Soil & Sediment Contamination 10(6) 2001


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Soil & Sediment Contamination 10(6) 2001

  1. 1. Soil and Sediment Contamination, 10(6):659-673 (2001) Co-Composting of Residual Fuel Contamination in Soil Turlough F. Guerin validation and reinstatement of two con- taminated areas on the site was first con- Shell Engineering Pty Ltd, NSW State Office, ducted, followed by development of a PO Box 26, Granville 2142 NSW, Australia. Email: composting treatment process. The total volume of contaminated soil (i.e., TPH con- centration > 1000 mg/kg C10-C 36) was ~4300 m3 with a concentration of 3100 ± 1270 mg/kg. The soil was stockpiled into four windrows, on a compacted, bunded clay base. Approximately 35% (v/v) of raw materials (green tree waste, cow manure, gypsum, and nutrients) were added to ini- tiate composting. The piles were kept moist during the summer months, but no other maintenance was conducted. Once the composting process was initiated, the wind- rows were sampled at 2 and 6 months. After 6 months treatment, the average TPH concentration (C10-C36) was 730 mg/kg (with a 95% CI of 1020 mg/kg), which met the A remediation program was designed and relevant clean fill criteria applicable to the implemented at a site in southeastern Aus- site. There were no other contaminants of tralia that had become contaminated with significance in the treated soil compost and nonvolatile, n-alkane total petroleum hy- it posed no unacceptable risk to human drocarbons (TPH). The remediation was health or the environment, allowing it to be conducted in two stages. The excavation, used as fill at the site. Key Words: bioremediation, composting, co-composting, soil, sludge, contamination, petroleum hydrocarbons, endpoint, phenol, mesophilic, field-scale, case study. 1532-0383/00/$.50 © 2001 by AEHS 659340306.pgs 659 11/13/01, 3:47 PM
  2. 2. INTRODUCTION M icrobial degradation of petroleum products in soil, either via naturally occurring or facilitated methods, is used to reduce soil concentrations of these products to acceptable levels (Atlas, 1988; Rhodes et al., 1994; Rhodes et al., 1994b). Microorganisms found in soils include naturally occurring populations that possess the ability to degrade petroleum products. Important soil factors that affect biodegradation processes, which apply to petroleum hydrocarbon and other organic contaminants, include (1) an energy source, (2) favorable soil pH, (3) soil temperature, (4) availability of soil moisture, (5) availability of essential macro and micro-nutrients, (6) the nature and bioavailability of the pollutants, and (7) aera- tion. The potential biodegradability of petroleum hydrocarbons can generally be estimated based on the structure of the chemicals comprising the source hydrocar- bon product causing the pollution. For example, branching structures typical of asphaltenes, generally reduce the biodegradation rate, and aromatic compounds are degraded more slowly than alkanes (Huesemann, 1997). A number of engineered treatment systems are available for bioremediation of petroleum products in soil. The selection of a bioremediation system should be based on the physical/chemical/biological properties of the product, site con- straints, cleanup criteria, and state or local regulatory requirements (Guerin, 1996). Land treatment, generally referred to as landfarming, has been widely used for treatment of petroleum products, both in liquid and solid waste forms. Land treatment is a bioremediation technique in which the petroleum product (liquid, solid, or contaminated soil) is spread on soil, mixed with nutrients, and biode- graded by soil microbes (either naturally occurring or amended to the soil). Land treatment is a relatively uncontrolled method of reducing petroleum hydrocarbons because volatilization is usually not prevented. Land treatment is now not gener- ally recognized as an industry best practice for treatment of petroleum hydrocar- bon-contaminated soils and wastes. A modification of land treatment that also utilizes microbial degradation is the biopile (which can be static or aerated). Composting is the addition of a soil amendment to contaminated soil to (1) supply energy for microbial growth and biodegradation of petroleum products as well as (2) a bulking agent to allow for enhanced microbial activity in the contaminated soil (Rhodes et al., 1994a; Rhodes et al., 1994b). Soil composting, or co-composting, an emerging bioremediation technology, relies on the actions of microorganisms to degrade organic pollutants, resulting in the formation of heat and inorganic and organic compounds (Semple et al., 2001). With changes in temperature, the structure of the microbial community changes, and this encourages a diversity of degradation reactions. Both aerobic and anaerobic processes can occur in windrow composting, depending on the frequency and extent of soil and organic matter mixing. Soil composting is now becoming widely recognized and adopted for the remediation of soils contaminated with recalcitrant organic compounds (Guerin, 660340306.pgs 660 11/13/01, 3:47 PM
