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Abstract

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

This study focused on the capacity of finished compost, often used as packing material in biofiltration units, to support microbial biodegradation of trichloroethylene (TCE). Finished compost was enriched with methane or propane (10% head space) to stimulate cometabolic biodegradation of gaseous TCE. Successful hydrocarbon enrichment, as indicated by rapid depletion of hydrocarbon gas and measurable growth of hydrocarbon-utilizing micro-organisms, occurred within a week. Within batch reactor flasks, approximately 75% of head space TCE (1–40 ppmv) was rapidly sorbed onto compost material. Up to 99% of the remaining head space TCE was removed via biodegradation in compost enriched with either hydrocarbon. Hydrocarbon enrichment with methane or propane corresponded to 10-fold increases in methanotrophic or propanotrophic populations, respectively. Based on growth assessment under different nutritional regimes, there appeared to be complex metabolic interactions within the microbial community in enriched compost. Five separate bacterial cultures were derived from the hydrocarbon-enriched compost and assayed for the ability to degrade TCE.

During the past decade, composting technology has received considerable attention in two distinct areas: microbial ecology of the composting process and potential uses of the end product compost. The composting process is ideally a controlled microbial digestion utilizing indigenous bacteria, actinomycetes and fungi to decompose an organic substrate typically under aerobic conditions. During the composting process, thermophilic temperatures are achieved at various stages because metabolic heat production exceeds losses (Miller 1993). Observed patterns of microbial succession are influenced by the relative degree of decomposition of the organic matter as well as the changing temperatures within the system. Physical and biochemical characteristics of finished compost vary depending on the starting material and the specific operational parameters maintained during the composting process. However, certain generalizations can be made. Finished compost typically consists of porous organic material, with an increased density and a decreased C/N ratio, which is rich in diverse microflora and suitable for a wide variety of applications.

Finished compost has historically been utilized as organic fertilizer and to modify texture and water retention capacity of agricultural soils (Finstein & Morris 1975; Miller 1993). Compost has also been successfully used as an economically feasible substrate for mushroom cultivation and as packing material in odour-removing biofilters for treatment of wastes from food and other industries (Carlson & Leiser 1966; Bohn 1975; Leson & Winer 1991; Segall 1995). More recently, gaseous organic contaminants have also been successfully treated using compost-packed biofiltration units (Leson & Winer 1991).

The capacity of compost microflora to degrade a wide variety of organic contaminants has been clearly demonstrated (Ottengraf & Van den Oever 1983; Leson & Winer 1991; Apel et al. 1994). Recent experiments in this laboratory have demonstrated that gaseous tricholoroethylene (TCE) can be effectively removed using compost derived from a variety of sources as packing material in batch scale biofilters (Watwood & Sukesan 1995; Sukesan & Watwood 1997).

Exposure of compost to gaseous hydrocarbon has been shown to stimulate TCE degradation, presumably by enriching for specific hydrocarbon-utilizing micro-organisms and by inducing the appropriate enzyme systems in these organisms, which are known to cometabolize the contaminant. Propane and methane have each been used for this purpose and have successfully stimulated TCE degradation in soil (Phelps et al. 1990) and in compost (Watwood & Sukesan 1995; Sukesan & Watwood 1997). The ability of methane monooxygenase and propane monooxygenase, produced by methanotrophs and propanotrophs, respectively, to cometabolize TCE under appropriate conditions has been described elsewhere in detail (Wackett et al. 1989; Oldenhuis et al. 1989; Semprini et al. 1990; Alvarez-Cohen & McCarty 1991; Ensley 1991; Speital & McLay 1993; Chang & Alvarez-Cohen 1995).

