Organic matter, a critical factor to immobilize phosphorus, copper, and zinc during composting under various initial C/N ratios

Bridging agro-science and human nutrition: zinc nanoparticles and biochar as catalysts for enhanced crop productivity and biofortification

The integration of zinc nanoparticles (Zn NPs) with biochar offers a transformative approach to sustainable agriculture by enhancing plant productivity and human nutrition. This combination improves soil health, optimizes nutrient uptake, and increases resilience to environmental stressors, leading to superior crop performance. Our literature review shows that combining Zn NPs with biochar significantly boosts the crop nutrient composition, including proteins, vitamins, sugars, and secondary metabolites. This enhancement improves the plant tolerance to environmental challenges, crop quality, and shelf life. This technique addresses the global issue of Zn deficiency by biofortifying food crops with increased Zn levels, such as mung beans, lettuce, tomatoes, wheat, maize, rice, citrus, apples, and microgreens. Additionally, Zn NPs and biochar improve soil properties by enhancing water retention, cation exchange capacity (CEC), and microbial activity, making soils more fertile and productive. The porous structure of biochar facilitates the slow and sustained release of Zn, ensuring its bioavailability over extended periods and reducing the need for frequent fertilizer applications. This synergy promotes sustainable agricultural practices and reduces the environmental footprint of the traditional farming methods. However, potential ecological risks such as biomagnification, nanoparticle accumulation, and toxicity require careful consideration. Comprehensive risk assessments and management strategies are essential to ensure that agricultural benefits do not compromise the environmental or human health. Future research should focus on sustainable practices for deploying Zn NPs in agriculture, balancing food security and ecological integrity and positioning this approach as a viable solution for nutrient-efficient and sustainable agriculture.

Effects and Mechanisms of Different Types of Biochar on Heavy Metal Passivation during Sludge Composting

The effects and mechanisms of the different types of biochar on heavy metal passivation are still not fully understood. This study compared the effects of three types of biochar on heavy metal passivation during sludge composting. Compared with composting without biochar, rice husk biochar was most effective for the passivation of Zn and Pb, with increased passivation rates of 1.90% and 20.43%, respectively. In contrast, sludge biochar was the most effective for the passivation of Cr and Hg, with increased passivation rates of 28.30% and 3.09%, respectively. Coconut shell biochar showed the best performance for the passivation of Cu, Ni, As, and Cd, and was enriched with micropore structures, which possibly led to the adsorption and reaction of heavy metals, organic matter, and microorganisms. The improved passivation effect of the rice husk and sludge biochar on heavy metals can be attributed to the improved humification of organic matter. This study suggests that specific types of biochar should be considered for the passivation of different types of heavy metals for practical applications.

Prediction models for bioavailability of Cu and Zn during composting: Insights into machine learning

Bioavailability assessment of heavy metals in compost products is crucial for evaluating associated environmental risks. However, existing experimental methods are time-consuming and inefficient. The machine learning (ML) method has demonstrated excellent performance in predicting heavy metal fractions. In this study, based on the conventional physicochemical properties of 260 compost samples, including compost time, temperature, electrical conductivity (EC), pH, organic matter (OM), total phosphorus (TP), total nitrogen, and total heavy metal contents, back propagation neural network, gradient boosting regression, and random forest (RF) models were used to predict the dynamic changes in bioavailable fractions of Cu and Zn during composting. All three models could be used for effective prediction of the variation trend in bioavailable fractions of Cu and Zn; the RF model showed the best prediction performance, with the prediction level higher than that reported in related studies. Although the key factors affecting changes among fractions were different, OM, EC, and TP were important for the accurate prediction of bioavailable fractions of Cu and Zn. This study provides simple and efficient ML models for predicting bioavailable fractions of Cu and Zn during composting, and offers a rapid evaluation method for the safe application of compost products.

Geochemical fractionation of iron in paper industry and municipal landfill soils: Ecological and health risks insights

Industrial processes and municipal wastes largely contribute to the fluctuations in iron (Fe) content in soils. Fe, when present in unfavorable amount, causes harmful effects on human, flora, and fauna. The present study is an attempt to evaluate the composition of Fe in surface soils from paper mill and municipal landfill sites and assess their potential ecological and human health risks. Geochemical fractionation was conducted to explore the chemical bonding of Fe across different fractions, i.e., water-soluble (F1) to residual (F6). Different contamination factors and pollution indices were evaluated to comprehend Fe contamination extent across the study area. Results indicated the preference for less mobile forms in the paper mill and landfill, with 26.66% and 43.46% of Fe associated with the Fe-Mn oxide bound fraction (F4), and 57.22% and 24.78% in the residual fraction (F6). Maximum mobility factor (MF) of 30.65% was observed in the paper mill, and 80.37% in the landfill. The enrichment factor (EF) varied within the range of 20 < EF < 40, signifying a high level of enrichment in the soil. The individual contamination factor (ICF) ranged from 0 to >6, highlighting low to high contamination. Adults were found to be more vulnerable towards Fe associated health risks compared to children. The Hazard Quotient (HQ) index showed the highest risk potential pathways as dermal contact > ingestion > inhalation. The study offers insights into potential Fe contamination risks in comparable environments, underscoring the crucial role of thorough soil assessments in shaping land use and waste management policies.

