Influence of total solids concentration on the anaerobic co-digestion of sugar beet by-products and livestock manures

Innovative approach for predicting biogas production from large-scale anaerobic digester using long-short term memory (LSTM) coupled with genetic algorithm (GA)

An artificial neural network (ANN) model called long-short term memory (LSTM), coupled with a genetic algorithm (GA) for feature selection, was used to predict biogas production of large-scale anaerobic digesters (ADs) of Tehran South Wastewater Treatment Plant (Iran), with a biogas production of approximately 30,000 Nm3/d. In order to employ the real conditions, the hydraulic retention time (HRT) of the ADs (21 days) was considered as the LSTM look-back window. To evaluate the model predictions, three different scenarios were defined. In the first scenario, the model predicted the produced biogas by using raw wastewater characteristics and reached the coefficient of determination of R2 = 0.84. The GA selected four out of eleven parameters of raw wastewater, including loads of BOD5, COD, TSS, and TN (kg/d), as the most informative data for the model. In the second scenario, the model predicted the produced biogas by employing the data of the thickened sludge streams entering the ADs and yielded a higher accuracy (R2 = 0.89). In this scenario, GA selected two out of six parameters of the sludge streams, including total flow rate (m3/d) and average solids content (w/w%). Finally, in the third scenario, by putting the parameters of the two previous scenarios together, the model's prediction accuracy increased slightly (R2 = 0.90). The results demonstrated that the GA-LSTM modeling technique could achieve reliable performance in predicting biogas production of large-scale ADs by including HRT in modeling procedure. It was also found that the raw wastewater characteristics severely affect AD behavior and can be successfully used as the input data of the AD models.

Wet anaerobic digestion of organic fraction of municipal solid waste: experience with long-term pilot plant operation and industrial scale-up

This paper presents the analysis of a pilot anaerobic digestion plant that operates with organic fraction of municipal solid waste (OFMSW) from a wholesale market and can treat up to 500 kg d-1. The process was monitored for a period of 524 days during which the residue was characterized and the biogas production and methane content were recorded. The organic load rate (OLR) of volatile solids (VS) was 0.89 kg m-3 d-1 and the Hydraulic Retention Time (HRT) was 25 d during the process. The yield was 82 Nm3 tons OFMSW-1 biogas, equivalent to 586 Nm3 tons CH4 VS-1. The results obtained in the pilot plant were used to carry out a technical-economic evaluation of a plant that treats 50 tons of OFMSW from wholesale markets. A production of 3769 Nm3 d-1 of biogas and 2080 Nm3 d-1 of methane is estimated, generating 35.1 MWh d-1 when converted to electricity.

Evaluación del desempeño de una celda de combustible microbiana con electrodo de grafito modificado para el tratamiento de agua residual del procesamiento del café

La actividad cafetalera en Costa Rica procesa aproximadamente 69.000 toneladas de café mediante la técnica de beneficiado húmedo. Esta actividad conlleva un alto impacto ambiental debido a la generación de 8 L de agua residual/kg de café oro producido. El presente trabajo tiene como objetivo utilizar el agua residual del procesamiento de café como sustrato en celdas combustibles microbianas (CCM), con el propósito de generar energía eléctrica a través de su uso y, a la vez, disminuir la carga orgánica del residuo. La CCM empleó un cátodo modificado con ftalocianinas de hierro (FePc), generó una eficiencia coulómbica de 0,7% y una densidad de potencia de 89 μW/cm2 en un ciclo de operación de cinco días. Además, se determinó que la CCM disminuye la demanda química de oxígeno (DQO) del residuo hasta en 27% bajo las condiciones de operación nativas del sustrato, a temperatura ambiente, sin mediadores químicos para la reacción anódica y con el uso de electrodos de platino para el cátodo. El estudio confirma la oportunidad de emplear el sustrato con una flora microbiana nativa apta para la operación de la tecnología de la CCM, y así perfilar el dispositivo como una opción novedosa para el tratamiento de este residuo en Costa Rica.