Although strict methanotrophy appears to be limited to a few specific bacterial genera, there have been reports of apparent facultative methanotrophy, in which isolates can grow on methane or on other organic carbon sources, such as glucose (Zhao & Hanson 1984). Propanotrophy is considered to be more widespread in the microbial world than true methanotrophy, and many reports of facultative propanotrophy have been cited for bacterial and actinomycetes species (Perry 1980). Methane and propane consumption in mixed cultures from subsurface and other environmental samples has been reported, even in cases where pure cultures exhibiting methanotrophy or propanotrophy were not isolated (Fliermans et al. 1988; Phelps et al. 1990). As the starting material for compost typically consists of organic debris, which has often had close and prolonged contact with the soil environment, conditions supporting mixed methanotrophic or propanotrophic populations would not be unexpected in finished compost.

The current research was conducted to test the efficacy of stimulating TCE biodegradation in compost material with methane or propane enrichment, and to characterize the effects of key parameters, such as TCE concentration and hydrocarbon concentration, on the biodegradation process. Additionally, effects of hydrocarbon enrichment on the numbers and types of compost micro-organisms in compost were investigated, and bacterial cultures derived from enriched compost were assayed individually for TCE degradation capacity.

Materials and methods

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Chemicals and reagents

Propane (99% pure) was purchased from Century Tool and Manufacturing Company, Cherry Valley, IL, USA. Methane (99·9% pure) was obtained from US Welding Gas Company, Pocatello, ID, USA. Spectrophotometric grade TCE (99+% pure) was obtained from Aldrich Chemical Company, Milwaukee, WI, USA. Media ingredients were purchased from Fisher Scientific Co., and gas chromatographic supplies were obtained from Hewlett Packard and Supelco, Inc.

Compost preparation and hydrocarbon enrichment

Finished compost, derived from deciduous leaf debris, was obtained from the Schenectady County Soil and Water Conservation District in Scotia, New York. Large twigs and debris were removed, and compost (200 g) was placed in 750 ml enrichment flasks fitted with Teflon-lined butyl septa. A portion of the head space in each flask was replaced with either propane or methane gas to a final hydrocarbon concentration of 10% v/v. Head space oxygen in the flasks was replenished daily by removing septa for 1 h. The flasks were then recapped, and head space was again replaced to achieve 10% propane or methane. Enrichment flasks were maintained in the dark at 25 °C. Periodically, enriched compost was removed from these flasks and used in TCE biodegradation experiments or for bacterial isolations. This procedure was utilized because of earlier results, which showed that hydrocarbon enrichment stimulated TCE degradation and resulted in shifts in compost microbial populations (Watwood & Sukesan 1995; Sukesan & Watwood 1997).

Trichloroethylene biodegradation in enriched compost

Enriched compost (20 g; prepared as described above) was added to 120 ml batch reactor flasks. The reactors were sealed with Teflon-lined butyl septa and aluminum crimp rings. Head space TCE concentrations of 1, 10, 20 and 40 ppmv were achieved in separate flasks. Using a multifactorial experimental design, methane or propane concentrations of 0·2, 1·0 and 2·0% v/v were used for each TCE concentration; all treatment combinations were run in triplicate. At t0 and every 48 h thereafter, oxygen (10% v/v) was added to the serum vials via a gas-tight syringe. Head space samples (100 μl) were collected periodically using gas-tight syringes, and TCE and methane or propane were quantified via gas chromatography. Control flasks for each treatment contained compost which had been subjected to six successive rounds of autoclaving (121 °C; 15 psi), with complete cooling between rounds and moisture adjustment to approximately 10 centibar following autoclaving.

