Profile of hydrolases and biogas production during two-stage mesophilic anaerobic digestion of solid potato waste

Dairy manure acidogenic fermentation at hyperthermophilic temperature enabled superior activity of thermostable hydrolytic enzymes linked to the genus Caldicoprobacter

In this study, fermentation experiments were conducted under mesophilic, thermophilic, and hyperthermophilic conditions to investigate adaptation of microbial communities and its effect on extracellular enzyme activities toward degradation of cellulose, hemicellulose and proteins in dairy manure. Hyperthermophilic conditions transformed the microbiome structure and stimulated activity of extracellular proteolytic, cellulolytic, and hemicellulolytic enzymes. Specifically, the activities of protease, cellulose 1,4-β-cellobiosidase, and β-glucosidase secreted by hyperthermophilic microbes were higher by 22%, 47% and 49% compared to those produced by mesophilic and thermophilic communities. Enhanced hydrolytic activity of hyperthermophilic microbes enabled improved feedstock solubilization and production of 39% and 22% more soluble COD than mesophilic and thermophilic microbes, respectively. Connections between hydrolytic function and microbial community structure at various temperatures were assessed using the PICRUSt2 computational tool. Genus Caldicoprobacter was identified as the primary candidate responsible for increased production of thermostable endo-1,4-β-glucanase, β-glucosidase and endo-1,4-β-xylanase, and enhanced hydrolytic performance of hyperthermophilic microbial community.

Release of cell-free enzymes by marine pelagic fungal strains

Fungi are ubiquitous organisms that secrete different enzymes to cleave large molecules into smaller ones so that can then be assimilated. Recent studies suggest that fungi are also present in the oceanic water column harboring the enzymatic repertoire necessary to cleave carbohydrates and proteins. In marine prokaryotes, the cell-free fraction is an important contributor to the oceanic extracellular enzymatic activities (EEAs), but the release of cell-free enzymes by marine fungi remains unknown. Here, to study the cell-free enzymatic activities of marine fungi and the potential influence of salinity on them, five strains of marine fungi that belong to the most abundant pelagic phyla (Ascomycota and Basidiomycota), were grown under non-saline and saline conditions (0 g/L and 35 g/L, respectively). The biomass was separated from the medium by filtration (0.2 μm), and the filtrate was used to perform fluorogenic enzymatic assays with substrate analogues of carbohydrates, lipids, organic phosphorus, sulfur moieties, and proteins. Kinetic parameters such as maximum velocity (Vmax) and half-saturation constant (Km) were obtained. The species studied were able to release cell-free enzymes, and this represented up to 85.1% of the respective total EEA. However, this differed between species and enzymes, with some of the highest contributions being found in those with low total EEA, with some exceptions. This suggests that some of these contributions to the enzymatic pool might be minimal compared to those with higher total EEA. Generally, in the saline medium, the release of cell-free enzymes degrading carbohydrates was reduced compared to the non-saline medium, but those degrading lipids and sulfur moieties were increased. For the remaining substrates, there was not a clear influence of the salinity. Taken together, our results suggest that marine fungi are potential contributors to the oceanic dissolved (i.e., cell-free) enzymatic pool. Our results also suggest that, under salinity changes, a potential effect of global warming, the hydrolysis of organic matter by marine fungal cell-free enzymes might be affected and hence, their potential contribution to the oceanic biogeochemical cycles.

Effect of fungal pretreatment by Pycnoporus sanguineus and Trichoderma longibrachiatum on the anaerobic digestion of rice straw

Rice straw is composed of complex lignocellulosic biomass, representing a major obstacle in its conversion to bioenergy. The objective of this study was to evaluate the usefulness of less explored fungal strains Trichoderma longibrachiatum (TL) and Pycnoporus sanguineus (PS) in improving hydrolysis and bioavailability of rice straw in anaerobic digestion (AD). The fungal treatment of rice straw for 10 days by PS and TL increased biogas production by 20.79% and 17.85% and reduced soluble chemical oxygen demand (sCOD) by 71.43% and 64.70%, respectively. The AD samples containing fungal-treated rice straw showed higher lignocellulolytic enzyme activities contributing to better process performance. The taxonomic profile of microbial communities in treated samples showed increased diversity that could sustain consistent system performance and exhibit enhanced resilience against pH fluctuations. Metagenomic analysis revealed 60.82% increase in Proteobacteria in PS and 11.58% increase in Bacteroidetes in TL-treated rice straw samples resulting in improved hydrolysis.

The Potential of Digested Sludge-Assimilating Microflora for Biogas Production from Food Processing Wastes

Food processing wastes (FPWs) are residues generated in food manufacturing, and their composition varies depending on the type of food product being manufactured. Therefore, selecting and acclimatizing seed microflora during the initiation of biogas production is crucial for optimal outcomes. The present study examined the biogas production capabilities of digested sludge-assimilating and biogas-yielding soil (DABYS) and enteric (DABYE) microflorae when used as seed cultures for biogas production from FPWs. After subculturing and feeding these microbial seeds with various FPWs, we assessed their biogas-producing abilities. The subcultures produced biogas from many FPWs, except orange peel, suggesting that the heterogeneity of the bacterial members in the seed microflora facilitates quick adaptation to FPWs. Microflorae fed with animal-derived FPWs contained several methanogenic archaeal families and produced methane. In contrast, microflorae fed with vegetable-, fruit-, and crop-derived FPWs generated hydrogen, and methanogenic archaeal populations were diminished by repeated subculturing. The subcultured microflorae appear to hydrolyze carbohydrates and protein in FPWs using cellulase, pectinase, or protease. Despite needing enhancements in biogas yield for future industrial scale-up, the DABYS and DABYE microflorae demonstrate robust adaptability to various FPWs.

Sustainable enzymatic treatment of organic waste in a framework of circular economy

Enzymatic treatment of food and vegetable waste (FVW) is an eco-friendly approach for producing industrially relevant value-added products. This review describes the sources, activities and potential applications of crucial enzymes in FVW valorization. The specific roles of amylase, cellulase, xylanase, ligninase, protease, pectinase, tannase, lipase and zymase enzymes were explained. The exhaustive list of value-added products that could be produced from FVW is presented. FVW valorization through enzymatic and whole-cell enzymatic valorization was compared. The note on global firms specialized in enzyme production reiterates the economic importance of enzymatic treatment. This review provides information on choosing an efficient enzymatic FVW treatment strategy, such as nanoenzyme and cross-linked based enzyme immobilization, to make the process viable, sustainable and cheaper. Finally, the importance of life cycle assessment of enzymatic valorization of FVW was impressed to prove this approach is a better option to shift from a linear to a circular economy.

