interaction of substrate nutrient balance and microbial community structure and function in anaerobic co-digestion

Anaerobic co-digestion generally results in a higher yield of biogas than mono-digestion, hence co-digestion has become a topic of general interest in recent studies of anaerobic digestion. Compared with mono-digestion, co-digestion utilizes multiple substrates. The balance of substrate nutrient in co-digestion comprises better adjustments of C/N ratio, pH, moisture, trace elements, and dilution of toxic substances. All of these changes could result in positive shifts in microbial community structure and function in the digestion processes and consequent augmentation of biogas production. Nevertheless, there have been few reviews on the interaction of nutrient and microbial community in co-digestions. The objective of this review is to investigate recent achievements and perspectives on the interaction of substrate nutrient balance and microbial community structure and function. This may provide valuable information on the optimization of combinations of substrates and prediction of bioreactor performance.

Anaerobic digestion of grape pomace: Effect of the hydraulic retention time on process performance and fibers degradability

To optimize the anaerobic digestion of grape pomace under mesophilic conditions, continuous digesters were operated at different hydraulic retention times (HRT) (30, 20, 15 and 10 days) equivalent to organic loading rates (OLR) of 2.5, 3.7, 5.7 and 7.3 kg COD m^-3 d^-1, respectively. At HRTs of 30 and 20 days, steady state conditions were observed with methane yields of 0.984 ± 0.013 NL d^-1 and 1.362 ± 0.018 NL d^-1, respectively. The HRT of 15 days was found critical because of acids accumulation through the experiments. When the OLR of 5.7 kg COD m^-3 d^-1 was reached, methane production was found to be instable. Finally, at HRT of 10 days, a failure of the system was observed due to the washing of the methanogenic microorganisms. Regarding the degradability of the lignocellulosic fractions, the maximum reduction yields for hemicellulose and cellulose were noted for HRTs of 30 and 20 days, while lignin was not degraded throughout the different experiments. For an optimization of the process, HRT of 20 days can therefore be recommended for productive use in large-scale applications.

Instability mechanisms and early warning indicators for mesophilic anaerobic digestion of vegetable waste

In order to elucidate the instability mechanism, screen early warning indicators, and propose control measures, the mesophilic digestion of vegetable waste (VW) was carried out at organic loading rates (OLR) of 0.5, 1.0, and 1.5g volatile solid (VS)/(Ld). The process parameters, including biogas components, volatile fatty acids (VFA), ammonia, pH, total alkalinity (TA), bicarbonate alkalinity (BA), and intermediate alkalinity (IA), were monitored every day. Digestion was inhibited at OLR of 1.5gVS/(Ld). The primary causes of instability are a high sugar and negligible ammonia content, in addition to the feed without effluent recirculation, which led to BA loss. The ratios of CH₄/CO₂, VFA/BA, propionate, n-butyrate and iso-valerate were selected as early warning indicators. In order to maintain the digestion of VW at a high OLR, control measures including effluent recirculation and trace element addition are recommended.

Meat Quality Traits of Pigs Finished on Food Waste

Despite the benefits associated with the use of food waste (FW), there are mixed consumer perceptions regarding pork quality harvested from pigs fed FW. Twenty crossbred pigs were selected for the present study. Ten pigs were fed a conventional diet (control group), and the other 10 pigs were given a conventional diet and FW (FW group) during different growth stages. Meat quality in the FW group showed deteriorative qualities with higher lightness and yellowness synonymous to pale soft exudative meat. Drip loss in the experimental group was significantly higher than that in the control group (p<0.01). The contents of polyunsaturated fatty acids in the FW group were higher and those of saturated and monounsaturated fatty acids were lower than those in the control group. The contents of thiobarbituric acid were significantly different between the control and FW groups (p<0.05). There was also a significant difference between the control and FW groups in terms of off-flavor (p<0.05) after sensory evaluation. To conclude, the off-flavor noted, including other inferior pork quality traits, in the FW group implies that FW should not be used as swine feed.

Synergistic effect of co-digestion to enhance anaerobic degradation of catering waste and orange peel for biogas production

Catering waste and orange peel were co-digested using an anaerobic digestion process. Orange peel is difficult to degrade anaerobically due to the presence of antimicrobial agents such as limonene. The present study aimed to examine the feasibility of anaerobic co-digestion of catering waste with orange peel to provide the optimum nutrient balance with reduced inhibitory effects of orange peel. Batch experiments were conducted using catering waste as a potential substrate mixed in varying ratios (20-50%) with orange peel. Similar ratios were followed using green vegetable waste as co-substrate. The results showed that the highest organic matter degradation (49%) was achieved with co-digestion of catering waste and orange peel at a 50% mixing ratio (CF4). Similarly, the soluble chemical oxygen demand (sCOD) was increased by 51% and reached its maximum value (9040 mg l^-1) due to conversion of organic matter from insoluble to soluble form. Biogas production was increased by 1.5 times in CF4 where accumulative biogas was 89.61 m³ t^-1_substrate compared with 57.35 m³ t^-1_substrate in the control after 80 days. The main reason behind the improved biogas production and degradation is the dilution of inhibitory factors (limonene), with subsequent provision of balanced nutrients in the co-digestion system. The tCOD of the final digestate was decreased by 79.9% in CF4, which was quite high as compared with 68.3% for the control. Overall, this study revealed that orange peel waste is a highly feasible co-substrate for anaerobic digestion with catering waste for enhanced biogas production.

Production of biogas from co-digestion of livestock and agricultural residues: A case study

This study was undertaken to determine the possible changes in the digester yield and performance for the anaerobic co-digestion under mesophilic conditions of strawberry residues (SRs) together with pig manure (PM). The first part of this paper deals with the digestion of SR as a single substrate. For organic loading rates (OLRs) of 4.4 (g L^-1 d^-1) or less, the experimental specific biogas and methane productions are 0.588 and 0.231 L g^-1, respectively. When higher OLRs (5.5 g L^-1 d^-1) are used the digester fails due to acidification. In the second part, the co-digestion of both residues is explored using a wide variety of SR:PM ratios and OLRs of 5.5 g L^-1 d^-1 with good stability. Therefore, it is demonstrated that co-digestion allows the improvement of the treatment capacity as compared with SR as a single residue. The methane and biogas productions increase as the SR:PM ratio increases. It may be concluded that, when a digester works with a certain OLR, the performance for co-digestion is always better than for single substrates because the presence of PM provides a better stability and the presence of SR improves the biogas and methane production.