Literature Review
2.3 Microbiology of Composting
or moderate-temperature phase, in which microorganisms acclimatize themselves into the surrounding environment (b) the thermophilic, or the highest temperature phase, which causes the highest degradation of the organic matter into stable pathogen free end product, it can last from a few days to several months, and finally, (c) Maturation and cooling phase, the temperature reduces down up to mesophilic and ambient level, it can last up to sev- eral months. The aerobic degradation of organic waste can be expressed by the simplified chemical formula (Eq. 2.1)
C6H10O4+ 6.5 O2= (C6H10O4)a−1+ 6 CO2+ 5 H2O (2.1) The (Eq. 2.1) is an exothermic reaction which generates 616 Kcal of heat per mole of the organic matters degraded, assuming 230 kcal/mol is the heat of formation of the organic matter. Thus, for degradation of a glucose molecule, -673 Kcal/mole of heat is generated followed by production of carbon dioxide and water (Themelis and Kim, 2002). Thus, for water hyacinth, composting is a better alternative in producing compost having valuable application for soil as compared with other treatment methods of water hyacinths. Gener- ally composting is divided into two types, as the open systems and the in-vessel systems.
An open system of composting includes windrow or pile composting, they are also called as non-reactor composting, being the most preliminary type of composting procedure (Roger, 1993). The in-vessel system of composting comprises of the tunnel systems, and the rotary drum composting (Gajalakshmi and Abbasi, 2008). The rotary drum composting is one of the promising decentralized composting techniques. It provides mixing and aeration of the waste organic matter, to produce a consistent and uniform compost of water hyacinth (Kalamdhad and Kazmi, 2009). The different phases of the composting period are predom- inated by varied microbial communities. At the mesophilic phase, microorganisms have the capability to rapidly break down the soluble, readily degradable compounds. A large variety of mesophilic and thermophilic microorganisms have been reported in composting of municipal and industrial solid waste.
2.3 Microbiology of Composting
Composting is a microbiological process of aerobic degradation of the heterogeneous organic matter by the metabolic activities of the successive mixed microbial population.
The populations and communities of microorganism vary continuously, which makes the description and differentiation of microorganisms participating in the composting process difficult, microbial populations change rapidly as a function of the surrounding environ- mental conditions such as temperature, nutrient availability, oxygen availability, water, and
Figure 2.2: Food web of the compost pile ((National Research Council, 1981)) pH in the process of composting. These conditions are responsible for microbial survival and their activities, additionally different types of microorganisms act at different stages of composting. It is because of the metabolic activities of these microbes in the compost- ing environment, the temperature fluctuations take place (Bhatia et al., 2012, 2013). The food web of composting process (Figure 2.2) explains that all other organisms also exist in compost but the major role of degradation is played by the microorganisms. Usually, the secondary level consumers appear during maturation stage. While tertiary consumers appear during final stages of composting or near end of composting. The microbes have known to do the degradation hence are studied extensively to understand the process of composting. Every microbe has peculiar characteristics features and can be distinguished.
These microbes require specific environmental conditions and sufficient substrate to grow and multiply in number. A large variety of mesophilic and thermophilic microorganisms have been reported in composting of municipal and industrial solid waste. Specific type of microorganisms i.e. bacteria (Bacillus sp., Thermus sp.), fungi (Cladosporium, Alternaria, Verticillium,Aspergillus,Eurotium,Penicillium,Trichoderma,Mucor, andRhizopussp.) and actinomycetes are involved in breaking of the organic material, also they have been reported as primary decomposers in composting (Strom, 1985; Beffa et al., 1996; Pedro et al., 2003).
2.3. Microbiology of Composting
2.3.1 Bacteria
Bacteria play an important role in organic matter degradation during the composting process, as they comprise of 80-90% of the billions of microorganisms present in a gram of compost material (Trautmann and Olynciw, 2012). Bacteria have more survival capa- bility as they are capable of utilizing organic materials for their metabolic actions, their broad range of enzymes make them capable of feeding on a wide range of organic mat- ter (Golueke, 1992; Epstein, 1997; Raut et al., 2008; Liu et al., 2011). Bacteria majorly participate in the mesophilic and thermophilic stages of composting. Bacteria targets the initial decomposition of the robust and hard to degrade organic matter in the initial stage of composting, also their action is majorly responsible for the temperature rise in com- posting process to a thermophilic phase (above 40◦C). In composting process, the bacterial species present commonly are such asBacillus, Pseudomonas, Enterobacter, andAcinetobac- ter(Strom, 1985; Bhatia et al., 2013).
