Composting different agri-industrial wastes with biochar: optimized lab-scale trials assessing ammonia emission and mass balance

In comparison with a control without biochar, the efficiency in reducing ammonia emission of 5% biochar, added in the initial organic mixtures, was evaluated in three optimized composting tests. Trials were carried out in a Lab pilot plant, named COMPOSTER, using livestock effluents, digestates and wet olive mill pomace as main ingredients, and adjusting the initial mixtures at a low C/N to stress the potential of ammonia emission. The system optimizes the aerobic biodegradation and allows for a mass balance assessment. It also prevents leaching, anoxia and production of undesired foul volatile compounds. All composting trials exhibited optimized thermal phases, yielding mature and stable compost in just 40-50 days. The process preserved most (>95%) of the nitrogen in the biomass, while carbon loss was around 47-50%. The addition of biochar significantly reduced ammonia emission, by ≈ 50% on trial with digestates, where the lowest emission occurred, and by ≈ 30% in the others. Regardless of the ingredients, composting required an air consumption of ≈ 7-9 m3 kgDM-1 (≈ 15-18 m3kgC-1), with hourly peaks of ≈ 21-24 L kgDM-1. The airflow need resulted quite lower than that reported in literature, thus preventing energy and water loss. All trials confirmed that biochar is completely “non-biodegradable”, even in the highly aggressive context of composting. Compost analyses showed high content of N, K, and, to a lesser extent, P; absence of phytotoxicity and Dynamic Respiration Index values < 25 mmol O2 (kgVS h)-1, with pH and electrical conductivity suitable for beneficial agronomic use, not only in open fields, as soil amendment/organic fertilizer, but also for high-value applications, such as ingredient in potting soils for nursery crops, partially or totally replacing peat.