Influence of substrate concentration and moisture content on the specific methanogenic activity of dry mesophilic municipal solid waste digestate spiked with propionate

Influence of Dairy Manure as Inoculum Source on Anaerobic Digestion of Swine Manure

Inoculation is a widely used method to improve the efficiency of anaerobic digestion (AD) with a high organic load. This study was conducted to prove the potential of dairy manure as an inoculum source for AD of swine manure. Furthermore, an appropriate inoculum-to-substrate (I/S) ratio was determined to improve methane yield and reduce the required time of AD. We carried out 176 days of anaerobic digestion for five different I/S ratios (3, 1, and 0.3 on a volatile solid basis, dairy manure alone, and swine manure alone) of manure, using solid container submerged lab-scale reactors in mesophilic conditions. As a result, solid-state swine manure inoculated with dairy manure could be digested without inhibition caused by ammonia and volatile fatty acid accumulation. The highest methane yield potential was observed in I/S ratios 1 and 0.3, as 133 and 145 mL CH4·g−1-VS, respectively. The lag phase of swine manure alone was more extended, 41 to 47 days, than other treatments containing dairy manure, directly related to tardy startup. These results revealed that dairy manure can be used as an inoculum source for AD of swine manure. The proper I/S ratios leading to successful AD of swine manure were 1 and 0.3.

New insights of anaerobic performance, antibiotic resistance gene removal, microbial community structure: applying graphite-based materials in wet anaerobic digestion

The addition of carbonaceous materials into anaerobic digestion (AD) has gained widespread attention due to their significant effects on anaerobic performance and antibiotic resistance gene (ARG) removal. This study selected graphite, graphene, and graphene oxide (GO) as additives to investigate variations in AD performance, ARG removal, microbial community diversity and structure in wet AD systems. The results indicated that the addition of graphite-based materials in wet AD systems could increase degradation of solid organic matters by 0.91%-3.41% and utilization of soluble organic fractions by 10.43%-13.67%, but could not stimulate methane production. After the addition of graphite and graphene, ARG removal rates were effectively increased to 90.85% and 94.22%, respectively. However, the total ARG removal rate was reduced to 77.46% with the addition of GO. In addition, the microbial diversity in the wet AD process was enhanced with the addition of GO only, graphite and graphene led to a reduction in it. As for bacterial community, graphite and graphene increased the abundance of Thermotogae from 43.43% to 57.42% and 58.74%, while GO increased the abundance of Firmicute from 49.90% to 56.27%. For the archaeal community, the proportion of hydrogenotrophic methanogens was improved when adding each graphite-based material; however, only GO increased Methanosaeta that was acetoclastic methanogens. Finally, methanogens were found as the ARG host, and ARGs that belong to the same subtype might exist in the same host bacteria.

Thermophilic Solid-State Anaerobic Digestion of Corn Straw, Cattle Manure, and Vegetable Waste: Effect of Temperature, Total Solid Content, and C/N Ratio

Thermophilic solid-state anaerobic digestion (SS-AD) of agricultural wastes, i.e., corn straw, cattle manure, and vegetable waste, was carried out in this study. The effects of temperature (40-60°C), initial solid content (ISC, 17.5-32.5%), and C/N ratio (15-32 : 1) on biogas production were evaluated using a Box-Behnken experimental design (BBD) combined with response surface methodology (RSM). The results showed that optimization of process parameters is important to promote the SS-AD performance. All the factors, including interactive terms (except the ISC), were significant in the quadratic model for biogas production with SS-AD. Among the three operation parameters, the C/N ratio had the largest effect on biogas production, followed by temperature, and a maximum biogas yield of 241.4 mL gVS-1 could be achieved at 47.3°C, ISC = 24.81%, and C/N = 22.35. After 20 d of SS-AD, the microbial community structure under different conditions was characterized by high-throughput sequencing, showing that Firmicutes, Bacteroidetes, Chloroflexi, Synergistetes, and Proteobacteria dominated the bacterial community, and that Firmicutes had a competitive advantage over Bacteroidetes at elevated temperatures. The biogas production values and relative abundance of OPB54 and Bacteroidia after 20 d of SS-AD can be fitted well using a quadratic model, implying that OPB54 and Bacteroidia play important roles in the methanogenic metabolism for agricultural waste thermophilic SS-AD.

