Comparative life cycle assessment of anaerobic co-digestion for dairy waste management in large-scale farms

Abstract

Dairy farms typically generate different types of wastes including cow manure, feed waste, sludge, and returned dairy products. In most developing countries, the conventional handling practices of these wastes constitute land application and stockpiling. This study presents a comparative life cycle assessment to evaluate the environmental performance of anaerobic digestion (AD) as an alternative to conventional practices in dairy farms. The analysis was conducted on a large-scale dairy farm (>10,000 cows) that handles manure and other organic feedstocks in a hot arid climate. Various environmental categories, including global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), and photochemical ozone creation potential (POCP) were assessed for the main processes within the dairy farm. The results showed that AD outperformed the conventional practice in all environmental categories; AD reduced GWP, AP, EP, and POCP by 25.7, 49.5, 18.1, and 16.1%, respectively. Accumulation of manure on sand beds was the highest contributor to GWP and POCP in both AD and conventional scenarios due to excessive amounts of methane emissions. The process that contributed the most toward AP in the conventional scenario was the outdoor storage of manure, whereas in the AD scenario, it was fertilization using stockpile and the digestate. Enteric emissions, landfilling, transportation, and feed waste processing had minimal environmental impacts as compared to manure management. The high ambient temperature had a significant negative impact on the overall environmental footprint, while the non-manure feedstock streams did not noticeably affect the results. Based on the sensitivity analysis, it was found that increasing the manure collection efficiency can reduce the environmental footprint in the AD scenario, and varying the methane yield of manure and other feedstocks did not affect the environmental impacts significantly.