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Article

Influence of Organic Loading Rates on the Treatment Performance of Membrane Bioreactors Treating Saline Industrial Wastewater

by
Majeb Alotaibi
1,*,
Ashraf Refaat
1,
Faris Munshi
1,
Mohamed Ali El-Said
2 and
Saber A. El-Shafai
3
1
Department of Civil and Environmental Engineering, King Saud University, Riyadh 11451, Saudi Arabia
2
Housing and Building National Research Center, Giza 3750250, Egypt
3
National Research Center, Department of Water Pollution Research, Giza 12622, Egypt
*
Author to whom correspondence should be addressed.
Water 2024, 16(18), 2629; https://doi.org/10.3390/w16182629
Submission received: 21 August 2024 / Revised: 7 September 2024 / Accepted: 11 September 2024 / Published: 16 September 2024
(This article belongs to the Section Wastewater Treatment and Reuse)
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Abstract

:
This study investigated the efficacy of membrane bioreactor (MBR) technology in treating saline industrial wastewater, focusing on the impact of the organic loading rate (OLR) and the food-to-microorganism (F/M) ratio on treatment performance. This research utilized saline industrial wastewater from Al-Hasa, which had salinity levels ranging from 5000 to 6900 mg/L. It explored treatment processes at varying Chemical Oxygen Demand (COD) concentrations of 800, 1400, and 2000 mg/L, corresponding to an OLR of 0.80 ± 0.05, 1.41 ± 0.07, and 1.98 ± 0.12 g COD/L, respectively. The average F/M ratios used were 0.20, 0.36, and 0.50 g COD/g MLSS·d, maintaining a constant Sludge Residence Time (SRT) of 12 days, a hydraulic retention time (HRT) of 24 h (hrs.), and a flux of 10 L/m2·h. The MBR system demonstrated high COD removal efficiencies, averaging 95.7 ± 1.6%, 95.5 ± 0.4%, and 96.1 ± 0.3%, alongside Biochemical Oxygen Demand (BOD) removal rates of 98.3 ± 0.2%, 99.8 ± 0.1%, and 98.5 ± 0.1%, respectively. However, an increased OLR led to elevated residual COD and BOD levels in the treated effluent, with COD concentrations reaching 34.2 ± 12.8, 63.3 ± 5.9, and 76.5 ± 5.4 mg/L, respectively. This study also reveals a significant decline in ammonia and Total Kjeldahl Nitrogen (TKN) removal efficiencies as OLR increases, dropping from 96.1 ± 0.5% to 80.2 ± 0.9% for ammonia and from 83.8 ± 3.4% to 65.8 ± 2.3% for TKN. Furthermore, higher OLRs significantly contribute to membrane fouling and elevate the transmembrane pressure (TMP), indicating a direct correlation between OLRs and operational challenges in MBR systems. The findings suggest that for optimal performance within the Saudi disposal limits for industrial wastewater, the MBR system should operate at an F/M ratio of ≤0.33 g COD/g of Mixed Liquor Suspended Solid (MLSS)·d. This study underscores the critical role of the OLR and F/M ratio in treating saline industrial wastewater using MBR technology, providing valuable insights for enhancing treatment efficiency and compliance with environmental standards.

