This work deals with the treatment of oily wastewater produced from the washing of oil-contaminated soil. Untreated oily wastewater contains toxic compounds that might be mutagenic or carcinogenic as total petroleum hydrocarbon (TPH) and heavy metals. Based on the water quality analysis, the tested samples contained a high concentration of TPH, chemical oxygen demand (COD) and turbidity with an average value of 67,500 mg/l, 48,240 mg/l and 176 (nephelometric turbidity unit, NTU), respectively. Several technologies were used, such as centrifuging, powdered activated carbon (PAC) and sawdust. The mean values of COD values for sawdust, centrifuging and PAC were 41,067, 25,600 and 13,133 mg/l, respectively. The present study indicated that the coagulation/flocculation processes were more efficient by using aluminium sulphate alum, while the preliminary conclusion derived was that the secondary treatment using an aeration system is capable of lowering the COD values as well as increasing the flocculent mass floc well equal to 4,784 mg/l and 0.69 g, respectively. The microbial seed was able to degrade the biosurfactant, which allows the stability of oil emulsion to be broken down and released easily.

  • Centrifuging sawdust and PAC were carried out to treat oily wastewater.

  • The mean values of COD for sawdust, centrifuging and PAC were 41,067, 25,600 and 13,133 mg/L.

  • The combination of aeration with coagulation/flocculation enhanced the treatment process.

  • Microbial seed effect on the stability of oil emulsion by degrading saponin in wastewater.

Graphical Abstract

Graphical Abstract
Graphical Abstract

As a result of rapid development and the colossal quantity of oil used in the industry, the issue of water pollution needs constant monitoring and review, given the inadequate technical and management expertise. The source of wastewater generated from oil within the industry is vast such as the oil refining process, oil storage, transportation and petrochemical production process (Ahmed et al. 2007; Javadian & Sadrpoor 2019; AlJaberi et al. 2020; Obotey Ezugbe & Rathilal 2020). Further, oily wastewater produced comprises varied types of organic pollutants and presents an extremely high content of salt. Some of the oily wastewater treatment technologies offered presently consist of vacuum evaporation, flocculation, adsorbents, coagulation, centrifugal devices, ultrafiltration and deep bed filtration, flotation, membrane separation technology, combined technologies, advanced oxidation process (Apostol et al. 2011; Peng et al. 2014; Putatunda et al. 2019; Zhao et al. 2020).

Adsorption treatments were used for the treatment of oily wastewater (Zapata Acosta et al. 2019; Mingming et al. 2021). These methods are principally adopted by chemical plants that generate wastewater with a high concentration of organic compounds and heavy metals (Rahmani et al. 2018; Le et al. 2021). Adsorption methods utilising non-conventional adsorbents, for instance sawdust, peat, wool, silk and powdered activated carbon (PAC), are also known to have been employed to remove organic substances and heavy metals from wastewater (Anirudhan & Sreekumari 2011; Hoang et al. 2018). In the industrial application, PAC was conducted as a pre-treatment step to remove the dissolved organic fraction and the refractory organic fraction from oily wastewater (Srivastava & Tyagi 1995).

In biodegradation processes, a chemical compound is decomposed or eliminated by the biological action of living microorganisms. Generally, biodegradability is the action of microorganisms to transform a substance into its constituent elements or new compounds (Soeder et al. 1996). In the case of petroleum compounds, coagulants are not able to destabilise the stabilised emulsions or dissolved oil, but they can be degraded easily by the biological treatment (Kang et al. 2015). The effectiveness of an aerobic biological wastewater treatment method is subject to the presence of adequate dissolved oxygen (DO) as the elimination of organic contaminants can be sped up with an increased concentration of DO (Travers & Lovett 1984; Liu et al. 2008; Corsino et al. 2018; Morgan-Sagastume et al. 2019). Alternatively, oxygen mass transfer usually reduces the acceleration of organic contaminant removal in view of its low solubility. As discovered by Liu et al. (2010), an air aeration method can offer great benefits to the development of aerobic digestion of organic pollutants. This can be achieved by improving the transfer of oxygen mass within the dyeing wastewater coagulation. However, the organisms in the anoxic (limited oxygen supply) treatment may utilise the nitrate acting as an electron acceptor and consequently discharging nitrogen oxides. According to Gallert & Winter (2005), methane and CO2 are considered as the main products in anaerobic or anoxic environments if sulphate is absent, but sulphide and CO2 are considered as the main products when sulphate is present.

The chemicals commonly used as inorganic salts include ferric chloride and alum, which have been used as a coagulant to breakdown the high molecular weight of organic compound from different types of water (Mohd-Salleh et al. 2019; Tang et al. 2019; AlJaberi et al. 2020; Zhao et al. 2020). Ferric chloride and alum are acknowledged as renowned coagulants and are frequently utilised as the first-choice reagents in treating wastewater due to a number of benefits such as affordability, effectiveness even with the least dosage, low toxicity and easy availability (Apostol et al. 2011).

In general, wastewater contaminated with heavy oil is considered a high chemical oxygen demand (COD) value, while an ordinary conventional oxygen sludge method proves to be inefficient to effectively remediate the wastewater in satisfying the discharge standard (Ji et al. 2009). As such, a mixture of biological processes would be necessary to enhance the effectiveness of the treatment (Silva et al. 2019).

According to Le et al. (2021), hydrocarbon compounds and heavy metals’ toxicity are well-documented and at some particular concentrations, they are lethal to higher organisms, microorganisms and plants. As such, their existence in wastewater is a real concern to the environment and substantially decreases the microbial activity. This results in the disturbance of the processes within the biological wastewater treatment (Al-Awadhi et al. 2016).

Many current methods are limited to specific types of contaminations. For the effective treatment of more complex characteristics, it might be necessary to combine several different treatment methods to improve the performance of removing oily particles and heavy metal of all compositions and particle sizes. Traditional methods of the treatment of wastewater give unsatisfactory results. Therefore, combining different types of wastewater treatment techniques might be able to reduce the cost of wastewater treatment as well as improve the wastewater treatment process, which generate less waste and minimise pollution.

