Inoculum To Substrate Ratio Research Articles

Evaluation of Biochemical Methane Potential and Kinetics of Organic Waste Streams for Enhanced Biogas Production

Organic waste has the potential to produce methane gas as a substitute for petrol-based fuels, while reducing landfilling and possible environmental pollution. Generally, anaerobic digestion (AD) is used only in wastewater treatment plants as a tertiary stage of sewage sludge treatment, generating a fraction of the energy that such process plants require. In this study, four different wastes—food waste (FW), dairy industry waste (DIW), brewery waste (BW), and cardboard waste (CBW)—were tested for biogas production. The biochemical methane potential (BMP) of each sample was evaluated using an automatic methane potential system (AMPTS). Operating parameters such as pH, temperature, total solids, and volatile solids were measured. Experiments on the anaerobic digestion of the samples were monitored under mesophilic conditions (temperature 37 °C, retention time 30 days). Specific methane yields (SMYs), as well as the theoretical methane potential (BMPth), were used to calculate the biodegradability of the substrates, obtaining the highest biodegradability for BW at 95.1% and producing 462.3 ± 1.25 NmL CH4/g volatile solids (VS), followed by FW at an inoculum-to-substrate ratio (ISR) of 2 at 84% generating 391.3 NmLCH4/g VS. The BMP test of the dairy industry waste at an inoculum-to-substrate ratio of 1 was heavily inhibited by bacteria overloading of the easily degradable organic matter, obtaining a total methane production of 106.3 NmL CH4/g VS and a biodegradability index of 24.8%. The kinetic modeling study demonstrated that the best-fitting model was the modified Gompertz model, presenting the highest coefficient of determination (R2) values, the lowest root means square error (RMSE) values for five of the substrates, and the best specific biogas yield estimation with a percentage difference ranging from 0.3 to 3.6%.

Anaerobic co‑digestion of bovine ruminal waste and brewery spent grain: Effects of inoculum to substrate ratio, mixing ratio, process stability, organic matter removal, and methane yield

The technological potential of co-digesting bovine rumen waste (BRW) with brewery spent grain (BSG) is significant, offering enhanced biogas production and effective waste management. This study's findings demonstrate that the anaerobic digestion process becomes significantly more efficient when difficult to degrade BRW is combined with easily degradable BSG. The highest methane yield obtained in the mono-digestion of BRW was 127.11 NmLCH4 gVS−1 with biodegradability of 33.89 % in inoculum to substrate ratio (ISR) 2, and for de BSG obtained was 304.18 NmLCH4 gVS−1 with biodegradability of 68.26 % in ISR 4. The overall average obtained in co-digestion for volatile solids (VS) removal was 26.28 %, and the concentration of methane present in the biogas was 57.18 %. The VS removal efficiency and the increase in biogas and methane yield were directly proportional to the rise in ISR and the proportion of BSG in the mixture. Specifically, the ideal mix of 25 % BRW with 75 % BSG at ISR 4 resulted in a notable methane yield of 255.30 NmLCH4 gVS−1. This process stabilizes digestion and significantly improves solids removal and methane concentration, making it a highly efficient solution for sustainable energy production and industrial waste management.

Evaluation of the Dark Fermentation Process as an Alternative for the Energy Valorization of the Organic Fraction of Municipal Solid Waste (OFMSW) for Bogotá, Colombia

In the context of valorizing the organic fraction of urban solid waste (OFMSW) in megacities, dark fermentation emerges as a central strategy alongside composting and anaerobic digestion. This article focuses on assessing the environmental, technical, and energy viability of dark fermentation using life cycle assessment (LCA) and circular economy principles. Dark fermentation for biohydrogen production is an active and promising research field in the quest for sustainable biofuels. In this context, defining operational parameters such as organic loading and the substrate-inoculum ratio is relevant for achieving better production yields. Laboratory tests were conducted using organic loading values of 5, 10, and 15 g of volatile solids per liter (gVS/L) and with substrate-inoculum ratios (s/x) of 1, 0.75, and 0.5 g of volatile solids of substrate per gram of volatile solids of inoculum (gVSs/gVSi). The combination with the best performance turned out to be an initial organic loading of 10 gVS/L and an s/x of 1 gVSs/gVSi. From this result, it was determined that the s/x had a greater impact on production. Finally, a valorization plant was dimensioned with the scaled-up process, starting from the municipal solid waste generated by Bogotá projected for 2042. The scaling was demonstrated to be energetically sustainable, producing a power of 2,368,358.72 kWh per day.

