Combination Of Biostimulation Research Articles

Biological remediation treatments improve the health of a mixed contaminated soil before significantly reducing contaminant levels.

The remediation of mixed contaminated soil is challenging as it often requires actions to minimize metal-induced risks while degrading organic contaminants. Here, the effectiveness of different bioremediation strategies, namely, rhizoremediation with native plant species, mycoremediation with Pleurotus ostreatus spent mushroom substrate, and biostimulation with organic by-products (i.e., composted sewage sludge and spent mushroom substrate), for the recovery of a mixed contaminated soil from an abandoned gravel pit was studied. The combination of biostimulation and rhizoremediation led to the most significant increase in soil health, according to microbial indicator values. The application of composted sewage sludge led to the highest reduction in anthracene and polychlorinated biphenyls concentrations. None of the strategies managed to decrease contamination levels below regulatory limits, but they did enhance soil health. It was concluded that the biological remediation treatments improved soil functioning in a short time, before the concentration of soil contaminants was significantly reduced.

A sustainable approach to enhance heavy hydrocarbons removal in landfarming treatment.

The present study aimed to evaluate the best strategy to enhance the degradation rate of heavy petroleum hydrocarbons (HPH) contaminated soil in a landfarming plant. Samples of real contaminated soil, further spiked with HPH, were treated in mesocosm reactors simulating the landfarming system. One reactor was operated without any modification compared to the real landfarming plant. The other three reactors were operated with different strategies to improve the removal rate: biostimulation (BS) through the addition of nitrogen and phosphorus; bioaugmentation (BA) with the inoculation of sludge produced in the treatment of the process water from the oil re-fining plant of the same industrial area; combination of biostimulation and bioaugmentation (BAS). The biostimulation (BS) was the most effective strategy, leading to a reduction of the remediation time by 35% as compared to the traditional treatment. Bioaugmentation (BA) also provided positive effects leading to a reduction of the remediation time by 24%; its performance improved further when the addition of sludge was combined with the increase of phosphorous (BAS). Therefore, the key tool was represented by the phosphorous availability, whereas the application of sludge was most useful to provide waste with a new possibility of reuse, thus fulfilling the principles of the circular economy. The final characterization showed that the treated soil was suitable for reuse in industrial areas according to the legislation in force.

Phytoremediation of soil contaminated with weathered petroleum hydrocarbons by applying mineral fertilization, an anionic surfactant, or hydrocarbonoclastic bacteria

This study evaluated the effect of the application of mineral fertilization (F), the anionic surfactant Triton X-100 (TX100), or the inoculation with a hydrocarbooclastic bacterial consortium (BCons) on the growth of Clitoria ternatea during the phytoremediation of a Gleysol contaminated with weathered petroleum hydrocarbons (39,000 mg kg−1 WPH) collected from La Venta, Tabasco (Mexico). The experiment consisted of a completely randomized design with seven treatments and four replications each under greenhouse conditions. The application of F (biostimulation) increased plant growth and biomass production; in contrast, TX100 only favored root biomass (11%) but significantly favored WPH degradation. Bioaugmentation with BCons did not show significant effects on plant growth. Nevertheless, the combination of biostimulation with bioaugmentation (BCons + F, BCons + TX100, and BCons + F+TX100) enhanced plant growth, hydrocarbonoclastic bacteria population, and WPH degradation when compared to treatments with the single application of bioaugmentation (BCons) or biostimulation (F).

Effects of two bacterial inoculants and biostimulants on the growth performance of Amaranthus cruentus at different levels of spent-engine-oil-contaminated soil

Abstract This study was conducted to assess the growth performance of Amaranthus cruentus on spent-engine-oil- (SEO) contaminated soil bioremediated with bacterial inoculants and processed biostimulants: compost, sole cocoa pod husk (CPH), and sole cow dung (CD). SEO with 5%, 10%, and 15% densities was artificially contaminated with sterilized soil. Subsequently, biostimulants and bacterial inoculants, Staphylococcus aureus and Bacillus cereus, were applied. The treatments were replicated three times, adopting a completely randomized design. After the biodegradation assay that lasted for 70 days, Amaranthus cruentus seeds were sown, while its growth parameters, plant height, number of leaves, and stem girth were monitored for 6 weeks after sowing (WAS). The results generated indicate that the applied biostimulants and bacterial inoculants accomplished significant (P < 0.05) reclamation of the SEO-contaminated soil as the test plant recorded substantial growth compared with the controls that recorded no growth. At 2 WAS, compost influenced the highest plant heights (6.5, 7.4, and 7.5 cm) compared with other biostimulants at 5%, 10%, and 15% SEO contamination levels respectively. At 4 WAS, all biostimulants produced the same number of leaves (5.0) at 5% SEO contamination level, the compost recorded the highest number of leaves (6.0) at 10% SEO contamination level, while all biostimulants produced the same number of leaves (4.0) at 15% SEO contamination level. The study showed that the combination of biostimulants and bacterial inoculants significantly reclaimed SEO contaminated soil. Therefore, reclamation of petroleum-impacted environments with the combination of biostimulation and bioaugmentation is recommended.

