Coupling thermophilic composting and vermicomposting processes to remove Cr from biogas residues and produce high value-added biofertilizers

Enhanced vermicomposting of rice straw and pressmud with biogas slurry employing Eisenia fetida: Production, characterization, growth, and toxicological risk assessment

The biogas plant plays a dual role: it directly provides energy and indirectly promotes organic farming through outlet slurry. However, agricultural biomass wastes such as rice straws (RS) and pressmud (PM), which can't be used as fertilizers on their own, were vermicomposted (60 days) with biogas slurry (BS), using earthworm, into four blends: T1(BS, 100%), T2(3:2, BS: RS), T3(3:2, BS: PM), and T4(3:1:1, BS: RS: PM). The characterization, elemental analysis, and toxicological risk assessment of derived vermimanure were carried out using various analytical tools, such as an organic elemental analyzer such as CHNS, FT-IR, FESEM-EDXA, XPS, and ICP-OES. The pH, electrical conductivity, and C/N values were within 7.1-7.8, 3.2-6.0 dSm-1, and 12-15, respectively, for all treatments. The proportions of N (38%), P (70%), K (58%), Mg (67%), Ca (42%), and ash (44%), increased significantly (P < 0.05) over the initial feedstocks. The ecological risks of heavy metals (Zn, Cu, Ni, Pb, Cd, and Cr) in all feedstocks were found to be under WHO-permitted levels. The growth performance of earthworms was also considerably higher (P < 0.05) over the control feedstock group. The analytical methods verified that feedstock T4 (3:1:1, BS: RS: PM) was more porous, containing NH4+, PO43-, K+, and other nutrients. Pellets of all vermimanure groups keep 65-75% of the original volume. As well, when these pellets have been employed for agronomy and dispersed in the field, they will cause less dust than traditional or powdered compost or manure. In comparison to the control group, the synergistic approach of RS, PM, and BS in vermimanure significantly (P < 0.05) enhanced seed germination (83%), vigour index (42.5%), and decreased mean germination time by 27%. Furthermore, pot trials with Abelmoschus esculentus seed indicated that seedlings cultivated with 40% vermimanure of T4 (3:1:1, BS: RS: PM) mixed soil showed high growth in shoot, root, and plant yield.

Hydrothermal carbonization aqueous phase promotes nutrient retention and humic substance formation during aerobic composting of chicken manure

The aqueous phase (AP) of hydrothermal carbonization is rich in humic substances (HSs), which could influence the poultry manure composting process and the product quality. Here, raw AP and its modified product (MAP) with different nitrogen (N) contents were added into chicken manure composting at low (5%) or high (10%) rate. Results showed that all APs addition decreased the temperature and pH but AP-10% increased total N, HSs, and humic acid (HA) of compost by 12%, 18% and 27%, respectively. MAP applications increased the total phosphorus by 8-9% and MAP-10% enhanced the total potussium content by 20%. Additionally, both AP and MAP additions increased the contents of three major components of dissolved organic matter by 20-64%. In conclusion, both AP and MAP can generally improve the chicken manure compost quality, which provides a new idea for the recycling of APs derived from agro-forestry wastes during hydrothermal carbonization.

Feasibility of housefly larvae-mediated vermicomposting for recycling food waste added digestate as additive

The development of methods for the efficient treatment and application of food waste digestate is an important research goal. Vermicomposting via housefly larvae is an efficient way to reduce food waste and achieve its valorization, however, studies on the application and performance of digestate in vermicomposting are rarely. The present study aimed to investigate the feasibility of the co-treatment of food waste and digestate as an additive via larvae. Restaurant food waste (RFW) and household food waste (HFW) were selected to assess the effects of waste type on vermicomposting performance and larval quality. Waste reduction rates of 50.9%-57.8% were observed in the vermicomposting of food waste mixed with digestate at a ratio of 25%, which were slightly lower than those for treatments without the addition of digestate (62.8%-65.9%). The addition of digestate increased the germination index, with a maximum value of 82% in the RFW treatments with 25% digestate, and decreased the respiration activity, with a minimum value of 30 mg-O₂/g-TS. The larval productivity of 13.9% in the RFW treatment system with a digestate rate of 25% was lower that without digestate (19.5%). Materials balance shows that larval biomass and metabolic equivalent had decreasing trends as the amount of digestate increased and HFW vermicomposting exhibited lower bioconversion efficiency than that of RFW treatment system regardless of the addition of digestate. These results suggest that mixing digestate at a low ratio (25%) during vermicomposting of food waste especially RFW could lead to considerable larval biomass and generate relatively stable residues.