Insight into the humification and carbon balance of biogas residual biochar amended co-composting of hog slurry and wheat straw

The impact of biogas residual biochar (BRB) on the humification and carbon balance process of co-composting of hog slurry (HGS) and wheat straw (WTS) was examined. The 50-day humification process was significantly enhanced by the addition of BRB, particular of 5% BRB, as indicated by the relatively higher humic acid content (67.28 g/kg) and humification ratio (2.31) than other treatments. The carbon balance calculation indicated that although BRB addition increased 22.16-46.77% of C lost in form of CO2-C, but the 5% BRB treatment showed relatively higher C fixation and lower C loss than other treatments. In addition, the BRB addition reshaped the bacterial community structure during composting, resulting in increased abundances of Proteobacteria (25.50%) during the thermophilic phase and Bacteroidetes (33.55%) during the maturation phase. Combined these results with biological mechanism analysis, 5% of BRB was likely an optimal addition for promoting compost humification and carbon fixation in practice.

Phytoremediation capacity of hybrid aspen at sites affected by industry and agriculture

Fast-growing Populus spp. are well-acknowledged to restore contaminated soils from heavy metals in industrial areas. Thus far, there is no knowledge about the phytoremediation capacity of Populus spp. plantations in hemiboreal Estonia conditions to restore industrially polluted areas. The objective of this study was to assess the soil contamination rate of heavy metals (As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb and Zn) and their uptake by mature hybrid aspen (Populus tremula × Populus tremuloides Michx.) in plantations in different industrial pollution areas (e.g. cement factory, oil shale mining). For the reference, industrially polluted plantations were compared with the low pollution area hybrid aspen plantation on former agricultural soil, which was influenced by fertilization and liming before afforestation. Twenty-one years after afforestation, soil samples were collected from the 0-10 cm topsoil layer. Aboveground biomass sampling was performed for bark and stem wood by ingrowth cores to separate wood formed during early (1-10 years) and late (11-21 years) stand development. Two decades after the afforestation of industrially polluted areas, the heavy metal concentrations in the soil were higher than the reference plantation and the standard reference for unpolluted soils in most cases. The highest concentrations of heavy metals in woody biomass were in the oil shale quarry spoil; because of poor growth, the accumulated pools in aboveground biomass were low. Cd differed from other metals and accumulated less in wood and more in bark. The concentration of heavy metals (Cd, Cr, Cu, Fe, Mn, Ni and Zn) was higher in the first decade of stand formation (1-10 years) than in the last 10 years (11-21 years). High pools of heavy metals were accumulated in aboveground biomass in the reference plantation, indicating the considerable removal of heavy metal residues from the previous fertilization and liming source with harvest. Two decades of afforestation with hybrid aspen is too short for complete ecosystem restoration from heavy metals in industrially polluted areas.

Application of thermally treated sludge residues on an e-waste contaminated soil: effects on PTE bioavailability, soil physicochemical and biological properties, and L. perenne growth

In the context of sustainable development, potentially toxic element (PTE) contamination of soil and large-scale disposal of sludge are two major environmental issues that need to be addressed urgently. It is of great significance to develop efficient and green technologies to solve these problems simultaneously. This study investigated the effects of a 5% addition of thermally treated sludge residues (fermentation and pyrolysis residues) in synergy with L. perenne on soil organic matter, mineral nutrients, PTE speciation, and PTE uptake and transport by L. perenne in an e-waste-contaminated soil through pot experiments. The results showed that the thermally treated sludge residues significantly increased soil electrical conductivity, cation exchange capacity, organic matter, available phosphorus, and exchangeable potassium contents. New PTE-containing crystalline phases were detected, and dissolved humic substances were found. Sludge fermentation residue significantly increased dissolved organic matter content, whereas sludge pyrolysis residue showed no significant effect. The combination of thermally treated sludge residues and L. perenne increased the residual fractions of Cu, Zn, Pb, and Cd. The thermally treated sludge residues promoted L. perenne growth, increasing fresh weight, plant height, and phosphorus and potassium uptake. The uptake of Cu, Zn, Pb, and Cd by L. perenne was significantly reduced. This approach has the potential for applications in the ecological restoration of e-waste-contaminated soils.

Optimization of organic solid waste composting process through iron-related additives: A systematic review

Composting is an environmentally friendly method that facilitates the biodegradation of organic solid waste, ultimately transforming it into stable end-products suitable for various applications. The element iron (Fe) exhibits flexibility in form and valence. The typical Fe-related additives include zero-valent-iron, iron oxides, ferric and ferrous ion salts, which can be targeted to drive composting process through different mechanisms and are of keen interest to academics. Therefore, this review integrated relevant literature from recent years to provide more comprehensive overview about the influence and mechanisms of various Fe-related additives on composting process, including organic components conversion, humus formation and sequestration, changes in biological factors, stability and safety of composting end-products. Meanwhile, it was recommended that further research be conducted on the deep action mechanisms, biochemical pathways, budget balance analysis, products stability and application during organic solid waste composting with Fe-related additives. This review provided guidance for the subsequent targeted application of Fe-related additives in compost, thereby facilitating cost reduction and promoting circular economy objectives.

Occurrence and transformation of heavy metals during swine waste treatment: A full scale study

This study tracked the fate of nine detected heavy metals in an industrial swine farm with integrated waste treatment, including anoxic stabilization, fixed-film anaerobic digestion, anoxic-oxic (A/O), and composting. Results show that heavy metals exhibited different transformation behaviors in the treatment streamline with Fe, Zn, Cu and Mn as the most abundant ones in raw swine waste. The overall removal of water-soluble heavy metals averaged at 30 %, 24 % and 42 % by anoxic stabilization, anaerobic digestion and A/O unit, respectively. In particular, anoxic stabilization could effectively remove Cu, Mn and Ni; while A/O unit was highly effective for Fe, Cr and Zn elimination from water-soluble states. As such, the environmental risk of liquid products for agricultural irrigation decreased gradually to the safe pollution level in swine waste treatment. Furthermore, heavy metals in the solid (slurry) phase of these bioprocesses could be immobilized with the passivation rate in the range of 42-70 %. Nevertheless, heavy metals preferably transformed from liquid to biosolids to remain their environmental risks when biosolids were used as organic fertilizer in agriculture, thereby requiring effective strategies to advance their passivation in all bioprocesses, particularly composting as the last treatment unit.

