The fate of the recombinant DNA in corn during composting

In order to make regulations that safeguard food and the environment, an understanding of the fate oftransgenes from genetically modified (GM) plants is of crucial importance. A compost experiment including mature transgenic corn plants and seeds of event Bt 176 (Zea mays L.) was conducted to trace the fate of the transgene cryIA(b) during the period of composting. In bin 1, shredded corn plants including seeds were composted above a layer of cow manure and samples from the corn layer were collected at intervals during a 12-month period. The samples were tested for the transgene persistence and microbial counts and also the compost was monitored for temperature. In bin 2, piles of corn seeds, surrounded by sheep manure and straw, were composted for 12 months. A method combining nested polymerase chain reaction (PCR) and southern hybridization was developed for detection of the transgene in compost. The detection sensitivity was 200 copies of the transgene per gram of dry composted corn material. Composting commenced on day 0, and the transgene was detected in specimens from bin 1 on days 0 and 7 but not on day 14 or thereafter. The transgene in corn seeds was not detectable after 12 months of composting in bin 2. Temperatures in both bins rose to about 50 degrees C within 2 weeks and remained above that temperature for about 3 months, even when the ambient temperature dropped below -20 degrees C. Extracts from compost were inoculated onto culture plates and then were incubated at 23 to 55 degrees C. Within the first 2 weeks of composting in bin 1, the counts of bacteria incubated at 55 degrees C increased from 3.5 to 7.5 log10, whereas those incubated at 23 degrees C remained at about 7.5 log10. The counts of fungi incubated at 45 degrees C increased slightly from 2.5 to 3.1 log10, but those incubated at 23 degrees C decreased from 6.3 to 3.0 log10. The rapid degradation of the transgene during composting of Bt corn plants suggested that the composting process could be used for safe disposal of transgenic plant wastes.     

The fate of recombinant plasmids during composting of organic wastes

Composting was investigated as a means for safe disposal of organic waste containing bacteria that carry transgenes in recombinant plasmids. To generate model recombinant plasmids, a mobile IncQ plasmid, RSF1010, and a non-mobile plasmid, pGFP, were genetically modified to carry a DNA segment encoding both green fluorescent protein and kanamycin resistance and were designated as RSF1010-GFPK and pGFPK. Escherichia coli (E. coli) C600 harboring these plasmids were inoculated into chicken manure specimens that were placed in compost at 20 and 60 cm from the bottom of a 1.0-m high compost bin. Control specimens were held at ambient temperature. By day 10, compost temperatures at the lower and upper levels of the bin had reached 45.3 and 61.5°C, respectively, and at both levels the target E. coli had been inactivated and the plasmids had lost their capacity to be transformed or mobilized. Furthermore, based on real time Polymerase chain reaction (PCR), the transgene fragments along with the host chromosomal DNA fragment from specimens at the upper level had been degraded beyond the detection limit. However, at the lower level where temperatures remained below 48°C these fragment persisted to day 21. At ambient temperatures (0–8°C), the E. coli, plasmids and the transgene fragments persisted in manure specimens throughout the 21 day test period. The study showed the potential for composting as a safe procedure for disposal of bacteria carrying transgenes in recombinant plasmids.     

Development of functional composts using spent coffee grounds,poultry manure and biochar through microbial bioaugmentation

Spent coffee grounds (SCG), poultry manure, and agricultural waste-derived biochar were used to manufacture functional composts through microbial bioaugmentation. The highest yield of tomato stalk-based biochar (40.7%) was obtained at 450°C with a surface area of 2.35 m² g^?1. Four pilot-scale composting reactors were established to perform composting for 45 days. The ratios of NH₄⁺–N/NO₃^?–N, which served as an indicator of compost maturity, indicate rapid, and successful composting via microbial bioaugmentation and biochar amendment. Moreover, germination indices for radish also increased by 14–34% through augmentation and biochar amendment. Microbial diversity was also enhanced in the augmented and biochar-amended composts by 7.1–8.9%, where two species of Sphingobacteriaceae were dominant (29–43%). The scavenging activities of 2,2-diphenyl-1-picrylhydrazyl (DPPH) were enhanced by 14.1% and 8.6% in the fruits of pepper plants grown in the presence of the TR-2 (augmentation applied only) and TR-3 (both augmentation and biochar amendment applied) composts, respectively. Total phenolic content was also enhanced by 68% in the fruits of the crops grown in TR-3. Moreover, the other compost, TR-L (augmentation applied only), boosted DPPH scavenging activity by 111% in leeks compared with commercial organic fertilizer, while TR-3 increased the phenolic content by 44.8%. Composting facilitated by microbial augmentation and biochar amendment shortened the composting time and enhanced the quality of the functional compost. These results indicate that functional compost has great potential to compete with commercially available organic fertilizers and that the novel composting technology could significantly contribute to the eco-friendly recycling of organic wastes such as spent coffee grounds, poultry manure, and agricultural wastes.     

