Laboratory-scale assessment of vacuum-degassed activated sludge for improved settling properties

Diversity among activated sludge in vacuum degassed laboratory systems

Vacuum degassing of activated sludge is a technology used to improve sludge settling. By improving the settling ability of the sludge, a higher amount of biomass can be kept in the bioreactor, which further results in better wastewater treatment results. However, the momentaneous vacuum exposition has been found a stress agent for activated sludge flocs and bacteria and may cause changes in sludge activity. However, no biological studies on the long-term intermittent application of vacuum to activated sludge have been published so far. The question arises whether the improvement in the degree of wastewater treatment results from an increase in the amount of biomass involved in the treatment process or does the change in pressure stimulate bacteria to increased activity? The study aimed to examine whether and how cyclic pressure reduction in the biological system affects the activity and composition of bacterial biocenosis of activated sludge. Three sequencing batch reactors were operated for almost three months. The work cycle of two of them included a vacuum degassing stage inserted between reaction and settling stage. Degassing was obtained with a pressure of 300 or 30 hPa. In addition to the wastewater quality analyzes, the microbial activity, number and variety of activated sludge bacteria and the characteristics of activated sludge flocs were determined. There were no significant differences between the reactors in the obtained effects of nutrient removal. All reactors showed organic compounds removal around 93%, and 40% and 58% of nitrogen and phosphorus removal, respectively. Obtained differences in respiratory and dehydrogenase activity were not significant. The biodiversity assessed with DNA sequencing revealed sludge enrichment with unclassified bacteria. Moreover, vacuum degassing caused flocs disintegration. In both the vacuum degassing reactors, the floc size range was much narrower than that of the control sludge. In the sludge degassed with a pressure of 30 hPa, the flocs were 25-80% smaller than in the sludge without the influence of a vacuum. The total number of bacteria was comparable among the reactors, however, in the reactor with degassing pressure of 30 hPa, the share of dead bacteria in the activated sludge (11%) was significantly lower than in other reactors (about 16%). The concentration of extracellular polymers in activated sludge was up to 87% higher when using vacuum degassing of 30 hPa than in other reactors. The results of the presented research show that the changes in the activated sludge occurring under the influence of vacuum degassing do not change the effectiveness of wastewater treatment, but may alter the community composition.

What do we know about the influence of vacuum on bacterial biocenosis used in environmental biotechnologies?

The article aims to show the increased interest in the applications of vacuum in the area of environmental biotechnology and the lack of research related to the effects of vacuum on bacteria and microbial communities. Information on the impact of vacuum on bacteria is limited and often comes from unrelated research fields. In most cases (astrobiology research, food preservation technologies), the exposure of microorganisms in vacuum is permanent for the whole life of a cell. In environmental science applications, the exposure of microorganisms containing media such as sludge or soil in vacuum is rather persistent, and lower values of vacuum are used. Vacuum is used or proposed to be used in wastewater treatment, anaerobic digestion, sludge treatment, soil remediation and mining. Usually, vacuum is used to remove gases from the test medium, so a purely physical process is applied. However, most reports show the influence of vacuum on biological processes and its efficiency, as well as on the community structure.