Application of a set of complementary techniques to understand how varying the proportion of two wastes affects humic acids produced by vermicomposting

A Critical Review on the Vermicomposting of Organic Wastes as a Strategy in Circular Bioeconomy: Mechanism, Performance, and Future Perspectives

AbstractTo meet the current need for sustainable development, vermicomposting (VC), a natural, eco-friendly, and cost-effective technology, can be a wise selection for the bioconversion of organic wastes into value-added by-products. However, no one has tried to establish the VC technology as an economically sustainable technology by exploring its linkage to circular bioeconomy. Even, no researcher has made any effort to explore the usability of the earthworms (EWs) as a protein supplement while assessing the economic perspectives of VC technology. Very few studies are available on the greenhouse gas (GHG) emission potential of VC technology. Still, the contribution of VC technology towards the non-carbon waste management policy is not yet explored. In the current review, a genuine effort has been made to inspect the contribution of VC technology towards the circular bioeconomy, along with evaluating its capability to bioremediate the organic wastes generated from domestic, industrial, and agricultural premises. The potential of the EWs as a protein source has also been explored to strengthen the contribution of VC technology towards the circular bioeconomy. Moreover, the linkage of the VC technology to the non-carbon waste management policy has been comprehensively demonstrated by highlighting its carbon sequestration and GHG emission potentials during the treatment of organic wastes. It has been observed that the cost of food production was reduced by 60--70% by replacing chemical fertilizers with vermicompost. The implication of the vermicompost significantly lessened the harvesting period of the crops, thereby helping the farmers attain higher profits by cultivating more crops in a single calendar year on the same plot. Furthermore, the vermicompost could hold the soil moisture for a long time, lessening the water demand up to 30-40%, which, in turn, reduced the frequency of irrigation. Also, the replacement of the chemical fertilizers with vermicompost resulted in a 23% increment in the grapes' yield, engendering an extra profit of up to 110000 rupees/ha. In Nepal, vermicompost has been produced at a cost of 15.68 rupees/kg, whereas it has been sold to the local market at a rate of 25 rupees/kg as organic manure, ensuring a net profit of 9.32 rupees/kg of vermicompost. EWs embraced 63% crude protein, 5-21% carbohydrates, 6-11% fat, 1476 kJ/100 g of metabolizable energy, and a wide range of minerals and vitamins. EWs also contained 4.11, 2.04, 4.43, 2.83, 1.47, and 6.26  g/kg (on protein basis) of leucine, isoleucine, tryptophan, arginine, histidine, and phenylalanine, respectively, enhancing the acceptability of the EW meal (EWM) as the protein supplement. The inclusion of 3 and 5% EWM in the diet of broiler pullets resulted in a 12.6 and 22.5% increase in their feed conversion ratio (FCR), respectively after one month. Similarly, when a 100% fish meal was substituted by 50% EWM and 50% fish meal, the FCR and growth rate of Parachanna obscura were increased substantially. The VC of maize crop residues mixed with pig manure, cow dung, and biochar, in the presence of Eisenia fetida EWs, yielded only 0.003-0.081, 0-0.17, and 130.40-189.10 g CO₂-eq.kg^-1 emissions of CO₂, CH₄, and N₂O, respectively. Similarly, the VC of tomato stems and cow dung ensured 2.28 and 5.76 g CO₂-eq.kg^-1 CO₂ emissions of CH₄ and N₂O, respectively. Additionally, the application of vermicompost at a rate of 5 t/ha improved the soil organic carbon proportion and aggravated carbon sequestration. The land application of vermicompost improved micro-aggregation and cut down the tillage, reducing GHG emissions and triggering carbon sequestration. The significant findings of the current review suggest that VC technology potentially contributes to the concept of circular bioeconomy, substantially negotiates potential GHG emissions, and complies with the non-carbon waste management policy, reinforcing its acceptability as an economically sound and environmentally benevolent organic waste bioremediation alternative.

Distinctive Features of Composts of Different Origin: A Thorough Examination of the Characterization Results

The potential of composts produced from different origin residues to be used in environmentally friendly agriculture is addressed in this work. Seven composts obtained from different raw materials and composting methodologies are compared using elemental, thermal and spectroscopic characterization data. Despite the stabilization of the organic matter in all composts being adequate for agricultural applications, they display distinct elemental and structural compositions. Likewise, the fertilisers have very different effects on lettuce growth. Despite the observed differences, some common features were found, namely a mass loss (TGA) of 25.2 g per mol C, association between groups of elements (Fe, Al, Ni, Co, Cr, Cu and S; Mg, Na, K and P, C, Coxi, N and Pb) and correlations between the amount of carbon nanostructures and the characteristic aromaticity parameters. These results suggest that the tuning of the compost features for specific cultures may be possible for sustainable food production.

