The uptake of anaerobic digestion for the organic fraction of municipal solid waste - Push versus pull factors

Implementation of anaerobic digestion for valorizing the organic fraction of municipal solid waste in developing countries: Technical insights from a systematic review

Anaerobic digestion (AD) as a waste management strategy for the organic fraction of municipal waste (OFMSW) has received attention in developed countries for several decades, leading to the development of large-scale plants. In contrast, AD of OFMSW has only recently drawn attention in developing countries. This systematic review was carried out to investigate the implementation of AD to treat the OFMSW in developing countries, focusing on assessing pilot and full-scale AD plants reported in the last ten years. Studies that met the selection criteria were analyzed and data regarding operating parameters, feedstock characteristics, and biogas, digestate, and energy production were extracted. As outlined in this systematic review, AD plants located in developing countries are mostly one-stage mesophilic systems that treat OFMSW via mono-digestion, almost exclusively with the aim of producing electrical energy. Based on the analysis done throughout this systematic review, it was noted that there is a large difference in the maturity level of AD systems between developing and developed countries, mainly due to the economic capacity of developed countries to invest in sustainable waste management systems. However, the number of AD plants reported in scientific papers is significantly lower than the number of installed AD systems. Research articles regarding large-scale implementation of AD to treat OFMSW in developed countries were analyzed and compared with developing countries. This comparison identified practices used in plants in developed countries that could be utilized in the large-scale implementation and success of AD in developing countries. These practices include exploiting potential products with high market-values, forming partnerships with local industries to use industrial wastes as co-substrates, and exploring different biological and physical pretreatment technologies. Additionally, the analysis of capital and operational costs of AD plants showed that costs tend to be higher for developing countries due to their need to import of materials and equipment from developed countries. Technical, economical, and political challenges for the implementation of AD at a large-scale in developing countries are highlighted.

Optimizing the effect of hydrochar on anaerobic digestion of organic fraction municipal solid waste for biogas and methane production

BACKGROUND

Anaerobic digestion (AD) is the biological waste treatment method for the organic fraction of municipal solid waste (OFMSW). AD is notable for its ability to reduce volume and produce biogas from waste. However, the conventional AD of OFMSW has a low degradation rate. In recent years, some treatment method has been used to promote the biogas and methane production of AD. One of these methods is hydrothermal carbonization (HTC).

PURPOSE

This study aimed to evaluate the effect of hydrothermal carbonization (HTC) temperature and hydrochar: OFMSW ratio as factors on biogas production, methane production, and methane content of anaerobic digestion (AD) as responses was investigated.

METHODS

This study determined the biomethane potential of raw and pretreated OFMSW (hydrochars) in 118 ml serum glass bottles. Based on the Hansen method, all tests were conducted at mesophilic temperature (37 ± 1 °C) in an incubator for 45 days. The response surface method and central composite model were used for designing experimental conditions. Quadratic models were used to estimate the correlation between factors and responses. Also, the optimal conditions for maximizing responses were determined.

RESULTS

Biogas production of mixing hydrochar and OFMSW was 41% more than control groups which contained OFMSW and inoculum. The optimal operating conditions to maximize all responses were applied in HTC temperature and hydrochar: OFMSW ratio of 179.366 °C and 2.406, respectively. In this condition, the maximum biogas production, methane production, and methane content were 394 mL/g VS, 284.351 mL/g VS, and 73.176%, respectively.

CONCLUSION

As an OFMSW HTC pretreatment for AD, hydrochar additive has a significantly positive and negative effect on biogas production, methane production, and methane content of biogas depending on operating conditions. Therefore. It is necessary to consider the individual and interaction effects of the temperature and hydrochar: OFMSW ratio, obtain the optimal conditions and determine responses.

Anaerobic digestate management, environmental impacts, and techno-economic challenges

Digestate is a nutrient-rich by-product from organic waste anaerobic digestion but can contribute to nutrient pollution without comprehensive management strategies. Some nutrient pollution impacts include harmful algal blooms, hypoxia, and eutrophication. This contribution explores current productive uses of digestate by analyzing its feedstocks, processing technologies, economics, product quality, impurities, incentive policies, and regulations. The analyzed studies found that feedstock, processing technology, and process operating conditions highly influence the digestate product characteristics. Also, incentive policies and regulations for managing organic waste by anaerobic digestion and producing digestate as a valuable product promote economic benefits. However, there are not many governmental and industry-led quality assurance certification systems for supporting commercializing digestate products. The sustainable and safe use of digestate in different applications needs further development of technologies and processes. Also, incentives for digestate use, quality regulation, and social awareness are essential to promote digestate product commercialization as part of the organic waste circular economy paradigm. Therefore, future studies about circular business models and standardized international regulations for digestate products are needed.

