Optimization of lignocellulolytic bacterial inoculum and substrate mix for lignocellulose degradation and product quality on co-composting of green waste with food waste

Screening of cellulose-degrading bacteria and optimization of cellulase production from Bacillus cereus A49 through response surface methodology

Cellulose-degrading microorganisms hold immense significance in utilizing cellulose resources efficiently. The screening of natural cellulase bacteria and the optimization of fermentation conditions are the hot spots of research. This study meticulously screened cellulose-degrading bacteria from mixed soil samples adopting a multi-step approach, encompassing preliminary culture medium screening, Congo red medium-based re-screening, and quantification of cellulase activity across various strains. Particularly, three robust cellulase-producing strains were identified: A24 (MT740356.1 Brevibacillus borstelensis), A49 (MT740358.1 Bacillus cereus), and A61 (MT740357.1 Paenibacillus sp.). For subsequent cultivation experiments, the growth curves of the three obtained isolates were monitored diligently. Additionally, optimal CMCase production conditions were determined, keeping CMCase activity as a key metric, through a series of single-factor experiments: agitation speed, cultivation temperature, unit medium concentration, and inoculum volume. Maximum CMCase production was observed at 150 rpm/37 °C, doubling the unit medium addition, and a 5 mL inoculation volume. Further optimization was conducted using the selected isolate A49 employing response surface methodology. The software model recommended a 2.21fold unit medium addition, 36.11 °C temperature, and 4.91 mL inoculant volume for optimal CMCase production. Consequently, three parallel experiments were conducted based on predicted conditions consistently yielding an average CMCase production activity of 15.63 U/mL, closely aligning with the predicted value of 16.41 U/mL. These findings validated the reliability of the model and demonstrated the effectiveness of optimized CMCase production conditions for isolate A49.

Effect evaluation of different green wastes on food waste digestate composting and improvement of operational conditions

This study attempted to determine the influence of diverse green wastes on food waste digestate composting and the improvement of operational conditions. Various effects of the green wastes (GW), with different types and sizes, initial substrate mixture C/N ratios, compost pile heights, and turning frequencies on the food waste digestate (FWD) composting were examined in the current work. The findings showed that the use of street sweeping green waste (SSGW) as an additive can maintain the thermophilic stage of the FWD composting for 28 days, while the end-product contained the greatest amounts of total phosphorus (TP, 2.29%) and total potassium (TK, 4.61%) and the lowest moisture content (14.8%). Crushed SSGW (20 mm) enabled the FWD composting to maintain the longest thermophilic period (28 days), achieving the highest temperature (70.2 °C) and seed germination index (GI, 100%). Adjusting the initial substrate mixture C/N ratio to 25, compost pile height to 30 cm, and turning frequency to three times a day could enhance the efficiency and improve the fertilizer quality of the co-composting of the FWD and SSGW. This study suggested that co-composting of FWD and SSGW (FWD/SSGW = 2.3, wet weight) is a promising technique for the treatment of municipal solid waste and provided significant theoretical data for the application of composting.

Enhanced humus synthesis from Chinese medicine residues composting by lignocellulose-degrading bacteria stimulation: Upregulation of key enzyme activity and neglected indirect effects on humus formation

Chinese medicine residues (CMHRs) resource is attracting widespread attention, as it is expected to be produced into Humus-rich fertilizer for soil application. This study aimed to promote effective humus (HS) production through lignocellulose-degrading bacteria (LDB) addition and explore the biological regulation mechanism of LDB affecting lignocellulose-to-humus conversion. The results showed higher HS production was achieved, with 109.73 and 111.44 g·kg-1, and HA/FA was raised by 12.70-16.02 % in compost products by LDB addition stimulation. Significant upregulation of β-glucanase and xylanase activities catalyzed higher decomposition of lignocellulose toward more HS potential precursors supply. Furthermore, exogenous LDB intervention induced microbial community restructure and microbial network establishment via enriching synergism functional bacteria, i.e., Thermobifida, Paenibacillus, Nonomuraea, etc. Mantel test results showed that it was variation of cellulose, hemicellulose and HS that affected microbial community succession (p < 0.01, r > 0.6), which represented the positive action of LDB addition stimulation on HS synthesis upregulation. Further exploration suggested LDB had an indirect effect on HS formation by enhanced lignin and hemicellulose conversion based on the Random Forest model and Partial least-squares path modeling results. This research provides new insights into the trigger effects of LDB introduction on upregulating HS synthesis and is expected to propose new perspectives for HS efficient production in CMHRs composting.

