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Spotsylvania County's Expanded Composting Facility Applies Aerated Static Pile Technology Advances

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Spotsylvania County embarked up 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 12,600 tons per year in 2009. Even with this rapid increase in quantities, all regulatory process criteria have been met and offsite odor impacts have been non-existent. Howeverer, residuals cake continued to be landfilled from a second WWTP, the FMC plant, in the amount of 5,000 – 6,000 tons per year. The County embarked upon a compost facility expansion program in 2006 with three main goals.

To manage the ever increasing quantities of residuals cake generated from both County WWTP's over the next 20 years.

To enhance and automate the compost process performance.

To accomplish this expansion with no offsite odor impacts.

Through growth projection analysis, it was determined that the expanded composting facility should be initially sized for 80 tons per day of dewatered solids on a 7 day basis, roughly three times the size of the original facility, with the capability to double in capacity to 160 tons per day in the future. 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. 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 below ground aeration piping and aeration risers with grates to deliver aeration to the compost piles. The facilities selected have a roof cover instead of totally enclosed building, and continuous negative aeration is used to capture and treat compost process air using 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 of the 18 fans provides aeration to one compost pile containing approximately 112.5 wet tons of biosolids. Three wireless temperature probes located in each pile continuously monitor pile temperatures to meet regulatory requirements for meeting process to further reduce pathogens (PFRP) and vector attraction reduction (VAR). The compost 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 throughout the 21-day composting process.

In 2006, odor sampling was performed on the original 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 original operation. Modeling was also performed for the ultimate build-out concept of 160 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 approximately 3 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 was completed in March, 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.

Keywords: Sludge; composting; composting odor control; odor dispersion modeling; residuals

Document Type: Research Article


Publication date: 2010-01-01

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