The various effect of cow manure compost on the degradation of imazethapyr in different soil types

Adding compost to soil is an effective strategy to promote the degradation of organic pollutants and reduce ecological risks. However, the effect of compost on the degradation of imazethapyr (IMET) in different soil types is not clear. To address this issue, a pot experiment was conducted, and high-throughput sequencing and mass spectrometry technology were used to identify the influence of cow manure compost on the degradation efficiency of IMET in black soil and saline-alkali soil and the role of key microorganisms. The results showed that adding compost to black soil increased the degradation rate of IMET by 12.58% and shortened the half-life by 53.37%, while in saline-alkali soil, the degradation rate of IMET decreased by 6.99% with no significant change in the half-life. High-throughput sequencing results showed that adding cow manure compost (mass ratio of 4%) significantly increased the abundance of bacterial families capable of degrading organic pollutants in black soil, but had an inhibitory effect on this bacterial community in saline-alkali soil. Redundancy analysis (RDA) results showed that total organic carbon (TOC), alkali-hydrolyzable nitrogen (AN), ammonia nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N) were the main factors driving microbial community variation. Mass spectrometry analysis indicated that IMET generated three metabolites during the degradation process. Sphingomonadaceae and Vicinamibacteraceae could accelerate the breaking of side-chain alkyl groups, while Chitinophagaceae could cause the rearrangement of the imidazole ring structure, gradually metabolizing IMET into small organic molecules. The application of appropriate cow manure compost can promote the development of IMET-degrading bacteria by adjusting the organic carbon and dissolved nitrogen content in black soil. In the future, the quantitative effects of organic fertilizer application on the IMET degradation process in different soil types should be further analyzed, and microbial isolation and purification should be used to enhance the ability of microorganisms to degrade herbicides.

Improving the humification by additives during composting: A review

Humic substances (HSs) are key indicators of compost maturity and are important for the composting process. The application of additives is generally considered to be an efficient and easy-to-master strategy to promote the humification of composting and quickly caught the interest of researchers. This review summarizes the recent literature on humification promotion by additives in the composting process. Firstly, the organic, inorganic, biological, and compound additives are introduced emphatically, and the effects and mechanisms of various additives on composting humification are systematically discussed. Inorganic, organic, biological, and compound additives can promote 5.58-82.19%, 30.61-50.92%, 2.3-40%, and 28.09-104.51% of humification during composting, respectively. Subsequently, the advantages and disadvantages of various additives in promoting composting humification are discussed and indicated that compound additives are the most promising method in promoting composting humification. Finally, future research on humification promotion is also proposed such as long-term stability, environmental impact, and economic feasibility of additive in the large-scale application of composting. It is aiming to provide a reference for future research and the application of additives in composting.

Simulation, prediction and optimization of typical heavy metals immobilization in swine manure composting by using machine learning models and genetic algorithm

Machine learning (ML) is a novel method of data analysis with potential to overcome limitations of traditional composting experiments. In this study, four ML models (multi-layer perceptron regression, support vector regression, decision tree regression, and gradient boosting regression) were integrated with genetic algorithm to predict and optimize heavy metal immobilization during composting. Gradient boosting regression performed best among the four models for predicting both heavy metal bioavailability variations and immobilization. Gradient boosting regression-based feature importance analysis revealed that the heavy metal initial bioavailability factor, total phosphorus, and composting duration were the determinant factors for heavy metal bioavailability variations (together contributing >75%). After genetic algorithm optimization, the maximum immobilization rates of Cu, Zn, Cd, As, and Cr were 79.53, 31.30, 14.91, 46.25, and 66.27%, respectively, superior to over 90% of the measured data. These findings demonstrate the potential application of ML to risk-control for heavy metals in livestock manure composting.

Dehydrated Food Waste for Composting: An Overview

Food waste disposal has recently received much attention worldwide due to its major impact on environmental pollution and economic costs. Using high moisture content of food waste has the highest negative environmental impact due to increased greenhouse gas emissions, odor, and leachate. Drying technologies play an important role in reducing the moisture content of food waste, which is necessary for environmental sustainability and safety. The first part of this review highlights that sun-drying is the most cost-effective drying method. However, it has not been widely recommended for food waste management due to several limitations, including the inability to control sunray temperature and the inability to control end-product quality. Thermal drying eliminates moisture from food waste quickly, preventing hydrolysis and biodegradation. Thermal dryers, such as the GAIA GC-300 dryer, and cabinet dryer fitted with a standard tray, are the best alternative to sun drying. The second part of this review highlights that dehydrated food waste products are slightly acidic (4.7–5.1), have a high electrical conductivity (EC) value (4.83–7.64 mS cm-1), with high nutrient content, due to low pH levels, dehydrated food waste is not suitable for direct use as a fertilizer for the plants. So, the dried food waste should be composted before application to the plants because the composting process will dominate the limitation of phytotoxins, anoxia, salinity, and water repellence. Trench compost can be a good choice for decomposing dried organic waste because trench compost relies solely on soil decomposing microorganisms and insects.