Stimulatory effects of biochar addition on dry anaerobic co-digestion of pig manure and food waste under mesophilic conditions

The stimulatory effect of biochar addition on dry anaerobic digestion (AD) has been rarely investigated. In this study, the effects of commonly used biochars (bamboo, rice husk, and pecan shell) on dry co-AD were investigated using mesophilic batch digesters fed with pig manure and food waste as substrates. The results show that the specific methane yield was mildly elevated with the addition of biochars by 7.9%, 9.4%, and 12.0% for bamboo, rice husk, and pecan shell-derived biochar additions, respectively. Biochar did facilitate the degradation of poorly biodegradable organics. In comparison, there was no significant effect on the peak methane production rate by the supplementation of the selected biochars. Among the three mechanisms of enhancing methanogenesis by biochar (buffering, providing supporting surface, and enhancing electron transfer), the first two mechanisms did not function significantly in dry co-AD, while the third mechanism (i.e., enhancing electron transfer) might play an important part in dry AD process. It is recommended that the utilization of biochar for the enhancement of biomethanation in dry AD should be more focused on mono digestion in future studies.

Waste to energy: valorization of spent tea waste by anaerobic digestion

The conversion of renewable resources into value-added products such as bioenergy is one of the growing concerns of bioeconomy strategy. Within this concept, assessing the proper combination of local wastes has major importance. This study set out to assess the feasibility of using spent tea waste as a single and co-substrate on anaerobic digestion and to explore the influence of the amount of microorganisms on the digester performance. For this purpose, biomethane potentials tests were conducted for seven different mixing ratios of spent tea waste and cow manure on a mass basis. The reactors operated under mesophilic conditions for 20 days with two inoculum/substrate ratios. The results revealed that using spent tea waste as a co-substrate did not reveal a significant effect on biomethane production in the reactors. Contrarily, the amount of inoculum had a remarkable effect on biomethane production, resulted in an increase in methane production between 28 and 32%. While the biomethane yields were in the range of 129-138 mL_N CH₄ g_VS^-1 for the co-digesters operated with inoculum/substrate ratio of 1, the range was 165-181 mL_N CH₄ g_VS^-1 for the co-digesters operated with inoculum/substrate ratio of 2. These findings represent the potential usage of the spent tea waste as a co-substrate within the sustainable waste management approach and are relevant to plant operators.

Anaerobic digestion of agricultural wastes from liquid to solid state: Performance and environ-economic comparison

Performance and environ-economic impacts were compared for anaerobic digestion (AD) of dairy manure and cucumber residues from liquid to solid state. Environ-economic evaluation of the overall AD process at different total solids (TS) was performed with the estimated treatment capacity of 9600 tons/year and service life of 20 years. Results showed that TS increase from 6% to 22% enhanced both cumulative and volumetric methane (CH₄) production. Further TS increase to 25%, however, reduced CH₄ yield. Environ-economic assessment indicated that TS increase enhanced volumetric waste treatment capacity and thus AD environmental footprints. Environmental credits from digestate and biogas utilization could compensate the adverse environmental impacts of other processes in AD plants. Furthermore, biogas and nutrients in digestate determined AD net-present value. As a result, solid state AD was more profitable with higher CH₄ yield and more nutrients in both biosolids and digested effluent of digestate than its liquid and hemi-solid counterparts.

Fate of antibiotic resistance genes during high solid anaerobic digestion with pig manure: Focused on different starting modes

As an emerging technology, high solid anaerobic digestion (HSAD) was usually hampered by the long lag phase of methane production. A reasonable starting mode enabled fast startup in HSAD, which was scarcely reported. This study established 5 starting modes for HSAD with pig manure. The results showed that system T4 (biogas slurry once and then autologous leachate reflux) had the shortest lag phase. Starting modes had a total effect of 36.6% on gas production, among which 17.1% affected gas production directly and 19.5% affected it through other factors. About 12/17 of antibiotic resistance genes (ARGs) and 3 mobile genetic elements (MGEs) were effectively reduced during HSAD. System T4 had the highest microbial diversity and the largest number of unique OTUs. MGEs explained most for ARGs variation (>50%), followed by microbial community. Most of the potential host genera for ARGs belonged to Firmicutes phyla, which could be decreased by starting modes.