Biodegradable and non-biodegradable fraction of municipal solid waste for multifaceted applications through a closed loop integrated refinery platform: Paving a path towards circular economy

An increase in population, rapid urbanization and industrialization has accelerated the rate of municipal solid waste generation. The current disposal of solid waste is a burgeoning issue and it's in immediate need to shift the existing disposal processes to a sustainable manner. Circular economy (CE) is a conceptual model which is been used for better use of resources and minimization of waste in a closed loop approach which could be appropriate for waste management. In this context, the present review illustrates the effective use of biodegradable and non-biodegradable fraction of solid waste in a closed loop integrated refinery platforms for the recovery of bioenergy resources and for the production of value added products. The biodegradable fraction of solid waste could be treated by advanced biological processes with the simultaneous production of bioenergy such as biohydrogen, biomethane, bioelectricity, etc., and other value added products like butanol, ethanol, methanol etc. The scheme illustrates the closed loop approach, the bioenergy generated from the biodegradable fraction of solid waste could be used for the operation of internal combustion engines and the energy could be further used for processing the waste. The non-biodegradable fraction of solid waste could be used for construction and pavement processes. Overall the study emphasizes the paradigm shift of solid waste management concepts from linear economy to a circular economy following the "Zero Waste" concept. The study also explains the circular economy policies practiced for solid waste management that stimulates the economy of the country and identify the pathways to maximize the local resources. In addition the review addresses the advanced information and communication technologies to unfold the issues and challenges faced in the solid waste management. The smart governance of managing waste using the "Internet of Things" (IoT) is one of the great precursors of technological development that could lead innovations in waste management.

Legacy effects override soil properties for CO2 and N2O but not CH4 emissions following digestate application to soil

The application of organic materials to soil can recycle nutrients and increase organic matter in agricultural lands. Digestate can be used as a nutrient source for crop production but it has also been shown to stimulate greenhouse gas (GHG) emissions from amended soils. While edaphic factors, such as soil texture and pH, have been shown to be strong determinants of soil GHG fluxes, the impact of the legacy of previous management practices is less well understood. Here we aim to investigate the impact of such legacy effects and to contrast them against soil properties to identify the key determinants of soil GHG fluxes following digestate application. Soil from an already established field experiment was used to set up a pot experiment, to evaluate N2O, CH4 and CO2 fluxes from cattle-slurry-digestate amended soils. The soil had been treated with farmyard manure, green manure or synthetic N-fertilizer, 18 months before the pot experiment was set up. Following homogenization and a preincubation stage, digestate was added at a concentration of 250 kg total N/ha eq. Soil GHG fluxes were then sampled over a 64 day period. The digestate stimulated emissions of the three GHGs compared to controls. The legacy of previous soil management was found to be a key determinant of CO2 and N2O flux while edaphic variables did not have a significant effect across the range of variables included in this experiment. Conversely, edaphic variables, in particular texture, were the main determinant of CH4 flux from soil following digestate application. Results demonstrate that edaphic factors and current soil management regime alone are not effective predictors of soil GHG flux response following digestate application. Knowledge of the site management in terms of organic amendments is required to make robust predictions of the likely soil GHG flux response following digestate application to soil.

Interpretable machine learning for predicting biomethane production in industrial-scale anaerobic co-digestion

The objective of this study is to apply machine learning models to accurately predict daily biomethane production in an industrial-scale co-digestion facility. The methodology involved applying elasticnet, random forest, and extreme gradient boosting to input-output data from an industrial-scale anaerobic co-digestion (ACoD) facility. The models were used to predict biomethane for 1-day, 3-day, 5-day, 10-day, 20-day, 30-day, and 40-day time horizons. These models were fit on four years of operational data. The results showed that elastic net (a model with assumptions of linearity) was clearly outperformed by random forest and extreme gradient boosting (XGBoost), which had out-of-sample R²values ranging between 0.80 and 0.88, depending on the time horizon. In addition, feature importance and partial dependence analysis demonstrated the marginal and interaction effects on biomethane of selected biowaste inputs. For instance, food waste co-digested with percolate were shown to have strong positive interaction effects. One implication of this study is that XGBoost and random forest algorithms applied to industrial-scale ACoD data provide dependable prediction results and may be a useful complement for experimental and mechanistic/theoretical models of anaerobic digestion, especially where detailed substrate characterization is difficult. However, these models have limitations, and suggestions for deriving additional value from these methods are proposed.

