Comparison of solid-state anaerobic digestion and composting of yard trimmings with effluent from liquid anaerobic digestion

Comparison and Evaluation of GHG Emissions during Simulated Thermophilic Composting of Different Municipal and Agricultural Feedstocks

Composting is widely used to recycle a variety of different organic wastes. In this study, dairy manure, chicken litter, biosolids, yard trimmings and food waste were selected as representative municipal and agricultural feedstocks and composted in simulated thermophilic composting reactors to compare and evaluate the GHG emissions. The results showed that the highest cumulative emissions of CO2, CH4 and N2O were observed during yard trimmings composting (659.14 g CO2 kg-1 DM), food waste composting (3308.85 mg CH4 kg-1 DM) and chicken litter composting (1203.92 mg N2O kg-1 DM), respectively. The majority of the carbon was lost in the form of CO2. The highest carbon loss by CO2 and CH4 emissions and the highest nitrogen loss by N2O emission occurred in dairy manure (41.41%), food waste (0.55%) and chicken litter composting (3.13%), respectively. The total GHG emission equivalent was highest during food waste composting (365.28 kg CO2-eq ton-1 DM) which generated the highest CH4 emission and second highest N2O emissions, followed by chicken litter composting (341.27 kg CO2-eq ton-1 DM), which had the highest N2O emissions. The results indicated that accounting for GHG emissions from composting processes when it is being considered as a sustainable waste management practice was of great importance.

Producing organic amendments: Physicochemical changes in biowaste used in anaerobic digestion, composting, and fermentation

Organic amendments (OAs) produced via composting, anaerobic digestion, or lactic acid fermentation, can be used to replenish soil carbon. Not all OAs production technologies preserve C and nutrients in the same way. In this study, we compared the influence of these technologies (i.e., treatments) on C and nutrient preservation and OAs chemical composition after production. We produced compost, digestate, and lactic-acid fermentation product using the same biowaste-resembling model substrate using three reactors under laboratory conditions. We compared the chemical conversions and end-products using mass balances over C, N, and P. Overall results show that losses are minimal under reducing production conditions. Fermentation and digestion conserved 99% and 64% of C; and 93% and 100% of N, respectively. While compost conservation of nutrients was limited to 25% of C and 38% of N. Digestate had the highest concentrations of C, N, and P in the water-soluble phase, enabling their accessibility for soil microbes. Concentrations in the fermentation product were one order of magnitude lower but still higher than in compost. The treatments also influence the final availability of C, N, and P, which could potentially improve the fertilising and soil-improving properties of produced OAs. Our results show that under reducing conditions, losses of C, N, and P can be decreased while increasing OAs applications in terms of sources for soil-microbial development.

Solid Fraction of Digestate from Biogas Plant as a Material for Pellets Production

One of the anaerobic digestion process products in an agricultural biogas plant is digestate (digested pulp). Large quantities of digestate generated in the process of biogas production all over the world require proper management. Fertilization is the main management of this substrate, so it is essential to look for new alternatives. The work aims to determine and discuss the possibilities of using digestate solid fraction (DSF) for pellets as biofuel production. Pellets from DSF alone and pellets with sawdust, grain straw additives were analyzed. The lower heating value (LHV) based on the dry matter for all analyzed pellets ranged from 19,164 kJ∙kg−1 to 19,879 kJ∙kg−1. The ash content was similar for all four samples and ranged from 3.62% to 5.23%. This value is relatively high, which is related to the degree of fermentation in the anaerobic digestion process. The results showed that the DSF substrate after the anaerobic digestion process still has energy potential. Analyzing those results, it seems that DSF can be a highly valuable substrate for solid biofuels production.

