Resource usage of integrated Pig–Biogas–Fish system: Partitioning and substitution within attributional life cycle assessment

•Life cycle assessment from cradle to farm gate of two Pig–Biogas–Fish systems.•Resource usage quantified as cumulative exergy extracted from natural environment.•Main resource usage is land (growth pig feed ingredients) and water (aquaculture).•Compared to monoculture reduced fish feed input but inefficient pond water use.•Recommended improvements are increased biogas use and a better water management. The integration of agriculture and aquaculture with anaerobic digestion is a popular practice at small Asian farms as a way of producing energy and fish, i.e. providing a better nutrient balance and resource recycling. Concerns are raised whether the resource efficiency of such system is better or worse relative to the monoculture system. In this study, we focused on quantifying the resource demand of two integrated Pig–Biogas–Fish farms in Vietnam. The analysis was performed by using the exergy concept: exergy flow analysis (ExFA) at process level and Cumulative Exergy Extraction from Natural Environment (CEENE) at life cycle level. Results showed that such integrated systems considerably relied on land (68% farm A and 54% farm B, mainly for pig feed production) and water (28% farm A and 42% farm B, mainly for aquaculture). It can also be concluded that the intensive aquaculture practice had a higher feed input than the semi-intensive one integrated with pig and biogas production; however, the latter system had a higher CEENE value to deliver a similar mass (i.e. one kilogram) of product at farm gate. This is mainly caused by a dramatically low areal yield of the integrated aquaculture, corresponding to an inefficient water use (16m3kg−1 fingerlings and 10m3kg−1 fish) therefore identified as the resource hotspot. Improvements could be achieved through a better water management in aquaculture and an increased biogas utilization, preventing any leakages. Fertilizing the fish pond with manure-based digestate instead of fresh manure and/or rising the application rate might be a more efficient way to reduce pelleted fish feed consumption, although further research on such options are needed.