Addition discarded tennis balls as an inert bulking agent to olive mill waste and pig slurry during composting reduces the energy consumption related to ventilation and the generation of anaerobic gases

Aeration of composting piles is a significant source of energy demand in large-scale composting. This study evaluated the use of inert bulking agents (IBA), which are made from discarded tennis balls, to improve air circulation and reduce the need for forced aeration and turning operations Two consecutive industrial-scale experiments were conducted under contrasting seasonal conditions: (A) a binary mixture of pig slurry solid fraction and urban pruning residues; and (B) a ternary mixture including olive mill waste. Four treatments were evaluated, combining aeration (Yes/No) and IBA addition (0 % or 5 % w/w). Inclusion of IBA enhanced internal airflow, increasing oxygen concentrations by 35 % and 50 % in Experiments A and B, respectively, while reducing ammonia emissions by up to ninefold and lowering carbon monoxide and methane formation. IBAs also lowered energy consumption by 160 kWh and 546 kWh in experiments A and B respectively, avoiding the emission of 31-135 kg of CO₂ equivalent. Compost quality improved with the addition of IBA, evidenced by higher pH levels, lower electrical conductivity, reduced polyphenol content, and higher germination indices. Using recycled tennis balls as IBA offers a circular solution that reduces energy expenditure, greenhouse gas emissions and operational requirements in composting. However, further optimization is required with regard to feedstock properties, seasonal conditions and pile management strategies.