Bacterial enumeration and replica plating between media

Subsamples (1 g) of enriched compost were removed from enrichment vials every 48 h over a 30 day period, subjected to serial dilution in sterile saline, and spread plated in triplicate onto nutrient agar (NA), Higgins agar, and L-salts medium. The composition of Higgins agar (per litre) was: 85 g NaNO3; 17 g K2SO4; 3·7 g MgSO4.7 H2O; 0·7 g CaCl2.2 H2O; 15 g Nobel agar-agar. The composition of L-salts medium (per litre) was: 0·21 g Na2HPO4; 0·09 g NaHPO4; 1 g NaNO3; 1 g NH4Cl; 0·001 g FeSO4; 0·098 g MgSO4.7 H2O; 0·04 g KCl; 0·015 g CaCl2; 15 g Nobel agar-agar and 10 ml of a trace element solution containing (per litre) 1 mg MoO3, 0·5 mg CuSO4.5H2O; 1 mg MnSO4.H2O, 1 mg CoCl2, 70 mg ZnSO4.7H2O and 1 mg HBO3.

Higgins agar and L-salts medium agar plates were incubated in the dark in dessicators or in BBL gas-tight jars at 25 °C. Higgins agar plates were incubated in a 10% methane atmosphere, and L-salts plates were incubated in an atmosphere of 10% propane. Control plates included (a) NA plates spread with sterile saline (b) Higgins and L-salts plates spread with sterile saline and incubated in the presence of methane or propane, respectively, and (c) Higgins and L-salts plates spread with compost dilutions but incubated without methane or propane.

Colony forming units were counted after 48 h for NA plates and after 6–7 d for Higgins or L-salts plates. Colony characteristics of visually distinct colonies were noted. Thirty random colonies were picked from each NA plate and replica plated onto either Higgins agar (with 10% head space methane) or L-salts agar (with 10% head space propane) using 1×1 cm overlay grids for guidance. Each colony was also replica plated onto a fresh NA plate. Thirty random colonies were picked from the initial Higgins-methane plates and replica plated on L-salts-propane medium, NA, and back onto fresh Higgins-methane plates. Similarly, colonies grown initially on L-salts-propane medium were replica plated onto Higgins-methane medium, NA, and back onto fresh L-salts-propane medium. Replica plates were incubated 48 h for NA plates and 6–7 d for the Higgins-methane and L-salts-propane plates. Following incubation, growth of replica plated colonies on each agar type was scored positive or negative. Relative proportions of replica plating success between each media type were recorded.

Characterization of cultures from enriched compost

Five distinct colonies which exhibited growth on all three media types were maintained as cultures for further study. Over a 10 month period, these cultures were repeatedly subcultured on Higgins agar (with 10% head space methane), L-salts agar (with 10% head space propane) and Bennets agar, which contains several sources of organic carbon. The composition of Bennets agar (per litre) is as follows: 10 g glucose; 2 g pancreatic digest of casein; 1 g yeast extract; 1 g beef extract; 15 g agar-agar. Colonies grown on each agar type were replica plated onto the other media repeatedly during this period to characterize the abilities of the cultures to grow on various carbon sources. Each culture was characterized with respect to morphological, physiological and basic biochemical properties. Periodic isolation streaking, microscopic examination and Gram staining were performed to assess culture purity. Each culture, grown separately on L-salts-propane and Higgins-methane medium, was subjected to fatty acid profile analysis using the MIDI/Hewlett Packard Microbial Identification system (MIS) by Analytical Services, Inc., Essex Junction, VT, USA. This system uses high resolution pattern recognition software to compare the profile with those in an extensive database.

Trichloroethylene biodegradation capacities of cultures

The five cultures from hydrocarbon-enriched compost were grown in 120 ml flasks sealed with aluminum caps and Teflon coated septa. Each flask contained 20 ml of liquid Higgins medium or L-salts medium with 10% methane or 10% propane, respectively, in the atmosphere. After 10 d of incubation, cells were harvested by centrifugation at 30 000 g for 30 min and introduced into batch reactors prepared by solidifying 20 ml of Higgins agar or L-salts medium in sterile 120 ml vials placed on their sides. Reactors were inoculated with 0·1 ml of the centrifugation pellet, and reactor atmospheres were amended with 10% propane (L-salts medium) or 10% methane (Higgins medium) and 3 ppmv TCE. Reactors were incubated in the dark at 25 °C; hydrocarbon consumption and TCE removal were monitored for 96 h. Controls were established with identical semi-solid medium and head space TCE and hydrocarbon concentrations but without inoculum; hydrocarbon consumption and TCE removal in control reactors were subtracted from these determinations in the test reactors.