Effects of Fe3O4 nanoparticles on anaerobic digestion enzymes and microbial community of sludge

Fe₃O₄ nanoparticles (NPs) have potential effects on the anaerobic digestion of excess sludge. The effects of different concentrations of Fe₃O₄ NPs (0, 100, 200, 400 and 600 mg/L) on enzymes and microorganisms in anaerobic digestion were studied to explore the mechanism of the effect of Fe₃O₄ NPs on anaerobic digestion. The results showed that 100, 200 and 400 mg/L Fe₃O₄ NPs could promote anaerobic digestion, and 200 mg/L Fe₃O₄ NPs had the most obvious promoting effect. The activities of protease, cellulase, dehydrogenase, acetic kinase and coenzyme F420 in the 200 mg/L Fe₃O₄ NPs group reached 120 U/mg VS, 71.75 U/g VS, 135 U/mL, 94 mol/L, 1.37 umol/g VSS, respectively, which were 3.8, 1.5, 1.2, 1.2 and 1.6 times of the blank group, respectively. However, when the concentration of Fe₃O₄ NPs reached 600 mg/L, the activities of cellulase, dehydrogenase and acetic kinase were lower than those of the blank group, and anaerobic digestion was inhibited. The above conclusions can also be confirmed by high-throughput sequencing. The abundance of longilinea and ornatilina in the 200 mg/L Fe₃O₄ NPs group was 8.8% and 4.1% respectively, and methanthrix abundance was 69%, which was more conducive to decomposition acetic acid into CH₄.Highlights To explore the effects of Fe3O4 NPS on enzyme activity and microorganisms.200mg/L Fe₃O₄ NPs could significantly promote the activity of enzyme.200mg/L Fe₃O₄ NPs could promote the diversity of bacterial and archaeal communities.600mg/L Fe₃O₄ NPs could inhibit some enzyme activities and microbial community diversity.

Effect of fruits and vegetables in the anaerobic digestion of food waste from university restaurant

The aim of this study was to evaluate the theoretical potential of methane production of the food waste generated by a university restaurant, as well as to verify the influence of the fruit and vegetable waste in the feeding composition of an anaerobic bioreactor treating this type of waste. Four feeding compositions combining three fractions of the food waste (fruit and vegetable fraction, soy protein and beans fraction, and rice fraction) at different concentrations were tested in anaerobic processes lasting 10 and 30 days. Additionally, a study of the theoretical potential of methane production from each fraction that composes the food waste was carried out, as well as the evaluation of the specific methanogenic activity of the anaerobic sludge. Despite its low theoretical potential of methane production (0.037 L_CH4/g), the presence of the fruit and vegetable mixture in three of the feeding compositions led to greater organic matter degradation (above 69%) and CH₄ yields (above 0.20 L_CH4/g_VS) in both periods tested, in comparison with the achieved by the feeding composition lacking this fraction. The results suggest that the presence of the fruit and vegetable mixture contributed with the supplementation of micro- and macroelements to the anaerobic sludge during the digestion of food waste.

Economic and environmental sustainability for anaerobic biological treatment of wastewater from paper and cardboard manufacturing industry

The sustainable application of an up-flow anaerobic baffled reactor (UABR) to treat real paper and cardboard industrial effluent (PCIE) containing bronopol (2-bromo-2-nitropropan-1, 3-diol) was investigated. At a hydraulic retention time (HRT) of 11.7 h and a bronopol concentration of 7.0 mg L^-1, the removal efficiencies of total chemical oxygen demand (COD_total), COD_soluble, COD_particulate, total suspended solids (TSS), volatile suspended solids (VSS), carbohydrates, and proteins were 55.3 ± 5.2%, 26.8 ± 2.3%, 94.4 ± 4.6%, 89.4 ± 2.6%, 84.5 ± 3.2%, 72.1 ± 1.8%, and 22.4 ± 1.8%, respectively. The conversion of complex organics (e.g., carbohydrates and proteins) into bio-methane (CH₄) was assisted via enzyme activities of, in U (100 mL)^-1, α-amylase (224-270), α-xylanase (171-188), carboxymethyl cellulase (CM-cellulase) (146-187), polygalacturonase (56-126), and protease (67,000-75300). The acidogenic condition was dominant at a short HRT of 2.9 h, where methane yield dropped by 32.5%. Under this condition, the growth of methanogenic bacteria could be inhibited by volatile fatty acids (VFA) accumulation. The analysis of Fourier-transform infrared (FTIR) spectra detected peaks relevant to methylene and nitro groups in the sludge samples, suggesting that entrapment/adsorption by the sludge bed could be a major mechanism for removing bronopol. The economic feasibility of UABR, as proposed to receive 100 m³ d^-1 of PCIE, showed a payback period (profits from environmental benefits, biogas recovery, and carbon credit) of 7.6 yr. The study outcomes showed a high connection to the environmental-, economic-, and social-related sustainable development goals (SDGs).

Volatile Fatty Acids (VFA) Production from Wastewaters with High Salinity—Influence of pH, Salinity and Reactor Configuration

The hydrocarbon-based economy is moving at a large pace to a decarbonized sustainable bioeconomy based on biorefining all types of secondary carbohydrate-based raw materials. In this work, 50 g L−1 in COD of a mixture of food waste, brine and wastewater derived from a biodiesel production facility were used to produce organic acids, important building-blocks for a biobased industry. High salinity (12–18 g L−1), different reactors configuration operated in batch mode, and different initial pH were tested. In experiment I, a batch stirred reactor (BSR) at atmospheric pressure and a granular sludge bed column (GSBC) were tested with an initial pH of 5. In the end of the experiment, the acidification yield (ηa) was similar in both reactors (22–24%, w/w); nevertheless, lactic acid was in lower concentrations in BSR (6.3 g L−1 in COD), when compared to GSBC (8.0 g L−1 in COD), and valeric was the dominant acid, reaching 17.3% (w/w) in the BSR. In experiment II, the BSR and a pressurized batch stirred reactor (PBSR, operated at 6 bar) were tested with initial pH 7. The ηa and the VFA concentration were higher in the BSR (46%, 22.8 g L−1 in COD) than in the PBSR (41%, 20.3 g/L in COD), and longer chain acids were more predominant in BSR (24.4% butyric, 6.7% valeric, and 6.2% caproic acids) than in PBSR (23.2%, 6.2%, and 4.2%, respectively). The results show that initial pH of 7 allows achieving higher ηa, and the BSR presents the most suitable reactor among tested configurations to produce VFA from wastes/wastewaters with high salinity.

