Evaluation of hydrothermal treatment in enhancing rice straw compost stability and maturity

The impact of microbial inoculants on large-scale composting of straw and manure under natural low-temperature conditions

Understanding large-scale composting under natural conditions is essential for improving waste management and promoting sustainable agriculture. In this study, corn straw (400 tons) and pig manure (200 tons) were composted with microbial inoculants. The thermophilic phase of composting lasted for fourteen weeks, resulting in an alkaline final product. Microbial systems with low-temperature initiation and high-temperature fermentation played a crucial role in enhancing lignocellulose degradation and humic substances (HS) formation. Adding microbes, including Rhodanobacter, Pseudomonas, and Planococcus, showed a positive correlation with degradation rates of cellulose, hemicellulose, and lignin. Bacillus, Planococcus, and Acinetobacter were positively correlated with HS formation. Microorganisms facilitated efficient hydrolysis of lignocelluloses, providing humic precursors to accelerate composting humification through phenolic protein and Maillard pathways. This study provides significant insights into large-scale composting under natural conditions, contributing to the advancement of waste management strategies and the promotion of sustainable agriculture.

Hydrochar drives reduction in bioavailability of heavy metals during composting via promoting humification and microbial community evolution

This study presented the effects of hydrochar on humification, heavy metals (HMs) bioavailability and bacterial community succession during composting. Results indicated that hydrochar addition led to elevated composting temperature, 7.3% increase in humic acid (HA), and 52.9% increase in ratio of humic acid to fulvic acid. The diethylene triamine pentacetic acid extractable Zn, Cu, Pb, and Ni were reduced by 19.2%, 36.3%, 37.8%, and 27.1%, respectively, in hydrochar-involved composting system. Furthermore, main mechanisms driving the reduced HMs bioavailability by hydrochar addition were revealed. The addition of hydrochar significantly modified the microbial community structure. Correlation analysis and microbial analysis demonstrated that relative abundance of bacterial groups connected with humification and HMs passivation were increased. Consequently, the HA formation was promoted and the HMs bioavailability were reduced through bacterial bioremediation and HA complexation. This study demonstrates the addition of hydrochar as a promising strategy to mitigate the HMs bioavailability during composting.

Recycling potassium from cow manure compost can replace potassium fertilizers in paddy rice production systems

The prevalence of K deficiency and negative K balance in rice production increases the demand for K fertilizer. However, the primary source of K fertilizer, potash rock, is limited. Recycling K from cow manure compost (CMC) is a sustainable solution. Nevertheless, the effects of substituting K fertilizer with CMC on rice yield, soil K fertility, and partial K balance (PKB) are not well understood. Therefore, a field experiment with four treatments (control - unfertilized, MNP K - CMC plus NPK fertilizer, MNP ½ K - CMC plus NP and 50 % K fertilizer, and MNP - CMC plus NP fertilizer) was conducted from 2020 to 2022 to study the effects of replacing K fertilizer with K from CMC on rice growth, yield, plant K uptake, soil K fertility, and PKB. The results indicated that K input from CMC exceeded the recommended K fertilizer level, sufficient for optimal rice growth and yield over three growing seasons and plant K uptake in the last two seasons. Plant K uptake increased with total K input and reached a plateau when total K input approached the maximum plant K uptake. In the MNP treatment, PKB was negative in the first two seasons but became positive in the last season, owing to the equivalence between K input from CMC and plant K uptake. Key factors influencing PKB in this treatment were K input from CMC and plant K uptake. Increasing the CMC application rate during the first two seasons could lead to a positive PKB. In this treatment, soil exchangeable K changed correspondingly with PKB, decreasing in the first two seasons but increasing in the last season. Overall, determining the appropriate amount of CMC application for a positive PKB is vital for the sustainability of substituting K fertilizer with K from CMC in paddy rice systems.

Lignite drove phenol precursors to participate in the formation of humic acid during chicken manure composting

This study set out to explore the impact of lignite on preserving organic matter and promoting the formation of humic acid (HA) during chicken manure composting. Composting test was carried out for control (CK), 5 % lignite addition treatment (L1), 10 % addition treatment (L2) and 15 % addition treatment (L3). The results demonstrated that lignite addition effectively reduced the loss of organic matter. The HA content of all lignite-added groups was higher than that of CK, and the highest was 45.44 %. L1 and L2 increased the richness of bacterial community. Network analysis showed higher diversity of HA-associated bacteria in L2 and L3 treatments. Structural equation models revealed that reducing sugar and amino acid contributed to the formation of HA during CK and L1 composting, while polyphenol contributed more to the HA formation during L2 and L3 composting. Furthermore, lignite addition also could promote the direct effect of microorganisms on HA formation. Therefore, the addition of lignite had practical significance to enhance compost quality.

