Comparison of Methods for Evaluating Stability and Maturity of Biosolids Compost

TL;DR: The authors compared several methods of evaluating the stability/maturity of biosolids compost: chemical properties, microbial respiration activity (CO₂ evolution), seed germination tests, and dissolved organic carbon (DOC) as determined by its concentration and mass-specific absorbence (MSA) at 420 nm.

Abstract: Compost stability/maturity has become a critical issue for land application of compost because immature compost can be detrimental to plant growth and the soil environment. We compared several methods of evaluating the stability/maturity of biosolids compost: chemical properties, microbial respiration activity (CO₂ evolution), seed germination tests, and dissolved organic carbon (DOC) as determined by its concentration and mass-specific absorbence (MSA) at 420 nm. Two to five biosolids compost samples were collected during compost curing stages from three full-scale composting facilities in Florida. The compost samples from two facilities were in the stabilization/maturation process. With curing time, electrical conductivity (EC), DOC concentration, and CO₂ evolution rate decreased, whereas pH, seed germination rate, and the MSA of DOC increased in these samples. Compost samples from the third facility exhibited distinctly different behavior. Relatively high seed phytotoxicity, with no consistent trends of other parameters, suggested that these samples may have not been in a real curing process, despite the generally low CO₂ evolution rate and DOC content. Based on these results, we concluded that pH, EC, CO₂ evolution rate, seed germination rate, and DOC (concentration and MSA) could be used to monitor stabilization and maturation processes. Compost stability and maturity are different characteristics and both are needed for compost quality control. Dissolved organic carbon analysis is promising as a simple but comprehensive index for compost stability/maturity. Approved for publication as Florida Agricultural Experiment Station Journal Series No. R-07121.