Water-heat balance characteristics of the sewage sludge bio-drying process in a full-scale bio-drying plant with circulated air

Enhanced microbial activity for moisture removal in biodrying with the assistance of stacked MFCs

Low microbial activity affected the organics degradation and limited the improvement of matrix temperature, leading to inefficient drying performance in conventional biordying (CB). In this study, two microbial fuel cells (MFCs) connected in series were applied on biodrying to facilitate water removal. Compared with CB process, moisture content (MC) of organic waste for stacked MFCs assisted biodrying (MB) process was rapidly decreased by 36.7 % within 6 days (CB: 13.8 %). Meanwhile, the assist of MFCs reshaped microbial communities and enriched electroactive bacteria Bacillus, 1.5 times and 3.4 times higher than those in CB during thermophilic and cooling phase, respectively. It could facilitate extracellular electron transfer and thus improving the reaction with O2. The analysis of O2 content also proved that electric field provided by stacked MFCs boosted O2 utilization and stimulated microbial metabolism. Therefore, organics biodegradation was greatly increased by 50.0 % and high-temperature duration was prolonged from 1.4 d to 2.3 d, which were essential driving forces for water removal. The dried product of MB was identified to be a satisfactory refuse-derived fuel (RDF) with low heating value of 9.35 MJ/kg, which was about 1.57-fold higher than that of CB. These results suggested that stacked MFCs assisted biodrying is an effective technology to ameliorate conventional biodrying, achieving rapid drying of municipal solid wastes with high MC and helping to improve the resource utilization of wastes. In particular, the integration of MFCs using reality organic wastewater as substrate and biodrying system could provide a feasible reference for the development of circular economy.

Effect of drying methods on phenolic compounds and antioxidant activity of Capparis spinosa L. fruits

BACKGROUND

Drying is a critical post-harvest process for medicinal plants, which are typically high in moisture and microorganisms. To prevent spoilage and quality loss, it is essential to dry these plants promptly. The drying method significantly impacts the levels of secondary metabolites and the organoleptic characteristics of medicinal plants. This study aimed to investigate the effects of various drying methods on the total phenolics, flavonoids, anthocyanins, antioxidant activity, and phenolic acids in caper (Capparis spinosa L.) fruits. The experiment was performed using a completely randomized design with three replications and included 11 treatments: shade drying, sun drying, oven drying (at 50 °C, 60 °C, and 70 °C), microwave drying (at 300 W, 600 W, and 900 W), freeze-drying, salt-drying, and a fresh plant sample as a control.

RESULTS

Among the drying methods tested, microwave drying consistently produced the highest levels of flavonoids, anthocyanins, and antioxidant activity, regardless of wattage. Specifically, the highest total phenol content was observed in samples dried at 900 W microwave, 600 W microwave, and 70 °C in the oven (5.3, 5.37, and 5.31 mg GAE/g DW, respectively). Drying at 600 W microwave yielded the highest levels of caffeic, cinnamic, ferulic, vanillic, and protocatechuic acids (13.03, 3.85, 4.28, 9.73, and 5.6 µg/g, respectively) while drying at 900 W microwave also resulted in elevated levels of caffeic, ferulic, protocatechuic, and p-coumaric acids. The 70 °C oven drying method also showed high levels of caffeic, cinnamic, ferulic, vanillic, and protocatechuic acids. Freeze-drying achieved the highest levels of rosmarinic, gallic, and m-coumaric acids (320.17, 175.3, and 12.99 µg/g, respectively), while shade drying produced high levels of p-hydroxybenzoic, cinnamic, ferulic, m-coumaric, protocatechuic, and p-coumaric acids.

CONCLUSIONS

Overall, microwave drying (especially at 600 W), oven drying at 70 °C, and freeze-drying emerged as effective alternatives to traditional drying methods. These methods not only preserved the color, texture, and taste of the fruits but also enhanced their bioactive compound levels.

Environmental impacts of cement kiln co-incineration sewage sludge biodried products in a scale-up trial

The biodrying technology as a pretreatment technology can overcome the limitations of cement kilns co-incineration sewage sludge (SS) on energy consumption. But the impact of SS biodried products on cement kilns and the route carbon reduction potential of biodrying + cement kilns have not been studied. In this study, SS biodrying and cement kiln co-incineration biodried product trials were conducted to highlight the matrix combustion characteristics, and the impact of biodried products on cement kilns (clinker capacity, coal consumption, and pollutant discharge). The carbon emissions of the four scenarios were assessed based on these results. The results showed that water removal rate reached 65.5 % after 11-day biodrying, and the wet-based lower heating value of the biodried product increased by 76.0 % compared with the initial matrix. Comprehensive combustibility index of the biodried product (0.745 × 10-7 %2℃-3min-2) was better than that of SS (0.433 × 10-7 %2℃-3min-2) although a portion of the organic matter was degraded. Cement kiln co-incineration of biodried products (150 t/d) resulted in per tonne of clinker saved 5.61 kg of coal due to the heat utilization efficiency of biodried products reached to 93.7 %. However, it led to an increase in the emission concentrations of NOX and SO2. Assessment results indicated that the biodrying + cement kiln pathway reduced CO2 emissions by 385.7 kg/t SS. Biodried products have greater potential to reduce emissions as alternative fuels than as fertilizers. This study indicated the advantages of SS biodrying + cement kiln co-incineration route.