Hydrothermal pretreatment and compound microbial agents promoting high-quality kitchen waste compost: Superior humification degree and reduction of odour

Present study investigated the effects of hydrothermal pretreatment (HTP) and addition of compound microbial agent (CMA) on humification, odour generation and metabolism functions of bacterial communities during composting of kitchen waste (KW). Surprisingly, HTP and CMA addition treatment could promote the humification of compost and the control of odour units in contrast to the control (without HTP and CMA addition). The humic acid to fulvic acid ratio of end compost increase by 187.30 %, while humification index (HIX) increased by 18.87 %. 3D-EEM fluorescence spectroscopy of dissolved organic matter (DOM) demonstrated that it facilitated the synthesis of humified compounds and the decomposition of biodegradable compounds. Moreover, the SUVA₂₅₄, SUVA₂₈₀ and E₂₅₃/E₂₀₃ increased by 118.6 %, 115.25 % and 42.11 % after HTP and CMA addition indicating an increase in aromatic carbon abundance. VFAs had the higher degradation rate (84.91 %) than other treatments (57.46-77.72 %). Meanwhile, the main contributor to the malodorous odour was isovaleric acid, followed by butyric acid and acetic acid during composting. Mantel test indicated that the humification degree was significantly influenced by environmental parameters (temperature, pH, etc.) and metabolic products (HA, DOC and VFAs). Metagenomic analysis indicated that the biodegradation processes at the thermophilic stage were controlled mainly through genes involved in microbial metabolism. HTP and CMA addition was an eco-friendly and efficient strategy to reduce odour emission and improve the compost quality.

Effects of carbon/nitrogen ratio and aeration rate on the sheep manure composting process and associated gaseous emissions

There are several issues such as low maturity degree of compost product and severe pollution gas emissions during the composting process. Carbon/Nitrogen (C/N) ratio and aeration rate (AR) are the most important factors affecting the composting performance. According to the results of previous studies, the proper C/N ratio and AR were 20-30:1 and 0.1-0.4 L kg^-1 DM·min^-1, respectively. Therefore, a lab-scale experiment was conducted to investigate the effects of C/N ratio and AR on sheep manure composting process and associated gaseous emissions. The initial C/N ratio in this experiment were set at 23, 26 and 29 to simulate the C/N ratio at low, medium and high levels. The AR were decided at 0.12, 0.24 and 0.36 L kg^-1 DM·min^-1 to simulate the aeration at low, middle and high levels. The results showed that as the C/N ratio or AR increased, the methane (CH₄) and hydrogen sulfide (H₂S) emissions decreased. The nitrous oxide (N₂O) emission peaked at the low C/N ratio or AR treatments. The total greenhouse gas (GHG) emissions decreased with the increase of C/N ratio or AR, and the maximum value occurred in the treatment with C/N ratio 23 and AR 0.24 L kg^-1 DM·min^-1. In the treatment with C/N ratio 26 and AR 0.36 L kg^-1 DM·min^-1, the GI value of compost product was the highest (about 250%), and the total greenhouse effect was the lowest (2.36 kg CO2-eq·t^-1 DM). Therefore, considering reduction of pollution gas emissions and improvement of the quality of compost products comprehensively, the optimum conditions were initial C/N ratio 26 and AR 0.36 L kg^-1 DM·min^-1 during the co-composting of sheep manure and cornstalks. In addition, the key physicochemical factors and eight key bacterial communities were determined to regulate compost maturity and pollution gas emissions during the sheep manure composting, which could provide scientific support and theoretical reference for controlling pollution gas emissions and obtaining high quality sheep manure compost products.

In situ generated oxygen distribution causes maturity differentiation during electrolytic oxygen aerobic composting

Electrolytic oxygen aerobic composting (EOAC) is an effective treatment with greater technical superiority and cost advantages for organic solid waste using in situ electrolytic oxygen as a feasible strategy to replace conventional aeration. However, the unclear effects of distribution and variation of in situ electrolytic oxygen on compost maturation in different depth zones of EOAC need further exploration. This study demonstrated that the humification of organic matter was faster at the bottom than in the middle and at the top. The main reason was that the higher oxygen content and lower moisture content in the bottom promoted microbial degradation and heat production, resulting in higher temperatures. The microbial analysis showed that the abundance of typical thermophilic bacteria (such as Cerasibacillus, Lactobacillus, and Pseudogracilibacillus) that could promote compost maturation was higher at the bottom than in the middle and at the top. The finding provided in-depth molecular insights into differentiated humification from bottom to top in EOAC and revealed its further practical engineering applications.

Co-composting of kitchen waste with agriculture and forestry residues and characteristics of compost with different particle size: An industrial scale case study

Since the implementation of domestic waste classification in China, the kitchen waste production has increased rapidly. The unique physical and chemical properties of kitchen waste make it impossible for direct composting for composting alone. This study investigated the co-composting of kitchen waste with agriculture and forest residues at an industrial scale at the Nangong Composting Plant (Located in Beijing). Cornstalks, garden waste, and watermelon seedlings were composted with kitchen waste, with the added agriculture and forestry residues comprising 5%, 10% and 20% of the weight. Industrial composting was performed 30 days at a scale of 165-180 tone. The mixed compost products were screened to different particle sizes, and the maturity, humification, and calorific value were analyzed. The kitchen waste mixed with 20% agricultural complementary materials reached hyperthermophilic temperature (82 °C), had reduced moisture content (45%), and resulted in better composting performance at an industrial scale. By adding 20% complementary materials to kitchen waste produced mature compost with a higher germination index (GI) (91%) by adjusting the pH, electrical conductivity (EC), carbon to nitrogen ratio (C/N), and moisture content. The compost in the 5% and 10% complementary materials treatments did not fully mature and had a GI of<10%, influenced by the higher EC and NH₄⁺-N content. The property of final compost with different particle size vary greatly. The small particle size compost (≤45 mm) had higher uniformity, maturity, and humification degree, and it was suitable to use as a fertilizer; the larger particle size (>45 mm) had more material with lower calorific value (8000-10,000 kJ·kg^-1), and could be used as refuse-derived fuel. To make better use of kitchen waste compost, 45-mm particle size screening is suggested at an industrial-scale composting plant. These results support industrial-scale kitchen waste composting in China.