1. Introduction

The scarcity of fresh water is a worldwide issue due to weak management strategies, inefficient wastewater treatment, the disposal of untreated sewage, the overuse of available resources, overpopulation, and climate change [1]. Globally, around 800 million people have no access to an appropriate source of drinking water, and in parallel, the industrial water demand is estimated to increase fourfold between 2000 and 2050 [2]. The scarcity of freshwater is the most significant challenge for Saudi Arabia [3]. The ongoing discharge of industrial wastewater into the environment and its impacts on living organisms represent a great concern [4]. Globally, around 80% of produced wastewater is discharged to the environment without treatment [5]. Land disposal of wastewater seriously affects groundwater, which is used by 1.7 billion people around the globe, putting them at high health risks. In Saudi Arabia, groundwater represents 80–89% of the water demand [5]. Groundwater is used as the main source of water in many regions in the kingdom, and groundwater abstraction reaches an extent of 17 billion cubic meters annually [6]. Importantly, aquatic environments are greatly affected by wastewater discharge [7]. However, the negative impacts of industrial wastewater discharge can be minimized by wastewater treatment [4]. Soil is a complex and diverse ecosystem that serves as the primary source of human food. However, excessive discharge of untreated wastewater seriously affects soil fertility and crop sustainability [8]. Globally, many industrial sectors generate large amounts of saline wastewater [9], discharged excessively with severe impacts on ecological systems [10]. The impacts of industrial wastewater discharge on wetlands, water bodies, and crop fields have been investigated, and the results revealed contamination of the food chain through soil and field crops [11]. Saline industrial wastewater is difficult to treat using conventional biological treatment methods [10,12], as well as physicochemical processes, because high-salinity wastewater is complicated, and the national and local discharge standards are increasingly strict, which makes it increasingly difficult to treat high-salinity wastewater [10]. Most microorganisms involved in the conventional activated sludge wastewater treatment are non-halophilic and non-adapted to stress conditions created by elevated salinity concentrations [10,12]. The performance of MBRs could be stable and efficient in treating saline wastewater with salinity up to 10 g/L. Still, beyond 10 g/L, salinity could exert osmotic stress on the microorganism, causing dehydration and cell damage [13]. However, adapting microbial communities to high salinity generates salt-tolerant bacteria. Furthermore, the bioaugmentation of biological treatment systems by salt-tolerant bacteria (halophilic) has been recommended and proposed to limit the detrimental impacts of salinity on treatment performance [10]. This process was successfully used to treat saline wastewater and alleviate the inhibitory effects of saline wastewater on the treatment performance of biological systems [14]. Moreover, an MBR has a better treatment performance than conventional activated sludge [15], showing very high efficiency in terms of organic carbon and ammonia while treating saline wastewater [16]. The gradual elevation in salinity concentration enables sludge biomass to attain good acclimation [16] and provide high treatment efficiency at variable salinity levels. Saline industrial wastewater contaminated with hardly biodegradable organic and recalcitrant materials is generally treated in an MBR with a long hydraulic retention time (HRT) and low flux rates. A moving-bed MBR was operated at a flux rate of 5 L/m2∙h to treat saline wastewater contaminated with the disinfectant NaClO [17]. The treatment of tannery wastewater was carried out over 280 days in an MBR working at a 4.2 L/m2∙h flux rate and a 1.3 g COD/L·d OLR, and good treatment performance was achieved with average ammonia, TKN, and COD removal efficiency rates of 70%, 68%, and 78%, respectively [7].
Vo et al. [7] recommended the optimum operation of an MBR treating tannery wastewater at a flux rate as low as 4.2 L/m2∙h and an OLR less than 1.3 kg COD m3∙d. Synthetic wastewater with variable salinity levels (5–20 g/L) was treated in an MBR working at a very low flux rate (3.5 L/m2∙h) and long HRT (18 h) [15]. Other researchers operated the MBR at a flux rate of 15 L/m2∙h during the treatment of saline wastewater [16,18]. However, an MBR operated at this substantially high flux rate (15 L/m2∙h) suffers from an elevated transmembrane pressure (TMP), approaching 0.6–0.7 bar [16]. During the treatment of saline wastewater, operation of the MBR at an OLR of 2.52–2.79 g COD/L∙d achieved COD removal of 98% and 90% at low (2.7 g/L) and high (9.9 g/L) salinity levels [18]. The system achieved 99% ammonia removal from an initial concentration of 85 mg N/L at low salinity and 80% ammonia removal from an initial concentration of 150 mg N/L at high salinity [18]. The estimated removal load of ammonia was higher (120 mg N/L) at a high salinity level than the removal load (84.1 mg N/L) at a low salinity level. During the operation of MBR to treat synthetic wastewater with variable salinity levels (5–20 g/L) at a 3.5 L/m2∙h flux rate over an 18 h HRT [15], Giacobbo et al. [19] achieved satisfactory carbon removal while treating tannery wastewater at 1.38–2.05 g COD/L·d.
Variation in the quantitative and qualitative characteristics of saline industrial wastewater makes it necessary to assess the impacts of OLR on the treatment performance of MBRs in a case-by-case fashion. Al-Hassa is a Saudi industrial city that generates saline industrial wastewater, and this wastewater should be effectively treated before environmental discharge. Therefore, the current study aims to assess the influence of OLRs on the treatment performance of MBRs in treating such wastewater and compare the residual concentration of various pollutants with Saudi disposal limits [20].