Dry anaerobic digestion of the fine particle fraction of mechanically-sorted organic fraction of municipal solid waste in laboratory and pilot reactor

High-solid anaerobic digestion of the very small particle fraction of mechanically-sorted organic fraction of municipal solid waste (OFMSW) was examined in mesophilic digestion tests in a conventional laboratory (0.013 m³) and a pilot (0.300 m³) reactor. The non-biodegradable and recalcitrant molecules together with the low protein and starch contents of the small-particles of OFMSW limited the methane generation potential of substrate. In the conventional AD system, methane yields remained low at 0.139 m³kg_VS^-1 due to formation of a non-reacting layer on digestate surface, which restricted utilization of the available in OFMSW digestible organics. The absence of surface solid crust in the pilot unit favoured consumption of a greater proportion of volatile solids of the OFMSW. Dry AD was remarkably stable over the entire period and negligibly effected by the toxic H₂S yields. Methane generation (0.167 m³kg_VS^-1) was increased 1.2-fold compared to the conventional system due to a better mixing of substrate and microorganisms achieved inside the pilot reactor, which led to an increase of the digested volatile organics. Digestate presented low stability and high heavy metal content, both of which restrain its implementation as soil conditioner or fertilizer in agriculture. A secondary co-digestion treatment may be required for the neutralization of digestate.

Fate of disposable face masks in high-solids anaerobic digestion: Experimental observations and review of potential environmental implications

Face masks became a part of our daily life amid the global COVID-19 (SARS-CoV-2) pandemic. Most of the face masks are made for single-use and primarily disposed of in garbage bins with other non-recyclable wastes. To date, little is known about how disposable face masks in municipal solid waste (MSW) would interfere with high-solids anaerobic digestion (HSAD) in waste management facilities. Here, we first report preliminary results from a lab experiment conducted with the organic fraction of municipal solid waste (OFMSW) amended with used disposable face masks. The lab-scale HSAD systems were operated with percolate recirculation comparable to commercial HSAD systems typically used for full-scale processing of OFMSW. The results suggested that the presence of face masks in OFMSW could negatively affect methane productivity and kinetics. In the digesters amended with face masks, total cumulative methane production decreased by up to 18%, along with a 12-29% decrease in maximum methane production rates than the control digester (without face masks). Moreover, lag phases increased by 7-14%. The results also suggested that the type of polymeric materials used in face masks would be more critical than their total number/loading in the digester, which warrants further investigation. The visual inspection of digestate showed that the face masks were mostly undegraded after 40 days of operation. Much remains unknown about how the undegraded face masks will affect the digestate management practices, such as composting, land application, and landfilling. However, the review of existing literature suggested that they can be a potential source of plastic and microplastic pollution and amplify transmission of antibiotic resistance genes to the ecosystem. In summary, this study underscores the importance of developing safe and reliable disposal guidelines and management plans for single-use face masks.

Single-phase anaerobic digestion of the organic fraction of municipal solid waste without dilution: Reactor stability and process performance of small, decentralised plants

Currently, centralised plants are the most favoured approach for the anaerobic treatment of the organic fraction of municipal solid waste (OFMSW). However, centralised solutions imply certain environmental impacts, which prevent large-scale implementation of the anaerobic digestion (AD). As a result, we are digesting <5% of organic waste both in Europe and the USA even today. Pursuing the criteria for maximising the balance between profit and impacts, an innovative layout with the ultimate goal of promoting the use of small, decentralised AD plants is proposed. In this study, source-separated OFMSW (SS-OFMSW) was treated in a mesophilic plug flow reactor by applying an atypical combination of conditions such as high SS-OFMSW solid content (214.5 g·kg^-1), high organic loading rate (6.2 kg VS·m^-3·d^-1), and no dilution or co-substrate addition. A suitable and an efficient mixing system is essential to control the process. Accordingly, the process was stable in a single-stage reactor, in the absence of digestate recirculation, obtaining specific gas production of 0.67 m³·kg^-1 VS in terms of biogas and 0.41 m³·kg^-1 VS in terms of methane. High reactor volume exploitation and small plant construction were feasible, reaching a gas production rate of 4.5 m³·m^-3 d^-1. The estimated costs in terms of capital and operating expenditure are expected to realize gross economic sustainability of full-scale installation.