  3. 3. 2000). It has particular application to soils and sludges that have a large proportion of nonvolatile petroleum hydrocarbons that are not readily degraded using conven- tional land treatment (Prado-Jatar et al., 1993) or bioventing processes. Semple (2001) provides an up-to-date overview of the application of soil composting and readers are referred to this for further details on soil composting. In summary, for recalcitrant petroleum hydrocarbons, soil composting is quicker; more controlled, results in lower pollutant endpoints, and requires less space than land treatment. There have been numerous reports on the effectiveness of composting processes for the remediation of petroleum hydrocarbons. Weathered, hydrocarbon-contami- nated soil containing 17,000 mg/kg oil and grease (O&G) of which 40% (w/w) was aliphatic, 32% (w/w) polar, and 28% (w/w) aromatic was composted with maple leaves and lucerne (Medicago sativa) in a laboratory-scale reactor. During this period the compost temperature had risen from ambient to 53°C (day 2) and subsequently, gradually returned to room temperature. The maximum rate of O&G degradation (600 mg/kg/day) occurred at the beginning of the experiment. Soxhlet extraction indicated that 50% of the O&G of soil origin was degraded in the first 105 days. O&G fractionation demonstrated that 60% of the aliphatics, 54% of the aromatics, and 83% of the polars were degraded during the first 180 days of co- composting. After 287 days, at least 73% of O&G of soil origin had been degraded (Beaudin et al., 1996). Previous research has shown, however, that static windrows can be as effective as turned windows in soil composting processes. Static and windrow composting techniques were compared with respect to their efficiency in the degradation of petroleum hydrocarbons. Static and windrow compost piles were constructed from diesel fuel-contaminated soil with a TPH of 11,000 mg/kg. Static aerated pile composting provided more effective treatment than windrow composting (Wong et al., 1993), which involved turning of the soil compost mix. In other studies, composting of horse manure was used as a means of degrading two oil wastes; oil sludge from petrol stations and petroleum residues from a refinery. Oil wastes decomposed to 78 to 93% during 4.5 months of composting. No difference was found between the two types of oil wastes concerning their decomposition. At the end of the experiment, most of the PAH compounds had been degraded except pyrene, chrysene, and dibenz(ah)anthracene. Gaseous losses of oil compounds through volatilization from composts were found not to be significant (Kirchmann and Ewnetu, 1998). In the current study, a site investigation was conducted at a fire fighting training facility. This site investigation identified several areas of soil contamination requir- ing remediation. The results of these investigations were then used to develop and evaluate remedial options. The remediation process was conducted in two stages. The excavation, validation, and reinstatement of two contaminated areas was con- ducted first, followed by development of a composting treatment process. The overall goal of the remedial works at the site was to enable the upgrade of the Flammable Liquid Pad (FLP)/Fuel Mix Areas (FMA) facilities, to provide better management of liquid fuels and hydrocarbon contaminated effluents and so prevent 661340306.pgs 661 11/13/01, 3:47 PM