Thermophilic Anaerobic Co-Digestion of Exhausted Sugar Beet Pulp with Cow Manure to Boost the Performance of the Process: The Effect of Manure Proportion

Sugar beet by-products are a lignocellulosic waste generated from sugar beet industry during the sugar production process and stand out for their high carbon content. Moreover, cow manure (CM) is hugely produced in rural areas and livestock industry, which requires proper disposal. Anaerobic digestion of such organic wastes has shown to be a suitable technology for these wastes valorization and bioenergy production. In this context, the biomethane production from the anaerobic co-digestion of exhausted sugar beet pulp (ESBP) and CM was investigated in this study. Four mixtures (0:100, 50:50, 75:25, and 90:10) of cow manure and sugar beet by-products were evaluated for methane generation by thermophilic batch anaerobic co-digestion assays. The results showed the highest methane production was observed in mixtures with 75% of CM (159.5 mL CH4/g VolatileSolids added). Nevertheless, the hydrolysis was inhibited by volatile fatty acids accumulation in the 0:100 mixture, which refers to the assay without CM addition. The modified Gompertz model was used to fit the experimental results of methane productions and the results of the modeling show a good fit between the estimated and the observed data.

Insights into Anaerobic Co-Digestion of Lignocellulosic Biomass (Sugar Beet By-Products) and Animal Manure in Long-Term Semi-Continuous Assays

Biogas production through anaerobic digestion has proven to be one of the most important pillars of the transition into the circular economy concept, a sustainable approach for biorefinery. This work aims to extend and improve knowledge in the anaerobic co-digestion of complementary substrates, given insights into wastes biodegradability and the influence of manure composition on the anaerobic process stability. Anaerobic co-digestion of sugar beet by-products with two kinds of animal manure (pig and cow) was investigated in semi-continuous assays, analyzing both common and non-classical parameters. Co-digestion with manure clearly mitigated the inhibitory effect of volatile fatty acids at high organic loading rates, leading to increases in methane production by 70% and 31% in comparison with individual digestion of sugar beet by-products, for co-digestion with pig and cow manure, respectively. Non-classical parameters could give more insight into the coupling/uncoupling of the anaerobic digestion phases and the involved microorganisms. Indirect parameters indicated that the process failure at the critical organic loading rates was mainly due to methanogenesis inhibition in the co-digestion with pig manure, while in co-digestion with cow manure or in individual digestion of sugar beet by-products, both hydrolysis–acidogenesis and methanogenesis phases were affected. Biomethanation degree refers to the maximum methane potential of organic wastes. Sugar beet by-products required a long digestion-time to reach high biodegradability. However, short digestion-times for co-digestion assays led to a high biomethanation degree.

Process performance and microbial community functional structure in a thermophilic anaerobic baffled reactor coupled with biocatalysed electrolysis

In this study, the performances of a conventional anaerobic baffled reactor (ABR) and an ABR combined with microbial electrolysis cells (MECs) for enhancing degradation of volatile fatty acids (VFAs) were evaluated in 55°C. The ABR-MECs system achieved a total chemical oxygen demand (COD) removal rate of 97.2% and a methane yield of 236 ± 5 mL g^-1 COD_removed at organic loading rate (OLR) of 6.9 kg COD m^-3 d^-1, which were higher than those of the ABR with 77.6% and 207 ± 5 mL g^-1 COD_removed, respectively, at OLR of 5.1 kg COD m^-3 d^-1. The pyrosequencing analysis confirmed that the introduction of MECs into ABR was conducive to establishing stable functional communities of syntrophic fatty acids oxidizing bacteria (SFOB), exoelectrogens and hydrogenotrophic methanogens, such as Syntrophobacter (5.4%), Thermodesulfovibrio (2.0%), Methanobacterium (43.8%), Methanolinea (20.4%). The content of unclassified bacteria increased from 12.4% in the ABR system to 52.3% in the ABR-MECs system. In contrast, the proportion of aceticlastic methanogens decreased from 50.1% in the ABR to 24.5% in the ABR-MECs system. The improved performance of the thermophilic ABR-MECs system resulted from phase separation, wide ecological niche and intensification of methanogenesis process via functional microbes, which significantly enhanced the degradation of propionic acid and acetic acid.

