Microbial succession associated with organic matter decomposition during thermophilic composting of organic waste.

Using dog food as a model of the organic waste, thermophilic composting was carried out for 14 days at a fixed temperature of 60 degrees C. The relationship between organic matter decomposition measured by CO2 evolution during the bio-stabilization process and microbial succession expressed as the changes over time in the restriction fragment length polymorphism (RFLP) patterns of 16S rDNA sequences, of micro-organisms associated with the composting material was also examined. The CO2 evolution rate peaked on day 3 and gradually decreased until it became extremely small after day 9 of composting, indicating that vigorous organic matter decomposition ceased around this time. On the other hand, the RFLP pattern changed drastically from day 0 to day 4 or 5, then remained stable until day 7 or 8, reaching its final configuration, with little variations, after day 9 of composting. RFLP analysis therefore indicates that microbial succession continued into the later stage of composting. Nevertheless, by day 9, the rate of organic matter decomposition was so low that its influence on microbial populations could be hardly recognized by conventional methods of dilution plating. Moreover, the compost produced by day 9 showed no inhibitory effect on the growth of Komatsuna (Brassica campestris L. var. rapiferafroug), indicating that the maturity of compost is sufficient for plant growth when the rate of organic matter decomposition has become extremely low and the RFLP patterns become stable.     

Dynamic of functional microbial groups during mesophilic composting of agro-industrial wastes and free-living (N2)-fixing bacteria application

Although several reports are available concerning the composition and dynamics of the microflora during the composting of municipal solid wastes, little is known about the microbial diversity during the composting of agro-industrial refuse. For this reason, the first parts of this study included the quantification of microbial generic groups and of the main functional groups of C and N cycle during composting of agro-industrial refuse. After a generalized decrease observed during the initial phases, a new bacterial growth was observed in the final phase of the process. Ammonifiers and (N₂)-fixing aerobic groups predominated outside of the piles whereas, nitrate-reducing group increased inside the piles during the first 23 days of composting. Ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), showed an opposite trend of growth since ammonia oxidation decreased with the increase of the nitrite oxidation activity. Pectinolytics, amylolytics and aerobic cellulolytic were present in greater quantities and showed an upward trend in both the internal and external part of the heaps.Several free-living (N₂)-fixing bacteria were molecularly identify as belonging especially to uncommon genera of nitrogen-fixing bacteria as Stenotrophomonas, Xanthomonas, Pseudomonas, Klebsiella, Alcaligenes, Achromobacter and Caulobacter. They were investigated for their ability to fix atmospheric nitrogen to employ as improvers of quality of compost. Some strains of Azotobacter chrococcum and Azotobacter salinestris were also tested. When different diazotrophic bacterial species were added in compost, the increase of total N ranged from 16% to 27% depending on the selected microbial strain being used. Such microorganisms may be used alone or in mixtures to provide an allocation of plant growth promoting rhizobacteria in soil.     

Effect of temperature on bacterial emissions in composting of swine manure

Swine manure was subjected to laboratory scale composting in order to quantify bioaerosols, i.e., airborne culturable bacteria and endotoxin, in the exhaust gas, which provided details on the effect of temperature on bacterial emissions. The concentration of airborne bacteria reached 31,250 colony-forming units (CFU)/m³ during the thermophilic stage of composting, and positively correlated with the temperature profile of the compost pile. Initially, the endotoxin concentration was 1820 endotoxin units (EU)/m³, but it decreased exponentially as the composting process proceeded. The temperature can be an excellent indicator of bacterial emissions during the composting process, indicating that the composting process requires a consistently high temperature to ensure sanitization of both compost and bacterial emissions. The cumulative emission data showed that emission factors was 11.2?13.5 CFU/g dry swine manure and that of endotoxin was 0.5?0.9 EU/g dry swine manure. The bacterial diversity in the bioaerosol was analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis, revealing the presence of various gram-negative bacterial consortia.     

