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Article

Investigation of the Impact of Wastewater from Waste Oil In-Stallation on the Activated Sludge Process, to Ensure the Proper Operation of Municipal Wastewater Treatment Plant

by
Agnieszka Bluszcz
1,2,*,
Krzysztof Barbusiński
1,
Barbara Pieczykolan
1 and
Mohamed Alwaeli
3,*
1
Department of Water and Wastewater Engineering, Silesian University of Technology, 18 Konarskiego Street, 44-100 Gliwice, Poland
2
PCC Energetyka Blachownia Limited Liability Company, 15 Szkolna Street, 47-225 Kędzierzyn-Koźle, Poland
3
Department of Technologies and Installations for Waste Management, Silesian University of Technology, 18 Konarskiego Street, 44-100 Gliwice, Poland
*
Authors to whom correspondence should be addressed.
Water 2026, 18(1), 108; https://doi.org/10.3390/w18010108
Submission received: 29 October 2025 / Revised: 10 December 2025 / Accepted: 19 December 2025 / Published: 1 January 2026
(This article belongs to the Special Issue Sustainable Biodegradation and Bioremediation of Organic Contaminants in Aquatic and Terrestrial Environments)
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Abstract

The study evaluated the feasibility of using the activated sludge process to treat real wastewater from used oil installations containing petroleum hydrocarbons, boron (B), and adsorbable organic halides (AOX). The aim was to determine the maximum ratio of this wastewater that could be added to the influent without impairing treatment efficiency. Tested shares ranged from 0.50% to 1.90%. An initial 1.30% of the tested share caused process instability, reflected in the elevated total nitrogen (TN) levels in treated wastewater. After reducing the share to 0.50%, an adaptation of the activated sludge was observed, manifested by a decrease in TN concentration to below 15.0 mg N/L. For the most favorable share of 1.60% (0.38 ± 0.10 kgBOD5/kgMLSS d, 0.51 ± 0.14 kgCOD/kgMLSS d), the removal efficiencies of chemical oxygen demand (COD), biochemical oxygen demand (BOD5), TN, and total phosphorus (TP) were 95.0% ± 1.5, 99.1% ± 0.2, 89.3% ± 2.7, and 94.0% ± 5.0, respectively. Increasing the share to 1.90% decreased treatment efficiency and exceedances of COD, BOD5, TN, and TP occurred. At this ratio, an increase in ammonium nitrogen (NH4+-N) and TN concentrations was observed, indicating the inhibition of nitrification. However, the average concentrations of mineral oil index, AOX and B in the treated wastewater remained within permissible levels throughout the study.