The influence of different solid-liquid ratios on the thermophilic anaerobic digestion performance of palm oil mill effluent (POME)

An alternative method was proposed to optimize the treatment process of palm oil mill effluent (POME) in an effort to address the poor removal efficiencies in terms of the chemical and biological oxygen demand (COD and BOD), total suspended solids (TSS) as well as oil and grease (O&G) content in treated POME along with many environmental issues associated with the existing POME treatment process. The elimination of the cooling ponds and the insertion of a dewatering device in the treatment process were recommended. The dewatering device should enhance the anaerobic digestion process by conferring a means of control on the digesters' load. The objective of this study is to identify the optimum solid: liquid ratio (total solids (TS) content) that would generate the maximum amount of biogas with better methane purity consistently throughout the anaerobic digestion of POME, all while improving the treated effluent quality. It was established that a 40S:60L (4.02% TS) was the best performing solid loading in terms of biogas production and methane yield as well as COD, BOD, TSS, and O&G removal efficiencies. Meanwhile, at higher solid loadings, the biogas production is inhibited due to poor transport and mass transfer. It is also speculated that sulfate-reducing bacteria tended to inhibit the biogas production based on the significantly elevated H₂S concentration recorded for the 75S:25L and the 100S loadings.

Dry anaerobic digestion of organic waste: A review of operational parameters and their impact on process performance

Dry digestion is a suitable technology for treating organic wastes with varying composition such as the organic fraction of municipal solids waste. Yet, there is a need for further research to overcome some of the disadvantages associated with the high total solids content of the process. Optimisation of inoculum to substrate ratio, feedstock composition and size, liquid recirculation, bed compaction and use of bulking agents are some of the parameters that need further investigation in batch dry anaerobic digestion, to limit localised inhibition effects and avoid process instability. In addition, further attention on the relation between feedstock composition, organic loading rate and mixing regimes is required for continuous dry anaerobic digestion systems. This paper highlights all the areas where knowledge is scarce and value can be added to increase dry anaerobic digestion performance and expansion.

Dry anaerobic co-digestion of roadside grass and cattle manure at a 60 L batch pilot scale

Roadside grass cuttings and solid cattle manure are resources that are available as input for dry anaerobic co-digestion. Two series of measurements were carried out, one in June 2016 and one in October 2016. The methane potentials were determined on a laboratory scale and revealed a high degree of seasonality, 202.9 and 167.9 Nm³_CH4.t_VS^-1, respectively. Moreover, these substrates were co-digested in reactors by the dry process on a pilot scale (60 L). Two strategies for filling and optimization, as layers or as a mixture, were compared. The seasonality also determined the physicochemical parameters and the hydrodynamic properties involved in percolation of the liquid phase recirculated in the dry digestion process. The production of methane depended on the filling method, the seasonality, and the nature of the input, which in some cases resulted in inhibition of 34.8-44.4 Nm³_CH4.t_VS^-1.

The auto fluorescence characteristics, specific activity, and microbial community structure in batch tests of mono-chicken manure digestion

Batch tests inoculated with granular and suspended sludge of mono chicken manure (CM) digestion were conducted. Kinetic analysis showed a maximum bio-CH₄ generation (6 mL/gVS/d) at an optimal TS of 10-12%. At a TS of 25%, serious inhibition was found for granular sludge and even greater inhibition for the suspended sludge caused by free ammonia. The auto fluorescence of Excitation-mission matrix with parallel factor analysis (PARAFAC) showed that the dissolved organic matter (DOM) varied between the form C1, C2, C3 and C4. The split component of the SMP-like C2 and protein-like C4 significantly related to the bio-methane production in time series. The canonical correlation analysis (CCA) indicated that ammonia, pH, and TS influenced the PARAFAC component significantly. The aceticlastic methanogens of the genus Methanosaeta and acetogens of the genus Syntrophobacter predominated in the CM sludge. The methanogens and acetogens formed a metabolic cooperation, making the process a stable methane produced activity.

