I had the pleasure of reviewing some classical data on composting biosolids from the LA County Sanitation District in a report published in 1982. The report is titled, Windrow and Static Pile Composting of Municipal Sewage Sludges by M.D Iacoboni, J.R. Livingston, T.J. LeBrun. 1982.
The report outlines how biosolids were effectively composted in aerated windrows 4-5 ft high and 14 ft wide starting in the mid 1970s in Los Angeles County. The optimal moisture content was 50%; at higher moisture contents, odor emissions increased and temperatures required for potential pathogen kill were not consistently met. Although they measured zero oxygen in the windrows shortly after turning, they determined the importance of the natural convection (chimney effect) of the windrow by covering the windrow with plastic and observing a rapid decrease in temperature until the cover was removed again.
The report concluded that turning windrows three times per week was optimal to increase porosity in the windrow to maintain aerobic conditions, promote sludge drying by exposure to air and sun, and ensure that all of the biosolids was consistently subject to temperatures required for pathogen kill.
The authors observed that odor emission from biosolids composting decreased logarithmically during the ten days of composting (from 14,000 odor units per cubic meter to less than 3,000 odor units per cubic meter). They observed that although turning events released significant odor, net odor emission during turning was about 15% of the overall odor emission from a turned windrow facility.
They also measured temperature and potential pathogen kill on aerated windrows (negative aeration) up to 24 ft wide and 10 ft high, and observed that the temperatures increased faster and were higher than temperatures measured in turned windrows that were not aerated. Two significant observations were that potentially pathogenic microbes were not consistently killed and potential pathogen regrowth occurred more readily. Although odor emission was reduced with larger aerated windrows, significant odor was emitted from the aeration blower (negative aeration).
The authors concluded that forced aerated composting of biosolids was not as effective as turned windrow composting, as measurd by pathogen reduction and total solids reduciton. It would be interesting to take this research further using larger windrows up to 12 ft high and 30 ft wide using the larger turners that are available. The most significant benefit of the larger windrows is the more efficient use of real estate.
With larger windrows, we can expect that the natural ventilation or “chimney effect” will not work as well as with smaller windrows. We can also expect that larger windrows will require lower moisture contents to maintain the same porosity as smaller windrows. We can also expect that the potential odor emission during turning may be higher because we can expect more anaerobic pockets in the center of the windrow.
We are now gaining experience with larger aerated and turned windrows. It appears that we are able to achieve the benefits of turned windrows that include the following:
- Consistent potential pathogen kill
- Completely homogenous end product
- Elimination of preferential air pathways that develop in static aerated windrows or piles
We also gain the benefits of forced aeration which include the following:
- More rapid temperature increases and higher temperatures
- Faster drying of the composting material.
We will likely see an increased use of turned and aerated windrow composting as it just makes sense to speed up the composting process, reduce real estate required for composting, reduce odor emissions and require less capital cost than some aerated composting technologies.