Bioconversion of Eichhornia crassipes into vermicompost on a large scale through improving operational aspects of in-vessel biodegradation process: Microbial dynamics

Eichhornia crassipes is a common, abundant aquatic weed biomass found globally. The present study examined optimum biodegradation procedures through batch studies (550 L rotating drum composter) and the resulting best combination on a large scale (5000 L rotary drum composter). The pilot scale rotary drum reactor was commenced with cow manure and then treated for 3 months with 250 kg/day of homogenously mixed E. crassipes and dry leaves. The rotary drum's inlet and outlet temperatures were 60 °C and 39 °C, respectively, suggesting thermophilic conditions with a 7-day waste retention duration. Eisenia fetida was used for vermicomposting the outlet material for 20 days, raising the nitrogen content to 3.2%. Bacterial diversity (16S-rRNA) sequencing revealed that Proteobacteria and Euryarchaeota are the most predominant. After 27 days, the volume dropped by 71%, and the product was stable and soil-safe. Large-scale optimised biodegradation may be a better way to handle aquatic weed biomass.

Evaluation of the synergistic effects of biochar and biogas residue on CO2 and CH4 emission, functional genes, and enzyme activity during straw composting

This study examined the effects of biochar, biogas residue, and their combined amendments on CO₂ and CH₄ emission, enzyme activity, and related functional genes during rice straw composting. Results showed that the biogas residue increased CO₂ and CH₄ emissions by 13.07 % and 74.65 %, while biochar had more obvious inhibition. Biogas residue addition enhanced functional gene abundance more than biochar. Biogas residue raised the methanogens mcrA gene by 2.5 times. Biochar improved the Acetyl-CoA synthase and β-glucosidase activities related to carbon fixation and decreased coenzyme activities related to methanogens. Biochar and biogas residue combined amendments enhanced the acsB gene abundance for CO₂ assimilation process and decreased methyl-coenzyme M reductase α subunit activity. Pearson correlation analysis indicated that organic matter was the significant variable affecting CO₂ and CH₄ emissions (P < 0.01). These results indicated biochar played significant roles in carbon loss and greenhouse emissions caused by biogas residue incorporation during composting.

Rapid and mass production of biopesticide Trichoderma Brev T069 from cassava peels using newly established solid-state fermentation bioreactor system

Converting agricultural waste into value-added biopesticides to replace chemical pesticides for plant protection is a good alternative for environmental sustainability and resource recycling. In this study, five tropical wastes (cassava peels, banana pseudostem, coconut shell, sugarcane bagasse, and pineapple peels) were screened as substrates for the rapid production of biopesticide Trichoderma Brev T069. Five single tests and a Box-Behnken design (BBD) with response surface methodology were used to optimize the culture conditions to improve the spore yield. The results showed that cassava peel was the optimal solid fermentation substrate, and the optimization enabled a spore yield of 9.31 × 10⁹ spores/g at 3rd day, which was equal to 93.19% of spore yield obtained at 5th day (9.99 × 10⁹ spores/g). A newly packed-bed bioreactor with agitation and ventilation system was developed and used to expand the production that 250 kg of biopesticide (2.89 × 10⁹ spores/g) could be available on the 3rd day. A pot experiment indicated that the biopesticide T. Brev T069 obtained under this production system, when applied at 1 × 10⁷ spores/g of soil had a 64.65% biocontrol efficiency on banana fusarium wilt. This study provides a practical solution for turning a tropical waste into an effective biopesticide which can prevent banana wilt disease, thereby helping to reduce disease management cost and overcome environmental hazards caused by synthetic pesticides.