All the experiments were carried out in triplicate, while the means of the results were required to determine the percent relative standard deviation. The data results were accepted when the standard deviation value was less than 5%.

Preparation of oily wastewater and seawater

Specimens of oily wastewater were obtained from washing oil-contaminated soil by using the author patent (Patent registered (A: 2018) Nos: US20170138135, (B; 2018) US201833100.78 and (C: 2017) GCC 2017-33018. The author patent recommended using seawater during the washing of contaminated soil; therefore, an actual water sample was collected from seawater (Portsmouth seafront). Furthermore, biosurfactant saponin has been employed considerably during the soil washing process to enhance the efficacy of oil removal for the rehabilitation of hydrophobic contamination. The properties of the water samples were analysed immediately after sampling. Prior to use, all samples were placed in 25-l dark jar bottles and retained in a refrigerator with a temperature of 4 °C.

Preparation of saponin

Saponins selected in this study were a large assembly of glycosides with foaming characteristics. The purified (98%) saponin was purchased from Fisher Scientific Ltd, Loughborough, UK. It was used without further purification as supplied. Five grams of saponin were added with 1 l of artificial seawater to obtain the required concentrations of saponin 0.5 wt%. The solution of artificial seawater and saponin was mixed for 10 min and stirred at a constant speed of 200 rpm to dissolve the saponin.

Seed inoculation

This experiment made use of the adapted seed, for instance, the influent from the biological purification phase of a wastewater treatment plant. Usually, it is not necessary for specimens obtained from wastewater treatment plants to be seeded with bacteria. The samples can normally be used straight away as the measurement solution. Most wastewater obtained from the municipal treatment plants have adequate minerals, trace elements and nutrients to degrade the carbon compounds at an optimum level. In this experiment, primary settled sewage (PSS) taken from the Petersfield Sewage Works (Southern Water) was utilised as the amendment substance.

Coagulation

In this study, aluminium sulphate hydrate (A12(SO4)3·16H2O) Al(III) and ferric chloride (FeCl3·6H2O) Fe(III) were used as coagulants in the coagulation/flocculation process to treat oily wastewater. The stock solutions of the ferric chloride and alum were employed at a concentration of 1,000 M (54,000 mg/l) by Fe and 0.238 M (12,848 mg/l) by Al, respectively. The distilled water was used for the preparation of stock solutions. Coagulants were supplied by Fisher Scientific, UK.

Chemical oxygen demand (COD)

COD was measured to estimate the oxidisable organic matter in the water sample. One hundred millilitres of the sample were homogenised for 5 min in an overhead stirrer (CP Cole-Parmer). The oily wastewater was diluted (distilled water to oily wastewater, 100:1) in a 100-ml conical flask and manually shaken for 5 min. The cap from a COD vial provided in the kit was removed and 2 ml of the diluted sample was micro-pipetted into the vial. The lids were sealed tightly, and the vial was mixed gently several times to mix the contents.

The vial was placed in the preheated digester block and allowed to react with the COD vial (0–1,500 ppm) containing 86% sulphuric acid and potassium dichromate. The vials were digested in a COD reactor hot block (DRB200, Hach, UK) and heated for 2 h at 150 °C. The block was turned off and allowed to cool for 15 min. The digested samples and reagent blanks were measured in a pre-programmed colorimeter (DR890, Hach, UK) photometer. The results are expressed as the number of milligrams of oxygen consumed per litre of the sample ((mg/l) COD). Subsequently, the colorimeter reading was multiplied by the ratio factor.

Potential hydrogen ions (pH) in the water sample

The pH of oily wastewater was measured based on BS EN ISO (10523: 2012). One hundred millilitres of oily wastewater sample were collected in a 150-ml beaker to submerge the tip of the probe. The probe was rinsed with the oily wastewater sample before the measurement for conditioning. The samples were then read using a pH meter (model, Jenway 3305, UK) after being calibrated at pH 7.

Electrical conductivity

The conductivity of the water samples was measured by using a Jenway 4010 conductivity (BS EN 27888: 1993).

Measurements of heavy metals

Seven heavy metals (Zn, Co, Cr, Mn, Pb, Fe and Ni), one metal (Ba) and two non-metals (B and Al) in the oily wastewater, were determined by using (SW-846, 6010B) atomic adsorption spectrophotometry (Perkin Elmer). The test for the concentration of metals and heavy metals in the oily wastewater sample was based on USEPA's (1996) method number (SW-846, 6010B). Seven heavy metals (Zn, Co, Cr, Mn, Pb, Fe and Ni), two metals (Ba and Al) and one non-metal (B) in the samples, were determined by using inductively coupled plasma-optical emission spectrometry (ICP-OES) after digestion with a microwave digester (MARS, CEM, USA) following the manual instructions with the addition of nitric acid, hydrofluoric acid and hydrogen peroxide for sample digestion.

Adsorption and physical methods

Adsorption processes have been studied for the removal of hydrocarbons from water through sorbent material such as PAC and sawdust; furthermore, another physical treatment process, such as centrifugation to evaluate the potential of separating the contamination residual form water, was implemented. All the selected technologies have been widely proved as the highest efficiency methods to remove dissolved organic matters or emulsified oils. The results of scientific experiments were determined by measuring COD, floc weight and wavelength in a fixed period of time.

Treatment using PAC

A 150-ml glass funnel was used for the preparation process, whereas fibre was packed into the glass tube of a conical flask. Five grams of PAC were weighted and then placed into the conical glass funnel for the adsorption process. Subsequently, 50 ml of oily wastewater was decanted into the graduated mark of the glass funnel. When the process was completed, the stock solutions were analysed immediately. The adsorption study was conducted at room temperature (25 ± 1 °C), while the values of COD were determined in the filtered water.

