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The high concentration of pig farms in NE Spain makes pig slurries an attractive fertilizer to reduce costs of agricultural production. However, inadequate management of fertilization with pig slurry can cause negative environmental consequences. In this context, a 4 year field trial was carried out to evaluate several fertilization strategies, using pig slurry, for a double-annual forage cropping rotation with barley (Hordeum vulgare L.) and sorghum (Sorghum bicolor L.) under sub-humid Mediterranean conditions. Four nitrogen (N) fertilization treatments applied as pig slurry (0, 170, 250 and 330 kg N ha−1 year−1) were applied and their effects on yield, N uptake, unrecovered N, and residual NO3-N in soil were evaluated for each crop and each rotation. The 4 year average dry matter (DM) forage yield of sorghum was 9.3 Mg ha−1 in all N fertilization treatments, except for the control (0 kg N ha−1) which was 6 Mg ha−1. However, barley DM yields varied among N treatments. The highest barley yield (8.7 Mg DM ha−1) was achieved with the application of 330 kg N ha−1 year−1. The barley yields were reduced by a 26% (6.9 Mg DM ha−1) and a 64% (5.3 Mg DM ha−1) with N rates of 250 and 170 kg N ha−1 year−1, respectively. The average total annual yield was 17.8 Mg DM ha−1 for the maximum N rate tested (330 kg N ha−1 year−1). Indeed, the application of N rates above the maximum amount allowed by the Nitrates Directive in areas vulnerable to nitrate contamination (NVZ) (170 kg N ha−1 year−1 in form of pig slurry), improved the total annual DM yield by 10–18%. However, N rates of 250 kg N ha−1 year−1 increased by 69% the unrecovered N compared to applying 170 kg N ha−1 year−1. This could lead to N losses to the environment, probably by nitrate leaching and/or volatilization.

This study assesses the technological, environmental and economic feasibility of biodrying to valorise cellulosic sludge as a renewable energy source. Specifically, three different aeration strategies were compared in terms of biodrying performance, energetic consumption, gaseous emissions, quality of end-products and technoeconomic analysis. These strategies were based on different combinations of convective drying with biogenic heat produced. Two innovative biodrying performance indicators (Energetic Biodrying Index and Biodrying Performance Index) were proposed to better assess the initial and operational conditions that favour the maximum energy process efficiency and the highest end-product quality. The end-products obtained consistently presented moisture contents below 40% and lower heating values above 9.4 MJ•kg-1.However, the best values achieved were 32.6% and 10.4 MJ•kg-1 for moisture content and lower heating value, respectively. Low N2O and CH4 emissions confirmed the effective aeration of all three strategies carried out, while NH4 and tVOCs were related either to temperature or biological phenomena. A techno-economic analysis proved the economic viability and attractiveness of the biodrying technology for cellulosic sludge in all the strategies applied.