Incorporation of solar-heated aeration and greenhouse in grass composting

Composting is an environment-friendly method for recycling organic waste, and incorporation of heat and aeration can enhance favorable conditions for microbial growth in the process. This research aimed to evaluate the influence of the introduction of solar heat and aeration to the waste grass exposed to the composting process. The compost piles studied were subjected to different processes: application of solar-heated aeration, only-aeration, solar heating with a greenhouse, and control. Solar-heated air was introduced to a compost pile of grass clippings and compared with a greenhouse compost system. The composting process of 70 days was monitored for temperature, oxygen, moisture, organic matter loss, and humification rate. Germination index has been used to evaluate the maturation of the composts produced. The highest temperature was obtained at the compost pile with the greenhouse. This system reached the highest temperature (68.2 °C) on day 15; the ambient temperature on that day was 20.6 °C. The decreases in the C/N ratios after day 70 of composting were 20% and 15% for the greenhouse and the system where solar-heated air was introduced, respectively. Although the temperature of the solar-heated air was higher than that of the greenhouse, thermophilic temperature levels could not be reached in the aerated compost pile, which indicated a cooling effect of excessive aeration even with the heated air. Composting of grass clippings resulted in a decrease in organic matter content and enhancement in seed germination and root growth, obtaining high GI levels, inferring no phytotoxicity. This study showed that composting of grass clippings with low C/N ratios and high humidity can still be possible by using solar energy.

Solid-state anaerobic co-digestion of organic fraction of municipal waste and sawdust: impact of co-digestion ratio, inoculum-to-substrate ratio, and total solids

Municipal solid waste contains mainly organic wastes that can be a good source for anaerobic digestion. Solid-state anaerobic digestion is an affordable and suitable technique to mitigate the organic fraction of municipal solid waste (OFMSW). However, as the organic loading of OFMSW is high, co-digestion with other materials can improve the system's performance. This study aimed to investigate the performance of the co-digestion of OFMSW and sawdust and study the parameters affecting its performance. Based on the experiments, the optimum sawdust/OFMSW ratio was achieved 1:2 with the methane production of 0.3 L/g VS. In addition, the inoculum-to-substrate ration (I/S) was investigated at 1:4, 1:2, 1:1, 2:1 ratios. The best result was obtained at 2:1 ratio with a total methane yield of 0.28 L/g VS. The results also indicated that I/S ratios less than 1:1 led to fatty acid accumulation and acidic pH condition. The effect of total solids content on the co-digestion process was also examined in this study. According to the results, as the total solids increased, the biomethane yield decreased while the biogas content increased.

A comprehensive review of the recent development and challenges of a solar-assisted biodigester system

The extensive use of fossil fuels and the environmental effect of their combustion products have attracted researchers to look into renewable energy sources. In addition, global mass production of waste has motivated communities to recycle and reuse the waste in a sustainable way to lower landfill waste and associated problems. The development of waste to energy (WtE) technology including the production of bioenergy, e.g. biogas produced from various waste through Anaerobic Digestion (AD), is considered one of the potential measures to achieve the sustainable development goals of the United Nations (UN). Therefore, this study reviews the most recent studies from relevant academic literature on WtE technology (particularly AD technology) for biogas production and the application of a solar-assisted biodigester (SAB) system aimed at improving performance. In addition, socio-economic factors, challenges, and perspectives have been reported. From the analysis of different technologies, further work on effective low-cost technologies is recommended, especially using SAB system upgrading and leveraging the opportunities of this system. The study found that the performance of the AD system is affected by a variety of factors and that different approaches can be applied to improve performance. It has also been found that solar energy systems efficiently raise the biogas digester temperature and through this, they maximize the biogas yield under optimum conditions. The study revealed that the solar-assisted AD system produces less pollution and improves performance compared to the conventional AD system.