Treatment using sawdust

Fifty millilitres of oily wastewater was decanted into a 250-ml glass beaker, and 5 g of sawdust was added. The flask was placed into a magnetic stirrer (CB302) at the room temperature. The mixture of oily wastewater with sawdust was mechanically stirred for 15 min at a constant speed of 100 rpm to provide a uniform distribution of the sawdust during the process, and then allowed to stand for 30 min, whereby the aggregation of oil residual was floated on the water surface. The chips that floated on the water surface were removed with a spoon, while the aggregates of oil residual that sank in the bottom of the glass beaker were taken out with a spoon after decanting the supernatant water. All the aggregate mixture that was removed manually from the beaker was allowed to filter for 1 h using weighed filter paper (Whatman Grade No. 2), while the COD values were measured in the filtrate water. After the filtration process was completed, the filter paper and the aggregate mixture were dried overnight at room temperature, and finally they were weighed.

Treatment using centrifugation

An Eppendorf centrifuge (model 5804 R, Eppendorf, UK) was used to separate the dispersion samples based on the density of fractions. Experiments were carried out using 50-ml glass vials. Each vial was filled with 50 ml of oily wastewater; during this experiment, three replicates were centrifuged each time. Then, the resolution of the organic phase was separated from the solution by centrifuging at 3,000 rpm for 10 min, while the supernatants were pipetted out to determine COD values. During the process of withdrawing the top emulsion phase, good care was taken to prevent distributing settled layers.

Procedures of coagulation and flocculation

Two standard conical flasks of 100 ml were used separately for the preparation process of the Al(III) and Fe(III). The coagulants were prepared by using 50 ml of distilled water with 7.5 g of alum and 50 ml of distilled water with 13.5 g of ferric chloride. The solution was mixed in a hot-plate magnetic stirrer (Fisher Scientific) for 10 min at 45 °C to allow the coagulants to be distributed and dissolved easily into the conical flask.

The coagulation tests were carried out using six beaker flasks (75 ml) with a magnetic stirrer (CB302). Each beaker was filled with 50 ml of oily wastewater samples. After the stock of coagulants’ dosage of ferric chloride and alum was prepared, then it was introduced to the water samples followed by the coagulation tests with rapid agitation at 700 rpm for 60 s to ensure that the effluent and the coagulant were well mixed. The speed of the mixing was then reduced to 50 rpm for 10 min with a 1-h settling time used. Upon settling, 1 ml of the supernatant interface was extracted at a predetermined distance of 20 mm beneath the air–liquid interface with a syringe to measure COD. The room temperature at which the experiments were undertaken was between 23 and 25 °C.

Subsequently, the precipitate was separated by using a funnel filter assembled with a weighed filter paper (Whatman 0.45 μm membrane filters, Water Conservation and Management (WCM)). Then, the filter paper was placed into the funnel filter's assembly. Consequently, the mixture of wastewater and precipitation was decanted into the filter unit, when the vacuum pump was turned on. The filtered paper was dried in an oven at 45 °C to obtain the total weight of filter papers with the remaining oil residual. As a result, the weight of the floc was determined. Finally, the filter sample was then placed in a fridge for the next analysis.

Enhancement treatment method

In this case, the oily wastewater sample was seeded by means of introducing the microorganism population into the oily wastewater by two different procedures, in order to compare whether the aerobic or anaerobic treatment was sufficient and evaluate the potential of combining the bio-treatment with coagulation/flocculation. Therefore, the coagulant dosage of ferric chloride and alum is required to be designed, to select the appropriate coagulant.

Experimental set-up for anoxic oxidation

Reagent dark bottles with a volume of 100 ml were used for the incubation of the anoxic process, and 5 ml of PSS was added and introduced up to 50 ml of oily wastewater. The samples were incubated in Heratherm Incubators (Thermo Scientific) at 37 °C. Then, 55 ml of the tested sample was carried out as explained after 3, 5, 10, 20 and 30 days of incubation.

Experimental set-up for aerobic oxidation

In order to simulate the treatment process for oily wastewater, the biodegradation study of organic compounds was conducted in a 2,000 ml of a measuring cylinder. The working cylinder was filled with oily wastewater (500 ml) and seeded with PSS (50 ml). In this work, 550 ml of the tested sample was carried out for 3 and 5 days at room temperature (25 ± 1 °C). Three replicates were carried out each time to verify replicates’ data. During this process, air was continuously supplied into the reactors using a compressor (model N810FT.18, KNF Neuberger UK Ltd) to provide aeration, acclimatisation and immobilisation for microorganisms. The aeration was measured during this process using an air velocity meter (model AVM501, Prosser Scientific Instruments, Ltd). It was controlled to 5.5 m/s (model VFB-80D-BV, Dwyer Instruments International) to avoid the froth from bubbling out of the cylinder. The COD was used as an indicator of the biodegradation rate.

Various oily wastewater treatment methods were assessed within the study to ascertain the best treatment method. The first method utilised was the adsorption technique in a conventional treatment using activated carbon, sawdust and the physical method including centrifugation. The second trial method was conducted by using a conventional coagulants/flocculation treatment. In this experiment, ferric chloride and alum were utilised as coagulants, to select the best coagulant and design the optimal doses. Subsequently, the combinations of bio-treatment such as aerobic and anoxic digestion with coagulation/flocculation were conducted to evaluate the efficiency of secondary treatment.

Wastewater quality analysis

The oily wastewater sample was evaluated by determining the COD, BOD5, total petroleum hydrocarbon (TPH) and turbidity, and the results are shown in Table 1. The TPH value was 67,500 mg/l, which indicated a characteristic of high organic compounds’ concentration, while the ratio of BOD5/COD for oily wastewater seeded with PSS was 0.17. Furthermore, the result indicated that the ratio of the BOD5/COD for the mixture solution of saponin seeded by PSS was 0.27 as well, leading that the biodegradation process occurred for only saponin compounds in the oily wastewater as explained previously. Based on the quality analysis of the wastewater, it could be concluded that the treatment of wastewater containing a high concentration of organic compounds using only a conventional biological process could be considered a difficult task.

