Irrigation of energy crops with urban wastewater: effects on biomass yield, soils and heating values

Exploitation of energy crops irrigated with non-conventional water resources on a large scale could decrease negative impacts from fossil fuel use, at the same time saving potable supplies and reducing disposal of polluting effluents in surface water, particularly in environments with water scarcity as Mediterranean basin. To this end, three energy crops (Typha latifolia, Arundo donax and Phragmites australis) were planted in experimental plots and irrigated over a 2-year period with the effluents of an urban wastewater depuration plant. Biomass and energy yields of the crops were evaluated; the effects on the main physical and chemical properties of soils (e.g. pH, EC, SOC, N, P) and leachate (pH, EC, COD) resulting from irrigation with wastewater were compared to those of irrigation with conventional water. In general, treatment with wastewater increased the biomass yield: this may be ascribed to the fertilising value of the wastewater. Irrigation with wastewater did not result in either notable changes of the soil’s physical and chemical properties, or risks of soil structure decay, when compared to irrigation with conventional water. Only a slight and insignificant reduction of the soil’s hydraulic saturated conductivity was measured. Percolation of contaminants (such as organic compounds and salts) into groundwater reserves was limited, as shown by the analysis of the leachate accumulated in lysimeters after irrigation with wastewater. Heating values measured by calorimetric tests were higher for T. latifolia in comparison to the other experimental species; nevertheless, the higher biomass productivity of A. donax makes the energy yield per unit of cultivated surface much higher than those of T. latifolia and P. australis. Even though the results achieved require further verification by mid- or long-term research, the present investigation shows that herbaceous crops irrigated with wastewater can produce appreciable biomass and energy yields. This is also an environmentally and economically sound way of wastewater disposal.