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Short-rotation Willow Biomass Plantations Irrigated and Fertilised with Wastewaters 1 Introduction
1.1 BackgroundHuman wastes have been used on land since ancient times. One of the first land treatment facilities documented in the literature was situated at Bunslaw, Germany (Reed and Crites, 1984). A sewage irrigation system commenced here in 1531 and was in operation for over 300 years. Many "sewage farms" existed in the latter half of the 19th century and during the first few decades of the last century. They were replaced gradually by in-plant alternatives starting round 1920 when the activated sludge process and other biological treatment methods were introduced. In the United States, land treatment methods in general, especially irrigation applications, have regained respect in recent years as cost-effective and competitive alternatives to conventional wastewater treatment processes in combination with recycling of wastewater nutrients. This result is mainly due to the pioneering and extension work carried out at the Penn State University in the 1960s and the early 1970s (Sopper and Kardos, 1973). In many countries throughout the world where water resources are scarce, reclaimed wastewater is used in agriculture as a replacement for natural water supplies. Cultivation of selected species of willows (Salix spp.) for energy purposes has rapidly increased in Sweden. About 15 000 hectares of short-rotation energy forestry have been established (Larsson, 2002). The total cost of chip production from willow biomass plantations by known technique amounts to 12-13 Euro/MWh (Melin, 2001), which figure is comparable to the price of chipped residuals from conventional forestry. Further development within the area of plant breeding and cultivation technology would most likely reduce the cost of biofuels from Salix plantations. Fertiliser costs are of great importance and corresponds to 15-20 % of the total chip production cost (Rosenqvist, 1997). As a complement or alternative to manufactured fertilisers recycling of waste products rich in nutrients, e.g. municipal or industrial wastewater, sewage sludge, leachate from sanitary landfills, and ashes from various combustion processes, have been discussed (Hasselgren, 1992; Aronsson and Perttu, 1994). In Sweden there are a few full-scale facilities where willow plantations are irrigated with pre-treated wastewater (Carlander et al., 2002). The first one began operation in 1997. The project background has bearing on both energy and agricultural policies within the EU member states. The stated need for replacement of fossil fuels by sustainable energy sources is well documented and bio-fuels take a central position in this respect. The development of alternative crops in European agriculture is being promoted partly due to a general over-production of cereals. Energy crops are on the agenda. A third aspect is that wastewater treatment in soil-plant systems combined with reuse of wastewater resources (water, organic material and nutrients) for crop production has potentials for saving finite resources in terms of manufactured fertilisers and less use of chemicals and energy compared with traditional wastewater treatment. 1.2 ObjectivesThe project group consisted of scientists and professionals with backgrounds in wastewater and/or willow applications that saw synergetic benefits in coupling together the two disciplines. The basic expected benefits were:
The aims of the 4-year project, based primarily on small-scale field experiments in four countries, were evaluations of:
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