Impact Of Anaerobic Digestion Research Articles

Impact of anaerobic digestion on reactive nitrogen gas emissions from dairy slurry storage

Biogas digesters are commonly used to treat animal manure/slurry, and abundant digested slurry is generated during the digestion process. Gas emissions from digested and raw slurry may vary with the change in slurry parameters after digestion, but the mechanism is not well understood. Gas emissions from raw dairy slurry (RS) and digested dairy slurry (BS) during 98 days of storage were investigated in this study to evaluate the effects of anaerobic digestion on reactive nitrogen emissions from slurry storage. Results showed that much higher N2O and NO emission and lower NH3 emission was achieved in BS than in RS. The mean gaseous emission of RS and BS accounted for 27.8% ± 6.9% and 17.1% ± 2.3% of the initial TN for NH3, 0.1% ± 0.1% and 3.5% ± 1.6% of the initial TN for N2O, and 0.0% ± 0.0% and 0.2% ± 0.0% of the initial TN for NO, respectively. Among all detected N2O-forming and reducing microbial genes, the abundance of amoA genes was the most closely related to N2O flux (r = 0.54, p < 0.01). More aerobic conditions occurred in BS, and dissolved oxygen (DO) increased to 0.4–1.6 mg L−1 after 35 days because the low organic matter of BS resulted in good infiltration of surface air into the slurry. The increased DO stimulated the growth of Nitrosomonas and the increase in amoA gene copies and contributed to the high N2O and NO emissions in BS through the nitrification process. Vulcanibacillus, Thauera, Castellaniella, and Thermomonas were the major denitrifying bacteria that occurred in BS and caused an incomplete denitrification process, which could be another reason for the increase in N2O and NO emissions from BS. Our study indicated that anaerobic digestion reduced the organic matter content of the slurry and caused an active microbial environment that facilitated the transformation of slurry N to N2O in BS storage, thus lowering the NH3 emission compared with RS storage. Therefore, aside from NH3, N2O should also be preferentially mitigated during BS storage because N2O is a greenhouse gas with high global warming potential.

Impact of anaerobic digestion and centrifugation/decanting processes in bacterial communities fractions

Sewage sludge can be treated by anaerobic processes that frequently are followed by physical separation processes. In this work, a high-throughput sequencing technology, based on variation in the bacterial 16S rRNA gene, has been used to characterise the bacterial populations present in samples taken from different points of an industrial anaerobic digestion process fed with sewage sludge. Relative abundances of phyla and classes throughout the biological process and the subsequent separation steps were determined. Results revealed that the Bacteroidetes, Firmicutes and Proteobacteria phyla were the most representative. However, significant changes in relative abundance were detected along treatments, showing the influence of operational parameters on the distribution of microorganisms throughout the process. After anaerobic digestion, phylum Firmicutes doubled its relative abundance, which seems to indicate that the anaerobic conditions and the nutrients favoured its growth, in contrast to other phyla that almost disappeared. After centrifugation, Proteobacteria went preferentially to the solid phase, in contrast to Firmicutes which was the dominant phylum in the liquid phase. After decanting the liquid phase during 45h, an important growth of Proteobacteria, Spirochaetes and Tenericutes was detected. At class level, only significantly changes were observed for Proteobacteria classes being α-proteobacteria dominant in the digestate, while γ-proteobacteria was the majority since this point to the final steps. To know the changes on the kind and abundance of microbial populations throughout the anaerobic and separation processes is very important to understand how the facilities design and operation conditions can influence over the efficiencies of next biological treatments.

Mitigation of greenhouse gas emissions by adopting anaerobic digestion technology on dairy, sow and pig farms in Finland

The impact of anaerobic digestion (AD) technology on mitigating greenhouse gas (GHG) emissions from manure management on typical dairy, sow and pig farms in Finland was compared. Firstly, the total annual GHG emissions from the farms were calculated using IPCC guidelines for a similar slurry type manure management system. Secondly, laboratory-scale experiments were conducted to estimate methane (CH 4) potentials and process parameters for semi-continuous digestion of manures. Finally, the obtained experimental data were used to evaluate the potential renewable energy production and subsequently, the possible GHG emissions that could be avoided through adoption of AD technology on the studied farms. Results showed that enteric fermentation (CH 4) and manure management (CH 4 and N 2O) accounted for 231.3, 32.3 and 18.3 Mg of CO 2 eq. yr −1 on dairy, sow and pig farms, respectively. With the existing farm data and experimental methane yields, an estimated renewable energy of 115.2, 36.3 and 79.5 MWh of heat yr −1 and 62.8, 21.8 and 47.7 MWh of electricity yr −1 could be generated in a CHP plant on these farms respectively. The total GHG emissions that could be offset on the studied dairy cow, sow and pig farms were 177, 87.7 and 125.6 Mg of CO 2 eq. yr −1, respectively. The impact of AD technology on mitigating GHG emissions was mainly through replaced fossil fuel consumption followed by reduced emissions due to reduced fertilizer use and production, and from manure management.

Bioavailability of Cu and Zn in raw and anaerobically digested pig slurry

The impact of anaerobic digestion on the bioavailability of copper and zinc from pig slurry was assessed. Both chemical and biological approaches were used independently on raw slurry (RS) and anaerobically digested pig slurry (DS). This work, using ultracentrifugation pellets from the same pig slurry before and after an anaerobic treatment, confirmed that Cu and Zn behave differently in terms of bioavailability, and contrasting results were obtained by chemical and biological assessments. A chemical approach combined a preliminary study of the pH effect on particulate/dissolved metal partitioning, sequential extraction, and biochemical fractionation. This approach tended to show a lower mobility of metals from digested slurry (DS). A biological approach was carried out with Zea mays and Vicia faba to study Cu and Zn uptake in soil amended with RS or DS. This assay could not differentiate the two slurries.

Impact of anaerobic digestion on organic matter quality in pig slurry

Changes in pig slurry organic matter (OM) during anaerobic digestion (AD) were studied in a reactor to characterize OM evolution through AD. OM maturity and stability were evaluated using different biological and physico-chemical methods. Germination and growth chamber experiments revealed a higher maturity of digested slurry (DS) than raw slurry (RS). Soil incubations showed that DS was more stable than RS with a C-mineralization of 12.0 g CO 2-C 100 g −1 C org after 49 days as compared to 17.6 g CO 2-C 100 g −1 C org. Biochemical fractionation showed a relative increase in stable compounds such as hemicellulose-like and lignin-like molecules. Fourier-transform infrared spectroscopy showed some changes in the chemical structures of OM with a reduction in the aliphatic chain, lipid and polysaccharide levels. A comparison between the evolution of OM during AD and the first weeks of a composting process showed almost identical changes. Finally a theoretical method called Fictitious Atomic-group Separation was applied to the elemental compositions of RS and DS. DS was less humified than RS and presented the properties of a fulvic acid, indicating that the observed stability in DS was mainly due to the biodegradation of the most labile compounds.

Bacterial removal by anaerobic digestion

The impact of anaerobic digestion and post composting of the digested slurry on total bacterial counts, total coliform, fecal coliform, and fecal streptococci was investigated. Anaerobic digestion removed up to 99% of the bacterial parameters; however, the residual numbers present in the digested slurry were still high. The residuals ranged from 10 5/g to 10 13/g for total bacterial counts, 10 2/g to 10 8/g for total coliform, 10 4/g to 10 8/g for fecal coliform, and 10 2/g to 10 8/g for fecal streptococci. The digested slurry was mixed with soil for one week and then for two months, but the density of bacterial parameters was not reduced. There was an increase in numbers during the two composting steps.