Effects and microbial mechanisms of phosphogypsum and medical stone on organic matter degradation and methane emissions during swine manure composting

The degradation of organic matter (OM) and CH₄ emissions during composting greatly influence the composting efficiency and greenhouse effect. This study evaluated the effects of adding phosphogypsum (PPG) and medical stone (MS) on OM breakdown, CH₄ emissions, and their underlying mechanisms. MS accelerated the breakdown of OM in the early composting stage, whereas PPG increased it in the cooling and maturation periods. At the ending of composting, humification was also significantly promoted by PPG and MS (P < 0.05). Moreover, MS and PPG reduced CH₄ emissions by 27.64% and 23.12%, respectively, and significantly inhibited the activities of methanogens in terms of their abundance (mcrA) and composition (dominant genera such as Methanobrevibacter, Methanocorpusculum, and Methanothermus) (P < 0.05). Interestingly, MS enhanced the activity of enzymes and bacterial metabolism related to OM degradation in the early composting stage, whereas PPG promoted them during the cooling and maturity stages. MS and PPG inhibited the activities of enzymes related to CH₄ release during the cooling and maturity stages. Therefore, PPG and MS may have influenced OM degradation and CH₄ releases during composting via changes in bacterial metabolism and enzyme activity levels. PPG and MS could have altered the activities of methanogens to influence the transformation of carbon and CH₄ emissions according to network analysis and partial least-squares path modeling analysis. These findings provide insights at the molecular level into the effects of adding PPG and MS on OM degradation and CH₄ emissions during composting, thereby facilitating the application of PPG and MS in composting systems.

Review on fate and bioavailability of heavy metals during anaerobic digestion and composting of animal manure

Anaerobic digestion and composting are attracting increasing attention due to the increased production of animal manure. It is essential to know about the fate and bioavailability of heavy metals (HMs) for further utilisation of animal manure. This review has systematically summarised the migration of HMs and the transformation of several typical HMs (Cu, Zn, Cd, As, and Pb) during anaerobic digestion and composting. The results showed that organic matter degradation increased the HMs content in biogas residue and compost (with the exception of As in compost). HMs migrated into biogas residue during anaerobic digestion through various mechanisms. Most of HMs in biogas residue and compost exceeded relevant standards. Then, anaerobic digestion increased the bioavailable fractions proportion in Zn and Cd, decreased the F4 proportion, and raised them more than moderate environmental risks. As (III) was the main species in the digester, which extremely increased As toxicity. The increase of F3 proportion in Cu and Pb was due to sulphide formation in biogas residue. Whereas, the high humus content in compost greatly increased the F3 proportion in Cu. The F1 proportion in Zn decreased, but the plant availability of Zn in compost did not reduce significantly. Cd and As mainly converted the bioavailable fractions into stable fractions during composting, but As (V) toxicity needs to be concerned. Moreover, additives are only suitable for animal manure treated with slightly HM contaminated. Therefore, it is necessary to combine more comprehensive methods to improve the manure treatment and make product utilisation safer.

Immobilization of Zn and Cu in Conditions of Reduced C/N Ratio during Sewage Sludge Composting Process

In this paper we present results of research on the transformation of chemical forms of two elements (Cu, Zn) that occurred at the highest concentration in sewage sludge being processed in a composting process. The factor that had impact on the direction of the observed transformation was the amount of straw added to the mix with sewage sludge at the batch preparation stage including elimination of an additional source of organic carbon (straw). The analysis of contents of Cu and Zn chemical forms was performed applying Tessiere’s methodology. It was ascertained that reduction of supplementation has positive impact on the allocation of tested elements in organic (IV) and residual (V) fractions with a simultaneous decrease of heavy metals mobile forms share in bioavailable fractions, mostly ion exchangeable (I) and carbonate (II). Using an artificial neural network (ANN), a tool was developed to classify composts based on Austrian standards taking into account only I ÷ IV fractions treated as a labile, potentially bioavailable, part of heavy metals bound in various chemical forms in compost. The independent variables that were predictors in the ANN model were the composting time, C/N, and total content of the given element (total Cu, Zn). The sensitivity coefficients for three applied predictors varied around 1, which proves their significant impact on the final result. Correctness of the predictions of the generated network featuring an MLP 3-5-3 structure for the test set was 100%.

Microbial inoculants and struvite improved organic matter humification and stabilized phosphorus during swine manure composting: Multivariate and multiscale investigations

The combined effects of microbial inoculants (MI) and magnesium ammonium phosphate (MAP; struvite) on organic matter (OM) biodegradation and nutrients stabilization during biowaste composting have not yet been investigated. Therefore, the effects of MI and MAP on OM stability and P species during swine manure composting were investigated using geochemical and spectroscopic techniques. MI promoted the degradation of carbohydrates and aliphatic compounds, which improved the degree of OM mineralization and humification. MI and MAP promoted the redistribution of P fractions and species during composting. After composting, the portion of water-soluble P decreased from 50.0% to 23.0%, while the portion of HCl-P increased from 18.5% to 33.5%, which mean that MI and MAP can stabilize P and mitigate its potential loss during composting. These findings indicate that MI can be recommended for enhancing OM biodegradation and stabilization of P during biowastes composting, as a novel trial for the biological waste treatment.

Immobilization pathways of heavy metals in composting: Interactions of microbial community and functional gene under varying C/N ratios and bulking agents

Excessive heavy metals (HMs) in livestock manure due to additives over-use limits its recycling through composting. However, few studies have investigated the interactive influence of microbial communities, functional genes, and environmental factors in HM immobilization during composting. Therefore, treatments with different C/N ratios (15:1, 20:1, 25:1) and bulking agents (maize straw, green waste, vinasse) were conducted to explore the HMs immobilization pathways using structural equation model (SEM). Results confirmed the promoting effect of C/N ratio of 20:1 and vinasse on microbial diversity, thus leading to greater HMs immobilization rate. Meanwhile, the dominant microbial phyla of Cu/Zn, Cd, As, and Cr were identified as Proteobacteria, Firmicutes, Chytridiomycota, and Bacteroidota, respectively. Moreover, the significant correlation between functional genes (copC, mt, cbh1, aoxB, yieF) and HMs illustrated potential immobilization effects of metal-binding proteins on Cu and Zn, humus complexation on Zn, and oxidase/reductase on As/Cr. Finally, SEM indicated that the redistribution of Cu, Zn, As, and Cr fractions was interactively influenced by environmental factors (organic matter, pH, electrical conductivity, and total phosphorus), microbial communities, and functional genes, while Cd was directly regulated by organic matter and total phosphorus. These results may provide a deeper understanding of HM immobilization pathways during swine manure composting.