  4. 4. further soil and water contamination from the fire training activities. The specific objective was to remove and treat the petroleum hydrocarbon contaminated soils on the site to <1000 mg/kg for the C10-C36 n-alkane hydrocarbon fractions (Victorian EPA Guidelines for low level contaminated soil). This study reports the results of a composting treatment process implemented to remediate this contamination. SITE BACKGROUND Location The site is located in southeastern Australia. The site (~150 ha) is in a rural pasture setting and currently is used as a training area for fire and emergency services personnel from various organisations. This principally involves fire fighting exer- cises at a number of “props”, using both gas and liquid fuels. The site has been used for such training for ~20 years. Prior to this few buildings existed on the site. The main areas of the site comprise a FLP and FMA used for fire training, bulk fuel storage area, light industrial facilities, including stores, workshops, and under- ground diesel storage tanks. All of the industrial facilities are located in the central part of the site where the contamination is present. Topography, Drainage, and Subsurface The site is located on a flat to gently undulating plateau, with lakes and wetlands formed in local depressions. A small lake is situated immediately southwest of the contaminated area and a central north-south ridge forms a break in the site drain- age. The site drains to two creeks surrounding the facility. Subsurface Conditions The site lies over quaternary olivine basalt bedrock. Surface soils are residual silts and clays, generally no more than 2 to 3 m deep, overlying very stiff, high plasticity residual clays, grading to variably weathered basalt. Shallow fill, comprising gravel or road base, is found on parts of the site, particularly in the area of the Flammable Liquids Pad. A summary of the site stratigraphy is given in Table 1. Eight bores (four deep bores to 20 m, and four shallow to 2 m) were installed. Groundwater was encountered in only two of these bores. The basalts are generally dense and unjointed without significant primary or secondary porosity to enable groundwater flow. The residual clays are also of low water bearing potential. As a result, the occurrence of any significant groundwater is precluded, and the potential for contaminant migration via groundwater is very limited. 662340306.pgs 662 11/13/01, 3:47 PM
  5. 5. TABLE 1 Generalized Subsurface Profile Soil Depth to Top of Thickness Description Unit Layer (m) (m) 1 0 0.1 - 0.8 Fill: fine to coarse grained sandy gravel, silty clay or medium plasticity red clay. 2 0.2 - 1.0 0.1 - 0.2 Residual silty clay: medium plasticity, grey to grey-brown, may comprise rounded buckshot gravel (2-5mm) with clay. 3 0.3 - 1.2 0.5 - 1.8 Silty clay: high plasticity, yellow-grey to yellow- brown, mottled orange-yellow. Residual clay formed on basalt. 4 0.8 - 2 14 - 18 Basalt 5 16 - 18.8 3.2 - 6.0 Volcanic ash Nature of Soil Contamination Soil and sediment contamination was present on the site, predominantly as petro- leum hydrocarbons, with lower concentrations of benzene, toluene, ethyl benzene, and xylenes (BTEX), phenols, and lead. This contamination principally resulted from storage and handling of fuels, use of liquid fuels in fire training activities, and disposal of fuel residues such as sludges. The petroleum hydrocarbons were generally medium to heavier fractions (C15-C36), as expected from the nature of the activities on the site with concentrations up to 20,000 mg/kg. Light hydrocarbons (C6-C9 and C10-C14), including BTEX constituents, were present in a drum burial area with concentrations up to 2000 mg/kg. This area also contained elevated concentrations of phenols. One sample from the old Fire Training Pits (FTP) contained elevated concentrations of lead. No significant concentrations of poly- cyclic aromatic hydrocarbons or other heavy metals were found at the site. No organochlorine pesticides or PCBs were detected in any samples tested in soil from the site. Extent of Soil Contamination Two areas of soil contamination were identified requiring remediation, as part of development plans for the site, as follows: (1) FLP. This large area contained obvious and unsightly superficial soil contamination with fuel residues from fire training activities. Crushed rock fill was contaminated with petroleum hydrocar- bons at depths of 0.1 to 0.5 m, but generally no deeper than 0.8 m. Total petroleum hydrocarbon concentrations ranged up to 1600 mg/kg; (2) Old FTP. Two decom- missioned fire training pits, east of the FLP contained a thin layer (less than 10 cm thick) of black petroleum hydrocarbon sludge, at a depth of 0.1 to 0.6 m. The 663340306.pgs 663 11/13/01, 3:47 PM
