Challenge Technology

Stability measurement of Biosolids Compost by Aerobic Respirometry

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Oxygen uptake of biosolids compost was measured during both laboratory and full-scale studies. Aerobic respirometry of solid samples of compost provided a precise measure of microbial activity. There was a noticeable decreasing trend in oxygen consumption over 25 days of composting, thereby indicating increasing stability. Moisture content also was found to affect the compost stability. During 48-hour respirometer tests, the compost sample did not dry to the point where respiration was inhibited. Measurement of volatile solids reduction alone during biosolids composting with large quantities of sawdust revealed little about stability.

Over the past 40 years, researchers have investigated many physical, chemical and biological parameters to monitor composting process performance and final end-product stability. Early on, experimental studies were conducted with municipal solid waste (MSW) to observe the effects of moisture content, pH and oxygen concentration on composting temperature and the quality (including stability) of the end-product. Tests on the composted materials were initially performed to evaluate trends or patterns and not to define actual characteristics of the end-product. Thus, parameters that showed increasing or decreasing patterns over specific composting periods (i.e., 30 to 120 days) were considered good stability measures.

Early measurements of compost respiration were published in the 1950s by Wiley (1955,1956,1957) who proposed using carbon dioxide production as a test parameter for high-rate composting. Wiley showed some correlation (e.g., r = 0.791-0.817) between carbon dioxide production and compost temperature. McCauley and Shell (1956) used oxygen uptake as an index for microbial activity and compared these findings to maximum oxygen diffusion rates. Those early studies used respiration rate as an operational parameter, but not as a stability index. That is, the test was not designed to provide a measure of the putrescibility, or biodegradability, of the residual material. One of the first individuals to correlate respiration rate with compost stability was Schulze (1961) who showed that finished MSW compost having different moisture levels had large differences in oxygen uptake. Schulze reported average oxygen uptake rates of 0.015,0.385 and 0.756 mg 02/g VS/hr at 19.6,51.3 and 60.4 percent moisture, respectively. Recently, improvements in equipment and understanding of fundamentals have led several investigators to use different approaches for respiratory measures and procedures (Pressel and Bidlingmaier, 1981; Usui et al, 1983; Willson and Dalmat, 1986; Haug and Ellsworth, 1991; Frost et al, 1992) and tests have been conducted on solid samples and slurries. The Composting Council of Alexandria, Virginia, a lead group in composting advocacy, is currently preparing guidance documents for measuring physical, chemical and biological parameters of compost products and oxygen consumption in a solid matrix form recently has received considerable attention.

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