Integrated biological and UV-Fenton processes for the treatment of automobile service station wastewater Open Access

Automobile service stations are integral to urban infrastructure, providing essential vehicle maintenance, repair, and cleaning services. However, a major byproduct of these operations is the generation of large volumes of wastewater. This wastewater contains a complex mixture of organic pollutants, oils, greases, surfactants, detergents, heavy metals, and hazardous substances. Hydrocarbons (oil and grease) are the predominant pollutants, significantly increasing the wastewater's chemical oxygen demand (COD). The concentration of hydrocarbons varies depending on the type and frequency of vehicle services (Mallick & Chakraborty 2019). A single vehicle wash generates approximately 150–600 L of wastewater, and improper management of this effluent can lead to severe environmental contamination, posing risks to human health, aquatic ecosystems, soil quality, and biodiversity (Masum & Islam 2020). As urbanization and industrialization continue to expand, the demand for effective and sustainable wastewater treatment solutions has become increasingly critical.

The environmental impacts of untreated automobile service station wastewater are multifaceted. When discharged into rainwater sewer systems or natural water bodies, this effluent not only pollutes water resources but also leads to the formation of excessive foam, which diminishes urban esthetic appeal and disrupts aquatic ecosystems (Nguegang et al. 2019). Pollutants such as heavy metals and hydrocarbons persist in the environment, causing long-term damage to terrestrial and aquatic ecosystems (Sibanda et al. 2017). In addition, contaminated water sources pose significant risks to public health. A major ecotoxicological concern is the cumulative release of chemically and microbiologically contaminated effluents from numerous commercial and informal service stations. Continuous discharge of untreated wastewater introduces chemical and microbial pollutants into aquatic environments, degrading water quality, reducing oxygen levels, and introducing toxic compounds. This can result in both lethal and sub-lethal toxic effects on aquatic organisms, particularly fish (Nguegang et al. 2019).

Conventional treatment methods for automobile service station wastewater include physical, chemical, and biological processes (Van Pham et al. 2020). Each method has its advantages and limitations. Physical treatments, such as sedimentation and filtration, effectively remove suspended solids but struggle with dissolved pollutants. Chemical treatments, while efficient in eliminating a broad range of contaminants, often require high energy consumption, costly reagents, and produce hazardous byproducts that necessitate further treatment. Moreover, many traditional methods are costly and unsustainable for long-term wastewater management (Huang et al. 2023). Thus, the development of cost-effective, eco-friendly, and sustainable treatment technologies is essential (Hublikar et al. 2024).

Integrated treatment systems, which combine biological and chemical processes, offer promising solutions to overcome the challenges associated with conventional methods. Biological treatment using microorganisms such as Pseudomonas aeruginosa is particularly advantageous due to its ability to biodegrade a wide range of pollutants into harmless byproducts like carbon dioxide and water (Kumar et al. 2014; Shi et al. 2022). These microbes can be isolated from contaminated environments using serial dilution techniques and identified via molecular methods such as rRNA sequencing (Church et al. 2020). The efficacy of biological treatment can be enhanced through the use of biofilm carriers, which provide a stable environment for microbial growth, improve pollutant degradation efficiency, and offer cost-effective, environmentally friendly treatment solutions (Singh et al. 2022).

Beyond biological treatments, advanced oxidation processes (AOPs) have gained prominence as effective methods for degrading recalcitrant organic pollutants. AOPs generate highly reactive hydroxyl radicals (·OH) from oxidants like hydrogen peroxide or ozone in the presence of catalysts or UV light. These hydroxyl radicals are highly effective in breaking down complex organic contaminants that are difficult to remove through conventional methods. Among AOPs, the UV-Fenton process stands out for its efficiency and versatility. This method utilizes UV irradiation to enhance the reaction between hydrogen peroxide and ferrous ions, generating hydroxyl radicals that degrade organic pollutants (Mazumder & Mukherjee 2011; Ikhlaq et al. 2023). The UV-Fenton process operates under mild conditions, treats a wide variety of wastewater types, and significantly improves wastewater quality (Ayat et al. 2021).

Existing integrated treatment approaches for automobile service station wastewater often combine biological and physicochemical methods, such as activated sludge, coagulation-flocculation, or membrane filtration. While these methods can achieve moderate pollutant removal, they often suffer from high operational costs, sludge generation, and incomplete degradation of recalcitrant pollutants. Given the urgent need for improved wastewater treatment, this study focuses on integrating biological treatment with AOP. Our approach enhances biological degradation efficiency by isolating and identifying oil-degrading bacteria and employing biofilm carriers. Additionally, the UV-Fenton process is incorporated to facilitate the oxidative breakdown of persistent pollutants. This combined approach offers a sustainable, cost-effective, and environmentally friendly solution that not only improves wastewater quality but also reduces the environmental footprint of automobile service stations.

