Compost age and sample storage effects on maturity indicators of biosolids compost

Compost product safety and quality assurance are required to meet the needs of the horticultural, agricultural, and silvicultural user markets. At present, there exist no industry-wide sampling and testing protocols for compost products, thus limiting the production sector. The objective of this research was to test three methods for determining compost maturity. The study followed the composting process of a locally successful commercial composting operation that had been producing lime-stabilized biosolids compost in the Washington, DC metro region for 12 yr. Change over time in the dependent variables--Dewar flask self-heating capacity, oxygen uptake rate, and cation exchange capacity (CEC)-during a 57-d composting of lime-stabilized biosolids was studied. Because cold storage at 4 degrees C is recommended when compost samples cannot be tested for maturity immediately, cold storage of up to 11 wk was included as a variable. Mathematical models were developed that predict change in the Dewar flask self-heating capacity, oxygen uptake rate, and CEC with composting time and storage at 4 degrees C. The Dewar flask self-heating test was the most useful indicator of compost maturity. This test showed change throughout the 57-d biosolids composting period while oxygen respirometry did not change after 29 d. The CEC was found to increase with age and storage. Storage effects varied for the different tests. Except for Days 1 and 57, composts continued to stabilize during storage. Testing stored composts may produce erroneous results that suggest the compost is mature.