Technical, environmental and cost-benefit assessment of manure management chain: A case study of large scale dairy farming

Improper management of livestock manure has resulted in loss of nutrients and organic matter available in manure in addition to negative environmental impacts. This study developed and compared eight manure management scenarios across their entire life cycles, rom excretion to transport to land, considering technical, environmental and economic aspects. The scenarios based on combinations of collection, sand separation, solid/liquid (S/L) separation, anaerobic digestion (AD), composting, and storage were compared. Mass balances, costs and benefits and greenhouse emissions were evaluated. The model framework was tested and validated for a large-scale dairy farm with 9000 heads of cattle and daily manure production of approximately 505 t in Iran. The study indicated that sand separation and S/L separation did not contribute to a change in manure nutrients or emissions but reduced sand, maintenance cost, and transport requirements. AD followed by separation achieved the highest emission reduction (27.7 kg CO2eq t−1) due to the avoided emissions from replacing fossil fuels by renewable energy. Composting method had the lowest costs; however it resulted in a low nutrient recovery efficiency and high nitrous oxide emission. The assessment revealed that AD is a promising management option yielding a high potential greenhouse gas savings, nutrients recovery and nitrogen availability in fertilizer for plants. In spite of the high investment costs of AD, it could be a profitable strategy due to the high subsidies paid to renewable energy projects in Iran. In conclusion, this study showed that the choice of manure treatment method has a strong influence on nutrients, profitability and greenhouse gas balances by performing sensitivity analysis. The results of this study and the application of this model further indicate the need to consider various significant impacts, farm specifications and local conditions to decide the best manure management options.