Evaluation of microbially enhanced composting of sophora flavescens residues

The effects of inoculants on the composting of Sophora flavescens residues were evaluated based on several physical, chemical and biological parameters, as well as the infrared spectra. Compared to the control compost without inoculants, the treatment compost with inoculants (Bacillus subtilis strain G-13 and Chaetomium thermophilum strain GF-1) had a significantly longer thermophilic duration, higher cellulase activity and a higher degradation rate of cellulose, hemicellulose and lignin (P < 0.05). Thus, a higher maturity degree of compost with apparently lower C:N ratio (15.88 vs. 17.77) and NH₄-N:NO₃-N ratio (0.16 vs. 0.20) was obtained with the inoculation comparing with the control (P < 0.05). Besides, the inoculants could markedly accelerate the composting process and increase the maturity degree of compost as indicated by the germination index (GI) in which the treatment reached the highest GI of 133.2% at day 15 while the control achieved the highest GI of 125.7% at day 30 of the composting. Inoculation with B. subtilis and C. thermophilum is a useful method to enhance the S. flavescens residues composting according to this study.     

Bioaerosol Generation at Large-Scale Green Waste Composting Plants

Abstract

Bioaerosol release from composting plants is a cause of concern because of the potential health impacts on site workers and local residents. A one-year monitoring was undertaken in a typical composting plant treating green wastes by windrowing in the open. Aspergillus fumigatus spores and mesophilic bacteria were used as monitoring parameters and were collected in a six-stage Andersen sampler impactor from the air at different locations and during different operational activities. Background concentrations of both microorganisms were generally below 1000 colony-forming units m^?3 when no vigorous activity was taking place. Shredding of fresh green wastes, pile turning, and screening of mature compost were identified as the activities generating the highest amounts of both bioaerosols 40 m downwind of the composting pad. These air concentrations were ～2 log units higher than background levels. Screening of mature compost generated lower amounts of A. fumigatus than the other two activities (an average of 1 log unit higher than background levels). Workers were identified as the main potential receptors of high bioaerosol concentrations in areas close to the composting pad, whereas no major risk for local residents was expected because the concentrations recorded at distances of 200 and 300 m downwind of the operational area were not significantly different from background levels.     

Recycling of wastes from fish beneficiation by composting: chemical characteristics of the compost and efficiency of their humic acids in stimulating the growth of lettuce

Waste from the beneficiation of fish was composted with crushed grass aiming to characterize their chemical composition and investigate the possibility of the use of the final compost as source of humic acids (HA) able to stimulate the growth of lettuce. Compost presented pH value, C/N ratio, and electrical conductivity that allow its use as an organic fertilizer. The element content was present in the following order of abundance in the compost: P?>?Ca?>?N?>?Mg?>?K?>?Fe?>?Zn?>?Mn?>?Mo?>?Cu, and the humus composition was similar to that observed in others kind of organic residues composted. The high content of oxygen pointed out a high level of oxidation of HA, in line with the predominance of phenolic acidity in the functional groups. The ¹³C-NMR spectra showed marked resonances due to the presence of lipids and other materials resistant to degradation as methoxy substituent and N-alkyl groups. A concentration of 20 mg L^?1 HA increased significantly both dry and wet root matter in lettuce but the CO₂ assimilation, stomatal conductance, and number of lateral roots of the plants were not affected. However, increases of 64% in the water-use efficiency was observed due to the HA addition, probably related to the root morphology alteration which resulted in 1.6-fold increase of lateral root average length and due to the higher H⁺ extrusion activity. Reuse of residues from the fish beneficiation activity by composting may represent a safe tool to increase the value of recycled organic residues and generate HA with potential use as plant growth stimulants.     