Analytical methods

Head space TCE, propane and methane were quantified via gas chromatography using a Hewlett-Packard 5890 Series II gas chromatograph (GC). Headspace samples (100 μl) were injected into the GC equipped with a Restek 30 m, 0·32 mm i.d. RTX-5 column with a 0·25 μm film thickness of crossbonded 95% dimethyl–5% diphenyl polysiloxane. Helium was the carrier gas at a flow rate of 1·1 ml min−1 and a split ratio of 1 : 14 for propane and methane analysis, and a flow rate of 88 ml min−1 and a split ratio of 1 : 44 for TCE analysis. Oven temperature was ramped during analysis from 50 °C to 135 °C over 6 min. Propane and methane were measured using a flame ionization detector (FID) at 275 °C, with an injection temperature of 225 °C and an isothermal oven temperature of 40 °C. Trichloroethylene was quantified using an electron capture detector (ECD) at 300 °C, nitrogen as the makeup gas, an injection temperature of 250 °C, and an isothermal oven temperature of 50 °C.

Results

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Trichloroethylene biodegradation in enriched compost

Trichloroethylene was rapidly adsorbed by viable and killed compost within the batch reactors. Regardless of starting concentration, approximately 75% of the added TCE (1–40 ppmv) was adsorbed within 30 min (data not shown). Data presented in Table 1 refer to biodegradation of the TCE remaining in the headspace after this initial removal due to sorption. With the exception of 1 ppmv TCE in the presence of 2·0% methane, TCE biodegradation rates were consistently over 65% and exceeded 95% in several cases. There was no statistically significant relationship between head space hydrocarbon identity or concentration and TCE removal. Head space TCE concentration did appear to influence the level of biodegradation to the extent that a starting concentration of 1 ppmv TCE corresponded with less efficient removal during 96 h. The highest overall levels of biodegradation were observed for the starting head space concentration of 40 ppmv; in this case TCE removal exceeding 99% was noted for 1% or 2% propane enrichment.

Table 1.  Biodegradation of TCE in compost enriched with 10% v/v head space methane or propane
TCE added to head space (ppmv)TCE remaining after T30 min (ppmv)Enrichment hydrocarbon
PropaneMethane
0·2%1%2%0·2%1%2%
  1. *Values reflect percentages of headspace TCE present at T30 min removed after 96 h, values are means ; n = 3. Standard errors were typically less than 10% of the means. ND, not determined.

1 0·2577·8*78·2ND76·7ND23·1
10 2·579·796·8ND66·182·397·7
20 5·080·293·196 81·397·288·1
4010·070·099·499·592·691·993·4

Bacterial enumeration

Figure 1 shows bacteria present in hydrocarbon-enriched compost over an 18 d period. Prior to enrichment (at t0) cfu on NA approached 107 g−1 compost; after 6 d of enrichment with either propane or methane, cfu numbered approximately 1–2×108 g−1 compost. These counts remained fairly constant throughout the compost incubation. No cfu appeared on Higgins-methane medium or L-salts-propane medium until after an enrichment period of 6 d. Subsequently, cfu on these media increased to approximately 1×108 g−1; similar growth was obtained on both of the hydrocarbons. Control plates with no added carbon showed only a few colonies, which were much smaller than those on the hydrocarbon-amended plates and appeared only after several additional days of incubation.

image

Figure 1&. emsp;Heterotrophic and hydrocarbon degrading bacterial counts in compost enriched with 10% (a) methane or (b) propane. Points represent means of triplicate plate counts±S.E. (•), Nutrient agar ; (▪), Higgins agar+methane