1. Introduction

The possibility of biotreating real industrial wastewater containing used oils and lubricants is essential due to their possible release into the natural environment. Residual oils in wastewater, because of their low affinity for water, exhibit poor biodegradability, which can consequently have adverse effects on human health and the environment [1,2]. The biodegradation of petroleum occurs slowly and may disrupt biological processes in living organisms. Petroleum-based contaminants can interfere with the functioning of the digestive systems of marine organisms, potentially leading to their death [3,4]. Effluents from the refinery industry adversely affect plants as well as aquatic organisms and may affect surface and groundwater. Even small quantities of oil in water can disturb the functioning of aquatic ecosystems by reducing light penetration and oxygen transport, consequently negatively affecting photosynthesis and plant growth [4,5,6]. Moreover, with the projected 50% increase in global energy demand by 2050, it is important to seek sustainable actions for the environment [7]. Oil-containing pollutants can enter ecosystems through ship accidents, oil extraction spills, transportation, tank leakiness, oil production facilities, as well as facilities for regenerating used oils [8].
Industrial development has led to the widespread presence of oils in wastewater, posing a significant challenge for their treatment [9]. Lubricating oils are produced from petroleum through dewaxing and solvent extraction processes. They consist of 85% base oil and additives [10,11]. Oils are classified into mineral, synthetic, and natural types [12]. Mineral oils of Group I (according to the API classification) have low biodegradability, whereas non-conventional mineral oils of Groups II–III exhibit higher susceptibility to biodegradation. Groups IV and V include poly(α-olefin) (PAO) oils [13]. Approximately 61% of waste oils undergo regeneration, while the remainder is used for energy recovery [14]. They also serve as alternative fuels [15]. Their main function is to minimize friction and prevent corrosion [10] but the presence in wastewater hinders the treatment processes [9]. Wastewater from oil regeneration consists of alkanes, as well as petroleum-derived hydrocarbons ranging from C20 to C40 [16], and also contain aromatic hydrocarbons, cycloalkanes, chlorinated compounds, and heavy metals [9]. The low solubility of oil in water limits its biochemical degradation [17], whereas used oils contain additional unidentified hazardous contaminants [18]. Industrial wastewater from used oil installations is characterized by high chemical oxygen demand (COD), high concentrations of ammonium nitrogen (NH4+-N), sulfides (S2−), petroleum hydrocarbons, and toxic substances [19]. These may include, among other substances, boron (B) derived from additives, such as boron nitride [20], which reduces friction and wear of equipment parts, consequently lowering CO2 emissions. At low concentration, boron is an essential micronutrient for both organisms and plants [21], however prolonged exposure to elevated boron levels can cause fatigue, dermatitis, hair loss, or hormonal disorders in humans [22], and also exert toxic effects on the environment [23]. Continuous consumption of water containing boron may increase the risk of neurological, reproductive, and cardiovascular disorders in humans [24]. Effluents from used oil installations may also contain adsorbable organic halides (AOX). Halogenated by-products pose toxic risks to living organisms [25]. AOX include compounds such as chlorinated hydrocarbons, phenols, catechols, chloroform, chlorates, and resin acids [26]. AOX exert negative effects on aquatic organisms, affecting respiratory stress, mutagenicity, and carcinogenicity [27]. Moreover, compounds formed through reactions between halogens and organic compounds have the potential for bioaccumulation in aquatic ecosystems [28].
Treatment of wastewater containing waste oils includes chemical approaches (e.g., adsorption, bioaccumulation), physical approaches (e.g., distillation, coagulation), and membrane-based techniques, but these methods are often insufficient [17]. Pretreatment of such industrial effluents is important, as lipids undergo hydrolysis, which consequently leads to the formation of solid deposits within the sewage system [6]. Physicochemical methods have certain limitations, such as high land requirements and significant energy consumption [29]. Besides substantial economic expenditures, they generate large amounts of secondary pollutants in the form of sludge [30,31]. Membrane filtration is characterized by high contaminant separation efficiency without the need for chemical additives. However, membrane fouling leads to a reduction in flux accompanied by increased energy consumption, consequently shortening membrane lifespan [32]. The development of novel membrane materials with high selectivity and permeability forms the basis for membrane separation [33]. Therefore, in recent years, guided by green chemistry principles, methods employing advanced oxidation processes, membrane separation, electrochemical treatments, or bioaugmentation have gained popularity. An alternative yet effective method for oil extraction from wastewater is electrocoagulation (EC) [34]. This process combines coagulation, flotation, and sedimentation with electrochemical treatment. EC offers multiple advantages over conventional treatment methods, including simple equipment design and operation, lower operating costs, and reduced sludge production [34].
The use of biological methods is justified due to their simplicity and low operating costs [6,35]. However, industrial wastewater, due to its complex chemical composition and potential for environment accumulation, represents a challenging matrix for conventional biological treatment [4]. In particular, the presence of fats, oils, or lubricants in wastewater often renders bacterial-based methods ineffective [6]. This is why a promising solution, in the context of a sustainable environmental approach, is presented by algal–bacterial consortia (ABC) [29]. The benefit of such systems lies in the photosynthetic activity of algae, which maintains stable oxygen concentrations. Algae provide bacteria with oxygen and organic compounds, while bacteria supply algae with carbon dioxide, nitrogen, and phosphorus [29]. Consequently, external aeration demand is significantly reduced, simultaneously decreasing energy requirements compared to conventional autotrophic systems [36]. In this symbiotic arrangement, bacteria efficiently biodegrade, through decomposition or adsorption, organic pollutants, which are often toxic to the natural environment [6]. Furthermore, the emerging algal biomass can subsequently be utilized for biodiesel production [29]. Natural catalytic materials such as chitosan display environmentally friendly approaches [18,37]. In research, chitosan derived from shrimp, mussel, or crab shells has been used for the degradation of used lubricating oils [18,37]. Hybrid techniques can support wastewater treatment as an additional step in removing contaminants. Adsorbers, particularly biocomposites made of natural and synthetic materials, are promising. Algae, such as Cladophora, can bind oils and dyes due to the structure of their cell walls [4]. Microalgae such as Scenedesmus vacuolatus have also been used for the removal of lubricating oils and hydrocarbon compounds [38]. Combination of algae with magnetic nanocomposites enables efficient separation and the recovery of the adsorbent [4]. However, for the treatment of refinery wastewater, the synergy of activated carbon adsorption and electrocoagulation (EC) has proven to be effective [5].
The presented research was carried out under laboratory-scale conditions to assess the feasibility of treating real industrial wastewater from used oil installations using the activated sludge process. This wastewater contained characteristic pollutants such as petroleum hydrocarbons, B, and AOX. Research on the treatment of this type of industrial wastewater is becoming a priority due to the stability of petroleum substances, which consequently affects their susceptibility to biodegradation [39]. Although biological treatment of hydrocarbon-rich industrial wastewater has been widely studied, research on real effluents from used oil recovery plants remains scarce. Existing studies often rely on synthetic wastewater, which does not fully capture the complexity or toxicity of actual industrial discharges [40,41,42]. Moreover, little is known about how combined pollutants such as hydrocarbons, B, and AOX influence activated sludge performance, particularly with respect to nitrification and microbial community stability. Therefore, further experimental work using real wastewater, such as that conducted in this study, is crucial to fill these knowledge gaps. The main objective was to determine the maximum concentrations of pollutant that can be introduced into the system without impairing activated sludge activity.

2. Materials and Methods

2.1. Real Wastewater

In this study, real industrial wastewater from used oil installations, pretreated by distillation, and domestic wastewater entering the wastewater treatment, located within industrial facilities in the southern region of Poland, were utilized. The distillation process is carried out continuously with flow 400 L/h, at temperature 35 °C, and pressure −950 hPa. Wastewater from this installation is generated by the recycling of used oil (such as engine oils, lubricants, and industrial oils), which may contain aromatic hydrocarbons, polycyclic aromatic hydrocarbons, and saturated and unsaturated hydrocarbons. The treatment plant has a design capacity of 14,400 m3/d. In addition to municipal wastewater, the plant also receives industrial wastewater of various characteristics. The biological treatment system employs activated sludge, operating in a denitrification reactor and two parallel nitrification reactors. The sludge operates at a solids retention time (SRT) of 18 days, which was considered in the study. Following the biological reactors, the sludge-containing wastewater is pumped to two radial secondary clarifiers.
During the research, the ratio of industrial wastewater in the mixture feeding the laboratory-scale treatment system was gradually increased until a distinct negative impact on the system’s operational stability was observed. Characteristics of industrial wastewater during the seven research phases (P1–P7) is presented in Table 1. The phases were defined by gradually increasing the share of industrial wastewater in the influent to the laboratory-scale treatment system. In addition to the characteristic contamination: adsorbable organic halides (AOX), boron (B), mineral oil index (MOI), indicators in wastewater, chemical oxygen demand (COD), total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP) were also monitored. In the present study, only industrial wastewater after the distillation treatment process was used. To emphasize the importance of pretreating industrial wastewater before direct introduction to the treatment system, pollutant concentrations in the wastewater prior to distillation are also presented (Table 1).