Table 1

Water characteristic analysis

ParameterUnitsSamples rangeLimiting range
pH  8.27 6.5–8.5 
EC μS/cm 60,800 2,500 
TPH mg/l 67,500  mg/l 
COD mg/l 48,215 100–350 
Turbidity NTU 176 50 
ParameterUnitsSamples rangeLimiting range
pH  8.27 6.5–8.5 
EC μS/cm 60,800 2,500 
TPH mg/l 67,500  mg/l 
COD mg/l 48,215 100–350 
Turbidity NTU 176 50 

Adsorption removal

The removal of emulsified oils was determined by using different types of sorbents such as activated carbon and sawdust, while a centrifuge was used to investigate the physical isolation efficiency (see Table 2).

Table 2

Preliminarily treatments of oily wastewater by centrifuging and adsorption

TreatmentCOD (mg/l) BeforeCOD (mg/l) afterTSS (g)pHTurbidity (NTU)
PAC 48,215 ± 2,720 13,133 – 8.55 18 ± 1.5 
Woodchips 48,215 ± 2,720 41,067 0.22 7.85 144 ± 5.5 
Centrifuging 48,215 ± 2,720 25,600 0.44 7.28 47 ± 3.0 
TreatmentCOD (mg/l) BeforeCOD (mg/l) afterTSS (g)pHTurbidity (NTU)
PAC 48,215 ± 2,720 13,133 – 8.55 18 ± 1.5 
Woodchips 48,215 ± 2,720 41,067 0.22 7.85 144 ± 5.5 
Centrifuging 48,215 ± 2,720 25,600 0.44 7.28 47 ± 3.0 

Table 2 presents a significant effective method for the reduction of COD as follows: PAC (M = 13,133; SD = 351) > centrifuging (M = 25,600, SD = 458) > sawdust (M = 41,067; SD = 1,550), (ANOVA F(2, 6) = 644, post-hoc Scheffe's test, p= 0.00). Further, the percentage reduction values of COD by using PAC, centrifuging and sawdust were 72, 46 and 13%, respectively. In the present research, the pH value of the oily wastewater was increased slightly after the PAC treatment. The initial pH value of oily wastewater was 8.27, which increased to 8.55 at 5 g/50 ml of PAC; there was a slight reduction in pH to 7.82 by using a centrifuge. However, there was no change in the pH value when sawdust was used. The results obtained in relation to turbidity showed that PAC has the ability to reduce the turbidity value from 176 to 18 nephelometric turbidity unit (NTU), which is considered the highest effective technique in colour removal than centrifuge or sawdust as well. The experiment found that the TSS for PAC was difficult to measure, since they stick together with wet PAC, while the centrifuging proved that was able to separate more suspended solid than sawdust. Wastewater treatment using a few inexpensive adsorbents and PAC had demonstrated that PAC was more effective in reducing COD compared with other pre-treated adsorbents such as bagasse, sawdust, sawdust, coconut coir and fly ash (Zapata Acosta et al. 2019). It happens that many of these procedures, due to high contamination level and high cost, become impracticable. These results suggest that the adsorption and centrifuge treatments are not the appropriate methods in this study.

Effect of coagulant dose

The assessment of coagulation is performed by means of a laboratory-scale test for preliminary appraisal in an attempt to choose the ideal coagulant for oily wastewater treatment. Coagulation/flocculation experiments were carried out using various inorganic types, such as ferric chloride and alum. The selection is based upon the floc weight and the values of COD for the solution upon the completion of the flocculation. The required coagulant dose to be added to the wastewater samples essentially depends upon the concentration of the organic content. The outcome of the experiment is shown in Figure 1.
Figure 1

Mean value of COD by various doses of alum. Note: the data are expressed as mean ± SD (n = 3).

Figure 1

Mean value of COD by various doses of alum. Note: the data are expressed as mean ± SD (n = 3).

Close modal
It can be seen from Figure 1 that the values of COD significantly decreased from 44,633 to 27,266 mg/l (ANOVA F(6,14) = 350, post-hoc Scheffe's test, p= 0.00) by increasing the coagulant dosage from 0.32 to 1.53 (mg-Al/l). Also, COD values were significantly reduced from 47,300 to 29,133 mg/l (ANOVA F(6,14) = 231, post-hoc Scheffe's test, p= 0.00) by reducing the coagulant dosage from 2.65 to 0.27 (mg-Fe/l). The result of the present work identified that the COD values were reduced by increasing dosages of Al; nevertheless, the COD values were reduced by decreasing dosages of Fe. The percentage of aluminium in alum is about 8% (weight (w)/weight (w)), while the percentage of iron in ferric chloride is about 35% (w/w); therefore, the addition of positive ions of Al is required. Further, the selection was based on the values of the floc weight obtained after the flocculation/coagulation process was completed. The outcome of the experiment is shown in Figure 2.
Figure 2

Mean value of flocs by various masses of Alum. Note: the data are expressed as mean ± SD (n = 3).

Figure 2

Mean value of flocs by various masses of Alum. Note: the data are expressed as mean ± SD (n = 3).

