Grantee Research Project Results
Final Report: Innovative Biomass Control Technology for Biotrickling Filters for Waste Air Treatment
EPA Grant Number: R825392Title: Innovative Biomass Control Technology for Biotrickling Filters for Waste Air Treatment
Investigators: Deshusses, Marc , Wistrom, Anders O.
Institution: University of California - Riverside
EPA Project Officer: Hahn, Intaek
Project Period: November 1, 1996 through October 31, 1998
Project Amount: $186,270
RFA: Exploratory Research - Air Engineering (1996) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Air , Safer Chemicals
Objective:
Interest in biological waste air treatment techniques is rapidly increasing in the United States, particularly with the implementation of the Clean Air Acts Amendments of 1990. The most promising bioreactor for air pollution control is the so-called biotrickling filter. Process air is contacted (co- or counter-currently) with a scrubbing solution containing essential nutrients) in a packed reactor. Absorbed pollutants are biodegraded by mixed cultures naturally immobilized on the packing. Waste air treatment in biotrickling filters is very competitive in terms of cost, with alternative treatment methods currently in use, such as catalytic oxidation and sorption onto granular activated carbon. However, process stability and maintenance of optimal operating conditions of biotrickling filters are difficult under field conditions; in particular, clogging of the bed by overgrowing biomass is a major problem. Research to improve long-term stability of biotrickling filters for waste air treatment is needed. This is required for biological waste air treatment to emerge as a reliable, cost-effective, and environmental friendly technology.
In this context, the main objective of this proposal was to develop a general strategy to control biomass growth in biotrickling filters for waste air treatment by engineering predation of the bacterial process culture by protozoa. The main advantage associated with this technique is that long-term stability would be obtained without reducing reactor performance. This is not the case for other techniques that currently are in their early phases of investigation. The supporting objectives for reaching this goal were: (1) quantification of predator-prey relationships between protozoa and pollutant degrading bacteria, (2) development of strategies to enhance and control grazing of biofilms in biotrickling filters, and (3) demonstration and optimization of the developed strategies in bench-scale biotrickling filters. After the first year of research, in light of the results obtained, the objectives were broadened to include a more general understanding of the factors that influence biomass growth and pollutant elimination in biotrickling filters, so that better biomass control strategies could be developed. Throughout the research, toluene served as a model volatile organic compound (VOC).
Summary/Accomplishments (Outputs/Outcomes):
The results of the study showed that protozoa played a very important role in mineralizing bacteria in biotrickling filters and that the frequency of clogging could, in most cases, be decreased by about 20 percent in the presence of protozoa. The costs associated with this reduction of clogging rate were modeled and found to be significant because biomass control costs were found to be about 30 percent of the total treatment costs. In our experiments, the lower rate of biomass accumulation in the presence of protozoa correlated with an increase of carbon mineralization indicating that protozoa indeed grazed on the primary pollutant degraders and had a beneficial effect on the overall carbon balance. Other results showed that protozoa helped in detaching biofilm from the substratum. Overall, the results showed that protozoa could help in controlling biomass in biotrickling filters for air pollution control. Depending on the pollutant elimination capacity of the biotrickling filter, a zero balance of net biomass growth could not be obtained. It suggested that for high-performance systems, further stimulation of predation of the biomass would be required. The means and techniques for protozoa stimulation still require further research. The use of higher organisms such as larvae or snails for biomass control could provide a means for higher rate of biomass control. Specific findings are reported below:
- While direct counting of bacteria and protozoa using light microscopy turned out to be the most simple technique, the Coulter Counter has been applied successfully to accurately quantify bacteria and protozoa in various types of samples. Sequential counting of bacteria and protozoa was necessary due to their differences in size, but the technique has proved to be robust and reliable. Mild sonication and the addition of a surfactant, Tween 80, to the culture samples worked reasonably well to prevent large aggregates of biomass to clog the aperture tube during counting. In many instances, direct counting was preferred, because it allowed simultaneous counting and basic identification of protozoa in complex mixed culture samples.
- Shake flask experiments were conducted with the protozoa Tetrahymena pyriformis fed on a bacterial culture growing on toluene vapors. The influence of various parameters such as pH, sodium chloride, and toluene concentration on the protozoan cultures was quantified first. Next, experiments were performed with flasks continuously aerated with air contaminated with toluene. Toluene gas phase concentrations commonly encountered in industrial waste gases did not significantly affect the predation rate of toluene degrading bacteria by T. pyriformis. Reducing the nitrate content of the liquid media had a strong influence on the metabolism of the cells and the predator-prey relationships. In all instances, exponentially growing cultures of bacteria (specific growth rate of 0.2-0.3 per hour) outgrew the protozoa. However, after the stationary phase was reached, bacteria concentration was reduced by growing protozoa by 98 to 99.9 percent compared to protozoa-free controls.
- Experiments also were performed with small scale (16 cm diameter) rotating disk contactors (RDCs) where the sole carbon source was toluene provided via the gas phase. The liquid phase was inoculated with various sources of protozoa: cow manure, pond sediment, aqueous phase or biofilm from laboratory biotrickling filters, activated sludge, a blank not inoculated, and a control containing cycloheximide, a common protozoa inhibitor. The reactors were operated for more than 5 weeks and selected parameters pertaining to the liquid phase and to the biofilm were monitored. In summary, these experiments showed that protozoa had a significant effect on the structure of the biofilm. The cycloheximide control compartment had a very firm, high carbon content biofilm, while those biofilms attached on disks inoculated with protozoa were much less dense and had a lower carbon content. Further, it was observed that biofilms had a marked tendency to detach by large patches in compartments where protozoa were present. Hence, it was hypothesized that one important mechanism was biomass removal triggered by detachment.