Production of rice bran oil (Oryza sativa L.) microparticles by spray drying taking advantage of the technological properties of cereal co-products

AIM

To evaluate the effects of the use of rice co-products (flour and protein) on the encapsulation process and on the stability of rice bran oil.

METHODS

The stability of the emulsions was evaluated by dynamic turbidimetry, zeta potential, and creaming index. Efficiency parameters, particle size, and densities were investigated in the particles. The study of oxidative stability was carried out by the determination of peroxides and volatiles (60 °C for 8 weeks).

RESULTS

Rice bran oil presented 1.75% ɣ-oryzanol. AG/RP (10% of rice protein): no phase separation after 24 h, higher zeta potential (- 29.09 mV ±0.67), encapsulation efficiency (73.90% ± 0.22) and real density (1.27 g/cm^-3), and smaller particle size (8.27 µm ± 0.13). Lower peroxide (AG/RF/RP) and hexanal (AG/RF) levels were associated with the use of rice flour.

CONCLUSION

The co-products improve the emulsion characteristics, particle properties and stability of the encapsulated oil.

Humic Substances Derived From Biomass Waste During Aerobic Composting and Hydrothermal Treatment: A Review

Humic substances (HSs) occupy 80% of organic matter in soil and have been widely applied for soil remediation agents, potential battery materials, and adsorbents. Since the HS extraction rate is very low by microbial degradation in nature, artificial humification processes such as aerobic composting (AC) and hydrothermal treatment (HT) have attracted a great deal of attention as the most important strategies in HS production. This article aims to provide a state-of-the-art review on the development of conversion of biomass waste into HSs based on AC and HT for the first time in terms of mechanisms, characteristics of HSs’ molecular structure, and influencing factors. In addition, some differences based on the aforementioned information between AC and HT are reviewed and discussed in the conversion of biomass waste into HSs in a pioneering way. For biomass waste conversion, a feasible strategy on effective humification processes by combining AC with HT is proposed.

Effect of thermophilic microbial agents on nitrogen transformation, nitrogen functional genes, and bacterial communities during bean dregs composting

This study explored how a thermophilic microbial agent altered nitrogen transformation, nitrogen functional genes, and bacterial communities during bean dregs composting with (T) and without (CK) a thermophilic microbial agent for 15 days. The results showed that the maximum temperature in T reached 73 °C and remained above 70 °C for 8 days, while that in CK was only 65 °C. The pH in T had essentially stabilized on day 7, while that in CK was still increasing. On day 15, the seed germination index (GI) of T (95%) reached maturity (defined by GI ≥ 85%), while the GI of CK was only 36%. The concentrations of total nitrogen, water-soluble nitrogen, ammonia nitrogen, and nitrate nitrogen in T (2.5%, 18.9 g/kg, 8.75 g/kg, and 1.69 g/kg) were all lower than those in CK (3.6%, 28.9 g/kg, 12.75 g/kg, and 6.82 g/kg). During composting, Bacillus played a major role in nitrogen reduction, nitrogen mineralization, denitrification, and the conversion between nitrite and nitrate. Weissella played a major role in nitrogen assimilation. Komagataeibacter and Bacillus played a major role in nitrogen fixation in CK and T, respectively. Nitrification was not observed during composting. The nosZ gene, which converts nitrous oxide to nitrogen, was found only in T. Network analysis suggested that the average number of neighbours in T was 3.30% higher than that in CK and the characteristic path length in T was 14.15% higher than that in CK. Therefore, the thermophilic microbial agents could cause nitrogen loss but promote the maturity of bean dregs, which have great potential application.

In-situ electrolytic oxygen is a feasible replacement for conventional aeration during aerobic composting

Aerobic composting is an effective recycling method for the disposal and resource utilization of organic solid waste. However, the inappropriate aeration mode used during conventional aerobic composting (CAC) often results in low oxygen utilization efficiency and loss of temperature, which further leads to a long maturation period and large odorous gas (NH₃) pollution. Herein, a novel electrolytic oxygen aerobic composting (EOAC) process was invented first using in-situ oxygen generation for aeration by the electrolysis of water in compost. Our results demonstrated that the germination index (GI) significantly increased during EOAC, and the maturation time of compost was shortened by nearly 50% during EOAC compared to CAC, indicating higher oxygen utilization efficiency during EOAC. Meanwhile, NH₃ emissions, N₂O emissions, and nitrogen loss during the EOAC process decreased by 61%, 46%, and 21%, respectively, compared to CAC. The total relative abundance of thermophilic and electroactive bacteria during EOAC increased remarkably. EOAC inhibited ammoniation, nitrification, and denitrification, and weakened N-associated functional genes. A techno-economic analysis indicated that EOAC had greater technical superiority and cost advantages compared to CAC. This study represents proof-of-principle for EOAC and suggests that in-situ electrolytic oxygen is a feasible replacement for conventional aeration during aerobic composting.