Combined biostimulation and bioaugmentation for chlorpyrifos degradation in laboratory microcosms.

The online version contains supplementary material available at 10.1007/s13205-021-02980-9.

Bioremediation perspective of historically contaminated sediment with polycyclic aromatic hydrocarbons

The purpose of the current study is to compare the influence of different aerobic conditions (biostimulation (BS), bioaugmentation (BA), and a combination of biostimulation and bioaugmentation (BB)) on polycyclic aromatic hydrocarbons (PAH) degradation and compare the degraded amount with single step XAD-4 extraction as a new tool for bioavailability assessment for chronically contaminated sediment samples obtained from territory of Autonomous Province Vojvodina of Serbia (S1, S2, and S3). A great number of papers dealing with biodegradation of PAHs in spiked sediment or soil have been published, but to the authors’ knowledge, a limited number of papers studied aged, historically polluted sediment and a sum of chosen U.S. Environmental Protection Agency (USEPA) PAHs. A significant reduction (up to 67%) in PAH concentration was observed, while the percentage of reduction varied depending on the sediment sample and treatment used. BS treatment successfully stimulated growth of indigenous bacteria. Further, PAH-degrading strain Sphingomonas paucimobilis F8 inoculated in BA and BB treatment survived for up to 7 weeks after it was suppressed by unfavorable conditions or native microbes. Degraded amounts generally showed good correlation with results obtained from XAD-4 extraction. Results obtained in the current study represent a good start for standardizing a XAD-4 extraction technique as a simplified, easier, and lower cost method for bioavailability assessment.

OPTIMIZATION BACTERIAL DENSITY AND FERTILIZER DOSAGE FOR BIOREMEDIATION OF OIL CONTAMINATED SANDY BEACH: A CASE OF CILACAP, INDONESIA

Bioremediation, involving biostimulation and/or bioaugmentation, is a promising method to overcome oil spills in Cilacap coastal waters. Cilacap coastal area has high risk on oil pollution. This study investigated the stimulatory effect of nitrogen concentration, bacterial density and the composition of bacterial culture in enhancing oil degradation in this area. The applications of 4 different concentrations of Slow Release Fertilizer (SRF) and 2 different densities of bacterial cells in the form of single (RCO/B/08_008) and mixed culture were employed in microcosm experiments for 28 days. The efficacy of combining bacterial culture and fertilizer application in various concentrations was also tested. Oil degradation, bacterial growth and environmental parameters were monitored periodically during the experiments. The results showed that oil degradation rate was more influenced by nutrient concentration (biostimulation) than bacterial number or culture composition (bioaugmentation) added. The efficacy of biostimulation in degrading oil was better than that of bioaugmentation. Biostimulation increased oil degradation up to 6.4 times higher than the control. The optimum of fertilizer concentration added was 7.5 mg N/g (C:N ratio of 1,000:75), which increased depletion rate both in biostimulation-only and the combination of biostimulation with bioaugmentation up to 6.4 and 7.5 times higher than the control, respectively. It is suggested that bioremediation of oil-contaminated sandy beach in Cilacap would be optimal by employing a combination of Slow Release Fertilizer at concentration having C/N ratio = 1,000 : 75 and RCO/B/08_008 culture at density of 0.5 x 108 cells/mL (100% homology with Alcanivorax sp. TE-9).

