Dynamic changes of microbial community and moisture ratio during bio-drying of sludge after electro-dewatering

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.

The new strategies for high efficiency removal of antibiotics and antibiotic resistance genes by direct bio-drying of biogas slurry: Microbiological mechanisms

High levels of antibiotics and antibiotic resistance genes (ARGs) still exist in biogas slurry after anaerobic digestion of cow manure. In this study, direct bio-drying strategies of cow manure biogas slurry without solid-liquid separation for the removal of antibiotics and ARGs were explored. The results showed that, after direct bio-drying of biogas slurry, the moisture contents decreased to 25.2 %-31.5 %. The maximum temperatures of the piles reached 76.1-77.4 °C, which is close to ultra-high temperatures (>80 °C). Direct biogas slurry bio-drying (CK treatment) achieved efficient removal of antibiotics, ARGs, and mobile genetic elements (MGEs) (95.4 %, 98.6 % and 86.7 % removal, respectively). Compared to the CK treatment, molecular membrane covering (MMC) alone was the most effective in further significantly decreasing the antibiotic concentration and the abundance of ARGs and MGEs in the final bio-dried samples, followed by food waste hydrochar (FHC) addition alone. Methanogenic archaea were identified as potential hosts for ARGs based on Network analysis. FHC addition-MMC increased the abundance of potential hosts for ARGs and promoted the expression of microbial methane metabolism function relative to the CK treatment during the later stages of bio-drying, thereby decreasing the removal efficiency of ARGs. The results of structural equation model and redundancy analysis showed that MGEs had the most significant direct effect on ARGs and moisture content had the highest relative contribution to changes in ARGs. In summary, direct bio-drying strategies were able to efficiently remove antibiotics and ARGs from cow manure biogas slurry and also achieve biological dewatering of the biogas slurry.

Determination of unbound-bound moisture interface of faecal sludges from different on-site sanitation systems

Non-sewered sanitation (NSS) has been adopted by a significant population, especially in developing countries. For the effective operation of NSS, it is important to ensure optimal treatment of the collected faecal sludge (FS) in on-site sanitation systems. Solid-liquid separation is among the most important steps in the treatment of FS in NSS. The ability of the FS for solid-liquid separation has assumed greater significance in the design and operation of sludge treatment process. Although dewatering indices such as specific resistance to filtration (SRF) and capillary suction time (CST) have been extensively used to determine the limits of sludge dewaterability, these parameters do not provide information on the limits of unbound moisture. The unbound and bound moisture fractions of the FS samples were determined using different methods, and each method was evaluated for reliability. Different FS samples were collected from ventilated improved pit (VIP) latrines, urine diversion dehydrating toilets (UDDT) and septic tanks (ST). Batch settling tests (BST), centrifugation, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and water activity (WA) measurements were performed on the samples. While the BST provided the settleability of FS at gravitational force, and centrifugation estimated the relative limit of solid-liquid separability upon application of a force, the boundary of unbound moisture or the interface of unbound and bound moisture fractions as determined by TGA, DSC and WA. Evidently there was a fine overlap of the unbound and bound moisture at the interface due to presence of some tightly held unbound moisture and loosely held bound moisture. The unbound-bound moisture interface, thus, can be experimentally determined to be between 51.07 and 60.35 % for VIP sludge, 51.48-64.38 % for UDDT sludge, 62.29-66.71 % for ST-wGW sludge and 59.11-60.37 % for ST-GW sludge. Although hypothesis testing revealed no statistically significant difference between the methods and sample types, it can be concluded that WA demonstrated the highest reliability in terms of accuracy, ease of measurement, rapidity and repeatability.

Machine learning modeling of thermally assisted biodrying process for municipal sludge

Preparation of activated carbons is an important way to utilize municipal sludge (MS) resources, while drying is a pretreatment method for making activated carbons from MS. In this study, machine learning techniques were used to develop moisture ratio (MR) and composting temperature (CT) prediction models for the thermally assisted biodrying process of MS. First, six machine learning (ML) models were used to construct the MR and CT prediction models, respectively. Then the hyperparameters of the ML models were optimized using the Bayesian optimization algorithm, and the prediction performances of these models after optimization were compared. Finally, the effect of each input feature on the model was also evaluated using SHapley Additive exPlanations (SHAP) analysis and Partial Dependence Plots (PDPs) analysis. The results showed that Gaussian process regression (GPR) was the best model for predicting MR and CT, with R2 of 0.9967 and 0.9958, respectively, and root mean square errors (RMSE) of 0.0059 and 0.354 ℃. In addition, graphical user interface software was developed to facilitate the use of the GPR model for predicting MR and CT by researchers and engineers. This study contributes to the rapid prediction, improvement, and optimization of MR and CT during thermally assisted biodrying of MS, and also provides valuable guidance for the dynamic regulation of the drying process.

