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The entrainment ratio is negatively correlated with the height and width of nozzle outlet, and positively with nozzle extension width. The primary mass had nothing to do with the diffuser configuration, while the secondary mass, the total mass and the entrainment ratio increased with the diffuser height, the diffuser width and the nozzle-diffuser distance. The research findings provide a valuable theoretical guidance for pulse jet cleaning system of filters. Air pollution control systems in WtE units: An overview.
Waste Management 5: Experimental study on the nozzle effect of the pulse cleaning for the ceramic filter candle. Powder Technology A novel in situ membrane cleaning method using periodic electrolysis. Journal of Membrane Science Three dimensional modeling of gas-solid coupled free and Porous flow in a filtration process.
International Journal of Heat and Technology Design and performance evaluation of dry cleaning process for syngas. Fuel Cleaning mechanisms in pulse jet fabric filters. Filtration and Separation 21 1 : Particulate control by pulsed-air bag-house filtration: describing equations and solutions.
Aspects of nozzle effect on the pulse jet cleaning of a ceramic filter. Comparison of numerical and experimental investigations of jet ejector with blower. International Journal of Thermal Sciences Experimental study of pleated fabric cartridges in a pulse-jet cleaned dust collector. Cleaning of filter media by pulsed flow — Establishment of dimensionless operation numbers describing the cleaning result. Journal of Food Engineering Effect of induced airflow on the surface static pressure of pleated fabric filter cartridges during pulse jet cleaning. The optimized relationship between jet distance and nozzle diameter of a pulse-jet cartridge filter.
Pulse jet cleaning of rigid filter elements at high temperature. Simulation of long-term behavior of regenerateable dust filters. Filtration and Separation 5: Simulation of the regeneration of dust filters. Mathematics and Computers in Simulation A non-metric multidimensional scaling nMDS plot was used to describe the root community structure while the degree of similarity was explored with the permutation-based hypothesis statistical test ANOSIM.
The Shannon—Wiener diversity index among the PE communities was calculated [ 35 ]. Next generation sequencing data analysis was performed on fastq files. After assembly of the paired-end reads, adapter trimming with a threshold of 0.
The default minimum quality threshold of 25 was used. Rarefied OTU tables were generated and all samples were subsampled to sequences per sample. The linear discriminant analysis LDA effect size LEfSe [ 39 ] was performed to identify the biomarker species between the initial and acclimated biofilm communities. A microcosm experiment was conducted in two phases in order to evaluate the ability of two different marine consortia the non-acclimated and acclimated marine community to degrade weathered PE films Fig 1D.
Seawater was selected as the aqueous medium in order to simulate the pelagic zone and polyethylene was the only carbon source to allow the growth of only potential PE consumers.
Successful adaptation of the microorganisms on weathered PE surface would lead to the development of a viable community. During phase I, the BIOG consortium developed a visible biofilm on the weathered PE flakes after 4 months of incubation Fig 1D while biofilm formation was merely detected by naked eye on the PE samples in the indigenous treatment until the end of this phase. At that time, the biofilm populations of both treatments were harvested and cultured in order to verify that there were still active cells.
Similar abundances were observed bioaugmented: 3. Taking into account that a population of 10 8 cells mL -1 was added in the inoculated microcosms, a decrease in their abundance was noticed. Afterwards, they decreased and 10 2 CFU mL -1 were counted in the water samples of month 6.
When they were incubated for two months, visible biofilm was detected on PE films in all treatments. Considering biodegradation of polyethylene as a slow procedure, the concentration of live biofilm cells was monitored to ensure metabolic activity during the experiment. As seen in Fig 2B , the bacteria were able to proliferate throughout the experiment. However, the two different consortia displayed variations concerning the colonization efficiency and biofilm development. Interestingly, the bioaugmented biofilm population decreased until month 3, then it increased until month 5 and decreased again.
At month 1 and 5, this population exhibited the maximum concentration of 10 9 CFU cm The indigenous biofilm community increased until month 3 to 10 9 CFU cm -2 , during month 4 and 5 remained stable and then it decreased to 10 4 CFU cm In this experiment, the surface of all the plastic pieces was 1 cm 2 , since the weight reduction is proportional to the surface of polymer. The percentage of the weight reduction of weathered PE films owing to biodegradation is presented in Fig 3A.