Biogas Production Optimization in the Anaerobic Codigestion Process: A Critical Review on Process Parameters Modeling and Simulation Tools

Many operational parameters, either discretely or collectively, can influence the biodegradation performance towards enhancing biogas yield and quality. Among the operating parameters, organic loading rate (OLR), inoculum-substrate ratio, and carbon-nitrogen ratio (C/N) are the most critical parameters in the optimization and enhancement of biogas yield. Optimization of the biogas production processes depends on the ability of anaerobic microorganisms to respond to variations in operational parameters such as pH, redox potential, and intermediate products to enhance the biogas yield. This review article focuses on the role of process parameters, kinetic models, artificial intelligence, Aspen Plus (AP), and anaerobic digestion model no. 1 (ADM1) in optimizing biogas yield via an anaerobic codigestion (AcoD) process. The review showed that biomaterials codigestion upgraded biogas yield to the extent of 400%, and organic removal efficiency reached up to 90% compared to a single substrate. In addition, the current work has verified that the kinetic model is the most effective tool for signifying that the hydrolysis phase is the rate-limiting step, whereas AP is the most effective tool in the design and optimization of the AcoD process parameters. The reviewed kinetic and AI models show strong correlation values ranging from 0.931 to 0.9991 and 0.8700 to 0.9998, respectively. The AcoD system involves complex chemical reactions, but AP might have limitations in representing such complex chemical processes with nonideal behavior and complicated reaction mechanisms. The design and optimization of AcoD with reliable input parameters are highly limited or nonexistent. The AcoD process design with AP opens fresh research opportunities, including improved efficiency, finding appropriate retention time, and saving time, as well as finding the optimum biogas yield. This review article gives an insightful understanding of AcoD process parameter optimization and valuable strategies for policy development enhancing sustainability in the biogas sector.

Methane Production from Poly(lactic Acid) (PLA) and Polyhydroxybutyrate (PHB) Biodegradable Plastics with Anaerobic Granulated Sludge

- In this study, using Methanosarcina barkeri DSM 800 and Methanococcus vannielii DSM 1224 methanogenic bacteria under mesophilic (38 ± 1oC) and thermophilic (58 ± 1oC) conditions in anaerobic granulated sludge taken from Pakmaya Yeast Factory in Izmir, Turkey; Methane production from biodegradable plastics with poly(lactic acid) (PLA) and polyhydroxybutyrate (PHB) was investigated. Effect of different operating parameters, increasing biodegradation times (from 10 days to 500 days), different inoculumsubstrate ratios (ISRs) (16, 8, 4, 2, 1) and increasing biochemical methane potential (BMP) times (between 10 day and 500 days) for the production of methane gas from PLA and PHB biodegradable plastics in anaerobic granular sludge waste; Methanosarcina barkeri DSM 800 and Methanococcus vannielii DSM 1224 methanogenic bacteria were operated during the anaerobic digestion process under anaerobic conditions at mesophilic (38 ± 1oC) and thermophilic (58 ± 1oC) experimental temperatures. PLA biodegradable plastics were operated at optimum pH=7.6. PHB biodegradable plastics were carried out at optimum pH=8.1. Predicting the biodegradation behavior of PLA and PHB biodegradable plastics with BMP tests; It is found that the ISR parameter plays a very important role. This study showed that temperature plays a key role in the aging of microorganisms (Methanosarcina barkeri DSM 800 and Methanococcus vannielii DSM 1224 methanogenic bacteria) during anaerobic digestion, the degradation of bioplastic materials (PLA and PHB) and the degradation of their natural properties. The increase in temperature from mesophilic conditions to thermophilic conditions increased the activities of methanogenic bacteria such as Methanosarcina barkeri DSM 800 and Methanococcus vannielii DSM 1224. The maximum cumulative CH4(g) production was measured at 630 NL CH4 / kgVS for PHB biodegradable plastics in anaerobic granulated sludge with inoculum culture (the mixture of Methanosarcina barkeri DSM 800 and Methanococcus vannielii DSM 1224 methanogenic bacteria), at ISR=16 value, after 100 days, at pH=8.1 and at 58±1oC, respectively. The maximum 97% biodegradation efficiency was observed for PHB biodegradable plastics after 100 days, at pH=8.1 and at 58 ± 1oC thermophilic conditions, respectively.

