Critical moisture point of sludge and its link to vapour sorption and dewatering.

The mechanical dewatering of sludge is important in order to achieve a high dry matter content, thereby lowering the transportation cost and the energy consumption during incineration. Thermogravimetric analysis is sometimes used to estimate the maximum dry matter content obtainable from mechanical dewatering, by measuring the critical moisture point. In this paper, the critical moisture point of digested sludge was measured and compared with vapour sorption curves. The critical moisture point was determined for raw and conditioned sludge to be 3.4 kg of water per kg of dry matter, corresponding to 23% w/w dry matter. This value was lower than the dry matter content obtained from the mechanical dewatering process, indicating that the dry matter content can exceed the critical moisture point. Moisture vapour sorption was measured for raw, conditioned, and dewatered sludges. The Blahovec and Yanniotis sorption isotherm fitted the experimental data well. Between 10 and 12 g of water was adsorbed as a monolayer per 100 g of dry matter. The rest of the moisture content was explained by the non-ideal Raoult's law, by including the effect of dissolved ions. At water activities above 0.95, the moisture content was determined by capillary condensation and cake compressibility. The water activity was higher than 0.95  at the critical moisture point and the capillary pressure was estimated to be 4-6 bars. This pressure was responsible for cake compression during drying, while the relatively low dry matter content at the critical moisture point may be due to the low capillary pressure.

Antimicrobial Susceptibility Testing and Tentative Epidemiological Cutoff Values for Five Bacillus Species Relevant for Use as Animal Feed Additives or for Plant Protection.

Bacillus megaterium (n = 29), Bacillus velezensis (n = 26), Bacillus amyloliquefaciens (n = 6), Bacillus paralicheniformis (n = 28), and Bacillus licheniformis (n = 35) strains from different sources, origins, and time periods were tested for the MICs for nine antimicrobial agents by the CLSI-recommended method (Mueller-Hinton broth, 35°C, for 18 to 20 h), as well as with a modified CLSI method (Iso-Sensitest [IST] broth, 37°C [35°C for B. megaterium], 24 h). This allows a proposal of species-specific epidemiological cutoff values (ECOFFs) for the interpretation of antimicrobial resistance in these species. MICs determined by the modified CLSI method were 2- to 16-fold higher than with the CLSI-recommended method for several antimicrobials. The MIC distributions differed between species for five of the nine antimicrobials. Consequently, use of the modified CLSI method and interpretation of resistance by use of species-specific ECOFFs is recommended. The genome sequences of all strains were determined and used for screening for resistance genes against the ResFinder database and for multilocus sequence typing. A putative chloramphenicol acetyltransferase (cat) gene was found in one B. megaterium strain with an elevated chloramphenicol MIC compared to the other B. megaterium strains. In B. velezensis and B. amyloliquefaciens, a putative tetracycline efflux gene, tet(L), was found in all strains (n = 27) with reduced tetracycline susceptibility but was absent in susceptible strains. All B. paralicheniformis and 23% of B. licheniformis strains had elevated MICs for erythromycin and harbored ermD The presence of these resistance genes follows taxonomy suggesting they may be intrinsic rather than horizontally acquired. Reduced susceptibility to chloramphenicol, streptomycin, and clindamycin could not be explained in all species.IMPORTANCE When commercializing bacterial strains, like Bacillus spp., for feed applications or plant bioprotection, it is required that the strains are free of acquired antimicrobial resistance genes that could potentially spread to pathogenic bacteria, thereby adding to the pool of resistance genes that may cause treatment failures in humans or animals. Conversely, if antimicrobial resistance is intrinsic to a bacterial species, the risk of spreading horizontally to other bacteria is considered very low. Reliable susceptibility test methods and interpretation criteria at the species level are needed to accurately assess antimicrobial resistance levels. In the present study, tentative ECOFFs for five Bacillus species were determined, and the results showed that the variation in MICs followed the respective species. Moreover, putative resistance genes, which were detected by whole-genome sequencing and suggested to be intrinsic rather that acquired, could explain the resistance phenotypes in most cases.