Moisture migration driven by the electric field causes the directional differentiation of compost maturity

Electric field-assisted aerobic composting (EAC) has been recently believed as a novel and effective process for the resource utilization of organic solid waste. However, the effect of electric field in composting process needs to be further clarified. Herein, moisture migration and compost maturity along electric-field-direction (from anode to cathode) in EAC was first to be explored. It was found that moisture content and compost maturity changed regularly from anode to cathode. At the end of composting, the moisture content of S3 (cathodic zone) was 30% and 62% higher than that of S2 (middle zone) and S1 (anodic zone), respectively. The germination index (a key parameter for compost maturity) in S3 (138.92%) was significantly higher than that of S2 (104.98%) and S1 (84.45%). However, temperatures in S3 were lower than that of S1 and S2, indicating the moisture content played a more important role than temperature for compost maturity in EAC. Furthermore, the microbial activities in S3 were also higher than that of S1 and S2, supporting the trend of compost maturity. This pioneering study demonstrates the electric field can drive moisture gradient migration to control the directional differentiation of compost maturity, showing a great application potential in aerobic composting.

Effect of hydrothermal treatment on organic matter degradation, phytotoxicity, and microbial communities in model food waste composting

Hydrothermal treatment (HTT) as a pretreatment method for compost raw material has multiple benefits such as enhanced solubility of organic material, improved bioaugmentation, and reduced biohazard by killing harmful microorganisms. In this study, we pretreated food waste via HTT at 180 °C for 30 min to investigate its effect on food waste composting. HTT generated 8.98 mg/g-dry solid (g-ds) of 5-hydroxymethylfurfural and 4.32 mg/g-ds furfural. These furan compounds were completely decomposed in the early stage of composting, subsequently the organic matter in the food waste started to be degraded. The HTT-pretreated experiment demonstrated less organic matter degradation during composting as well as lower compost phytotoxicity compared to the non-HTT-pretreated experiment, where the conversion of carbon was 25.2% and the germination index value was 55%. HTT probably denatured part of the organic matter and making it more difficult to decompose, thereby preventing the rapid release of high concentrations of phytotoxic compounds such as organic acids and ammonium ions during composting. High-throughput microbial community analysis revealed that only Firmicutes appeared in the HTT-pretreated experiment, however, other bacterial groups also appeared in the non-HTT-pretreated experiment. This was possibly influenced by furan compounds and the changes of easily degradable organic matter to hardly degradable. Bacillus and Lysinibacillus were dominant in both composting experiments during vigorous organic matter degradation, suggesting that these bacterial groups were the main contributors to food waste composting. This study suggests that HTT is advantageous for the pretreatment of easily degradable food waste, as compost with less phytotoxicity was produced.

Phosphorus recovery and recycling from model animal wastewaters using materials prepared from rice straw and corn cobs

Anthropogenic loss of phosphorus to surface waters not only causes environmental problems but depletes valuable phosphorus reserves. In this study, magnesium amended biochars and magnesium silicate, synthesized from corn cobs and rice straw, respectively, were evaluated for phosphorus uptake including the effects of pH and alkalinity. The overall goal was to close the phosphorus loop by recovering phosphorus from animal waste and reusing it as fertilizer. After phosphorus uptake, spent materials were tested for phosphorus release using modified soil tests representing different soil pH and alkalinity conditions. In experiments using model animal wastewaters containing both ammonia and bicarbonate alkalinity, dissolved phosphorus was removed by struvite (MgNH₄PO₄·6H₂O) formation, whereas in deionized water, dissolved phosphorus was removed by adsorption. Alkalinity in the model animal wastewaters competed with phosphate for dissolved or solid-associated magnesium, thereby reducing phosphorus uptake. Spent materials released significant phosphorus in waters with bicarbonate alkalinity. This work shows that abundant agricultural wastes can be used to synthesize solids for phosphorus uptake, with the spent materials having potential application as fertilizers.

Rice Husk Compost Production and Use in Mitigating Ammonia Volatilization from Urea

Using value-added products such as compost in farming systems could enable optimization of nitrogen (N) fertilizers whose world-wide demand is on the increase. The objectives of this study were to: (i) produce compost through co-composting rice husk (RH) with chicken dung slurry (CDS), chicken feed, and molasses, (ii) determine the effects of optimum rate of urea and RH compost on minimizing ammonia (NH3) volatilization, and (iii) determine total N, exchangeable ammonium (NH4+), and available nitrate (NO3−) retained in soil following co-application of urea and RH compost. Compost was produced for 60 days by mixing RH, CDS, chicken feed, and molasses at a ratio of 20:1:1:1. The color of RH compost was dark brown and had significant amounts of major nutrients such as N (1.15%), phosphorus (3101 mg kg−1), potassium (2038 mg kg−1), calcium (863 mg kg−1), magnesium (276 mg kg−1), organic matter (OM) (60.67%), organic carbon (35.17%), and humic acids (5.87%). The C/N ratio of the RH compost was 30. The electrical conductivity and pH of the RH compost were 2.79 µS cm−1 and 6.55, respectively, and they were not phytotoxic because paddy seeds were successfully germinated in all of the RH compost extractants. The high cation exchange capacity (CEC) of the RH compost (100.67 cmolc kg−1) at the end of composting was one of the determinant factors that controlled NH3 loss from urea. The effectiveness of the RH compost in minimizing urea-N loss was determined using a close-dynamic air flow system. The RH compost significantly minimized NH3 volatilization because of the high affinity of the RH compost for NH4+. An attestation of this reaction was that the high negative charges due to high CEC and OM of the RH compost temporarily protected NH4+ from being transformed into NH3 gas. Further evidence is the higher soil total N and exchangeable NH4+ for the treatments with RH compost than with urea alone. High quality compost can be produced from RH to reduce urea-N from being lost from urea. For the optimum rate, co-application of 60 g RH compost and 2.9 g urea per trough is recommended to mitigate NH3 volatilization instead of the existing practice (7.3 g urea alone per trough).

