by: Brian M. Bell, P.E.
Envirocon has a long history of designing and implementing a variety of odor, dust, and emission control systems on contaminated sites across the U.S. Such systems are critical to ensure environmental compliance and safety for our workers, clients, and the residents of the surrounding communities we serve. At Envirocon, we rely on lessons learned from past project experiences, innovative technology, process improvement, and assessment of means and methods to provide our clients safe, reliable project execution. Our project team recently used a unique odor/emission control system at a former Manufactured Gas Plant (MGP) site on Lake Superior.
Data Evaluation and Management for an Odor / Emissions Control System
Envirocon recently performed mechanical dredging of impacted sediments on Lake Superior. Field crews offloaded and processed dredged materials within a 376-foot by 150-foot Sediment Processing Tent (SPT). After the screening of large debris, quick lime was used to stabilize sediments so that they were stackable within the SPT and could pass a paint filter test, allowing for transport and disposal to the landfill. The SPT was ventilated to produce approximately four air changes per hour. The ventilated air stream contained lime dust, volatile organic compounds (VOCs), and potentially odiferous compounds, and was treated before being released into the atmosphere. The main components of treatment included particulate filtration followed by granular activated carbon (GAC) media adsorption of VOCs. The vapor phase GAC is 4×8 mesh which is ideal for vapor phase applications (mass transfer), whereas liquid phase GAC is typically smaller (e.g., 8×30 mesh).
In conjunction with our joint venture partner, Foth Infrastructure and Environment (Foth), Envirocon developed a means to evaluate and manage data using an emissions monitoring program. The management effort assesses GAC media performance as an aid to determining when GAC breakthrough is occurring so that we may remove and replace it in a timely manner, thus ensuring all performance requirements are continually met. This type of data assessment and management may be used as a tool for future use on a variety of liquid phase and vapor phase treatment systems that employ GAC media to track GAC media performance.
The Odor / Emissions Control System Units
The total air flow rate ventilated and treated was 155,000 actual cubic feet per minute (ACFM), which included five modular Air Filter Units (AFUs) each at 32,000 ACFM capacity. Each unit measured 8-feet wide by 10-feet tall and 17.5-feet long with a 42-inch spiral duct stack that was 20-feet tall with a rain cap. A stack was mounted on top of each modular unit, and guy wires were attached to each unit to steady the stack. The stacks’ exhaust was 29.5 feet above the ground surface.
The GAC beds were contained by 304 SS perforated sheets 48-inches by 104-inches by 6-inches deep or about 17.3 cubic feet (cf) of carbon per bed and six beds per unit for a total volume of 103.8 cf. At an average weight of 32.5 lbs per cubic foot, the total carbon weight per unit minimum was 3,400 lbs. The unit was constructed of 14-gauge G-90 galvanized metal on a six channel epoxy-coated base. GAC removal and fill was from the top through two flip up panels. The particulate pre-filters each had a magnehelic gauge to measure the pressure loss across the filters. The pre-filters were changed based on particulate loading. Daily static pressure readings were taken and used to determine daily average air flow rates per unit, which in turn were used to calculate VOC mass flow rates, including those captured on GAC media. Controls for the 40 horsepower (HP) motor were mounted in a 3R control box for a VFD and start-stop station.
Each of the air filters’ module blowers required a 40 HP motor. This relatively low energy motor was sufficient since the GAC bed depth was low, but deep enough to meet a required mass transfer bed depth of at least 4 inches. The amount of GAC required for the project was estimated at 79,200 lbs, which was a total of four GAC changes per unit for the anticipated project duration (initial change and three subsequent change-outs). The particulate pre-filters were assessed and changed-out as required, based on the lime particulate build-up on the pre-filters.
Monitoring Program and Data Assessment
A detailed air monitoring program was required for pre-treatment and post-treatment air stream characterization of each AFU. The air monitoring locations included both of the air sampling ports: one on the intake ductwork to each AFU representing the influent, and the other on the treated effluent, located after the GAC panels. The air monitoring program consisted of the following measurement, monitoring, and data interpretation steps:
- 1. Photoionization detector (PID) operational measurements were conducted for pre- and post- particulate and vapor phase GAC treatment for total volatile organic compounds (TVOC). The monitoring was conducted every operational day at each of the five AFUs. TVOC operational measurements were recorded from the influent and effluent of each AFU three to four times (e.g., approximately every 3 to 4 hours) over a 12-hour operational day, with results averaged.
- 2. The influent to and effluent from the five AFUs were sampled twice per week using SUMMA® canisters over a 24-hour period. Samples were analyzed for total VOCs (TVOC).
- 3. PID operational measurements (TVOC influent and effluent) and SUMMA® TO-15 PF analysis (TVOC influent and effluent) were evaluated to determine GAC removal efficiencies and tracked together with effluent concentration trends. The data was used to determine when the GAC was exhausted, requiring removal and replacement. Assessment of TVOC concentrations between PID operational measurements and SUMMA® TO-15 PF results for the odor/emission control system unit effluent emissions did not allow us to make an accurate correlation.
- 4. Total GAC removal efficiencies for TVOC by PID measurement and SUMMA® TO-15 PF analysis (influent versus effluent) was used to determine total volatile organic mass loading on GAC (mass of volatile organics captured per mass of GAC).
- 5. GAC breakthrough is defined when GAC adsorption capacity for organics of concern is exhausted (i.e., the GAC is saturated with organic loading at a state of equilibrium). Correlations of PID operational measurements were assessed versus SUMMA® TO-15 PF results for TVOC to help evaluate breakthrough, or imminent breakthrough.
- 6. The key monitoring information needed to determine the air action level potentially indicating the need for GAC change-out for any AFU included: 1) removal efficiency % of TVOC and per each VOC; and 2) total VOC mass rate loading on GAC for TVOC and for each VOC.
- 7. Data was assessed after each monitoring event with both PID and SUMMA® TO-15 PF analysis.
- 8. Appropriate actions regarding GAC removal and replacement were taken based mainly on tracking the removal efficiencies of TVOC and some key indicator VOCs, including BTEX and naphthalene.
The hand-held PID measurements and time-weighted composite SUMMA® sampling results were used to assist in evaluating removal efficiencies and effluent concentration trending. This program may be utilized in the future for assessing the GAC media performance in a wide variety of liquid and vapor phase based treatment systems.