Evaluating Ammonia Reduction Efficiency of Natural Covers in Buffalo Manure Digestate Management: A Techno-Economic-Environmental Perspective on Biochar Implementation

Storage of livestock effluents represents a major source of ammonia (NH₃) emissions to the atmosphere. Therefore, identifying efficient and sustainable mitigation strategies is crucial. Conventional floating covers are commonly used to reduce emissions; however, they often present limitations in terms of management, durability, and cost. This study proposes a novel approach by comparing traditional floating cover materials like natural crust, straw, and light expanded clay with biochar as an innovative mitigation strategy to reduce ammonia volatilization from the liquid fraction of buffalo digestate obtained from an anaerobic digestion plant in southern Italy. All cover materials were applied at a uniform thickness of 2 cm under laboratory conditions using a dynamic chamber technique. Additionally, a cost analysis was performed considering the material purchase cost for an average storage tank of 700 m² and two hypothetical reduction efficiencies (50% and 70%). Results indicated that biochar was the most effective cover, achieving a 67% reduction in ammonia emissions compared with the natural crust. Light expanded clay exhibited the lowest efficiency, likely due to its insufficient sealing capacity at the applied thickness. From an economic perspective, biochar becomes increasingly competitive when emission reduction efficiency is accounted for, owing to its favorable physical–chemical properties. Further research is recommended to evaluate the long-term durability of biochar as a cover material and to investigate potential synergistic effects when applied later as a soil amendment.