Functional changes in culturable microbial communities during a co-composting process: Carbon source utilization and co-metabolism

Microbial communities in sewage sludge and green waste co-composting were investigated using culture-dependent methods and community level physiological profiles (CLPP) with Biolog Microplate. Different microbial groups characterized each stage of composting. Bacterial densities were high from beginning to end of composting, whereas actinomycete densities increased only after bio-oxidation phase i.e. after 40 days. Fungal populations become particularly high during the last stage of decomposition. Cluster analyses of metabolic profiles revealed a similar separation between two groups of composts at 67 days for bacteria and fungi. Principal component analysis (PCA) applied to bacterial and fungal CLPP data showed a chronological distribution of composts with two phases. The first one (before 67 days), where the composts were characterized by the rapid decomposition of non-humic biodegradable organic matter, was significantly correlated to the decrease of C, C/N, organic matter (OM), fulvic acid (FA), respiration, cellulase, protease, phenoloxidase, alkaline and acid phosphatases activities. The second phase corresponding to the formation of polycondensed humic-like substances was significantly correlated to humic acid (HA) content, pH and HA/FA. The influent substrates selected on both factorial maps showed that microbial communities could adapt their metabolic capacities to the particular environment. The first phase seems to be focused on easily degradable substrate utilization whereas the maturation phase appears as multiple metabolisms, which induce the release of metabolites and their polymerization leading to humification processes.     

Composting in small laboratory pilots: performance and reproducibility

Small-scale reactors (<10 l) have been employed in composting research, but few attempts have assessed the performance of composting considering the transformations of organic matter. Moreover, composting at small scales is often performed by imposing a fixed temperature, thus creating artificial conditions, and the reproducibility of composting has rarely been reported. The objectives of this study are to design an innovative small-scale composting device safeguarding self-heating to drive the composting process and to assess the performance and reproducibility of composting in small-scale pilots. The experimental setup included six 4-l reactors used for composting a mixture of sewage sludge and green wastes. The performance of the process was assessed by monitoring the temperature, O₂ consumption and CO₂ emissions, and characterising the biochemical evolution of organic matter. A good reproducibility was found for the six replicates with coefficients of variation for all parameters generally lower than 19%. An intense self-heating ensured the existence of a spontaneous thermophilic phase in all reactors. The average loss of total organic matter (TOM) was 46% of the initial content. Compared to the initial mixture, the hot water soluble fraction decreased by 62%, the hemicellulose-like fraction by 68%, the cellulose-like fraction by 50% and the lignin-like fractions by 12% in the final compost. The TOM losses, compost stabilisation and evolution of the biochemical fractions were similar to observed in large reactors or on-site experiments, excluding the lignin degradation, which was less important than in full-scale systems. The reproducibility of the process and the quality of the final compost make it possible to propose the use of this experimental device for research requiring a mass reduction of the initial composted waste mixtures.     

Effect of commercial mineral-based additives on composting and compost quality

The effectiveness of two commercial additives meant to improve the composting process was studied in a laboratory-scale experiment. Improver A (sulphates and oxides of iron, magnesium, manganese, and zinc mixed with clay) and B (mixture of calcium hydroxide, peroxide, and oxide) were added to source-separated biowaste:peat mixture (1:1, v/v) in proportions recommended by the producers. The composting process (T, emissions of CO₂, NH₃, and CH₄) and the quality of the compost (pH, conductivity, C/N ratio, water-soluble NH₄–N and NO₃–N, water- and NaOH-soluble low-weight carboxylic acids, nutrients, heavy metals and phytotoxicity to Lepidium sarivum) were monitored during one year. Compared with the control, the addition of improver B increased pH by two units, led to an earlier elimination of water-soluble ammonia, an increase in nitrates, a 10-fold increase in concentrations of acetic acid, and shortened phytotoxicity period by half; as negative aspect it led to volatilization of ammonia. The addition of improver A led to a longer thermophilic stage by one week and lower concentrations of low-weight carboxylic acids (both water- and NaOH-extractable) with formic and acetic of similar amounts, however, most of the aspects claimed by the improver’s producer were not confirmed in this trial.     

