Uptake of metals in willow biomass plantations fertilised with sewage sludge

Abstract

Despite the main part of the sewage sludge (biosolids) production in Sweden meets the
high quality requirements set up for recycling in agriculture, amounts of sludge spread to
traditional crops are decreasing. This is mainly due to the negative attitude within the
food industry to sludge use. However, sludge managers gradually develop other routes of
sludge disposal.
For instance, sludge fertilisation of willow (Salix spp.) plantations for biofuel production
has increased in recent years. The increase can be described by the fact that the willow
farmers measure positive effects after sludge application and/or because willow is
excluded from the food industry business.
The primary project objective is to clarify the practical extent of uptake of heavy metals
in Salix wood through direct measurements of commercial willow plantations fertilised
with biosolids. Some 20 plantations in SkAne and Orebro counties (south Sweden) were
included in the investigation.
In general, uptake rates of metals in the Salix crop were high compared to traditional
agricultural crops. Uptake of Cd in Salix stems was greatly exceeding (ca 10 times) the
application of Cd with sludge (general sludge type and application rates), also including
atmospheric deposition of Cd. This means that a willow crop fertilised with sludge could
work as a biological filter for remediation of Cd contaminated land. Uptake of Zn in Salix
was fairly well balanced with the Zn application with sludge, whereas for other metals
uptake in Salix wood was lower than the amount added with biosolids.
Sludge from the Swedish wastewater treatment plants can potentially supply the future
willow-to-energy plantations (I 00 000's of hectares) in the country with full amounts of
phosphorus and with parts of the other nutrient requirements. Further, application of
biosolids increases the content of organic matter in the soil.