Pål BÖRJESSON a and Göran BERNDES b
a Environmental and Energy Systems Studies, Dept. of Technology and Society. Lund Universtiy, Gerdagatan 13, SE-223 62 Lund, Sweden. Fax: +46-46-2228644, E-mail: Pal.Borjesson@miljo.lth.se
b Dept. of Physical Resource Theory, Chalmers Univ. of Technology/Göteborg Univ., Sweden *Corresponding author
This paper presents one concept of multifunctional bioenergy systems –the use of willow vegetation filters for the treatment of nutrient rich municipal wastewater and drainage water in Sweden. The concept is evaluated considering cost and efficiency in water treatment compared to alternative options. The concept is also evaluated from a farmer and energy sector point of view, where biomass supply costs and land use efficiency are in focus.
Wastewater irrigation will reduce willow cultivation costs in two ways, by increased biomass yields and by eliminating, or reducing, commercial fertilizers requirements. Compared to conventional cultivation methods, the land-use efficiency increases 30-100% when nutrient rich wastewater is used for irrigation.
The wastewater treatment efficiency (nitrogen and phosphorus removal) of willow vegetation filters is high. For municipal wastewater, the estimated treatment cost is lower than for conventional treatment in sewage treatment plants. In the case of drainage water treatment, the cost is in most cases estimated to be higher than for the alternative treatment option using restored wetlands. The reductions in willow cultivation cost are in most regions smaller than the cost difference between restored wetlands and willow vegetation filters.
When the value of water treatment is included, willow vegetation filters for municipal wastewater treatment have negative production costs. When willow vegetation filters are used for treatment of polluted drainage water, the biomass production cost will be reduced or increased depending on system design and geographic location in Sweden. Willow plantations that are established as buffer strips along open streams can provide biomass at approximately half the cost when the economic value of N retention is included.