Reducing H2S production by O2 feedback control during large-scale sewage sludge composting

Hydrogen sulfide (H₂S) production patterns and the influence of oxygen (O₂) concentration were studied based on a well operated composting plant. A real-time, online multi-gas detection system was applied to monitor the concentrations of H₂S and O₂ in the pile during composting. The results indicate that H₂S was mainly produced during the early stage of composting, especially during the first 40 h. Lack of available O₂ was the main reason for H₂S production. Maintaining the O₂ concentration higher than 14% in the pile could reduce H₂S production. This study suggests that shortening the interval between aeration or aerating continuously to maintain a high O₂ concentration in the pile was an effective strategy for restraining H₂S production in sewage sludge composting.     

The effects of apple pomace,bentonite and calcium superphosphate on swine manure aerobic composting

The effects of additives such as apple pomace, bentonite and calcium superphosphate on swine manure composting were investigated in a self-built aerated static box (90 L) by assessing their influences on the transformation of nitrogen, carbon, phosphorous and compost maturity. The results showed that additives all prolonged the thermophilic stage in composting compared to control. Nitrogen losses amounted to 34–58% of the initial nitrogen, in which ammonia volatilization accounted for 0.3–4.6%. Calcium superphosphate was helpful in facilitating composting process as it significantly reduced the ammonia volatilization during thermophilic stage and increased the contents of total nitrogen and phosphorous in compost, but bentonite increased the ammonia volatilization and reduced the total nitrogen concentration. It suggested that calcium superphosphate is an effective additive for keeping nitrogen during swine manure composting.     

Kinetic Study Of The Composting Of Evergreen Oak Forestry Waste

The successive stages in the composting process of forestry waste from evergreen oak (Quercus ilx sbsp. ballota) were studied under controlled conditions (initial) carbon to nitrogen ratio = 30, T = 27°C). The original material was composted for 6 months and sampled every 15 days. The variables measured on the oak biomass in the course of the experiment showed different kinetics: the weight loss and germination index underwent a monotonic increase whereas the reducing sugars, phenols and E₄₆₅/E₆₆₅ extinction ratio of the water-soluble fraction stabilized at their lowest values after the first 2 weeks. Other variables, such as alkali solubility, water repellency, pH and particle size, showed maximum or minimum values at intermediate stages of the experiment. In contrast to the adverse agrobiological effects of the direct application to soil of the original waste, germination biotests and greenhouse experiments showed that plant response improved from the 2 first weeks of composting. The kinetics observed for the parameters studied suggested that the less favourable effect on plant yield may come from phytotoxic substances in compost but also from the microbial use of soil N required for the transformation of the most biodegradable compost fractions in special hemicelluloses.     

Effect of the mixing ratio on the composting of OFMSW digestate: assessment of compost quality

This study presents the results obtained in compostability tests of organic fraction of municipal solid waste (OFMSW) digestate. The final aim was to obtain mature compost without phytotoxic effects. For the evaluation of the composting process, a novel parameter describing the performance of the composting process, the relative heat generation standardized with the initial volatile solid content (RHGVS₀), was defined and evaluated at laboratory-scale. From these laboratory-scale test, the optimum operational conditions were obtained, a mixing ratio (v/v) of 1:1:0 (bulking agent:digestate:co-substrate) and with 15% of mature compost as inoculum. Subsequently, these optimum operational conditions were applied in the active phase of the composting pilot-scale reactor. The active composting stage took 7 days, subsequently a curing phase of 60 days was carried out at ambient conditions. After 30 days of curing, the mature compost showed a specific oxygen uptake rate (SOUR) of 0.14 mg O₂/g VS·h, a germination index (GI) of 99.63% and a low volatile fatty acids (VFA) concentration (41.3 AcH mg/kg_dm), being indicative of the good compost stability and maturity of the compost. The very good quality of the final compost obtained indicated that the RHGVS₀ accurately describes the performance of the composting process.

