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Spotsylvania County's New Improved Composting Facility With State of The Art Odor Control

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Spotsylvania County embarked upon an aerated static pile composting program in 2002 to manage undigested dewatered wastewater treatment plant (WWTP) residuals cake from their Massaponax WWTP in conjunction with brush collected through a convenience center and at the Livingston Landfill. The initial compost facility included a covered aerated static pile process that provided intermittent positive aeration only. The quantity of dewatered residuals being composted has increased from approximately 8,800 tons per year in 2003 to in excess of 12,000 tons per year in 2008. Even with this rapid increase in quantities, all process criteria have been met and offsite odor impacts have been non-existent. Further, residuals cake continued to be landfilled from a second WWTP, the FMC plant, in the amount of 5,000 – 6,000 tons per year. As a result, the County embarked upon a compost facility expansion program in 2006 to manage the ever increasing quantities of residuals cake generated and to minimize the quantities of materials being landfilled.

One of the fatal flaws of composting facility expansions has been the lack of good odor control analysis and proper decision science in the scaling up process. All too often, demonstration sized facilities have worked well only to be fraught with odor problems upon expansion to larger full-scale operations that have been inadequately designed to manage the increased amount of odors produced. At the other end of the spectrum, total enclosure of compost facilities with 100% odor capture and treatment has been included in some facilities raising capital and operating costs to an extreme. In order to provide the right balance of technology for odor control and process performance while minimizing costs to the greatest extent possible, Spotsylvania County engaged CH2M HILL to assist in evaluating technical options available and to analyze the odor impact of expanding the facility.

The compost facility capacity needed to be increased from 37.5 wet tons per day (WTPD) of residuals to 112.5 WTPD on a five day per week basis, or three times the size. Information on technology options was presented, the advantages and disadvantages considered and tours of operating systems similar in design were visited to aid the County in the selection process. An aerated static pile layout was developed utilizing the technologies chosen with capability for a phased expansion in the future. The facilities are to be covered only, and negative aeration used to capture and treat compost process gas through biofiltration. The aeration system is designed to be operated continuously using variable frequency drive (VFD) motors on fans so that airflow delivered can be adjusted to meet changing demands of the process at both partial and fully loaded conditions. Each fan provides aeration to one compost pile containing approximately 112.5 wet tons of biosolids. Three temperature probes located in the pile continuously monitor pile temperatures to meet regulatory requirements for pathogen and vector attraction reduction. The pile temperatures are monitored by a central computer that adjusts the VFD speed to deliver the amount of air required to keep pile temperatures at desired levels.

Odor sampling was performed on the existing composting facility at each stage in the process and by modifying aeration regimes to mimic a full scale negative aeration process design. The resulting data was used to perform odor dispersion modeling to compare the odor impact of the planned expansion to the existing operation. Modeling was also performed for the ultimate build-out concept of 225 WTPD capacity. From the odor analysis performed, it was determined that by operating in a continuously negative aeration mode, greater than 95% of fugitive compost emissions are captured for treatment. This analysis showed that with a covered only (not fully enclosed) facility, and treatment through biofiltration, the offsite odor impact of the expanded facility would be no more than from the existing facility. In fact, designing and building a covered only facility instead of a totally enclosed facility actually reduced the projected odor impact on adjacent property owners and saved over 1.5 million in capital costs. Data regarding actual odor sampling, testing and modeling will be presented as will the trend of odor production at various stages in the composting process.

Construction of the new facilities is complete as of December 2009 with start-up of the new operation beginning in January, 2010. This paper presents data on the process flow, process controls, and the odor management system of this successfully expanded aerated static pile composting operation.
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Keywords: Sludge; composting; composting odor control; odor dispersion modeling; residuals

Document Type: Research Article

Publication date: 2010-01-01

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