2.2. Experimental System

The laboratory-scale installation based on an activated sludge system was designed as a replica of the actual biological wastewater treatment plant (Figure 1). The model includes:
  • Raw wastewater tank with a working volume of 30.0 L, daily replenished with fresh wastewater, to prevent putrefaction. A peristaltic pump (KAMOER X1 PRO T2) operating at a flow rate of 1.0 L/h was used to feed wastewater into the denitrification reactor. During the adaptation (P0), only domestic wastewater was introduced into the raw wastewater tank. Subsequently, during the research phases on industrial wastewater (P1–P7), domestic wastewater was mixed with industrial wastewater after distillation in the tank. Since the wastewater was phosphorus-deficient, an orthophosphoric acid solution of 73% concentration (v/v) was added.
  • A denitrification reactor with a working volume of 8.0 L was equipped with a mechanical stirrer (JOANLAB OS-15S), maintaining a rotational speed of 300–400 rpm. The denitrification reactor was connected to the nitrification reactor via gravitational flow. In the denitrification reactor, bacteria use the carbon source to reduce nitrates to N2.
  • Nitrification reactor with a working volume of 16.0 L was equipped with a Sicce Voyager Nano 100 stirrer, with a capacity of 1000.0 L/h. The reactor was aerified using a dual-channel air pump with aeration curtains. In the nitrification reactor, ammonia is oxidized to nitrites, then to nitrates.
  • Secondary clarifier with a working volume of 20.0 L. A coagulant, in the form of a 35% (v/v) ferric chloride (III) solution, was added to the sludge-containing wastewater to enhance sludge sedimentation. The tank was equipped with a magnetic stirrer operating at 120 rpm.
  • For internal and external recirculation, a KAMOER X1 PRO T2 peristaltic pump was used, operating at flow rates of 4.0 and 1.5 L/h, respectively. Internal recirculation allows for the reduction in total nitrogen, while the external recirculation maintains a stable bacterial population.
All tanks were constructed from transparent material to allow observation of the activated sludge process. The activated sludge was collected from a biological treatment plant located in southern Poland. The average mixed liquor suspended solids (MLSS) concentration in the nitrification reactor during P0 was 3889 ± 672 mg/L. During the main experiments (P1–P7), the average MLSS concentration ranged from 3968 ± 452 mg/L to 4865 ± 468 mg/L. The intended adaptation period was 36 days, corresponding to two SRT in the actual wastewater treatment plant. After the adaptation period, the main experiments were initiated, involving feeding the system a mixture of domestic wastewater and industrial wastewater from used oil installations.

2.3. Data and Study Area

The operation of the laboratory-scale installation was monitored twice a week. All measured parameters were performed three times. The treated wastewater was analyzed based on the retention time of the raw wastewater in the experimental installation. The planned duration of each percentage share phase was 1.5–2.0 SRT (27–36 days). The duration and activated sludge loading of the adaptation and the respective phases of the main experiment are presented in Table 2. Phase P0 (76 days) was extended to allow the activated sludge to acclimate to laboratory conditions. Phases P1 (7 days) and P7 (21 days) were shortened due to the clearly observed negative impact of industrial wastewater on the performance of the activated sludge.
Measurement of pH [43] and electrical conductivity [44] were conducted in the wastewater using HACH LANGE probes (Loveland, CO, USA) coupled with an HQ4300 meter (HACH LANGE, Loveland, CO, USA). To assess wastewater treatment efficiency, COD, TN, and TP were measured using a spectrophotometric method with HACH LANGE rapid test kits (Berlin, Germany). Additionally, biological oxygen demand (BOD5) was monitored using the respirometric method with the WTW OxiTop system (Xylem, Weilheim in Oberbayern, Germany).
The concentrations of NH4+-N and orthophosphates (PO43−-P) were determined using the continuous flow analysis (CFA) method with a SAN++® analyzer (SKALAR) (Breda, The Netherlands). During the determination of orthophosphates, the procedure was based on the PN-EN ISO 15681-2:2006 standard [45], while the determination of NH4+-N was performed in accordance with the PN-EN ISO 11732:2007 standard [46]. To analyze the process of ammonium nitrogen oxidation to nitrates, nitrate (NO3−-N) in treated wastewater was determined using the colorimetric method with sodium salicylate [47]. Additionally, nitrite (NO2−-N) concentrations in treated wastewater were monitored using the colorimetric method with sulfanilic acid and 1-naphthylamine [48].
The efficiency of the secondary clarifier was determined by measuring suspended solids as TSS, according to PN-EN 872:2007 [49], using glass fiber filters (ALCHEM, Torun, Poland).
Characteristic contamination indicators, such as B and AOX, were identified using the spectrophotometric method with HACH LANGE rapid test kits (Berin, Germany). The concentration of petroleum hydrocarbons C10–C40 as the MOI was determined according to PN-EN ISO 9377-2:2003 [50], using solvent extraction and gas chromatography (GC-FID).