In phase I, after six months incubation, both non acclimated consortia decreased the PE films by a small fraction which corresponded to approximately 0. A Percentage of weight reduction by the different marine consortia A: phase I, B: phase II ; B Percentage of weight reduction of the different polyethylene pieces during the phase II by both treatments. A significant boost in the weight reduction was detected when the acclimated consortia were utilized.
After one month incubation, the bioaugmented community decreased already 2. A similar pattern was observed for the indigenous treatment, whereas this community reduced 4. It is important to mention that no weight reduction was recorded when the sterile plastic pieces abiotic control were incubated with sterile saline water for a period of six months. Moreover, significant interaction effects F: 9. At the end of phase I, the extent of colonization on the polymer surfaces was visually observed and a dense layer covering the plastic surfaces was observed.
In order to monitor the microbial attachment, samples were taken every month during phase II. After 6 months incubation, the plastic surfaces were also fully covered by a thick multi-layer matrix of material. This technique was also used in order to verify any potential erosion on the surface due to microbial activity.
The surface topography of non-treated samples and samples subjected to the consortia was compared and defects were detected on the treated samples. As seen in Fig 4C , the weathered PE films had a wavy appearance with some small cracks probably because they were exposed to UV radiation and temperature changes before they were collected. With the aid of recent tube-model theories it is possible to determine the molecular weight distribution of polymers with rheological measurements [ 40 ]. For the present samples we restrict the discussion on the qualitative features of the measured viscoelastic spectra.
A similar pattern was obtained for the weathered HDPE treated with the indigenous consortium. A shift to lower cross over point was noticed, although this shift occurred to a lesser extent. These rheological measurements suggest that the molar mass distribution of the microbially-treated polymers has been likely broadened implying a shorter and less branched molecule.
Moreover, the treated polymers have a marginally lower average molar mass, indicating a biodegradation effect as opposed to fragmentation due to abiotic stresses. Changes of the functional groups on surface of polyethylene films were monitored with FTIR spectroscopy Fig 6 and S3 Fig throughout the experimental period. Interestingly, the spectra of microbially treated films at the end of experimental period are similar to the spectra of the virgin polymers. Moreover, crystallinity increased in LDPE weathered films in comparison to virgin ones and progressively decreased in the microbially treated films.
With respect to HDPE pieces, crystallinity decreased in weathered films, increased in films at month 1 and again progressively decreased. ARISA analysis was performed in the biofilm samples collected at the end of the phase I and during phase II, in order to monitor the succession of bacterial communities. It seems that the communities of the first two months were more similar to the initial while the biofilm structure of the month 5 was the most discrete.
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When the initial consortia were the selecting factor, no significant differences were detected among the communities adhered to the PE at various time intervals. When comparing the diversities Fig 7B , no significant effects were exhibited due to months of cultivation F: 0. Moreover, no significant interaction effect was detected between the two factors.
The indigenous assemblages tend to be less diverse along time while the diversity of bioaugmented community increased until the month 2 and then reduced. Next to ARISA analysis, samples from the seawater used as the aqueous media as well as from the biofilm that was developed at the end of the two phases were sequenced with next generation sequencing techniques 16S rRNA gene sequencing using the MiSeq platform. Phyla Bacteroidetes and Actinobacteria were also abundant. Moreover, the relative abundance of p. Bacteroidetes was higher in samples collected from the water column and from the biofilm samples of phase I over the p.
Actinobacteria while the opposite occurs in biofilm samples collected at the end of phase II. Community composition of major A bacterial phyla and classes B of the biofilm communities, C PCoA plot of the PE adhered communities and D alkB gene copy numbers in biofilm communities. Moreover, the inoculated strains were not detected in the biofilm communities harvested already at the end of the phase I, indicating that they could not dominate over the indigenous marine species or even survive.
Based on these results, we infer that both acclimated biofilm communities are comprised of indigenous marine species. It seems that alkB exhibits higher concentration in bioaugmented communities in comparison to the indigenous ones. The gene was not detected in any community at the end of first month, while this was also the case for indigenous communities at the end of the second month. The highest abundance of alkB was measured at month 4 and 5 in bioaugmented and indigenous communities respectively. The discriminant OTUs in final bioaugmented biofilm community were also assigned to the genera Cellulosimicrobium and Ochrobactrum.