The seasonal evolution of fruit, vegetable and yard wastes by mono, co and tri-digestion at Hyderabad, Sindh Pakistan

The contribution of biowastes in municipal solid waste (MSW) is increasing day by day and being dumped in open atmosphere along with other wastes in every city of Pakistan. This study was formulated to evaluate the feasibility of biowastes such as fruit, vegetable and yard wastes of different seasons individual and mixing at different ratios to optimize methane production at Hyderabad Sindh, Pakistan. Batch digestion of selected samples was conducted for 40 days under mesophilic condition. Methane yield of individual fruit, vegetable and yard wastes (FrVYW) of summer and winter season was obtained in the range of 0.36-0.40 L/g VS and 0.39-0.44 L/g VS added respectively. The results of co-digestion of FrVYW of summer and winter season were observed in the range of 0.42-0.45 L/g VS added and 0.46 to 0.54 L/g VS added respectively. The results of tri-digestion of FrVYW of summer and winter season were achieved in the range of 0.46-0.53 L/g VS added and 0.56-0.62 L/g VS added respectively. Findings of study showed that methane production potential of tri-digestions were highest than all of others and that of co-digestion were higher than mono-digestion of FrVYW. Overall results of study concluded that tri-digestion of FrVYW at the equal blending ratio reported highest methane potential. Therefore, the study recommended that tri-digestion of FrVYW at equal mixing ratio is an optimal ratio for anaerobic digestion process to yield maximum methane production from FrVYW.

Solid anaerobic digestion: State-of-art, scientific and technological hurdles

In this paper, a state-of-art about solid anaerobic digestion (AD), focused on recent progress and trends of research is proposed. Solid anaerobic digestion should be the most appropriate process for degradation of by-products with high total solid (TS) content, especially lignocellulosic materials like agricultural waste (straw, manure), household waste and food waste. Solid AD is already widely used in waste water treatment plant for treating plant for sewage sludge but could be more developed for lignocellulosic materials with high TS content. Many research works were carried out in Europe on solid AD, focused on current hurdles (BMP, codigestion, inhibition, microbial population, rheology, water transfers, inoculum, etc.) in order to optimize the solid AD process. In conclusion, hurdles of solid AD process should and must be solved in order to propose better productivity and profitability of such system operating with high TS content (>15%), favouring reliable industrial processes.

The effects of pre-aeration and inoculation on solid-state anaerobic digestion of rice straw

Pre-aeration was investigated for enhancing biodegradation of recalcitrant lignocellulosic structure of rice straw under various low temperatures regimes (25, 35 and 45°C) and aeration durations (0, 2, 4, 6 and 8days). It was demonstrated aerated rice straw for 2days at 35°C resulted in highest hydrolytic efficiency and biochemical methane potential (BMP) (355.3±18.7mlCH₄/gVS). Furthermore, both methane yields and initiation speeds of the solid-state anaerobic digestion (SS-AD) were inversely proportional to substrate-to-inoculum ratios due to the accumulation of volatile fatty acids (VFAs) and poor mass transfer. The highest methane yield achieved under SS-AD was 234mlCH₄/gVS at TS of 16% which 72% of the BMP. Inoculum dilution with recycled water improved buffering capacity and mitigated accumulation of VFAs, resulting in an improved SS-AD performance. The combined pre-aeration and SS-AD was therefore established as a viable option to accelerate methane production for lignocellulosic biomass.

Fractal-like kinetics of the solid-state anaerobic digestion

Total solid content (TS) negatively impacts the methane production efficiency (MPE) of solid-state anaerobic digestion (SS-AD), to which the classic mass action-based kinetics failed to provide a unified explanation. This study revealed that SS-AD reactions actually follow the fractal-like kinetics in light of the surface reactions in crowded SS-AD environment packed with heterogeneous media. The fractal characteristics of the SS-AD kinetics were found increasingly pronounced as TS increased. This study represents the first attempt to resolve the dilemma in SS-AD kinetics with the application of fractal theory. Employing this new concept allows explaining the reduced MPE at high TS and offers an easy assessment of the fractal characteristics of the SS-AD media.