3. Results and Discussion

The activated sludge adaptation stage was the first phase of the study (referred to as P0) and served as the control period. Concentrations of pollutants and their removal rates are given in Table 3 and Table 4. The 76-day duration of the P0 phase was chosen to ensure stable operation of the laboratory purification system, particularly in maintaining consistent removal efficiency of carbon, nitrogen, and phosphorus compounds. The results obtained during this phase served as a reference point for evaluating the performance and trends observed in phases P1–P7. After stabilizing the work of microorganisms in the nitrification reactor in P0, the main experiments were initiated, gradually increasing the percentage ratio of industrial wastewater in the mixture feeding the treatment system until a distinct negative impact of the tested wastewater on the operational stability of the system was observed. As a result, six ratios of industrial wastewater to domestic wastewater were examined: 0.50%, 0.75%, 1.00%, 1.30%, 1.60%, and 1.90%, corresponding to the experimental phases P2–P7. The duration of each research phase is given in Table 2.
During the experiments (P1–P7), the average pH and electrical conductivity of the influent to the experimental system were 7.2 ± 0.1 and 2266 ± 444 µS/cm, respectively, whereas those of the treated effluent were 7.0 ± 0.1 and 1828 ± 350 µS/cm, respectively. The wastewater pH remained stable throughout all experimental phases. However, the electrical conductivity of both the influent and treated wastewater remained stable during phases P1–P6: 2196 ± 418 µS/cm and 1760 ± 296 µS/cm. In phase P7, the highest average electrical conductivity was recorded in the influent and treated wastewater, at 2830 ± 132 µS/cm and 2375 ± 271 µS/cm, respectively, due to the largest share of industrial wastewater.
The average value of TSS was 25.0 ± 14.5 mg/L in the outflow, observed across all experimental phases (P1–P7). Two exceptions were noted, in which TSS exceeded the permissible limit during phase P1 (an average value of 35.5 ± 0.7 mg/L) and phase P7 (an average value of 54.8 ± 11.2 mg/L) (permissible limit: 35 mg/L). The observed TSS values may reflect changes in the characteristics of the activated sludge flocs due to the gradual increase in the ratio of the tested industrial wastewater.
Changes in COD values in the influent and treated wastewater for all research phases are presented in Figure 2. The average COD values in the influent during phases P2–P7 gradually increased from 1103.4 ± 240.9 mg O2/L to 2810.8 ± 427.0 mg O2/L. During phases P1–P6, the treated wastewater met the permissible COD limit of 125 mg O2/L, with one exception. The following research phase, P7, confirmed the negative impact of industrial wastewater on treatment efficiency, with an average COD value of 185.8 ± 61.2 mg O2/L. The average BOD5 values in the influent to the experimental system also gradually increased during phases P2–P7, from 718.2 ± 140.1 mg O2/L to 2791.7 ± 280.0 mg O2/L. The permissible BOD5 value (15 mg O2/L) in the treated wastewater was achieved in phases P2–P6. In phases P1 and P7, exceedances of this parameter were observed, with average values of 25.0 ± 0.5 mg O2/L and 43.3 ± 9.8 mg O2/L, respectively. Concentration and the treatment efficiencies for individual pollution indicators (COD, BOD5, TN, NH4+-N, TP, and PO43−-P) are presented in Table 3 and Table 4.
The initial industrial wastewater ratio was set at 1.30%, based on information from the water permit (Q_avg = 30 m3/d; 10,950 m3/year). During period P1, the average values of COD, BOD5, TN, TP, AOX, and B in the treated wastewater were 95.9 ± 18.5 mg O2/L, 25.0 ± 0.5 mg O2/L, 39.8 ± 5.1 mg N/L, 0.19 ± 0.01 mg P/L, 0.362 ± 0.097 mg/L, and 0.201 ± 0.004 mg B/L, respectively (Figure 2 and Figure 3a,b). This share negatively affected the operational stability of the activated sludge, as reflected by an exceedance of the TN concentration in the treated wastewater. Therefore, after 7 days, the industrial wastewater share was reduced to 0.50% (P2), which led to stabilization of the activated sludge treatment process after 27 days. During the 27–45 day period, the average COD and TN values in the treated wastewater were 50.2 ± 5.1 mg O2/L (Figure 2) and 8.4 ± 0.9 mg N/L (Figure 3a), respectively.
During phases P3, P4, and P5, no adverse impact of the industrial wastewater on the activated sludge process was observed. The average concentrations of COD, BOD5, TN, TP, AOX, and B in the treated wastewater over periods P3–P5 were 79.4 ± 12.1 mg O2/L, 10.7 ± 1.9 mg O2/L, 7.2 ± 1.8 mg N/L, 0.50 ± 0.44 mg P/L, 0.249 ± 0.102 mg/L, and 0.315 ± 0.216 mg B/L, respectively (Figure 2 and Figure 3a,b).
After increasing the share of the industrial wastewater to 1.60% (P6), permissible values for COD, BOD5, and TN (125 mg O2/L, 15 mg O2/L, and 15 mg N/L) were obtained, and only one exceedance was recorded for COD 157.0 mg O2/L, BOD5 16.0 mg O2/L, and TN 15.4 mg N/L, respectively. In addition, two exceedances of TP were noted, amounting to 2.25 mg P/L and 2.05 mg P/L (the permissible concentration is 2 mg P/L). Decreases in the removal efficiencies of COD, BOD5, TN, NH4+-N, TP, and PO43−-P were also observed by 0.6%, 0.1%, 3.4%, 3.5%, 3.2%, and 2.8%, respectively (Table 4). The results obtained for the 1.60% share of industrial wastewater were insufficient to clearly confirm an adverse effect on activated sludge performance or to justify terminating the study; hence, the research was continued. After increasing the share to 1.90% (P7), a distinct adverse effect of this stream on the activated sludge system’s stability was observed. The average concentrations of COD, BOD5, TN, TP, AOX, and B in the treated wastewater increased to 185.8 ± 61.2 mg O2/L, 44.3 ± 9.8 mg O2/L, 30.2 ± 12.7 mg N/L, 2.32 ± 1.36 mg P/L, 0.701 ± 0.539 mg/L, and 0.250 ± 0.046 mg B/L, respectively (Figure 2, Figure 3a and Figure 3b). In addition to the observed exceedances of COD, TN, and TP values, one exceedance of AOX concentration (1.680 mg/L; permissible limit 1 mg/L) was recorded. Throughout all research phases, no exceedances of B concentration were observed (permissible limit 1 mg B/L). The average MOI values in the influent to the laboratory-scale system were as follows: P1—1.0 ± 0.1 mg/L; P2—1.0 ± 0.1 mg/L; P3—1.5 ± 1.2 mg/L; P4—22.2 ± 12.4 mg/L; P5—27.0 ± 11.5 mg/L; P6—10.9 ± 9.6 mg/L; and P7—50.7 ± 10.7 mg/L. For this parameter, the average concentration in treated wastewater remained approximately 0.1 mg/L (permissible limit: 15 mg/L).