2. Materials and Methods

2.1. MBR System and Experiment

The experiment was carried out using a pilot-scale MBR system, including an influent storage tank, an online feed tank, an aeration tank, an MBR tank, a permeate storage tank, and a backwash tank (Figure 1). The effective volume of the aeration tank was 240 L. An Ultra-Filtration (UF) membrane module with a 0.06 µm pore size and a 1 m2 effective filtration area was made of polyvinylidene fluoride (PVDF) by HINADA Water Treatment Tech. Co., Ltd. (North of Junya Road Number 3, Huangpu District, Guangzhou City, China). The manufacturer recommended a permeate flux range of 10–25 L/m2∙h. The mixed industrial wastewater was collected after the Dissolved Air Floatation (DAF) unit at the Al-Hasa wastewater treatment plant. It originates from various factory sources and exhibits high salt concentrations, as detailed in Table 1. The wastewater was stored in a 6 m3 storage tank equipped with a mechanical stirrer to ensure complete mixing of the contents and prevent the settlement of particulate matter. A peristaltic dosing pump drives influent from the storage tank to an online feed tank, which continuously feeds the aeration tank via gravity. Flow from the online feed tank is controlled by a float valve at the inlet point of the aeration tank. The aeration tank is connected to the MBR tank, and water flows by gravity from the aeration tank to the MBR tank; accordingly, the permeate flow from the MBR tank controls the influent flow to the aeration tank via the float valve at the inlet point. The membrane tank hosts a membrane module connected to a permeate vacuum pump and a backwash pump.
The vacuum pump drives permeate through the membrane filter. To control sludge deposition on the membrane surface, the membrane module conducts air purging at an aeration rate of 0.6 m3/min. At the same time, automatic backwash with clean water is carried out automatically by a backwash pump (30 L/h∙Day) connected to the backwash cleaning tank. Two air blowers, one in operation and one on standby, are used to secure continuous airflow to the system. A sludge recirculation pump was installed to recycle sludge from the MBR tank to the aeration tank. The SRT was maintained at 12 days using a sludge-wasting pump.

2.2. System Start-Up and Operation

The influent was stored in the influent storage tank and fully characterized before being fed to the pilot plant system. Glucose was used to enrich the industrial wastewater with organic matter to achieve a specific concentration of COD. Ammonium chloride and sodium hydrogen phosphate were used to adjust COD: N:P at 250:5:1 if needed [21,22]. Mixed Liquor Suspended Solid (MLSS) can act as an adsorbent for colloidal particles of raw wastewater, so inoculating the aeration tank with activated sludge was necessary to prevent pore clogging [23]. Hence, the aeration tank was inoculated prior with activated sludge from the Al-Hasa industrial wastewater treatment plant at 4 g MLSS/L.
The system acclimated to the experimental conditions for 2 weeks, after which the influence of three different proposed OLRs (0.8, 1.4, and 2.0 g COD/L∙d) on the treatment performance was assessed by feeding influent wastewaters with three different COD concentrations (800 mg/L, 1400 mg/L, and 2000 mg/L). However, the organic matter concentration, represented by COD and BOD, along with the measured flux rate, was used to estimate the actual daily OLRs. The SRT was maintained at 12 days, while the permeated flux rate was around 10 L/m2∙h, providing an average HRT of 24 h.

2.3. Monitoring Parameters and Analytical Methods

The analytical parameters, including the pH (pH meter), COD (APHA 5220 B), BOD (APHA 5210 B), Total Suspended Solid (TSS) (APHA 2540 B), turbidity (Hach Turbidity Meter, 2100 Q, Colorado/USA), TDS (APHA 2540 C), EC (HACH MP-6, Colorado/USA), salinity (HACH Salinity meter, 9531600, Colorado/USA), TKN (APHA 4500-N), ammonia (APHA 4500-NH3), Total Phosphate (TP) (APHA 4500 PB), and total coliform count (APHA 9221 B), were analyzed in the influent wastewater before and after enrichment with glucose and nutrients according to the standard methods for the examination of water and wastewater [24]. In addition to the previous parameters, nitrite and nitrate (HACH Spectrophotometer DR 3900, Colorado/USA) were analyzed in the treated effluent. Activated sludge samples were collected from the aeration tank and MBR tanks and subjected to the measurement of sludge volume after 1 h, MLSS (APHA 2540 D) concentration, and Mixed Liquor Volatile Suspended Solid (MLVSS) (APHA 2540 E) concentration, followed by the calculation of the Sludge Volume Index (SVI). Online measurement of TMP was carried out 24 h a day using a pressure gauge. The flux rate was measured hourly during the whole day using a graduated cylinder and stopwatch, and the value was maintained at 10 L/m2∙h. The average daily values of TMP and flux rate were extracted from the hourly measurements. A sample from the membrane with the fouling layers was investigated with a scanning electron microscope (SEM). Samples during system start-up and from each trail of an OLR were subjected to SEM analysis. Membrane sample preparation was performed using a Quattro S model SEM [25].

