Using treated wastewater to irrigate energy crops is a favourable alternative for traditional irrigation, especially on marginal lands or in seasons when water is scarce. Although irrigating crops with reused wastewater (even when used to irrigate non-food crop) not always have social acceptance, considering the amount of wastewater daily available, and its high nutrient loads, it is a sustainable and cheap method – and might be very necessary. From all the crops identified as suitable for combining high biomass production with wastewater Arundo donax is highlighted because this perennial crop is well adapted to water-scarce regions and marginal lands, with an ability for wastewater phytoremediation and nutrients removal.
Crop productivity
As different studies highlighted, no significant differences in yields between Arundo donax plants irrigated with wastewater and giant reed plants irrigated with tap water could be observed.
Yields are directly proportional to different water regimes, that is: the lower the water provided the lower the yields. In ideal conditions, Arundo donax has a yield of 50-80 dry tonnes of biomass yield per hectare every year, meaning that giant reed is way ahead of its competitors.
Carbon sequestration
Carbon sequestration in the leaves, roots and rhizomes of perennials, like Arundo donax, improve soil structure and soil aeration, and these factors highly contribute to the reduction of soil erosion and to prevent desertification.
Carbon sequestration results are coincident with crops yields, which means the higher the yield, more carbon is sequestered and stored in the soil. Again, no significant differences were witnessed between fields irrigated with wastewaters and fields irrigated with fresh waters. Naturally, the lower the amount of water provided the lower the yields and the carbon sequestered.
Arundo donax is able to store ca. 2, 4-4 metric tons of soil organic carbon/hectare/year under Southern European climate, belowground.
Emissions to soil, air and water
According to estimated results, the use of stems as solid fuel is relevant to reduce greenhouse gases emissions as the released of CO2 emissions is balanced by the CO2 sequestered by biomass in the photosynthetic process. Higher water regimes (and consequently higher yields) represent higher CO2 savings. Most importantly, no significant differences were found between giant reed plants irrigated with wastewater and those irrigated with fresh water.
As for nitrogen emission, plants irrigated with wastewater present higher N emissions due to higher N application (derived from a wastewater rich in ammonium and nitrate ions).
These results could represent an environmental constriction for the irrigation of perennial crop fields with treated wastewaters. Still, using wastewater for non-food crops irrigation can decrease the amount of N-based nutrients artificially added to the soil, thus contributing to the pollutant load reduction once the soil-biomass system represents a depuration system.
When lowering the amount of water used in the irrigation, nitrogen emission – which is pollutant to air, soil and water – were also reduced.
Economic and social considerations
Needless to say that wastewater irrigation is much more profitable than freshwater irrigation, since it provides a higher and positive income.
Furthermore, the use of wastewaters in the irrigation of energy crops, such as Arundo donax, represents sustainable environmental and economic opportunity, providing many social benefits to human communities. The use of wastewaters in the irrigation of non-food crops, contributes to recycling of liquid wastes, returning them back to environmental systems in a non-harmful way, and thus lessening the risk of human and environment exposure to pollutants.
Also, the making of biomass for energy or for other applications, when irrigated with wastewaters, reduces the emissions of several pollutants and thus the effects on health and environmental damage.
Wastewater irrigation: Conclusions
Overall it is suggested that the reuse of wastewater on the production of perennial crops have more benefits than traditional irrigation, namely regarding use of water and mineral resources and costs savings.
Irrigation of perennial grasses – like giant reed – with wastewaters could be an appropriate tool to lessen water and mineral resources consumption and the associated environmental concerns due to the burning of fossil fuels. At the same time, it could establish an opportunity to develop rural areas and marginal lands. Combining wastewater irrigation with biomass production may also contribute to improve the effluents quality, with environmental, economic and social benefits.
To find out more about Arundo, its cultivation techniques, and effects on the environment, visit our website or get in touch with us at info@arundobioenergy.com
