Effects of Initial C/N Ratio on Changes in Physicochemical Properties of Co-Composting of Three-Phase Olive Mill Solid Waste and Wastewater with Dairy and Poultry Manure

Abstract

Co-composting of olive pomace, olive mill waste water, dairy and poultry manure, and straw was conducted using 15 identical cylindrical 100-L reactors under laboratory conditions at five different C/N ratios. The changes in physical and chemical parameters of composting such as dry matter, organic matter, total phosphorus, ammonium nitrogen, nitrate nitrogen, pH, reduction in volume, and relative heat generation were evaluated based on the initial and final conditions. The results showed that when considering the maximum changes in physical and chemical parameters of the composting process, the ideal initial C/N ratios were determined to be between 25.97 to 33.07 applying regression analysis using the Gaussian equation. Conversely, the results based on ANOVA analysis suggested that the initial C/N ratio of the most ideal mixture (46% olive pomace, 36% separated dairy manure, 12% poultry manure, 5% straw, 1% olive mill wastewater) was 27.5 for this study. Comparing this composting recipe to the other one tested in this study, the use of straw, which is more valuable for agricultural use, is limited to 5%. Consequently, more separated dairy manure and olive pomace could be used in composting.