Determination of Compost Biomaturity. II. Optical Density of Water Extracts of Composts as a Reflection of their Maturity

Abstract

This study attempted to devise a simple, reliable, and scientifically valid test for determining the biomaturity of composts. Four types of mixtures containing various fresh animal manures and shredded waste paper were composted indoors during January to March 1992, in drums with basal air intake pipes. The experiment had five replicates. Samples of the compost mixtures were extracted with hot (60°C) water at six intervals. The filtered extracts were investigated to determine their BOD₅ (Biological Oxygen Demand for 5 days); dissolved organic carbon content (DOC); concentrations of ammonium and nitrate ions; effect on germination of cress seeds; and absorption (E) at 280 nm, 465 nm and 665 nm wavelengths of ultraviolet and visible light (UV, VL). The BOD5 of the extracts increased as decomposition began and then declined as the composts matured because both the total carbon content and the biodegradability of the organic matter in the water extracts decreased after the thermophilic phase of the composting. The DOC contents of the extracts correlated at more than 99.9% level of confidence with absorption at all of the three, 280, 465, and 665 nm wavelengths of UV or visible light. Consequently both DOC and the absorption values increased initially and then declined, but they declined well below the value for zero time only for 665 nm. This was perhaps because many individual organic compounds and humic substances strongly absorb light at 280 and 465 nm so that the presence of even small amounts of individual organic compounds can exhibit strong absorption at the lower wavelengths. Temperature, % O₂, BOD₅, DOC, NH₄, NO₃ and 665 nm absorption data indicated that the composts matured between days 40 and 59 after initiation. This was confirmed by germination tests with seeds of cress. The conclusion of this study is that absorption by water extracts of composts at 665 nm of visible light promises to be the basis of a single, facile, reliable, and scientifically sound test for regulatory determination of compost biomaturity, both during and after the process. Compared to other scientifically sound tests, the colorimetric test would be neither amenable to dishonest manipulation, nor expensive.