Composting duck excreta enriched wood shavings: C and N transformations and bacterial pathogen reductions

Composting of agricultural and domestic wastes is used increasingly to reduce weight, volume, and odor; destroy animal and plant pathogens; and improve the quality of end-products to be used as soil amendments and growth substrates. The objective of this study was to investigate the transformation of C and N and the survival of bacterial populations and pathogenic bacteria during in-vessel composting of duck excreta enriched wood shavings. Two feedstocks, collected on different dates, were composted (C1 and C2) in an enclosed hall system equipped with an electromechanical turner. Temperature was continuously recorded, whereas moisture content and bacterial counts were determined twice a week. Data showed that, although the N content of C2 was only half of that of C1, both materials were fully biostabilized at the end of the composting period as indicated by extractable lipid ratios. In the compost with the low C/N ratio (C1), all bacterial populations were eliminated, whereas fecal streptococci, total coliforms, and gram-negative bacteria were still present in C2 at the end of the composting period. Our results emphasize that the composting of manures and other organic wastes needs to be properly managed to stabilize C and N and to eliminate or reduce bacterial populations.     

Thermophilic Toluene Biofiltration

ABSTRACT

Thermophilic biodégradation of toluene with active compost biofilters was studied. Thermophilic conditions were maintained either by daily substrate addition (semicontinuous composting) or with a heating system (batch thermophilic composting). The semicontinuous system was designed for the treatment of cool (less than approximately 35 °C) gases under thermophilic conditions, while the extended batch approach was developed for the treatment of warmer gases. When the semicontinuous system was operated at 50 °C (after a one-day start-up period) at an average inlet concentration of 5.5 g m^-3, toluene was degraded at a rate ranging from 73 to 110 g C m^-3 hr^-1. Batch thermophilic treatment was somewhat less effective at the same inlet concentration. Semicontinuous toluene biofiltration at 60 °C was also investigated, but biodegradation rates were significantly lower than at 50 °C. In all systems, toluene biodegradation was proportional to the inlet concentration. Rates of up to 289 g C m^-3 hr^-1 (at an inlet concentration of 14.7 g m^-3) were achieved for semicontinuous and batch operation and 251 g C m^-3 hr^-1 (at an inlet concentration of 18.4 g m^-3) for batch thermophilic at 50 °C. Semicontinuous thermophilic operation at 60 °C showed a maximum rate of 119 g C m^-3 hr^-1. Active compost ther-mophilic biofiltration was found to be very effective when concentrations are high. At lower concentrations, rates were similar to those obtained with mesophilic biofiltration. Mixing, humidity, and the presence of cosubstrate were important parameters in maintaining high degradation rates. Biofiltration in the batch thermophilic mode could be useful when conventional biofiltration is ineffective due to elevated gas temperatures. Biofiltration in the semicontinuous thermophilic could reduce the biofilter size necessary for treatment of cooler gases containing high concentrations of volatile organic compounds.     

Detection of Escherichia coli in a cattle manure composting process by selective cultivation and colony polymerase chain reaction

Livestock manure is suitable for use as a composting material. However, various intestinal microbes, such as Escherichia coli, are significant components of such manures. Thus, it is desirable that the level of intestinal microbes, and particularly opportunistic pathogens, in compost is inspected and counted regularly. The sensitivity and specificity of detection of E. coli in compost have been improved by selective cultivation followed by colony polymerase chain reaction (PCR) using the ECO primer. Indeed, the sensitivity of this method is higher than that of DNA extraction from compost and PCR. In this study, changes in numbers of E. coli present in a field-scale composting process over time was assessed using selective cultivation and colony PCR. Numbers of ECO-positive colonies after 24 h decreased, with a concomitant rise in compost temperature. ECO-positive colonies were not detected from 33 to 48 h. However, ECO-positive colony numbers increased beginning on day 4 and continuing until day 42. Thus, it seems likely that the high temperatures reached during the composting process did not affect E. coli numbers in the final compost. Additionally, selective cultivation followed by colony PCR using specific primers is an appropriate method of determining levels of cultivable pathogens in composted materials.     

Effects of lime amendment on availability of heavy metals and maturation in sewage sludge composting

A batch composting study was performed to evaluate the feasibility of co-composting sewage sludge with lime, aiming at reducing the availability of heavy metals in the sludge compost. Sewage sludge with sawdust as bulking agent was amended with lime at 0, 0.63, 1.0, and 1.63% w/w, and composted for 100 days in laboratory batch reactors. The changes in temperature, pH, electrical conductivity (EC), and extractable heavy metal contents were measured while compost maturity was determined by C/N(organic) and cress seed germination during the composting period. Liming raised pH of compost effectively at the initial stage of composting and caused a decrease in EC through precipitation of soluble ions. Lime amendment also significantly reduced water-soluble and Diethylene triamine pentracetic acid (DTPA)-extractable metal contents. The maximum reductions were 60 and 40% for Cu, 80 and 40% for Mn, 55 and 10% for Zn, and 20 and 25% for Ni at the end of the composting period for the lime-amended sludge as compared to the control. In spite of the inhibitory effect of lime amendment on the decomposition activity of sewage sludge, all treatments reached maturation after 63 days of composting as indicated by the results of C/N(organic) ratio and cress seed germination test results. A lime amendment of < or =1.0% is recommended to co-compost with sewage sludge.     