Reconsidering hydrolysis kinetics for anaerobic digestion of waste activated sludge applying cascade reactors with ultra-short residence times

Hydrolysis is considered to be the rate-limiting step in anaerobic digestion of waste activated sludge (WAS). In this study, an innovative 4 stages cascade anaerobic digestion system was researched to (1) comprehensively clarify whether cascading configuration enhances WAS hydrolysis, and to (2) better understand the governing hydrolysis kinetics in this system. The cascade system consisted of three 2.2 L ultra-short solids retention times (SRT) continuous stirred tank reactors (CSTRs) and one 15.4 L CSTR. The cascade system was compared with a reference conventional CSTR digester (22 L) in terms of process performance, hydrolytic enzyme activities and microbial community dynamics under mesophilic conditions (35 °C). The results showed that the cascade system achieved a high and stable total chemical oxygen demand (tCOD) reduction efficiency of 40-42%, even at 12 days total SRT that corresponded to only 1.2 days SRT each in the first three reactors of the cascade. The reference-CSTR converted only 31% tCOD into biogas and suffered process deterioration at the applied low SRTs. Calculated specific hydrolysis rates in the first reactors of the cascade system were significantly higher compared to the reference-CSTR, especially at the lowest applied SRTs. The activities of several hydrolytic enzymes produced in the different stages revealed that protease, cellulase, amino peptidases, and most of the tested glycosyl-hydrolases had significantly higher activities in the first three small digesters of the cascade system, compared to the reference-CSTR. This increase in hydrolytic enzyme production by far exceeded the increase in specific hydrolysis rate, indicating that hydrolysis was limited by solids-surface availability for enzymatic attack. Correspondingly, high relative abundances of hydrolytic-fermentative bacteria and hydrogenotrophic methanogens as well as the presence of syntrophic bacteria were found in the first three digesters of the cascade system. However, in the fourth reactor, acetoclastic methanogens dominated, similarly as in the reference-CSTR. Overall, the results concluded that using multiple CSTRs that are operated at low SRTs in a cascade mode of operation significantly improved the enzymatic hydrolysis rate and extend in anaerobic WAS digestion. Moreover, the governing hydrolysis kinetics in the cascading reactors were far more complex than the generally assumed simplified first-order kinetics.

Anaerobic co-digestion of agricultural wastes toward circular bioeconomy

A huge amount of agricultural wastes and waste activated-sludge are being generated every year around the world. Anaerobic co-digestion (AcD) has been considered as an alternative for the utilization of organic matters from such organic wastes by producing bioenergy and biochemicals to realize a circular bioeconomy. Despite recent advancement in AcD processes, the effect of feedstock compositions and operating conditions on the biomethane production processe has not been critically explored. In this paper, we have reviewed the effects of feedstock (organic wastes) characteristics, including particle size, carbon-to-nitrogen ratio, and pretreatment options, on the performance of an anaerobic digestion process. In addition, we provided an overview of the effect of key control parameters, including retention time, temperature, pH of digestate, volatile fatty acids content, total solids content, and organic loading rate. Lastly, based on the findings from the literature, we have presented several perspectives and prospects on priority research to promote AcD to a steppingstone for a circular bioeconomy.

Novel strategy for enhancing acetic and formic acids generation in acidogenesis of anaerobic digestion via targeted adjusting environmental niches

Optimization of acetic acid and formic acid production efficient methanogenesis is always the research hot spot in anaerobic digestion. It is a promising approach to adjust the operation parameters to influence the functional microorganisms for better acetic acid and formic acid production in acidogenesis. Herein, the effects of pH, oxidation-reduction potential (ORP) and carbon-nitrogen (C/N) ratio were determined in batch experiments to probe acetic and formic acids production, and were further verified in continuous stirred tank reactor (CSTR). The results revealed that the content of volatile fatty acids (VFAs) reached to maximum at pH 6.0 or ORP -350 mV, while the production of acetic and formic acids was the highest at pH 7.0 or ORP -450 mV in 9 h fermentation. Also, fermentation products dominated by acetic and formic acids were adjusted in the CSTR under the operating conditions of pH 7.0 and ORP -450 mV. Microbiological analysis from batch test showed that fermentation at pH value of 7.0 enriched the diversity of microorganism, and provided a niche for microbes (Petrimonas, norank_f__Synergistaceae, vadinBC27_wastewater-sludge_group, and Trichococcus) to produce acetic and formic acids. Correspondingly, 78.70% of the carbon was converted to acetic and formic acids in pH 7.0. This study provides a promising strategy for the targeted regulation of acetic and formic acids production in acidogenesis of anaerobic digestion.

Biotransformation of Citrus Waste-I: Production of Biofuel and Valuable Compounds by Fermentation

Citrus is the largest grown fruit crop on the globe with an annual production of ~110–124 million tons. Approximately, 45–55% of the whole fruit post-processing is generally discarded as waste by the food processing industries. The waste is a huge problem to the environment in terms of land and water pollution along with displeasure from aesthetic viewpoint and spread of diseases owing to its huge content of fermentable sugars. The waste can be utilized as a raw material feedstock for producing a number of valuable chemicals and products, such as bioethanol, biogas, bio-oil, organic acids, enzymes, and so on. The production of these chemicals from waste biomass gives an inexpensive alternative to the harsh chemicals used during industrial synthesis processes as well as the possibility of controlling pollution from the waste discarded to the environment. The derived chemicals can be further utilized in the production of industrially important chemicals, as solvents and building blocks of newer chemicals. Furthermore, organic acids, pectin, enzymes, prebiotics, etc., derived from citrus wastes have an edge over their synthetic counterparts in practical applications in the food processing and pharmaceutical industries.

One- and two-stage anaerobic co-digestion of cucumber waste and sewage sludge

The demand for uniformly sized and shaped produce that are aesthetically pleasing results in significant food waste throughout the world. Cucumber waste is a major agricultural waste product in a number of countries, especially areas with high pickle production. Opportunity exists for wastewater treatment plants containing anaerobic digesters to utilize cucumber agricultural and industrial waste for biogas production. The biomethane potential of cucumber waste as a substrate for co-digestion with sewage sludge was assessed. The impact of long-term co-digestion of cucumber was then evaluated using mesophilic continuously stirred tank reactors (CSTRs), in both single- and two-stage anaerobic co-digestion with sewage sludge. Ground cucumber waste was added to sewage sludge at 8% of the volume (4.5-4.6% of the organic load) and CSTRs were maintained for five hydraulic retention times (HRTs). One-stage co-digestion of cucumber waste produced comparable gas levels as CSTRs without cucumbers (averaging 219 and 221 m³/kgVS/h, respectively) after two HRTs. The two-stage cucumber co-digestion CSTR averaged 64% higher specific gas than the control and single-stage digester, although the volumetric gas produced was lower (averaging 152 m³/kgVS/h) likely due to gas loss in the first stage resulting in a lower organic load rate. After four HRTs, relative methanogen content showed dramatic differences in levels of hydrogenotrophic methanogens for the two-stage digester, while the one-stage digester containing cucumber waste showed minor differences relative to the control. Cucumber waste co-digestion with sewage sludge is effective although numerous conditions could be utilized to optimize gas production.

