Performance of A Large-Scale Biotrickling Filter and Activated Carbon Odor Scrubber System At the Joint Water Pollution Control Plant

In the spring of 2005 the Los Angeles County Sanitation Districts (Districts) began operation of a large-scale odor control system at its 320 million gallon per day Joint Water Pollution Control Plant (JWPCP) in Carson, CA. This odor control system, also known as the Central Odor Control Station (COCS), replaced nine chemical and activated carbon scrubbers that were used to treat odorous air emissions from headworks, aerated channels and grit chambers. The project was designed to allow the consolidation of odor treatment into one facility to help minimize labor and materials cost and to increase the amount of odorous air that could be treated. The COCS was designed to treat up to 60,000 cubic feet per minute of odorous air. The COCS consists of two stages. The first stage system contains three biotrickling filters and was designed principally to remove hydrogen sulfide (H2S). The second stage system includes three activated carbon units and is run so that two are always in operation with the third providing a backup. The activated carbon units were installed to remove the remaining odorous compounds and volatile organic compounds (VOC).Sampling has been conducted over the last two years to evaluate the performance of the COCS with respect to H2S, odors and VOC removals. With the start-up of the COCS the Districts established a comprehensive monitoring program that includes daily H2S measurements and weekly analysis of VOCs and volatile sulfur compounds of the air entering and leaving the COCS. In addition to compound analysis an onsite odor panel was used to determine odor concentrations. The sampling was conducted concurrently for VOCs, volatile sulfur compounds and odor analyses. The sampling has assisted in determining the efficiency of the COCS in removing the compounds of concern and in correlating specific compounds in the air stream to odors.The monitoring results indicate that the biotrickling filters achieved an average H2S removal efficiency >99% over an approximately two year monitoring period (March 2005 – May 2007). Both stages of the COCS completely removed H2S from inlet air. During the monitoring period the average inlet H2S concentration has been below 10 ppmv/v. The low inlet concentration is below the projected average inlet concentration of approximately 50 ppmv/v. The system was designed to handle an average H2S loading of over 50 g m−3h−1. The lower inlet H2S concentrations are due the high ventilation rates used to maintain the contained primary treatment areas under vacuum. Analysis of the containment systems indicated that the negative pressures being used draw in a significant amount of ambient air through the seams and joints in the covers and diluted the odorous air with clean ambient air. So far the highest H2S elimination capacity achieved by the biotrickling filter system is 7.0 g m−3h−1. This is a relatively low H2S loading rate for biotrickling filter systems. The low sulfur-loading rate indicates that the COCS has ample H2S treatment capacity left. Many biotrickling filter systems have reported exceeding H2S elimination capacities of 100 g m−3h−1.To determine the ability of COCS to remove organics over 25 VOCs are typically analyzed in the inlet air stream using GC/MS analysis techniques. Only ten compounds have been detected above the JWPCP laboratories GC/MS detection limits, which vary from 2.1 to 4.2 ppbv/v. The concentrations of the detected VOCs entering the COCS were all well below 1 ppmv/v. Some of the detected VOCs, such as toluene, could contribute to offsite odors. The biotrickling filters were able to remove approximately 20% of aromatic VOCs from the inlet air and both stages of the COCS removed approximately 90%. In addition to the VOCs, the COCS was also able to removed odorous organic sulfur compounds such as methyl mercaptan. The first stage biotrickling filters removed over 40% of the inlet methyl mercaptan, which had inlet concentrations averaging 500 ppbv/v, and the second stage activated carbon scrubbers eliminated the rest.By removing more than 99% of the H2S, the biotrickling filters were able to reduce the odor concentrations by an average of 60%. Overall, both stages of the COCS removed an average of 91% of inlet odors. However, these results also indicated that at times the overall removal efficiency could drop to approximately 60%. These low removal efficiencies occurred when the inlet H2S concentrations and inlet odor concentrations were low (inlet H2S less than 1 ppmv/v and inlet odor concentration less than 1,000 detection threshold). Through an intensive investigation, it was revealed that even though the COCS was able to completely remove H2S, there were still a small amount of other odorous compounds that escaped treatment. A thorough analysis of compounds entering and leaving the COCS determined that the compounds contributing to odor being discharged from the COCS were dimethyl sulfide (DMS), dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS). These compounds were typically present in concentrations below 100 pbbv/v. It should be noted that any odors getting through the COCS are considered minor and periodic odor analyses near the JWPCP facility boundaries have indicated that there is no detectable odors from the COCS.Based on the monitoring results of the past 26 months, the Districts concluded that the performance of the COCS has exceeded its original design goals. The system has effectively treated H2S emissions from grit chambers, aerated channels and headworks and has significantly reduced odor concentrations (>90%). With the success of the COCS, the Distrcits are currently constructing new odor control systems using the same two-stage design of biotrickling filters and activated carbon to treat 180,000 cfm of air from the headspaces of covered primary sedimentation tanks at the JWPCP.