A comprehensive review on the decentralized composting systems for household biodegradable waste management

Municipal solid waste primarily consists of household biodegradable waste (HBW). HBW treatment is a crucial step in many countries due to rapid urbanization. Composting is an effective technique to treat HBW. However, conventional composting systems are unable to produce matured compost (MC), as well as releasing huge amounts of greenhouse and odorous gases. Therefore, this review attempts to suggest suitable composting system to manage HBW, role of additives and bulking agents in composting process, identify knowledge gaps and recommend future research directions. Centralized composting systems are unable to produce MC due to improper sorting and inadequate aeration for composting substrate. Recently, decentralized compost systems (DCS) are becoming more popular due to effective solid waste reduction at the household and/or community level itself, thereby reducing the burden on municipalities. Solid waste sorting and aeration for the composting substrate is easy at DCS, thereby producing MC. However, Mono-composting of HBW in DCS leads to production of immature compost and release greenhouse and odorous gases due to lower free air space and carbon-to-nitrogen ratios, and higher moisture content. Mixing HBW with additives and bulking agents in DCS resulted in a proper initial substrate for composting, allowing rapid degradation of substrate due to longer duration of thermophilic phase and produce MC within a shorter duration. However, people have lack of awareness about solid waste management is the biggest challenge. More studies are needed to eliminate greenhouse and odorous gases emissions by mixing different combinations of bulking agents and additives (mainly microbial additives) to HBW in DCS.

Incorporation of Substrates and Inoculums as Operational Strategies to Promote Lignocellulose Degradation in Composting of Green Waste—A Pilot-Scale Study

Composting is a sustainable alternative for green waste (GW) valorization contributing to the circular bioeconomy. However, the processing time must be reduced and the end-product quality must be improved. This study determined the effect of the incorporation of processed food waste (PFW), unprocessed food (UPFW), sawdust (SW), phosphate rock (PR) and a specific bacterial inoculum on GW-composting process parameters and product quality. Three treatments were evaluated in 120 kg piles: (i) TA: (GW + UPFW + PFW + inoculum), (ii) TB (GW + UPFW + PFW), and (iii) TC (GW). An inoculum of Bacillus sp. and Paenibacillus sp. was incorporated in the cooling phase for TA. On the other hand, the effect of the inoculum at the laboratory scale (20 kg reactors) was compared with that found at the pilot scale (120 kg piles). The incorporation of FW, SW, PR and the inoculum increased the amount of lignocellulose biodegradation (TA: 29.1%; TB: 22.7%; TC: 18.2%), which allowed for a reduction of up to 14 days of processing time. The product obtained for TA had a similar quality to the other two treatments, although a lower phytotoxicity was determined according to the germination index (TA: 95%; TB: 85%; and TC: 83%). The final product of TA showed the best agricultural characteristics with pH 8.3, TOC of 24.8%, TN of 1.32%, and GI of 98.8%. Finally, the scaling effect with the bacterial inoculum was shown to affect parameters such as the TOC, TN, GI, and, to a lesser extent, temperature and pH. The results obtained in this paper highlight the importance of optimizing the composting of GW, specifically with the use of co-substrates and specific inocula, which can be of interest for composting materials with a high content of lignocellulose such as GW.

Integrated system of anaerobic digestion and pyrolysis for valorization of agricultural and food waste towards circular bioeconomy: Review

Agricultural and food waste have become major issue affecting the environment and climate owing to growing population. However, such wastes have potential to produce renewable fuels which will help to meet energy demands. Numerous valorization pathways like anaerobic digestion, pyrolysis, composting and landfilling have been employed for treating such wastes. However, it requires integrated system that could utilize waste and promote circular bioeconomy. This review explores integration of anaerobic digestion and pyrolysis for treating agricultural and food waste. Proposed system examines the production of biochar and pyro-oil by pyrolysis of digestate. The use of this biochar for stabilizing anaerobic digestion process, biogas purification and soil amendment will promote the circular bioeconomy. Kinetic models and framework of techno-economic analysis of system were discussed and knowledge gaps have been identified for future research. This system will provide sustainable approach and offer carbon capture and storage in form of biochar in soil.