Waste-to-energy nexus: A sustainable development

An upsurge in global population due to speedy urbanization and industrialization is facing significant challenges such as rising energy-demand, enormous waste-generation and environmental deterioration. The waste-to-energy nexus based on the 5R principle (Reduce, Reuse, Recycle, Recovery, and Restore) is of paramount importance in solving these Gordian knots. This review essentially concentrates on latest advancements in the field of 'simultaneous waste reduction and energy production' technologies. The waste-to-energy approaches (thermal and biochemical) for energy production from the agricultural residues are comprehensively discussed in terms environmental, techno-economic, and policy analysis. The review will assess the loopholes in order to come up with more sophisticated technologies that are not only eco-friendly and cost-effective, but also socially viable. The waste-to-energy nexus as a paradigm for sustainable development of restoring waste is critically discussed considering future advancement plans and agendas of the policy-makers.

Thermotolerant cellulolytic Clostridiaceae and Lachnospiraceae rich consortium enhanced biogas production from oil palm empty fruit bunches by solid-state anaerobic digestion

Thermotolerant cellulolytic consortium for improvement biogas production from oil palm empty fruit bunches (EFB) by prehydrolysis and bioaugmentation strategies was investigated via solid-state anaerobic digestion (SS-AD). The prehydrolysis EFB with Clostridiaceae and Lachnospiraceae rich consortium have maximum methane yield of 252 and 349 ml CH₄ g^-1 VS with total EFB degradation efficiency of 62% and 86%, respectively. Clostridiaceae and Lachnospiraceae rich consortium augmentation in biogas reactor have maximum methane yield of 217 and 85.2 ml CH₄ g^-1 VS with degradation efficiency of 42% and 16%, respectively. The best improvement of biogas production was achieved by prehydrolysis EFB with Lachnospiraceae rich consortium with maximum methane production of 113 m³ CH₄ tonne^-1 EFB. While, Clostridiaceae rich consortium was suitable for augmentation in biogas reactor with maximum methane production of 70.6 m³ CH₄ tonne^-1 EFB. Application of thermotolerant cellulolytic consortium into the SS-AD systems could enhance biogas production of 3-11 times.

Valorisation of biowaste digestate through solid state fermentation to produce biopesticides from Bacillus thuringiensis

The main goal of this work is the production of a biopesticide through solid-state fermentation of biowaste digestate inoculated with Bacillus thuringiensis (Bt) at pilot scale using different configurations of reactors. Fermentations were carried out using insulated and non-insulated, stirred and non-stirred reactors at different scales (10, 22 and 100 L) in order to assess the influence of the reactor configuration on the biopesticide production process. A maximum temperature of 60 °C was reached in 10-L insulated non-stirred reactors where increments of Bt viable cells and spores with respect to initial values of 1.9 and 171.6 respectively, were attained. In contrast, when temperature was regulated by using 22-L non-insulated stirred reactors the increment of viable cells and spores were 0.8 and 1.9, respectively, at a stable temperature of 27 °C. When the non-insulated stirred reactor was scaled up to 100-L, the increase of viable cells and spore counts were 1.2 and 3.8 respectively, with an average temperature of 28 °C. These results demonstrated that the election of a proper reactor configuration is important when considering the development of a new SSF process, especially when dealing with non-conventional substrates as digestate.

Techno-economic analyses of solid-state anaerobic digestion and composting of yard trimmings

Solid-state anaerobic digestion (SS-AD) and composting are two potential alternatives to divert yard trimmings from landfills. This study aimed to evaluate the techno-economic feasibility of commercial-scale SS-AD and composting systems (20,000 metric tons (MT)/year) that received both yard trimmings and liquid AD effluent using a modeling software, SuperPro Designer. Both the SS-AD and composting systems were shown to be economically feasible. While their revenues were comparable ($48/MT), SS-AD with digestate drying showed a higher capital cost ($256/MT vs. $84/MT) but a lower non-facility-dependent operating cost ($11/MT vs. $21/MT) than composting. The payback time, internal rate of return (IRR), and net present value (NPV) were estimated to be ∼10 years, 8%, and $0.2 million, respectively, for SS-AD, and ∼4.9 years, 33%, and $1.8 million, respectively, for composting. Digestate drying was necessary to make SS-AD profitable via the sale of byproduct, but it was also the most energy intensive step, relying on heat recovery to reduce costs. Moreover, the economics of SS-AD were highly improved (NPV $2 million) with financial incentives (i.e. investment tax credits), indicating that incentives were critical to the economic feasibility of current SS-AD systems that utilize lignocellulosic biomass. However, renewable identification numbers (RINs) and renewable energy certificates (RECs) had minor effects. Furthermore, the economics of both systems were most sensitive to plant size, tipping fees, and byproduct/compost price. The results suggest SS-AD may be favored for centralized management while composting for de-centralized management of yard trimmings. Alternative ways to valorize digestate should be evaluated in future studies.