     

Effects of pH and microbial composition on odour in food waste composting

A major problem for composting plants is odour emission. Slow decomposition during prolonged low-pH conditions is a frequent process problem in food waste composting. The aim was to investigate correlations between low pH, odour and microbial composition during food waste composting. Samples from laboratory composting experiments and two large scale composting plants were analysed for odour by olfactometry, as well as physico-chemical and microbial composition. There was large variation in odour, and samples clustered in two groups, one with low odour and high pH (above 6.5), the other with high odour and low pH (below 6.0). The low-odour samples were significantly drier, had lower nitrate and TVOC concentrations and no detectable organic acids. Samples of both groups were dominated by Bacillales or Actinobacteria, organisms which are often indicative of well-functioning composting processes, but the high-odour group DNA sequences were similar to those of anaerobic or facultatively anaerobic species, not to typical thermophilic composting species. High-odour samples also contained Lactobacteria and Clostridia, known to produce odorous substances. A proposed odour reduction strategy is to rapidly overcome the low pH phase, through high initial aeration rates and the use of additives such as recycled compost.     

Fertilizer efficiency and environmental risk of irrigating Impatiens with composting leachate in decentralized solid waste management

The reduction and reuse of composting leachate is an issue of importance in the field of decentralized solid waste management. In this study, composting leachate from source-separated food waste was treated and subsequently used as liquid fertilizer to irrigate Impatiens (Impatiens balsamina). The leachate was altered by adjusting storage time and dilution, and through addition of microbial inocula. For each test case, the effects of irrigation were monitored by analyzing the Impatiens extension degree, numbers of leaves and flowers, dry weight, and photosynthetic pigment content to assess fertilizer efficiency. The main results obtained revealed that the addition of microbial inocula and lengthening of storage times may lower COD concentrations, adjust pH value and maintain a comparatively high level of nutrient contents. By adding microbial inocula, a COD concentration of 9.6% and BOD₅ concentration of 6.7% were obtained for non-treated leachate with the same storage time. COD concentrations in leachate decreased to 69.4% after 36 weeks storage. Moreover, composting leachate promoted growth of Impatiens. The dry weight biomass of Impatiens irrigated using treated diluted leachate was 1.15–2.94 times that obtained for Impatiens irrigated using tap water. Lastly, following the irrigation of Impatiens over a short period, soil did not accumulate VOCs and heavy metals to levels exceeding relative standards. Further research on heavy metal and salinity accumulation in plants should be undertaken to meet the needs of large-scale applications.     

Chemical characterization of emissions from a municipal solid waste treatment plant

Gaseous emissions are an important problem in municipal solid waste (MSW) treatment plants. The sources points of emissions considered in the present work are: fresh compost, mature compost, landfill leaks and leachate ponds. Hydrogen sulphide, ammonia and volatile organic compounds (VOCs) were analysed in the emissions from these sources. Hydrogen sulphide and ammonia were important contributors to the total emission volume. Landfill leaks are significant source points of emissions of H₂S; the average concentration of H₂S in biogas from the landfill leaks is around 1700 ppmv. The fresh composting site was also an important contributor of H₂S to the total emission volume; its concentration varied between 3.2 and 1.7 ppmv and a decrease with time was observed. The mature composting site showed a reduction of H₂S concentration (<0.1 ppmv). Leachate pond showed a low concentration of H₂S (in order of ppbv). Regarding NH₃, composting sites and landfill leaks are notable source points of emissions (composting sites varied around 30–600 ppmv; biogas from landfill leaks varied from 160 to 640 ppmv).Regarding VOCs, the main compounds were: limonene, p-cymene, pinene, cyclohexane, reaching concentrations around 0.2–4.3 ppmv.H₂S/NH₃, limonene/p-cymene, limonene/cyclohexane ratios can be useful for analysing and identifying the emission sources.     

