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Composting process as a means of bioremediating the harmful waste can be assessed in terms of its hygienic aspect since the effect of its quality is indicative of its essentiality and feasibility in the environment. Hygienic relevance of composting is primarily related to the microbes functioning as composters, the dust aerosols in the ambient air of the compost pile, and the type, concentration and state of the waste to be degraded. In terms of its hygienic feature, the compost may pose threat to human health as it generates immune response in living systems possibly triggered by leftover microbes, dust and target compounds to be treated in the compost matter. Although many of the toxins and pathogens are diminished to a great number, the presence of pathogens in the final compost product can contaminate the food chain. Composting is an acknowledged pathogen reducing technology, but process management and heterogeneous pile conditions may pose particular challenge concerning the biosafety of this process. Composting has been successfully adopted but enough research is lacking on biosecurity of this process. Accordingly regarding the microbial profile of the compost, the experimental studies and characterization of microbes with respect to hygienic relevance by various scientists are discussed and reviewed as under.
Composting is the process of biodegrading the waste material in which an enormous number of materials like hydrocarbons, nitrogenous compounds, acids, their derivatives and even other organic and inorganic substances can be remediated from the environment (Finstein et al., 1986). Compost process is marked by a number of stages that actually are driven by the microbes that invade, feed on, survive, multiply and precede or succeed their fellow microbes. This indicates that compost supports a number and a diverse variety of microbes inside that are shown by studies to be bacteria, fungi and actinomycetes, while among the macro-organisms are insects and worms, as in vermicomposting. All of these living bodies can be infectious, toxic, chronic or acute disease-causing, or may be injurious to human health (Pella, 1997, Lemunier et al., 2005 and Zaleski et al., 2005). Therefore, the know-how of the entire microbial flora associated with the composting process is essential so that we can actually understand which of these species of microbes are safe and which are potentially harmful to the human health or even to the environmental well being.
Scientists report that especially thermophilic and spore-forming resisitant microbes can persist in the compost environment and even their spores, if spread, can promulgate inside the human body. The spores spread easily from the compost source and probable chances are their dispersion when the compost is mixed, transported, shifted or stored. Microbial content can be assessed by ATP content, viable count, enzymatic activity assays and is followed by identification through Density Gradient Gel Electrophoresis and Immune Awakening Response Assays (Principi et al., 2003). Particular considerations here are when the microbes are studied well for their role, their occurrence or peak activity, sporulation or hibernation in the compost and their pathogenicity. In this regard particular the earlier record of disease evidence in the people associated with compost maintenance and even the diagnosis for the people working on is vital to arbitrate the hygiene significance of the compost procedure.
Composting Itself as a Disinfecting Process
An important factor to determine the good quality of the composting process is the absence of pathogenic bacteria at the end of the process: in fact by literature it is known that the thermophilic phase within the compost with temperature values higher than 50°C to 60°C hygienizes during the process. For example, it has been found that Salmonella sp. disappeared completely from compost by the 25th day as the temperature reached 60°C (Hassen et al., 2001). Also the disappearance of both Salmonella enteridis and E. coli occurs during the similar process after initial experimental purpose inoculation (Lung et al., 2001).
Another method developed to evaluate and estimate the margins of pathogen inactivation during thermal composting. In this pilot-scale experiment, well functioning, well insulated composting proved effective for disinfection of Enteroviruses and Ascaris eggs in the compost pile (Vinneras et al., 2003). Non-thermotolerant organisms, including pathogens and parasites, are inhibited during this thermophilic and overheating stage (Steger et al., 2004).
Table I: Time-Temperatures Required for Organism Destruction
Ent. Histolytica cysts
Ascaris lumbricoides eggs
Source: Wiley, 1962.
Minimizing the Risk of Microbial Sprawl
Composting is a natural decomposition process carried out by microbial entities taking place under aerobic and thermophilic conditions. In order to study the time-temperature relationship and moisture content analysis, Salmonella enterica were inoculated on a meat carrier and monitored subsequently (Ceustermans et al., 2007). Different types of composting processes were investigated with temperature was found to be the most important parameter to eradicate S. enterica from compost. In addition low moisture content microbial antagonism, toxic compounds etc also reduced this content of S. enterica which is a highly infectious bacterium responsible for severe digestive disorders in human gut.
For the metal detoxification, a general increase in the removal of Pb, Cu, Cr, and to a much smaller extent Zn marks the composting process normally, however, there was no decrease in the overall concentration of Cd (Whittle and Dyson, 2002). Another example of detoxification shows that the plant infectious molds and pathogen Botrytis cinerea were totally destroyed by composting and thus soil can be easily rotated or reused once it goes through a compost turn in greenhouses wherever the routine transfer of soils for cultivation purposes are done (Ghaly et al., 2005).