Advanced composting technologies promotes environmental benefits and eco-efficiency: A life cycle assessment

Growing demand for intensive animal farms and increased public awareness of environmental friendliness, have led to continuous iteration and refinement of the initially crude composting technology. However, the impact of the composting facility and energy input on eco-efficiency is limited. In this study, a LCA approach was conducted to investigate the eco-efficiency of four widely applied composting strategies: static heaps (SH), windrow composting (WC), membrane-covered composting (MC) and reactor composting (RC). The results showed that the environmental benefits of RC's were decreased by 11.3%, 21.7%, and 6.5% compared to SH, WC, and MC, respectively. Advanced composting technologies didn't substantially reduce direct economic costs, however, the eco-efficiency of RC was increased by 296.9%, 54.7%, and 87.6% compared to SH, WC, and MC, respectively. Overall, the results demonstrate that RC is a promising solution with high ecological efficiency that can contribute to the sustainable development of intensified livestock production.

Elevated temperatures drive abiotic and biotic degradation of organic matter in a peat bog under oxic conditions

Understanding the effects of elevated temperatures on soil organic matter (SOM) decomposition pathways in northern peatlands is central to predicting their fate under future warming. Peatlands role as carbon (C) sink is dependent on both anoxic conditions and low temperatures that limit SOM decomposition. Previous studies have shown that elevated temperatures due to climate change can disrupt peatland's C balance by enhancing SOM decomposition and increasing CO₂ emissions. However, little is known about how SOM decomposition pathways change at higher temperatures. Here, we used an integrated research approach to investigate the mechanisms behind enhanced CO₂ emissions and SOM decomposition under elevated temperatures of surface peat soil collected from a raised and Sphagnum dominated mid-continental bog (S1 bog) peatland at the Marcel Experimental Forest in Minnesota, USA, incubated under oxic conditions at three different temperatures (4, 21, and 35 °C). Our results indicated that elevated temperatures could destabilize peatland's C pool via a combination of abiotic and biotic processes. In particular, temperature-driven changes in redox conditions can lead to abiotic destabilization of Fe-organic matter (phenol) complexes, previously an underestimated decomposition pathway in peatlands, leading to increased CO₂ production and accumulation of polyphenol-like compounds that could further inhibit extracellular enzyme activities and/or fuel the microbial communities with labile compounds. Further, increased temperatures can alter strategies of microbial communities for nutrient acquisition via changes in the activities of extracellular enzymes by priming SOM decomposition, leading to enhanced CO₂ emission from peatlands. Therefore, coupled biotic and abiotic processes need to be incorporated into process-based climate models to predict the fate of SOM under elevated temperatures and to project the likely impacts of environmental change on northern peatlands and CO₂ emissions.

Nutrient dynamics during composting of human excreta, cattle manure, and organic waste affected by biochar

Ecological sanitation via thermophilic composting could be a promising solution to the lack of sanitation and limited access to fertilizers, particularly in developing countries. Here, we conducted a 185-d thermophilic composting experiment with human excreta, and separately with cattle manure, mixed with kitchen scraps, teff [Eragrostis tef (Zuccagni) Trotter] straw, sawdust, and biochar (BC) by using an appropriate-technology approach. We followed the dynamics of the most important macronutrients (N, P, K), temperature, moisture, pH, electrical conductivity, cation exchange capacity, as well as content of organic matter, organic C, Ca, Mg, and micronutrients throughout the process. Low N (<47%), P (<9%), K (<11%), Ca (<18%), and Mg (<21%) losses and the temperature profile indicated a well-functioning thermophilic composting process. Compost temperature was >60 °C for 7, 6, 5, and 8 consecutive days for treatments containing human excreta, human excreta amended with BC, cattle manure, and cattle manure amended with BC, respectively, suggesting a final compost product free of pathogens. The compost mixture with cattle manure and BC reached a significantly higher temperature than the same variant without BC, with a maximum value of 65.9 °C on Day 6. For all treatments, final germination index values >100% indicated compost maturity and the absence of phytotoxic substances. Biochar addition reduced losses of organic matter (18-23%), C (33-42%), and N (49-100%) and decreased the amount of extractable NO₃ ^- (32-36%) in the final compost. The tested ecological sanitation concept via thermophilic composting is thus a promising strategy to improve access to cheap fertilizer by safe and sustainable sanitation and waste management.