Solid-state anaerobic digestion of lignocellulosic biomass: Recent progress and perspectives

Solid-state anaerobic digestion (SS-AD), which has gained popularity in the past decade as an environmentally friendly and cost-effective technology for extracting energy from various types of lignocellulosic biomass, is reviewed in this paper. According to data of biomass and methane yields of lignocellulosic feedstocks, crop residues have the highest methane production potential in the U.S., followed by the organic fraction of municipal solid waste (OFMSW), forestry waste, and energy crops. Methane yield and process stability of SS-AD can be improved by different strategies, such as co-digestion with other organic wastes, pretreatment of lignocellulosic biomass, and optimization of operating parameters. Different models for SS-AD have been developed, and insights into SS-AD processes have been obtained via microbial community analysis, microscope imaging, and tracer techniques. Future research and development in SS-AD, including feedstock identification and co-digestion, feedstock storage and pretreatment, SS-AD reactor development, digestate treatment, and value-added production, are recommended.

Comparison of digestate from solid anaerobic digesters and dewatered effluent from liquid anaerobic digesters as inocula for solid state anaerobic digestion of yard trimmings

To select a proper inoculum for the solid state anaerobic digestion (SS-AD) of yard trimmings, digestate from solid anaerobic digesters and dewatered effluent from liquid anaerobic digesters were compared at substrate-to-inoculum (S/I) ratios from 0.2 to 2 (dry basis), and total solids (TS) contents from 20% to 35%. The highest methane yield of around 244L/kg VSfeed was obtained at an S/I ratio of 0.2 and TS content of 20% for both types of inoculum. The highest volumetric methane productivity was obtained with dewatered effluent at an S/I ratio of 0.6 and TS content of 24%. The two types of inoculum were found comparable regarding methane yields and volumetric methane productivities at each S/I ratio, while using dewatered effluent as inoculum reduced the startup time. An S/I ratio of 1 was determined to be a critical level and should be set as the upper limit for mesophilic SS-AD of yard trimmings.

Dry co-digestion of sewage sludge and rice straw under mesophilic and thermophilic anaerobic conditions

Dry anaerobic digestion of sewage sludge can recover biogas as energy; however, its low C/N ratio limits it as a single substrate in the anaerobic digestion. Rice straw is an abundant agricultural residue in China, which is rich in carbon and can be used as carbon source. In the present study, the performance of dry co-digestion of sewage sludge and rice straw was investigated under mesophilic (35 °C) and thermophilic (55 °C) conditions. The operational factors impacting dry co-digestion of sewage sludge and rice straw such as C/N ratio, moisture content, and initial pH were explored under mesophilic conditions. The results show that low C/N ratios resulted in a higher biogas production rate, but a lower specific biogas yield; low moisture content of 65 % resulted in the instability of the digestion system and a low specific biogas yield. Initial pH ranging 7.0-9.0 did not affect the performance of the anaerobic digestion. The C/N ratio of 26-29:1, moisture content of 70-80 %, and pH 7.0-9.0 resulted in good performance in the dry mesophilic co-digestion of sewage sludge and rice straw. As compared with mesophilic digestion, thermophilic co-digestion of sewage sludge and rice straw significantly enhanced the degradation efficiency of the substrates and the specific biogas yield (p < 0.05) at the conditions of C/N ratio 26:1, moisture content 80 %, and natural initial pH. Although high concentrations of ammonia-nitrogen (NH4-N, 1500 mg/kg wet weight) were formed during thermophilic digestion, there was no obvious inhibition occurred. The results indicated that rice straw can be used as carbon source for the dry co-digestion of sewage sludge under mesophilic and thermophilic conditions.

Modified Anaerobic Digestion Model No.1 for dry and semi-dry anaerobic digestion of solid organic waste

The role of total solids (TS) content in anaerobic digestion of selected complex organic matter, e.g. rice straw and food waste, was investigated. A range of TS from wet (4.5%) to dry (23%) was evaluated. A modified version of the Anaerobic Digestion Model No.1 for a complex organic substrate is proposed to take into account the effect of the TS content on anaerobic digestion. A linear function that correlates the kinetic constants of three specific processes (i.e. disintegration, acetate and propionate up-take) was included in the model. Results of biomethanation and volatile fatty acids production tests were used to calibrate the proposed model. Model simulations showed a good agreement between numerical and observed data.