Figure I: Microbial Evaluation related to Compost-associated Hygiene
Source: Boulter et al., 2000.
Inactivating Methods for Pathogens to Viable Hygiene
Composting process is basically a phenomenon of bioutilization or bioconversion with enormous number of microbes working, which may be pathogens; hence the compost workers need to ensure safety, and take proper measures when it comes to working, collecting, sorting, packaging and even transporting the compost. As a precaution, the optimal turning method of composting regarding hygienic safety as suggested by is that turning of a composting pile was although essential in terms of hygienic aspects; the number of turning should be minimized (Tateda et al.,2005).
Composted barks that are inoculated with Trichoderma sp. after a primary degradation stage are known to possess the potential to suppress fellow cellulolytic species such as Aspergillus and Fusarium species (Smith et al., 1987). Also the compost windrows had emitting gas concentration profiles (O2, CO2, CH4, and N2O) determined and hence analyzed to affect the quality of ambient air of compost (Xu et al., 2007). Phenolic compounds have been observed to have antimicrobial activity and this kill the most of harmful microbes. Also if sewage sludge is composted along with sawdust, the viable content of Aspergillus fumigatus, Geotrichum candidum, Mycobacterium tuberculosis and eggs of Trichuris sp. and Ascaris lumbricoides reduced significantly as all of other methods had these microbes surviving in the final compost (Reeves, 1959). Moreover this compost was ready in a short span of 11 weeks only.
Table II: Comparison of Sample Depth and Numbers of Pathogens
Isolated from Composted Sewage Sludge and Sawdust
Depth of Sample (inches)
Strains of Pathogenic Intestinal Bacteria Isolated
Source: Reeves, 1959.
Deeming the Type of Composting Methods' Effect on Hygiene
Studies have shown that the type of composting i.e., the method adopted may also effect the pathogen reduction count or vice versa. In an experiment illustrated, the qualitative and quantitative composition of the mycoflora of a green compost that is thermophilically produced from plant debris, versus a vermicompost which is mesophilically produced by the action of earthworms on plant and animal wastes were demonstrated and assessed (Anastasi et al., 2005). The fungal count came up to be 8.2-105 CFU/g (dwt) in compost and 4.0-105 CFU/g (dwt) in vermicompost. Composting mode of operations were also studied in another experiment with static pile method, complete mix turning method, and layer turning method, and as a result, the layer turning method showed an excellent performance and ought to be employed in a composting operation to minimize contamination risk.
Onion waste being anti-microbial was the most effective waste type in reducing Sclerotia viability in all soils, while the incorporation of the raw or composted vegetable waste mixtures into sandy loam, silt and peat soils reduced the viability of Sclerotium cepivorumin, soil-borne fungus, in phytotoxic assays conducted (Coventry et al., 2004). In another study, after the prepared compost, of the plant pathogens, Plasmodiophora brassicae (clubroot of Brassica sp.), Fusarium oxysporum sp. lycopersici (tomato wilt) and Macrophomina phaseolina (dry root rot), Synchytrium endobioticum (potato wart disease) and Tobacco Mosaic Virus were found to be temperature-tolerant (Noble and Rowerts, 2004). Also another phytofungus Fusarium oxysporum was found viable in a common horticulture waste composting (Estrella et al., 2003). Even above high-temperatures (above 60°C), in an experimental synthetic food waste compost, species of Saccharomonospora, Gordonia, Rhodococcus and Corynebacterium survived (Dees and Ghiorse, 2001). For simple measures to avoid the release of aerosols from the compost assemblies included the sealing with rubber mats, wetting of piles before and after turning, and regular cleaning and wetting drive-ways during the dry season (Reinthaler et al., 2004).
With tons of solid and semi-solid form of waste being generated daily worldwide, composting is a quintessential process to get a safer rid of this nuisance. Composting can reduce and even eradicate microbial load of pathogenic nature to a greater limit, still however many respiratory, pulmonary, digestive, allergic and skin related diseases have been reported to have spread from the compost aerosols. Studies have shown that the presence of pathogens in a final compost may contaminate the food chain with the incomplete compost product provoking the growth of pathogenic microbes or keeping them alive and vigorous. Most notably, human pathogens like Aeromonas, Aspergillus fumigatus, A. terreus, Cryptosporidium and Giardia cysts, Enterococcus faecalis, Geotrichum candidum, GeoBacillus stearothermophilus, G. thermoglucosidasi, Klebsiella, Listeria monocytogenes, Pseudomonas, Saccharomonospora and Saccharopolyspora, Salmonella typhimurium, Shigella, Thermoactinomyces, Thermobifida, and Bacillus sp. being thermophilic and sporulating species still persist in a variety of compost. Studies are going on to give a better and ultimate solution view on the parameters for the eradication of various pathogenic strains activated or evolved during the process of composting.