Volatile fatty acid production from saline cooked mussel processing wastewater at low pH

The production of VFA using as substrate the wastewater produced in a cooked mussel processing factory, containing large COD (13.7 ± 3.2 g COD/L), salt concentrations (21.8 ± 2.8 g NaCl/L) and characterized by low pH (4.6 ± 0.6) was evaluated. This wastewater was fed to a 5-L completely stirred tank reactor operated in continuous mode. The conversion efficiency of its COD content into volatile fatty acids (VFA) was evaluated. The maximum acidification of 43% (total VFA on soluble COD basis) was obtained when an organic loading rate of 2.5 ± 0.4 g COD/(L·d) was applied to the reactor and corresponded to a VFA volumetric productivity of 0.72 ± 0.07 g COD_VFA/(L·d). Under steady-state conditions, the obtained mixture of VFA was composed by 80:18:2 as acetic:propionic:butyric acids (percentage of VFA on soluble COD basis). Carbohydrates were degraded up to 96% while protein fermentation did not take place, probably due to the low pH value, limiting the maximum acidification of the wastewater. Batch experiments showed that the increase of the pH from 4.2 to 4.9 by the addition of NaHCO₃ resulted in the improvement of the acidification and changed the VFA mixture composition. Thus, this study demonstrates the opportunity of using complex substrates, as cooked mussel processing wastewater, to produce rich-VFA streams under unfavourable operational conditions, such as high salinity and low pH.

Extracellular enzyme and microbial activity in MSW landfills with different gas collection and leachate management practices

The performance of simulated municipal solid waste (MSW) landfills with two different biogas collection practices - (1) upward and upward-downward biogas flow collection (L_T-TB) in sequence and (2) simultaneous upward-downward biogas flow collection (L_TB) from the beginning of the anaerobic degradation process - was investigated in terms of landfill gas and leachate, enzyme activity, and microbial community structure associated with MSW compression and leachate recirculation. The cumulative methane volume in L_TB was 1.5 times higher than that in L_T-TB. With MSW compression and leachate recirculation, amylase and lipase activity were enhanced in L_TB. In L_T-TB, the activities gradually decreased after reaching a peak with compression. The two biogas collection strategies influenced the community structure and activity of bacteria and archaea. The upward and downward gas collection flow with waste compression and leachate recirculation improved the environment for enriching bacterial phyla Firmicutes, Proteobacteria, and Synergistetes and genus Methanosarcina in Archaea.

Enhanced bioremediation of heavy metals and bioelectricity generation in a macrophyte-integrated cathode sediment microbial fuel cell (mSMFC)

Sediment microbial fuel cell (SMFC) and constructed wetlands with macrophytes have been independently employed for the removal of heavy metals from polluted aquatic ecosystems. Nonetheless, the coupling of macrophytes at the cathode of SMFCs for efficient and synchronous heavy metal removal and bioelectricity generation from polluted river sediment has not been fully explored. Therefore, a novel macrophyte biocathode SMFC (mSMFC) was proposed, developed, and evaluated for heavy metals/organics removal as well as bioelectricity generation in an urban polluted river. With macrophyte-integrated cathode, higher heavy metal removals of Pb 99.58%, Cd 98.46%, Hg 95.78%, Cr 92.60%, As 89.18%, and Zn 82.28% from the sediments were exhibited after 120 days' operation. Total chemical oxygen demand, total suspended solids, and loss on ignition reached 73.27%, 44.42 ± 4.4%, and 5.87 ± 0.4%, respectively. A maximum voltage output of 0.353 V, power density of 74.16 mW/m³, columbic efficiency of 19.1%, normalized energy recovery of 0.028 kWh/m³, and net energy production of 0.015 kWh/m³ were observed in the Lemna minor L. SMFC. Heavy metal and organic removal pathways included electrochemical reduction, precipitation and recovery, bioaccumulation by macrophyte from the surface water, and bioelectrochemical reduction in the sediment. This study established that mSMFC proved as an efficient system for the remediation of heavy metals Pb, Cd, Hg, Cr, As, and Zn, and TCOD in polluted rivers along with bioelectricity generation.

Genome-Wide and Functional View of Proteolytic and Lipolytic Bacteria for Efficient Biogas Production through Enhanced Sewage Sludge Hydrolysis

In this study, we used a multifaceted approach to select robust bioaugmentation candidates for enhancing biogas production and to demonstrate the usefulness of a genome-centric approach for strain selection for specific bioaugmentation purposes. We also investigated the influence of the isolation source of bacterial strains on their metabolic potential and their efficiency in enhancing anaerobic digestion. Whole genome sequencing, metabolic pathway reconstruction, and physiological analyses, including phenomics, of phylogenetically diverse strains, Rummeliibacillus sp. POC4, Ochrobactrum sp. POC9 (both isolated from sewage sludge) and Brevundimonas sp. LPMIX5 (isolated from an agricultural biogas plant) showed their diverse enzymatic activities, metabolic versatility and ability to survive under varied growth conditions. All tested strains display proteolytic, lipolytic, cellulolytic, amylolytic, and xylanolytic activities and are able to utilize a wide array of single carbon and energy sources, as well as more complex industrial by-products, such as dairy waste and molasses. The specific enzymatic activity expressed by the three strains studied was related to the type of substrate present in the original isolation source. Bioaugmentation with sewage sludge isolates–POC4 and POC9–was more effective for enhancing biogas production from sewage sludge (22% and 28%, respectively) than an approach based on LPMIX5 strain (biogas production boosted by 7%) that had been isolated from an agricultural biogas plant, where other type of substrate is used.

Anaerobic digestion process modeling using Kohonen self-organising maps

Anaerobic digestion is a versatile method for wastewater treatment as it not only reduces the waste but also leads to production of renewable energy. Modeling of the anaerobic process requires knowledge of biological and physico-chemical conditions, bacterial growth kinetics, substrate utilization, and product synthesis. However, the complexity of the process calls for highly sophisticated models requiring very high level of expertise and knowledge in the subject. This paper presents an approach for modeling of anaerobic digestion process through which the correlation between various process parameters can be studied, knowledge can be extracted, and system behaviour can be predicted. The datasets have been generated using a synthetic Matlab-Simulink-Excel model and process modelling is done using Kohonen Self organizing maps (KSOM). The resulting KSOM provided a visual interpretation of the inter-relationships between parameters (OLR, Sac, pH, Shco3, Q, Sglu_in, Qgas_out, Sglu_out, and Sch4_gas_out) which would help semi-skilled operators for operation and control of such plants. The model accurately predicts the variations in methane and total gas output with respect to changes in input parameters as the correlation is more than 90% for most of the parameters. This methodology offers a platform for scientists and researchers in comprehending the system behaviour under various operating conditions, even with missing data.

Effect of fipronil on biogas production performance during anaerobic digestion of chicken manure and corn straw

Fipronil is a broad-spectrum insecticide that has a good control effect on pests of commercial poultry. Although many studies have reported the environmental fate of fipronil, the influence of residual fipronil in poultry waste on biogas production has not been further explored yet. In this article, an experimental comparative study on anaerobic digestion (AD) of chicken manure (CM) and corn straw (CS) with different fipronil concentrations (FCs) was carried at 8% of total solid (TS) and mid-temperature (35 ± 1)°C. The results showed that fipronil had a significant effect on biogas production during AD of CM and CS. When the FC is at a low level (≤10 mg·kg^-1), the biogas production rate is increased and the digestion period was shortened, while higher FC (≥ 20 mg·kg^-1) showed an inhibitory effect. During the monitoring of enzyme activity, low FC showed no significant effect on cellulase and saccharase, but the urease activity increased in the early stage. High FC showed inhibition of activity of cellulase and urease, but the saccharase activity was significantly inhibited until FC reached 40 mg·kg^-1. This study also confirms that the environment in anaerobic digester is favorable for the degradation of fipronil, and its half-life is about 15.83 days.