Assessment of high-solid mesophilic and thermophilic anaerobic digestion of mechanically-separated municipal solid waste

Mechanically-sorted organic fraction of municipal solid waste (OFMSW) was tested to determine its biogas and biomethane generation efficiency. Methane production capability of OFMSW was examined in biochemical methane potential (BMP) tests. The factors affecting the high-solid anaerobic digestion (AD) of feedstock were investigated in a series of long-term semi-continuous digestion tests performed at dry mesophilic and thermophilic conditions in a continuously rotating drum reactor with working volume of 0.013 m³. OFMSW presented low biogas and methane generation capacity due to its contained non-biodegradable components and the low proteins and starch proportions. Dry mesophilic AD allowed only a relatively limited fraction of OFMSW volatile solids to be consumed for biogas and methane production. Reducing particle size favoured utilization of higher proportions of the available digestible organic substances, and concurrently promoted biogas and biomethane generation rate. Stability of methane generation was also significantly improved by particle downsizing. Small particles compensated the limited mass transfer and restricted distribution of methane production intermediate metabolites caused by water absence in the dry AD system. Dry thermophilic AD converted sufficient quantity of OFMSWs biodegradable content. The average methane released from dry thermophilic AD (0.176 m³kg_VS^-1) was higher than that of dry mesophilic AD of fine particles (0.148 m³kg_VS^-1) and much higher than that of dry mesophilic AD of same grain size (0.114 m³kg_VS^-1). High temperature proved more suitable for anaerobically digesting mechanically-sorted OFMSW.

Giving credit to residual bioresources: From municipal solid waste hydrolysate and waste plum juice to poly (3-hydroxybutyrate)

Municipal solid waste (MSW) is massively generated all over the world. Its organic fraction (OFMSW), which represents a high percentage of MSW, mainly contains biodegradable materials, namely food waste, paper and garden waste. The social cost of OFMSW treatment and/or disposal is a serious and widespread problem, particularly in highly populated areas. Thus, effective and innovative solutions, which include the upgrading of OFMSW, are being currently sought. In fact, the OFMSW abundance, availability and average composition suggest its considerable potential within the circular economy desideratum, paving the way to valorisation approaches. In this context, an OFMSW sugar-rich hydrolysate and its validation as a substrate for the production of the polyester poly(3-hydroxybutyrate) (P(3HB)), to date the only bioplastic easily biodegradable in marine environment, were successfully obtained in a previous study. Based on those results, this work addresses the upscaling of the fermentative production, in fed-batch mode, of P(3HB) by Burkholderia sacchari. The OFMSW hydrolysate was used as cultivation medium due to its balanced nutrient composition, while a plum waste juice, also rich in sugars, was applied as feed to the bioreactor. By implementing this strategy, a maximum P(3HB) production of 30 g·L^-1 with an accumulation of 43% g (P(3HB))/g cell dry weight (CDW) after 51 h, was achieved. The use of the hydrolysate as initial medium resulted in higher CDW (71 g·L^-1) than that of the simulated hydrolysate (62 g·L^-1 in average), probably because the OFMSW hydrolysate favours biomass growth in detriment of P(3HB) production.

