The stability of twenty-seven composts and organic substrates (including raw, less stable and stable materials) was assessed using two different liquid phase tests. One of the tests was introduced in 1998 and was based on the calculation of a Specific Oxygen Uptake Rate (SOUR). The newly introduced liquid phase test presented here is simpler to set-up and to perform than the older liquid phase test and is more representative of the composting process due to its longer experimental duration. It is based on the quantification of oxygen consumption in the headspace of a BOD bottle that contains the liquid-solid solution. The results indicate that a marginal correlation does exist between the main indices calculated from both tests. The correlation was slightly stronger for the indices calculated from the raw materials than from the processed ones. The correlation calculated from the processed substrates was statistically insignificant. The SOUR ranged from 1520 to 3650 mg O2
/kg VS-h for the raw materials and from 110 to 1150 mg O2
/kg VS-h for the processed materials, respectively. The corresponding stability rate related index (LSRI24
) of the new liquid phase test introduced here ranged from 240 to 1180 mg O2
/dry kg-h for the raw materials and from 64 to 792 mg O2
/dry kg-h for the processed ones.
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