Characterization of microbial regrowth potential shaped by advanced drinking water treatment

Microbial regrowth in premise plumbing is a threat to water safety. Disinfectant residuals are often diminished during water transportation and stagnation, leading to the regrowth of opportunistic pathogens. Although microbial regrowth potential is mostly determined by water treatment, little is known about how each treatment step affects two key factors that contribute to microbial regrowth potential: biodegradable organic matter and microbial abundance. In this study, we operated annular reactors to evaluate the microbial regrowth potential of water shaped after each treatment step in a full-scale drinking water treatment plant with ozonation and biological activated carbon filtration. The assimilable organic carbon and total cell count (TCC) were stable at all treatment steps during the sampling period from July to October 2015. The assimilable organic carbon consumption and TCC net increase in the annular reactors indicated that apparent growth yields (cell number base) of microbial communities were different in each reactor. Regrowth potential evaluated by indigenous microbial community in finished water was reduced to 22% of that in raw water, while 75% of assimilable organic carbon in raw water remained in finished water. It suggested that treatment performance evaluated by indigenous microbial communities was better than that evaluated by assimilable organic carbon.

Increased Likelihood of High Nitrous Oxide (N2O) Exchange in Soils at Reduced Microbial Diversity

Rare soil organisms are normally considered of less importance for ecosystem functioning. We present results that oppose this view. In otherwise well-aerated soils, anaerobic/microaerophilic production or consumption of the trace gas N2O occurs in small soil volumes, when intense decomposition activity at the site leads to local oxygen depletion. At such patch scales, the control of microbial growth and oxygen consumption may depend on the specific organisms present. We assessed N2O turnover in an experiment, where soil dilution from 10−2 over 10−4 to 10−6 followed by microbial regrowth resulted in similar microbial biomass and respiration but reduced diversity. We found an increasing number of very high N2O turnover rates when soil dilution increased from 10−2 over 10−4 to 10−6, as revealed from a significantly increased skewness of the frequency distribution of N2O turnover levels. N2O turnover also tended to increase (p = 0.08) by 20–30% when soil was diluted from 10−2 to 10−6. This suggests that rare soil organisms regulate the local activity of fast-growing microorganisms and thus reduce the probability that anoxic/microaerophilic soil volumes develop. Future studies may reveal which less abundant organisms prevent development of anoxic/microaerophilic conditions in well-aerated soils.

Aeromonas species from non-chlorinated distribution systems and their competitive planktonic growth in drinking water

Aeromonas is included in the Dutch Drinking Water Decree as an indicator for elevated microbial regrowth in non-chlorinated drinking water distribution systems (DWDS). The temporal and spatial diversity of Aeromonas species in ten DWDS and their planktonic growth characteristics for different carbon sources was investigated. Genotyping of the gyrB gene of isolates showed a non-systematic temporal and spatial variable prevalence of seven different Aeromonas species in these DWDS and no correlation with AOC-P17/NOX and Aeromonas concentrations. Pure cultures of these seven species showed a high affinity to low concentrations (μg/L) of individual amino acids and fatty acids, compounds associated with biomass. Growth occurred at 0.5 μg-C/L of an amino acid mixture. Growth of a mixed community of A. rivuli, A. salmonicida, A. sobria and A. veronii in drinking water occurred in pasteurized samples, however, no growth and decay occurred in competition with the autochthonous bacteria (non-pasteurized samples). This community also failed to grow in non-pasteurized distribution samples from a location with clear increase in planktonic Aeromonas concentrations in the transported drinking water. For competitive planktonic growth of Aeromonas an amino acid concentration of ≥5 μg-C/L is required. AOC-P17/NOX concentrations showed that such concentrations are not expected in Dutch drinking water. Therefore, we suspect that competitive planktonic growth is not the major cause of the observed non-compliance with the Aeromonas standard in non-chlorinated DWSD. Importance The occurrence of the bacterial genus Aeromonas in non-chlorinated drinking water in the Netherlands is regarded as an indication for elevated microbial regrowth in the distribution system. Identification of the prevalent species in ten distribution systems by genotyping yielded seven different species, with A. rivuli, A. veronii and A. sobria as the most dominant ones. Planktonic growth experiments of pure cultures confirmed former published affinity of Aeromonas for certain biomass compounds (amino and fatty acids). In competition with the autochthonous microflora, however, planktonic growth was not observed, only after addition of a threshold amino acid concentration of 5 μg-C/L. Based on our results and further observations we deduced that planktonic growth of Aeromonas in the DWDS is not very likely. Benthic growth in loose deposits and planktonic release is a more plausible explanation for the observed planktonic increase of Aeromonas.

Safety and passivation of faecal contamination in waste

Absorbent hygiene wastes like nappies and incontinence pads are ubiquitous in municipal and healthcare waste streams around the world as they are convenient products used in child-care and adult incontinence management. Absorbent Hygiene Product (AHP) manufacturing is resource-intensive as the products are required to be of the highest value as they are in almost-constant contact with sensitive body parts. The potential for recovering such valuable resources such as cellulose-based fibres and super-absorbent polymers for reuse in non-food sectors like the construction and wastewater industries has been considered in this study. Appropriate decontamination via chemical methods have been examined using AHPs contaminated with human-associated bacteria. Findings suggest that for simulated AHP wastes inoculated with 108–109 CFU g-1 of human-associated bacteria like Escherichia coli, Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus pyogenes, a 1:1 ratio of 0.5% calcium hypochlorite/AHP waste is adequate to inactivate the bacteria particularly when combined with an inorganic salt for at least 60 min. Specifically, 4 to 5 log10 reductions were observed. Following such disinfection, material storage and temperatures above 25ºC minimise incidences of microbial regrowth. The disinfection protocol was not found to adversely affect the AHP quality. Overall, such findings suggest that AHP recycling is a potential alternative to current AHP waste disposal practices like incineration (with or without energy recovery) and landfilling.

Stimulatory effects on bacteria induced by chemical cleaning cause severe biofouling of membranes

Chemical cleaning with hypochlorite is routinely used in membrane-based processes. However, a high-transient cleaning efficiency does not guarantee a low biofouling rate when filtration is restarted, with the physiological mechanisms largely remaining unknown. Herein, we investigated the microbial regrowth and surface colonization on membrane surfaces after NaOCl cleaning had been completed. Results of this study showed that the regrowth of model bacteria, Pseudomonas aeruginosa, was initially subject to inhibition due to the damage of key enzymes' activity and the accumulation of intracellular reactive oxygen species although the oxidative stress induced by NaOCl had been removed. However, with the resuscitation ongoing, the stimulatory effects became obvious, which was associated with the enhanced production of N-acyl homoserine lactones and the secretion of eDNA that ultimately led to more severe biofouling on the membrane surface. This study elucidates the inhibition–stimulation mechanisms involved in biofilm reformation (membrane biofouling) after membrane chemical cleaning, which is of particular significance to the improvement of cleaning efficiency and application of membrane technologies.