Grantee Research Project Results
1998 Progress Report: Optimization of Transient Response of Vapor Phase Biofiltration Systems
EPA Grant Number: R825390Title: Optimization of Transient Response of Vapor Phase Biofiltration Systems
Investigators: Schroeder, Edward D. , Chang, Daniel Y.
Institution: University of California - Davis
EPA Project Officer: Hahn, Intaek
Project Period: November 1, 1996 through October 30, 1998
Project Period Covered by this Report: November 1, 1997 through October 30, 1998
Project Amount: $168,012
RFA: Exploratory Research - Air Engineering (1996) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Air , Safer Chemicals
Objective:
The objective of this project is to develop operating strategies for packed bed biological air treatment systems (biofilters and biotrickling filters) subjected to transient volatile organic contaminant (VOC) loadings. Because total airflow is normally constant in air treatment systems, transient loadings result from changes in VOC concentration (intensity transients) and from changes in the VOC constituent mixture (compositional transients). Both transient types can result in breakthrough and emission of VOCs.
Transient loadings occur during startup and during changes in operation throughout an operating shift of manufacturing and commercial operations such as paint spraying, baking, coating, and chemical manufacturing. A second type of intensity transient has been observed in emissions from wastewater treatment plants, where relatively small diurnal transients were overshadowed by occasional large spikes at intervals as long as several months. Compositional transients occur when VOC mixtures are being treated and the mixture composition changes. Components of VOC mixtures are removed at rates dependent on individual gas-liquid transfer rates, vapor pressure, solubility, intrinsic biodegradation rates, and amount of active biomass. In some cases, significantly different microbial populations develop that are associated with the components, as is the case for aromatics and dichloromethane. We have found that the gasoline oxygenate methyl tert-butyl ether (MTBE) is also a compound with an apparently restricted degradative population. Developing operating strategies to minimize breakthrough will allow more extensive application of biological gas treatment technology. Moreover, information developed in this project will provide a more complete basis for establishing monitoring regulations for biological gas treatment systems.
Progress Summary:
Three problems are addressed in this study: (1) management of regular transients, (2) management of periodic transients, and (3) management of compositional transients. The experimental systems used are 1-m deep, 0.15-m I.D. columns similar to those used in previous studies at the University of California at Davis. Air flux has been maintained at 1m3/m2 min, and VOC concentrations and mixtures are varied temporally with the type of experiment. The direction of airflow is switched periodically to provide a more uniform distribution of microorganisms and to maintain a high level of microbial activity throughout the bed1.
The work accomplished to date has addressed the first and third objectives (regular and compositional transients). Toluene has been the principal VOC used, and inlet toluene concentrations have ranged from 107 to 428 ppmv in the single component studies. In the multicomponent experiments, toluene has been varied from 8 to 55 ppmv, and MTBE has been varied from 35 to 70 ppmv. Regular transients were simulated by considering selected feed on/off periods that gave peak-to-mean ratio loading ratios of 2 and 8 for cycle lengths of 4, 8, 16, and 32 hours. Contaminant concentrations were measured at the inlet, the 250 mm points along the bed, and at the bed outlet.
Results from the regular transient studies are: (1) switching times of 12 hours or less have little impact on performance relative to continuous, monodirectional flow processes. Longer periods between flow direction switching result in loss of activity at the downstream end of the column and susceptibility to breakthrough under transient loads; (2) breakthrough did not occur in any of the experiments, that is, outlet toluene concentrations have not exceeded the quantification limit of 0.2 ppmv for all loading conditions; and (3) for a given peak-to-mean loading ration value, toluene removal in the inlet region decreased (and toluene penetration into the packed bed increased) with off-period length.
The experiments with toluene and MTBE have been extremely interesting. The compound was initially reported to be non-biodegradable. The first report in the refereed literature of microbial biodegradation was in 1994. Since that time, a number of research groups have developed established cultures and two incidences of microbial cultures developing serendipitously in compost biofilters have been reported. Work on MTBE degradation in our group at Davis has evolved from one of these serendipitous cultures, and we are involved with the monitoring of the second culture. Development of wild MTBE degrading cultures requires extremely long periods?often up to 1 year. Laboratory mixed cultures derived from the wild cultures respond much more quickly, and process start-up times are 2 to 3 weeks. The response is similar, but more extreme, than our experience with dichloromethane. A pure culture capable of mineralizing MTBE has been isolated from the mixed culture. Identification of the pure culture using 16S rRNA has not been possible, although our colleague, Professor K.M. Scow, made Sphingomonas a tentative allocation to the genus. The mixed culture was grown in a biofilter for 9 months at a selected range of inlet concentrations between 1 and 35 ppmv. The empty bed contact time was 1 minute, as in the toluene studies. Toluene was then introduced in a series of step functions: 0 to 8 ppmv, 8 to 23 ppmv, 23 to 55 ppmv. The first transient was the most interesting because the culture had not received any organic material other than MTBE for 9 months. Response to toluene addition was immediate. Removal of MTBE decreased from 100 percent to approximately 70 percent in the first hour, and toluene removal was at approximately 60 percent after 1 hour. Removals of both compounds exceeded 97 percent within 12 hours. Response to the later increases in inlet toluene concentration was less dramatic and the recovery times were shorter. At 55 ppmv toluene nitrogen became limiting, and a decrease in removal of both compounds was observed. Increasing nitrogen in the feed stream eliminated the limitations. Spatial variation in MTBE removal occurred in response to toluene addition, as occurred in the case of dichloromethane. When MTBE was the sole carbon source, approximately 90 percent of the removal occurred in the first 25 cm from the inlet. Following toluene addition, MTBE penetration into the column increased and the second 25 cm of the column removed a greater fraction of the compound. The rapid response of the culture to the introduction of toluene was surprising and suggests that a sizable fraction of the consortium, maintained when MTBE was the sole carbon source, was capable of metabolizing toluene. Experiments with the pure culture of MTBE metabolizing bacteria have not been run at this time, so we do not know if substrate competition was a factor in the response. However, the deeper penetration of MTBE into the column remained constant over a period of weeks.
References:
1Kinney KA, Wright WF, Chang DPY, Schroeder ED. Biodegradation of vapor phase contaminants. In: Sikdar SK, Irvine RL, eds. Bioremediation: Principles and Practice, Technomics Press, 1997;I:601-632.
Future Activities:
The principal remaining work to be done on this project is to investigate the response to periodic transients. This work is being conducted at the present time and will be completed by May 1999. The multiple substrate studies are continuing but much of the work has been completed. We expect that the project will be completed in June 1999.Journal Articles:
No journal articles submitted with this report: View all 2 publications for this projectSupplemental Keywords:
VOC's, MTBE, biofilters, biotrickling filters, degradation, Scientific Discipline, Air, Toxics, POLLUTANTS/TOXICS, Ecology, air toxics, Environmental Chemistry, Chemicals, HAPS, VOCs, Engineering, 33/50, Engineering, Chemistry, & Physics, transient response, gasoline, vapor extraction, Toluene, ceramic packing, diurnal loading, biomass, vapor phase, soil, dichloromethane, biofiltration systems, exhaustProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.