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PERSPECTIVE

CSU Autumn, 2005, Vol. 13, No. 4, p. 231

Solid Waste Composting Trends - And Research Needs

MUNICIPAL solid waste composting has taken many twists and turns in the past two decades, as early projects utilized preprocessing lines with screens, ferrous removal, pickers and shredders. Most received mixed municipal solid waste, with project proponents advocating composting as a viable

alternative to landfill disposal. Along the way, a handful of projects started that were based on residential separation of the compostable stream from recyclables and trash - a model that today is quite widespread in Europe and increasingly in Canada.
The challenge of producing quality compost from mixed waste was blamed in part on shredding in the preprocessing phase. Therefore, the next generation of facilities began installing rotary drums ahead of composting. The tumbling action of the drum - versus mechanical shredding - was perceived as adequate to open bags of garbage, mix wet and dry feedstocks, and initiate the composting process. Composting facilities in the U.S. processing a mixed waste stream - e.g., Rapid City, South Dakota; Pinetop-Lakeside, Arizona; and Marlborough, Massachusetts - selected rotary drums on the front end.
Today, 20 years into this generation of MSW composting - some trends have become apparent. One is that mixed waste composting seems to have a specific niche - servicing rural areas and/or tourist destinations where existing landfills have limited capacity. Recent examples in the United States are West Yellowstone, Montana and Mariposa County, California where nearby national parks result in seasonal peaks in waste flows. At the gateway to Yosemite National Park, Mariposa County is preparing to launch its 50 tons/day mixed waste site. The Yellowstone facility is designed to also process 50 tons/day of mixed MSW along with several other organic streams.
Still another trend is the improved quality of compost from mixed waste composting facilities. This result can be attributed to a better understanding of how to manage the composting process to optimize contaminant separation along with different configurations of screens, destoners, air separators, et al. to maximize removal of inerts.
Another evolving trend - usually in cities and suburban communities - is to add source separated residential organics to existing yard trimmings collection programs. This makes it feasible for municipalities to “upgrade” their existing organics collection programs. As we enter the third decade of MSW composting, facilities permitted to receive food residuals are making it suitable to process the residential organics stream.
This issue of Compost Science & Utilization includes several research studies on how different approaches are improving results with MSW compost applications. A two-year study in the south of Italy showed how MSW compost can be combined with mineral N fertilizer and deeper soil tillage to increase yields of tomato plants. “The results indicated a positive effect of MSW application on organic carbon content in the soil and did not show any significant increase of heavy metals,” conclude researchers at the Istituto Spermimentale Agronomico in Bari, Italy.
In their work (p. 288) at the Ohio Agricultural Research and Development Center, Keener, Ekinci and Michel analyzed how consistent performance of composting systems required that blended products be correctly proportioned to achieve good chemical, physical and biological properties. In addition, process conditions (temperature, moisture, oxygen, mixing schedule ... ) must also be controlled to maintain these high rates. These are some areas that require continued careful research to fulfill the goals of both practitioners and scientists in the composting field. - N.G., J.G.