  6. 6. sludge was covered by a 0.1- to 0.8-m-thick layer of surface fill comprising silty clay, silt and gravel. High concentrations of total petroleum hydrocarbons (TPH), up to 88,000 mg/kg, were found in the sludge layer and in soil from 0.6 to 1.0 m. Phenols were present at concentrations up to 50 mg/kg. The average initial phenol concentration in the segregated, uncomposted, material was 2.5 mg/kg and the maximum was 9.3 mg/kg. In the soil that was actually composted (not stockpiled only), the initial average phenol concentration was even lower, at 1.6 mg/kg, with a maximum 4.1 mg/kg. No post-composting analysis were conducted to determine phenol in the current study. A guideline is provided by the Dutch soil intervention value of 40 mg/kg (RIVM, 1994). The current Australian risk-based health inves- tigation levels for phenol are higher still (i.e., 8500 mg/kg for a residential, i.e., a sensitive setting) (NEPC, 1999). Given the current land use at the site, and the absence of any significant groundwater or surface water resource, the maximum phenol concentrations of the treated soil do not represent a significant risk to human health or the environment. Elevated lead (710 mg/kg) was found in one sample. The TPH, BTEX, phenol, and lead contamination was distributed within the top 1 to 1.5 m of the soil profile. MATERIALS AND METHODS Excavation Criteria Remediation of the soil contamination involved excavation of all soil exceeding specified criteria for organic contaminants and lead, and transport to a treatment area constructed elsewhere on the site. The criteria adopted for the excavation work were the relevant guidelines for off site disposal of contaminated soil as clean fill: total petroleum hydrocarbons (TPH) ([C9) 100 mg/kg; total petroleum hydrocar- bons (TPH) (C10-C36) 1000 mg/kg; phenols 1 mg/kg; BTEX 7 mg/kg; lead 300 mg/ kg. Soil Treatment The excavated contaminated soil was treated by on site composting. Given the contaminants were predominantly medium-heavy hydrocarbons, the aim of the treatment was to stabilize the soil so that it could be reused on site as fill material, for example, under a new training pad. Two criteria were proposed. Either the average concentration of TPH should fall below 1000 mg/kg (C10-C36) or the composting process should reach completion. The selection of an environmentally acceptable endpoint for treatment of petro- leum hydrocarbons in soil is a complex process involving numerous factors (Linz and Nakles, 1997). A plateauing of biodegradation activity was considered to 664340306.pgs 664 11/13/01, 3:47 PM
  7. 7. represent a natural, or environmentally acceptable endpoint for the composting process and was used to assess the status of the soil remediation process. At this stage, the residual petroleum hydrocarbons can be considered to be ‘biostabilized’, effectively immobile and unavailable for further uptake and biodegradation, and thus unlikely to adversely affect the quality of the soil or water with which it may come into contact’(Huesemann, 1995; Huesemann, 1997). Changes in the tem- perature regime were not used as an indicator of biostabilization since other factors, such as soil mixing, could reactivate the biodegradation reaction and again decrease the residual petroleum hydrocarbons. The temperature was measured (see section “Monitoring and Maintenance) in order to assess whether a mesophilic or thermophilic process was occurring in the windrows. Construction of Treatment Area The facility consisted of a bunded area with final dimensions of 200 m × 40 m. The area was scraped and graded, before stockpiling and processing the contaminated soil. The 500 mm bunds were constructed of compacted local clay soil. Surface drainage external to the treatment facility, was diverted around the facility via a perimeter drain. Any run-off within the facility area was collected and pumped to the nearby dam. This dam was a retention pond and retained any leachate on site. This was achieved with a shallow drain within the bund wall, draining to a deeper sump (500 to 600 mm) at the low southern end of the treatment area. Though no data were collected, only very small volumes of leachate were actually generated during the composting process because of judicious application of water to the windrows. Once operational, access to the site was controlled by site personnel, so that only machinery directly involved in the process would contact contaminated soil. Trucks, excavators, and other machinery were washed down after handling contaminated soil and compost, and before these machines were allowed to leave the site. This wash down water, which was of a low volume (no data collected), was colleted in the on site retention pond. Establishment of Compost Windrows Soil was excavated from the FLP (including the FMA) and the FTP, and stockpiled in 7 rows in the bunded treatment area. The total volume of soil excavated was ~6000 m3. This material was sampled to determine TPH contamination. Any soil that was determined at >1000 mg/kg TPH (C10-C36) was segregated and designated for treatment by soil composting. Excavated soil determined at <1000 mg/kg TPH was set aside. As well as low concentrations of TPH, this material was found to contain low concentrations of total phenol compounds (up to 9 mg/kg). These low concentrations were not considered to represent any unacceptable risk to health or 665340306.pgs 665 11/13/01, 3:47 PM