Wastewater samples were collected at an automobile service station in Kodakara, Thrissur, Kerala, India. The samples were stored in a refrigerator before analysis. The physicochemical parameters analyzed in this study included COD, turbidity, and oil and grease concentrations (Kumar et al. 2014). The dichromate method (APHA 5220D) was used to quantify COD, and a calibrated nephelometer was used to assess turbidity in accordance with APHA 2130B. The APHA 5520B gravimetric method, which involves extracting samples with n-hexane and measuring them after solvent evaporation, was used to quantify oil and grease.

Post the characterization tests, the samples were homogenized by shaking and serially diluted for plating. A total of 100 μL of the sample was spread onto a freshly prepared nutrient agar medium (Himedia, India) for isolation of the organisms. Nutrient agar was used as the isolation medium due to its effectiveness in supporting the growth of a wide range of heterotrophic bacteria, including those capable of degrading organic pollutants present in automobile service station wastewater. The selection of nutrient agar was based on its well-documented ability to facilitate the isolation of oil-degrading and hydrocarbon-utilizing bacteria, as reported in previous studies (Kumar et al. 2014). Serial dilutions were performed up to 10⁵, and the plates were incubated at 27 °C for 24 h. Colonies displaying vigorous growth were selected as isolates and stored as pure culture slants at 4 °C until further study (Kumar et al. 2014). The selected bacterial isolates were subcultured on agar plates, and colonies from these subcultures were transferred onto tributyrin agar using the swab technique. The plates were incubated at 37 °C for 48 h to assess lipolytic activity. Tributyrin agar (Himedia, India), formulated with 0.5% peptone, 0.3% yeast extract, 1% tributyrin, and 2% agar, served as the medium for detecting oil-degrading capabilities. Clear zones surrounding bacterial colonies indicated lipase activity, demonstrating their potential for oil degradation (Martha et al. 1995). Samples exhibiting the largest clear zones were selected for subsequent analysis. The selected colonies were screened for morphological and biochemical identification. 16S ribosomal RNA (rRNA) sequencing was carried out for molecular-level identification (Hussein et al. 2023). The selected clone was identified, and this strain was used for all the further studies. The growth was optimized by selecting a nutrient-rich medium, maintaining a pH of approximately 7.0, and maintaining the temperature at 37 °C (Kumar et al. 2014). Aeration and agitation were carefully controlled to prevent oxygen limitation. The nutrient concentrations of carbon, nitrogen, and phosphorus were fine-tuned, and further optimization was performed using response surface methodology (RSM).

RSM, a robust statistical tool, was employed to optimize bacterial growth conditions by evaluating the effects and interactions of multiple factors. This method utilizes mathematical modeling to design experiments that investigate the influence of variables such as pH and incubation time on key response variables, including bacterial growth and pollutant degradation efficiency. The study incorporated analysis of variance (ANOVA) to assess the statistical significance of individual factors and their interactions. Visualizations, including response surface and contour plots, were generated to elucidate the relationships between variables and response outcomes, enabling the determination of optimal operational ranges (Ayat et al. 2021). The experimental parameters for biological treatment and the UV-Fenton process are detailed in Supplementary 1 – Tables 1 and 2. The experimental ranges of parameters were fixed based on a review of the literature and preliminary experiments.

Biological treatment was carried out as laboratory-scale batch experiments. Biofilm carriers coated with selected bacteria were employed in the biological treatment process due to their demonstrated efficacy in degrading organic pollutants. These carriers, characterized by a high surface area, enhance microbial adhesion and biofilm growth, thus promoting efficient pollutant breakdown. The optimal pH and incubation time for the biological treatment process were determined using RSM. Immobilized biofilms of selected bacteria were introduced into 13 wastewater samples, which were continuously aerated and agitated. The performance of pollutant degradation was evaluated through parameters such as COD, turbidity, and reductions in oil and grease content. For the second treatment approach, a UV-based advanced oxidation process (UV/H₂O₂) was utilized, building on previous methodologies. RSM was applied to identify the optimal concentrations of Fe²⁺ and H₂O₂. In laboratory-scale batch experiments, wastewater samples were adjusted to specific pH levels before the addition of FeSO₄·7H₂O and 30% hydrogen peroxide, which facilitated the generation of hydroxyl radicals (Ayat et al. 2021). The mixture was subjected to magnetic stirring and UV radiation at 254 nm for 20–30 min in a controlled environment. Degradation efficiency was assessed by monitoring COD and other physicochemical parameters, which confirmed the breakdown of complex organic pollutants by hydroxyl radicals (Yang et al. 2014).