Cheese whey and dairy manure anaerobic co-digestion at psychrophilic conditions: Technical and environmental evaluation

Cheese whey (CW) and dairy manure (DM) are the main residues from the dairy industry, both of which can led to significant negative environment impacts if not properly managed. However, their combined anaerobic digestion represents an opportunity to obtain bioenergy and a stabilised material as a soil improver on the farm. Biochemical potential of methane (BMP) assays were carried out at psychrophilic conditions (20 °C) to analyse the influence on biomethane production of different CW:DM mixtures (% w/w) at different of inoculum-to-substrate ratios (ISR). Based on the BMP results, a life cycle assessment (LCA) of the cheese manufacturing process was carried out considering two scenarios (i) considering the current process, where propane gas and electricity are used for cheese production (ii) the incorporation of the biogas generated in the cheese production process in the company. BMP results showed that the best mixture between CW and DM was 65:35 (weight basis) at an organic load of 0.6 gVS/L (ISR of X). The LCA showed that CW and DM anaerobic digestion allowed to reduce the cheese manufacturing carbon footprint from through the substitution of propane by the biogas produced, changing from 5.5 to 3.1 kg CO2-eq/kg cheese produced, which indicates that according to the monthly production (633.6 kg) it would stop emitting about 1519 kg CO2-eq, i.e. a saving in terms of emissions of approximately 43,6% of the total currently generated.

Enhancing Methane Yield in Anaerobic Co-Digestion of Primary Sewage Sludge: A Comprehensive Review on Potential Additives and Strategies

Traditionally, anaerobic digestion has been applied to mixed sludge, combining primary sludge (PS) with secondary sludge. However, recent research has unveiled the advantages of dedicated PS digestion due to its higher energy content. Anaerobic digestion (AD) of primary sewage sludge can offer a sustainable solution for managing sewage sludge while generating renewable energy. The present study provides a comprehensive examination of the current state of knowledge regarding the anaerobic digestion of PS. Co-digestion of PS with organic substrates, including food waste and agro-industrial residues, emerges as a promising approach to boost biogas production. Additionally, the utilization of additives such as glucose and clay minerals has shown potential in improving methane yield. Critical factors affecting AD, such as pretreatment methods, carbon-to-nitrogen (C/N) ratio, temperature, pH, volatile fatty acids (VFAs) levels, organic loading rates (OLR), inoculum-to-substrate ratio (ISR), and the role of additives, have been meticulously studied. Finally, this review consolidates existing knowledge to advance our understanding of primary sewage sludge anaerobic digestion, fostering more efficient and sustainable practices in sludge management and renewable energy generation.

Removal of organic matter during adaptation of Nannochloropsis oculata in livestock waste

Microalgae, like plants, contribute significantly to the development of the oxygen biogeochemical cycle due to their high photosynthetic efficiency. In addition, they provide high yields of polyunsaturated fatty acids, sterols, proteins, terpenoids, and pigments, among others. Therefore, different species of microalgae have been studied and used on a laboratory scale to carry out processes such as wastewater treatment or aerobic bioconversion, which are presented as sustainable and viable alternatives for the treatment and recovery of organic waste (OW), usually rich in carbohydrates, lipids and proteins. In the present investigation, the removal of organic matter was evaluated during the adaptation of Nannochloropsis oculata in residues of poultry wastewater and swine origin, obtained from technical and semi-technical plants, respectively, located in the high mountain zone of the state of Veracruz, Mexico. The experiment was carried out in 250 mL discontinuous photobioreactors with a working volume of 200 mL, where 3 inoculum-substrate ratios were studied for each organic residue: 10, 15 and 20% inoculum in poultry wastewater (PWW) and 30, 50 and 70% inoculum in pig manure (PM). In addition, the conditions of temperature (20 ± 2 °C), illumination (2000 lx), photoperiod of 12/12 (light/dark) and continuous aeration were controlled. It was shown that N. oculata can tolerate alkaline conditions of pH ≥ 10 and contributes to the reduction of soluble organic matter in OW. PWW and PM were found to be viable media for the survival of N. oculata. Finally, regarding the inoculum concentrations studied, the most appropriate were 10% for PWW and 70% for PM.