Composting and bioremediation process evaluation of wood waste materials generated from the construction and demolition industry

The suitability of using bioremediation and composting techniques for diverting construction and demolition (C&D) waste from landfill has been validated in this study. Different timber products from C&D waste have been composted using various composting approaches. The present work demonstrates the quality of compost produced as a result of composting of mixed board product wood waste, which is frequently obtained from the construction and demolition industry. Three compost mixes were prepared by mixing shredded chip board, medium density fibre, hardboard and melamine. Poultry manure, Eco-Bio mixture and green waste were used as nutrient supplements. The results revealed that compost produced from mixtures of poultry manure and green waste used as nutrient supplements improved the performance in plant growth trials (phytotoxicity tests). Results obtained from the experimental study clearly indicate that the composts produced comply with the criterion suggested in BSI PAS 100 (A specification for compost materials) for use in different applications. Composting can also be demonstrated to be a very practical approach to material management including transport reduction to and from the site. The economic suitability of the process will be improved with the increase in landfill tax. In the current regulatory scenario, it is recommended that these materials should be composted at a centralised facility.     

Composting of vegetable waste in subtropical climates

This experiment examined the possibility of using composting as a method of waste disposal in subtropical countries, such as the United Arab Emirates (UAE), where temperatures are high and humidity is low. In order to simulate conditions in the UAE, a representative mixture of wastes generated in the UAE was prepared and tested at ambient temperatures resembling those of subtropical countries. The mixture consisted of tomatoes, cabbage, eggplant and grass. The addition of grass (40% of the mixture by weight) adjusted the carbon/nitrogen ratio and the moisture content of the compost to the necessary level for initiating decomposition. To maintain the moisture content of the compost under severe dry hot weather, a saturated insoluble polymer (IP) was added at 20% and 60% concentration (wet weight). The mixture was then composted at three different temperatures. Results showed that the mixture requires about two weeks for stabilisation. High temperature is an advantage to the compost process; at temperatures between 40 and 60°C, the waste mixture (regardless of IP concentration) decomposed quicker than at low temperature (25°C). The addition of 20% saturated IP to the compost increases the moisture content and decomposition rate. It also increases the organic chemical content of the compost, which will be reflected by microbial activity.     

Effect of Inoculating Flower Stalks and Vegetable Waste with Ligno-cellulolytic Microorganisms on the Composting Process

Abstract

A lab-scale composting experiment was carried out using vegetable and flower stalks waste to study the effectiveness of ligno-cellulolytic microorganisms (LCMs) obtained from the previous isolation on composting process, especially on enhancement of biodegradation rate of these organic materials. The addition of LCMs to compost showed promised to be a valuable asset by rendering timely benefits in efficiency, maturity, and quality of the composting. This was evidenced by a significant increase of temperature, O₂ consumption and CO₂ emission, and population density of LCMs in compost mass compared with that of biotic (addition of culture of horse feces) and abiotic (1% molasses amendment) treatments, as well as control trial. The phytotoxicity assay showed that the substrate became mature after 60 days’ composting. The LCMs inoculation enhanced the biodegradation of the composting materials as evidenced by an increasing screening ratio (1.2 cm sieve pore) of 34.5% in the treated trail, compared with that of control, which elucidated that big advantage of adding selected inoculants over other treatment, and screening ratio is a reasonable index to compare the quality of different compost. However, the inoculation seemed to have no significant effect on the moisture content, pH, and the final organic carbon of the composting materials.     

Enumerating actinomycetes in compost bioaerosols at source—Use of soil compost agar to address plate ‘masking’

Actinomycetes are the dominant bacteria isolated from bioaerosols sampled at composting facilities. Here, a novel method for the isolation of actinomycetes is reported, overcoming masking of conventional agar plates, as well as reducing analysis time and costs. Repeatable and reliable actinomycetes growth was best achieved using a soil compost media at an incubation temperature of 44 °C and 7 days’ incubation. The results are of particular value to waste management operators and their advisors undertaking regulatory risk assessments that support environmental approvals for compost facilities.     

