Determination of compost respiration rates using pressure sensors

Compost stability is one of the most important criteria used to evaluate compost quality. Several of the most popular methods to quantify stability are based on measurements of aerobic respiration of oxygen to carbon dioxide. This study compared two closed-vessel respiration techniques. In both methods, the evolved CO2 was absorbed by sodium hydroxide (NaOH), leading to a pressure drop in the container. The conventional technique measures absorbed CO2 by titrating the NaOH trap, limiting measurement frequency by the need to manually remove and replace the trap. The pressure sensor method (PSM) uses a datalogger to record pressure differences in the vessel, allowing several observations each hour. Samples of bedded swine manure compost (n=125) were analyzed using both the PSM and the titration method (TM) during 48 h incubation at 30°C. Each compost sample was also analyzed for total solids, volatile solids (VS), pH and electrical conductivity. Respiration results obtained using the PSM were compared with those obtained using the TM, with each method run on split compost samples under the same environmental conditions. Respiration rates measured by PSM and TM were highly correlated (R² = 0.92). Respiration rates ranged from 3.64 to 10.77 mg CO²-C gVS⁻¹ d⁻¹, varying with feedstock and the extent of prior composting. Decreases in volatile solids and/or increases in composting time resulted in decreases in the respiration rate, but the effect of pH and electrical conductivity was not clear. For the PSM, detailed time series data also allowed semi continuous observation of important changes in the respiration rate throughout the course of incubation. PSM is a sensitive and reliable method to quantify compost respiration and stability.