Full-scale thermophilic aerobic co-composting of blue-green algae sludge with livestock faeces and straw

A high incidence of harmful algal bloom in eutrophic surface waters causes many environmental problems. Thermophilic aerobic composting enables effective treatment and disposal of algal sludge that remains after the dewatering of algae slurries, and provides a value-added organic fertiliser. Previous studies have either only dealt with the composting of a single waste component or were conducted at a lab-/pilot-scale; however, this work is a comprehensive assessment of full-scale mechanized thermophilic aerobic co-composting of algal sludge and other typical biomass-based wastes, including chicken faeces and rice straw, in a water-rich rural area in the Tai lake basin, China. With the optimised feedstock material mass ratio (6.0:1.8:1.0 for straw:algae:faeces; initial C/N ratio of 20; and initial moisture of 60 wt%), the co-composting process effectively achieved the reduction, harmlessness, and reuse of waste. The moisture content (28.36 wt% of wet weight), organic matter content (57.91 wt% of dried weight), total nutrient content (6.59 wt% for TN + TP + TK of dried weight), and heavy metal contents as well as the pH of the final product fully met the Chinese National Agricultural Organic Fertiliser Standard requirements. The reduction rates of microcystin and toxic volatile fatty acid contents were higher than 99.5%, and the seed germination index of the product was 114.5%. A notable economic benefit with a gross profit margin of 167-434% of the process was highlighted. Investigation of the associated mechanisms, including statistical analysis, spectral characterisation, micro-morphological observation, and microbial community analysis, revealed that a decreased particle sizes with a looser structure and an efficient humification effect, resulting from the work of several identified dominant microbial species, contributed to the high product quality. The current study provided a demonstration of the promising full-scale co-composting technology for comprehensive management of the environment in water-rich rural areas and the construction of a sustainable watershed.

Evaluating the Aqueous Phase From Hydrothermal Carbonization of Cow Manure Digestate as Possible Fertilizer Solution for Plant Growth

Improving the agronomic use of recycled nutrients derived from organic waste is one of the priorities within the measures adopted by the European community to reduce environmental issues but remains an unexplored area of research. This study focused on investigating the possibility of using innovative fertilizer solutions in hydroponic systems for the growth of agricultural plants. To this purpose, a liquid fraction [aqueous hydrothermal carbonization (HTC) liquid (AHL)] derived from HTC of cow manure digestate was chemically characterized (pH, electrical conductivity, mineral elements, and organic compounds such as phytotoxins), diluted with distilled water (1:30, 1:60, and 1:90, v/v) to reduce its potential phytotoxicity, and used to grow hydroponic maize (Zea mays L.) plants instead of the classical full-strength nutrient solution. The results indicated that the dilution ratio 1:30 of the AHL solution maintained a high level of toxicity for the plants (phytotoxic substances, especially Na and alkalinity), inducing the arrest of their growth. Differently, the two other dilution ratios (i.e., 1:60 and 1:90) seemed to considerably limit the levels of toxicity, since they allowed the plants to develop. However, these dilution ratios were poor in nutrient elements, inducing alteration in photosynthesis and an onset of deficiency symptoms such as pronounced leaf chlorosis. In view of an eco-friendly approach, future studies are, therefore, needed to identify the correct species-specific dilution ratio to supply both low levels of phytotoxins and adequate content of essential nutrients for appropriate plant growth and development. Furthermore, in order to lower specific Na phytotoxicity, treatments are of utmost importance before using AHL as a fertilizer solution.

Lignite Improved the Quality of Composted Manure and Mitigated Emissions of Ammonia and Greenhouse Gases during Forced Aeration Composting

Lignite amendment of livestock manure is considered a viable ammonia (NH3) emission mitigation technique. However, its impact on the subsequent composting of the manure has not been well studied. This work compared changes in biochemical parameters (e.g., organic matter loss and nitrogen (N) transformation) and also the emissions of NH3 and greenhouse gases (GHGs) between lignite-amended and unamended cattle manure during forced aeration composting. Amending manure with lignite did not alter the time to compost stability despite delaying the onset of the thermophilic temperatures. Lignite treatments retained N in the manure by suppressing NH3 loss by 35–54%, resulting in lignite-amended manure composts having 10–19% more total N than the unamended compost. Relative to manure only, lignites reduced GHG emissions over the composting period: nitrous oxide (N2O) (58–72%), carbon dioxide (CO2) (12–23%) and methane (CH4) (52–59%). Low levels of CH4 and N2O emissions were observed and this was attributed to the continuous forced aeration system used in the composting. Lignite addition also improved the germination index of the final compost: 90–113% compared to 71% for manure only. These findings suggest that lignite amendment of manure has the potential to improve the quality of the final compost whilst mitigating the environmental release of NH3 and GHGs.

Use of an in situ thermoelectric generator for electric field-assisted aerobic composting

Electric field-assisted aerobic composting (EAAC) is a simple and effective process. To further improve the EAAC process and make good use of waste heat during composting, in this study, we designed an in situ thermoelectric generator using thermoelectric sheets and applied it for EAAC. The findings show that the voltage generated was 8.8-18.6 V, and the maximum power was over 7 W. A direct current-to-direct current (DC-DC) voltage converter was used to stabilize the output at 6.0 V. Self-powered EAAC (sp-EAAC) enhanced compost maturity compared to conventional aerobic composting (CAC). The germination index reached 118% and 88% in sp-EAAC and CAC, respectively, at the end of composting. This work verified that the temperature gap between compost and the environment could be used for the EAAC process, opening a new way to recover waste heat during aerobic composting and accelerate compost maturity.

