Can Composting BMPS Reduce Air Emissions?

Imagine a scenario where all yard trimmings collected for recycling have to be received, processed and composted under the cover of a building. It would be tough for most yard trimmings composting facilities to survive the expense. Yet this situation could have occurred in Southern California. At one point, the South Coast Air Quality Management District (SCAQMD), which regulates air emissions in the vicinity of Los Angeles, Orange, San Bernardino, and Riverside counties, proposed that all composting facilities should be enclosed to reduce emissions of ammonia and volatile organic compounds (VOC). After gathering responses from the composting industry, local governments and the California Integrated Waste Management Board (CIWMB), the SCAQMD reconsidered its proposal to require enclosed facilities for composting of yard trimmings, or “green waste” as it called in California. However, the need to improve air quality in the South Coast District still remains, and the SCAQMD continues to seek reductions from green waste composting facilities.

The CIWMB has suggested that air emissions can be improved, without severely impacting a facility’s costs, simply by changing some operational practices – by adopting best management practices (BMPs). However, regulatory agencies, like SCAQMD, need to see the data. Thus, the CIWMB implemented a project to investigate the effect of management practices on air emissions from green waste composting. Tests were conducted at a composting facility to measure the ammonia and VOC emissions for different feedstock mixes and aeration strategies. The results add much needed insight into the relationship between operational factors and specific air emissions and pose important consideration of alternative regulatory strategies.

Testing BMPs



The objectives of the CIWMB tests were to determine whether and how management practices impact air emissions. The tests were not intended to be a rigorous experiment – time and funding were both limited. Instead, CIWMB sought a measured and realistic assessment of the potential for reducing air emissions by adjusting management practices.



A large number of factors can potentially affect air emissions, including feedstock composition, temperature, pH, moisture, aeration and pile geometry. Because all of these variables could not be investigated by this project, investigators chose to focus on two factors – feedstock composition and aeration -while keeping the other variables relatively constant. Two green waste feedstock blends were tested, one with a high percentage of grass clippings (curbside green waste) and the second with a high percentage of woody materials (mostly leaves, brush and wood with some grass clippings). These feedstock blends were characterized by their carbon to nitrogen (C:N) ratios. Similarly two aeration strategies were investigated – static windrows and turned windrows.



To test the influence of these factors, full-scale experimental windrows were established at the Tierra Verde Industries green waste composting facility in Irvine, California. The windrows were constructed in late October 2002 and composting continued into February 2003. Four full-scale test windrows were established to evaluate the four variables (Figure 1). Two windrows were constructed with the high C:N ratio feedstock blend (woody) and two contained the low C:N blend (grass-heavy). As measured during the first week, the C:N ratios of the grass-prominent blends ranged from 16 to 28 and averaged 18.5 and 26 for the static and turned windrows, respectively. In comparison, the C:N ratio of the woody blends ranged between 51 and 100, with averages of 54 and 74 for the static and turned windrows. Two of the windrows were static while two were turned regularly. The static windrows were not turned at all throughout the 102-day composting cycle. The turned windrows were turned with a Scarab machine when windrow temperatures were high enough to m