  8. 8. environmental receptors at the site since these concentrations fell within the regulatory guidelines of 10 mg/kg for low-level contaminated soil at the facility. Additionally, it was decided that biodegradation would be stimulated through aeration and mixing during excavation and placement, resulting in a further reduc- tion in the concentration of phenols and TPH. In total, 4300 m3 of soil was designated for soil composting. Hydrocarbon contaminated soil was then stockpiled in four windrows (~1000 m3 each). The approximate dimensions of the windrows were that of rectangles of 200 m long × 5 m wide × 1 m high. To each soil stockpile a mixture of freshly shredded green tree waste (-20 mm fraction), cow manure, gypsum, and nutrients (MaxBac, Scotts, Sydney) were added. Approximately 35% by volume of raw materials were added to the soil to initiate composting. Table 2 provides details for the volumes of soil and raw materials used. The shredded green tree waste provided necessary bulking agent into the soil mix, which in turn allowed for improved aeration by diffusion if only contaminated soil was in the windrows. Cow manure also contrib- uted as a bulking agent. The shredded green tree waste and cow manure also contributed an energy source to the soil mixture that allowed microbial populations to flourish. Gypsum was added at ~4% (v/v) and has previously been shown to enhance soil properties in bioremediation processes (Rhodes and Guerin, 1996; Guerin, 2000). The MaxBac, a proprietary mixture of slow release micro nutrients only, was added at the recommended rates of 50 g/m3 of contaminated soil. Monitoring and Maintenance For the first 2 months of composting, the internal temperature of the compost windrows was monitored using thermocouples linked to a field datalogger (Datataker Model DT50). The soil compost windrows were kept moist (20 to 25% w/w) during the dry summer months with regular watering. No other maintenance on the windrows was performed. The windrows were therefore static for the remainder of the trial. Sampling Protocol General. Once the composting process was initiated, the windrows were sampled after 2 months, and followed up with a second round of sampling after 6 months composting. The samples were collected by taking multiple composite samples from the loader bucket. In order to determine the variation associated with this sampling technique a set of 20 separate samples were taken from one bulldozer bucket and analyzed. Each bucket load was taken as a cross section through the short length of each windrow. Each sampling event introduced oxygen into the windrow, at least in the adjoining 1 to 2 m of windrow length. 666340306.pgs 666 11/13/01, 3:47 PM
  9. 9. 340306.pgs TABLE 2 Composition of Soil Compost Windrows Compost Green Cow Gypsum Total Raw Contaminated Total Initial667 Windrow Tree Waste Manure (m3) Materials Soil Volume (m3) (m3) (m3) (m3) (m3) 1 270 75 50 395 940 1335 2 280 75 50 405 1035 1440 3 265 75 60 400 1185 1585 4 280 75 50 405 1180 1585 Total 1,605 4,340 5,945 66711/13/01, 3:47 PM
  10. 10. Sampling After 2 Months. Four windrows were established in the composting process. From windrow 1, samples were taken at seven locations along the wind- row, and at three different depths (top, middle, and bottom). Additionally, in order to determine the variation associated with sampling, additional samples were taken from this windrow. Field duplicate samples were taken 0.5 m from the original samples. This resulted in a total of 42 samples being collected from Windrow 1. From each of the other three windrows, samples were taken from the middle depth and at three locations along each windrow. Sampling After 6 Months. Samples were taken at seven intervals along each windrow, again at three different depths. This resulted in a total number of 84 samples. Analytical Protocols Solvent extraction of the total petroleum hydrocarbons was conducted using DCM (dichloromethane):acetone (3:1) as the same method as