Abstract
This paper reports a fully automatic respirometric technique with low maintenance costs that allows the routine determination of the specific oxygen uptake rate (SOUR), as well as the cumulative oxygen demand at 20 h, (OD20). The procedure involves the measurement of the dissolved oxygen concentration in an aqueous suspension of compost that is supplied with various nutrients, none of which contains organic carbon. The proposed method was used to monitor composting in a compost pile consisting of 65% organic matter obtained by selective collection of municipal solid waste (MSW), 30% sewage sludge and 5% plant trimming residues. Based on the results, the maximum SOUR value reached, (SOURmax), decreases gradually with time, to values near 1 mgO2/gVS.h after 130 days; indicating that the final product is stable. Linear correlations between OD20 and the absorbances at 465 and 665 nm of aqueous extracts of compost, and its soluble chemical organic demand (CODsol), and soluble organic carbon (TOCsol) are significant at the p ≤ 0.01 level. However, the use of such variables instead of respirometric data requires prior experimentation with the type of waste to be composted in order to identify the absolute values at which biological activity ceases.
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