Towards centralized biogas plants: Co-digestion of sewage sludge and pig manure maintains process performance and active microbiome diversity

The aim of this study is to assess the performance of anaerobic digestion against co-digestion systems during the start-up stages based on key process parameters and biological indicators. Two parallel experiments treating sewage sludge alone or co-digested with low concentration of pig manure (8% vol., 2-3% in COD basis) were carried out in two lab-scale CSTR at mesophilic conditions. Same inoculant and organic loading rate sequences were applied for two consecutive runs of 79 and 90 days. According to the removal efficiencies achieved, no significant differences were encountered amongst mono-digestion and co-digestion. This observation was reinforced with the analysis of the total/active microbiome, sequencing 16S rRNA genes and transcripts. The addition of a co-substrate at low concentration had a negligible effect on the total/active microbial communities; they evolved following the same pattern. This might be an advantage in order to upgrade existing wastewater treatment plants to become centralized biogas facilities.

Interactive Effects of Chemical Composition of Food Waste during Anaerobic Co-Digestion under Thermophilic Temperature

The effects of chemical composition (carbohydrates, lipids, and protein) on the anaerobic co-digestion performance of food wastes (FW) were investigated from the viewpoints of methane production, dynamic parameters, and microbial community structure. The results of this study showed that a notable gasification rate was positively correlated with the proportion of the composition. A T2 reactor, which consisted of 60% carbohydrates, 20% lipids, and 20% protein, held a higher gasification rate of 65.09% compared to other groups, while its process parameters showed some deficiency regarding the stability of digestion, especially for low biochemical methane potential (BMP), which was not beneficial for the actual practice. A T4 reactor, with a highest gasification rate of 70.68%, held the maximum BMP (497.44 mL/g VS). The stable chemical parameters achieved the optimal proportion, consisting of 40% carbohydrates, 40% lipids, and 20% protein. Furthermore, its microbial populations were rich and achieved a balance of the two main dominant communities of acetoclastic methanogens and hydrogenotrophic methanogens, whose relative abundance was close. It was obvious that interactive effects were caused by different proportional composition, which led to constantly changing chemical parameters and microbial community.

Adjusting the rheological properties of corn-straw slurry to reduce the agitation power consumption in anaerobic digestion

Agitation power consumption (P) in the anaerobic digestion of biogas plants is a major consumer of electric energy. To reduce P by adjusting the rheological properties, in this work, the rheological properties of the corn-straw slurry were studied systematically considering the effects of TS, temperature and particle-size, and P was calculated based on the rheological behavior of the corn-straw slurry. The investigation shows that the corn-straw slurry is a non-Newtonian fluid and exhibit shear-thinning behavior, and the rheological properties can be well described with the power law model. The size-reduction is more effective compared to the option of temperature-increase to improve the agitation power efficiency, and the value of P can be reduced by up to 48.11%. Since the size-reduction can also increase the methane yield, the reduction of the particle-size is a promising option to save P, especially at relatively high TSs and for the thermophilic AD process.

Effects of endogenous inhibitors on the evolution of antibiotic resistance genes during high solid anaerobic digestion of swine manure

Livestock manure is a reservoir of antibiotic resistance genes (ARGs). The endogenous inhibitors that emerge during high solid anaerobic digestion (HSAD) greatly influence the bioprocess performance and the ARGs. The effect of endogenous inhibitors including volatile fatty acids (VFAs) and free ammonia (FA) on the ARG occurrences during HSAD of swine manure was investigated in this study. The ARG properties during HSAD (8%-14% total solids (TS)) largely differed from the low TS control (4%). The endogenous inhibitors and microbial communities greatly contributed to the three-phase changes in ARGs. The concentrations of VFAs and abundances of Proteobacteria and intI1 were correlated with the changes in ARGs. FA inhibition and VFA accumulation (especially propionate) delayed and restricted the decline of ARGs. The relatively slow rate of changes in ARGs and high ARGs in the end product suggested the high risk of the HSAD digestate for land application.

Evaluation of biomethane potential from by-products and agricultural residues co-digestion in southern Italy

The suitability of the co-digestion of feedstock-mixtures (by-products and agricultural residues) depends on their ability to produce biogas. In this study, the effects of mixing five feedstocks (citrus pulp, olive pomace, poultry manure, Italian sainfoin silage and opuntia fresh cladodes) on anaerobic digestion for biogas production have been investigated by carrying out biomethane potential (BMP) tests on six different mixing ratios of the selected five biomasses. The BMP test results demonstrated that all the six studied feedstock-mixtures could be potentially used for renewable energy generation by biogas plants. More in detail, two mixing ratios of the studied feedstock-mixtures showed the best biomethane potential of 249.9 and 260.1 Nm³CH₄/tVS, respectively. Since this research study made it possible to screen the suitability and technical feasibility of the feedstock-mixtures analysed, the results provide the basis for subsequent pilot scale evaluation of anaerobic digestion in Mediterranean area, where by-products and agricultural residues are profuse and necessary to produce advanced biofuels.

