Liangcheng Yang

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.

Biological conversion of methane to liquid fuels: Status and opportunities

Methane is the main component of natural gas and biogas. As an abundant energy source, methane is crucial not only to meet current energy needs but also to achieve a sustainable energy future. Conversion of methane to liquid fuels provides energy-dense products and therefore reduces costs for storage, transportation, and distribution. Compared to thermochemical processes, biological conversion has advantages such as high conversion efficiency and using environmentally friendly processes. This paper is a comprehensive review of studies on three promising groups of microorganisms (methanotrophs, ammonia-oxidizing bacteria, and acetogens) that hold potential in converting methane to liquid fuels; their habitats, biochemical conversion mechanisms, performance in liquid fuels production, and genetic modification to enhance the conversion are also discussed. To date, methane-to-methanol conversion efficiencies (moles of methanol produced per mole methane consumed) of up to 80% have been reported. A number of issues that impede scale-up of this technology, such as mass transfer limitations of methane, inhibitory effects of H2S in biogas, usage of expensive chemicals as electron donors, and lack of native strains capable of converting methane to liquid fuels other than methanol, are discussed. Future perspectives and strategies in addressing these challenges are also discussed.

Qualitative and quantitative analysis on aroma characteristics of ginseng at different ages using E-nose and GC-MS combined with chemometrics

Aroma profiles of ginseng samples at different ages were investigated using electronic nose (E-nose) and GC-MS techniques combined with chemometrics analysis. The bioactive ginsenoside and volatile oil content increased with age. E-nose performed well in the qualitative analyses. Both Principal Component Analysis (PCA) and Discriminant Functions Analysis (DFA) performed well when used to analyze ginseng samples, with the first two principal components (PCs) explaining 85.51% and the first two factors explaining 95.51% of the variations. Hierarchical Cluster Analysis (HCA) successfully clustered the different types of ginsengs into four groups. A total of 91 volatile constituents were identified. 50 of them were calculated and compared using GC-MS. The main fragrance ingredients were terpenes and alcohols, followed by aromatics and ester. The changes in terpenes, alcohols, aromatics, esters, and acids during the growth year once again confirmed the dominant role of terpenes. The Partial Least Squares (PLS) loading plot of gas sensors and aroma ingredients indicated that particular sensors were closely related to terpenes. The scores plot indicated that terpenes and its corresponding sensors contributed the most in grouping. As regards to quantitative analyze, 7 constituent of terpenes could be accurately explained and predicted by using gas sensors in PLS models. In predicting ginseng age using Back Propagation-Artificial Neural Networks (BP-ANN), E-nose data was found to predict more accurately than GC-MS data. E-nose measurement may be a potential method for determining ginseng age. The combination of GC-MS can help explain the hidden correlation between sensors and fragrance ingredients from two different viewpoints.

Anaerobic digestion of giant reed for methane production

As a fast growing plant, giant reed has good potential to be used as a feedstock for methane production via anaerobic digestion (AD). The effect of total solids (TS) content, an AD operating parameter, was studied. Results showed that increasing TS from 8% to 38% decreased methane yield, due to the inhibition of volatile fatty acids (VFAs) and total ammonia nitrogen (TAN); while the maximum volumetric methane production was obtained at 20-23% TS. Comparison of solid-state AD (SS-AD) at 20% TS and liquid AD (L-AD) at 8% TS was conducted at feedstock to effluent (F/E) ratios of 2.0, 3.5, and 5.0. The best performance was achieved at an F/E of 2.0, with methane yields of 129.7 and 150.8L-CH4/kg-VS for SS-AD and L-AD, respectively. Overall organic components were degraded by 17.7-28.5% and 24.0-26.6% in SS-AD and L-AD, respectively; among which cellulose showed the highest degradation rate and the highest contribution to methane production.

Anaerobic digestion of food waste - challenges and opportunities.

The disposal of large amounts of food waste has caused significant environmental pollution and financial costs globally. Compared with traditional disposal methods (i.e., landfilling, incineration, and composting), anaerobic digestion (AD) is a promising technology for food waste management, but has not yet been fully applied due to a few technical and social challenges. This paper summarizes the quantity, composition, and methane potential of various types of food waste. Recent research on different strategies to enhance AD of food waste, including co-digestion, addition of micronutrients, control of foaming, and process design, is discussed. It is envisaged that AD of food waste could be combined with an existing AD facility or be integrated with the production of value-added products to reduce costs and increase revenue. Further understanding of the fundamental biological and physicochemical processes in AD is required to improve the technology.