2.4. Statistical Analysis

All the data are presented as average ± standard deviations. The results of each analytical parameter within the different OLRs were subjected to a One-way Analysis of Variance (One-way ANOVA) to check the significance of differences between the OLRs. A probability level of 95% (p-value < 0.05) was chosen to indicate significance.

3. Results and Discussion

3.1. Characteristics of Industrial Wastewater

The data in Table 2 show the characteristics of industrial wastewater before and after enrichment with synthetic wastewater. The influent industrial wastewater was categorized as low-to-medium strength, with a COD value between 300 and 400 mg/L. Similarly, the influent wastewater before enrichment with synthetic wastewater had a limited nutrient content. There was a significant variation in the BOD/COD ratio, which was 29.3% in the second and third runs (1400 and 2000 mg/L initial COD) and a higher value of 39.7% in the first run (800 mg COD/L initial COD). There was no consistent correlation between COD and BOD; sometimes, they were inversely related (−0.75). This variation between COD and BOD is normal for mixed industrial wastewater, which may have variable ratios of biodegradable and non-biodegradable organic matter or the presence of inhibitory or toxic compounds that may affect the biodegradation process. There was a significant correlation (0.6) between ammonia and TKN.
The correlation between TKN and ammonia is variable and depends on the wastewater source and environmental conditions. The TKN contains ammonia and organic nitrogen, so an increase in the TKN concentration in the mixed industrial wastewater could be attributed to either an increase in organic nitrogen (dairy wastewater, egg processing wastewater, etc.), an increase in the ammonia concentration (ammonia industry), or an increase in both ammonia and organic nitrogen (meat-processing industry, slaughterhouse industry, livestock wastewater, or nitrogen fertilizer industry, which produce both ammonia and urea). In some cases, ammonification (the conversion of organic nitrogen into ammonia) takes place in the sewerage network, especially during summer, so ammonia might increase in the wastewater due to an increase in the ammonification rate, not an increase in ammonia in the main source. Thus, there will be no correlation between ammonia and TKN since the reduction in organic nitrogen will equal the increase in ammonia, but TKN will remain the same. In some cases, organic nitrogen might increase, and ammonification might increase as well, so there will be a strong correlation between ammonia and TKN. The best correlation was between TSS and turbidity, reaching 0.95. A weak correlation between TSS and turbidity was reported for wastewater collected from a university campus in Saudi Arabia [26].