Evolution of phytotoxicity during the active phase of co-composting of chicken manure, tobacco powder and mushroom substrate

This study systematically investigated the phytotoxicity of chicken manure co-composted with tobacco powder and mushroom substrate on seed germination during active phase of composting. All compost products met the sanitation requirements specified in the Chinese national standard; however, only the mushroom substrate compost satisfied the maturity standard. From day 28, the composting entered the end of active phase and the concentrations of K+, Zn2+, Na+, Cu2+ and Fe3+ decreased gradually. Redundancy analysis indicated that the germination index, catalase and peroxidase activities was positively correlated with K+, Zn2+, Na+, Cu2+, Fe3+ and NO3--N, and negatively correlated with NH4+-N, Mg2+ and Ca2+, among which the most significant ions were Fe3+, Mg2+ and Zn2+ for all treatments. The malondialdehyde concentration of germinated seeds had adverse correlation with the above ions parameters.

Lignocellulosic crop residue composting by cellulolytic nitrogen-fixing bacteria: A novel tool for environmental sustainability

Lignocellulosic crop residue (LCCR) composting is a cost-effective and sustainable approach for addressing environmental pollution associated with open biomass burning and application of chemical fertilizers in agriculture. The value-added bio-product of the composting process contributes to the improvement of the soil properties and plant growth in an environment-friendly way. However, the conventional process employed for composting LCCRs is slow and becomes an impediment for farmers who plant two or three crops a year. This concern has led to the development of different techniques for rapid composting of LCCRs. The use of cellulolytic nitrogen-fixing microorganisms for composting has emerged as a promising method for enhancing LCCR composting and quality of the compost. Therefore, this review addresses the recent progress on the potential use of cellulolytic nitrogen-fixing bacteria (CNFB) for LCCR composting and discusses various applications of nutrient-rich compost for sustainable agriculture to increase crop yields in a nature-friendly way. This knowledge of bacteria with both cellulose-degrading and nitrogen-fixing activities is significant with respect to rapid composting, soil fertility, plant growth and sustainable management of the lignocellulosic agricultural waste and it provides a means for the development of new technology for sustainability.

Bioelectrochemically-assisted vermibiofilter process enhancing stabilization of sewage sludge with synchronous electricity generation

Bioelectrochemically-assisted vermifilter (VBF_BE) with sewage sludge as the anode fuel was constructed to accelerate composting of sewage sludge, which could increase the quality of the compost and harvest electric energy in comparison with vermicomposting and electrochemical only. Results revealed that the sludge stabilization with a higher soluble chemical oxygen demand (SCOD) and lower NH₄⁺-H during 40 days of composting. At the composting, pH, C/N, electrical conductivity (EC) and germination index (GI) results demonstrated that the maturity degree of VBF_BE4 was higher than that of other VBF_BE. The VBF_BE4 yielded a voltage of 1.024 V and maximum power density of 105.28 mW/m² on 3th day. The bacteria in VBF_BE4 were richer and higher in terms of diversity than those in other VBF_BE, that was demonstrated that combination vermicomposting and electrochemistry could improve the sludge stabilization degree, accelerate sludge composting process and enhance composting maturity.

Pilot-scale composting of typical multiple agricultural wastes: Parameter optimization and mechanisms

In this work, pilot-scale (100 kg of mixed wastes each time) composting of typical agricultural wastes, including chicken manure, vegetable leaves and rice husks with a mass ratio of 6:3:1, was studied. Effects of thermal phases and transformation time on performance, including moisture, nutrient, and carbon contents and C/N ratios of compost, were investigated. The optimal parameters were 75 ± 5 °C and 18 h; the compost met the requirements of Chinese National Agricultural Organic Fertilizer Standard (NY525-2012). Mechanisms investigations demonstrated that, Bacillus and Sinibacillus played key roles in degrading high-molecular-weighted organic substances into small-molecular-weighted humic- and fulvic-acid-like matters, resulting in smaller particle size and loose structure of the product; rice husk particles acted as a conditioning agent and remained their originally morphology. The mechanism provided informative guidance for optimizing the process in practical application.

Electric field induces electron flow to simultaneously enhance the maturity of aerobic composting and mitigate greenhouse gas emissions

The long maturation period and greenhouse gas (GHG) emission are two major problems that arise during aerobic composting, mainly due to the low efficiency of O₂ transmission and utilization. In this study, a novel electric-field-assisted aerobic composting (EAC) process was tested by simply applying a direct-current voltage of 2 V to a conventional aerobic composting (CAC) process. Compared with the CAC process, the maturation time and the total GHG for the EAC process were reduced by 33% and 70%, respectively. Furthermore, the analyses of O₂ consumption and microbial communities demonstrated that the electric field had enhanced O₂ utilization by 30 ± 9% and increased the relative abundance of electroactive bacteria by about 3.4-fold compared to CAC. This work has represented a proof of principle for EAC and suggests that the electric field is an effective and environmentally friendly strategy for enhancing compost maturity and mitigating GHG emissions during aerobic composting.