An overview of the recent trends on the waste valorization techniques for food wastes

A critical and up-to-date review has been conducted on the latest individual valorization technologies aimed at the generation of value-added by-products from food wastes in the form of bio-fuels, bio-materials, value added components and bio-based adsorbents. The aim is to examine the associated advantages and drawbacks of each technique separately along with the assessment of process parameters affecting the efficiency of the generation of the bio-based products. Challenges faced during the processing of the wastes to each of the bio-products have been explained and future scopes stated. Among the many hurdles encountered in the successful and high yield generation of the bio-products is the complexity and variability in the composition of the food wastes along with the high inherent moisture content. Also, individual technologies have their own process configurations and operating parameters which may affect the yield and composition of the desired end product. All these require extensive study of the composition of the food wastes followed by their effective pre-treatments, judicial selection of the technological parameters and finally optimization of not only the process configurations but also in relation to the input food waste material. Attempt has also been made to address the hurdles faced during the implementation of such technologies on an industrial scale.

Microbial pretreatment of lignocellulosic biomass for enhanced biomethanation and waste management

Biogas obtained from organic remains entails a developed technology and an appreciable methane yield, but its use may not be sustainable. The potential methane yield of various lignocellulose biomass and the operational conditions employed are inherently reviewed. Although of lower methane yields compared to conventional substrates, agricultural biomass is a cheap option. The major challenges encountered during its biogasification are its recalcitrance nature primarily due to the presence of crystalline cellulose and lignin. This necessitates an essential pretreatment step through physical, chemical or biological interventions for enhanced biomethanation potential. Various pretreatment-physical, chemical, and biological-strategies have been developed to overcome the inherent recalcitrance of lignocellulose to anaerobic degradation. Biological pretreatment approach, however, outcompete other pretreatments due to their application in milder conditions, little corrosiveness, and lower byproduct formation. Such pretreatment importantly aids in selectively reducing the lignin content and crystalline nature of the lignocellulosic biomass, which would evidently enhance the hydrolysis and production of monomers for their further anaerobic digestion (AD) for methanation. A variety of applied biological pretreatment strategies comprises microaerobic treatments, ensiling or composting, separation of digestion stages, and pretreatments using various lignocellulolytic fungi alongside. The net energy output through such approaches is substantially more and relatively inexpensive compared to other established chemical and mechanical approaches. The present review highlights the use of biological agents including bacterial, fungal and/or their enzymes which trigger biodegradation of wastes and utilization of lignocellulose for biofuel production. Additionally, the different physical, chemical, and biological pretreatment strategies for biogas yield enhancement are presented.

Products of sugar beet processing as raw materials for chemicals and biodegradable polymers

This paper presents an overview of alternative uses for products of sugar beet processing, especially sucrose, as chemical raw materials for the production of biodegradable polymers. Traditionally, sucrose has not been considered as a chemical raw material, because of its use in the food industry and high sugar prices. Beet pulp and beetroot leaves have also not been considered as raw materials for chemical production processes until recently. However, current changes in the European sugar market could lead to falling demand and overproduction of sucrose. Increases in the production of white sugar will also increase the production of waste biomass, as a result of the processing of larger quantities of sugar beet. This creates an opportunity for the development of new chemical technologies based on the use of products of sugar beet processing as raw materials. Promising methods for producing functionalized materials include the acidic hydrolysis of sugars (sucrose, biomass polysaccharides), the catalytic dehydration of monosaccharides to HMF followed by catalytic oxidation of HMF to FDCA and polymerization to biodegradable polymers. The technologies reviewed in this article will be of interest both to industry and science.

This paper presents an overview of alternative uses for products of sugar beet processing, especially sucrose, as chemical raw materials for the production of biodegradable polymers.

Effect of total solids content on biohydrogen production and lactic acid accumulation during dark fermentation of organic waste biomass

Production of biohydrogen and related metabolic by-products was investigated in Solid State Dark Fermentation (SSDF) of food waste (FW) and wheat straw (WS). The effect of the total solids (TS) content and H₂ partial pressure (pp_H2), two of the main operating factors of SSDF, were investigated. Batch tests with FW at 10, 15, 20, 25 and 30% TS showed considerable effects of the TS on metabolites distribution. H₂ production was strongly inhibited for TS contents higher than 15% with a concomitant accumulation of lactic acid and a decrease in substrate conversion. Varying the pp_H2 had no significant effect on the conversion products and overall degradation of FW and WS, suggesting that pp_H2 was not the main limiting factor in SSDF. This study showed that the conversion of complex substrates by SSDF depends on the substrate type and is limited by the TS content.

A comparative study on the process efficiencies and microbial community structures of six full-scale wet and semi-dry anaerobic digesters treating food wastes

The purpose of this study was to investigate the effect of different types of food wastes on the process efficiency and microbial community structures in full-scale anaerobic digesters and to identify parameters that affect these criteria. Six full-scale anaerobic digesters were investigated; three were operated under "wet" condition (total solids TS≤10%), and three were run under "semi-dry" condition (10%≤TS≤20%). Removal efficiency of volatile solids was much higher in the wet digesters (75.2±3.8%) than in the semi-dry digesters (42.6±5.5%). The bacterial and archaeal communities were distinctly characterized by families Porphyromonadaceae, Sphingobacteriaceae, Syntrophomonadaceae, and Methanobacteriaceae in the wet digesters; and of Clostridiaceae, Patulibacteraceae, Pseudonocardiaceae, Lachnospiraceae, Rikenellaceae, and Methanomicrobiaceae in the semi-dry digesters. The discriminant parameters identified were TS content of influent, concentration of total ammonia nitrogen and the ratio of soluble chemical oxygen demand (COD) to COD in the digester.

Biomass waste-to-energy valorisation technologies: a review case for banana processing in Uganda

BACKGROUND

Uganda's banana industry is heavily impeded by the lack of cheap, reliable and sustainable energy mainly needed for processing of banana fruit into pulp and subsequent drying into chips before milling into banana flour that has several uses in the bakery industry, among others. Uganda has one of the lowest electricity access levels, estimated at only 2-3% in rural areas where most of the banana growing is located. In addition, most banana farmers have limited financial capacity to access modern solar energy technologies that can generate sufficient energy for industrial processing. Besides energy scarcity and unreliability, banana production, marketing and industrial processing generate large quantities of organic wastes that are disposed of majorly by unregulated dumping in places such as swamps, thereby forming huge putrefying biomass that emit green house gases (methane and carbon dioxide). On the other hand, the energy content of banana waste, if harnessed through appropriate waste-to-energy technologies, would not only solve the energy requirement for processing of banana pulp, but would also offer an additional benefit of avoiding fossil fuels through the use of renewable energy.