The abundances of taxa increased in the acclimated biofilm communities are presented in the S4 Fig. Biofilm biomarkers of the initial consortium and the final developed communities. Although polyethylene is considered non-biodegradable, studies have shown that weathering by exposure to photo-oxidation or thermal oxidation favor microbial attachment and degradation [ 41 , 42 ]. This process leads to carbonyl residues that can be used as carbon source by microorganisms.
Besides thermal or light degradation, the abiotic pre-treatment also involves the mechanical and chemical impact of various factors on polymers [ 43 ]. In this context, naturally weathered PE pieces were collected from various beaches in Crete. A two-phase microcosm experiment was further conducted in order to evaluate the ability of different consortia to degrade polyethylene in the marine environment.
During the phase I, the bioaugmented and indigenous marine communities were incubated with weathered PE pieces for 6 months. Since the biofilm formation is a prerequisite in polymer biodegradation process, only the viable biofilm cells were harvested and were further inoculated with PE pieces again for 6 months phase II. It was demonstrated that biofilm cells possess structural and physiological characteristics that offer them high chances for adaption to LDPE surfaces in comparison to the planktonic cells [ 44 ].
The bacteria of both treatments were able to survive and thrive using weathered polyethylene as a sole carbon source.
High numbers of biofilm cells were enumerated at the end of the first month, the population tended to increase until it reached a plateau and then decreased. The abundance of free cells decreased through time, but a population was maintained despite the carbon starvation. It can be hypothesized that the mature biofilms release dispersal cells to the water column during the experiment [ 45 ]. The polymer served not only as the sole carbon source but also as a substrate, underlying that the hydrophobicity of the planktonic consortia should have been enhanced by the carbon limitation.
The efficient colonization of non-soluble surfaces is the first step in the polymer biodegradation and the excretion of extracellular enzymes follows [ 18 ]. Bacteria should overcome the hydrophobicity of the polymer with surfactant production or the strains with hydrophobic surfaces are the first colonizers [ 19 ]. Microorganisms adhere to the polymer and the breakdown of the big chain to smaller molecules initiates due to physical, chemical or enzymatic processes.
Both consortia developed a dense matrix of material, similar to those visualized of biofilms in literature [ 46 ] on the weathered polymer surfaces, as being visualized by the scanning electron microscopy. The formation of biofilm was was consistently observable on inoculated films in all the treatments independently of the type of polymer and the extent of weight loss. In general, HDPE films are not very attractive substances to adhere to and most bacteria display dispersed patterns of colonization [ 42 , 47 ].
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The biofilm development was extensive in the first 30 days of incubation and no significant changes were further detected. Rapid biofilm formation was also noticed on polyethylene plastic food bags submersed in seawater [ 48 ]. A visible layer was developed on these bags already after one week exposure and kept increasing throughout the experiment. In ocean, immersed or buoyant plastics are susceptible to biofouling that leads to sinking and degradation [ 22 , 49 ].
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Plastic niche presents a floating habitat thus harbors a variety of microorganisms and few invertebrate taxa [ 50 , 51 ]. Moreover, the plastic residents are metabolically active and distinct from the surrounding planktonic microbiota, comprising members of Bryozoa , Cyanobacteria , Alphaproteobacteria , and Bacteroidetes [ 52 ]. In this experiment, the planktonic bacterial community was not significantly different from the biofilm communities developed on PE surfaces. Members of Proteobacteria dominate all the bacterial assemblages while members of Alphaproteobacteria and Gammaproteobacteria are the most abundant classes in the biofilm samples.
It is difficult to identify the pioneer species and their role in the colonization process since the community composition alters within the first day [ 53 ] or over a longer period of time [ 54 ]. In accordance, monitoring of the microbial succession on weathered PE surfaces revealed a time dependent community structure, with shifts towards less diverse communities over months.
During phase II where the degradation is more prominent, the well-developed biofilm communities differ significantly from the initial consortia, implying that an efficient microbial network has been developed on PE surfaces.