Strong influence of medium pH condition on gas-phase biofilter ammonia removal, nitrous oxide generation and microbial communities

Effects of pH on gas-phase biofilter performance including NH3 removal efficiency (RE), N2O generation, and microbial communities of ammonia oxidizers and denitrifies, are examined. A two-step experiment was carried out on four biofilters for 130 days. In step 1 with pH 8.0, NH3 REs were 85-95% and N2O concentrations were 0.1-0.4 ppm. In step 2, pH was adjusted to 4.5, 6.0, 8.0, and 9.5 in four biofilters, respectively. The acidified biofilters showed higher NH3 REs than the alkalized biofilters. N2O concentration in biofilters with pH 4.5 and 6.0 was increased to 1.5 and 0.5 ppm, respectively, while no change in the alkalized biofilters. Comparing to communities in step 1, the amoA and nosZ structures were altered when pH was changed to 4.5 and 6.0, but not at 9.5. Abundance of amoA was reduced at pH 4.5, while nosZ abundance was increased with considerably less changes in acidified biofilters compared to alkalized biofilters.

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

Solid-state anaerobic digestion (SS-AD) and composting of yard trimmings with effluent from liquid AD were compared under thermophilic condition. Total solids (TS) contents of 22%, 25%, and 30% were studied for SS-AD, and 35%, 45%, and 55% for composting. Feedstock/effluent (F/E) ratios of 2, 3, 4, 5, and 6 were tested. In composting, the greatest carbon loss was obtained at 35% TS, which was 2-3 times of that at 55% TS and was up to 50% higher than that in SS-AD. In SS-AD, over half of the degraded carbon was converted to methane with the greatest methane yield of 121 L/kg VS(feedstock). Methane production from SS-AD was low at F/E ratios of 2 and 3, likely due to the inhibitory effect of high concentrations of ammonia nitrogen (up to 5.6g/kg). The N-P-K values were similar for SS-AD digestate and compost with different dominant nitrogen forms.

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.

Transport and Fate of Nitrogen-Containing Compounds in Gas-Phase Biofilters: A Swing Test to Mitigate Ammonia

This work carried out a three-month experiment to study the transport, transformation, and fate of nitrogen (N) in two gas-phase ammonia mitigation biofilters using agricultural products (a mixture of wood chips and compost) as the packing media. Two nitrogen enriching steps and one nitrogen depleting step were included as a swing test. In the first nitrogen enriching step, a high ammonia inlet concentration (~70 ppm) was applied. The ammonia removal efficiencies reached 94%, and 73.5% to 86.6% of N-NH3 was transformed into NH4+-N, NO2--N, or NO3--N. The nitrogen depleting step partially cleaned nitrogen compounds with limited disturbance to the media and restored ammonia removal efficiency for a short time. The second nitrogen enriching step used a lower (40 ppm) ammonia inlet concentration. Only 33.4% to 46.2% of NH3-N was transformed into NH4+-N, NO2--N, or NO3--N in the media, and the ammonia removal efficiency fell quickly after ten days. The pH values in the biofilter media were high and changed only slightly during the test (8.5 to 8.2). The free ammonia concentration reached 784 mg L-1 in the media moisture. The nitrogen mass balance analysis showed that the NH4+-N, NO2--N, and NO3--N accumulated in the media accounted for 50% to 100% of the nitrogen captured from the inlet gas. The nitrification process was modeled as consecutive first-order reactions. The kinetic time constants of the two first-order reactions decreased with time and with accumulated nitrogen in the media, suggesting a decline of the nitrogen transformation rates. One very likely reason that caused the decrease of ammonia removal efficiency and kinetic time constants is the accumulated free ammonium in the biofilter media.

Co-digestion of spent mushroom substrate and corn stover for methane production via solid-state anaerobic digestion

Performance of batch solid-state anaerobic co-digestion of spent mushroom substrates (SMS) and corn stover (CS) was investigated. Digestion with SMS alone (SMS/CS = 100:0) resulted in excessive volatile fatty acids (VFAs) accumulation and low methane yields during the start-up phase. Co-digestion of SMS and CS significantly enhanced digesting performance. Compared to the digesters with 100% SMS, the start-up phase of the digesters with SMS/CS = 75:25 was shortened from 11 days to 4 days, and the methane yield increased by 40%. It was also observed that the peak of daily biogas yields showed up earlier in the co-digestion reactors than in the digesters with SMS alone. A similar phenomenon happened to the methane content curves. The VFAs and pH were shown to be important driving factors for determining the population of methanogenic communities (methanosaeta and methanobacteriaceae), which were obtained by using the Fluorescent in situ Hybridization method. The changes of the methanogenic communities, in return, affected the methane production in digesters. This study showed that co-digesting SMS with CS is a feasible method to handle mushroom wastes and produce bioenergy.