Energetic environmental and economic assessment of three tree species and one herbaceous crop irrigated with primary treated sewage effluent

The energetic, environmental, and economic performance of four bioenergy plant species (Αcacia cyanophylla, Eucalyptus camaldulensis, Populus nigra, and Arundo donax), irrigated with pre-treated effluent in a three-year-field trial, was investigated. Results showed that irrigation was more energy demanding compared to other practices. A. cyanophylla achieved 34, 83, and 189% higher above ground biomass energy yield than E. camaldulensis, A. donax, and P. nigra, respectively. A. cyanophylla and E. camaldulensis had higher energy requirements but they achieved higher net energy yield and energy ratio. P. nigra (3.49 € GJ−1) and A. donax (2.34 € GJ−1) were found to have higher energy production cost compared to E. camaldulensis (1.65 € GJ−1) and A. cyanophylla (1.24 € GJ−1). P. nigra and A. donax had the highest nutrient use efficiency for nitrogen (NUE-N) and phosphorus (NUE-P), respectively. It was concluded that species with low NUE, which is opposite to current directions, and high water use efficiency (WUE) (i.e. plant species with relatively low water demands and great biomass allocation in leaves), are more suitable when irrigation with strong effluent occur and N removal is primary objective. Such plant species have comparatively lower energy requirements, which is consistent with the current trend for low energy inputs in agriculture. To mitigate the losses in plant biomass and reduce the adverse environmental impacts from nutrients, additional practices should be adopted. Our results help us to draw important conclusions in order to set criteria in the selection of suitable bioenergy crops and practices.