Close modal
The present research work identified that the mean values of flocs were significantly increased from 0.22 to 0.43 g by increasing the coagulant dosage from 0.32 to 1.53 (mg-Al/l) (ANOVA F(6,14) = 54, post-hoc Scheffe's test, p= 0.00). However, the mean values of flocs were significantly increased from 0.25 to 0.41 g by decreasing the coagulant dosage from 2.65 to 0.27 (mg-Fe/l) (ANOVA F(6, 14) = 290, post-hoc Scheffe's test, p= 0.00). Furthermore, the outcome shows that the generated results from overdosing charge of Fe and Al, for instance increasing the Al dose to 1.81 (mg-Al/l) or decreasing the Fe dose to 0.162 (mg-Fe/l), developed from a further increase of COD and decrease of the floc weight. In addition, Figure 3 reveals that a reverse impact on the water clarity by altering the dosages of ferric chloride or alum could be identified. Therefore, design doses have a limitation of overdosing, as such a higher charge of Fe or Al is able to increase the possibility of restabilisation by causing charge reversal. Additionally, Figure 3 shows that further issues could be generated by using ferric chloride. The outcome of the experiment demonstrated that turbidity was affected by transferring the ion colour to the water; as a result, the generated water required further treatment.
Figure 3

Preliminary study on selecting the best dose of Fe.

Figure 3

Preliminary study on selecting the best dose of Fe.

Close modal
It can be evidently shown that the reduction of COD and the weight of floc dramatically increased by increasing the dosage of coagulants up to reaching the optimal coagulant dosages; moreover, these significant reductions may be due to the charge neutralisation. Increasing the presence of cationic ions of Fe or Al can enhance the attraction of the negative charge colloids from emulsified oils through electrostatic interaction. Therefore, the colloids and floc particles were starting to agglomerate during the neutralisation phase until they become heavy enough to settle down by gravity. Moreover, a paired sample t-test was conducted to examine whether there was a significant difference between cationic ions of Fe and Al in relation to their COD reduction, as shown in Figure 4.
Figure 4

Mean value of COD reduction by using the selected dose of Fe and Al. Note: The data are expressed as mean ± SD (n = 3).

Figure 4

Mean value of COD reduction by using the selected dose of Fe and Al. Note: The data are expressed as mean ± SD (n = 3).

Close modal

It is clearly shown that the mean value of COD was significantly reduced by using Al (M = 27,266; SD = 152) than Fe (M = 29,133; SD = 165) (t = 21, df = 2, paired sample test, p= 0.002). As a result, the efficiency of COD reduction was significantly enhanced by about 6.5% with alum. Further, the values of the floc weight obtained after the flocculation/coagulation process were measured. The outcome has shown that the agglomerations of floc weights were significantly higher by using Al (M = 0.44; SD = 0.010) more than Fe (M = 41; SD = 0.006) (paired sample test, t = 4, df = 2, p= 0.0057). Also, it was noted that the weight of flocs changes in tandem with the tendencies shown in the COD; moreover, this suggests that the optimal coagulant dosage contributes mainly to achieving the desired results. As such, the conclusion is that the most optimum coagulant was achieved with alum at a concentration of 1.53 mg-Al/l. At present, alum is the favoured choice as a coagulant over ferric chloride due to the ferric ion colour that is transferred to water, while the use of ferric chloride could limit the improvement of the coagulation–flocculation process by minimising the flocs produced and decreasing the COD value. This study demonstrated that the coagulant type and optimal coagulant dose will not only reduce the treatment costs but also improve the water characteristics.

Efficiency of anoxic digestion with coagulation and flocculation

In this section, the effects of alum with coagulation/flocculation on the anoxic treatment were investigated in the present study. The results of the combination system were generated after 3 and 5 days. The combination of the anoxic treatment followed by coagulation/flocculation was carried out in the current work. Alum was used as a coagulant after 3 and 5 days of the anoxic process. Figure 5 indicates the mean values of COD for oily wastewater during the anoxic process.
Figure 5

Mean value of COD reduction using the anaerobic digestion process after 3 and 5 days. Whereas (a) oily wastewater; (b) oily wastewater with PSS; (c) anoxic treatment; and (d) combination of anoxic treatment with coagulation/flocculation. The data are expressed as mean ± SD (n = 3).

Figure 5

Mean value of COD reduction using the anaerobic digestion process after 3 and 5 days. Whereas (a) oily wastewater; (b) oily wastewater with PSS; (c) anoxic treatment; and (d) combination of anoxic treatment with coagulation/flocculation. The data are expressed as mean ± SD (n = 3).

Close modal

The result demonstrated that the mean value of COD for oily wastewater with PSS was significantly reduced from 48,240 mg/l (SD = 378) to 25,720 mg/l (SD = 497) after 3 days and to 22,034 mg/l (SD = 749) after 5 days (one-way ANOVA F(5,12) = 3,174, post-hoc Scheffe's test, p= 0.00). However, the COD values after the anoxic treatment were still high, as exhibited in the same Figure 5; therefore, further treatment is required to remove the recalcitrant organic compounds to an acceptable level. The current work identified that the alum was able to reduce the mean values of COD by about 39%. These results encourage us to consider the combined use of coagulation/flocculation.

The present work recognised that the COD values for incubated samples were significantly decreased by using the coagulation and flocculation process to 17,700 mg/l (SD = 483) after 3 days and to 14,110 mg/l (SD = 384) after 5 days (paired sample test, t = 13.6, df = 2, p= 0.00). This study identified that the combination technique of conventional anoxic treatment with coagulation/flocculation processes enhanced the reduction of COD values by about 64% after 3 days and 70% after 5 days. The outcome of this study proposed that the contact time (treatment duration) between the microbes and oily wastewater exhibits a direct relationship on to the removal efficiency of oil residue. The combination treatment of anoxic treatment followed by coagulation/flocculation does not require electrical energy; therefore, this treatment is considered an environmentally friendly treatment method. However, the result indicated that there is a slight increase in aggregation of floc weight after 5 days of the incubation period. The outcome showed that the average floc weight was significantly changed to 0.50 (SD = 0.016) after 3 days and to 0.61 g after 5 days (SD = 0.015) (paired sample test, t = −15.24, df = 2, p= 0.00), respectively. It seems that floc weight increases with longer incubation times. This oily wastewater sample contained high COD in the average of 48,050 mg/l and a large amount of colourants that provide dark brown colour, which is considered slightly basic (pH 7.5–8.0). Even after being treated with anoxic procedures, the colour of the oily wastewater becomes a darker brown with high odour.