Probing changes in humus chemical characteristics in response to biochar addition and varying bulking agents during composting: A holistic multi-evidence-based approach

Despite the various benefits of humus, the changes in its chemical characteristics during composting in response to biochar addition and varying bulking agents remain to be further explored. In this study, three treatments were conducted, in which swine manure, bulking agent, and biochar were mixed at ratios of 4:1:0, 8:1:0, and 8:1:1. Fourier transform infrared spectroscopy (FTIR), carbon nuclear magnetic resonance spectroscopy (¹³C-NMR), three-dimensional excitation-emission matrix fluorescence spectroscopy (3D-EEM), and near-edge X-ray absorption fine structure (NEXAFS) were employed to characterize the chemical and structural properties of humus from multiple perspectives. The 3D-EEM spectra in this study showed a larger increase in humic acids (HAs) content (56%) and HAs to fulvic acids ratio (128%) during composting, indicating stronger humification in biochar-amended treatment. FTIR, ¹³C-NMR, and NEXAFS all confirmed the essential properties of HA as the core agronomic functional substance with rich aromatic and carboxyl groups, and that its aromaticity increased gradually during composting. In addition, ¹³C-NMR demonstrated that biochar addition and a relatively higher bulking agent ratio aided an increase in the carboxyl C proportion in HA after composting. In particular, NEXAFS revealed that biochar addition promoted the diversification of C, N, and O species in HA, with the emergence of quinone C and O-alkyl C as the main representatives. This work suggests that biochar addition and a relatively high bulking agent ratio could enhance humification and improve the agronomic function of humus.

Factors that affect physicochemical and acid-base properties of compost and vermicompost and its potential use as a soil amendment

Composting and vermicomposting have attracted attention in relation to both waste management and the potential to produce organic amendments that could improve soil quality. The main differences between compost depend on the feedstock, the production process, and the degree of maturity. In the present study, samples of compost of different origin (food and green waste, livestock waste, algae waste, urban waste or sewage sludge) or subjected to different composting methods (traditional or using earthworms) were collected for analysis. Additionally, samples collected at various stages of the composting process were compared (raw material, 15 and 30 days of composting, and final compost). Different analysis and techniques were used to establish the chemical composition, physicochemical and acid-base properties of compost samples and the organic matter extracts. The correlations obtained (between the abundance of acid groups in different extracts of the compost or between the cation exchange capacity and the C/N atomic ratio) would allow for predicting the compost behaviour based on certain characteristics, and a reduction in the number of parameters determined experimentally, thus facilitating comparisons between different compost. In addition, the potential value of the compost as amendment was tested with a Haplic Cambisol from a mining area. The application of compost increased the pH, the organic matter and nutrient content, and promoted seed germination and root growth.

Effect of C/N ration on disposal of pig carcass by co-composting with swine manure: experiment at laboratory scale

Disposal of animal carcasses by co-composting with animal waste usually selected conventional carbon to nitrogen (C/N) ratio around 25:1, in which the compost is widely used throughout the world. In this study, the pig carcass tissue blocks were sampled for composting at a laboratory scale to evaluate the effect of C/N ratio on the pig carcass compost. The time of thermophilic phase between 60 °C - 70 °C at a lower C/N ratio of 20:1 was significantly longer than that at the conventional C/N ratio, and it was the only one with the temperature beyond 70 °C that lasted for 2 days. Germination index and T value (the final C/N ratio / the initial C/N ratio) of the treatment with a C/N ratio of 20:1 were 94.67% and 0.69, respectively, meeting the standards of animal carcass compost. The degradation rate was 75.67%, and no significant difference was obtained as compared to the conventional C/N ratio groups. Organic fertilizer produced from the treatment with a C/N ratio of 20:1 was selected to evaluate the fertility by pot experiment of Cayenne pepper compared with chemical fertilizer. The results showed that organic fertilizer from this treatment could significantly improve the growth of Cayenne pepper. Overall, the use of the lower C/N ratio of 20:1 in the disposal of pig carcass by co-composting with swine manure could achieve the similar degradation rate as well as the maturity and stability of organic fertilizer as compared with the traditional C/N ratio at lab scale.

Effects of C/N Ratio on Lignocellulose Degradation and Enzyme Activities in Aerobic Composting

Lignocellulosic materials have a complex physicochemical composition and structure that reduces their decomposition rate and hinders the formation of humic substances during composting. Therefore, a composting experiment was conducted to evaluate the effects of different C/N ratios on lignocellulose (cellulose, hemicellulose and lignin) degradation and the activities of corresponding enzymes during aerobic composting. The study had five C/N ratios, namely, T1 (C/N ratio of 15), T2 (C/N ratio of 20), T3 (C/N ratio of 25), T4 (C/N ratio of 30) and T5 (C/N ratio of 35). The results showed that treatments T3 and T4 had the highest rate of degradation of cellulose and hemicellulose, while treatment T3 had the highest rate of degradation of lignin. Among the five treatments, treatment T3 enhanced the degradation of the lignocellulose constituents, indicating a degradation rate of 6.86–35.17%, 15.63–44.08% and 31.69–165.60% for cellulose, hemicellulose and lignin, respectively. The degradation of cellulose and lignin occurred mainly at the thermophilic and late mesophilic phases of composting, while hemicellulose degradation occurred at the maturation phase. Treatment T3 was the best C/N ratio to stimulate the activities of manganese peroxidase, lignin peroxidase, polyphenol oxidase and peroxidase, which in turn promoted lignocellulose degradation.

Metal contamination and toxicity of soils and river sediments from the world's largest platinum mining area

Mining activities in the world's largest platinum mining area in South Africa have resulted in environmental contamination with Pt (e.g., the Hex River's vicinity). The present study compared a Pt mining area with a non-mining area along this river in terms of (1) metal concentrations in different grain size fractions from soils and aquatic sediments; (2) the toxicological potential of aquatic sediments based on the Consensus-Based Sediment Quality Guideline (CBSQG); and (3) the chronic toxicity of aqueous eluates from soils and sediments to Caenorhabditis elegans. Platinum concentrations were higher in the mining area than in the non-mining area. For most metals, the sediment silt and clay fraction contained the highest metal concentrations. Based on the CBSQG, most sampling sites exhibited a high toxicological potential, driven by Cr and Ni. Eluate toxicity testing revealed that C. elegans growth, fertility, and reproduction inhibition were not dependent on mining activities or the CBSQG predictions. Toxicity was instead likely due to Cd, Fe, Mn, Ni, Pt, and Pb. In conclusion, the investigated region is loaded with a high geogenic background resulting in high reproduction inhibition. The mining activities lead to additional environmental metal contamination (particularly Pt), contributing to environmental soil and sediment toxicity.

