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Using a second-order polynomial model to determine the optimum dregs/bark ratio for industrial-biosolid composting: the initial conditioning stage

Forest Sciences Faculty, University of Concepción, P.O. Box 160 — C, Concepción, Chile, mazambrano{at}udec.cl

J. Freer

Forest Sciences Faculty, University of Concepción, P.O. Box 160 — C, Concepción, Chile

J. Baeza

Forest Sciences Faculty, University of Concepción, P.O. Box 160 — C, Concepción, Chile

M. Villarroel

Chemical Engineering Department, University of La Frontera, P.O. Box 54 - D, Temuco, Chile

G. Vidal

Environmental Science Centre EULA-Chile. Universidad de Concepción, P.O. Box 160-C, Concepción, Chile

Increasing pulp industry production has generated additional solid waste (i.e. biological sludge, dregs, etc.) and therefore an increasing disposal problem. On the other hand, composting is emerging as an interesting alternative for the disposal of residues. The objective of this study was to evaluate the physical and chemical properties of sludge, dregs and bark and the effect on their initial conditioning stage prior to composting, using pine chips as bulking agent. After their characterization, these solid wastes were combined in different mixture proportions defined by a 32 experimental design based on the response surface methodology (RSM), in which 18 observations were required for the independent variables (dregs and bark) and the C : N ratio, moisture content and pH were the dependent variables. The characterization indicated that the dregs have alkaline properties with the presence of some essential plant nutrients, such as phosphorous (0.37%), potassium (0.76%), magnesium (1.4%) and calcium (27%). The combination of the macronutrients (phosphorous: 0.39%, potassium: 0.24%, calcium: 1.7%, magnesium: 0.44%) and micro-nutrients such as ferrous material (0.47%) and zinc (0.12%) found in the sludge suggest a promising alternative despite the potential problem due to the high pH of the dregs. The RSM design indicated a feasible region that satisfied the optimal dregs : bark ratio of 0.25 without exceeding the addition of 12.5% dregs, due to the alkaline properties of these inorganic wastes and the quadratic influence over the C : N ratio. The experimental results indicated that the composting process of dregs, bark and sludge is technically suitable, although the use of a rapidly available C source needs to be evaluated.

Key Words: dregs • bark • start-up conditions • composting process • sludge • wmr 1003—6

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