Perhaps, a chemical reaction occurred which changed the physical priorities of oily wastewater. Recent studies have reported that the colour of the blackening of wastewater can increase with an anaerobic process due to the formation of ferrous sulphide (Morgan-Sagastume et al. 2019; Muzzammil & Loh 2020). Under anaerobic conditions, hydrogen sulphide can be produced and combined with divalent metal, such as Fe, which is often black or brown in colour.

Efficiency of aerobic digestion with coagulation and flocculation

Alum was used as a coagulant after 3 and 5 days of the aeration process. The result demonstrated that the mean values of COD for oily wastewater with PSS was significantly reduced from 48,240 mg/l (SD = 378) to 16,466 mg/l (SD = 406) after 3 days and to 10,516 mg/l (SD = 697) after 5 days (ANOVA F(5,12) = 7,778, post-hoc Scheffe's test, p = 0.00). Additionally, this investigation identified that the COD values for aerated samples were significantly decreased by using the coagulation and flocculation process to 7,992 (SD = 205) after 3 days and to 4,784 mg/l (SD = 270) after 5 days (paired sample test, t = 28.3, df = 4, p= 0.00). The present work indicated that the combination of aeration followed by the coagulation/flocculation process was able to reduce the COD value by about 83 and 90% after 3 and 5 days, respectively. In addition, the result indicated that there is a slight increase in the aggregation of the flocs’ weight after 3 and 5 days of the aeration process. The results showed that there was a significant change in the aggregation of the floc weight to 0.55 (SD = 0.015) after 3 days aeration and to 0.69 g (SD = 0.013) after 5 days aeration (paired sample test, t = −54.7, df = 2, p= 0.00). Basically, the physical property of oily wastewater was changed after employing the aeration system such as the colour as shown in Figure 6.
Figure 6

Oily wastewater sample after 5 days of the aeration process: (a) oily wastewater; (b) after aerobic digestion; and (c) after added ‘alum’.

Figure 6

Oily wastewater sample after 5 days of the aeration process: (a) oily wastewater; (b) after aerobic digestion; and (c) after added ‘alum’.

Close modal

It is thought that this change of colouration of the oily wastewater is due to the loss of colloid particles’ stability by the flocculants and thus emulsifies oil within the wastewater. A study carried out by Cruz et al. (2020) stated that the destabilisation of colloidal particles by using a high aluminium dosage, an adequate level of oversaturation prompts fast precipitation of a large amount of aluminium hydroxide by the formation of ‘sweep floc’. They identified that the addition and precipitation of alum or ferric chloride can create a dense and readily separable floc that can enmesh and capture particles in a process known as a sweep floc. As explained by Sharma & Paliwal (2013), the hydrophilic part of saponin is polar due to the several functional groups such as OH, COOH and carboxylate group of sugar, acetate group and esteric band. Since these compounds carry a negative charge, the removal of oil emulsion may be due to the complexation of Al with hydroxyl ion (OH). Moreover, these significant enhancements in COD values and the flocs’ weight perhaps due to the flocculant enmeshment (sweep–floc coagulation) are produced from overdosing of the flocculants, which tend to give a fairly thick layer of the flocculant around the suspended solids, and therefore accelerate the setting of floc, as explained by licsko (2004). Regardless of the high dose of coagulant leading to the assumption of a sweep floc coagulation mechanism, the dependence of the observed stoichiometric or quasi-stoichiometric relation inclines to suggest that the charged adsorption–neutralisation mechanism is performed by a high charge of Al.

Comparing aeration and anoxic treatments

This investigation was evaluated by measuring the reduction of COD values, as shown in Figure 7.
Figure 7

Mean value of COD reduction: (A) aeration treatment; (An) anoxic treatment; and (B) coagulation/flocculation. The data are expressed as mean ± SD (n = 3).

Figure 7

Mean value of COD reduction: (A) aeration treatment; (An) anoxic treatment; and (B) coagulation/flocculation. The data are expressed as mean ± SD (n = 3).

Close modal

Based on Figure 7, there is a significant enhancement in the percentage of COD reduction by the combination of the aeration system more than the combination of the anoxic system, where it increased to 64 and 83% after 3 days and then increased dramatically after 5 days to 70 and 90% (ANOVA F(7,16) = 443, post-hoc Scheffe's test, p= 0.00), respectively. Further, this study was carried out to measure the aggregation of the flocs’ weight. The present work demonstrated that the average floc weight for aerobic and anoxic digestion was measured, where it was 0.55 and 0.50 g after 3 days and then increased significantly after 5 days to 0.69 and 0.61 g (ANOVA F(3, 8) = 155, post-hoc Scheffe's test, p= 0.00), respectively. It can be seen that the removal quantity of COD and floc were often higher in the aerobic treatment than in the anoxic treatment. This observation leads to the suggestion that the charge adsorption–neutralisation mechanism was achieved with high efficiency by introducing aerobic digestion. Consequently, the reduction in the COD values upon the aeration system is probably due to the breakage of stable colloid particles and thus presented as emulsified oil within the wastewater. Therefore, the aeration system for 5 days was recommended in this study.

Saponin removal

The aeration process was carried out to measure the removal of saponin during the aeration treatment. Two volumetric flasks were used during this research. The first flask was filled with distillate water and saponin (0.5 wt%), while the second flask was filled with artificial seawater and saponin (0.5 wt%). The experiment aimed to compare the initial COD values with the COD values obtained at the end of Day 5 in order to find the decrease in saponin due to biodegradation in terms of COD reduction. The outcome of the experiment is shown in Figure 8.
Figure 8

Mean value of COD values for artificial seawater with saponin and distilled water with saponin. The data are expressed as mean ± SD (n = 3).

Figure 8

Mean value of COD values for artificial seawater with saponin and distilled water with saponin. The data are expressed as mean ± SD (n = 3).