MAIN BODY

The potential waste-to-energy technologies that can be used in valorisation of banana waste can be grouped into three: Thermal (Direct combustion and Incineration), Thermo-chemical (Torrefaction, Plasma treatment, Gasification and Pyrolysis) and Biochemical (Composting, Ethanol fermentation and Anaerobic Digestion). However, due to high moisture content of banana waste, direct application of either thermal or thermo-chemical waste-to-energy technologies is challenging. Although, supercritical water gasification does not require drying of feedstock beforehand and can be a promising thermo-chemical technology for gasification of wet biomass such as banana waste, it is an expensive technology that may not be adopted by banana farmers in Uganda. Biochemical conversion technologies are reported to be more eco-friendly and appropriate for waste biomass with high moisture content such as banana waste.

CONCLUSION

Uganda's banana industrialisation is rural based with limited technical knowledge and economic capability to setup modern solar technologies and thermo-conversions for drying banana fruit pulp. This review explored the advantages of various waste-to-energy technologies as well as their shortfalls. Anaerobic digestion stands out as the most feasible and appropriate waste-to-energy technology for solving the energy scarcity and waste burden in banana industry. Finally, potential options for the enhancement of anaerobic digestion of banana waste were also elucidated.

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

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

Modeling and optimization of anaerobic codigestion of potato waste and aquatic weed by response surface methodology and artificial neural network coupled genetic algorithm

A novel approach to overcome the acidification problem has been attempted in the present study by codigesting industrial potato waste (PW) with Pistia stratiotes (PS, an aquatic weed). The effectiveness of codigestion of the weed and PW was tested in an equal (1:1) proportion by weight with substrate concentration of 5g total solid (TS)/L (2.5gPW+2.5gPS) which resulted in enhancement of methane yield by 76.45% as compared to monodigestion of PW with a positive synergistic effect. Optimization of process parameters was conducted using central composite design (CCD) based response surface methodology (RSM) and artificial neural network (ANN) coupled genetic algorithm (GA) model. Upon comparison of these two optimization techniques, ANN-GA model obtained through feed forward back propagation methodology was found to be efficient and yielded 447.4±21.43LCH4/kgVSfed (0.279gCH4/kgCODvs) which is 6% higher as compared to the CCD-RSM based approach.

Enhanced biohydrogen and subsequent biomethane production from sugarcane bagasse using nano-titanium dioxide pretreatment

Nano-titanium dioxide (nanoTiO2) under ultraviolet irradiation (UV) followed by dilute sulfuric acid hydrolysis of sugarcane bagasse was used to enhance the production of biohydrogen and biomethane in a consecutive dark fermentation and anaerobic digestion. Different concentrations of 0.001, 0.01, 0.1 and 1g nanoTiO2/L under different UV times of 30, 60, 90 and 120min were used. Sulfuric acid (2%v/v) at 121°C was used for 15, 30 and 60min to hydrolyze the pretreated bagasse. For acidic hydrolysis times of 15, 30 and 60min, the highest total free sugar values were enhanced by 260%, 107%, and 189%, respectively, compared to samples without nanoTiO2 pretreatment. The highest hydrogen production samples for the same acidic hydrolysis times showed 88%, 127%, and 25% enhancement. The maximum hydrogen production of 101.5ml/g VS (volatile solids) was obtained at 1g nanoTiO2/L and 120min UV irradiation followed by 30min acid hydrolysis.

Lactic acid fermentation from food waste with indigenous microbiota: Effects of pH, temperature and high OLR

The effects of pH, temperature and high organic loading rate (OLR) on lactic acid production from food waste without extra inoculum addition were investigated in this study. Using batch experiments, the results showed that although the hydrolysis rate increased with pH adjustment, the lactic acid concentration and productivity were highest at pH 6. High temperatures were suitable for solubilization but seriously restricted the acidification processes. The highest lactic acid yield (0.46g/g-TS) and productivity (278.1mg/Lh) were obtained at 37°C and pH 6. In addition, the lactic acid concentration gradually increased with the increase in OLR, and the semi-continuous reactor could be stably operated at an OLR of 18g-TS/Ld. However, system instability, low lactic acid yield and a decrease in VS removal were noticed at high OLRs (22g-TS/Ld). The concentrations of volatile fatty acids (VFAs) in the fermentation mixture were relatively low but slightly increased with OLR, and acetate was the predominant VFA component. Using high-throughput pyrosequencing, Lactobacillus from the raw food waste was found to selectively accumulate and become dominant in the semi-continuous reactor.

Continuous fermentation of food waste leachate for the production of volatile fatty acids and potential as a denitrification carbon source

This study investigated the simultaneous effects of hydraulic retention time (HRT) and pH on the continuous production of VFAs from food waste leachate using response surface analysis. The response surface approximations (R(2)=0.895, p<0.05) revealed that pH has a dominant effect on the specific VFA production (PTVFA) within the explored space (1-4-day HRT, pH 4.5-6.5). The estimated maximum PTVFA was 0.26g total VFAs/g CODf at 2.14-day HRT and pH 6.44, and the approximation was experimentally validated by running triplicate reactors under the estimated optimum conditions. The mixture of the filtrates recovered from these reactors was tested as a denitrification carbon source and demonstrated superior performance in terms of reaction rate and lag length relative to other chemicals, including acetate and methanol. The overall results provide helpful information for better design and control of continuous fermentation for producing waste-derived VFAs, an alternative carbon source for denitrification.

Hydrolysis-acidogenesis of food waste in solid-liquid-separating continuous stirred tank reactor (SLS-CSTR) for volatile organic acid production

The use of conventional continuous stirred tank reactor (CSTR) can affect the methane (CH4) recovery in a two-stage anaerobic digestion of food waste (FW) due to carbon short circuiting in the hydrolysis-acidogenesis (Hy-Aci) stage. In this research, we have designed and tested a solid-liquid-separating CSTR (SLS-CSTR) for effective Hy-Aci of FW. The working conditions were pH 6 and 9 (SLS-CSTR-1 and -2, respectively); temperature-37°C; agitation-300rpm; and organic loading rate (OLR)-2gVSL(-1)day(-1). The volatile fatty acids (VFA), enzyme activities and bacterial population (by qPCR) were determined as test parameters. Results showed that the Hy-Aci of FW at pH 9 produced ∼35% excess VFA as compared to that at pH 6, with acetic and butyric acids as major precursors, which correlated with the high enzyme activities and low lactic acid bacteria. The design provided efficient solid-liquid separation there by improved the organic acid yields from FW.