Comparison of thermophilic anaerobic and aerobic treatment processes for stabilization of green and food wastes and production of soil amendments

The management of organic wastes is an environmental and social priority. Aerobic digestion (AED) or composting and anaerobic digestion (AD) are two organic waste management practices that produce a value-added final product. Few side-by-side comparisons of both technologies and their digestate products have been performed. The objective of this study was to compare the impact of initial feedstock properties (moisture content and/or C/N ratio) on stabilization rate by AED and AD and soil amendment characteristics of the final products. Green and food wastes were considered as they are two of the main contributors to municipal organic waste. Stabilization rate was assessed by measurement of CH₄ and CO₂ evolution for AD and AED, respectively. For AD, CH₄ yield showed a second-order relationship with the C/N content (P < 0.05); the optimal C/N ratio indicated by the relationship was 25.5. For AED, cumulative CO₂ evolution values were significantly affected by the C/N ratio and moisture content of the initial feedstock (P < 0.05). A response surface model showed optimal AED stabilization for a C/N of 25.6 and moisture of 64.9% (wet basis). AD final products presented lower soluble chemical oxygen demand (COD) but lower humification degree and aromaticity than the products from AED. This lower stability may lead to further degradation when amended to soil. The results suggest that composting feedstocks with higher C/N produces an end-product with higher suitability for soil amendment. The instability of end products from AD could be leveraged in pest control techniques that rely on organic matter degradation to produce compounds with pesticidal properties.

Spatial and temporal distribution of pore gas concentrations during mainstream large-scale trough composting in China

With the advantages of high treatment capacity and low operational cost, large-scale trough composting has become one of the mainstream composting patterns in composting plants in China. This study measured concentrations of O₂, CO₂, CH₄ and NH₃ on-site to investigate the spatial and temporal distribution of pore gas concentrations during mainstream large-scale trough composting in China. The results showed that the temperature in the center of the pile was obviously higher than that in the side of the pile. Pore O₂ concentration rapidly decreased and maintained <5% (in volume) for 38 days or more in both the center and side of the pile and effective O₂ diffusion occurred at most in every two contiguous layers. Pore CO₂ and CH₄ concentrations at each measurement point were positively correlated (0.436 ≤ r ≤ 0.570, P < 0.01) and the concentrations in the side of the pile were obviously lower than those in the center. The top layer exhibited highest pore O₂ concentration and lowest CO₂ and CH₄ concentrations, and the bottom layer was on the contrary. No significant differences in pore NH₃ concentrations between different layers or between different measurement points in the same layer were found. Therefore, mixing the center and the side of the pile when mechanical turning and adjusting the height of the pile according to the physical properties of bulking agents are suggested to optimize the oxygen distribution and promote the composting process during large-scale trough composting when the pile was naturally aerated, which will contribute to improving the current undesirable atmosphere environment in China.

