Evaluation of Organic Matter Stability During the Composting Process of Agroindustrial Wastes
Composting of wastes from citrus industrial processing (pastazzo and sludge) was studied in order to evaluate the evolution of organic matter during the process and to individuate chemical and/or biochemical techniques able to set the stability of the fin
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Introduction During the last years the citrus-processing industry, in Italy, has been increasing its economical importance. Nowadays, about 800 000 tons year- 1 of citrus fruits are processed, with a production of 500000 tons year- 1 ofpastazzo, a mixture of citrus pulp and skins, which represents the main residue of citrus-processing squeezing treatment. In addition to pastazzo, citrus-processing industry produces also a high amount of effluents, mainly constituted by fruits and factory plant-washing waters, which undergo a purification treatment with the production of sludge. The use of these residues as matrices for the production of a quality compost could constitute a typical example of transformation of a waste into a resource with a market value (Sequi and Tittarelli 1998). One of the key issues in defining compost quality is the evaluation of its organic matter stability. For this reason, it is necessary to individuate and set up analytical methods able to follow the organic matter transformation during the composting process.
lIstituto Sperimentale per la Nutrizione delle Piante, Roma, Italy 2Istituto Sperimentale per I' Agrumicoltura, Acireale (CT), Italy H. Insam, N. Riddech, S. Klammer (Eds.) Microbiology of Composting © Springer-Verlag Berlin Heidelberg 2002
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During composting, for a combination of biological and chemical transformations, total organic carbon content decreases, but the relative content in humic (or humic-like) compounds increases. Determination of the humification rate (HR%) and the humification index (HI%) demonstrated to be effective indicators of the formation of humic-like substances during compost maturation, being able to establish with accuracy the moment of complete stabilisation (Ciavatta et al. 1990). Another chemical technique able to characterise organic matter of composted amendments is isoelectric focusing (IEF) (Ciavatta et al. 1993). This technique allows one to fractionate the organic compounds on the basis of their isoelectric point and their electrophoretic mobility. IEF was usefully utilised in order to obtain information on the qualitative characteristics of organic matter in soils (Ciavatta and Govi 1993), amendments and organic fertilisers (Ciavatta et al. 1997; Canali et al. 1998). An effective biochemical method able to define, for a compost, the level of organic matter stability is the study of organic carbon mineralisation after addition of compost to soil (Kirchrnann 1991; Tittarelli et al. 1998). Since organic matter resistance to soil microbial degradation is directly correlated to its chemical stabilisation, it is possible to follow organic matter evolution, during a composting process, by detecting evolved CO 2 after soil amendments with compost sampled from the heap at prefixed time. The aim of this work is to study organic matter evolution of two composts from agroindustrial wastes (the first obtained mixing pastazzo, citrus-processing sludge and green residues, the second mixing only pastazzo and green residues), by means of c
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