Extraction of bioethanol from fermentation broth and valorization of the vinasse for biogas production

An effective ethanol extraction from fermentation broths and further utilization of the waste streams for biogas production can enhance the overall efficiency of the biorefinery process. In the present study, ethanol was extracted from a fermentation broth by liquid-liquid extraction (LLE) after ultrafiltration. Thereafter, the vinasse was subjected to batch and continuous anaerobic digestion (AD). Batch AD was carried out at inoculum-to-substrate ratios (ISR) of 0.1, 0.2 and 0.3 (gVS/gCOD), while continuous AD was carried out at hydraulic retention times (HRT) of 2, 3 and 5 days as well as at organic loading rates (OLR) of 2, 5 and 7 gCOD/L/day. Results showed that during ultrafiltration, fouling mechanism was primarily due to the formation of a cake layer on the surface of the membrane. LLE efficiency was 97.13% with an ethanol purity of 98.84%. During the batch AD, the highest cumulative biomethane potential of 236.43 mL/gCOD (± 13) was obtained at an ISR of 0.2 (gVS/gCOD). During the continuous AD, both OLR and HRT significantly affected biogas and biomethane production with the highest biogas yield being 415.50 mL/gCOD at an HRT of 5 days and OLR of 2 gCOD/L/day.

Influence of Inoculum to Substrate Ratio and Substrates Mixing Ratio on Biogas Production from the Anaerobic Co-digestion of Phragmites australis and Food Waste

This study focused on determining the effect of the inoculum to substrate ratio (ISR) on biogas production efficiency from the anaerobic co-digestion of two substrates: synthetic food waste and common reeds (Phragmites australis) that were ground and pre-treated using sodium hydroxide at a concentration of 2% to increase access to their cellulose. It also studied the role of different mixing ratios of the two substrates in improving the stability of the digestion process and increasing biogas production. A series of batch tests were carried out under mesophilic conditions using three ratios of ISR: 1:4, 1:2, and 1:1, and five substrate mixing ratios (synthetic food waste: pre-treated P. australis): 25:75, 50:50, 75:25, 100:0, and 0:100. The results showed low biogas production at the ISR 1:4 (21.58±0.00–44.46±0.01 mL/g volatile solid (VS) added), and the reactors suffered from acidification at the different substrates mixing ratios, while the biogas production increased at an ISR of 1:2, where the reactors with the substrate mixing ratio of 25:75 presented the highest biogas production (82.17±0.62 mL/g VS added), and the digestion process was stable. However, the reactors with substrate mixing ratios of 50:50, 75:25, and 100:0 suffered from acidification effects at this ISR. In contrast, at ISR of 1:1, the reactors did not expose to acidification inhibition at all the substrates mixing ratios, and the highest biogas production was found at synthetic food waste: pre-treated P. australis mixing ratios of 75:25 and 100:0 (76.15±1.85 and 82.47±1.85 mL/g VS added, respectively).

A novel solid-state submerged fermenter (3SF) for acidogenic fermentation of food waste at high volumetric loading: Effect of inoculum to substrate ratio, design optimization, and inoculum enrichment