Figure 4 presents the concentration changes of NH4+-N, NO3−-N, NO2−-N, and organic nitrogen (Norg) in the treated wastewater during all research phases (P1–P7). At phases P1 and P2, the average NH4+-N concentrations in the treated wastewater were 38.04 ± 5.61 mg/L and 11.31 ± 13.19 mg/L, respectively. The low efficiency of ammonium nitrogen oxidation was further confirmed by the NO3−-N results, which showed an average concentration of 0.600 ± 0.064 mg/L during phase P1. The disruption of ammonium nitrogen oxidation in phase P1 was caused by the lack of acclimation of the activated sludge in the nitrification reactor to the characteristic contaminants present in the industrial wastewater. During phase P2, as a consequence of the substantial reduction in the proportion of industrial wastewater, a decreasing trend in ammonia concentration in the treated wastewater was observed. Results obtained on day 27 confirmed the adaptation of the activated sludge microorganisms to the contaminants present in the wastewater. In the subsequent periods P3–P5, the average NH4+-N concentrations in the treated wastewater ranged from 0.86 ± 0.32 mg/L to 1.37 ± 0.70 mg/L, indicating that the microorganisms gradually adapted to the increasing ratio of industrial wastewater. During phase P6, an increase in NH4+-N to an average concentration of 4.18 ± 2.31 mg/L was observed, which could be caused by higher concentrations of industrial pollutants. The final research phase (P7) confirmed the toxic effect of industrial wastewater at a 1.90% proportion relative to the remaining influent wastewater. The average concentrations of NH4+-N and NO3−-N were 27.43 ± 14.04 mg/L and 0.731 ± 1.426 mg/L, respectively, confirming the inhibition of the nitrification process. Consequently, the treated wastewater during phase P7 did not meet the permissible norms. The main reason for the change in NH4+-N was the acclimatization of the activated sludge to industrial wastewater from the used oil installation. Moreover, by the end of the study, this type of wastewater had a negative impact on the activated sludge biocenosis in the nitrification reactor, as evidenced by the exceedance of total nitrogen (TN) in the effluent.
The activated sludge stabilized under laboratory conditions only after 76 days (corresponding to 1.5–2.0 SRT), which was 33 days longer compared to the study by Zabermawi et al. [51]. The bacterial adaptive cellular mechanism to pollutant was also reported by Hamimed et. al., which is crucial in biological methods [52]. During the research conducted by Zabermawi et al. [51], real industrial wastewater from an oil refinery was treated using activated sludge (AS) in a sequencing batch reactor (SBR). To enhance the AS performance, modified Fe3O4/silica NC (nanocomposite) was applied. The implemented modification improved the decomposition of oily wastewater for BOD5, COD, and OG (oil and grease) by 37.65%, 44.95%, and 36.08%, respectively (Table 5). When comparing these results with the outcomes obtained during phase P6 in the present study—under influent wastewater feeding the biological treatment system parameters comparable to those reported by Zabermawi et al. [51]—a higher removal efficiency was achieved for BOD5 (99.1%) and COD (95.0%).
Barani et al. [53] conducted studies using industrial oily wastewater from a refinery plant. Collected activated sludge was also sourced from an industrial wastewater treatment plant and applied for contaminant removal through a conventional activated sludge process. Compared with the results reported by Barani et al. [53], the present study achieved higher removal efficiencies of 31.1% and 28.0% for BOD5 and COD, respectively. These findings confirm that activated sludge microorganisms require an appropriate adaptation period to acclimate to the specific substances present in industrial wastewater effectively.
Traditional biological methods utilizing activated sludge face numerous challenges due to the complex matrix of industrial wastewater. Therefore, enhancing traditional approaches has become a solution, as demonstrated by Wang et al. [42], who employed the strain Rhodococcus erythropolis KB1 and a filler material in a sequencing batch reactor (SBR). During our study, at a 1.60% proportion, a higher COD removal efficiency by 0.8% and a lower NH4+-N removal efficiency by 3.0% were achieved compared to the results reported by Wang et al. Hybrid techniques employing two reactors, such as a sequencing batch reactor (SBR) combined with a ceramic membrane bioreactor (CMBR) [54], also represent a promising direction. Biological methods, due to their environmental friendliness and economic advantages, have an advantage over physicochemical approaches. However, due to the presence of characteristic chemical compounds in oily wastewater, the acclimation phase of the activated sludge is crucial. Table 5 presents the efficiencies of oily wastewater treatment obtained using various biological methods reported in different studies.
The aim of our research was to determine, whether and to what extent a municipal wastewater treatment plant can accept and treat wastewater from a waste recycling installation, along with municipal wastewater, to ensure proper and efficient operation. As shown in a review paper [19], there are many methods for treating industrial oily wastewater. Most of these advanced methods are expensive and require the construction of dedicated installations. Therefore, in most cases, they can only be applied to such wastewater in separate treatment systems. This leads to an increase in the amount of chemical waste due to the use of various reagents and, in some cases, the application of additional radiation sources (e.g., UV), which in turn increases energy consumption and treatment costs. This increases the carbon footprint and violates the circular economy paradigm and sustainable development objectives. The advantage of using biological methods, in turn, is their environmentally friendly impact, as they do not lead to secondary pollution. The generated sludge from the activated sludge method can be used to recover heat and electricity, as well as to produce organic fertilizer for agriculture. Naturally, despite its many advantages, the activated sludge method also has drawbacks, such as the potential negative or toxic impact of industrial wastewater (e.g., from waste oil processing plants) on microorganisms, resulting in a reduction or inhibition of their metabolic activity and, consequently, serious difficulties with maintaining the proper operation of the treatment plant. The solution presented in our study not only minimizes treatment costs (it does not require expansion of the treatment plant or modernization of the process line) but can also be replicated by other municipal wastewater treatment plants receiving various types of industrial wastewater.