The effects of initial substrate concentration, C/N ratio, and temperature on solid-state anaerobic digestion from composting rice straw

This study investigated the possibilities of improving the biogasification from solid-state anaerobic digestion (SS-AD) of composting rice straw (RS) based on the optimized digestion temperature, initial substrate concentration (ISC) and C/N ratio. RS compounds, such as lignin, cellulose, and hemicellulose, were significantly degraded after composting. A significant interactive effect of temperature, ISC and C/N ratio was found on the biogasification of SS-AD of composting RS, and a maximum biogas production was achieved at 35.6°C, with a 20% ISC and a C/N ratio of 29.6:1. The verification experiment confirmed the optimization results. High-throughput sequencing analysis indicated that microbial communities in the SS-AD mainly consist of Methanobacteria, Bacteroidia, Clostridia, Betaproteobacteria, and Gammaproteobacteria. A dominant Methanobacteria was shifted from Methanobacterium to Methanoculleus during the SS-AD process. This study provides novel information about the interdependent effects and microbial behavior of AD.

Effect of total solids content on methane and volatile fatty acid production in anaerobic digestion of food waste

This work investigates the role of the moisture content on anaerobic digestion of food waste, as representative of rapidly biodegradable substrates, analysing the role of volatile fatty acid production on process kinetics. A range of total solids from 4.5% to 19.2% is considered in order to compare methane yields and kinetics of reactors operated under wet to dry conditions. The experimental results show a reduction of the specific final methane yield of 4.3% and 40.8% in semi-dry and dry conditions compared with wet conditions. A decreasing trend of the specific initial methane production rate is observed when increasing the total solids concentration. Because of lack of water, volatile fatty acids accumulation occurs during the first step of the process at semi-dry and dry conditions, which is considered to be responsible for the reduction of process kinetic rates. The total volatile fatty acids concentration and speciation are proposed as indicators of process development at different total solids content.

Chemical characteristics of biomass from nature conservation management for methane production

The aim of the current study was to assess the biochemical methane potential (BMP) of different functional groups harvested from different semi-natural grassland types that are valuable for nature conservation purposes. Ensiling of particular biomass did not significantly influence its methane yield, however, the ranking of functional groups by their methane yield varied during the experiment. During the first days of the experiment, methane was released most rapidly by legumes and other forbs with higher N and P contents. At the end of the BMP experiment the quantity of methane produced was higher in grasses and sedges/rushes with lower K, Mg and lignin content. Hence, measurement of feedstock chemical composition is an essential input to develop suitable technology for anaerobic digestion of late harvested biomass from semi-natural grasslands.

A mass diffusion-based interpretation of the effect of total solids content on solid-state anaerobic digestion of cellulosic biomass

In solid-state anaerobic digestion (SS-AD) of cellulosic biomass, the volumetric methane production rate has often been found to increase with the increase in total solids (TS) content until a threshold is reached, and then to decrease. This phenomenon cannot be explained by conventional understanding derived from liquid anaerobic digestion. This study proposed that the high TS content-caused mass diffusion limitation may be responsible for the observed methane production deterioration. Based on this hypothesis, a new SS-AD model was developed by taking into account the mass diffusion limitation and hydrolysis inhibition. The good agreement between model simulation and the experimental as well as literature data verified that the observed reduction in volumetric methane production rate could be ascribed to hydrolysis inhibition as a result of the mass diffusion limitation in SS-AD.

Effect of moisture on disintegration kinetics during anaerobic digestion of complex organic substrates

The role of the moisture content and particle size (PS) on the disintegration of complex organic matter during the wet anaerobic digestion (AD) process was investigated. A range of total solids (TS) from 5% to 11.3% and PS from 0.25 to 15 mm was evaluated using carrot waste as model complex organic matter. The experimental results showed that the methane production rate decreased with higher TS and PS. A modified version of the AD model no.1 for complex organic substrates was used to model the experimental data. The simulations showed a decrease of the disintegration rate constants with increasing TS and PS. The results of the biomethanation tests were used to calibrate and validate the applied model. In particular, the values of the disintegration constant for various TS and PS were determined. The simulations showed good agreement between the numerical and observed data.

Total solids content: a key parameter of metabolic pathways in dry anaerobic digestion

BACKGROUND

In solid-state anaerobic digestion (AD) bioprocesses, hydrolytic and acidogenic microbial metabolisms have not yet been clarified. Since these stages are particularly important for the establishment of the biological reaction, better knowledge could optimize the process performances by process parameters adjustment.