This study reports the development and operation of a solid-state submerged fermenter (3SF) to produce short-chain fatty acids (SCFAs) from food waste at high volumetric loading of 55 gVS/Lreactor. To maximize food waste degradation and SCFA production in the 3SF, three factors were optimized: inoculum to substrate ratio (16%, 9%, 6%, and 4%), mesh pore size of the food waste holding chamber (4.5 mm, 2 mm, and 1 mm), and enrichment of the inoculum. The 3SF operation at higher inoculum-to-substrate ratio (ISR) of 16% resulted in 13–20% higher hydrolysis (600 ± 6.6 gCOD/kgVSadded) and SCFA yields (480 ± 8.7 gCODSCFA/kgVSadded) than those obtained at lower ISRs of 4–6%. Optimization of the mesh pore size (2 mm) further improved hydrolysis and SCFA yield to 628 ± 6.1 gCOD/kgVSadded and 517 ± 7.3 gCODSCFA/kgVSadded, respectively. Enrichment of the inoculum increased the hydrolysis and acidification rates resulting in significantly higher (P ≤ 0.05) hydrolysis and SCFA yields than the non-enriched reactor in just 8 days, implying a 43% reduction in the fermentation time. SCFA composition comprised of acetate, propionate, and butyrate for all reactors. However, an increase in ISR and the use of enriched inoculum resulted in a gradual increase in butyrate composition within the SCFA mix with a maximum butyrate composition of 46% on day 8. Microbial composition showed a high relative abundance of Enterococcus, Clostridium Sensu Stricto 1, and Bacteroides.

Solid-State Co-digestion of Food Waste and Highland Barley Straw: Effect of Inoculum-Substrate Ratio on the Performance and Microbial Community Structure

Solid-State Co-digestion of Food Waste and Highland Barley Straw: Effect of Inoculum-Substrate Ratio on the Performance and Microbial Community Structure

Zero-Valent Iron and Activated Carbon Coupled to Enhance Anaerobic Digestion of Food Waste: Alleviating Acid Inhibition at High Loads

Anaerobic digestion (AD) has the advantages of utilizing complex substrates and producing renewable energy and is currently one of the mainstream technologies for food waste (FW) resourcing. However, at high organic loads and low inoculum-to-substrate ratios (ISRs), AD with FW as substrate is prone to acid accumulation, resulting in a drastic decrease in gas production and system collapse. This study investigated the effect of the coupled addition of zero-valent iron (ZVI) and activated carbon (AC) on the AD of FW at three low ISRs of 0.715, 0.625, and 0.5. The results showed that the control group acidified and stopped producing biogas when the ISR decreased to 0.625 and 0.5, but ZVI coupled with AC alleviated the acidification and increased the cumulative biogas yield. Especially at ISR = 0.5, the cumulative biogas yield for the ZVI + AC group was 31.5%, 99.5%, and 11.43 times higher than that of the ZVI, AC, and control groups, respectively. ZVI coupled with AC also increased the degradation of volatile fatty acids (70.5–84.4%) and soluble chemical oxygen demand (50.0–72.9%) while decreasing propionate concentration and improving the stability of the AD system. COD mass balance analyses indicated that the coupled addition of ZVI and AC promoted the conversion of particulate organic matter to soluble organic matter and increased the conversion of carbon sources to methane.

Anaerobic Digestion of Spoiled Maize, Lucerne and Barley Silage Mixture with and without Cow Manure: Methane Yields and Kinetic Studies

The effect of different inoculum-to-substrate ratios (ISRs) and feed mix (FM) ratios on the kinetics of methane production and yields during anaerobic digestion of spoiled silage mixture (SM) alone or co-digestion with cow manure (CM) was investigated in batch experiments at 37 °C. The silage mixture was prepared from spoiled silages of maize, lucerne and barley in equal proportions of 33% by wet weight. The effect of ISRs of 0.5, 1, 2 and 4 showed that methane yields increased with an increased ISR ratio. At ISRs of 1, 2 and 4, methane yields of 262.18 ± 14.96, 387.77 ± 14.43 and 482.23 ± 38.47 NmL CH4/gVSadded were obtained, respectively. Incubation at ISR 0.5 resulted in low methane yields (174.49 ± 9.29 NmL CH4/gVSadded) due to build-up of volatile fatty acids (VFAs). Further, co-digestion of spoiled SM with CM showed that the highest methane yields of 387.77 and 382.86 NmL CH4/gVSadded were obtained at SM:CM feed mix ratios of 100–0 and 75–25, respectively. The corresponding volatile solids (VS) removal rates were 72.80% and 70.82%, respectively. However, the best synergistic effect was noticed at a SM:CM = 50–50 feed mix ratio. Thus, this study shows that anaerobic digestion of spoiled silages is feasible and co-digestion of spoiled silage mixed with cow manure at a SM:CM feed mix ratio of 75–25 is recommended.

