Clostridium cluster I and their pathogenic members in a full-scale operating biogas plant.

A biogas production plant operating with main and secondary digesters (MD, SD) was analysed for the diversity of bacteria from Clostridium cluster I and its pathogenic members. The plant was run in two parallel lines, both receiving silages, and one, in addition, cattle manure (CM). Quantitative PCR of 16S rRNA genes from directly extracted DNA indicated that cluster I represented 0.2 to 5.6 % of the total bacterial communities. Its prevalence was particularly low in CM and also in SD compared to MD, indicating its decline during fermentation. In contrast, another highly abundant clostridial group, i.e. the "faecal" cluster XIVa, remained quantitatively unaffected during fermentation. A total of 85.1 % of 581,934 rRNA gene sequences gathered by group-specific PCR from the silages, CM and digesters could be assigned to cluster I. All remaining sequences fell into other clostridial groups. The three most dominant operational taxonomic units (OTUs) introduced with CM were from cluster I, and they declined during fermentation. Fermentation with CM significantly increased OTUs of clostridia outside of cluster I but not within. The only OTUs related to pathogens were detected for Clostridium botulinum with 0.18 % of all cluster I sequences in maize silage and less than 0.01 % in the other substrates and digester materials. These OTUs could be assigned to all four established C. botulinum groups, thus, potentially covering all seven neurotoxins. Mouse lethality tests of samples with suspected presence of C. botulinum, however, indicated no toxigenic potential and, thus, no risk associated with the rare occurrence of these OTUs.

Anaerobic co-digestion of paper sludge: Feasibility of additional methane generation in mechanical-biological treatment plants.

In this work, the feasibility of the anaerobic digestion of paper sludge as a co-substrate in anaerobic digestion mechanical-biological treatment (MBT) plants is investigated. In the first phase, the biochemical properties, biomethane potential (BMP), and pollutant contents of 20 different industrial paper sludges are determined. Following the general evaluation in the BMP tests, the second phase of the project involves the semi-continuous co-digestion of six paper sludges in continuous stirred reactors (CSTR). Paper sludges are categorized according to their origin within the pulp and paper mills: Deinking Sludge (DS), Primary Sludge (PS) and Biological Sludge (BS). The analysis of potentially inhibiting elements shows that the concentrations of chlororganic compounds, mineral oil and some heavy metals are highest in DS, while the mean heavy metal loads in all paper sludges are relatively low compared to other industrial sludges. Large differences in total solids (TS) and volatile solids (VS) contents are observed among the different paper sludges investigated, with DS having the highest TS due to the high inorganic contents. The BMP of the investigated sludges ranges from 90 to 355 NL CH4 kg-1 VS. In subsequent semi-continuous co-digestion experiments simulating MBT conditions, three DS and two fiber sludges (a mixture of PS and BS) show good methane generation rates, while one fiber sludge causes inhibition and indicates an increase in viscosity. In general, co-digestion of paper sludge in anaerobic digestion MBT plants can be a viable option for energy production and also facilitates a safe disposal of the paper sludge digestates.