Heavy metals in source-separated compost and digestates

Area of Science:

• Environmental Science
• Soil Science
• Agronomy

Background:

• Compost and digestate production from organic residues is common in Europe.
• These organic products can potentially introduce pollutants, specifically heavy metals, into agricultural soils.

Purpose of the Study:

• To analyze heavy metal concentrations (Cd, Co, Cr, Cu, Ni, Pb, Zn) in Swiss composts and digestates.
• To investigate factors influencing heavy metal levels, such as treatment process, input material characteristics, and degradation degree.
• To assess the contribution of composts and digestates to overall heavy metal loads in agricultural soils.

Main Methods:

• Analysis of heavy metals in composts (n=81), solid digestates (n=20), and liquid digestates (n=5) from Switzerland.
• Statistical analysis to explore relationships between heavy metal content and influencing factors.
• Mass balance calculations to compare heavy metal loads from compost/digestate application with other sources.

Main Results:

• Compost heavy metal levels were below 60% of legal thresholds.
• Solid digestates showed lower Cd, Co, Pb, and Zn but similar Cr, Cu, and Ni compared to composts.
• Liquid digestates had approximately double the heavy metal concentrations of composts for most elements.
• No clear relationships were found between analyzed factors and heavy metal concentrations.
• Composts and digestates contribute 2-22% of total annual heavy metal loads to Swiss agricultural soils.

Conclusions:

• Compost and digestate application generally results in lower heavy metal loads than manure, mineral fertilizers, or aerial deposition at a regional scale.
• However, on individual fields, composts and digestates can lead to higher heavy metal loads than equivalent nutrient inputs from manure or mineral fertilizers.
• Factors beyond those studied likely influence heavy metal contamination in these organic products.