Vienna's biowaste compost--quality development and effects of input materials

Extensive chemical analyses were conducted during the last decade to assess the heavy metal content of the finished compost as well as of biowaste and other input materials. Twenty six percent of the compost lots were class I according to ONORM S2200, 70% class II and 4% did not reach class II. If the compost lots were classified according to the new Austrian compost ordinance, 22% would conform to class A+ and 78% to class A. These data are put into relation with heavy metal contents of soil, of yard wastes and of biowastes without contaminants. In the soil samples particularly nickel and lead exceeded the limits for class A+ (compost ordinance). In the yard wastes, more than 90% of the samples met the limits for all heavy metals. Biowaste without contaminants conformed to class A+ on average. In biowaste without contaminants no influence of the housing structure on the heavy metal content was observed. The compost produced only from biowaste from areas with high building density, after undergoing the normal process of metal removal, however, was significantly higher contaminated than that of compost of the whole city of Vienna. The impact of specific input materials, such as Christmas trees or wooden crates, was investigated by chemical analyses before and after controlled composting experiments with those materials. In both experiments the compost quality did not reach the limits for heavy metals of class I (ONORM S2200).