Determination of nitrous oxide, methane, and ammonia emissions from a swine waste composting process

The amounts of harmful gas emissions from the process of composting swine waste were determined using an experimental composting apparatus. Forced aeration (19.2–96.1 l/m³/min) was carried out continuously, and exhaust gases were collected and analyzed periodically. With weekly turning and the addition of a bulking agent in order to decrease the moisture content and increase air permeability, the temperature of most of the contents rose to 70°C and composting was complete within 3–5 weeks. NH₃, CH₄, and N₂O emissions were high in the early stage of composting. About 10%–25% of the nitrogen in the raw material was lost as NH₃ gas during composting. The emission rate of NH₃ mainly depended on the aeration rate, so that as the aeration rate rose, the level of NH₃ emissions increased. The CH₄ and N₂O emissions could be kept lower with adequate treatment at more than 40 l/m³/min aeration. N₂O may be mainly the result of the denitrification of NO _x -N in the additional matured compost used as a composting accelerator. Received: September 11, 1998 / Accepted: November 8, 1999     

Degradation of a Terephthalate-containing Polyester by Thermophilic Actinomycetes and <Emphasis Type="Italic">Bacillus</Emphasis> Species Derived from Composts

We intended to find thermophilic degraders of terephthalate-containing Biomax^® films. Films in mesh bags were buried in composts (inside temperature: approximately 55–60 °C), resulting in the degradation of them in 2 weeks. Fluorescent microscopy of films recovered from composts showed that microorganisms gradually covered the surface of a film during composting. DGGE analysis of microorganisms on the composted film indicated the presence of Bacillus species as main species (approximately 80% of microbial flora) and actinomycetes (approximately 10–20%) as the second major flora. Isolation of Biomax^®-utilizing bacteria was focused on these two genera: two actinomycetes and one Bacillus species were isolated as pure best degraders from the composted polymer films, which were fragmented into small pieces. All the strains were thermophilic and identified, based on their 16S rDNA analyses. Degradation of polymer films was confirmed by (1) accelerated fragmentation of films in composts, compared with a control (no inoculum) and resultant decrease in molecular weights, (2) growth in a powdered Biomax^® medium, compared with a control without powdered Biomax^®, and (3) production of terephthalate in a powdered Biomax^® medium. In this way, we concluded that these bacteria were useful for degradation of thermostable Biomax^® products.     

Microbial communities and greenhouse gas emissions associated with the biodegradation of specified risk material in compost

Provided that infectious prions (PrP^Sc) are inactivated, composting of specified risk material (SRM) may be a viable alternative to rendering and landfilling. In this study, bacterial and fungal communities as well as greenhouse gas emissions associated with the degradation of SRM were examined in laboratory composters over two 14 day composting cycles. Chicken feathers were mixed into compost to enrich for microbial communities involved in the degradation of keratin and other recalcitrant proteins such as prions. Feathers altered the composition of bacterial and fungal communities primarily during the first cycle. The bacterial genera Saccharomonospora, Thermobifida, Thermoactinomycetaceae, Thiohalospira, Pseudomonas, Actinomadura, and Enterobacter, and the fungal genera Dothideomycetes, Cladosporium, Chaetomium, and Trichaptum were identified as candidates involved in SRM degradation. Feathers increased (P < 0.05) headspace concentrations of CH₄ primarily during the early stages of the first cycle and N₂O during the second. Although inclusion of feathers in compost increases greenhouse gas emissions, it may promote the establishment of microbial communities that are more adept at degrading SRM and recalcitrant proteins such as keratin and PrP^Sc.     

Contaminated land clean-up using composted wastes and impacts of VOCs on land

This paper describes experiments that demonstrate the effects and potential for remediation of a former steelworks site in Wales polluted with polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs). Under field conditions, PAH-contaminated soil was composted in-vessel, with or without organic feedstocks, receiving forced aeration for 80 days followed by 4 months maturation. Treatments compared PAH removal in contaminated soil to contaminated soil mixed with three different organic waste mixes after composting and after composts were spread to land. After composting, PAH concentrations declined in all treatments, by up to 38%. Sixteen months after the composts were landspread and vegetation was established, only those containing contaminated soil with organic additions exhibited further PAH removal, by up to 29%. Composting resulted in a decline in the relative concentration of small PAHs, whereas the landspreading-vegetation phase saw a decline in the relative concentration of medium PAHs in two of the three composts exhibiting PAH removal. Under controlled glasshouse conditions, vegetated soil columns of differing depths were exposed to VOCs from beneath. VOC vapour affected both shoot and root growth and soil microbial activity; effects varied with distance from the VOC source. This work demonstrated that on-site remediation of aged PAH-contaminated land can be successfully initiated by in-vessel co-composting followed by land spreading and vegetation, within a practical timeframe.     