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Replica plating between media

The results of replica plating experiments are shown in Table 2. Of the 30 random colonies originally grown on NA, approximately 8·9 and 7·0% were successfully replica plated on L-salts-propane medium and Higgins-methane medium, respectively. Of the colonies originally grown on L-salts-propane medium, over 90% of the replica transfers onto the same medium, Higgins-methane medium and NA were successful. The same pattern was observed for those colonies originally grown on Higgins-methane medium (Table 2). Of the colonies originally grown on L-salts-propane medium, approximately 11% were capable of growth on this medium exclusively; transfer attempts to other media types failed. Approximately 6% of colonies originally grown on Higgins-methane medium would grow on this medium but not on propane or NA (data not shown).

Table 2.  Effect of carbon substrate on growth of replica plated colonies derived from compost
Original mediumReplica plate medium
Nutrient agarL-salt-propaneHiggins-methane
  • *

    Percent of replica plated colonies which re-grew in replica plate medium. Values are means±S.E. with the total number of colonies transferred given in parentheses.

Nutrient agar100·0±0 (480)* 8·9±1·5 (270) 7·0±0·9 (270)
L-salts-propane 93·3±1·6 (210) 94·3±2·6 (210)83·0±6·1 (240)
Higgins-methane 91·8±1·9 (210) 90·0±3·4 (240)93·3±1·3 (210)

To address the possibility that growth on alternate substrates was simply due to growth on residual carbon transferred with the colony, repeated colony transfers were performed over a period of several months. These results were completely reproducible over repeated transfers. Furthermore, control plating onto mineral media with no carbon source provided resulted in insignificant growth. The few colonies that occasionally appeared under these conditions were much smaller, morphologically distinct and required longer incubation times for growth; they were considered to be the result of contamination.

Characterization of cultures from enriched compost

Five separate cultures derived from enriched compost were each able to grow on L-salts-propane, Higgins-methane media and NA. Based on gross colonial characteristics and cellular morphology, the cultures were designated as actinomycetes. All of the cultures appeared to be pure, based on microscopic inspection of repeated subcultures. However, two of the cultures showed variable characteristics when grown L-salts-propane vs Higgins-methane medium. Culture no. 1 demonstrated growth in 6·5% NaCl only on Higgins-methane medium and H2S production only when grown on L-salts-propane medium. Culture no. 2 demonstrated growth in 6·5% NaCl and lipolytic activity only when grown on L-salts-propane medium, and H2S production only when grown on Higgins-methane medium (data not shown). Fatty acid analysis also indicated that cultures nos 1 and 2 were actually mixed cultures due to different probable identifications assigned to the same culture when grown on the two distinct carbon sources. Tentative identities of Streptomyces rochei and S. violaceusniger were assigned to the two members of culture no. 1 grown on Higgins-methane medium and L-salts-propane medium, respectively. The two members of culture no. 2 were tentatively identified as S. violaceusniger and S. lavendulae grown on L-salts-propane medium and Higgins-methane medium, respectively.

Fatty acid profile identities for cultures nos 3, 4 and 5 were also assigned within the genus Streptomyces, with no species level differences detected for any of the cultures grown separately on L-salts-propane medium or Higgins-methane medium. In fact, none of the analyses during this study were sufficient to eliminate the possibility of pure cultures for cultures nos 3, 4 and 5.

Trichloroethylene biodegradation capacities of cultures

Table 3 shows the TCE biodegradation capacities of five cultures derived from enriched compost and grown separately on Higgins-methane medium and L-salts-propane medium. Also shown are concomitant hydrocarbon consumption levels noted for each culture. Cultures nos 1 and 3 exhibited the most substantial TCE removals and also consumed more propane than the other cultures grown on propane. However, the methane utilization rates for these cultures remained very low. Cultures nos 4 and 5 exhibited relatively low propane utilization levels, but did show some TCE degradation. Culture no. 2 showed negligible propane or methane utilization. In no case did any of the cultures degrade TCE when grown on Higgins-methane medium.