The characterization of wastewater collected from the automobile service station indicated high levels of pollutants. The COD was recorded at 650 mg/L, while turbidity was measured at 150 NTU. Additionally, the oil and grease concentration was found to be 87 mg/L. The primary parameters assessed were COD, turbidity, and oil and grease contents (Kumar et al. 2014). Studies indicate that wastewater from these facilities typically contains high levels of COD, with concentrations reported as high as 7,054 mg/L (Suwannarat et al. 2022). The presence of toxic elements such as lead, zinc, and nickel has also been documented, posing risks to both environmental and human health (Le et al. 2019; Singh et al. 2021). Additionally, the wastewater often includes petroleum hydrocarbons, which can be effectively degraded by specific microbial consortia (Amran et al. 2022). The treatment of such wastewater is critical, as improper disposal can lead to severe ecological impacts, including degradation of surface water quality and harm to aquatic life (Rai et al. 2020).

During the biological treatment process, optimal pH and incubation time were determined using RSM. P. aeruginosa was immobilized on biofilm carriers and introduced into 13 wastewater samples. Biofilm carriers made of high-density polyethylene (HDPE) with a specific surface area of 500 m²/m³ (Figure 2(b)). Following incubation, the reductions in COD, turbidity, and oil and grease contents were measured. Optimal degradation occurred at pH 6 with a 45-h incubation period. Contour and surface plots illustrated the degradation trends, and ANOVA was performed to validate the significance of the model (Kumar et al. 2014).

In ANOVA, the F-value represents the ratio of the mean square of the regression to the mean square of the error. Probability values (P-values) were used to assess model significance, with values below 0.05 indicating acceptability. In this case, the F-value of 17.98 and a low probability value of 0.001 suggest model significance for COD removal % versus pH and time. The R² value, indicating goodness of fit, was 92.78%, exceeding the threshold of 80% for acceptance. Hence, the model was statistically significant and acceptable (Sirisha et al. 2012). The F-values and P-values are given in Table 1.

The model is deemed statistically significant for turbidity removal % versus pH and time, supported by an F-value of 51.23 and a low probability value shown in Table 1. A regression coefficient R² value of 97.34% further confirms the significance.

The regression coefficient R² value of 94.61% further confirms the significance (Sirisha et al. 2012).

RSM contour and surface plots show that pH and time affect wastewater oil and grease removal efficiency. Oil and grease removal is most effective at pH levels of 6–8, when microbial activity is highest and breakdown is accelerated (Emara et al. 2019). Electrocoagulation at ideal pH levels removed 99% of oil and grease, highlighting the importance of pH control (Emara et al. 2019). Time-pH interaction is also important; longer treatment periods usually result in better removal efficiencies. Studies reported that extending treatment time enhanced oil and grease removal by up to 90% under ideal conditions. RSM contour plots show that both parameters must be optimized to maximize pollution removal (Kamaruddin et al. 2019).

In the UV-Fenton system, the combination of ferrous sulfate (FeSO₄) and hydrogen peroxide (H₂O₂) produces hydroxyl radicals, which are potent oxidants that effectively oxidize a wide range of organic pollutants in wastewater. These radicals break down complex organic molecules into simpler and less harmful compounds, leading to significant reductions in COD, turbidity, and oil and grease levels. Maximum degradation percentages of 88, 94, and 71% were achieved for COD, turbidity, oil, and grease, respectively. The graphs illustrating these results were generated using RSM (Sirisha et al. 2012; Ayat et al. 2021).

While highly effective, the UV-Fenton process may also generate various intermediate byproducts, some of which could pose environmental or health risks. Partial oxidation of complex organic pollutants can result in the formation of aldehydes, ketones, carboxylic acids, and short-chain hydrocarbons. However, complete mineralization ensures that no harmful intermediates remain in the treated effluent (Sirisha et al. 2012).

The composite desirability chart illustrates the optimization of three response variables in relation to pH and time: turbidity removal %, COD removal %, and oil and grease removal % (Figure 7(a)). The graph's data points indicated that the optimal desirability values were 93.18, 82.76, and 67.38, respectively. These optimal results were achieved at a pH of 6.25 and an incubation time of 45.64 h. (Sirisha et al. 2012; Sibanda et al. 2017).

Although both biological and UV-Fenton treatments showed promising results, they each had limitations. The biological treatment only achieved a 67% reduction in oil and grease, whereas the UV-Fenton treatment reached 71%. However, the reductions in COD and turbidity were similar between the two methods. To improve treatment efficiency and wastewater quality, we combined both approaches. This combined method yielded significantly better results than either treatment alone did. The combined treatment led to substantial reductions in COD (89%), turbidity (96%), and oil and grease levels (78%). Combining biological treatment and UV-Fenton methods to remove organic contaminants from wastewater seems promising. The UV-Fenton process generates hydroxyl radicals (·OH) through the synergy of ultraviolet light, hydrogen peroxide (H₂O₂), and ferrous ions (Fe²⁺), effectively decomposing various pollutants (Bensalah et al. 2019; Ran & Li 2019; Ilhan 2024). Compared to standard Fenton procedures, this technique greatly enhances the degrading efficiency of persistent organic pollutants (Gomez-Herrero et al. 2019; Hassan 2024).