Gaseous emission during the composting of pig feces from Chinese Ganqinfen system

The Ganqinfen system – a process of manually cleaning animal feces by means of a shovel – is a widely used manure separating method in Chinese pig farms. Ganqinfen pig feces and chopped corn stalks were mixed at the ratio of 7:1, and composted in 1.5 m³ rotting boxes for 70 d. Evolution of CH₄, N₂O and NH₃ during composting, and the effects of turning and covering, were studied in this research. Results showed that 20–39% and 0.5–4% of total nitrogen were lost in the form of NH₃ and N₂O respectively, and 0.1–0.9% of initial organic carbon was emitted as CH₄. Turning enhanced air exchange in the piles, thus decreasing CH₄ emission by 83–93% and shortening the maturing period. When trials were finished, all non-turned piles were separated to three layers by moisture content. This structure caused the N₂O losses of non-turning treatments to be 6–12.7 times higher than that of turning treatments. Covering materials reduced air exchange at the surface of the pile, thus decreasing the O₂ supply and consequently increasing CH₄ production by 33–45%. Covering also reduced NH₃ emission by 4–34%. For the composting of Ganqinfen pig feces, we suggest that a program of turning twice weekly without covering will result in compost that is sufficiently matured after 6 wk with the lowest resultant greenhouse gas emission.     

The effects of high metal concentrations in soil-compost mixtures on soil enzymes

The study was undertaken to determine the impact of high-metal composts on the activities of four soil enzymes. High concentrations of metal salts (Cr, Cu, Ni or a Co-Mo-Pb combination) were added to feedstocks during the thermophilic stage of composting. These four metal-enriched composts and an unamended control compost were then mixed with soil collected from long-term agriculture plots under organic management or conventional management. The compost-soil mixtures were prepared at two rates (1:1 or 1:3 compost:soil, v/v) and incubated at 20°C for three weeks. These 20 combinations plus the five composts and the two soils were added to pots and incubated for three weeks. Following incubation, soil enzyme activities (acid phosphatase, arysulfatase, dehydrogenase, phosphodiesterase) were measured using traditional assay procedures. Compared to the control, none of the high-metal composts inhibited soil enzyme activity. Notably, the Cu compost treatment produced significantly higher activity of all four enzymes in the soil compared to the control. Previous soil management influenced the activity of three enzymes, arysulfatase and dehydrogenase had greater activity in the organic soil while phosphatase activity was greater in the conventional soil. Increasing the proportion of compost in the pot had a positive effect on phosphodiesterase activity only. In conclusion, the high-metal compost treatments either enhanced or caused no adverse effects on soil enzyme activity.     

Bioaerosol generation at large-scale green waste composting plants

Bioaerosol release from composting plants is a cause of concern because of the potential health impacts on site workers and local residents. A one-year monitoring was undertaken in a typical composting plant treating green wastes by windrowing in the open. Aspergillus fumigatus spores and mesophilic bacteria were used as monitoring parameters and were collected in a six-stage Andersen sampler impactor from the air at different locations and during different operational activities. Background concentrations of both microorganisms were generally below 1000 colony-forming units m(-3) when no vigorous activity was taking place. Shredding of fresh green wastes, pile turning, and screening of mature compost were identified as the activities generating the highest amounts of both bioaerosols 40 m downwind of the composting pad. These air concentrations were approximately 2 log units higher than background levels. Screening of mature compost generated lower amounts of A. fumigatus than the other two activities (an average of 1 log unit higher than background levels). Workers were identified as the main potential receptors of high bioaerosol concentrations in areas close to the composting pad, whereas no major risk for local residents was expected because the concentrations recorded at distances of 200 and 300 m downwind of the operational area were not significantly different from background levels.     