Whereas, no significant difference was detected between the biofilm compositions of both treatments, underlying a convergence of the PE associated communities. These results imply that the substrate weathered PE films is responsible for shaping the biofilm bacterial community structure. Significant increase in the abundance of specific bacterial genera such as Bacillus in the mature biofilm was observed, that has previously been associated with PE degradation [ 19 , 20 ]. At the same time, species participating in hydrocarbon or natural polymers degradation have been enriched in the acclimated biofilm community.
For example, the genus Pseudonocardia carries the gene responsible for encoding AlkB -rubredoxin fused proteins, which is one of the key enzymes in alkane degradation pathway in bacteria [ 55 ]. Interestingly, the concentration of genus Cellulosimicrobium comprising of hydrocarbon and cellulose degraders [ 56 , 57 ] was only increased in the acclimated bioaugmented assemblages, where the highest weight reduction was recorded. Moreover, these two genera exhibit higher abundances in the bioaugmented community in accordance with the higher concentration of alkB gene.
More specifically, it appears that alkB harboring bacteria are significantly stimulated within the bioaugmented biofilm population. The alkB gene encodes the alkane 1-monooxygenase and is considered one of the key participants in polyethylene degradation [ 19 , 58 ]. Since successful colonization and population establishment does not verify polymer degradation [ 59 ], weight reduction, surface images, monitoring of chemical changes on the surface and changes in the molecular weight were obtained in order to evaluate the degradation accomplished by the consortia.
The extent of weight reduction varied and depended on several factors such as the type of polymer and the degree of acclimatization of the microorganisms. LDPE is a more branched polymer in comparison to HDPE, the intermolecular forces are weaker, the tensile strength as well as density are lower and hence, it is more susceptible to degradation [ 20 ]. Higher weight reduction was demonstrated in phase II when acclimated consortia were exploited. Similarly, 4. These results imply that the assimilation of weathered plastics by a previously exposed to them community overcomes the fragmentation to microplastics since HDPE films were found resistant to abiotic fragmentation when immersed in seawater after 6-months incubation [ 60 ].
Various results with respect to weight reduction have been reported until now [ 61 — 63 ]. Marine strains, affiliated to Arthrobacter sp. Microbial activity induces changes on the surface chemistry that can be elucidated with FTIR spectroscopy. In detail, a decrease or increase in the concentration of functional groups serve as indication of biological activity which is further enhanced in oxidised substrates [ 19 ]. Modification on the intensity of bands of HDPE films subjected to microbial activity was demonstrated [ 63 ]. In this experiment, several bands were detected on the surface of weathered plastic films in comparison to virgin ones.
Interestingly, these bands were depleted and the chemical structure of the pieces at the end of phase II was similar to the profile of virgin polymers. We believe that microorganisms consumed the weathered part of the PE film and then as they reached the virgin molecules which are much more resistant to biodegradation their concentration decreased. Besides the presence of functional groups on the surface, crystallinity is an important parameter in monitoring PE biodegradation.
In general, abiotic parameters increase crystallinity by degrading the amorphous regions [ 65 ] while similar effects have also been reported in case of biodegradation [ 20 ]. A progressive decrease in crystallinity has been observed in phase II, in accordance with other studies [ 21 , 66 ]. Once microorganisms consume completely the amorphous regions of the polymer, they start to degrade the smaller crystals thus increasing the proportion of larger crystals [ 19 , 20 ].
Alteration of the rheological properties and molecular mass distribution serve as an indicator of polymer biodegradation [ 20 , 43 ]. However, the effect of microbial activity on the molecular weight of polyethylene is a controversial issue since many researchers demonstrate no change of the molecular weight while others observe decrease or increase [ 19 ]. For example, Hadad et al. Whereas incubation with various microorganisms did not have a significant effect on molar mass of PE films [ 41 ].
In our experiments, the rheological results demonstrate that a shorter molecule with wider molecular mass distribution was produced after microbial attack. Changes on the surface topography of the weathered polymer films further support the biodegradation hypothesis. Progressive erosion on the plastic substances was observed during the experiment; the longer the incubation period was the more fissures and cracks appeared.
Alteration of the initial film surface due to microbial growth has been elucidated by many studies [ 19 , 62 , 68 ], where grooves and pits were detected superficially after polymers were subjected to biodegradation. Unravelling the underlying mechanisms of plastic colonization and subsequently the role of platisphere community in PE degradation will help towards successful remediation strategies.