Humic acid characterization and heavy metal behaviour during vermicomposting of pig manure amended with 13C-labelled rice straw

Aiming to reveal the humification process of organic waste and its contribution to the heavy metal behaviour affected by earthworm activity, it was studied about the variation of humic acid (HA) and heavy metal behaviour during vermicomposting of the mixed pig manure and ¹³C-labelled rice straw. The results showed that earthworms could well adapt to the culturing environment and feed organic matter for its growth and reproduction, the vermicomposting process increased the content of humic substances (HS), HA, and fulvic acid (FA) in substrate residues, but led to less transformation of HA into FA. The elemental, ultraviolet absorption spectroscopy, Fourier transform infrared (FTIR) and fluorescence excitation-emission matrix (EEM) analysis indicated that vermicomposting led to more aromatic structures and much higher humification degree in HA, whereas less protein, FA-like substances and plastein in HA. Vermicomposting could enhance the total Cu content and decrease Cu/Zn bioavailability in the substrate residues, and vermicomposting especially can help stabilize Cu in the substrate residues by forming more complexed HA-Cu.

Evaluating the interaction of soil microorganisms and gut of soil fauna on the fate and spread of antibiotic resistance genes in digested sludge-amended soil ecosystem

Earthworms have shown their effectiveness in reducing the abundances of antibiotic resistance genes (ARGs) from solid waste. However, the mechanisms of the reduced ARGs by earthworm and whether the solid waste would affect the ARGs profile in earthworm gut were poorly understood. Herein, the patterns of ARGs and microbial communities in digested sludge-amended soil and earthworm gut after 80-day cultivation were investigated. Results show that the enrichment of ARGs (e.g., tetA, tetQ, and sulII) in soil caused by digested sludge-amendment was temporary and would recover to their original levels before amendment. In addition, earthworms could contribute to the further reduction of ARG abundances, which was mainly attributed to their gut digestion via shifting the microbial community (e.g., attenuating the anaerobes). However, the amended soil could significantly increase ARGs abundance in the earthworm gut, which may enhance the potential risk of ARGs spread via the food chain. These findings may provide a new sight on the control of ARGs occurrence and dissemination in sludge-amended soil ecosystem with consideration of earthworms.

Effects of TiO2 and ZnO nanoparticles on vermicomposting of dewatered sludge: studies based on the humification and microbial profiles of vermicompost

Nanoparticles (NPs) are prevalent in dewatered sludge, and their presence increases the environmental risks associated with the subsequent sludge treatment process. However, until now, their potential effects on sludge vermicomposting have not been clarified. This study investigated the effects of NPs on sludge humification and microbial profiles during vermicomposting by comparing fresh dewatered sludge substrates with substrates mixed with 0 mg/kg NPs (control), 100 mg/kg TiO₂, 500 mg/kg TiO₂, 100 mg/kg ZnO, and 500 mg/kg ZnO. The results showed that addition of TiO₂ and ZnO NPs to sludge did not significantly affect the growth rate of earthworms and the superoxide dismutase activity in their guts during vermicomposting. Moreover, higher concentrations of the selected NPs promoted the humification index of sludge by 20.7-49.6%, through the formation of polysaccharides, aromatic substances, and organic acids in final vermicomposts. Compared with the control without NP addition, bacterial community diversity was enhanced in treatments with TiO₂ and ZnO NPs, and dominant genera differed according to the type and concentration of NPs. This study suggests that the presence of TiO₂ and ZnO NP residuals modify the microbial community of sludge, thus promoting sludge humification during vermicomposting.

Assisted phytostabilization of Pb-spiked soils amended with charcoal and banana compost and vegetated with Ricinus communis L. (Castor bean)

A greenhouse experiment was performed to elucidate the potency of Prosopis juliflora charcoal (PJC) and banana waste compost (BWC) to improve soil fertility and enhance plant growth rate. Plantlets of Ricinus communis were grown in 0, 400, and 800 mg kg^-1 Pb-spiked soil ameliorated with P. juliflora charcoal and banana waste compost at 0, 5%, and 10% (w/w) for 60 days. PJC and BWC significantly (p < 0.05) increased plant growth parameters, that is, number of leaves, node number, plant height, and leaf diameter and reduced oxidative stress manifested by the lesser production of proline, hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) with respect to control plants. Soil usage of PJC at 10% decreased the Pb accumulation by 61%, whereas BWC decreased Pb concentration in roots by 56% concerning control. Field emission scanning electron microscope (FE-SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) showed high macro and microspores on the surface of charcoal while banana compost showed significant raise in the nutrient content (N, P, K, Zn, Ca, Fe, and Mg). Thermogravimetric (TG) and Fourier-transform infrared spectroscopy (FTIR) analysis of banana compost showed enhanced molar convolution of carbohydrate composites and nitrogen content. These findings pave a clear understanding that PJC and BWC are recalcitrant for Pb phytotoxicity and can also be used as nutrient-rich composites for increased crop production.