Close modal
This work identified that the COD reductions for saponin prepared with distilled water and PSS were decreased significantly from 6,488 (SD = 222) to 2,018 mg/l (SD = 75) (paired test, t = 62, df = 4, p < 0.05). Therefore, around 69% of saponin seems to be removed during the aeration system. However, the study identified that the mean values of COD for saponin prepared with artificial seawater and PSS was significantly reduced from 8,753 (SD = 173) to 4,415 mg/kg (SD = 148) (paired test, t = 37, df = 4, p < 0.05). The mean value of COD for artificial seawater is 2,550 mg/l (SD = 55), while the mean value of COD for saponin, once the COD value of artificial seawater is subtracted, becomes 6,203 and 1,856 mg/l before and after the aeration system, respectively. For this reason, it could be concluded that the average of the COD reduction for saponin in artificial seawater was about 68%. It seems that there is no significant difference between the final COD values of saponin prepared with artificial seawater and saponin prepared with distilled water (paired test, t = −2.1, df = 4, p= 0.106). This experiment is a rule of thumb for measuring the percentage removal of saponin during the aeration process. The outcome of the aeration system is a normal distribution (skewness and kurtosis, z-value = ±1.96). The percentage removal of saponin removal was calculated by:
formula
(1)
where COD final is the COD of oily wastewater after 5 days of aeration, the value of COD for artificial seawater is 2,550 mg/l, and the value of COD for saponin is 6,203 mg/l.

Removal of metals

Numerous techniques have been considered to remove or reduce the metal contents from water. The encounter between microbes and metal contents within the environment takes place in a few forms. Somehow, they likely interact with each other. In this investigation, the combination of an aeration system and coagulation/flocculation was carried out to measure the removal of metal contents from oily wastewater, an aerated sample and a treated sample, as illustrated in Table 3.

Table 3

The average concentration of metals during the water treatment process (n = 3)

ParameterOily wastewater (mg/l)Aerated sample (mg/l)Treated sample (mg/l)KEPA limit (mg/l)
Zinc (Zn) 2.302 1.684 0.752 2.0 
Aluminium (Al) 7.260 1.008 9.600 5.0 
Cobalt (Co) 0.520 0.510 0.440 0.2 
Chromium (Cr) 18.610 9.390 0.611 1.0 
Barium (Ba) 8.880 0.388 0.181 2.0 
Manganese (Mn) 0.588 0.300 ND 0.2 
Lead (Pb) 0.920 0.090 ND 0.5 
Iron (Fe) 52.240 6.308 6.197 5.0 
Boron (B) 24.720 9.520 7.742 2.0 
Nickel (Ni) 1.149 0.900 0.020 0.2 
ParameterOily wastewater (mg/l)Aerated sample (mg/l)Treated sample (mg/l)KEPA limit (mg/l)
Zinc (Zn) 2.302 1.684 0.752 2.0 
Aluminium (Al) 7.260 1.008 9.600 5.0 
Cobalt (Co) 0.520 0.510 0.440 0.2 
Chromium (Cr) 18.610 9.390 0.611 1.0 
Barium (Ba) 8.880 0.388 0.181 2.0 
Manganese (Mn) 0.588 0.300 ND 0.2 
Lead (Pb) 0.920 0.090 ND 0.5 
Iron (Fe) 52.240 6.308 6.197 5.0 
Boron (B) 24.720 9.520 7.742 2.0 
Nickel (Ni) 1.149 0.900 0.020 0.2 

The outcome of the present work recognised that the mean value of cobalt was almost the same in oily wastewater (0.52 mg/l) and the aerated sample (0.51 mg/l), while it was slightly less in the treated sample (0.44 mg/l). Iron content was also highest in oily wastewater with a mean value of 72.24 mg/l, while it becomes 6.31 and 6.20 mg/l in an aeration process and a combination system of aeration and coagulation/flocculation, respectively. Nickel content became the lowest in the treated sample with a mean value of 0.02 mg/l, while it was 1.15 mg/l in the oily wastewater sample. Furthermore, the aerated sample showed a much lesser mean value of nickel content (0.90 mg/l). Chromium was observed with a higher mean in the oily wastewater sample with a value of 19.61 mg/l, while it becomes 9.40 and 0.61 mg/l by using the aeration process and the combination system of aeration and coagulation/flocculation, respectively. One of the interesting features of the present investigation was the absence of manganese and lead in aerated and treated samples. While in metal, the average reduction of Ni and Ba in the treated sample was recorded to be 98%. The average reduction in a non-metal concentration such as B after the combination process was 69%. During this project, the alum has been used for the coagulation process, as expected aluminium was observed to be present in these treated samples with a maximum mean value of 9.60 mg/l.

A study carried out by Xia & Yan (2010) reported that the surfactants are able to increase the bioavailability of metals, which allows the biosorption to be more accessible through enhancing uptake mechanisms of microorganisms. Generally, the cell walls of microbial biomass, mainly composed of proteins, polysaccharides and lipids, have abundant metal binding groups such as amino groups, carboxyl, sulphate and phosphate. This type of biosorption, i.e., non­metabolism dependent, is relatively rapid and can be reversible. In the case of precipitation, the metal uptake may take place both in the solution and on the cell surface of microorganisms (Ercole et al. 1994). In most cases, these metalloids and metals are present in nature in the form of oxyanions, cations or both in an aqueous solution, and mostly in salts or oxides in crystalline (mineral) form or as amorphous precipitates in an insoluble form. The phenomenon of biosorption is vital in the microbial removal of heavy metals.

The concentration of cobalt in the oily wastewater was the lowest and its removal from the wastewater was the lowest, whereas those of barium and nickel were the highest removal. Hence, the removal of heavy metals from wastewater is influenced by their initial contents. According to Sheng-lian et al. (2006), there are many factors that can influence the removal efficiency of heavy metals during the biological processes, of which pH is regarded as the most critical factor for biosorption performance of toxic metals with microbial communities. As shown previously (Table 1), the oily wastewater sample is considered slightly basic, which may affect the removal efficiency of heavy content from wastewater.