Biodrying of biogas residue through a thermophilic bacterial agent inoculation: Insights into dewatering contribution and microbial mechanism

Biogas residue (BR) is difficult to transport and compost due to its high moisture content. The purpose of this study was to elucidate the dewatering and microbial mechanisms underlying the inoculation of a thermophilic bacterial agent (TBA) onto BR with a high moisture content (i.e., 90.4%). TBA accounted for 78.7% of the water loss rate in BR, dramatically higher than the effects of aeration, external heat, or indigenous microorganisms (i.e., 1.8%, 0.1%, and 19.4%, respectively). Furthermore, TBA inoculation resulted in a stable product [with a low moisture content (9.4%) and a high seed germination index (107.3%)]. Finally, TBA increased microbial diversity and the abundance of functional bacteria (Proteobacteria and Bacteroidota), which might be beneficial for refractory organic compound decomposition and plant growth. Thus, biodrying BR via inoculation with a TBA is recommended economically.

Temperature impacts fate of antibiotic resistance genes during vermicomposting of domestic excess activated sludge

Effect of temperature on antibiotic resistance genes (ARGs) during vermicomposting of domestic excess sludge remains poorly understood. Vermicomposting experiment with excess sludge was conducted at three different temperatures (15 °C, 20 °C, and 25 °C) to investigate the fate of ARGs, bacterial community and their relationship in the process. The vermicomposting at 25 °C did not significantly attenuate the targeted ARGs relative to that at 15 °C and 20 °C. The dynamics of qnrA, qnrS, and tetM genes during vermicomposting at 15 °C and 20 °C followed the first-order kinetic model. Temperature remarkably impacted bacterial diversity of the final products with the lowest Shannon index at 25 °C. The presence of the genus (Aeromonas and Chitinophagaceae) at 25 °C may contribute to the rebound of the genes (qnrA, qnrS and tetM). The study indicates that 20 °C is a suitable vermicomposting temperature to simultaneously reach the highest removal efficiency of the ARGs and the good biostability of the final product.

Process optimization by combining in-vessel composting and vermicomposting of vegetable waste

The process parameters of in-vessel rotary drum composting (RDC) with vermicomposting (VC) were investigated for the conversion of vegetable waste into vermicompost. After 7-day initial thermophilic exposure (maximal 51.5 °C in 24 h), the partially degraded RDC waste was divided into R1 (no vermiculture), R2, R3, and R4 (with Eudrilus eugeniae; Eisenia fetida; and Perionyx excavates monocultures, respectively). R3 derived vermicompost displayed maximum optimal process parameters and desirable compost qualities. Against the constant 2.2% nitrogen content of R1, an increase from 1.4 to 4.15% was seen in R3, with a 52.5% reduction in total organic carbon (TOC). A clear testimony to the enhanced nutritional content and fitness of the novel combination of RDC thermophilic biodegradation and E. fetida based vermicomposting. In an environmentally compatible mode, the faster organic deconstruction in 27 days could substantially alter organic waste treatment in the immediate future.

Anaerobic digestion beyond biogas

Anaerobic digestion (AD) is a matured technology for waste (water) remediation/stabilization and bioenergy generation in the form of biogas. AD technology has several inherent benefits ranging from generating renewable energy, remediating waste (water), and reducing greenhouse gas emission to improving health/hygiene and the overall socio-economic status of rural communities in developing nations. In recent years, there has been a paradigm shift in applications of AD technology beyond biogas. This special issue (SI) entitled, "Anaerobic Digestion Beyond Biogas (ADBB-2021)," was conceptualized to incorporate some of the recent advances in AD in which the emphasis is beyond biogas, such as anaerobic biorefinery, chain elongation, treatment of micropollutants, toxicity and system stability, digestate as biofertilizer, bio-electrochemical systems, innovative bioreactors, carbon sequestration, biogas upgrading, microbiomes, waste (water) remediation, residues/waste pre-treatment, promoter addition, and modeling, process control, and automation, among others. This VSI: ADBB-2021 contains 53 manuscripts (14 critical reviews and 39 research). The key findings of each manuscript are briefly summarized here, which can serve as a valuable resource for AD researchers to learn of major advances in AD technology and identify future research directions.