Table 3.  Hydrocarbon consumption and TCE removal by individual cultures
CultureGrowth medium
L-salts+propaneHiggins+methane
Propane consumedTCE removedMethane consumedTCE removed
  • *

    Values are percentages of hydrocarbon (10% v/v) or TCE (3 ppmv) removed after 96 h.

149*2401
201000
3493303
401007
5201607

Discussion

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Considerable research has focused on the stimulation of TCE biodegradation in soil by methane or propane. Wilson & Wilson (1985) first established the feasibility of methane enrichment for stimulating TCE removal in soils. Phelps et al. (1990) successfully enriched subsurface sediments with methane and propane under pulse feeding and continuous feeding conditions, and observed up to 90% TCE removal (20 ppm) within 5 d. These researchers found propane to be superior to methane as an enrichment gas, and considered this to be due to the higher energy value of propane vs methane. Conversely, Fliermans et al. (1988) enriched contaminated aquifer sediments with a variety of carbon sources, including methane and propane. They reported substantial TCE degradation (91% of 50 ppm within 3 weeks) in sediments spiked with 50 ppm TCE and enriched with methane, but negligible degradation with propane enrichment. Likewise, Lanzarone & McCarty 1990) were unable to enrich subsurface sediments with propane; propane was never consumed in their column reactor, and TCE was not removed as it was in methane-enriched reactors. They concluded that propanotrophic populations were non-existent in their sediment samples. In the current study, compost was successfully enriched with either propane or methane, but previous work demonstrated that certain compost materials are more amenable to this purpose than others (Sukesan & Watwood 1997). Enrichment with hydrocarbon gas is an excellent approach to consider for TCE remediation needs, but assessment of each system is obviously necessary to ensure suitability.

With respect to finished compost as a biofiltration matrix, both absorption and biological degradation contribute to initial TCE removal. It is likely that sorption is actually the initial step in the biodegradation process, as it increases contact between cells and chemical (Barrio-Lage et al. 1987). However, even an extremely sorptive matrix such as compost has a finite sorptive capacity which limits the amount of TCE which can be removed in this manner from a gaseous waste stream. Beyond this point biological degradation of the contaminant must proceed in order for removal to continue. It has been demonstrated that this process can be stimulated by enriching compost material with hydrocarbon gas (Watwood & Sukesan 1995; Sukesan & Watwood 1997). The current results confirm that exposure of compost material to propane or methane gas results in elevated populations of propanotrophs and methanotrophs, respectively. It is therefore considered that the stimulation of TCE removal is due to the cometabolic activity of propane monooxygenase and methane monooxygenase, respectively.

For such a complex matrix as finished compost, however, the situation may not be this straightforward. In this study, there was apparent overlap between methane, propane and complex carbon utilization in compost microbial populations. In separate TCE degradation experiments where the compost was first enriched with methane then switched to propane enrichment, there was a 6 d lag period, during which propane consumption and TCE removal were considerably lower than in compost continuously exposed to propane (data not shown). However, even during this lag period, low but measurable levels of propane and TCE depletion did occur. These observations indicate that methane enrichment may impact propanotrophic activity within the compost. Replica plating experiments revealed that significant numbers of propane-utilizing colonies could grow successfully on Higgins-methane medium, and vice versa. Many colonies originally grown on Higgins-methane or L-salts-propane medium could also grow on NA, although very few colonies could be successfully transferred from NA to Higgins-methane or L-salts-propane medium. This type of apparent metabolic overlap has been described by other researchers. For example, Brockman et al. (1994) found that 1% methane injections into subsurface sediments resulted in an elevated population of propanotrophs able to degrade TCE.