Biological Treatment of Real Oilfield-Produced Water by Bioaugmentation with Sophorolipid-Producing Candida catenulata

This study focuses on biological removals of petroleum hydrocarbons from an oilfield-produced water generated in Naft Shahr resource by natural attenuation, biostimulation, and bioaugmentation processes. The results of the natural attenuation experiments indicated low population of existing environmental bacterial cells which could not remove total petroleum hydrocarbon (TPH) and initial chemical oxygen demand (COD) at 30 °C and 180 rpm. However, biostimulation by adding nitrogen, sulfur, and phosphorus sources resulted to 19.9% removal of TPH, mainly corresponding to the low chain n-alkanes, and 15.9% removal of COD in 6 days. Degradation of hydrocarbons with longer chain than C20 was limited mainly due to the low solubility of the hydrophobic compounds in the process. Bioaugmentation with the locally-isolated biosurfactant-producing Candida catenulata provided a companionship of the yeast with existing bacterial environmental cells and resulted in increase of the biodegradability of TPH by 16.1% even in the absence of the additional nutrient. A uniform degradation for all hydrocarbon fractions was only observed in the combination of biostimulation with bioaugmentation processes. The study on initial pH yielded the highest removals of 93.39% of TPH and 66.85% of COD in the combined process when initial pH was set at 5.0.

Development of bioreactors for comparative study of natural attenuation, biostimulation, and bioaugmentation of petroleum-hydrocarbon contaminated soil

Bioremediation of soil and groundwater sites contaminated by petroleum hydrocarbons is known as a technically viable, cost-effective, and environmentally sustainable technology. The purpose of this study is to investigate laboratory-scale bioremediation of petroleum-hydrocarbon contaminated soil through development of eight bioreactors, two bioreactors for each bioremediation mode. The modes were: (1) natural attenuation (NA); (2) biostimulation (BS) with oxygen and nutrients; (3) bioaugmentation (BA) with hydrocarbon degrading isolates; (4) a combination of biostimulation and bioaugmentation (BS-BA). Total petroleum hydrocarbons (TPH) mass balance over the bioreactors showed about 2% of initial 20,000mgkg-soil−1 TPH was removed by advection due to synthetic groundwater which was flowing through the soil, and the rest of decrease in TPH was caused by biodegradation. The BS-BA mode showed the highest TPH biodegradation percentage (89.7±0.3%) compared to the NA (51.4±0.6%), BS (81.9±0.3%) and BA (62.9±0.5%) modes. Furthermore, an increase in microbial population was another evidence of TPH biodegradation by microorganism. Reaction rate data from each bioremediation mode were fitted with a first-order reaction rate model. The Monod kinetic constants including maximum specific growth rate of microorganisms (μmax) and substrate concentration at half-velocity constant (Ks) were estimated for each bioremediation modes.

Combined iron and sulfate reduction biostimulation as a novel approach to enhance BTEX and PAH source-zone biodegradation in biodiesel blend-contaminated groundwater

The use of biodiesel as a transportation fuel and its growing mandatory blending percentage in diesel increase the likelihood of contaminating groundwater with diesel/biodiesel blends. A 100L-field experiment with B20 (20% biodiesel and 80% diesel, v/v) was conducted to assess the potential for the combined biostimulation of iron and sulfate reducing bacteria to enhance BTEX and PAH biodegradation in a diesel/biodiesel blend-contaminated groundwater. A B20 field experiment under monitored natural attenuation (MNA) was used as a baseline control. Ammonium acetate and a low-cost and sustainable product recovered from acid mine drainage treatment were used to stimulate iron and sulfate-reducing conditions. As a result, benzene and naphthalene concentrations (maximum concentrations were 28.1μgL−1 and 10.0μgL−1, respectively) remained lower than the MNA experiment (maximum concentrations were 974.7μgL−1 and 121.3μgL−1, respectively) over the whole experiment. Geochemical changes were chronologically consistent with the temporal change of the predominance of Geobacter and GOUTA19 which might be the key players responsible for the rapid attenuation of benzene and naphthalene. To the best of our knowledge, this is the first field experiment to demonstrate the potential for the combined iron and sulfate biostimulation to enhance B20 source-zone biodegradation.

Biodegradation of Polycyclic Aromatic Hydrocarbons in Mangrove Sediments Under Different Strategies: Natural Attenuation, Biostimulation, and Bioaugmentation with Rhodococcus erythropolis T902.1

Polycyclic aromatic hydrocarbons (PAHs) are pollutants that occur in mangrove sediments. Their removal by bacteria often depends on specific characteristics as the number of benzene rings they possess and their solubility. Their removal also depends on environmental factors, such as pH, temperature, oxygen, and the ability of the endogenous or exogenous microflora to metabolize hydrocarbons. With the aim of treating mangrove sediments polluted by hydrocarbons in a biological way, a biodegradation experiment was conducted using mangrove sediments artificially contaminated with a mixture of four PAHs. The study used Rhodococcus erythropolis as an exogenous bacterial strain in order to assess the biodegradation of the PAH mixture by natural attenuation, biostimulation, bioaugmentation, and a combination of biostimulation and bioaugmentation. The results showed that the last three treatments were more efficient than natural attenuation. The biostimulation/bioaugmentation combination proved to be the most effective PAH degradation treatment.