Temperature and deactivation of microbial faecal indicators during small scale co-composting of faecal matter

Small scale co-composting of faecal matter from dry toilet systems with shredded plant material and food waste was investigated in respect to heat development and deactivation of faecal indicators under tropical semiarid conditions. Open (uncovered) co-composting of faecal matter with shredded plant material alone did not generate temperatures high enough (<55 °C) to reduce the indicators sufficiently. The addition of food waste and confinement in chambers, built of concrete bricks and wooden boards, improved the composting process significantly. Under these conditions peak temperatures of up to 70 °C were achieved and temperatures above 55 °C were maintained over 2 weeks. This temperature and time is sufficient to comply with international composting regulations. The reduction of Escherichia coli, Enterococcus faecalis and Salmonella senftenberg in test containment systems placed in the core of the compost piles was very efficient, exceeding 5 log₁₀-units in all cases, but recolonisation from the cooler outer layers appeared to interfere with the sanitisation efficiency of the substrate itself. The addition of a stabilisation period by extending the composting process to over 4 months ensured that the load of E. coli was reduced to less than 10³ cfu^?g and salmonella were undetectable.     

Changes in the microbial communities during co-composting of digestates

Anaerobic digestion is a waste treatment method which is of increasing interest worldwide. At the end of the process, a digestate remains, which can gain added value by being composted. A study was conducted in order to investigate microbial community dynamics during the composting process of a mixture of anaerobic digestate (derived from the anaerobic digestion of municipal food waste), green wastes and a screened compost (green waste/kitchen waste compost), using the COMPOCHIP microarray. The composting process showed a typical temperature development, and the highest degradation rates occurred during the first 14 days of composting, as seen from the elevated CO₂ content in the exhaust air. With an exception of elevated nitrite and nitrate levels in the day 34 samples, physical–chemical parameters for all compost samples collected during the 63 day process indicated typical composting conditions. The microbial communities changed over the 63 days of composting. According to principal component analysis of the COMPOCHIP microarray results, compost samples from the start of the experiment were found to cluster most closely with the digestate and screened compost samples. The green waste samples were found to group separately. All starting materials investigated were found to yield fewer and lower signals when compared to the samples collected during the composting experiment.     

Production of well-matured compost from night-soil sludge by an extremely short period of thermophilic composting

The effect of various operational conditions on the decomposition of organic material during the composting of night-soil treatment sludge was quantitatively examined. The optimum composting conditions were found to be a temperature of ca. 60 °C and an initial pH value of 8. Rapid decomposition of organic matter ceased by the sixth day of composting under these optimum conditions, and the final value of the cumulative emission of carbon (E_C), which represents the degree of organic matter decomposition, was less than 40%, indicating that the sludge contained only a small amount of easily degradable organic material. A plant growth assay using Komatsuna (Brassica campestris L. var. rapiferafroug) in a 1/5000a standard cultivation pot was then conducted for the compost at various degrees of organic matter decomposition: the raw composting material, the final compost obtained on day 6, and the 2 intermediate compost products (i.e., E_C = 10% and 20%). It was found that the larger the E_C, the greater the yield of Komatsuna growth. It was also found that 6 days of composting is sufficient to promote Komatsuna growth at the standard loading level, which is equivalent to a 1.5 g N/pot, since the promotion effect was as high as that obtained using chemical fertilizer. It can therefore be concluded that well-matured compost could be obtained in a short period of time (i.e., as early as 6 days), when night-soil sludge is composted under optimum conditions.