One likely explanation for the apparent metabolic overlap observed here and by other researchers is the existence of complex microbial interactions. In the current study, two of five cultures, derived from enriched compost and exhibiting apparent metabolic overlap, proved to be mixed. The tentative genera assigned to the other three cultures, based on fatty acid analysis, have never been cited as methanotrophic. This is difficult to reconcile with the fact that these cultures grew consistently on Higgins-methane medium. There have been several well publicized claims of new facultative methanotrophs (Patt et al. 1974; Lynch et al. 1980), which later proved difficult to verify and in certain cases, were recanted (Lidstrom-O’Connor et al. 1983). Dalton & Leak (1985) therefore recommended caution when asserting new claims of facultative methanotrophy, and no such claim is made here. Rather, based on the unusual metabolic patterns exhibited by these cultures and the apparent metabolic discrepancies in identification, it appears that each of the five compost-derived cultures was mixed, although the methods employed in this study could not separate the components. It is likely that complex interactions between members in the mixed cultures prevented their separation.

This type of complexity has frustrated other researchers in this field. Fliermans et al. (1988) observed TCE degradation in methane and propane enrichment cultures from subsurface sediments, but they were unable to establish stable methanotrophic enrichment cultures and could not isolate methanotrophic TCE degraders in pure culture. These researchers theorized that methane enrichment might serve to enrich the general microbial community slowly, promoting diverse populations capable of tolerating and degrading TCE.

It is also important to note in the current study that the separate cultures derived from enriched compost exhibited substantially lower levels of TCE biodegradation than those exhibited by the consortium present in the compost material. When taken together, these observations suggest that microbial populations present in enriched compost participate in intricate, and perhaps obligatory, metabolic interactions. In this system, the complexity of the microbial community may contribute significantly to cometabolic TCE removal; syntrophic, commensalic or symbiotic associations may result in conversions that are not feasible for separate cultures.

Another issue of interest is the optimum hydrocarbon concentration to target in the treatment system concomitant to contaminant introduction. In theory, some minimal hydrocarbon concentration must be maintained, even after successful enrichment, to ensure continuous induction of the appropriate cometabolic enzymes. However, high hydrocarbon concentrations could theoretically result in substrate competition which could lead to decreased TCE degradation. An additional factor to consider for actual treatment systems is the expense associated with the hydrocarbon supply. In the current study, hydrocarbon enrichment was routinely performed with 10% hydrocarbon, based on previous results, but different hydrocarbon concentrations were assessed during TCE degradation experiments. The results indicate that for methane and propane, TCE degradation in previously enriched compost was independent of hydrocarbon concentration within the range of 0·2 to 2·0%. This result contrasts with a report by Strandberg et al. (1989). This group studied TCE removal in a fixed film bioreactor infused with aqueous growth medium and methane and inoculated with an enrichment culture from contaminated subsurface material. While they found no evidence of substrate competition between methane and TCE with atmospheric methane concentrations between 4 and 20%, when methane was reduced to 2%, TCE biodegradation efficiency declined, presumably due to a lack of sustained induction of methane monooxygenase. These divergent results may not be totally unexpected for two such different systems, again highlighting the difficulty inherent in extrapolation of results between systems.

More research is needed to address specific aspects of biofiltration microbiology. While it is not surprising that such a complex process will vary between systems and materials, it is extremely important that researchers and practitioners continue to report observations and results with compost packed systems so that this promising remediation technology can be further improved and refined. Critical to advancing this type of technology is a more thorough understanding of the microbial ecology and metabolic constraints within the biofiltration matrix.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

This research was supported by the NSF-Idaho EPSCoR Program, by National Science Foundation Cooperative Agreement number OSR–9350539, and by subcontracts from the Idaho National Engineering and Environmental Laboratory. Salary for S.S. was provided by a DOE traineeship. The authors are grateful to Dr Bill Apel and Mr Brady Lee of the INEEL Research Center for valuable discussions and technical assistance.

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  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
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