Investigation of technology for composting mixed deer manure and straw

Composting is an effective method for utilizing agricultural straw waste and livestock manure resources. Using deer manure and corn straw as raw materials, the changes in various indexes were studied during composting under different initial C/N ratios, initial moisture contents, and particle sizes of corn straw, and compost maturity was evaluated. Moisture content, total organic carbon content, and C/N ratio all declined during composting, while total nitrogen, total phosphorus, total potassium, pH, germination index, and electrical conductivity increased. The grey relational analysis method was used to evaluate maturity. The results showed that a mixture of stalk and deer manure with initial moisture content of 55%, initial C/N ratio of 30:1, and a straw particle size of 1.5-3.5 cm constituted the optimal experimental conditions. Taguchi analysis indicated that initial moisture content exerted the greatest influence on compost maturity, followed by initial C/N ratio and crushed straw particle size. This study provides an important reference for the utilization of compost derived from a mixture of livestock manure and straw.

Additive grain-size: An innovative perspective to investigate the transformation among heavy metal and phosphorus fractions during aerobic composting

Considerable researches have been devoted to ascertain the transformation among heavy metal (HM) or phosphorus (P) fractions during aerobic composting. However, available information that additives with different grain-sizes regulate the activation mechanism on P through influencing the passivation effect on HMs remains limited. Thus, this work aimed to investigate the dynamic changes in HM-fractions and P-fractions, and ascertain the interaction pathway between HMs and P during aerobic composting amended with medical stone (Coarse medical stone, 3-5 mm; Fine medical stone, < 0.1 mm). Medical stone, especially for coarse-grained medical stone, significantly enhanced the HM-passivation and P-activation during the composting (P < 0.05). The bioavailability factor of HMs decreased by 48.05% (Cu), 20.65% (Pb), 15.58% (Cd) and 6.10% (Zn), and the content of labile available P (LAP) increased by 6.45%. HMs, with the explanatory capacity of 65.9%-84.9%, was important parameter superior to temperature (0.8%-5.4%), moisture content (MC, 0.1%-1.7%), pH (0.1%-8.7%), electric conductivity (EC, 0.8%-9.8%), carbon-to-nitrogen (C:N, 0.3%-2.3%) ratio and dissolved organic carbon (DOC, 0.4%-3.1%), to evaluate the transformation among P-fractions. Our results cast a new light on P-activation with respect to HM-passivation during aerobic composting.

The process of biotransformation can produce insect protein and promote the effective inactivation of heavy metals

An experiment was performed to study the inactivation effect of aerobic composting on heavy metals in maggot, pig and chicken manures. After composting, Cu mainly occurred in the oxidizable (OXI) fraction with a percentage distribution above 54%. Zn and Cd mainly existed in the bioavailable factor (BF), which has strong activity, with percentage distributions greater than 88.3% and 82.7%, respectively. Cr and Pb mainly existed in the stable residual (RES) fraction with a percentage distribution of approximately 50%. The aerobic composting process had a clear inactivation effect on heavy metals. For maggot manure compost in particular, the inactivation effects of Cu, Cr, Zn, Cd, and Pb were very good throughout the composting process, and the inactivation effect of Pb reached 54.42%. In addition, the process of biotransformation by housefly maggots promoted the conversion of fulvic acid (FA) to humic acid (HA) in pig manure, and the final increase in HA/FA after maggot manure composting was the largest among the different types of manure and beneficial to the inactivation of heavy metals. Compounds containing -CH₃ and -CH₂ groups were reduced, and aromatic structures were enhanced. Moreover, a maggot yield equivalent to 13.2% of the fresh pig manure was achieved during the process of biotransformation. The correlation analysis results showed that moisture content was an important factor affecting the inactivation rates of heavy metals in the three manure composts. Our results highlight that the process of biotransformation by housefly maggots can promote composting maturity and the inactivation of heavy metals, and produce a large amount of insect protein, yielding beneficial ecological and economic benefits.

Critical passivation mechanisms on heavy metals during aerobic composting with different grain-size zeolite

Available information about the passivation effect on heavy metals (HMs) through adsorption and humification during zeolite-amended composting remains limited. Thus, this study explored the dynamic changes in HM-fractions (Zn, Cu, Cd, Cr and Pb) during aerobic composting added with different grain-size zeolite (Fine zeolite, < 0.1 mm, ZF; Coarse zeolite: 3-5 mm, ZC). Compared to the control (without zeolite, CK) and ZF treatments, ZC treatment got the highest temperature in the thermophilic phase, and significantly reduced the bioavailability factor (BF) of HMs, especially for Cu (45.13%), Cd (16.11%) and Pb (25.49%). Redundancy analysis (RDA) and structural equation models (SEMs) indicated that zeolite accelerated the passivation effect on Cd and Pb through regulating the electrical conductivity (EC) as a result of surface adsorption, and on Cu by influencing total carbon (TC) under the function of humification. These results increase our understanding of the passivation mechanisms of HMs during aerobic composting.

