Respiration profiles in monitoring the composting of by-products from the olive oil agro-industry

The composting of olive press cake (OPC) repeatedly mixed either with olive mill wastewater (OPC+OMW) or with tap water (OPC+W) was studied using the thermogradient respirometer, an apparatus that determines the respiration rates from a substrate over a wide range of different temperatures (respiratory profile). The composting processes took place over a period of five months during which nine moistenings of the OPC were performed with the respective liquids. The composting resulted in detoxification of the materials used in both treatments, as indicated by seed germination tests. However, the repeated applications of OMW resulted in recurring thermophilic phases (following each application) and in greater pH and conductivity increases in the final product, as compared to water applications. Respiration measurements performed at 35 degrees C were good indicators of the mean metabolic potential in the compost piles (the mean respiration derived from the whole respiration profile over a wide range of environmental temperatures). However, respiration measurements at higher temperatures (48.5 degrees C) were better indicators of the respiration activity occurring in situ. Following the initial thermophilic phase, the respiration potential of the composts at high temperatures (42-63 degrees C) increased drastically compared to their respiration potential at lower temperatures (17-42 degrees C) indicating the establishment of a thermophilic microflora. Subsequently, only the periodic new substrate-C applications in the form of OMW resulted in increased ratios of low temperature-to-high temperature respiration potential. These ratios decreased again following the respective thermophilic phase that each new OMW application had induced.