Session- Yard Trimmings Composting
Effects of Turning Frequency and Mixture Composition on Process Conditions and Odor Concentrations During Grass Composting
Stuart C BUCKNER (1)
(1) Town of Islip, Development of Environmental Control, USA
Odor concentrations were quantified and compared among treatments in a turned windrow yard waste composting system. Experiments focused on identification of windrow treatments and process conditions that were associated with low odor levels during grass composting. Treatments consisted of variations in both turning frequency and feedstock mixture ratios of grass with either leaves or woodchips. The composting process was evaluated primarily by monitoring temperature, oxygen concentration, and decomposition rate. In addition, changes in moisture content, pH, C:N ratio, and bulk weight were followed over time. Rapid dissipation of initially high odor levels and maintenance of low odor concentrations were observed for a wide variety of yard waste mixtures and turning frequencies. High rate decomposition and adequate conditions of oxygenation were primary factors associated with these observations. Although oxygen concentration exhibited spatial and temporal variations, low odor levels were generally maintained only in windrows with mean oxygen concentrations of approximately 10% or more. Turning frequency significantly affected the rate of decomposition. Without periodic agitation, the rate of decomposition slowed dramatically. Turning frequencies consistent with low odor levels varied with the feedstock mixture and bulking ratio. The initial feedstock mixture strongly influenced the windrow's ability to sustain oxygenated conditions. At equivalent turning frequencies, mixtures of partially decomposed leaves and grass ranging from one to three parts leaves to one part grass (v:v) were less consistent in maintaining low odor concentrations than woodchip/grass mixtures ranging from one to two parts woodchips to one part grass. Although the study focused on process management for odor control during grass composting, it is noted that additional factors need to be considered to minimize the potential for odor emissions off-site. Practical aspects of windrow management are discussed briefly.
Sequential Composting to Determine Compost Maturity Levels
KIMBERLY K. MOORE(1) and George E. Fitzpatrick (1)
University of Florida, Fort Lauderdale Research and Education Center, 3205 College Ave Fort Lauderdale FL 33314, USA
Analytical determination and confirmation of minimum compost processing times and curing times can aid commercial growers in selecting compost materials that should give them reliable and consistent results in their operations. Five cubic yards of Ficus spp. yard trimmings were assembled into three 1.25 cubic yard compost piles at 60 day intervals. The piles were sampled in June 2000 and December 2000. At the first sampling there were three piles of each compost of the following ages: 14, 13, 9, 5, 2 months old. At the second sampling there were three piles of each compost of the following ages: 20, 18, 15, 11, 8 and 1 month old. Samples of the reference compost from each pile were screened to pass through a 0.75 inch screen. Total porosity, water-holding capacity, air-filled porosity, C:N, C:P, and organic matter content were measured on the samples. The samples also were used to perform a bioassay using radish seeds. In March 2000, samples from the piles were blended with perlite, vermiculite, and Sphagnum peat to create substrates that contained 30, 60, or 100% reference compost, by volume. Impatiens and petunia plugs were grown in these substrates. Results suggest that a minimum composting time of 11 to 12 months is required but greater plant growth was obtained from reference compost piles that were 18 to 23 months old. This study also suggests that blending younger reference compost with other media components can amend some of the negative properties of immature compost.
Maturity and stability evaluation of composted yard trimmings
Linda J. Brewer and Dan M. Sullivan
Department of Crop and Soil Science, Oregon State University, USA
Reliable methods of compost quality assessment are needed for the development and expansion of high-value markets for compost. The objective of this research was to evaluate a variety of stability and maturity indices for yard trimmings compost produced in the Puget Sound region of western Washington State. We collected compost samples periodically during a 133-d composting cycle at a commercial composting facility. We found that indices of compost respiration rate were sensitive indicators of compost quality. All respiration rate indices identified a period of high respiration rates during active composting (first 27 d), and a period of relatively stable respiration rates during the latter part of curing (70 to 133 d). Chemical tests of compost solids showed less promise as maturity indicators, but provided valuable information on final compost quality. Mature yard trimmings compost had a C:N of 12, an NH4-N to NO3-N ratio of less than 4, a cation exchange capacity (CEC) of 400 cmol per kg of compost-C, and a pH between 6.5 and 7.0. Seed germination tests and sensory tests (color and odor) were of limited value in assessing compost maturity. Fully-cured compost produced with forced aeration had a Solvita CO2 test value of 6 to 7 and a respiration rate via the alkaline trap method of 2 mg CO2-C g compost-C d-1. It reheated less than 2 oC in an insulated Dewar flask in a 7 d incubation. We recommend further evaluation and calibration of respiration test protocols for compost quality assurance testing programs.