- A very efficient bacterial phage (T7) was tested for its ability to lyse toluene mixed bacterial culture from the biotrickling filter in shake flasks. This would have offered a drastic treatment option, to biologically remove biomass. The results were negative, the T7 phage did not lyse the process culture. The enrichment of potential phages for biomass control was not pursued.
- A rapid protocol to test the usefulness of various chemicals to remove biomass in the event of a clogging was developed. Seventeen different chemical treatments were evaluated for their ability to remove excess biomass and for their effect on the pollutant degrading activity of the process culture. One disclosure of invention has been filed, but the University of California decided not to seek patents. The best chemical for removing excess biomass was sodium hypochlorite (see Water Research, 1999).
- It was fortuitously found that one biotrickling filter reactor had achieved constant weight and that another had actually lost biomass with no detrimental effect on the pollutant elimination. Fly larvae were noticed in the biofilm while flies colonized the headspace of these reactors. These were identified as belonging to the species Colbodia fuscipes and seemed to be the origin of the observed phenomena. A number of experiments were conducted to quantify the larvae in the reactors and possibly link the presence of the larvae to the low biomass yield. The difficulty was that larvae go through a cycle (fly eggs -> larvae -> pupae -> flies) and that it was unknown what factors affected the cycle and what form of the insect could have an effect on biofilms. In one experiment, larvae were taken out of the reactor and were fed biofilms, but results were not conclusive. Other experiments tried to eradicate the larvae and the fly using a potent organophosphate insecticide from one bioreactor while monitoring the reactor weight, but the results were inconclusive. Unfortunately, the complexity of the systems prevented us from firmly demonstrating that a cause and effect relationship existed between the presence of the fly larvae and the low biomass yield.
- We have looked further into the question of how much of the pollutant was actually degraded in the biofilm and how much was degraded in the recycle liquid (suspended organisms), because this might affect the choice of a biomass control strategy. Using two different methods, we demonstrated that up to 25 percent of the pollutant degraded in biotrickling filter could be degraded in the recycle liquid, and we showed that the average specific activity of the suspended cells was up to 20-30 times higher than the specific activity of the immobilized cells. This may have important implication for future reactor design.
- Biotrickling filtration experiments with various mineral nutrient concentrations (nitrate, ammonia, urea, and other mineral nutrient limitation) were performed and the influence on biomass growth and on the efficiency of pollutant elimination was determined. It was found that biomass yield and pollutant elimination do not vary linearly from one another. This has profound implications as far as the selection of a method for the control of biomass is concerned. A mathematical model was developed, which considered nutrient supply as a key parameter limiting pollutant removal rate and biomass accumulation. Integration of the experimental results in a general treatment cost model showed that there was an optimum nutrient loading that minimized the overall treatment costs. The optimum was a relatively nitrogen limited case. Interestingly, the major part of the treatment costs were capital cost and the cost for controlling biomass. This confirmed that the problem of biomass control is a very important problem, and that cost-effective solutions are needed to reduce treatment costs.
Journal Articles on this Report : 8 Displayed | Download in RIS Format
Other project views: | All 20 publications | 8 publications in selected types | All 8 journal articles |
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Cox HHJ, Deshusses MA. Biological waste air treatment in biotrickling filters. Current Opinion in Biotechnology 1998;9(3):256-262. |
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Cox HHJ, Nguyen TT, Deshusses MA. Predation of bacteria by the protozoa Tetrahymena pyriformis in toluene-degrading cultures. Biotechnology Letters 1999;21(3):235-239. |
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Cox HHJ, Deshusses MA. Innovative experimental setup for the parallel operation of multiple bench scale biotrickling filters for waste air treatment. Environmental Technology 2000;21(4):427-435. |
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Cox HHJ, Nguyen TT, Deshusses MA. Toluene degradation in the recycle liquid of biotrickling filters for air pollution control. Applied Microbiology and Biotechnology 2000;54(1):133-137. |
R825392 (Final) |
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Cox HHJ, Deshusses MA. Effect of starvation on the performance and re-acclimation of biotrickling filters for air pollution control. Environmental Science & Technology 2002;36(14):3069-3073. |
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Cox HHJ, Deshusses MA. Biomass control in waste air biotrickling filters by protozoan predation. Biotechnology and Bioengineering 1999;62(2):216-224. |
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Cox HHJ, Deshusses MA. Chemical removal of biomass from waste air biotrickling filters: screening chemicals of potential interest. Water Research 1999;33(10):2383-2391. |
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Deshusses MA, Cox HHJ. A cost benefit approach to reactor sizing and nutrient supply for biotrickling filters for air pollution control. Environmental Progress 1999;18(3):188-196. |
R825392 (Final) |
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Supplemental Keywords:
biomass, air, treatment, bacteria, waste, engineering, waste air treatment., Scientific Discipline, Air, Environmental Chemistry, Engineering, Chemistry, & Physics, bacteria filtration, protozoa, biomass, biofilms, waste air treatment, pollutant degrading bacteria, mathematical formulationsRelevant Websites:
http://engr.ucr.edu/~mdeshuss Exit
Progress 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.