Microbial biodegradation of waste materials for nutrients enrichment and heavy metals removal: An integrated composting-vermicomposting process
Introduction
Today, the growing world population and large scale urbanization has created a global environmental issue for adopting a sustainable approach in handling solid waste, with solutions such as recycle of organic waste, to promote enduring agriculture as well as a pollution-free environment. Vermicomposting is a non-thermophilic process (Elvira et al., 1996), stabilizing organic matter and converting important plant nutrients in the presence of earthworms and microflora into a more soluble state to become more available to plants (Atiyeh et al., 2002). Vermicompost is considered an excellent product since it is homogenous with desirable aesthetics, reduced level of contaminates; besides, containing plant growth hormones and high level of soil enzymes, it has a larger microbial population and tends to hold more nutrients over a longer period without adversely impacting the environment than regular compost (Sharma et al., 2005). There is growing interest in the use of an integrated systems approach, starting with pre-composting followed by vermicomposting to achieve specific technical objectives. This approach is characterized by greater microbial activity of more uniform sized microbes (Ndegwa and Thompson, 2000), hastened degradation rate (Frederickson et al., 1997, Ndegwa and Thompson, 2000) and enhanced pathogen control compared to either of the individual processes. Also, for the development of sustainable farming, waste enrichment is of interest. This could be done by treating the waste initially with certain prolific microflora. Inoculation of suitable strains have been reported to hasten the rate of vermicomposting, which leads to reduced time needed to complete the process of composting and to enrichment of nutrients in the final product (Singh and Sharma, 2002). This was revealed that inoculation of microorganisms influences the chemical and biochemical properties of organic substrates significantly during vermicomposting (Pramanik et al., 2007). They demonstrated an increase in nutrient content in vermicompost and also phosphates and urease activities (Pramanik et al., 2007). Kumar and Sing reported that an increased percentage of nitrogen and phosphorus was observed when organic waste was treated with a colony of nitrogen fixing and phosphate solubilizing bacteria (Kumar and Singh, 2001). The combination of bioinoculants was best in terms of quality of the vermicompost (Saha et al., 2008). Many authors have investigated the characteristics of vermicompost derived from various feed stocks (Kaviraj and Sharma, 2003, Garg et al., 2006, Pramanik et al., 2007). Over the past decades, extensive research has been conducted to evaluate the feasibility of using corn stalk for various applications including for livestock feed, paper making, energy source, biofertilizer and direct use as soil amendment (Li et al., 2007, Ioannidou et al., 2009).
The aim of the present study was to address sustainability goals through (1) recycling corn stalk residue through vermicomposting, (2) improving the quality of vermicompost using different substrates and (3) adding activated sludge as source of N-fixing and P-solubilizing bacteria in order to speed up the composting process. Keeping all this in view, the main objective of the present research work was to study the role of inoculation of activated sludge into a corn stalk vermicompost in order to determine how the inoculation of microorganism would be able to bring about changes in the nutrient contents, and further, to evaluate the changes of heavy metal content possibly present in the substrate in order to determine potential environmental hazards. Another purpose of the study was to test the technical viability of this system, initially utilizing wheat straw to be employed later on other substrates.
Section snippets
Material and methods
The experimental setup included 15 L reactors which were used for pre-composting and vermicomposting. Four different biodegradable organic wastes, viz. corn waste, cow dung, compost of municipal solid waste and paper with variable C/N ratio were used as vermicomposting substrate. Corn residue was procured from a corn farm close to the city of Mashhad, Iran. Cow manure was procured from a dairy farm adjacent to Mashhad Compost Plant, and paper was collected from the waste collection site of the
pH and electrical conductivity
The pH value of the vermicasts of E. fetida, reared (for 70 days) in activated sludge inoculation was nearly neutral. The nutrients found in the vermicasts such as TKN, TP, heavy metals and the C/N ratio indicate that the produced vermicomposts using E. fetida were appropriate for an agricultural application.
The results related to the variations in the pH and EC during the substrate stabilization phase are depicted in Fig. 1. Estimating the pH periodically revealed that there was an increase in
Conclusion
The information presented here provides a basis for the vermicomposting process becoming the mode of management of biodegradable waste. The chemical analysis of the vermicompost produced by aforementioned pre-composted organic waste with different concentrations of activated sludge as bioinoculants pointed to the feasibility of the process. The vermicompost from a substrate with 80% corn waste showed the highest nutrient and the least heavy metal content. Increasing the concentration of sludge
Acknowledgments
The authors wish to thank the Ministry of Science and Tarbiat Modares University for their financial support to carry out this research project. The authors wish to thank also the Recycle Organization of Mashhad Municipality for considerable laboratory support, Miss Haghdoust for her technical assistance during the experiments and Ellen Vuosalo Tavakoli (University of Mazandaran) for editing the English text.
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