Grantee Research Project Results
1997 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, P. Y.
Current 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, 1996 through October 30, 1997
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 overall objective of this project is to develop operating strategies for packed bed biological air treatment systems (biofilters and biotrickling filters) subjected to transient VOC loadings. Because total air flow 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 types of transient can result in breakthrough and emission of VOCs.
Transient loadings occur during start-up and changes in operation throughout an operating shift of manufacturing and commercial operations such as paint spraying, baking, coating, and chemical manufacture. 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, and intrinsic biodegradation rates, and amount of active biomass. In some cases significantly different microbial populations develop, as is the case for aromatics and dichloromethane.
Both intensity transients and compositional transients have been studied by Deshusses and his colleagues in laboratory studies. However, the experimental conditions used were limited and neither mechanisms of competition nor control strategies were addressed. 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 will are addressed in this study; (1) management of diurnal transients, (2) management of periodic transients, and(3) management of compositional transients. The experimental systems used will be 1 m deep, 0.15 m I.D. columns similar to those used in previous studies at UC Davis. Air flux will be maintained at 1m3/m2*min and VOC concentrations and mixtures will be varied with the type of experiment.
Diurnal transient studies will be conducted to study performance of biofilters subjected to regular variation in VOC concentrations such as might result from commercial operation of one 8 hour shift per day or variations in loading during an operating shift. Our hypothesis is that the microbial population will adjust to diurnal variations within a limited range of inlet concentration values and times between transients, and that within this range outlet concentration range will not be significantly impacted. Some experimental support for this hypothesis exists Currently available models cannot be used to predict transient response because adequate characterization of the microbial population is impossible.
Periodic spike studies will be conducted to study the management of large increases in the VOC flux relative to the nominal steady state flux. Large increases will be defined as a ratio of transient to average VOC flux of 8 or greater. Breakthrough is hypothesized to result from decreasing biological activity along the length of the bed in systems subjected to nearly constant loadings. Under constant loadings the microbial population increases near the inlet until the mass transfer rate limits growth. The result is a roughly first order decrease in microbial population with distance from the inlet. When a high concentration transient occurs, the mass transfer rate increases but the available reaction capacity may become limiting due to lack of microorganisms. The proposed management strategy is to reverse direction of flow in the column at regular intervals and thus maintain an active microbial culture through the length of the bed.
Compositional transients will be studied using two types of VOC mixture; (1) contaminants degraded by separate microbial groups and (2) contaminants degraded by the same microbial groups. The target compounds proposed as contaminants degraded by separate microbial groups are toluene and methylene chloride and the compounds proposed as contaminants degraded by the same microbial groups are toluene and hexane
In the case of compounds degraded by the same groups of organisms, relative availability of the VOCs and/or microbial preference of one compound over another based on molecular structure are hypothesized to be the principal determinants in biodegradation. For example, both hexane and toluene are easily degraded in compost biofilters at empty bed contact times of less than 1 minute and mass loading rates greater than 500 g/m3*d when fed as pure substrates. However, hexane (and other light aliphatics) is not always removed, even at empty bed contact times of two minutes, when included in mixtures such as gasoline vapor. Toluene is easily removed from mixtures containing aliphatic, alicyclic and aromatic compounds in the C5 to C19 range and straight-chain aliphatics are more easily degraded than branched-chain aliphatics. Both toluene and hexane are metabolized by a large number of the microbial species present in compost, including a significant number that degrade both aromatics and aliphatics. While hypotheses explaining preferential degradation of particular compounds in a mixture can be developed (and are relatively common in the biological wastewater treatment literature), experience in wastewater treatment has been that cultures evolve that remove a wide array of easily degraded materials. Henry's law coefficient values, solubility, and structure are probably key factors but the difference between the performance of vapor and liquid phase treatment is not understood. Thus the experience with vapor phase VOC degradation is surprising and determination of the limiting rate mechanisms is very desirable because biofiltration of gas mixtures such as gasoline is economically attractive.
Specific Project Objectives
1. Determine the relationship between transient loading peak and breakthrough in a system subjected to "regular" transients.
2. Determine optimal directional switching strategy for management of periodic "spike" transients.
3. Determine the controlling mechanisms in the competitive replacement of dichloromethane degraders by toluene degraders in a dichloromethane/toluene treatment system.
4. Determine the requirements for simultaneous removal of a complete spectrum of compounds in gasoline.
The experimental program schedule given in the proposal is below. At present we are about one month behind schedule. However, we now are developing considerable data from a parallel project operating in parallel using methyl tert-butyl ether, a common gasoline oxygenate which is somewhat recalcitrant, and several easily degradable compounds (benzene, toluene, isopentane). Availability of this data will allow meeting the project objectives for compositional transients studies.
Summary of Results to Date
A laboratory-scale study has been conducted to characterize and optimize the response of a directionally-switching rigid mineral medium (Celite R-635) biofilter subject to regular diurnal and periodic transients. Off-period duration is expected to be more influential on toluene breakthrough than will be peak-to-mean loading ratios (case a) or toluene concentration (case b). It is known from other research that breakthrough is positively correlated with contaminant loading. The hypothesis stated above may not apply if the system is loaded to the point where mass transfer limitations and media clogging become significant.
The biofilter was fed toluene and operated on a 12-hour switching interval with an empty-bed contact time of one-minute. The relationship between transient loading and system breakthrough was investigated by conducting feed-on/off experiments with variable peak-to-mean loading ratios (PTMLR) of 2, 4, and 8 and variable feed-off periods of 2, 4, 8, 16, and 32-hours using two approaches: a) constant feed concentration, decreasing average loading (by increasing off-time) and b) increasing feed concentration, constant average loading (by decreasing off-time). Air and toluene vapor were passed through the biofilter during the "feed-on" periods while only air was passed through the biofilter during "feed-off" periods. Breakthrough was measured at several points along the length of the bed after system response to continuous and transient loading approached steady-state values.
Results indicate that active cell mass (as measured by cell activity) is distributed symmetrically across the biofilter with most of the activity concentrated near the ends of the column as a result of direction switching. In contrast, past experience has shown that biofilters fed unidirectionally concentrate cell mass (and activity) at the inlet end of the biofilter and therefore are subject to a greater degree of mass transfer limitation and clogging.
Relative to continuous feeding performance, loss in performance (increase in breakthrough) resulting from discontinuous feeding was largest near the biofilter inlet and decreased along the length of the bed to the point where loss in performance at the outlet was too small to quantify. For a given PTMLR breakthrough increased with cycle length (and therefore the off-period) illustrating the relationship between loss in performance and off-period.
The first publication from this work will be presented at the June, 1998, annual meeting of the Air and Waste Management Association. We expect to have at least two more publications completed by June, 1998.
Journal Articles:
No journal articles submitted with this report: View all 2 publications for this projectSupplemental Keywords:
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.