Life-Cycle Greenhouse Gas Emissions and Human Health Trade-Offs of Organic Waste Management Strategies

Waste-to-energy systems can play an important role in diverting organic waste from landfills. However, real-world waste management can differ from idealized practices, and emissions driven by microbial communities and complex chemical processes are poorly understood. This study presents a comprehensive life-cycle assessment, using reported and measured data, of competing management alternatives for organic municipal solid waste including landfilling, composting, dry anaerobic digestion (AD) for the production of renewable natural gas (RNG), and dry AD with electricity generation. Landfilling is the most greenhouse gas (GHG)-intensive option, emitting nearly 400 kg CO_2e per tonne of organic waste. Composting raw organics resulted in the lowest GHG emissions, at -41 kg CO_2e per tonne of waste, while upgrading biogas to RNG after dry AD resulted in -36 to -2 kg CO_2e per tonne. Monetizing the results based on social costs of carbon and other air pollutant emissions highlights the importance of ground-level NH₃ emissions from composting nitrogen-rich organic waste or post-AD solids. However, better characterization of material-specific NH₃ emissions from landfills and land-application of digestate is essential to fully understand the trade-offs between alternatives.

Organic waste biorefineries: Looking towards implementation

The concept of biorefinery expands the possibilities to extract value from organic matter in form of either bespoke crops or organic waste. The viability of biorefinery schemes depends on the recovery of higher-value chemicals with potential for a wide distribution and an untapped marketability. The feasibility of biorefining organic waste is enhanced by the fact that the biorefinery will typically receive a waste management fee for accepting organic waste. The development and implementation of waste biorefinery concepts can open up a wide array of possibilities to shift waste management towards higher sustainability. However, barriers encompassing environmental, technical, economic, logistic, social and legislative aspects need to be overcome. For instance, waste biorefineries are likely to be complex systems due to the variability, heterogeneity and low purity of waste materials as opposed to dedicated biomasses. This article discusses the drivers that can make the biorefinery concept applicable to waste management and the possibilities for its development to full scale. Technological, strategic and market constraints affect the successful implementations of these systems. Fluctuations in waste characteristics, the level of contamination in the organic waste fraction, the proximity of the organic waste resource, the markets for the biorefinery products, the potential for integration with other industrial processes and disposal of final residues are all critical aspects requiring detailed analysis. Furthermore, interventions from policy makers are necessary to foster sustainable bio-based solutions for waste management.

Challenges and An Implementation Framework for Sustainable Municipal Organic Waste Management Using Biogas Technology in Emerging Asian Countries

Due to its ability to recover both material and energy from organic waste, biogas technology is considered the best technology for treating organic waste. While in many emerging Asian countries more than 50% of municipal waste is organic waste, the amount of organic waste treated with biogas technology remains very limited. This study identified key challenges faced by practitioners in sustaining biogas plants from literature and interviewed a number of sustainably operating biogas plant managers and, based on the findings, developed an implementation framework to help decision makers and practitioners in planning a sustainable municipal organic waste biogas plant facility.

Destruction of Staphylococcus aureus and the impact of chlortetracycline on biomethane production during anaerobic digestion of chicken manure

Research was undertaken to ascertain the effect on biogas potential during the anaerobic digestion of chicken manure containing Staphylococcus aureus and chlortetracycline (antibiotic) from infected chicken flocks. S. aureus is a pathogenic bacteria in chicken flocks that is usually treated with the broad-spectrum antibiotic, chlortetracycline. Veterinary antibiotics are often prescribed in the poultry sector for on-farm use at the flock level to control disease; consequently, significant quantities of antibiotics are excreted from the bird into the manure. Subsequent anaerobic digestion of this chicken manure could lead to pathogens and antibiotics affecting the digestion process. Anaerobic digestion biochemical methane potential assays were completed at 35°C for 39 days, with some assays receiving S. aureus and some receiving S. aureus and chlortetracycline. No viable S. aureus cells were detected after Day 0 of the experiment. A further experiment utilising an order of magnitude greater concentration of S. aureus demonstrated a significant reduction (>400 fold) in S. aureus within 24 h when inoculated into anaerobic digestate, with no viable S. aureus cells detected by the end of 3 days. Furthermore, the efficacy of chlortetracycline was significantly reduced when applied to anaerobic digestate compared to water alone. Total biogas yields from chicken manure were significantly lowered by the addition of S. aureus, with and without chlortetracycline. However, there was no significant difference in methane yields between treatments. The cellulose control assays showed a lag phase in methane production after receiving chlortetracycline. In comparison, the absence of a lag phase when the antibiotic were added to chicken manure may have been due to the relatively high nitrogen content of the feedstock reducing the inhibition of chlortetracycline on methanogens. Therefore, this study demonstrates that the addition of S. aureus and chlortetracycline does not have a commercially relevant effect on the digestion of chicken manure.