[Agroindustrial wastes methanization and bacterial composition in anaerobic digestion]

The tons of organic waste that are annually generated by agro-industry, can be used as raw material for methane production. For this reason, it is important to previously perform biodegradability tests to organic wastes for their full scale methanization. This paper addresses biodegradability, methane production and the behavior of populations of eubacteria and archaeabacteria during anaerobic digestion of banana, mango and papaya agroindustrial wastes. Mango and banana wastes had higher organic matter content than papaya in terms of their volatile solids and total solid rate (94 and 75% respectively). After 63 days of treatment, the highest methane production was observed in banana waste anaerobic digestion: 63.89ml CH4/per gram of chemical oxygen demand of the waste. In the PCR-DGGE molecular analysis, different genomic footprints with oligonucleotides for eubacteria and archeobacteria were found. Biochemical methane potential results proved that banana wastes have the best potential to be used as raw material for methane production. The result of a PCR- DGGE analysis using specific oligonucleotides enabled to identify the behavior of populations of eubacteria and archaeabacteria present during the anaerobic digestion of agroindustrial wastes throughout the process.

Anaerobic Leaching-Bed Reactor Treating Food Waste for Organic Acid Production: Effect of Bulking Agent

Food waste was a troublesome organic waste stream, but a highly desirable substrate for anaerobic digestion to recover energy. Among the diverse reactor configurations, the leaching-bed reactor was reported to be best choice to treating the particular organic waste for platform compounds (volatile fatty acid). In this study, we carried out a series of experiments to investigate the bulking agent and pH control on process performance, and special focus was put on the biodegradability of bulking agent. The supplement of bulking agent greatly improved the leaching rate by 3.6 folds. Although the hydrolytic and acidogenic bacteria were resistant to low pH, the pH control (neutralization) caused a significantly increased volatile fatty acid (VFA) productions from 39.1 g COD/kg VSaddedto 183.4 g VFA/kg VSadded. Comparing with the undegradable bulking agent, the case with supplementing corncob as a bulking agent showed superior VFA yields (225 g COD/kg VSadded) which could be ascribed to the good adhesive properties for microorganisms and biodegradability. The ultimate analysis of the substrate (excluding the bulking agent) also showed that most of food waste was degraded with relative short reaction time. In addition, the TG/DTA and FTIR of residual corncob results indicated some components (cellulose, hemi-cellulose) were degraded, which could contribute to the additional VFA production (14 g COD/kg VSadded). This study suggested that the corncob could be considered as a good bio-compatible bulking agent for leaching-bed reactor, which not only increased the VFA productivity, but provided additional VFA production.

Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure

This study investigated the effects of different initial pH (6.0, 6.5, 7.0, 7.5 and 8.0) and uncontrolled initial pH (CK) on the lab-scale anaerobic co-digestion of kitchen waste (KW) with cow manure (CM). The variations of pH, alkalinity, volatile fatty acids (VFAs) and total ammonia nitrogen (NH4(+)-N) were analyzed. The modified Gompertz equation was used for selecting the optimal initial pH through comprehensive evaluation of methane production potential, degradation of volatile solids (VS), and lag-phase time. The results showed that CK and the fermentation with initial pH of 6.0 failed. The pH values of the rest treatments reached 7.7-7.9 with significantly increased methane production. The predicted lag-phase times of treatments with initial pH of 6.5 and 7.5 were 21 and 22 days, which were 10 days shorter than the treatments with initial pH of 7.0 and 8.0, respectively. The maximum methane production potential (8579 mL) and VS degradation rate (179.8 mL/g VS) were obtained when the initial pH was 7.5, which is recommended for co-digestion of KW and CM.

Microbial community distribution and extracellular enzyme activities in leach bed reactor treating food waste: effect of different leachate recirculation practices

This study aimed at understanding the relationship between microbial community and extracellular enzyme activities of leach bed reactor (LBR) treating food waste under different leachate recirculation practices (once per day and continuous) and liquid to solid (L/S) ratios (1:1 and 0.5:1). Microbial community analysis using PCR-DGGE revealed that Lactobacillus sp., Bifidobacter sp., and Proteobacteria were the most abundant species. Number of phylotypes was higher in LBRs with intermittent recirculation; whereas, lower number of phylotypes dominated by the key players of degradation was observed with continuous recirculation. The L/S ratio of 1:1 significantly enhanced the volatile solids removal compared with 0.5:1; however, this effect was insignificant under once a day leachate recirculation. Continuous leachate recirculation with 1:1 L/S ratio significantly improved the organic leaching (240 g COD/kgvolatile solid) and showed distinct extracellular enzyme activities suitable for food waste acidogenesis.

Enhancement of biogas production by co-digestion of potato pulp with cow manure in a CSTR system

Anaerobic digestion (AD) process is a well-established method to generate energy from the organic wastes both from the environmental and economical perspectives. The purpose of present study is to evaluate energy production from potato wastes by incorporating cow manure into the process. Firstly, a laboratory pilot of one-stage biogas production was designed and built according to continuously stirred tank reactor (CSTR) system. The setup was able to automatically control the environmental conditions of the process including temperature, duration, and rate of stirring. AD experiment was exclusively performed on co-digestion of potato peel (PP) and cow manure (CM) in three levels of mixing ratio including 20:80, 50:50, 80:20 (PP:CM), and 0:100 as control treatment based on the volatile solid (VS) weight without adding initial inoculums. After hydraulic retention time (HRT) of 50 days on average 193, 256, 348, and 149 norm liter (LN) (kg VS)(-1), methane was produced for different mixing ratios, respectively. Statistical analysis shows that these gas productions are significantly different. The average energy was determined based on the produced methane which was about 2.8 kWh (kg VS)(-1), implying a significant energy production potential. The average chemical oxygen demand (COD) removal of treatments was about 61%, showing that it can be leached significantly with high organic matter by the employed pilot. The energy efficiency of 92% of the process also showed the optimum control of the process by the pilot.

Anaerobic digestion of food waste for volatile fatty acids (VFAs) production with different types of inoculum: effect of pH

Food waste anaerobic fermentation was carried out under acidic conditions using inocula based on aerobic activated sludge (Inoculum AE) or anaerobic activated sludge (Inoculum AN) for volatile fatty acids (VFAs) production. The results showed that food waste hydrolysis increased obviously when Inoculum AN was used relative to Inoculum AE at any pH investigated. Hydrolysis at pH 4.0 and uncontrolled pH was higher than that at other pHs when either inoculum was used. Additionally, VFAs production at pH 6.0 was the highest, regardless of the inoculum used. The optimum VFA yields were 0.482g/gVSSremoval with Inoculum AE and 0.918g/gVSSremoval with Inoculum AN, which were observed after 4d and 20d of fermentation, respectively. VFAs composition analysis showed that butyrate acid was the prevalent acid at pH 6.0, followed by acetate acid and propionic acid.

