Anaerobic digestion of crude glycerol from biodiesel production for biogas generation: Process optimization and pilot scale operation

Anaerobic digestion of wastes and wastewater is a complex process that can be affected by many operational parameters. In this context, the purpose of the present study was to optimize biogas production using crude glycerol (GLY) generated in biodiesel production from waste cooking oil without pretreatment or nutrient supplementation. The study was divided into two parts: the first phase consisted of an experimental design based on central composite design (CCD) with two variables (food to microorganism (F/M) ratio and cycle time) at five levels (F/M of 0.20; 0.51; 1.02; 1.53 and 2.04 gCOD/gVS; tc of 3, 4, 5, 6, 7 days) focusing on optimizing the biogas production from crude GLY in lab-scale batch reactors (500 mL). The second phase was conducted on a pilot-scale biodigester (1.2 m3) based on the optimized variables obtained from the CCD. The optimized results showed that the F/M ratio of 2.04 gCOD/gVS and a cycle time (tc) of 6 days reached the highest specific methane production (SMP) of 46 LCH4/kgVS. However, the highest SMP of 14.7 LCH4/kgVSd was obtained during the operation of the pilot-scale biodigester for the optimized conditions of F/M ratio of 0.23 gCOD/gSV and a tc of 7 days. Therefore, pilot-scale biogas production from crude GLY was demonstrated to be feasible without the use of nutrients or GLY pretreatment at 0.15 LGLY/m3 d.

Virulence profile of Candida spp. isolated from an anaerobic biodigester supplied with dairy cattle waste

Anaerobic biodigesters play a crucial role in enhancing animal waste management. However, the presence of pathogens in the biodigestion process poses a significant concern. Candida spp., a widespread fungus known for its opportunistic nature and adaptability to diverse environmental conditions, including reciprocal transmission between humans and animals, is one such pathogen of concern. Therefore, it is imperative to assess the virulence profile of Candida spp. originating from anaerobic biodigestion processes. Here we demonstrate that strains isolated from the biodigestion process of dairy cattle waste exhibit noteworthy virulence mechanisms, surpassing the virulence of clinical control strains. After we identified strains from affluent, effluent, and biofertilizer, we observed that all analyzed isolates produced biofilm. Additionally, a substantial proportion of these isolates demonstrated phospholipase production, while only a few strains exhibited protease production. Furthermore, all strains exhibited resistance or dose-dependent responses to amphotericin B and itraconazole, with the majority displaying resistance to fluconazole. In the in vivo test, we observed a significant correlation (p < 0.05) between the LT50 and biofilm formation as well as hyphae/pseudohyphae production. Additionally, some isolates demonstrated a quicker nematode-killing capacity compared to clinical controls. Our findings underscore the considerable pathogenic potential of certain Candida species present in the dynamics of anaerobic biodigestion. Importantly, the anaerobic biodigester system did not eliminate Candida strains from dairy cattle waste, highlighting the need for caution in utilizing biodigester products. We advocate for further studies to explore the virulence of other microorganisms in various animal production contexts. Furthermore, our results emphasize the urgency of enhancing waste treatment methods to effectively eliminate pathogens and curb their potential dissemination.

Toxic risk evaluation of effluents from a swine biodigester in the plant models Lactuca sativa and Allium cepa

Pig farming is recognized as an activity with great polluting potential. The aim was to investigate possible environmental risks of effluents from the stabilization pond (SP) and the raw effluent (RE) from the biodigestion process of swine residues, in different concentrations in the models Lactuca sativa and Allium cepa. Seeds were germinated in different dilutions, 100% (C1), 50% (C2), 25% (C3), 12.5% (C4), 6.25% (C5), 3.12% (C6), 0.78% (C7), and 0.39% (C8). Distilled water was used as the negative control (CN) and trifluralin (0.84 g/L-1) as the positive control. Germination (GR), root growth (RG), cell cycle, and oxidative stress (OS) were analyzed. To assess OS, the activity of the enzymes superoxide dismutase (SOD), catalase (CAT), and glutathione S-transferase (GST) and the quantification of glutathione (GSH) and lipid peroxidation (LPO) were analyzed. Data were submitted to ANOVA (one way), followed by the Kruskal-Wallis mean test (P ≤ 0.05). Chemical analysis showed high values of Cu, Fe, Mn, and Zn. Dilutions (C1, C2, C3 RE) and (C1 and C2 SP) inhibited GR and RG of L. sativa and A. cepa than other concentrations. The mitotic index showed a reduction in C5 (RE), C6, and C7 (SP) of L. sativa and C3 and C4 (SP) of A. cepa in relation to CN and higher frequencies of chromosomal alterations. Regarding the OS, only the concentrations of SP treatment showed statistical difference in relation to the NC: in L. sativa model, GSH at (C5 and C8) concentrations and LPO (C7); in A. cepa model, SOD (C3 and C4), GST (C4, C5 and C6), GSH (C5 and C8), and CAT (C3 and C7). The alterations in metabolism are possibly related to the metals, such as zinc and copper, observed in high amounts in the raw waste. The results allowed us to conclude that the raw and stabilization pond effluents offer environmental risks, requiring caution and monitoring in the use of these effluents.

Hygienization and control of Diplodia seriata fungus in vine pruning waste composting and its seasonal variability in open and closed systems

After the ban on sodium arsenite, waste management alternatives to the prevalent burning method, such as the hygienization and biodegradation in solid phase by composting, are required for the pruned material from grapevines affected by various fungi. In this work the dynamics of a fungus associated with vine decay (Diplodia seriata) during the composting process of a mixture of laying hen manure and vine pruning waste (2:1w/w) have been investigated in an open pile and a discontinuous closed biodigester. Through the optimization of the various physical-chemical parameters, hygienization of the infected waste materials was attained, yielding class-A organo-mineral fertilizers. Nevertheless, important differences in the efficiency of each system were observed: whereas in the open pile it took 10days to control D. seriata and 35 additional composting days to achieve full inactivation, in the discontinuous biodigester the fungus was entirely inactivated within the first 3-7days. Finally, the impact of seasonal variability was assessed and summer temperatures shown to have greater significance in the open pile.

The financial and economic feasibility of rural household biodigesters for poor communities in South Africa

Given the persistence of systemic poverty in, most notably, the rural parts of South Africa, the question is whether the use of biodigesters as a source of energy offers potential solutions to some of the difficulties and development needs faced by people in these areas. At the core, this translates into whether this technology would be financially and economically feasible for installation and use by rural households. Here we conduct both a financial and an economic cost-benefit analysis in one such community based on survey data from 120 households. Analysis of these data and supporting literature reveals that a biodigester is not a financially feasible investment for a rural household. Substantial economic benefits are, however, found to make a biodigester a worthwhile investment from a broader societal perspective. This is a compelling argument for further study and the consideration of government support in the light of broader economy-wide benefits.