Use of municipal wastewater and composted wastewater sludge in willow short rotation coppice in Estonia

The establishment of short crop rotation (SRC) plantations of willows was initiated to provide raw materials for the paper and pulp industries. Such plantations are now widely used in Europe for commercial, environmental and scientific purposes. The first experimental willow SRCs were established in Estonia in 1993. The Swedish planting material consisted of clones from two willow species-Salix viminalis and S. dasyclados. During the next years most clones were found to be suitable for Estonian weather conditions and showed remarkably wood biomass yields the best growth yield reached 15 tonnes of dry mass per hectare. However, high wood yields were achieved only due to additional fertilisation. The use of mineral fertilisers increases the SRC management cost approximately by a third. Therefore the application of residues like wastewater and associated sludge is reasonable as these promote the wood biomass production in a similar way to mineral fertilisers. The study reported herein shows that due to high evapotranspiration rate the volume of wastewater that can be utilised in willow SRC without any groundwater pollution is very high. In local climate 170 mm of wastewater (average concentration of 16.5 mg N and 2.7 mg P per litre) irrigation during the vegetation period caused no pollution of groundwater in the area. The application of sludge that contained up to 300 kg of N and 220 kg of P per ha caused no groundwater pollution either. Both applications increased the wood biomass yield in the case of the most productive test areas up to two- or three-fold.