Semi-continuous mono-digestion of OFMSW and Co-digestion of OFMSW with beech sawdust: Assessment of the maximum operational total solid content

In this study, mono-digestion of the organic fraction of municipal solid waste (OFMSW) and co-digestion of OFMSW with beech sawdust, simulating green waste, were used to investigate the maximum operational total solid (TS) content in semi-continuous high-solids anaerobic digestion (HS-AD). To alleviate substrate overloading in HS-AD, the effluent mass was relatively reduced compared to the influent mass, extending the mass retention time. To this aim, the reactor mass was daily evaluated, permitting to assess the reactor content removal by biogas production. During mono-digestion of OFMSW, the NH₃ inhibition and the rapid TS removal prevented to maintain HS-AD conditions (i.e. TS ≥ 10%), without exacerbating the risk of reactor acidification. In contrast, the inclusion of sawdust in OFMSW permitted to operate HS-AD up to 30% TS, before acidification occurred. Therefore, including a lignocellulosic substrate in OFMSW can prevent acidification and stabilize HS-AD at very high TS contents (i.e. 20-30%).

Anaerobic digestion of municipal solid waste: Energy and carbon emission footprint

Anaerobic digestion (AD) serves as a promising alternative for waste treatment and a potential solution to improve the energy supply security. The feasibility of AD has been proven in some of the technologically and agriculturally advanced countries. However, development is still needed for worldwide implementation, especially for AD process dealing with municipal solid waste (MSW). This paper reviews various approaches and stages in the AD of MSW, which used to optimise the biogas production and quality. The assessed stages include pre-treatment, digestion process, post-treatment as well as the waste collection and transportation. The latest approaches and integrated system to improve the AD process are also presented. The stages were assessed in a relatively quantitative manner. The range of energy requirement, carbon emission footprint and the percentage of enhancement are summarised. Thermal hydrolysis pre-treatment is identified to be less suitable for MSW (-5% to +15.4% enhancement), unless conducted in the two-phase AD system. Microwave pre-treatment shows consistent performance in elevating the biogas production of MSW, but the energy consumption (114.24-8,040 kW_eh t^-1) and carbon emission footprint (59.93-4,217.78 kg CO₂ t^-1 waste) are relatively high. Chemical (∼0.43 kW_eh m^-3) and membrane-based (∼0.45 kW_eh m^-3) post-treatments are suggested to be a lower energy consumption approach for upgrading the biogas. The feasibility in terms of cost (scale up) and other environmental impacts (non-CO₂ footprint) needs to be further assessed. This study provides an overview to facilitate further development and extended implementation of AD.

Food waste enhanced anaerobic digestion of biologically pretreated yard waste: Analysis of cellulose crystallinity and microbial communities

Solid waste treatment through anaerobic digestion (AD) technology contributes to energy recycling and reuse of various solid organic wastes. However, yard waste (YW) is generally recalcitrant to AD due to the presence of high cellulose and hemicellulose content, which are difficult to be hydrolyzed. In this study, to enhance hydrolysis efficiency, YW was biologically pretreated with digested sludge and supplemented with food waste (FW) before AD process. Effects of FW supplementation on pH, SCOD, cellulose and hemicellulose content and cellulose crystallinity were examined. The optimal amount of FW supplementation was determined to be 10 wt%. An increase of 6.5-20.3% in cellulose reduction and an increase of 14.8-53.1% in hemicellulose reduction in digesters was achieved within the optimal pretreatment time of 4 days. After hydrolysis, cellulose crystallinity decreased by 23% from 71% in the control digester, which was responsible for improved biodegradability of cellulose in YW. FT-IR analysis of hydrolysis mixture confirmed that partial hydrogen bonds were destroyed in digesters with supplementation of 10 wt% FW, leading to a higher extent of degradation of the feedstock. In the batch AD of FW supplemented YW, results indicated that methane yield was 35% higher than that of the control digester without FW supplementation. Pyrosequencing analysis indicated that the abundance of bacterial genus Sphaerochaeta and Cellulosibacter in subsequent digestion were enhanced by 10- and 5-folds by 10 wt% FW supplementation, respectively, and were deemed to be responsible for the enhanced anaerobic digestion performance.