previously described (Guerin, 1999c). Total phenols were analyzed using APHA 5530C (Anon., 1992). Soil pH (1:5 soil water extract) was also monitored during the process. Soil pH, soil lead, PCBs, organochlorine pesticides, and TPH analyses were taken from standard US EPA protocols as described elsewhere (USEPA, 1983; USEPA, 1996). RESULTS AND DISCUSSION The initial average concentration of TPH (C10-C36) in the excavated contaminated soil was 3075 mg/kg, ranging between 1000 to 7900 mg/kg. After 2 months of composting, the second sample set indicated that the average TPH concentration for the four windrows had been reduced to 900 mg/kg. There was variation in the TPH concentrations between the four windrows, and also within the windrows. This variation was analysed statistically and the total error was found to be ± 40%, that is, the TPH concentration overall was 900 ± 370 mg/kg (Table 3). While the average TPH concentration was below the target of 1000 mg/kg C10-36 TPH after 2 months, the 95% upper CI for the total compost mass for this sampling program was 1270 mg/kg. A third sampling program was performed 6 months after the windrows were established. At this time, the average TPH concentration was 730 mg/kg (Table 3). Error analysis was again performed that also indicated a total error of ±40%, as found in the initial error analysis. While the upper 95% CI for windrows 1 and 2 exceed 1000 mg/kg TPH (1500 and 1100, respectively), the average TPH concentration across the 4 windrows after 6 months of treatment was below the clean fill criterion of 1000 mg/kg C10-36 TPH. 668340306.pgs 668 11/13/01, 3:47 PM
  11. 11. TABLE 3 TPH Concentrations in Soil Compost TPH concentration Compost (mg/kg total dry mattera) Windrow Average Average Average Upper 95% CI at 6 Initial 2 Months 6 Months monthsb 1 2,800 1,200 1,080 1,510 2 4,500 850 810 1,130 3 3,400 780 530 740 4 1,800 770 510 710 mean 3,100 ± 1,270 900 ± 370 730 ± 300 1,020 a All data are given to 2 significant figures. Concentrations are not adjusted for dilution from added organic matter. The TPH reported here is in the range of C10-C36. b 95% Upper Confidence Interval (CI) = Average Concentration + 40%. There were no other contaminants of significance in the composted soil. For example, there were no volatile petroleum hydrocarbons (C6-C9) present in the soil-composting mix in the windrows, prior to treatment. Elevated lead concentra- tions were identified in one sample taken from the FTP, which represents ~30% of the total material in the compost windrows. However, the rest of the compost windrows consist of material sourced from the FLP and the FMA which reported acceptable concentrations of lead. After mixing the material it was expected that the lead “hotspot” would have been dispersed and diluted. Results of the lead analysis (at T = 6 months) showed that the average lead concentration of all the material was 55 mg/kg, which is well below the 300 mg/kg clean fill criteria for lead. All of the samples assayed were below these guidelines (Table 4). In the lead sampling program, 20 replicate samples were analyzed (the same samples were used as those used for TPH analysis at 6 months). This found that the subsampling and analytical error for the lead analysis was 13%. There are several important soil factors that affect the biodegradation processes, which also apply to composting or co-composting of petroleum hydrocarbon- contaminated soil. These are an energy source, favorable soil pH, soil temperature, availability of soil moisture, availability of essential macro and micro-nutrients, bioavailability of the pollutants, and aeration. These are discussed as follows: • Energy Source. In the current study, the energy source was provided through the added organic matter and the TPH contaminants. • Mixture pH. The pH of soil/OM mixture remained within the range of 6.5 to 7.5 throughout the process. • Windrow Temperature. There was only a relatively small increase in the temperature profile in the soil composting mix during the 6 month treatment 669340306.pgs 669 11/13/01, 3:47 PM