Critical evaluation of biochar effects on methane production and process stability in anaerobic digestion

Biochar is an emerging biomaterial for managing residual biomass while simultaneously sequestering carbon. To extend the biochar value chain, applying biochar to enhance anaerobic digestion (AD) processes is gaining attention in the context of a circular economy and cascading use of biomass. However, the comparative effects of various biochar dosages under normal and severe AD conditions are still unclear. To further our understanding of its potential application, this work investigated the impact of adding various biochar dosages on AD processes under normal and high substrate loadings. Three inoculum-to-substrate ratios (ISRs): one representing normal substrate loading (ISR 2) and two representing substrate overloading (ISR 1 and 0.5) were investigated. Each substrate loading rate was tested with a biochar dosage of 0% (control), 10%, and 25% based on substrate volatile solids. The results revealed that under the severe condition of high substrate overload (ISR 0.5), a high biochar dosage of 25% significantly increased cumulative methane production by 5.6% (p = 0.06) when compared to the control. Under the same condition (ISR 0.5, 25%), the time required to achieve a particular extent of ultimate methane potential was significantly reduced (p = 0.04), indicating that the methane production rate was increased. At ISR 0.5, the increase of process stability was also significant with 25% biochar addition, while the control (0%) and 10% biochar addition exhibited high variance among replicates. However, biochar did not affect AD processes under normal substrate loading (ISR 2) and mild substrate overload (ISR 1). Thus, a positive effect of biochar on the AD process was only observed under severe conditions with the highest biochar dosage. Future works should consider optimising substrate loadings and biochar dosages under real conditions when testing the practical application of biochar addition in AD processes.

Achieving efficient methane production from protein-rich organic waste in anaerobic digestion: Using conductive materials or regulating inoculum-to-substrate ratios?

The contribution of inoculum-to-substrate ratios (ISRs) and conductive materials (CMs) on the productivity of anaerobic digestion (AD) remains unclear, particularly for protein-rich organic waste. This study investigated whether the addition of CMs, i.e., biochar and iron powder, can overcome the limitations imposed by varying ISRs for the AD of protein as the sole substrate. Results indicate the ISR plays a decisive role in hydrolysis, acidification, and methanogenesis for protein conversion, irrespective of CMs addition. Methane production increased stepwise as the ISR escalated to 3:1. The addition of CMs provided limited improvement, and iron powder even inhibited methanogenesis at a low ISR. Bacterial community variations were contingent on the ISR, while iron powder supplementation significantly elevates the proportion of hydrogenotrophic methanogen. This study demonstrates that the addition of CMs could affect methanogenic efficiency but can not overcome the limitation of ISRs for the AD of protein.

Improved Recovery of Overloaded Anaerobic Batch Reactors by Graphene Oxide

Anaerobic digestion reactors may suffer from acidification when overloading occurs. Carbon-based materials are amended to mitigate the souring effects of excessive loading. This study aims to test if graphene oxide (GO) helps overloaded anaerobic reactors recover faster. Batch tests were conducted following a fed-batch strategy at different GO levels (0, 10, and 20 mg GO per g of volatile solid (VS)) and different inoculum substrate ratios (ISRs) of 2, 1, and 0.75 based on VS. While an ISR of 2 was initially applied, the ISR was decreased to 1 and 0.75 in two parallel sets of experiments to simulate overloading conditions at the fourth feeding cycle. Lastly, an ISR of 2 was restored in all assays. First-order model kinetic constants confirmed a significant (p < 0.05) effect by GO from the third feed on. Although the GO-amended assays did not alleviate the acidification effects, during the final phase the kinetic constants reached values similar to or even above the controls (without GO). Moreover, a GO concentration up to 20 mgGO/gVS had no impact on FOS/TAC. Overall, this study broadens the understanding of the design and operation of anaerobic reactors amended with GO.