In the mesophilic and thermophilic stages of composting bacterial species ofBacillus ba- dius, Bacillus pumilus, Bacillus sphaericus, andBacillus thuringiensisdominate the process (Ryckeboer et al., 2003). High surface to volume ratio, a broad range of organic matter degrading enzymes and less generation time make bacteria more capable of surviving the rapid changes of substrate availability and other surrounding conditions during the com- posting process. Consequently, bacteria are major role players in initial decomposition and temperature rise during the composting procedure (Ryckeboer et al., 2003). There are re- ports which state that CO2 evolution rate of mesophilic bacteria such as Bacillus sp. and Azotobacter sp. were highest in the initial phase of composting (Temperature less than 40
◦C), they have caused major degradation of the mixed raw organic material (Nakasaki et al., 1985). Although composting is an aerobic process, facultative anaerobic bacterial species of Bacillus andThermoactinomyces genera have been reported in rotary drum composting of biodegradable organic wastes (Bhatia et al., 2013). It has been observed that at the be- ginning of composting process gram positive rod shaped bacteria are dominantly present, while at the end of composting period gram-negative bacilli shaped bacteria dominate (Bha- tia et al., 2012). Relevant changes in microbial population occur with the availability and complexity of the organic material present as well as the temperature variation plays an important role in the activity and type of microbial species of the composting cycle.
2.3.2 Actinomycetes
Actinomycetes resemble the characteristics of bacteria as well as fungi, they lack nuclei like bacteria and grows into mycelium like fungi and with no chitin and cellulose in the cell
wall. The earthy smell of soil is because of the production of sesquiterpenoid compounds i.e. geosmine by the actinomycetes, and are capable of degrading complex organic material such as lignin and cellulose, they also possess enzymes to break down the tough woody debris, bark and other cellulolytic material. Actinomycetes develop slowly as compared to most bacteria and fungi and can inhibit microbial growth by producing antibiotics, lytic enzymes and can also parasitism (Ryckeboer et al., 2003).
Moreover, they can tolerate higher pH than fungi, the optimum pH range of growth is be- tween 7 and 8 and they can also develop spores in adverse surrounding conditions. The op- timum temperature for growth of actinomycetes is between 25- 30◦C, except some species which are resistant to higher temperatures above 60◦C (Nakasaki et al., 1985). However, maximum lignin degradation by actinomycetes along with thermophilic fungi takes place between the temperature range of 40-50◦C (Gajalakshmi and Abbasi, 2008). These mi- croorganisms are not only present majorly during thermophilic stage of composting but also during maturation and curing stage. A white film of actinomycetes species such as Thermoactinomyces sp. and Streptomyces sp. have been reported after the maturation of compost (Strom, 1985).
2.3.3 Fungi
Fungi comprise of molds and yeasts and are majorly causing the degradation of complex plant polymers such as lignin. Fungi are the dominant active degraders of fresh organic matter in the mesophilic and thermophilic stage of composting within the temperature range of 20-40◦C, a pH range of 4-5 supports the growth of fungi and yeasts (Ryckeboer et al., 2003). Their tendency of spreading into many cells and filaments make them capable of degrading organic matter that is acidic or low in nitrogen composition, which is difficult for bacteria to degrade. With further degradation, ammonification, and increase in pH, the fungi community reduces. In composting process, the majorly found fungal species are such asAcremoniumsp. Actinomucor sp.,Aspergillussp.,Candida sp. Fusarium,Penicillium, and Graphium(Ryckeboer et al., 2003; del Carmen Vargas-García et al., 2012).
The high temperature is unfavorable for fungal growth, the optimal temperature for growth of thermophilic fungi ranges from 45-50◦C. Above 65◦C, no fungal growth has been observed, however, thermophilic fungi have been observed to have cellulolytic or ligni- nolytic activity at higher temperatures (Nakasaki et al., 1985; Hellmann et al., 1997). Apart from temperature, other factors such as pH and availability of carbon and nitrogen, most fungi prefer acidic conditions and high nitrogen content, except the wood rot fungi which requires low nitrogen (Dix and Webster, 1995). Usually, fungi grow in form of unseen filaments as well as colonies on the compost surface.