Biogas recovery from two-phase anaerobic digestion of food waste and paper waste: Optimization of paper waste addition

In order to optimize the biogas recovery from the co-digestion of food waste (FW) and paper waste (PW), the effect of PW content on two-phase anaerobic digestion (TPAD) was investigated. The mixtures of FW and PW, with the ratios of 10:0, 8:2, 6:4 and 5:5 (total solids), were fed into TPAD to recover biomethane. After the long-term expriment, it is elucidated that the methanogenesis in TPAD was stable for PW ≤ 40%. When PW = 50%, NH₄HCO₃ was added to the methanogenic phase to provide nitrogen. As the indicators of the stability of the anaerobic process, the ammonia and alkalinity in the methanogenic phase were simulated for their decreasing trend. The simulation results quantified the nitrogen deficiency in the methanogenic phase for PW = 50%. Also, the comparison of alkalinity and ammonia revealed that ammonia was the major contributor to the alkalinity. Furthermore, via stoichiometric calculations, high C/N ratios were found to increase the microbial yield and exacerbated the nitrogen deficiency. In the energy estimation, adding PW showed significant increase only when PW ≥ 40%. It was concluded that 40% was the optimal PW content for bioenergy augmentation from co-digestion of FW and PW using TPAD.

Anaerobic co-digestion of multiple agricultural residues to enhance biogas production in southern Italy

To valorize agricultural wastes and byproducts in southern Italy, anaerobic co-digestion of six feedstocks (citrus pulp, olive pomace, cattle manure, poultry litter, whey, and corn silage) was studied to produce biogas for renewable energy generation. Both batch and semi-continuous co-digestion approaches were adopted to carry out the investigation. The feedstocks were mixed at different percentages according to their availabilities in southern Italy. The batch anaerobic co-digestion demonstrated that six studied feedstock mixtures generated an average of 239 mL CH₄/g VS loading without significant difference between each other, which concluded that the feedstock mixtures can be used for biogas production. Considering the feedstock availability of citrus pulp and olive pomace in Sicily, three feedstock mixtures with the highest volatile solids concentration of citrus pulp (42% citrus pulp, 17% corn silage, 4% cattle manure, 8% poultry litter, and 18% whey; 34% citrus pulp, 8% olive pomace, 17% corn silage, 4% cattle manure, 8% poultry litter, and 18% whey; and 25% citrus pulp, 16% olive pomace, 17% corn silage, 4% cattle manure, 8% poultry litter, and 18% whey, respectively) were selected to run the semi-continuous anaerobic digestion. Under the stabilized culture condition, the feed mixture with 42% citrus pulp, 17% corn silage, 4% cattle manure, 8% poultry litter, and 18% whey presented the best biogas production (231 L methane/kg VS loading/day). The corresponding mass and energy balance concluded that all three tested feedstock mixtures have positive net energy outputs (1.5, 0.9, and 1.2 kWh-e/kg dry feedstock mixture, respectively).

Biomethanation macrodynamics of vegetable residues pretreated by low-frequency microwave irradiation

The effects of microwave irradiation on the digestibility and biogas production of cauliflower (Brassica oleracea var. botrytis) and cabbage (Brassica oleracea var. capitata) leaves were investigated using biochemical methane potential (BMP) assays. Cow dung was utilised as inoculum. Different microwave powers (87.5, 175 and 350W) were applied in a first set of runs for 15min. The second set consisted of 20, 25 and 30min irradiation at 350W. Based on ANOVA analysis (α=0.05), biogas production was significantly higher for the irradiated substrates compared to controls. The peak biogas production was 700ml for 36days HRT for 350W/25min. Peak COD, SCOD, volatile and total solids removals were 54.84%, 39.08%, 34.60% and 71.96%, respectively. Phosphate and total nitrogen increased significantly. Cumulative biogas production data fitted the modified Gompertz equation well. The highest biogas yield was 0.271L/g VS_removed at a 350W microwave irradiation for 30min.