Biodegradation of trinitrotoluene (TNT) by indigenous microorganisms from TNT-contaminated soil, and their application in TNT bioremediation

ABSTRACTSoil and groundwater contaminated by munitions compounds is a crucial issue in environmental protection. Trinitrotoluene (TNT) is highly toxic and carcinogenic; therefore, the control and remediation of TNT contamination is a critical environmental issue. In this study, the authors characterized the indigenous microbial isolates from a TNT-contaminated site and evaluated their activity in TNT biodegradation. The bacteria Achromobacter sp. BC09 and Citrobacter sp. YC4 isolated from TNT-contaminated soil by enrichment culture with TNT as the sole carbon and nitrogen source (strain BC09) and as the sole nitrogen but not carbon source (strain YC4) were studied for their use in TNT bioremediation. The efficacy of degradation of TNT by indigenous microorganisms in contaminated soil without any modification was insufficient in the laboratory-scale pilot experiments. The addition of strains BC09 and YC4 to the contaminated soil did not significantly accelerate the degradation rate. However, the addition of an additional carbon source (e.g., 0.25% sucrose) could significantly increase the bioremediation efficiency (ca. decrease of 200 ppm for 10 days). Overall, the results suggested that biostimulation was more efficient as compared with bioaugmentation. Nevertheless, the combination of biostimulation and bioaugmentation using these indigenous isolates is still a feasible approach for the development of bioremediation of TNT pollution.

Treatment of petroleum drill cuttings using bioaugmentation and biostimulation supplemented with phytoremediation

ABSTRACTThis study sought to compare the effectiveness of bioaugmentation and biostimulation, as well as the combination of both techniques, supplemented with phytoremediation, in the decontamination of petroleum drill cuttings. Drill cuttings with relatively low concentration of total petroleum hydrocarbons (TPH) and metals were mixed with soil in the ratio 5:1 and treated with three different combinations of the bioremediation options. Option A entailed bioaugmentation supplemented with phytoremediation. Option B had the combination of biostimulation and bioaugmentation supplemented with phytoremediation. While biostimulation supplemented with phytoremediation was deployed in option C. Option O containing the drill cuttings–soil mixture without treatment served as untreated control. Fertilizer application, tillage and watering were used for biostimulation treatment, while spent mushroom substrate (Pleurotus ostreatus) and elephant grass (Pennisetum purpureum) were employed for bioaugmentation and phytoremediation treatment, respectively. The drill cuttings–soil mixtures were monitored for TPH, organic carbon, total nitrogen, pH, metal concentrations, and fungal counts, over time. After 56 days of treatment, there was a decline in the initial TPH concentration of 4,114 mg kg−1 by 5.5%, 68.3%, 75.6% and 48% in options O, A, B and C, respectively. Generally, higher TPH loss resulted from the phytoremediation treatment stage. The treated options also showed slight reductions in metal concentrations ranging from 0% to 16% of the initial low concentrations. The results highlight the effectiveness of bioaugmentation supplemented with phytoremediation. The combination of bioaugmentation and biostimulation supplemented with phytoremediation, however, may prove better in decontaminating petroleum drill cuttings to environmentally benign levels.

Bioremediation strategies of hydrocarbons and microbial diversity in the Trindade Island shoreline — Brazil

This study analyzed the microbial diversity colonizing the surface of an oil sample during its contact with water, off the Trindade Island coast and simulated the efficiency of eight different bioremediation strategies for this environment. The diversity analysis was performed using acrylic coupons that served as the support for an oil inclusion at sea. The coupons were sampled over 30days, and T-RFLP multiplex was employed to access the diversity of fungi, Bacteria and Archaea present on the oil surface. The bioremediation strategies were simulated in a respirometer. The results showed that the bacterial domain was the most dominant in oil colonization and that the richness of the species attached to the oil gradually increases with the exposure time of the coupons. The combination of biostimulation and bioaugmentation with a native population was proven to be an effective strategy for the remediation of oil off the Trindade Island shoreline.

