Municipal wastewater application to Short Rotation Coppice of willows – Treatment efficiency and clone response in Estonian case study

• We evaluated the purification efficiency of willow vegetation filter.
• We describe the response of eight willow clones to pre-treated wastewater irrigation.
• The soil-plant system reduced the concentration of total N and P efficiently.
• Wastewater irrigation increased the average shoot number, plant DW and wood yield.
• Higher planting density and denser irrigation network did not increase the wood yield.

Wastewater application to Short Rotation Coppice (SRC) of fast growing trees (willows and poplars) is an attractive alternative to conventional forestry and water purification systems to meet the environmental and renewable energy goals set in European Union (EU). To evaluate the purification efficiency of willow vegetation filter and to describe the response of different commercial willow clones to pre-treated wastewater application, the study was carried out in experimental SRC during last three years of the first five-year-long harvest cycle. Water samples collected from the lysimeters after percolating through the vegetation filter soil (at 40 cm) indicated substantial reduction of pollutants (58% and 70% for total N and P, respectively). The purification efficiency of studied vegetation filter was sufficient to avoid groundwater pollution and meet the legislative limits of Estonian environmental legislation. Wastewater application did not reduce the average plant survival but increased the average number of shoots, plant dry weight (DW) and wood yield per area significantly. The comparison of average shoot DW of clones indicated quite stable growth pattern of different genotypes irrespective of various growing conditions. The average number of shoots of clones 83, 90, 97, Gudrun and Tora was smaller, the shoots were heavier and clones produced more wood than the rest of the studied varieties. Higher planting density or denser irrigation pipe network did not result in substantially higher wood yield. We conclude that willow SRC functioned efficiently as vegetation filter and when combined with growing more productive clones the economically viable yields should be realistic.