Multivariate relationships between microbial communities and environmental variables during co-composting of sewage sludge and agricultural waste in the presence of PVP-AgNPs

This study evaluated the contributions of environmental variables to the variations in bacterial 16S rDNA, nitrifying and denitrifying genes abundances during composting in the presence of polyvinylpyrrolidone coated silver nanoparticles (PVP-AgNPs). Manual forward selection in redundancy analysis (RDA) indicated that the variation in 16S rDNA was significantly explained by NO₃^--N, while nitrifying genes were significantly related with pH, and denitrifying genes were driven by NO₃^--N and TN. Partial RDA further revealed that NO₃^--N solely explained 28.8% of the variation in 16S rDNA abundance, and pH accounted for 61.8% of the variation in nitrifying genes. NO₃^--N and TN accounted for 34.2% and 9.2% of denitrifying genes variation, respectively. The RDA triplots showed that different genes shared different relationships with environmental parameters. Based on these findings, a composting with high efficiency and quality may be conducted in the future work by adjusting the significant environmental variables.

Abundance of ammonia-oxidizing bacteria and archaea under different ventilation strategies during cattle manure composting

Composting of cattle manure was conducted under four ventilation strategies, i.e., no-aeration (A-00), continuous aeration (B-44), non-aeration for 14 d and then aeration for 42 d (C-04), aeration for 14 d and then no-aeration for 42 d (D-40). Physicochemical parameters and potential ammonia oxidation (PAO) indicated that continuous and intermittent ventilation provide favourable conditions for ammonia-oxidizing bacteria (AOB) and archaea (AOA) to oxidize ammonia. Quantitative PCR (qPCR) analysis showed AOB amoA gene abundance of all treatments on every sampling day ranged from 2.25 × 10⁵ to 2.76 × 10⁹copies/g, was significantly lower than that of archaeal amoA gene from 2.71 × 10⁸ to 9.05 × 10¹¹copies/g. There was also a significantly positive relationship between PAO rates and AOB (r² ≥ 0.066, p < 0.05) and AOA (r² ≥ 0.300, p < 0.05) abundance. These data suggested that ammonia oxidation is driven by both AOA and AOB in cattle manure composting.

Role of psychrotrophic bacteria in organic domestic waste composting in cold regions of China

To study the influence of psychrotrophic bacteria on organic domestic waste (ODW) composting in cold regions, twelve new efficient psychrotrophic composting strains were isolated. Together with the published representative composting strains, a phylogenetic tree was constructed showing that although the strains belong to the same phylum, the genera were markedly different. The twelve strains were inoculated into the ODW in the composting reactor at 13°C. After treatment, the indices of temperature, moisture content, pH, electrical conductivity, C/N, ammonium nitrogen, and nitrate nitrogen indicated that the compost had reached maturity. The thermophilic phase was reached at 17d, and composting was completed at 42d, a markedly shorter composting time than that in previous studies. High-throughput sequencing indicated that the inoculative strains became the dominant community during the mesophilic phase and that they enhanced the stability of the microbial community structure. Thus, psychrotrophic bacteria played a key role in low-temperature composting.

Composting technology in waste stabilization: On the methods, challenges and future prospects

Composting technology has become invaluable in stabilization of municipal waste due to its environmental compatibility. In this review, different types of composting methods reportedly applied in waste management were explored. Further to that, the major factors such as temperature, pH, C/N ratio, moisture, particle size that have been considered relevant in the monitoring of the composting process were elucidated. Relevant strategies to improve and optimize process effectiveness were also addressed. However, during composting, some challenges such as leachate generation, gas emission and lack of uniformity in assessing maturity indices are imminent. Here in, these challenges were properly addressed and some strategies towards ameliorating them were proffered. Finally, we highlighted some recent technologies that could improve composting.

Insights into mechanisms of hexavalent chromium removal from aqueous solution by using rice husk pretreated using hydrothermal carbonization technology

Hydrothermal carbonization technique enhanced the adsorption capacity of rice husk towards Cr(vi).

Rapid production of organic fertilizer by dynamic high-temperature aerobic fermentation (DHAF) of food waste

Keep composting matrix in continuous collision and friction under a relatively high-temperature can significantly accelerate the progress of composting. A bioreactor was designed according to the novel process. Using this technology, organic fertilizer could be produced within 96h. The electric conductivity (EC) and pH value reached to a stable value of 2.35mS/cm and 7.7 after 96h of fermentation. The total carbon/total nitrogen (TC/TN) and dissolved carbon/dissolved nitrogen (DC/DN) ratio was decrease from 27.3 and 36.2 to 17.4 and 7.6 respectively. In contrast, it needed 24days to achieve the similar result in traditional static composting (TSC). Compost particles with different size were analyzed to explore the rapid degradation mechanism of food waste. The evidence of anaerobic fermentation was firstly discovered in aerobic composting.

Bioelectrochemically-assisted anaerobic composting process enhancing compost maturity of dewatered sludge with synchronous electricity generation

Bioelectrochemically-assisted anaerobic composting process (AnCBE) with dewatered sludge as the anode fuel was constructed to accelerate composting of dewatered sludge, which could increase the quality of the compost and harvest electric energy in comparison with the traditional anaerobic composting (AnC). Results revealed that the AnCBE yielded a voltage of 0.60 ± 0.02 V, and total COD (TCOD) removal reached 19.8 ± 0.2% at the end of 35 d. The maximum power density was 5.6 W/m(3). At the end of composting, organic matter content (OM) reduction rate increased to 19.5 ± 0.2% in AnCBE and to 12.9 ± 0.1% in AnC. The fuzzy comprehensive assessment (FCA) result indicated that the membership degree of class I of AnCBE compost (0.64) was higher than that of AnC compost (0.44). It was demonstrated that electrogenesis in the AnCBE could improve the sludge stabilization degree, accelerate anaerobic composting process and enhance composting maturity with bioelectricity generation.