Bioaugmentation of anaerobic sludge digestion with iron-reducing bacteria: process and microbial responses to variations in hydraulic retention time.

Although anaerobic digestion (AD) is a widely used option to manage waste activated sludge (WAS), there are some drawbacks related to its slow reaction rate and low energy productivity. This study examined an anaerobic WAS digester, augmented with an iron-reducing microbial consortium, relative to changes in microbial community structure and process performance at decreasing hydraulic retention times (HRTs) of 20 to 10 days. The enhanced methanation performance (approximately 40 % increase in methane yield) by the bioaugmentation was sustained until the HRT was decreased to 12.5 days, under Fe(3+)-rich conditions (ferric oxyhydroxide, 20 mM Fe). Enhanced iron-reducing activity was evidenced by the increased Fe(2+) to total Fe ratio maintained above 50 % during the stable operational phases. A further decrease in HRT to 10 days resulted in a significant performance deterioration, along with a drop in the Fe(2+) to total Fe ratio to <35 %, after four turnovers of operation. Prevailing existence of putative iron-reducing bacteria (IRBs) was identified by denaturing gradient gel electrophoresis (DGGE), with Spirochaetaceae- and Thauera-related organisms being dominant members, and clear dominance shifts among them with respect to decrease in HRT were observed. Lowering HRT led to evident shifts in bacterial community structure likely associated with washout of IRBs, leading to decreases in iron respiration activity and AD performance at a lower HRT. The bacterial community structure shifted dynamically over phases, and the community transitions correlated well with the changes in process performance. Overall, the combined biostimulation and bioaugmentation investigated in this study proved effective for enhanced methane recovery from anaerobic WAS digestion, which suggests an interesting potential for high-rate AD.

Bioremediation strategies for removal of residual atrazine in the boreal groundwater zone

Strategies for bioremediation of atrazine, a pesticide commonly polluting groundwater in low concentrations, were studied in two boreal nonagricultural soils. Atrazine was not mineralized in soil without bioremediation treatments. In biostimulation treatment with molasses, up to 52% of atrazine was mineralized at 10 °C, even though the degradation gene copy numbers did not increase. Incubations with radioactively labeled atrazine followed by microautoradiographic analysis revealed that bioremediation strategies increased the relative proportion of active degraders from 0.3 up to 1.9% of the total bacterial count. These results indicate that atrazine degradation might not solely be facilitated by atzA/trzN-atzB genes. In combined biostimulation treatment using citrate or molasses and augmentation with Pseudomonas citronellolis ADP or Arthrobacter aurescens strain TC1, up to 76% of atrazine was mineralized at 30 °C, and the atrazine degradation gene numbers increased up to 10(7) copies g(-1) soil. Clone libraries from passive samplers in groundwater monitoring wells revealed the presence of phylogenetic groups formerly shown to include atrazine degraders, and the presence of atrazine degradation genes atzA and atzB. These results show that the mineralization of low concentrations of atrazine in the groundwater zone at low temperatures is possible by bioremediation treatments.

Enhanced in situ bioremediation of groundwater contaminated by petroleum hydrocarbons at the location of the Nitex textiles, Serbia

This study provides insight into the enhanced in situ bioremediation applied for remediation of groundwater contaminated by petroleum hydrocarbons. Activities prior to the application of this remediation approach included removal of the source of groundwater contamination—an underground storage tank and contaminated sediments—from the unsaturated zone. The hydraulic feasibility of this remediation approach was proved by hydraulic conductivity characterization of the site. Enhanced in situ bioremediation was performed by a combination of biostimulation and bioaugmentation within the closed bipolar system (one extraction and two injection wells). Biostimulation was conducted by addition of nutrients, and stimulation of oxidation processes by injection of H2O2, into the aquifer. Bioaugmentation/re-inoculation was achieved by injection of a zymogenous consortium of hydrocarbon degrading microorganisms isolated from the contaminated groundwater. The average extraction capacity was 0.5 l/s, with an average injection capacity of 0.25 l/s per well. The efficiency of the remediation approach was measured by changes in the content of total petroleum hydrocarbon (TPH), total chemoorganoheterotrophic (TC) and hydrocarbon degrading (HD) microorganisms. After biostimulation and bioaugmentation, the number of TC and HD microorganisms started to increase. This was followed by a decrease of TPH concentration in the groundwater from an initial 6.8–0.5 mg/l at the end of the bioremediation. The applied remediation approach was highly efficient and very effective in reducing TPH to acceptable levels. Together, these facts provide strong evidence of its potential for remediation of groundwater contaminated by petroleum hydrocarbons.

