Evaluation of two different aeration systems for composting two-phase olive mill wastes

Composting of two-phase olive mill waste (TPOMW) mixed with sheep litter and grape stalks was performed using two different aeration systems: forced aeration and windrow turning. The aim was to find out which of these technologies was the most appropriate for the composting of these materials. The efficiency of each aeration method was evaluated by monitoring the evolution of parameters such as temperature, biodegradation of organic matter fractions (carbohydrates, lipids, phenols, lignin, cellulose and hemicellulose) and nitrogen loss. Besides, the quality of the final composts was compared in terms of their main chemical characteristics (total N, N-NH4+, N-NO3−, P2O5, K2O, pH, electrical conductivity, polymerisation degree, humification degree and germination index) and their hygienisation degree (faecal coliforms and faecal streptococci). The study revealed that forced aeration of static pile presents several drawbacks for the composting of two-phase olive mill waste due to its physical properties. Several windrow turnings were required to avoid the formation of preferential air path-flows, the compaction and heterogeneous drying of the material. Piles elaborated by windrow turning showed a longer thermophile phase which lead to a higher degradation of fibres and a greater loss of nitrogen. The quality of the end products obtained by both methods was quite similar, although the organic matter of composts prepared by windrow turning had a higher humification degree. Turnings of TPOMW composting piles are necessary for the normal development of the process and the high investment necessary to implement the forced aeration could be avoided. The benefits that forced aeration introduces by means of the control of temperature can be obtained by optimising pile size and turning frequency.