Study of the rice straw biodegradation in mixed culture of Trichoderma viride and Aspergillus niger by GC-MS and FTIR

This study was conducted to investigate the biodegradation ability of the mixed culture of Trichoderma viride and Aspergillus niger through the study of the organic matter extracted from rice straw and the lignocellulose structure by using gas chromatography-mass spectrometer (GC-MS) and Fourier transform infrared spectroscopy (FTIR). The results of the GC-MS showed that the mixed culture possessed shorter alkane (heptane) at the end of the incubation and more kinds of organic matter (except the alkanes, 29 kinds of organic matter were detected) than the pure cultures. It could be deduced that the organic matter could indicate the degradation degree of the lignocellulose to some extent. Moreover, pinene was detected in the mixed culture on days 5 and 10, which might represent the antagonistic relationship between T. viride and A. niger. The analysis of FTIR spectrums which indirectly verified the GC-MS results showed that the mixed culture possessed a better degradation of rice straw compared with the pure culture. Therefore, the methods used in this research could be considered as effective ones to investigate the lignocellulose degradation mechanism in mixed culture.

Struvite for composting of agricultural wastes with termite mound: Utilizing the unutilized

Although, compost is the store house of different plant nutrients, there is a concern for low amount of major nutrients especially nitrogen content in prepared compost. The present study deals with preparation of compost by using agricultural wastes with struvite (MgNH4PO4·6H2O) along with termite mound. Among four composting mixtures, 50kg termite mound and 2.5kg struvite with crop residues (stover of ground nut: 361.65kg; soybean: 354.59kg; potato: 357.67kg and mustard: 373.19kg) and cow dung (84.90kg) formed a good quality compost within 70days of composting having nitrogen, phosphorus and potassium as 21.59, 3.98 and 34.6gkg(-1), respectively. Multivariate analysis of variance revealed significant differences among the composts. The four composts formed two (pit 1, pit 2 and pit 3, pit 4) different groups. Two principal components expressed more than 97% of the total variability. Hierarchical cluster analysis resulted two homogeneous groups of composts.

Composting of food waste subjected to hydrothermal pretreatment and inoculated with Paecilomyces sp. FA13

Food waste collected from restaurants, convenience stores, and food-processing factories was mixed with sawdust and subjected to hydrothermal pretreatment at 180°C for 30min to prepare compost raw material. Furan compounds such as 5-HMF (5-hydroxymethyl furfural) and furfural were produced at concentration levels of approximately 8 and 0.5mg/g-ds, respectively, through hydrothermal pretreatment. The furan compounds inhibited the activity of composting microorganisms, thus delaying the start of organic matter degradation during composting. A newly identified fungus, Paecilomyces sp. FA13, which possesses the ability to degrade furan compounds, was isolated and used as an inoculum for the composting of the raw material prepared by hydrothermal pretreatment. By inoculating the FA13 into the compost raw material at 10(5)CFU/g-ds, the degradation of furan compounds was accelerated. As a result, bacterial activity, which contributed to composting, was enhanced, significantly promoting the start of vigorous degradation of organic materials.

Review of the rice production cycle: by-products and the main applications focusing on rice husk combustion and ash recycling

One of the consequences of industrial food production activities is the generation of high volumes of waste, whose disposal can be problematic, since it occupies large spaces, and when poorly managed can pose environmental and health risks for the population. The rice industry is an important activity and generates large quantities of waste. The main solid wastes generated in the rice production cycle include straw, husk, ash, bran and broken rice. As such, the aim of this article is to present a review of this cycle, the waste generated and the identification of opportunities to use them. Owing to impacts that can be minimised with the application of rice husk ash as a by-product, this work is focused on the recycling of the main wastes. In order to achieve that, we performed theoretical research about the rice production cycle and its wastes. The findings point to the existence of an environmentally suitable use for all wastes from the rice production cycle. As rice, bran and broken rice have their main use in the food industry, the other wastes are highly studied in order to find solutions instead of landfilling. Straw can be used for burning or animal feeding. The husk can be used for poultry farming, composting or burning. In the case of burning, it has been used as biomass to power reactors to generate thermal or electrical energy. This process generates rice husk ash, which shows potential to be used as a by-product in many different applications, but not yet consolidated.

Enhanced hydrogen production from anaerobic fermentation of rice straw pretreated by hydrothermal technology

This study tested the effect of hydrothermal treatment (HTT) at different peak temperatures (150 °C and 210 °C, i.e. HTT150 and HTT210, respectively) and holding time (0-30 min) on the solubilization of rice straw at total solids (TS) of 20% and then subsequent H2 production from resultant substrates. No obvious degradation was detected in lignin content under all tested HTT conditions which did open up the surface structure and have efficient solubilization effect on rice straw. Soluble carbohydrates produced from straw particles during HTT210 was found to have strongly (r=0.9987) positive correlation with the subsequent H2 yield. The maximum soluble carbohydrates, 80 mg per gram of volatile solids (VS) was achieved under HTT210 and 0 min of holding condition, correspondingly yielding the highest hydrogen production (28 ml/g-VS), about 93-fold higher than the control. Results suggest that holding time is crucial for HTT pretreatment when taking subsequent H2 production into consideration.