4. Summary and Future Research

The results of the study demonstrated that activated sludge can adapt and effectively treat industrial wastewater from used oil installations, provided that the share of these industrial effluents in the mixture with domestic wastewater remains within an appropriate permissible ratio. The initial application of a 1.30% share immediately disrupted the operational stability of the activated sludge, manifested by an excessive increase in TN concentration in the treated wastewater. After a significant reduction of the share to 0.50% during days 10–27, gradual adaptation of the activated sludge was observed, reflected in a rapid decrease in TN concentrations to below 15.0 mg N/L. Subsequently, until the end of phase P2 and throughout phases P3, P4, P5, and P6 (with industrial wastewater shares of 0.75%, 1.00%, 1.30%, and 1.60%, respectively), TN concentrations remained below 15.0 mg N/L, with one exception in phase P6, when the TN value slightly exceeded the permissible limit. It should be noted that during phase P6, NH4+-N concentration constituted a significant fraction of TN, indicating disturbances in the nitrification process. Increasing the industrial wastewater share to 1.90% caused a rapid rise in NH4+-N and TN concentrations, confirming inhibition of nitrification.
During the research phases P1–P6, COD values remained below the permissible limit of 125 mg O2/L, with a trend of increasing in phase P6 and a sharp rise above 125 mg O2/L in phase P7.
Based on the results, the maximum permissible share of the investigated industrial wastewater stream relative to the remaining influent wastewater is 1.60%. The results also demonstrate the rationale and necessity of conducting such studies for biological wastewater treatment plants receiving additional industrial wastewater to ensure proper activated sludge functioning. The literature analysis shows that research was conducted only on industrial wastewater originating from refineries, petrochemical installations, or synthetic oily wastewater. In our case, wastewater from an oil recycling installation was tested, which had not been done before. The comparison of our research with previous studies was made due to the similar composition of petroleum hydrocarbons. However, wastewater from waste oil installation is more difficult to treat due to additional contaminants, which makes this research innovative. Moreover, the activated sludge method is the most cost-effective and widely used, which justifies testing its applicability.
Since the wastewater treatment plant is located in an industrial area and receives wastewater containing toxic substances, studies were planned to assess the impact of successive streams of industrial wastewater on the physiological state of the activated sludge and the efficiency of pollutant degradation. The results presented in this publication provide insights into improving the treatment of incoming wastewater, particularly in emergency situations. The limitations are emergency discharges of industrial wastewater to sewage treatment plants and the concentration of pollutants in the inflow during phase P7. This is why the research aims to avoid operational problems in biological reactors within the actual sewage treatment plant system. In perspective, the results of this research will improve treatment efficiency at the sewage treatment plant, thereby improving the quality of aquatic ecosystems. The wastewater treatment plants are facing significant challenges in the field of achieving the high removal efficiency of chemical pollutants, which is why it is important to develop ecological methods that are friendly to the natural environment. The balance between industry and the natural environment was demonstrated by Hamimed et al. using the immobilization of Yarrowia lipolytica within a polyvinylpyrrolidone (PVP)/polyethylene glycol (PEG)/agar matrix [55]. Consequently, studies into hybrid techniques appear crucial for the advancement of green technologies.