Biotransformation of paper mill sludge and tea waste with cow dung using vermicomposting

Vermicomposting of paper mill sludge (PMS) and tea waste (TW) using cow dung (CD) in five different combinations was carried out using Eisenia fetida. The aim of this study was to manage the waste disposal problem of PMS using the environment-friendly technology of vermiconversion. The changes in physico-chemical parameters were observed at 30-day intervals up to 90 days. The final pH was within 6.09-6.95 among all units. The TN, TP and TK contents increased 0.30-0.87, 0.53-3.23, 0.33-0.63 times, respectively in all mixtures after vermicomposting with increase in EC and ash Content. Maximum reduction in Total Organic Carbon (23.91%) was observed in treatment with highest PMS content, attributed to earthworm activity. Reduction in C: N ratio (38.63%-54.05%) was significantly observed in all the treatments. It was finally inferred that the paper mill sludge and tea waste in combination with cow dung can be successfully biotransformed into useful manure employing earthworms.

Assessment of qualitative enrichment of organic paper mill wastes through vermicomposting: humification factor and time of maturity

The process of bioconversion of solid organic wastes through vermicomposting justifies the environmental message for sustainability such as reduce, recycle and reuse of wastes. In the present study, wastes derived from two different types of paper mill sludge (primary and secondary), was used for their bioconversion through the vermicomposting process using an indigenous species of earthworm (Perionyx excavatus). The maturity and stability stage of vermicompost production was assessed using FT-IR, GC-MS and TG analyses. During vermicomposting, different biochemical functional groups present in the wastes have shown differential chemical alteration and turnover as revealed by FT-IR spectroscopy. This study has also confirmed the trend of biodegradation of complex substances like lignin, cellulose, proteins etc. and thereby demonstrates the extent of mineralization. TG spectral analysis had revealed a mass loss of 80% and 71% in vermicompost produced from primary and secondary sludge respectively. GC-MS studies have also shown the presence of several humic acids like octadecanoic acid, heptadecanoic acid etc. in the decomposing substances demonstrating as an indicator of the maturity of products. This was further confirmed by the decrease of humification index which focuses the combined action of both earthworms and microbes in the degradation of organic wastes. The present study has highlighted the role of an indigenous earthworm in converting specific industrial wastes especially by recording the point of maturation using humic acids an indicator of the quality of decomposing of wastes following several instrumental applications.

The impact of varying abiotic humification conditions and the resultant structural characteristics on the copper complexation ability of synthetic humic-like acids in aquatic environments

Humic acid (HA) has a high complexation ability with metal ions due to its functional groups. In this study, 11 synthetic humic-like acids (SHLAs) were prepared under varying abiotic humification conditions: precursor species (glycine-catechol and glycine-catechol-glucose), precursor concentrations (from 0.25 M:0.25 M to 1 M:1 M), pH (6-8), temperature (25-45 °C) and mass of MnO₂ catalyst (1.3-2.5% w/v). The effect of the varying humification conditions on the complexation ability of the SHLA for Cu²⁺ were investigated together with the relationships between Cu complexation ability and the structure of the SHLAs (elemental composition, type and content of functional groups, AL/AR, E₄/E₆). Conditional stability constants (log K) of the SHLAs ranged from 6.00 to 6.42 and complexation capacities ranged from 1.76 mmol/g to 2.61 mmol/g. SHLAs synthesized at lower temperature (25 ℃), pH 8, low precursor concentrations (glycine:catechol= 0.25 M:0.25 M) and a larger proportion of catalyst (2.5% w/v) had a larger copper complexation ability. Log K values of SHLAs had significant positive correlations with carboxylic carbon (r = 0.671, p < 0.05), carboxylic group content (r = 0.890, p < 0.01) and O/C ratio (r = 0.618, p < 0.05), and significant negative correlations with aliphatic carbon (r = -0.616, p < 0.05), total C (r = -0.685, p < 0.05) and total H contents (r = -0.654, p < 0.05). Complexation capacities had a significant positive correlation with total N (r = 0.826, p < 0.01) and a significant negative correlation with C/N ratio (r = -0.823, p < 0.01).

