The physical, chemical and microbial characteristics of biodegradable municipal waste derived composts

A comparative study of the physical, chemical and microbial properties of 12 composts sourced from United Kingdom commercial composting plants was carried out. The aim was to ascertain whether these composts could be used as growing media or for application to agricultural land. The composts were produced from either source segregated biodegradable municipal waste (BMW) or mixed municipal solid waste (MSW). Ten composts contained 100% source segregated BMW including green, fruit, vegetable, meat and kitchen waste, paper and cardboard. One compost was 72% mixed MSW plus 18% source segregated BMW, and one compost was 100% mixed MSW. Composts were analyzed for pH, electrical conductivity, carbohydrates, nutrients and contaminants. The composts sourced from mixed MSW contained higher levels of physical contaminants (glass, plastic and metal) than the source segregated BMW composts. Nitrogen concentrations (as % dry weight) were 1.7-2.2% where kitchen or meat waste was included and 1.0-1.6% otherwise. Phosphorus concentration ranged from 23 to 247 mg kg-1, and K from 1851 to 6615 mg kg-1. Total salts were higher in mixed waste composts (15-23 g kg-1), predominantly due to high concentrations of K, Ca, S and Na. Electrical conductivity varied from 670 µS cm-1 to 3320 µS cm-1.

The levels of the heavy metals examined in the 10 source segregated BMW composts were much lower than the limits for composts in the United Kingdom (PAS 100). However, the 100% mixed MSW compost exceeded the PAS 100 levels in four of the seven heavy metals tested (Cu, Ni, Pb and Zn) which may have implications for repeated applications to agriculture. Salmonella was absent from all twelve composts, however, five composts contained levels of E. coli which exceeded the UK PAS 100 limits. Five composts produced from source segregated BMW could be applied to agricultural land, however, high levels of E. coli would exclude the other five. Moreover, high levels of heavy metals and physical contaminants would render the two mixed MSW composts unsuitable.

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