Other external elements such as the concentration of the metal, metal species, solubility, contact time (treatment duration), temperature, presence of other cations and activated sludge dosage concentration of organic materials in wastewater can also contribute to the heavy metal toxicity (Le et al. 2021).

Subsequently, this also affects the heavy metals removal, despite the fact that many studies have demonstrated that heavy metals can be removed in the biological wastewater treatment processes (Chipasa 2003; Nanda et al. 2011; Agoro et al. 2020; Ida & Eva 2021). Consequently, the removal level of metal contents from oily wastewater may be difficult to estimate. Based upon the previous explanations, it is apparent that the treatment of wastewater by an aeration process seems like an intricate process that relies heavily upon various biological and physicochemical conditions in addition to process operating design and conditions.

It is well known that the varying ionic strength can affect the efficiency of the coagulation process and then flocs’ weight. Saponins are categorised as a non-ionic biosurfactant but contain an acidic and ionisable group of glucuronic acid as part of its head group (hydrophilic fraction) (Mitra & Dungan 1997), and some charged species would exist in saponin solutions from the artificial seawater. So, it seems that the ionic strength of the saponin solution may have a potential effect on the destabilisation of colloidal particles during the coagulation/flocculation process. There are advantages and disadvantages in each treatment, as all are subjected to the difficulty of operations and capital and operational expenditures. While the secondary treatment by the chemical process could be achieved by neutralising the negative charge of emulsion oil through using alum, polymer or acidification; furthermore, the pH needs to be modified into an alkaline range to encourage the formation of inorganic salt followed by gravity separation (Dick 1982).

On the other hand, the applicability of the secondary treatment with the aeration system was able to destabilise colloidal particles by breaking down the long chain of complex organic compounds in oil emulsion and convert them into small molecules by increasing the surface area ‘colloid concentration’ (Corsino et al. 2018; Tian et al. 2022). The outcome is in agreement with Ma et al. (2020) that the contact of the microorganisms with the wastewater could be improved which allows the biosurfactant to be ready for biodegradation. This point leads that the reduction in the COD reading is perhaps due to the biodegradation that occurred to the saponin. The biodegradation allows the breakage of colloid particles’ stability, which was presented as emulsified oil within the wastewater. Soeder et al. (1996) confirmed that the solubility of hydrocarbon fractures improves with the presence of saponins which makes them easily obtainable for the degrading bacteria. Therefore, during the biodegradation of colloidal particles during 5 days of aeration system, an adequate level of microorganism is able to break down the stability of oil emulsion by saponin.

The current work finds that the saponin can be easily degraded, while the degradation for oily wastewater was difficult, the behaviour characteristics of emulsions stabilised were changed by adapting the aeration system for 5 days; this point indicated that the microbial seed was able to degrade saponin compounds in the oily wastewater, which allowed the stability of oil emulsion to be broken down and released easily. Moreover, the conductivity of oily wastewater in this study was 60,800 μS/cm, so the ionic strength of the oily wastewater was considered as high. Based on the literature, the sodium chloride (NaCl) aids in double-layer compression. The study suggested that the ions with the opposite charge play a significant role in improving the flocculation process by bridging among the particles and allow the flocculate to be formed. As mentioned by Metcalf & Eddy (2002), the suspended solids have a negative charge, so that coagulant chemicals are able to allow the colloids to be neutralised by the positively charged coagulant; as a result, the net electrical force at particles is reduced during coagulation. The two means whereby high coagulant dosage can improve the rate of coagulation are: (a) increasing the metal hydroxide precipitate concentration and hence the rate of aggregation, and (b) enmeshing particulates to enlarge the size of aggregates using sweep–floc coagulation (Duan & Gregory 2003). In this case, adding a high dosage of coagulant precipitates a heavy, sticky and high quantity of flocs, while the settling time was accelerated as well.

The outcome of this study demonstrated that the settling time for colloidal particles by adapting the oily wastewater sample under an aeration system for 5 days required 5 min, while the sample without aeration required overnight to be settled. This scenario suggested the sweep–floc coagulation whereby the colloidal particles are removed from the suspension by means of enmeshment into the aluminium hydroxide Al(OH)3 precipitate. The process resulted in the creation of a suspension with a smaller size and high quantity flocs. This will, therefore, eliminate a higher quantity of organic particles in view of the increased surface area offered for adsorption. In contrast, low doses of coagulant boost larger and fewer quantity flocs owing to the faster rate of growth compared to the rate of nucleation. This causes the reduced surface areas for the adsorption of organic compounds to take place. This study suggested that the destabilisation improvement of colloidal particles occurred due to the linkage of inter-particles. This effect results in the generation of a large amount of precipitation that permits the colloidal particles to be enmeshed while settling. In such cases, it is not straightforward to define the main mechanism for destabilisation.

Oily wastewater generated by washing of oil-contaminated sand harms human health and the environment. Therefore, various wastewater treatments were performed during this research. This experiment found that the adsorption could not be the appropriate method for this study, due to the high contamination level and high cost, and becomes inapplicable.

It was noted that the BOD/COD ratios for all samples were 0.4 or lower. This indicates that the constituents in the wastewater samples were somewhat non-biodegradable and cause high toxicity in the samples that could affect the aquatic living. The coagulation/flocculation process showed better results when alum was utilised as a coagulant, with optimum concentrations of 1.53 mg/l. It has been concluded that the most suitable treatment method for oily wastewater is the combination of aeration with coagulation/flocculation.

The authors gratefully acknowledge the support of the Ministry of Higher Education, Kuwait, for providing the PhD scholarship, which helped the successful accomplishment of this research. Special thanks are also due to the honourable counsellor Mr Naif al Rukaibi for his support rendered in conducting this research. My deep gratitude to my lovely mother's soul for her loving support during this work. This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

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

The authors declare there is no conflict.

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