Author Contributions

Conceptualization, A.B. and B.P.; methodology, A.B. and K.B.; validation, K.B. and B.P.; writing—original draft preparation, A.B.; writing—review and editing, K.B., B.P. and M.A.; visualization, A.B. and M.A.; funding acquisition, K.B. and M.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded and supported by the Ministry of Science and Higher Education Grant No. DWD/6/0514/2022 (Poland).

Data Availability Statement

The data presented in this study are available on request from the corresponding author as we are bound by professional secrecy.

Acknowledgments

The authors would like to thank the PCC Energetyka Blachownia Sp. z o.o.

Conflicts of Interest

Author Agnieszka Bluszcz was conducting research between the Industrial PhD Program by Silesian University of Technology and the company PCC Energetyka Blachownia. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
BBoron
AOXAdsorbable organic halides
MOIMineral oil index
CODChemical oxygen demand
BOD5Biological oxygen demand
TSSTotal suspended solids
TNTotal nitrogen
TPTotal phosphorus
NH4+-NAmmonium nitrogen
PO43−-POrthophosphates
CFAContinuous flow analysis
NO3−-NNitrate
NO2−-NNitrite
NorgOrganic nitrogen
SRTSolids retention time
P1–P7Phases
S2−Sulfides
PAOPoly(α-olefin)
ABCAlgal–bacterial consortia
ECElectrocoagulation

