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The Hyperion Treatment Plant began full-scale pilot testing of thermophilic digestion in June 2000 in preparation for a new land application ordinance requiring Class A biosolids. Because of the “reputation” that thermophilic sludge is significantly more odorous than conventional mesophilic digestion process based upon literature searches and the operational experience of other agencies, the City of Los Angeles conducted an odor study for the treatment processes from digestion through land application. City of Los Angeles Engineering and Operations Staff coordinated odor testing on the solids unit processes and digester gas, and also implemented odor mitigation methods for the solids unit processes to enhance odor capture and treatment.

The City of Los Angeles contracted with air sampling and analysis companies to conduct odor sampling with odor panel analysis, Tentatively Identified Compounds (TIC, i.e. organics) with GC/MS analysis and Total Reduced Sulfur (TRS) compounds with SCAQMD Method 307–91 analysis. Samples were taken of digester gas from a thermophilic and mesophilic digester, and of foul air from the screening facility, the dewatering centrifuges (wetcake and centrate emissions), the storage silos and during land application at the City-owned farm in Kern County, California. The foul air samples were drawn from identical process trains for both mesophilic and thermophilic sludge at concurrent times. The results for the odor analysis indicate the thermophilic sludge odor is orders-of-magnitude higher than the mesophilic sludge during active processing in the plant (i.e. screening, dewatering and associated centrate). However, the results surprisingly indicated that the thermophilic sludge is orders-of-magnitude less odorous in storage and during land application than the mesophilic sludge. This is completely contrary to current industry thought, and a definite benefit for land application concerns regarding odor and air emissions. The results from the TIC and TRS analysis of the foul air samples and the digester gas samples indicate validation of the odor trends. The speciated, odorous sulfur compounds and the speciated organic compounds generally indicate a higher concentration in the thermophilic-digested sludge during processing in the plant, but the mesophilic biosolids concentrations are orders-of –magnitude higher than the thermophilic biosolids when stored and/or land applied. This data will be essential in the design of odor control facilities and assessment of the digestion process at thermophilic temperatures.

Hyperion operations was also concerned with upgrading or enhancing the existing odor control capture and treatment in the screening facility, the dewatering facility, and the storage facility, even prior to having the odor panel results. The plant engineering and maintenance groups designed and installed odor mitigation equipment or optimized existing odor control systems for better capture of foul air specifically for the short-term thermophilic dewatering testing. For the full-scale thermophilic digestion conversion that will take place over the next year and a half, the City will also design odor control upgrades to the screening facility, the wetcake storage facility and the truck loading facility. Future plans will include upgrades to the digester gas desulfurization facility to handle possible increases in hydrogen sulfide in the digester gas, and a centralized solids odor control facility that will treat foul air from the screening facility, dewatering facility, storage facility and truck loading facility.
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Document Type: Research Article

Publication date: 2002-01-01

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