The integration of biological treatment with the UV-Fenton process presents a synergistic approach to efficiently degrade complex pollutants in automobile service station wastewater. Biological treatment effectively reduces organic load through microbial metabolism, targeting biodegradable contaminants such as oils and greases, while UV-Fenton treatment ensures the degradation of persistent and non-biodegradable organic compounds through hydroxyl radical oxidation (Ayat et al. 2021). The sequential combination of these processes enhances overall treatment efficiency by leveraging their distinct mechanisms, leading to higher pollutant removal rates compared to individual treatments. The synergistic effect arises from the pretreatment role of biological degradation, which lowers the initial COD and organic content, making the subsequent UV-Fenton oxidation process more efficient (Huang et al. 2023). By first reducing biodegradable organic matter, microbial treatment minimizes the scavenging effect of organic compounds on hydroxyl radicals in the UV-Fenton process, allowing for more targeted degradation of recalcitrant pollutants (Shi et al. 2022). As a result, the combined treatment achieved 89% COD removal, 96% turbidity reduction, and 78% oil and grease removal, surpassing the efficiency of either method applied separately.

Although biological treatment is cost-effective and environmentally friendly, its efficiency in removing recalcitrant pollutants is limited, often requiring extended retention times. In contrast, the UV-Fenton process achieves rapid degradation but involves higher chemical costs and sludge generation, making long-term operation expensive (Ikhlaq et al. 2023). By integrating both methods, the overall operational costs are reduced, as biological treatment decreases the required chemical dosage for the UV-Fenton process. Additionally, the hybrid approach minimizes sludge production, reducing disposal costs while improving water quality (Van Pham et al. 2020).

UV-Fenton followed by biological treatment or biological treatment followed by UV-Fenton. Research shows that combining biological and UV-Fenton treatments improves therapy efficacy. UV-Fenton pretreatment of wastewater can improve biodegradability, making it more suitable for biological treatment (Gomez-Herrero et al. 2019; Ayed et al. 2021). Beneficial for wastewater with resistant substances, the first oxidative treatment simplifies complex molecules for biodegradability (Metin & Çifçi 2023; Hassan 2024). Studies have shown that the UV-Fenton technique can remove dyes and pharmaceuticals with COD reductions above 90% (Benassi et al. 2021; Wang 2024). UV radiation promotes reactive species formation and organic matter oxidation, improving treatment outcomes (Sadiq & Saleh 2023; Watcharenwong et al. 2023). Biological treatment degrades biodegradable organic matter, significantly reducing COD and other pollutants. UV-Fenton oxidation of complex organic pollutants can produce aldehydes, ketones, and carboxylic acids, which may be toxic to microbial populations (Sirisha et al. 2012). Applying biological treatment first prevents these toxic intermediates from accumulating before microbial degradation, allowing bacteria to function optimally without inhibition. This ensures that the UV-Fenton step is focused on breaking down recalcitrant pollutants, rather than wasting hydroxyl radicals (·OH) on easily degradable organic matter (Shi et al. 2022). Treating wastewater biologically first reduces the amount of chemicals (H₂O₂ and Fe²⁺) needed for UV-Fenton treatment, making the process more economical and sustainable (Van Pham et al. 2020). In conclusion, biological treatment combined with UV-Fenton processes is a powerful wastewater treatment technique that overcomes persistent organic contaminants and improves efficiency.

This study demonstrated that the integration of biological and UV-Fenton processes offers an efficient and effective approach for treating wastewater from automobile service stations. By isolating, characterizing, and optimizing oil-degrading microorganisms, particularly P. aeruginosa, the biological treatment achieved significant reductions in wastewater contaminants, including 83% COD, 93% turbidity, and 67% oil and grease. The UV-Fenton process further enhanced the treatment, yielding reductions of 89% in COD, 95% in turbidity, and 72% in oil and grease production. The combined use of the biological and UV-Fenton treatments resulted in an even more pronounced improvement, with reductions of 89% in COD, 96% in turbidity, and 78% in oil and grease. This clearly demonstrates the synergistic potential of the combined approach, outperforming individual methods. Biological treatment effectively initiates the degradation of pollutants, whereas the UV-Fenton process further breaks down residual organic matter and other contaminants. Despite these operational obstacles, this integrated approach improves sustainability by maximizing resource consumption and diversifying treatment options, making it a feasible and environmentally benign alternative.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.