Abstract
Despite the main part of the sewage sludge (biosolids) production in Sweden meets thehigh quality requirements set up for recycling in agriculture, amounts of sludge spread totraditional crops are decreasing. This is mainly due to the negative attitude within thefood industry to sludge use. However, sludge managers gradually develop other routes ofsludge disposal.For instance, sludge fertilisation of willow (Salix spp.) plantations for biofuel productionhas increased in recent years. The increase can be described by the fact that the willowfarmers measure positive effects after sludge application and/or because willow isexcluded from the food industry business.The primary project objective is to clarify the practical extent of uptake of heavy metalsin Salix wood through direct measurements of commercial willow plantations fertilisedwith biosolids. Some 20 plantations in SkAne and Orebro counties (south Sweden) wereincluded in the investigation.In general, uptake rates of metals in the Salix crop were high compared to traditionalagricultural crops. Uptake of Cd in Salix stems was greatly exceeding (ca 10 times) theapplication of Cd with sludge (general sludge type and application rates), also includingatmospheric deposition of Cd. This means that a willow crop fertilised with sludge couldwork as a biological filter for remediation of Cd contaminated land. Uptake of Zn in Salixwas fairly well balanced with the Zn application with sludge, whereas for other metalsuptake in Salix wood was lower than the amount added with biosolids.Sludge from the Swedish wastewater treatment plants can potentially supply the futurewillow-to-energy plantations (I 00 000's of hectares) in the country with full amounts ofphosphorus and with parts of the other nutrient requirements. Further, application ofbiosolids increases the content of organic matter in the soil.
Highlights
The Swedish quality requirements for sewage sludge recycling in agriculture are among the hardest in the world
One way to show the potential of Salix for phytoremediation can be to present the amount of sludge which is needed to balance the uptake of each metal in willow stems (Table 6)
Based on the studies of metal uptake in commercial short rotation energy forests fertilized with sewage sludge the following conclusions could be drawn
Summary
The Swedish quality requirements for sewage sludge recycling in agriculture are among the hardest in the world. Today there are about 15 000 ha of willow plantations in Sweden, out of which the majority are fertilised with sewage sludge. Almost IO % of the biosolids from the Swedish sewage treatment plants, corresponding to I 00 000 tons of dewatered biosolids with ca 20 % dry solid content, is utilised annually in commercial willow biomass plantations (Alven et al, 2003). Different studies have shown that the willow crop is effective concerning accwnulation of metals in the stem wood (e.g. Greger and Landberg, 1999; Bertholdsson, 200 I; Hassel gren, 2003). This ability has been used at several places for remediation of polluted water and land. KALMAR ECO-TECH'03 Biorernediation and Leachate Treatment KALMAR, SWEDEN, November 25-27, 2003
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