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In this study, composting of olive pomace from three-phase oil production system, sewage sludge, dairy manure, and tomato stalks were carried out. The effects of carbon/nitrogen ratio on decomposition rate of composting were investigated with constant free air spaces of composting mixtures. Composting process was carried out in the aerobic reactors made of stainless steel which were monitored for 32 days. Temperature-controlled feedback system was operated based on the Rutgers strategy. Temperature, moisture, organic matter, pH, electrical conductivity total carbon, total nitrogen, ammonium nitrogen and nitrate nitrogen, total phosphorus as well as potassium were monitored during the composting process. In addition, mass and volume changes of the mixtures were determined. The highest degree of decomposition was determined for the mixture (carbon/nitrogen ratio of 20) based on the dry matter loss where the maximum amount of sewage sludge was used. On the other hand, maximum decomposition occurred at a carbon/nitrogen ratio of 23 for mixtures containing dairy manure. The increase in the use of olive pomace in mixtures had adverse effects on the decomposition process. The stabilization process in the mixture containing dairy manure lasted shorter than the mixtures containing sewage sludge.

Composting plants should be operated at a high degradation rate in order to lower their capital and operational costs. Initial C/N ratio is one of the most influential factors on the composting process. This study aimed to determine the impacts of initial C/N ratio on operational composting parameters of two‐phase olive mill pomace with separated dairy manure. Five initial C/N ratios ranging from 29.43 to 43.33 were examined using fifteen 100 L cylindrical reactors. The highest decomposition rate was estimated at 0.02 day−1 with the C/N ratio of 34.49. The maximum relative heat generation was obtained with the C/N ratio of 36.07. The C/N ratio of ≅33 reduced the composting cost of process‐based total airflow supplied per dry matter and organic matter loss. The maximum throughput capacity was obtained when the compost blend had the C/N ratio of ≅37.24. It was concluded that operating the composting of two‐phase olive mill waste, separated dairy manure, and straw with the boundary of the initial C/N ratios between 33 and 37 was recommended.

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