Graduate Thesis Or Dissertation
 

Maturity and stability evaluation of composted yard debris

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  • Compost maturity is an important determinant of end use for composted municipal yard debris, and generally refers to the effect the compost has on plants. The rate of microbial respiration is an indicator of compost stability. The objectives of this research were to: i) determine whether continuous aeration resulted in more rapid maturity of composted yard debris than windrow turning; ii) determine which maturity indicators distinguish between mature and immature compost; iii) measure rates of CO₂ evolution during active composting and curing; iv) adapt the CO₂ detection tube technique for compost; and v) evaluate rapid compost stability tests (Solvita test, self-heating test and CO₂ detection tubes) for potential use by commercial composters. Land Recovery, Inc. of Puyallup, WA, composted yard debris under careful process control. Two compost piles were studied for 113 d; one was subjected to continuous forced aeration and periodic turning. The other was managed as a turned windrow. We found that forced aeration resulted in mature compost about 20 days before windrowing. Compost pH, C content and respiration rate were all useful indicators of compost maturity. Carbon fell from 400 k kg⁻¹ to 250 g kg⁻¹ and pH rose from 5 to 7. The CO₂ evolution rate fell from 16 to 2 mg CO₂-C g C⁻¹ d⁻¹. Maturity correlated somewhat with compost odor. Neither percent germination nor an odor/color scale were reliable indicators of maturity for theses composts. All the rapid tests were correlated with alkaline trapping of microbially respired CO₂. The Solvita test took 4 h to administer; values (1 to 8 Solvita scale) were 2 to 4 during active composting, 3 to 6 during early curing and 6 to 7 during late curing. Self-heating test values decreased from 20°C above ambient at the start of composting to ambient (no heat production) at the end of composting. This test took two to six days to administer. We developed a method to measure CO₂ evolution rates of samples in sealed containers using CO₂ detection tubes. This method took 4 h to administer, correlated well with alkaline trapping, was quantitative and had excellent sensitivity at extreme compost maturity. The methods shows promise as a raid test for compost respiration and should be evaluated on a wider range of composts.
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