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Water 18 00108 g001
Figure 1. The laboratory-scale wastewater treatment system.
Figure 1. The laboratory-scale wastewater treatment system.
Water 18 00108 g001
Water 18 00108 g002
Figure 2. Value of the chemical oxygen demand (COD) in the raw and treated wastewater.
Figure 2. Value of the chemical oxygen demand (COD) in the raw and treated wastewater.
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Water 18 00108 g003
Figure 3. Concentration of: (a) total nitrogen (TN) and total phosphorus (TP); (b) adsorbable organic halides (AOX) and boron (B) in the treated wastewater.
Figure 3. Concentration of: (a) total nitrogen (TN) and total phosphorus (TP); (b) adsorbable organic halides (AOX) and boron (B) in the treated wastewater.
Water 18 00108 g003
Water 18 00108 g004
Figure 4. Concentration profiles of ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), nitrite nitrogen (NO2−-N), and organic nitrogen (Norg) in the treated wastewater.
Figure 4. Concentration profiles of ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), nitrite nitrogen (NO2−-N), and organic nitrogen (Norg) in the treated wastewater.
Water 18 00108 g004
Table 1. Characteristics of real wastewater from waste oil installation.
Table 1. Characteristics of real wastewater from waste oil installation.
Wastewater Before Distillation
PhasesAverage Concentration of Pollutants (mg/L)
CODTSSTNTPAOXBMOI
P1218,000472954.01.2247.0626.0229.0
P2178,000232479.00.8329.1524.0100.0
P3224,000402140.00.5329.4636.0100.0
P4219,500461783.30.6161.1590.5364.0
P5209,500371870.00.65116.4532.5961.5
P6190,000351900.00.5373.0442.5246.0
P7220,000691983.00.5057.7423.03093.0
Min154,000231358.00.3229.1403.092.0
Max238,000692954.01.22168.0636.03093.0
SD25,01914458.00.2740.087.6995.3
Wastewater after distillation
PhasesAverage concentration of pollutants (mg/L)
CODTSSTNTPAOXBMOI
P151,200202647.01.1127.820.697.0
P251,367212093.30.4121.933.661.7
P355,838381316.50.4521.726.965.6
P460,240261475.80.3122.017.185.4
P557,774201466.10.3551.223.467.6
P658,054331471.80.3854.413.191.4
P758,025291572.60.3954.212.8111.8
Min45,2004950.00.0211.62.516.0
Max63,000942960.01.11137.082.9145.0
SD392519485.60.2533.419.834.3
Table 2. Duration and activated sludge loading of research phases.
Table 2. Duration and activated sludge loading of research phases.
PhasesShare of Industrial Wastewater Relative to Domestic Wastewater (% v/v)Activated Sludge Loading (kgBOD5/kgMLSS d)Activated Sludge Loading (kgCOD/kgMLSS d)Duration of the Research (Days)
P00.000.09 ± 0.030.14 ± 0.0476
P11.300.24 ± 0.010.34 ± 0.027
P20.500.17 ± 0.400.27 ± 0.0838
P30.750.23 ± 0.050.34 ± 0.0739
P41.000.26 ± 0.030.39 ± 0.0428
P51.300.31 ± 0.060.43 ± 0.0928
P61.600.38 ± 0.100.51 ± 0.1428
P71.900.71 ± 0.080.71 ± 0.0621
Table 3. Concentration of pollutants in the inflow.
Table 3. Concentration of pollutants in the inflow.
PhasesDaysConcentration of Pollutants (mg/L)
CODBOD5TNNH4+-NTPPO43−-P
P076509.8 ± 136.8337.1 ± 133.250.49 ± 9.8043.98 ± 8.9312.10 ± 3.2510.85 ± 3.22
P171648.0 ± 42.41175.0 ± 35.4110.50 ± 3.54108.49 ± 2.8927.40 ± 11.1725.68 ± 11.46
P2381103.4 ± 240.9718.2 ± 140.174.85 ± 11.7569.83 ± 10.2219.72 ± 8.7418.15 ± 8.68
P3391400.8 ± 238.1954.5 ± 203.074.58 ± 8.1967.38 ± 7.3512.80 ± 2.0910.37 ± 1.99
P4281749.3 ± 142.51181.3 ± 113.277.79 ± 11.3672.70 ± 9.3214.76 ± 1.6113.62 ± 2.62
P5281973.3 ± 257.21402.5 ± 157.488.46 ± 14.3680.18 ± 12.2715.79 ± 2.5313.60 ± 3.07
P6282277.8 ± 474.41693.8 ± 380.393.69 ± 22.6989.36 ± 24.0215.31 ± 1.9013.51 ± 1.02
P7212810.8 ± 427.02791.7 ± 280.0101.86 ± 8.4698.46 ± 7.6413.18 ± 3.1911.73 ± 3.45
Table 4. Degree of pollutant removal in particular research phases.
Table 4. Degree of pollutant removal in particular research phases.
PhasesDaysRemoval (%)
CODBOD5TNNH4+-NTPPO43−-P
P07692.1 ± 2.896.2 ± 1.873.2 ± 9.898.7 ± 1.099.0 ± 0.699.2 ± 0.4
P1794.2 ± 1.397.9 ± 0.164.0 ± 3.565.0 ± 4.299.3 ± 0.399.3 ± 0.4
P23895.0 ± 1.198.9 ± 0.478.0 ± 13.784.4 ± 17.897.4 ± 1.797.6 ± 1.7
P33994.5 ± 1.398.9 ± 0.389.0 ± 2.498.0 ± 1.194.4 ± 3.996.0 ± 5.0
P42895.4 ± 0.799.1 ± 0.291.3 ± 1.098.4 ± 0.798.0 ± 1.798.5 ± 1.9
P52895.6 ± 0.699.2 ± 0.192.7 ± 1.898.9 ± 0.597.2 ± 3.997.7 ± 3.2
P62895.0 ± 1.599.1 ± 0.289.3 ± 2.795.4 ± 2.094.0 ± 5.094.9 ± 4.8
P72193.3 ± 2.398.4 ± 0.570.6 ± 12.372.6 ± 14.182.5 ± 9.989.4 ± 7.8
Table 5. Efficiency of oily wastewater treatment.
Table 5. Efficiency of oily wastewater treatment.
Oil WastewaterPollutionAverage Initial Concentration (mg/L)Methods of DegradationEfficiency/ConcentrationRef.
Industrial wastewater from an oil refineryBOD51200 ± 0.38 The activated sludge (AS) using sequencing batch reactor (SBR) 50.0 ± 0.07%[51]
COD2342 ± 0.5740.22 ± 0.17%
OG (oil and grease)380 ± 0.6156.84 ± 0.36%
BOD51700 ± 0.37The activated sludge (AS)—modified Fe3O4/silica nanocomposite (NC), using sequencing batch reactor (SBR)87.65 ± 0.44%[51]
COD2900 ± 0.4185.17 ± 0.44%
OG (oil and grease)480 ± 0.3792.92 ± 0.84%
Refinery plantBOD5247The activated sludge (AS) 68%[53]
COD26467%
N (nitrate)13178%
P (phosphate)20567%
TOG (total oil and grease)2265%
Synthetic oily wastewater COD3162.3–3280.4The biofortification of sequencing batch reactor (SBR) using strain Rhodococcus eryth ropolis KB194.2%[42]
NH4+-N47.9–53.398.4%
TP6.3–7.794.4%
Oil137.9–150.291.6%
Petrochemical industry wastewaterCOD30,000–40,000A full-scale sequencing batch reactor and a ceramic membrane
bioreactor (SBR + CMBR)		<250 mg O2/L[54]
TN1400–1620<70 mg N/L
TP22–25<5 mg P/L
Oil600–2200<2 mg/L
Real wastewater from waste oil installation with domestic wastewater (present research)COD2277.8 ± 474.4The activated sludge (AS)95.0 ± 1.5
BOD51693.8 ± 380.399.1 ± 0.2
TN93.69 ± 22.6989.3 ± 2.7
NH4+-N89.36 ± 24.0295.4 ± 2.0
TP15.31 ± 1.9094.0 ± 5.0
PO43−-P13.51 ± 1.0294.9 ± 4.8
MOI10.9 ± 9.698.5 ± 1.0
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Bluszcz, A.; Barbusiński, K.; Pieczykolan, B.; Alwaeli, M. Investigation of the Impact of Wastewater from Waste Oil In-Stallation on the Activated Sludge Process, to Ensure the Proper Operation of Municipal Wastewater Treatment Plant. Water 2026, 18, 108. https://doi.org/10.3390/w18010108

AMA Style

Bluszcz A, Barbusiński K, Pieczykolan B, Alwaeli M. Investigation of the Impact of Wastewater from Waste Oil In-Stallation on the Activated Sludge Process, to Ensure the Proper Operation of Municipal Wastewater Treatment Plant. Water. 2026; 18(1):108. https://doi.org/10.3390/w18010108

Chicago/Turabian Style

Bluszcz, Agnieszka, Krzysztof Barbusiński, Barbara Pieczykolan, and Mohamed Alwaeli. 2026. "Investigation of the Impact of Wastewater from Waste Oil In-Stallation on the Activated Sludge Process, to Ensure the Proper Operation of Municipal Wastewater Treatment Plant" Water 18, no. 1: 108. https://doi.org/10.3390/w18010108

APA Style

Bluszcz, A., Barbusiński, K., Pieczykolan, B., & Alwaeli, M. (2026). Investigation of the Impact of Wastewater from Waste Oil In-Stallation on the Activated Sludge Process, to Ensure the Proper Operation of Municipal Wastewater Treatment Plant. Water, 18(1), 108. https://doi.org/10.3390/w18010108

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