Sequential batch thermophilic solid-state anaerobic digestion of lignocellulosic biomass via recirculating digestate as inoculum - Part I: Reactor performance

Sequential batch thermophilic (55°C) solid-state anaerobic digestion (SS-AD) of yard trimmings was evaluated at a total solids (TS) content of 22% via recirculating digestate as the inoculum. The substrate-to-inoculum (S/I) ratio of 1 (TS basis) was favored over 2 and 3 due to significantly higher methane yield and volumetric productivity. At an S/I ratio of 1, sequential batch SS-AD gradually reached steady state by 3 runs (30days/run) with increases in both methane yields (up to 11.5%) and cellulose degradation (up to 55%), indicating that recirculated digestate could be a feasible inoculum to establish long term stable SS-AD of lignocellulosic biomass. The initial sharp increases of volatile fatty acids during runs 2-4 indicated faster hydrolysis of organic matter than during run 1, suggesting that microbes were probably more acclimated due to digestate recirculation. At steady state, 51% (w/w) of the digestate was recirculated as the inoculum.

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

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

Beyond land application: Emerging technologies for the treatment and reuse of anaerobically digested agricultural and food waste

Effective treatment and reuse of the massive quantities of agricultural and food wastes generated daily has the potential to improve the sustainability of food production systems. Anaerobic digestion (AD) is used throughout the world as a waste treatment process to convert organic waste into two main products: biogas and nutrient-rich digestate, called AD effluent. Biogas can be used as a source of renewable energy or transportation fuels, while AD effluent is traditionally applied to land as a soil amendment. However, there are economic and environmental concerns that limit widespread land application, which may lead to underutilization of AD for the treatment of agricultural and food wastes. To combat these constraints, existing and novel methods have emerged to treat or reuse AD effluent. The objective of this review is to analyze several emerging methods used for efficient treatment and reuse of AD effluent. Overall, the application of emerging technologies is limited by AD effluent composition, especially the total solid content. Some technologies, such as composting, use the solid fraction of AD effluent, while most other technologies, such as algae culture and struvite crystallization, use the liquid fraction. Therefore, dewatering of AD effluent, reuse of the liquid and solid fractions, and land application could all be combined to sustainably manage the large quantities of AD effluent produced. Issues such as pathogen regrowth and prevalence of emerging organic micro-pollutants are also discussed.

Effect of limited air exposure and comparative performance between thermophilic and mesophilic solid-state anaerobic digestion of switchgrass

Switchgrass is an attractive feedstock for biogas production via anaerobic digestion (AD). Many studies have used switchgrass for liquid anaerobic digestion (L-AD), but few have used switchgrass for solid-state anaerobic digestion (SS-AD). Limited air exposure to the reactor headspace has been adopted in commercial scale anaerobic digesters for different applications. However, little research has examined the effect of limited air exposure on biogas production during SS-AD. In this study, the effects of air exposure and total solids (TS) content on SS-AD performance were evaluated under mesophilic (36±1°C) and thermophilic (55±0.3°C) conditions. Limited air exposure did not significantly influence the methane yield during SS-AD. Thermophilic SS-AD had greater methane yields (102-145LCH4kg(-1)VSadded) than mesophilic SS-AD (88-113LCH4kg(-1)VSadded). Both mesophilic SS-AD (73-136GJ) and thermophilic SS-AD (2-95GJ) produced positive net energy based on a theoretical 'garage-type' SS-AD digester operating in a temperate climate.

Comparison of premixing methods for solid-state anaerobic digestion of corn stover

The development of solid-state anaerobic digestion (SS-AD) has prompted studies to resolve practical challenges such as mixing of feedstock and inoculum. This study compared the performance of SS-AD using three premixing methods. Results showed that at feedstock to inoculum (F/I) ratios of 4 and 6, the two-layer partial premixing method obtained the highest methane yield, followed by one-layer partial premixing and complete premixing methods. Partial premixing methods also showed wider daily methane yield peaks than the complete premixing method. The volatile fatty acid (VFA) concentration was affected by the premixing method, and was highly correlated to methane yield; while the concentration of remaining holocellulose was correlated to pH and alkalinity. SS-AD digesters failed at an F/I ratio of 8, regardless of the premixing method. Adding extra inoculum to the top of failed digesters resulted in recovery of methane production.