ESTROM—a Romanian–Swiss research programme

Background, aim, and scope  Composting is an effective treatment process to realize sludge land application. However, nitrogen loss could result in the reduction of nutrient value of the compost products and the stabilization effect of composting on heavy metal concentration and mobility in sludge has been shown to be very limited. Materials and methods  Laboratory-scale experiments were carried out to investigate the effects of bamboo charcoal (BC) on nitrogen conservation and mobility of Cu and Zn during sludge composting. Results  The result indicated that the incorporation of BC into the sludge composting material could significantly reduce nitrogen loss. With 9% BC amendment, total nitrogen loss at the end of composting decreased 64.1% compared with no BC amendment (control treatment). Mobility of Cu and Zn in the sludge may also have been lessened, based on the decline in diethylenetriaminepentaacetic acid-extractable Cu and Zn contents of composted sludge by 44.4% and 19.3%, respectively, compared to metal extractability in the original material. Discussion  Ammonia adsorption capability of BC might be the main reason for the retention of nitrogen in sludge composting materials. Decrease of extractable Cu²⁺ and Zn²⁺ in the composting material mainly resulted from the adsorption of both metals by BC. Conclusions  Incorporation of BC into composting material could significantly lessen the total nitrogen loss during sludge composting. Mobility of heavy metals in the sludge composting material could also be reduced by the addition of BC. Recommendations and perspectives  Bamboo charcoal could be an effective amendment for nitrogen conservation and heavy metal stabilization in sludge composts. Further research into the effect of BC-amended sludge compost material on soil properties, bioavailability of heavy metals, and nutrient turnover in soil needs to be carried out prior to the application of BC-sludge compost in agriculture.     

Concentrations and microbial mineralization of four to six ring polycyclic aromatic hydrocarbons in composted municipal waste

Contents of four to six ring polycyclic aromatic hydrocarbons (PAHs) were estimated in twelve composted municipal wastes of different origin and age. By means of clean-up procedures and subsequent gaschromatography nine different PAHs or isomeric mixtures of PAHs could be separated in extracts. Concentrations of PAHs ranged from 0.17 μg perylene to 56.75 μg benz(a)anthracene/chrysene g^?1 compost (dwt). In each compost the same distinct relation between the amounts of individual PAHs was found. In spite of total weight reduction during compost processing (40–60% loss) no accumulation of PAH concentrations in ripe composts was detected. This points to a decay of PAHs by microbial activities during composting. Degradation studies carried out with four ¹⁴C-labelled PAHs indicated that in fresh composts only minor amounts of PAHs can be degraded. However, microbial populations of ripe composts possess considerable capabilities to mineralize these recalcitrant molecules.     

Assessment of microbial contamination within working environments of different types of composting plants

The objective of the study was to determine the degree of microbiological contamination, type of microflora, bioaerosol particle size distribution, and concentration of endotoxins in dust in different types of composting plants. In addition, this study provides a list of indicator microorganisms that pose a biological threat in composting facilities, based on their prevalence within the workplace, source of isolation, and health hazards. We undertook microbiological analysis of the air, work surfaces, and compost, and assessed the particle size distribution of bioaerosols using a six-stage Andersen sampler. Endotoxins were determined using gas chromatography–mass spectrometry (GC-MS). Microbial identification was undertaken both microscopically and using biochemical tests. The predominant bacterial and fungal species were identified using 16S rRNA and ITS1/2 analysis, respectively.?The number of mesophilic microorganisms in composting plants amounted to 6.9 × 10²–2.5 × 10⁴ CFU/m³ in the air, 2.9 × 10²–3.3 × 10³ CFU/100 cm² on surfaces, and 2.2 × 10⁵–2.4 × 10⁷ CFU/g in compost. Qualitative analysis revealed 75 microbial strains in composting plants, with filamentous fungi being the largest group of microorganisms, accounting for as many as 38 isolates. The total amount of endotoxins was 0.0062–0.0140 nmol/mg of dust. The dust fraction with aerodynamic particle diameter of 0.65–1.1 μm accounted for 28–39% of bacterial aerosols and 4–13% of fungal aerosols. We propose the following strains as indicators of harmful biological agent contamination: Bacillus cereus, Aspergillus fumigatus, Cladosporium cladosporioides, C. herbarum, Mucor hiemalis, and Rhizopus oryzae for both types of composting plants, and Bacillus pumilus, Mucor fragilis, Penicillium svalbardense, and P. crustosum for green waste composting plants. The biological hazards posed within these plants are due to the presence of potentially pathogenic microorganisms and the inhalation of respirable bioaerosol. Depending on the type of microorganism, these hazards may be aggravated or reduced after cleaning procedures.

Implications:?This study assessed the microbial contamination in two categories of composting plants: (1) facilities producing substrates for industrial cultivation of button mushrooms, and (2) facilities for processing biodegradable waste. Both workplaces showed potentially pathogenic microorganisms, respirable bioaerosol, and endotoxin. These results are useful to determine the procedures to control harmful biological agents, and to disinfect workplaces in composting plants.