Dynamics of organic matter decomposition during vermicomposting of banana stem waste using Eisenia fetida

A better understanding of how dynamics of physical and chemical changes occur during vermicomposting process would be helpful for determining the stability and maturity of vermicompost. For improving the knowledge about this issue several instrumental techniques were used in the present study to analyse the physical and chemical changes as a function of vermicomposting time of banana stem waste (BS) spiked with cow dung (CD) in different proportions using earthworm Eisenia fetida. Chemical analysis by ICP-AES showed gradual increase in the plant nutrients (P, Ca, K, Mg, Fe) up to 60 day of vermicomposting in all the treatments. But among different treatments, K, Mg and Fe were considerably higher in the BS2CD1 blend. The FTIR showed strong NO stretching vibration with increasing BS content signifying the presence of nitrate in the final compost. The TG analysis of final BS-CD composts described the lower mass loss (52-55%) in the final compared to the initial stage due to high level of humification by earthworms. The maturity of the final compost was confirmed by DSC analysis which exhibited lowering of relative intensity of exothermic peaks related to the easily degradable material at 320-330 °C and complex organic moieties at 495-530 °C. Decrease in the humification index (Q_4/6, Q_2/4, Q_2/6) at 60 day confirmed the stability of vermicomposts. All the treatments showed <2 mg CO₂-C g^-1 vermicompost C day^-1 respiration rates and >70% germination indices (GI) for rice and pea seeds. These findings defined a clear comparison between the treatments during vermicomposting in terms of stability and maturity and revealed that BS2CD1 can be utilized as nutrient-rich stable compost for enhanced crop production.

The copper complexation ability of a synthetic humic-like acid formed by an abiotic humification process and the effect of experimental factors on its copper complexation ability

Humic acids have an important impact on the distribution, toxicity, and bioavailability of hazardous metals in the environment. In this study, a synthetic humic-like acid (SHLA) was prepared by an abiotic humification process using catechol and glycine as humic precursors and a MnO₂ catalyst. The effect of physico-chemical conditions (ionic strength from 0.01 to 0.5 M NaNO₃, pH from 4 to 8, temperature from 25 to 45 °C, and humic acid concentration from 5 to 100 mg/L) on the complexation ability of SHLA for Cu²⁺ were investigated. A commercial humic acid (CHA, CAS: 1415-93-6) from Sigma-Aldrich was also studied for comparison. The results showed that for pH 4 to 8, the conditional stability constants (log K) of SHLA and CHA were in the range 5.63-8.62 and 4.87-6.23, respectively, and complexation capacities (CC) were 1.34-2.61 and 1.42-2.31 mmol/g, respectively. The Cu complexation ability of SHLA was higher than that of the CHA due to its higher number of acidic functional groups (SHLA 19.19 mmol/g; CHA 3.87 mmol/g), extent of humification and aromaticity (AL/AR: 0.333 (SHLA); 1.554 (CHA)), and O-alkyl functional groups (SHLA 15.56%; CHA 3.45%). The log K and complexation efficiency (fraction of metal bound to SHLA) of SHLA were higher at higher pH, lower ionic strength, higher temperature, and higher SHLA concentration. Overall, SHLA was a good and promising complexation agent for copper in both soil washing of copper contaminated soil and the treatment of copper-containing wastewater.

Effects of earthworms on nitrification and ammonia oxidizers in vermicomposting systems for recycling of fruit and vegetable wastes

Although it is known that earthworms enrich the nitrate content in their final products, the detailed mechanisms behind this are not well understood, and this is important for determining the agricultural value of vermicomposting. Hence, this study aimed to investigate the effects of earthworms on ammonia oxidization and to clarify the functions of ammonia-oxidizing bacteria and archaea (AOB and AOA) during vermicomposting of fruit and vegetable wastes (FVWs). For this, two dry systems using dry FVWs and a fresh system using fresh FVWs were adopted and compared during 60days of vermicomposting. Each system included two treatments, with earthworms and without earthworms. The results revealed that vermicomposting could facilitate the stabilization of FVWs, forming high value-added products. Based on the results of fluorescent excitation-emission matrix analysis, humification indices of the dry and fresh vermicomposts were 4.0 and 4.2, respectively. Moreover, compared to the minus net nitrification rates in groups without worm treatment, the net nitrification rates of 17.5mgN/kg/d and 9.3mgN/g/d, respectively, were found in dry and fresh vermicomposting systems, indicating that earthworms could significantly accelerate the nitrification process. Compost treated with earthworms exhibited elevated numbers of ammonia oxidizers (AOA and AOB) and greater community diversity in final products, compared to the counterparts without earthworms. Final vermicompost products were abundant in the AOB members of Nitrosomonas and Nitrosospira along with AOA groups including Crenarchaeota and Thaumarchaeota. By contrast, AOA were the dominate members completing ammonia oxidization during vermicomposting of dry and fresh FVWs. This study suggests that earthworms facilitate the ammonia oxidization process by promoting both numbers and diversity of AOA and AOB during vermicomposting of FVWs.