The response and factors of microbial aerosol emission from the sludge bio-drying process

Exposure to high levels of microbial contaminants during waste disposal leads to the development of various diseases, including respiratory symptoms and gastrointestinal infections. In this study, the emissions of airborne bacteria and fungi during the process of sludge bio-drying were investigated. The recorded emission levels of airborne bacteria and fungi were 2398 ± 1307 CFU/m3 and 1963 ± 468 CFU/m3, respectively. Viable bacteria were sized between 1.1 and 3.3 μm, while fungal particles were concentrated between 2.1 and 4.7 μm. High-throughput sequencing was used to conduct a microbial population assay, and correlation analysis was performed to estimate the relationship between key factors and bioaerosol emissions. The main bacteria identified were Bacillus sp., Lysinibacillus sp. YS11, unclassified Enterobacteriaceae, Brevundimonas olei, and Achromobacter sp.; the primary types of fungi were Aspergillus ochraceus, Gibberella intricans, Fusarium concentricum, Aspergillus qinqixianii, and Alternaria sp.; and the dominant opportunistic pathogens were Bacillus anthracis and Aspergillus ochraceus. At lower moisture and temperature levels, airborne bacterial concentrations were higher, especially the release of fine particles. In addition, moisture content had a significant impact on the microbial population in bioaerosols. This study provides insights into strategies for controlling bioaerosols in the exhaust gases of the sludge bio-drying process.

Inoculation with thermophiles enhanced the food waste bio-drying and complicated interdomain ecological networks between bacterial and fungal communities

Bio-drying is a practical approach for treating food waste (FW). However, microbial ecological processes during treatment are essential for improving the dry efficiency, and have not been stressed enough. This study analyzed the microbial community succession and two critical periods of interdomain ecological networks (IDENs) during FW bio-drying inoculated with thermophiles (TB), to determine how TB affects FW bio-drying efficiency. The results showed that TB could rapidly colonize in the FW bio-drying, with the highest relative abundance of 5.13%. Inoculating TB increased the maximum temperature, temperature integrated index and moisture removal rate of FW bio-drying (55.7 °C, 219.5 °C, and 86.11% vs. 52.1 °C, 159.1 °C, and 56.02%), thereby accelerating the FW bio-drying efficiency by altering the succession of microbial communities. The structural equation model and IDEN analysis demonstrated that TB inoculation complicated the IDENs between bacterial and fungal communities by significantly and positively affecting bacterial communities (b = 0.39, p < 0.001) and fungal communities (b = 0.32, p < 0.01), thereby enhancing interdomain interactions between bacteria and fungi. Additionally, inoculation TB significantly increased the relative abundance of keystone taxa, including Clostridium sensu stricto, Ochrobactrum, Phenylobacterium, Microvirga and Candida. In conclusion, the inoculation of TB could effectively improve FW bio-drying, which is a promising technology for rapidly reducing FW with high moisture content and recovering resources from it.

Insights into the effect mechanism of back-mixing inoculation on sewage sludge biodrying process: Biodrying characteristics and microbial community succession

Back mixing was frequently used to replace conventional bulking agenting, however, however, the internal effect mechanism was unclear. This study compared four bulking agents: mushroom residue (MR), MR + primary BM (BM-P), BM-P, and secondary BM (BM-S). The effect mechanism of back mixing (BM) inoculation was assessed based on biodrying performance and microbial community succession. Four trials (Trial A, Trial B, Trial C, and Trial D) reached maximum temperatures of 61.9, 68.8, 73.7, and 69.9 °C on days 6, 3, 2, and 2, respectively. Application of BM increased pile warming rate and resulted in higher temperatures. Temperature changes and microbial competition lead to decline in microbial diversity and richness during the biodrying process. Microbial diversity increased of four biodried products. The number of microorganisms shared by Trial A, Trial B, Trial C, and Trial D were 90, 119, 224, and 300, respectively. The addition of BM improved microbial community stability, and facilitating the initiation of biodrying process. Microbial genera that played an important role in the biodrying process included Ureibacillus, Bacillus, Sphaerobacter, and Tepidimicrobium. Based on these results, it was concluded that BM was efficient method to enhanced the microbial activity and reduced the usage of bulking agent.

Inhibitory Effects of the Addition of KNO3 on Volatile Sulfur Compound Emissions during Sewage Sludge Composting

Odor released from the sewage sludge composting process often has a negative impact on the sewage sludge treatment facility and becomes a hindrance to promoting compost technology. This study investigated the effect of adding KNO₃ on the emissions of volatile sulfur compounds, such as hydrogen sulfide (H₂S), dimethyl sulfide (DMS), and carbon disulfide (CS₂), during sewage sludge composting and on the physicochemical properties of compost products, such as arylsulfatase activity, available sulfur, total sulfur, moisture content, and germination index. The results showed that the addition of KNO₃ could inhibit the emissions of volatile sulfur compounds during composting. KNO₃ can also increase the heating rate and peak temperature of the compost pile and reduce the available sulfur loss. The addition of 4% and 8% KNO₃ had the best effect on H₂S emissions, and it reduced the emissions of H₂S during composting by 19.5% and 20.0%, respectively. The addition of 4% KNO₃ had the best effect on DMS and CS₂ emissions, and it reduced the emissions of DMS and CS₂ by 75.8% and 63.0%, respectively. Furthermore, adding 4% KNO₃ had the best effect from the perspective of improving the germination index of the compost.