Integrated Solid–Liquid Separation and Digestate Recycling in Anaerobic Digestion of Beef Cattle Manure: Implications for Methane Production and Wheat Fertilization

The intensification of beef cattle feedlot systems generates large volumes of manure, whose inadequate management can lead to negative environmental impacts and nutrient losses. Anaerobic digestion is an effective strategy for mitigating greenhouse gas emissions while producing renewable energy and nutrient-rich by-products. However, the high solids content of beef cattle manure may limit process performance and operational stability. This study evaluated an integrated management approach combining solid–liquid separation and digestate recycling during semi-continuous anaerobic digestion of beef cattle manure, followed by the agronomic use of the resulting by-products in wheat cultivation. Four treatments were evaluated in 60 L horizontal tubular digesters operated under mesophilic conditions and a hydraulic retention time of 30 days: solid–liquid separation followed by water dilution; solid–liquid separation combined with 100% digestate recycling; raw manure diluted with water; and raw manure diluted with water and digestate, with 60% recycling. Biogas and methane production were monitored, and the digestate and separated solid fractions were chemically characterised. A greenhouse experiment was subsequently conducted to compare wheat growth and grain yield under mineral, digestate-based, and organomineral fertilisation strategies. Solid–liquid separation combined with full digestate recycling produced the highest specific methane yields, whereas treatments without separation showed higher volumetric methane production due to their higher organic loading rates. Digestate-based fertilisation resulted in wheat grain yields comparable to or higher than those obtained with mineral fertilisation. These findings suggest that integrating solid–liquid separation and digestate recycling has potential to reduce freshwater demand, promote nutrient recovery, and support the agronomic reuse of anaerobic digestion by-products.