RESULTS

This study demonstrated the effect of total solids (TS) content on microbial fermentation of wheat straw with six different TS contents ranging from wet to dry conditions (10 to 33% TS). Three groups of metabolic behaviors were distinguished based on wheat straw conversion rates with 2,200, 1,600, and 1,400 mmol.kgVS-1 of fermentative products under wet (10 and 14% TS), dry (19 to 28% TS), and highly dry (28 to 33% TS) conditions, respectively. Furthermore, both wet and dry fermentations showed acetic and butyric acid metabolisms, whereas a mainly butyric acid metabolism occurred in highly dry fermentation.

CONCLUSION

Substrate conversion was reduced with no changes of the metabolic pathways until a clear limit at 28% TS content, which corresponded to the threshold value of free water content of wheat straw. This study suggested that metabolic pathways present a limit of TS content for high-solid AD.

Liquid mixing and solid segregation in high-solid anaerobic digesters

An experimental procedure (Residence Time Distribution technique) was used to characterize the macro-mixing of both liquid and solid phases of a laboratory-scale dry anaerobic digester using appropriate tracers. The effects of the waste origin and total solid content were studied. An increase in TS content from 22% to 30% TS (w/w) induced a macro-mixing mode closer to a theoretical Plug Flow Reactor. The segregation of particles having different densities was investigated regarding the RTD of the solid phase. Segregation of dense particles occurred at low TS content. By using different TS content and waste origins, it was also determined that the yield stress was a key parameter in the mechanism of segregation. At high yield stress, dense particles were more stable and thus less subjected to settling. As a consequence, operating at high TS content may permit to prevent the sedimentation of the denser particles.

Dynamic effect of total solid content, low substrate/inoculum ratio and particle size on solid-state anaerobic digestion

Among all the process parameters of solid-state anaerobic digestion (SS-AD), total solid content (TS), inoculation (S/X ratio) and size of the organic solid particles can be optimized to improve methane yield and process stability. To evaluate the effects of each parameter and their interactions on methane production, a three level Box-Behnken experimental design was implemented in SS-AD batch tests degrading wheat straw by adjusting: TS content from 15% to 25%, S/X ratio (in volatile solids) between 28 and 47 and particle size with a mean diameter ranging from 0.1 to 1.4mm. A dynamic analysis of the methane production indicates that the S/X ratio has only an effect during the start-up phase of the SS-AD. During the growing phase, TS content becomes the main parameter governing the methane production and its strong interaction with the particle size suggests the important role of water compartmentation on SS-AD.

Bio-methanization of organic fraction from municipal solid waste: temperature effects

The main objective of this study was to analyse the development of dry anaerobic digestion process of OFMSW in batch reactors under two temperature ranges, thermophilic (55°C) and mesophilic (35°C). The experimental results lead to the conclusion that the thermophilic range has a greater rate of hydrolysis and is therefore more effective to degrade wastes, shortening the overall operating time. For example, the hydrolytic step in the thermophilic (T) process lasted an average of 8 days versus 14 days in the mesophilic (M) range. The methanogenic phase lasted for 18 and 29 days in the T and M processes, respectively. The mesophilic range showed higher removal of the organic effluent but with greater uptime requirements. Moreover, the thermophilic range showed greater productivities than the mesophilic range, and the productivities were approximately doubled in terms of the produced biogas from a given amount of consumed organic matter.

Stabilisation of biodried municipal solid waste fine fraction in landfill bioreactor

The biodrying process of solid waste is a pre-treatment for the bio-stabilisation of the municipal solid waste. This study aims to investigate the fate of the municipal solid waste fine fraction (MSWFF) resulting from a biodrying treatment when disposed in landfills that are operated as bioreactors. Biodried MSWFF was apparently stable due to its low moisture content that slows down the microbial activity. The lab-scale anaerobic bioreactors demonstrated that a proper moisture content leads to a complete biodegradation of the organic matter contained in the biodried MSWFF. Using a pilot-scale landfill bioreactor (LBR), MSWFF stabilisation was achieved, suggesting that the leachate recirculation could be an effective approach to accomplish the anaerobic biodegradation and biostabilisation of biodried MSWFF after landfilling. The biostabilisation of the material resulting from the LBR treatment was confirmed using anaerobic and aerobic stability indices. All anaerobic and aerobic indices showed a stability increase of approximately 80% of the MSWFF after treatment in the LBR. The similar values of OD7 and BMP stability indices well agree with the relationship between the aerobic and anaerobic indices reported in literature.