It was demonstrated that tailored indigenous marine communities comprising of polymer and hydrocarbon degrader species have the potential to degrade naturally weathered PE films in the marine environment before they are turned into microplastics. The bacterial populations were able to develop a dense biofilm on the weathered PE surfaces and induced alterations on the surface topography and chemistry and on rheological properties along with the weight decrease of the samples.
At the end of the phase II, it appears that the developed consortium has depleted most of the weathered polymer as confirmed by FTIR spectra and the remaining PE film is lightly or not at all weathered as its FTIR spectrum is very similar to the virgin PE spectrum. Correlation between the molar mass distribution MMD and the viscoelastic behavior of the polymer.
We thank S. Costanzo and D. Chatzidoukas Chemical Eng. National Center for Biotechnology Information , U. PLoS One. Published online Aug Zhili He, Editor. Author information Article notes Copyright and License information Disclaimer. Competing Interests: The authors have declared that no competing interests exist.
Received Mar 28; Accepted Aug This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article has been cited by other articles in PMC. S2 Fig: SEM analysis. S4 Fig: Biomarkers. Abstract This study investigated the potential of bacterial-mediated polyethylene PE degradation in a two-phase microcosm experiment.
Introduction Plastics are synthetic organic polymers that are manufactured from petrochemicals and show various characteristics such as plasticity and high molecular mass. Materials and methods Samples collection and preparation Naturally weathered PE plastics were collected from two coastal sites in Northern Crete; Agios Onoufrios coordinates: Open in a separate window. Fig 1. Experimental design.
Bacterial strains Bacterial isolates belonging to the genera Lysinibacillus and Salinibacterium were provided from Prof. Experimental design Biodegradation tests were performed in triplicates in pre-sterilized beakers Fig 1. Development of a consortium The acclimated biofilm communities were cultured in Standard I nutrient broth until the late log phase the growth curves of each microbial community were previously performed by measuring the absorbance and cell numbers at same time intervals.
Weight loss measurements The measure of weight loss is a quick method to estimate the biodegradation of polymers assuming no abiotic processes take place that would cause weight reduction. Scanning electron microscopy SEM Both non-treated and microbially treated plastic pieces were subjected to SEM analysis in order to observe potential bio-erosion on the surface of the polymers pieces as well as biofilm formation on the latter pieces. Rheology measurements The samples were shaped into discotic specimens at room temperature using a home-made vacuum mold along with a mechanical press with applied pressure of 0.
Data analysis Statistical analysis was carried out with the automatic R software package [ 33 ]. Results Microbial growth in microcosms A microcosm experiment was conducted in two phases in order to evaluate the ability of two different marine consortia the non-acclimated and acclimated marine community to degrade weathered PE films Fig 1D. Fig 2. Cell densities. Weight reduction due to biodegradation In this experiment, the surface of all the plastic pieces was 1 cm 2 , since the weight reduction is proportional to the surface of polymer.
Fig 3. Weight reduction of PE films. Fig 4. SEM analysis. Rheological behavior of weathered and microbial-treated samples With the aid of recent tube-model theories it is possible to determine the molecular weight distribution of polymers with rheological measurements [ 40 ]. Fig 5.
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Rheological analysis. Fig 6. FTIR analysis. PE-associated communities ARISA analysis was performed in the biofilm samples collected at the end of the phase I and during phase II, in order to monitor the succession of bacterial communities. Fig 7. ARISA results. Fig 8. Microbial communities. Fig 9. Discussion Although polyethylene is considered non-biodegradable, studies have shown that weathering by exposure to photo-oxidation or thermal oxidation favor microbial attachment and degradation [ 41 , 42 ]. Conclusions It was demonstrated that tailored indigenous marine communities comprising of polymer and hydrocarbon degrader species have the potential to degrade naturally weathered PE films in the marine environment before they are turned into microplastics.
Supporting information S1 Fig Qualitative explanation of intersection movement. TIF Click here for additional data file. S2 Fig SEM analysis. S4 Fig Biomarkers. Data Availability All relevant data are within the paper and its Supporting Information files. References 1. Our plastic age. Plastics Europe. Plastics Brussels, Belgium; Biological degradation of plastics: A comprehensive review. Biotechnol Adv. Prieto A. To be, or not to be biodegradable??? Microb Biotechnol.