Comparison among the Efficiency of Different Bioremediation Technologies of Atrazine–Contaminated Soils

In a previous study by the authors, toxicity screening of Atrazine-resistant soil bacteria from different contaminated soils resulted in 23-soil isolate best grown in the presence of herbicide Atrazine. They were identified according to their 16S rDNA sequencing into Enterobacter (E. cloacae), Bacillus (B. cereus and B. anthracis), Pseudomonas (P. aeruginosa, P. balearica, P. indica and P. otitidis), Ochrobactrum (O. intermedium) and Providencia (P. vermicola). The 23 resistant isolates were enriched in nutrient broth medium amended with 2 folds the recommended dose (RD) of Atrazine. The enrichment technique resulted in the selection of seven bacterial species belong to 4 genera (Enterobacter, Pseudomonas, Bacillus and Providencia) that were superior in their resistance to Atrazine and exhibited remarkable growth stimulation (70.7-88.7%). These four acclimatized and highly Atrazine-resistant strains have been selected and efficiently used for the degradation of Atrazine-contaminated soil. They were employed in different proposed bioremediation technologies including biostimulation (addition of nutrients to enhance the growth and activity of the indigenous microorganisms), bioaugmentation (seeding the most promising indigenous and exogenous Atrazine degraders to accelerate and help the indigenous bacterial population to achieve high and fast remediation of the contaminated soil) and finally a combination of biostimulation and bioaugmentation technologies to investigate the synergistic or suppressive effects of the two techniques. Results proved that bioaugmentation coupled with biostimulation is the most promising bioremediation technology since it is powerful, economical and environmentally friendly technique for decontamination of Atrazine-contaminated soils.

Biostimulation and Phytoremediation Treatment Strategies of Gasoline-Nickel Co-Contaminated Soil

This study investigated the potential effect of poultry dung (biostimulation) and stubborn grass (Sporobolus pyramidalis) (phytoremediation) on microbial biodegradation of gasoline and nickel uptake in gasoline-nickel-impacted soil. In addition, the potential stimulatory effects of nickel on hydrocarbon utilization were investigated over a small range of nickel concentrations (2.5–12.5 mg/kg). The results showed that an increase in nickel concentration increased hydrocarbon degraders in soil by a range of 8.4–17.2% and resulted in a relative increase in gasoline biodegradation (57.5–62.4%). Also, under aerobic conditions, total petroleum hydrocarbons’ (TPH) removal was 62.4% in the natural gasoline-nickel microcosm (natural attenuation), and a maximum of 78.5%, 85.7%, and 95.8% TPH removal was obtained in phytoremediation, biostimulation, and a combination of biostimulation- and phytoremediation-treated microcosms, respectively. First-order kinetics described the biodegradation of gasoline and nickel uptake very well. Half-life times obtained were 28.88, 18.24, 14.44, and 8.56 days for gasoline degradation under natural attenuation, phytoremediation, biostimulation, and combined biostimulation and phytoremediation treatment methods, respectively. The results indicate that these remediation methods have promising potential for effective remediation of soils co-contaminated with petroleum hydrocarbons and heavy metals.

Bioremediation of petroleum-contaminated soil by a combined system of biostimulation–bioaugmentation with yeast

This paper presents a study of the effect of a combined biostimulation–bioaugmentation treatment applied to a clay-loam soil contaminated with 16,300 mg/kg of total petroleum hydrocarbons (TPH), which comprised 51% saturated hydrocarbons and 31% aromatic hydrocarbons. The bioaugmentation was performed with yeast Candida tropicalis SK21 isolated from petroleum-contaminated soil. The strain was able to grow in a pH range of 3–9 in liquid culture, and the optimum pH was found to be 6 for both growth and biosurfactant production. At pH 6, 96% and 42% of TPH were degraded by the strain at the initial diesel oil concentrations of 0.5% and 5% (v/v), respectively. The remediation via inoculating the yeast removed 83% of TPH in 180 days while the experiment with the indigenous microorganisms alone removed 61%. Microbial enumeration showed that the yeast SK21 could grow good in the soil. It was also found that dehydrogenase and polyphenoloxidase activities in soil were remarkably enhanced by the inoculation of the yeast.