Relationships between arsenic biotransformation genes, antibiotic resistance genes, and microbial function under different arsenic stresses during composting

Although the arsenic contamination and antibiotic resistance genes (ARGs) during composting have been studied separately, there is limited information on their interactions, particularly, the relationship between arsenic biotransformation genes (ABGs) and ARGs. Therefore, the present study used different forms of arsenic stress (organic and inorganic arsenic at 10 and 50 mg/kg) in pig manure and straw co-composting, to evaluate the effects of arsenic stress on microbial community structures, metabolic function, ABGs, and ARGs. The results showed that arsenic stress had different effects on different parameters and promoted the microbial formation of humic acid and the biodegradation of fulvic acid. Inorganic arsenic showed more rapid effects on microbial community structure, visible within about 20 days, while the effects of organic arsenic were later (about 45 days) due to the necessity of transformation. Moreover, the addition of organic roxarsone and inorganic arsenic resulted in higher expression of ABGs and ARGs, respectively. Arsenic addition also caused increased expression of genes associated with replication and repair. A significant relationship was observed between ABG and ARG expression, for instance, genes involved in arsenic reduction and oxidation were influenced by genes involved in aminoglycoside and chloramphenicol resistance genes (p < 0.05). These complex interactions among microorganisms, functional genes, and external parameters contribute to the understanding of the mechanisms underlying cross-contamination.

Effects of complex pollution by microplastics and heavy metals on soil physicochemical properties and microbial communities under alternate wetting and drying conditions

Microplastics (MPs) broadly coexist with heavy metals (HMs) in soil, Cd and Cu are the main types of soil HMs contamination, in addition to polystyrene (PS), which is also widely present in the environment and prone to aging. However, differences in the effects of MPs and HMs on soil properties and microbial characteristics under alternating wetting and drying (AWD) remain unclear. Thus, this study investigated the effects of four conventional (0.2% (w/w)) and aged MPs in indoor incubation experiments on soil properties under desiccation (Dry) and AWD. We found that with the influence of the "enzyme lock" theory, the coexistence of MPs and HMs under Dry had a more pronounced effect on soil physicochemical properties, whereas the effects on soil enzyme activity under AWD were more significant. In addition, MPs decreased the available Cu by 4.27% and, conversely, increased the available Cd by 8.55%. Under Dry, MPs affected microbial function mainly through physicochemical properties, with a contribution of approximately 72.4%, whereas under AWD enzyme activity and HMs were significantly greater, with increases of 28.2% and 7.9%, respectively. These results indicate that the effects of MPs on environmental variation and microbial profiles under AWD conditions differed significantly from those under Dry.

Sodium alginate/sinter gel spheres immobilized lysozyme producing strain SJ25 enhanced sludge reduction: Optimization and mechanism

In order to promote sludge hydrolysis and improve the efficiency of aerobic digestion, the sodium alginate immobilized gel spheres pellet B (SIP B) were prepared using sodium alginate (SA) and sinter as carrier to immobilize lysozyme producing strain SJ25. The optimal conditions for SIP B to promote sludge hydrolysis were 5.6 mg SS^-1 dosage and pH of 9.0. Under the optimal condition compared with the control group, the reduction efficiency of suspended solids (SS) in 24 h was increased by 26.89 %, the release of soluble chemical oxygen demand (SCOD) was increased by 517.79 mg L^-1, polysaccharide (PS) and protein (PN) concentrations were increased by 186.69 and 368.68 mg L^-1, respectively. SIP B enhanced the degradation efficiency of sludge by promote the release of lysozyme, prolonging the action time of the enzyme, enhancing the metabolism and membrane transport of xenobiotics, carbohydrate and amino.