  12. 12. TABLE 4 Extent of Lead Contamination in Compost Windrows Compost Lead Concentration (mg/kg) Windrow Average Minimum Maximum 1 60(a) 42 69 2 38 20 82 3 48 22 82 4 55 33 97 a One sample from windrow 1 assayed at ~360 mg/kg lead. Another sub-sample was taken and assayed, this yielded a result of 60 mg/kg lead. The first assay is considered an outlier, and has been omitted from the statistical analysis. period (Table 5). This is unlike other studies of the composting of soil (also referred to as co-composting in the literature), where composting tempera- tures have increased to >40°C (Guerin 1999a). Typically composting reac- tions cause windrow temperatures to increase to as high as 45 to 65°C (Semple et al., 2001). These lower temperatures recorded in the current study may be a reflection of lower levels of aerobic microbial activity. • Windrow Moisture. Moisture content of the soil/OM mix in each windrow was kept within the target moisture content of 20 to 25%. The soil-compost mixture was also shown to have a strong water absorbing capacity as there was only very small volumes of leachate released after each water period. Furthermore, after heavy rainfall, only small amounts of rain water re- mained free within the bunded treatment area. • Bioavailability. No specific tests were conducted to evaluate this property of the TPH compounds in the soil composting mixtures. However, from the 6 month composting data, it was likely that the composting process may have increased the bioavailability of the TPH in the soil as this contamina- tion had been present for periods of up to 30 to 40 years. • Windrow Aeration. Given that the windrows were only mixed during the initial stages of the process, it is likely that there would have been anaerobic processes in the windrows. Therefore, even under conditions where oxygen was likely to be limiting, petroleum hydrocarbon degradation still occurred, attaining the target criteria of <1000 mg/kg C10-36 TPH. Further research would be needed to determine to what extent anaerobic and aerobic pro- cesses were occurring in the windrows. After 6 months, the composting was considered to be complete, with the soil- compost mix posing no unacceptable risks to human or environmental health. It is expected that without further maintenance of the compost windrows (i.e., watering, 670340306.pgs 670 11/13/01, 3:47 PM
  13. 13. 340306.pgs TABLE 5 Temperature Changes in Compost Windrows (°C) Days Windrow Mean Windrow Maximum Ambient Maximum Variation(a)671 0 16 16 14 2 3 28 32 14 4 5 28 32 13 4 10 27 30 13 6 20 24 27 12 5 40 22 25 11 5 80 16 17 10 4 a variation across 4 treatment windrows 67111/13/01, 3:47 PM
  14. 14. aeration) contaminant degradation will continue, albeit slowly. Grasses colonised the compost (90 to 100% surface coverage) in the 6 month period immediately after the end of the processing. This growth will stimulate phytoremediation processes that will further promote contaminant degradation (Chang and Corapcioglu, 1998). The bund walls will be maintained, until the compost wind- rows are sufficiently vegetated, to prevent potential run-off of sediment-laden water. CONCLUSIONS/IMPLICATIONS The composting process was successful in reaching its primary objective of reduc- ing TPH concentrations to acceptable clean fill criteria of <1000 mg/kg C10-36 TPH. The process observed is best described as mesophilic because high temperatures (i.e., >45°C) were not achieved. The composted soil remaining from the composting process was used to rehabilitate the treatment area, leveling the soil and compost windrows within the bund walls, and allowing revegetation to take place. Co- Composting, with minimal windrow turning (i.e., aeration), can be effective in bioremediatin in long-chain, nonvolatile n-alkanes in clay soil. ACKNOWLEDGMENTS The contributions of my colleagues in Rio Tinto Technical Services (Sydney and Melbourne, Australia) are gratefully acknowledged, in particular Stuart Rhodes. The information and opinions expressed in this paper are those of the author and do not necessarily reflect those of Shell Engineering REFERENCES Anon 1992. Standard Methods for the enumeration of water and waste water. Washington, D.C., APHA, AWWA, WEF. Atlas, R. M. 1988. Biodegradation of hydrocarbons in the environment. Basic Life Sci 45, 211–222. Beaudin, N., Caron, R. F., Legros, R., Ramsay, J., Lawlor, L., and Ramsay, B. 1996. Cocomposting of weathered hydrocarbon-contaminated soil. Comp. Sci. Utiliz. 4, 37–45. Chang, Y.-Y. and Corapcioglu, M. Y. 1998. Plant-enhanced subsurface bioremediation of nonvola- tile hydrocarbons. J. Environ. Eng. 124, 162–169. Guerin, T. F. 1996. Selecting the appropriate technology for treating a contaminated site or waste stream. Austral. J. Min. 11, 82–86. Guerin, T. F. 1999a. A comparison of mesophilic composting and unamended land treatment for the bioremediation of aged PAH residues in soil. Remediation 9, 59–78. 672340306.pgs 672 11/13/01, 3:47 PM
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