Inoculum-to-substrate ratio and solid content effects over in natura spent coffee grounds anaerobic digestion

Coffee is the second most traded commodity worldwide, and its production is associated with the generation of a large number of residues, which are underused and disposed of in landfills. Notably, the coffee industry annually generates approximately 6 million tons of industrial spent coffee ground (ISCG) when extracting coffee flavorings to produce soluble coffee. That resource loss scenario has been highlighted in sustainable waste management contexts as an opportunity to improve the coffee circular economy. Despite ISCG bioconversion to methane potentially meets the waste-to-energy purposes of reducing residues disposal in landfills, decreasing greenhouse gas (GHG) emissions, and increasing renewable energy sources, data about anaerobic digestion (AD) of ISCG remains quite restricted. That limitation becomes more apparent owing to the lack of data focusing on AD key parameters for ISCG as substrate. This study assessed the influence of inoculum-to-substrate ratio (ISR) and the solid content influences on mesophilic (37 °C) ISCG-AD throughout the Response Surface Methodology (RSM) and Central Composite Design (CCD). Results revealed that both factors, ISR and solid content, should be kept above a certain threshold of 0.5 and 6.0 gTVS L−1 to ensure experimental reliability, as well as reproductively and above 1.0 and 8.0 gTVS L−1 to avoid underestimation on the MY potential achieved. Concerning ISCG-AD kinetics, the quadratic model optimum condition was at 1.36 and 14.83 gTVS L−1 for ISR and solid content, respectively. This optimum range for ISR and solid content could guide further development of process configurations for mono- and co-digestion of ISCG, avoiding underestimation of the MY potential and extended incubation periods.

Anaerobic co-digestion of food waste with sewage sludge from wastewater treatment plant of Sequele, Luanda-Angola

This work aimed to evaluate the potential for methane production from anaerobic co-digestion of the organic fraction of urban solid wastes with sewage sludge. The tests were carried out using Bioprocess AMPTS II batch reactors operated at 37 °C in two different runs. In each run, a different food waste sample, inoculated with sludge, was tested varying the inoculum to substrate ratio (ISR) in each reactor. The experimental cumulative methane production by the mixture at the end of the tests was compared with the expected methane production in order to identify synergistic effects. The modified Gompertz model was fitted to experimental substrate cumulative methane yields. The results of this study suggest the suitability of this model to describe the degradation kinetics. ISR was found to be an important parameter for an efficient methane production, values ≥ 2 are recommended in order to prevent the pH drop in reactors. For one of the food waste samples, the highest substrate methane yield achieved was 91% of the theoretical value. For the other one, synergistic effects were observed in the mixtures of inoculum and substrate, that were attributed to an enhanced methane production by the sewage sludge when mixed with the food waste. The anaerobic co-digestion of food waste with sewage sludge from the Sequele WWTP is a sustainable treatment option for both organic wastes, which can be implemented in this WWTP where the sludge is simply discarded after dehydration.

Biochemical methane potential and active microbial communities during anaerobic digestion of biodegradable plastics at different inoculum-substrate ratios

The influence of the inoculum-substrate ratio (ISR) on the mesophilic and thermophilic biochemical methane potential test of two biodegradable plastics was evaluated. Poly(lactic acid) (PLA) and polyhydroxybutyrate (PHB) were selected for this study, the first for being recalcitrant to mesophilic anaerobic digestion (AD) and the second, by contrast, for being readily biodegradable. Several ISRs, calculated on the basis of volatile solids (VS), were tested: 1, 2, 2.85, 4, and 10 g(VS of inoculum).g(VS of substrate)−1. A high ISR was associated with an enhanced methane production rate (i.e., biodegradation kinetics). However, the ultimate methane production did not change, except when inhibition was observed. Indeed, applying the lowest ISR to readily biodegradable plastics such as PHB resulted in inhibition of methane production. Based on these experiments, in order to have reproducible degradation kinetics and optimal methane production, an ISR between 2.85 and 4 is recommended for biodegradable plastics. The active microbial communities were analyzed, and the active bacteria differed depending on the plastic digested (PLA versus PHB) and the temperature of the process (mesophilic versus thermophilic). Previously identified PHB degraders (Ilyobacter delafieldii and Enterobacter) were detected in PHB-fed reactors. Thermogutta and Tepidanaerobacter were detected during the thermophilic AD of PLA, and they are probably involved in PLA hydrolysis and lactate conversion, respectively.