Effects of organic loading rate and effluent recirculation on the performance of two-stage anaerobic digestion of vegetable waste

The effects of organic loading rates (OLR) and effluent recirculation on dynamics of acidogenic and methanogenic processes in two-stage anaerobic digestion of vegetable waste were investigated. Two systems were performed at OLRs of 1.3, 1.7, 2.1 and 2.6 g VS/L/d. One system recirculated the effluent from the methanogenic reactor to acidogenic reactor. With increasing OLRs, total volatile fatty acid (VFA) concentration increased to approximately 8500 mg/L in acidogenic digester, where pH decreased from 6.4 to 5.2. Daily biogas production and methane content in methanogenic reactor increased from 1.2 to 4.4 L/d and from 27.4% to 60.5%, respectively. However, inhibition of hydrolysis in acidogenic reactor was demonstrated under the OLR of 2.6 g VS/L/d without recirculation, thus indicating system overloading. Effluent recirculation shown a considerable positive effect on alleviating VFA inhibition and improving biogas production in acidogenic reactor because of the effect of dilution and pH adjustment, particularly at high OLRs.

Volatile fatty acids production from food waste: effects of pH, temperature, and organic loading rate

The effects of pH, temperature, and organic loading rate (OLR) on the acidogenesis of food waste have been determined. The present study investigated their effects on soluble chemical oxygen demand (SCOD), volatile fatty acids (VFAs), volatile solids (VS), and ammonia nitrogen (NH4(+)-N). Both the concentration and yield of VFAs were highest at pH 6.0, acetate and butyrate accounted for 77% of total VFAs. VFAs concentration and the VFA/SCOD ratio were highest, and VS levels were lowest, at 45 °C, but the differences compared to the values at 35 °C were slight. The concentrations of VFAs, SCOD, and NH4(+)-N increased as OLR increased, whereas the yield of VFAs decreased from 0.504 at 5 g/Ld to 0.306 at 16 g/Ld. Acetate and butyrate accounted for 60% of total VFAs. The percentage of acetate and valerate increased as OLR increased, whereas a high OLR produced a lower percentage of propionate and butyrate.

Optimization of liquid fermentation of microbial consortium WSD-5 followed by saccharification and acidification of wheat straw

The microbial consortium WSD-5 is composed of bacteria and fungi, and the cooperation and symbiosis of the contained microbes enhance the degradation ability of WSD-5. Experiment results showed that the highest cellulase and hemicellulase were obtained when ventilation volume was 4 L/min, stirring rate was 0 rpm, and substrate loading rate was 3%. After 6 days of cultivation, a 67.60% loss in wheat straw dry weight was observed. The crude enzyme secreted from WSD-5 after optimization was evaluated by experiments of saccharification and acidification. The maximum concentration of reducing sugars was 3254 mg/L after 48 h saccharification. The concentration of sCOD peaked on day 2 with a value of 4345 mg/L during acidification, and the biogas yield and methane yield were 22.3% and 32.3% higher than un-acidified samples. This study is the first attempt to explore both the saccharification and the acidification ability of crude enzymes secreted by microbial consortium.

Hydrolysis and acidification of grass silage in leaching bed reactors

Hydrolysis and acidification of grass silage (GS) was examined in leaching bed reactors (LBRs) under organic loading rates (OLRs) of 0.5, 0.8 and 1.0 kg volatile solids (VS)/m(3)/day. The LBRs were run in duplicate over five consecutive batch tests (Batch tests 1-5) to examine the effects of pH, leachate dilution and addition of inoculum on the process of hydrolysis and acidification. The highest GS hydrolysis yields of 52-58%, acidification yields of 57-60% and VS removals of 62-66% were obtained in Batch test 4. Increasing OLRs affected the hydrolysis yield negatively. In Batch test 4, the reduction of lignocellulosic materials was up to 74.4% of hemicellulose, 30.1% of cellulose and 9.3% of lignin within 32 days. Cellulase activity can be used as an indicator for the hydrolysis process. Methane production from the LBRs only accounted for 10.0-13.8% of the biological methane potential of GS.

Enhancement of methane production from cassava residues by biological pretreatment using a constructed microbial consortium

In the study, a stable thermophilic microbial consortium with high cellulose-degradation ability was successfully constructed. That several species of microbes coexisted in this consortium was proved by DGGE (denaturing gradient gel electrophoresis) and sequence analysis. The cooperation and symbiosis of these microbes in this consortium enhanced their cellulose-degradation ability. The pretreatment of cassava residues mixing with distillery wastewater prior to anaerobic digestion was investigated by using this microbial consortium as inoculums in batch bioreactors at 55 °C. The experimental results showed that the maximum methane yield (259.46 mL/g-VS) of cassava residues was obtained through 12h of pretreatment by this microbial consortium, which was 96.63% higher than the control (131.95 mL/g-VS). In addition, it was also found that the maximum methane yield is obtained when the highest filter paper cellulase (FPase), carboxymethyl cellulase (CMCase) and xylanase activity and soluble COD (sCOD) are produced.

Enzyme research and applications in biotechnological intensification of biogas production

Biogas technology provides an alternative source of energy to fossil fuels in many parts of the world. Using local resources such as agricultural crop remains, municipal solid wastes, market wastes and animal waste, energy (biogas), and manure are derived by anaerobic digestion. The hydrolysis process, where the complex insoluble organic materials are hydrolysed by extracellular enzymes, is a rate-limiting step for anaerobic digestion of high-solid organic solid wastes. Biomass pretreatment and hydrolysis are areas in need of drastic improvement for economic production of biogas from complex organic matter such as lignocellulosic material and sewage sludge. Despite development of pretreatment techniques, sugar release from complex biomass still remains an expensive and slow step, perhaps the most critical in the overall process. This paper gives an updated review of the biotechnological advances to improve biogas production by microbial enzymatic hydrolysis of different complex organic matter for converting them into fermentable structures. A number of authors have reported significant improvement in biogas production when crude and commercial enzymes are used in the pretreatment of complex organic matter. There have been studies on the improvement of biogas production from lignocellulolytic materials, one of the largest and renewable sources of energy on earth, after pretreatment with cellulases and cellulase-producing microorganisms. Lipids (characterised as oil, grease, fat, and free long chain fatty acids, LCFA) are a major organic compound in wastewater generated from the food processing industries and have been considered very difficult to convert into biogas. Improved methane yield has been reported in the literature when these lipid-rich wastewaters are pretreated with lipases and lipase-producing microorganisms. The enzymatic treatment of mixed sludge by added enzymes prior to anaerobic digestion has been shown to result in improved degradation of the sludge and an increase in methane production. Strategies for enzyme dosing to enhance anaerobic digestion of the different complex organic rich materials have been investigated. This review also highlights the various challenges and opportunities that exist to improve enzymatic hydrolysis of complex organic matter for biogas production. The arguments in favor of enzymes to pretreat complex biomass are compelling. The high cost of commercial enzyme production, however, still limits application of enzymatic hydrolysis in full-scale biogas production plants, although production of low-cost enzymes and genetic engineering are addressing this issue.