Application of machine learning methods for the prediction of organic solid waste treatment and recycling processes: A review

Conventional treatment and recycling methods of organic solid waste contain inherent flaws, such as low efficiency, low accuracy, high cost, and potential environmental risks. In the past decade, machine learning has gradually attracted increasing attention in solving the complex problems of organic solid waste treatment. Although significant research has been carried out, there is a lack of a systematic review of the research findings in this field. This study sorts the research studies published between 2003 and 2020, summarizes the specific application fields, characteristics, and suitability of different machine learning models, and discusses the relevant application limitations and future prospects. It can be concluded that studies mostly focused on municipal solid waste management, followed by anaerobic digestion, thermal treatment, composting, and landfill. The most widely used model is the artificial neural network, which has been successfully applied to various complicated non-linear organic solid waste related problems.

Development, current state and future trends of sludge management in China: Based on exploratory data and CO2-equivaient emissions analysis

This study statistically reported the current state of sludge treatment/disposal in China from the aspects of sources, technical routes, geographical distribution, and development by using observational data after 1978. By the end of 2019, 5476 municipal wastewater treatment plants were operating in China, leading to an annual sludge productivity of 39.04 million tons (80% water content). Overall, 29.3% of the sludge in China was disposed via land application, followed by incineration (26.7%) and sanitary landfills (20.1%). Incineration, compost, thermal hydrolysis and anerobic digestion were the mainstream technologies for sludge treatment in China, with capacities of 27,122, 11,250, 8342 and 6944 t/d in 2019, respectively. Incineration and drying were preferentially constructed in East China. In contrast, sludge compost was most frequently used in Northeast China (46.5%), East China (22.4%) and Central China (12.8%), while anaerobic digestion in East China, North China and Central China. The capacities of sludge facilities exhibited a sharp increase in 2009-2019, with an overall greenhouse gas emissions in China in 2019 reached 108.18 × 10⁸ kg CO₂-equivaient emissions, and the four main technical routes contributed as: incineration (45.11%) > sanitary landfills (23.04%) > land utilization (17.64%) > building materials (14.21%). Challenges and existing problems of sludge disposal in China, including high CO₂ emissions, unbalanced regional development, low stabilization and land utilization levels, were discussed. Finally, suggestions regarding potential technical and administrative measures in China, and sustainable sludge management for developing countries, were also given.

Suitability of Black Soldier Fly Frass as Soil Amendment and Implication for Organic Waste Hygienization

Because of its nutritious properties, the black soldier fly has emerged as one of the most popular species in advancing circular economy through the re-valorization of anthropogenic organic wastes to insect biomass. Black soldier fly frass accumulates as a major by-product in artificial rearing set-ups and harbors great potential to complement or replace commercial fertilizers. We applied frass from larvae raised on different diets in nitrogen-equivalent amounts as soil amendment, comparing it to NH4NO3 fertilizer as a control. While the soil properties did not reveal any difference between mineral fertilizer and frass, principal component analysis showed significant differences that are mainly attributed to nitrate and dissolved nitrogen contents. We did not find significant differences in the growth of perennial ryegrass between the treatments, indicating that frass serves as a rapidly acting fertilizer comparable to NH4NO3. While the abundance of coliform bacteria increased during frass maturation, after application to the soil, they were outcompeted by gram-negatives. We thus conclude that frass may serve as a valuable fertilizer and does not impair the hygienic properties of soils.

Potential mechanisms involving the immobilization of Cd, As and Cr during swine manure composting

This study aims to investigate the relationship between key physicochemical parameters related to composting process and bioavailability of Cd, As and Cr during swine manure composting through regulating different initial carbon to nitrogen (C/N) ratios (15:1, 20:1, 25:1) and bulking agent types (straw, green waste). Results showed that higher initial C/N ratio of 20:1 or 25:1 and straw as bulking agent were optimal to reduce the bioavailability of Cd, As and Cr (62.4%, 20.6% and 32.2% reduction, respectively). Redundancy analysis implied that the bioavailability of Cd was significantly associated with total phosphorus and total nitrogen, deducing the formation of phosphate precipitation and biosorption might participated in the reaction process, while that of As and Cr were mainly influenced by organic matter (OM), cation exchange capacity (CEC) and OM, CEC, electric conductivity, respectively. A total of 48.5%, 64.6% and 62.2% of Cd, As and Cr redistribution information could be explained by the above parameters. Further correlation analysis revealed that bioavailable As and Cr were negatively correlated with humic acid to fulvic acid ratio. In summary, this study confirms that the mechanisms of phosphate precipitation, biosorption and humification played critical role in reducing Cd, As and Cr bioavailability during swine manure composting.

Evaluation of Organic Substrates and Microorganisms as Bio-Fertilisation Tool in Container Crop Production

Microorganisms are only effective when adequate conditions for their survival and development are provided. Among the factors that influence its effectiveness includes the type of soil or culture substrate, which works as an energy source reserve. Therefore, a tomato and a melon crop were established in different cycles to assess the effect of the physicochemical properties of organic substrates based on coconut fibre and vermicompost in three proportions, 0:100, 40:60 and 60:40 (% v:v), on the microbial activity in the rhizosphere when the bacteria Azotobacter vinelandii, Bacillus megaterium and Frateuria aurantia were applied. Concentrations of NO3−, H2PO4−, K+ and Ca2+ in the petiole cellular extract (PCE) were quantified at 60, 90 and 120 days after transplantation (DAT) for tomato and 45 and 65 DAT for melon. We analysed dehydrogenase activity (DHA), acid phosphatase activity (FTA) and β-glucosidase activity (β-GLU). In order to maintain optimal volumetric moisture for the survival of microorganisms, automatic control was used to manage the irrigation frequency between 22%–28%. The results showed that physicochemical substrate properties, by incorporating 40% vermicompost into the coconut fibre mixture, increased enzymatic activity. Plants that were inoculated with Azotobacter vinelandii and Frateuria aurantia showed an improvement in NO3− and K+ assimilation achieving highest yields.