3.2. Organic Loading Rates (OLRs) and Their Impacts on the Treatment Performance

The organic matter concentration represented by COD and BOD, along with the measured flux rate, was used to estimate the actual daily OLRs (Figure 2). The average COD loading rates were 0.795 ± 0.051, 1.401 ± 0.072, and 1.983 ± 0.122 g COD/L∙d in COD 800, COD 1400, and COD 2000, respectively. The corresponding BOD loading rates were 0.441 ± 0.031, 0.785 ± 0.031, and 1.140 ± 0.033 g BOD/L∙d. These values are within the range that has been investigated by many authors. A range of an OLR between 1.3 and 2.6 g COD/L·d was used during the treatment of real tannery wastewater in an MBR [7], which achieved a better residual concentration and removal efficiency for COD at 1.3 g COD/L∙d. One study reported satisfactory COD removal results during the operation of an MBR treating tannery wastewater at an OLR of 1.38–2.05 g COD/L·d [19]. During the treatment of slightly saline industrial wastewaters (vegetable canning and winery wastewater), two MBR systems were operated at a similar OLR (1.2 g COD/L·d), while the F/M ratio was 0.29 and 0.44 in the reactors [27]. While treating saline pharmaceutical wastewater, Zhang and his colleagues operated an MBR at a long HRT (30 h) with an estimated OLR of less than 1 g COD/L·d [28]. Xie and his research team [29] operated an intermittent aerated MBR for the treatment of saline synthetic wastewater with a 12 h HRT and a low estimated OLR of 0.25 g COD/L·d. Cartagena et al. [30] reported an OLR of 1.2 g COD/L∙d during the treatment of saline wastewater in an MBR, with an F/M ratio of 0.15 g COD/g VSS∙d (0.13 g COD/g TSS∙d).
MBR performance testing on 240 L at different inlet COD concentrations of 800, 1400, and 2000 mg/L was carried out between December 2023 and February 2024. For this duration, the SRT and HRT were maintained at 12 days and 24 h. The data presented in Table 3 show the average residual concentrations of TSS, turbidity, COD, and BOD in the treated effluent. The range of residual TSS in the effluents was 1.84–2.4 mg/L on average, with the highest value at the highest OLR. However, the statistical analysis in Table 3 shows no significant difference between the TSS concentrations in the effluents. Similarly, statistical analyses of turbidity did not show any significant difference between the three OLRs. Essentially, membrane filtration in the MBR system is a physical separation between treated effluent and sludge biomass and may not be affected by organic loading rates and other operating parameters related to the biological fluid. The complete removal of TSS was reported during the treatment of saline wastewater in an MBR [30], and Vergine et al. [27] did not find evidence of particulate COD in the treated effluent of MBR-treating winery wastewater, which indicates the complete removal of TSS. Furthermore, there were no significant differences between residual turbidity and TSS at different operating conditions in an MBR treating municipal sewage [26]. These results confirm the excellent effluent quality from the MBR with respect to TSS and turbidity, regardless of the OLR.
With respect to COD and BOD, the results of effluent quality indicated high removal efficiency for both. The BOD removal range was 98.3–99.8%, while the COD removal range was 95.5–96.1% (Table 4). The corresponding removal loads of COD were 0.761, 1.338, and 1.906 g COD/L·d, while BOD recorded 0.434, 0.783, and 1.123 g BOD/L∙d. The estimated daily specific removal loads were 0.179, 0.336, and 0.330 g COD/g MLSS for COD and 0.149, 0.272, and 0.407 g BOD/g MLVSS. The COD% removal in this study is better than the reported range (78–89%) for an MBR system treating real tannery wastewater [7] and the reported range (65–90%) for an MBR treating saline pharmaceutical wastewater with a 30 h HRT [28]. Furthermore, the COD% removal in this study is better than the average of 79.8% achieved in MBR-treated tannery wastewater with a 70 h HRT and a 2–5 g/L salinity level [31]. For the treatment of mixed residential sewage and water produced at different ratios and salinities (0.5–6.4 mS/cm) in an SBR-MBR operated with a 12 h HRT and an OLR of 0.26–1.0 g COD/L·d, Frank et al. [32] reported a decline in COD concentration by 79.5–90.4%, which is less than the performance of the current study. The results of this study are comparable to the data of a carrier MBR hosted with halophilic bacteria, which achieved 90–95% removal for COD using synthetic wastewater with zero ampicillin with a 30 h HRT and a 25 °C working temperature [28]. Furthermore, the current results are better than the results (85.2–87.9%) for an MBR operated at 0.197 g COD/L·d with an 18 h HRT for treating petrochemical wastewater [33]. The better results of this study compared to the others could be attributed to the origin of the sludge inoculum obtained from the Al-Hasa wastewater treatment plant, which receives and treats saline industrial wastewater. This sludge might be better adapted to saline industrial wastewater than other inocula from municipal sewage treatment plants. This could also be attributed to the co-metabolic effect of glucose and nutrient amendments to the industrial wastewater. The co-metabolic process has been reported using different easily biodegradable substrates to enhance and accelerate the biodegradation of high-salinity industrial wastewater [34].
In the current study, an increase in the OLR enhanced the daily removal load (0.761–1.906 g COD/L·d and 0.434–1.23 g BOD/L∙d), which aligns with the results found by Vo et al. [7], who reported an enhancement in COD removal after raising the OLR from 1.3 to 2.6 g COD/L·d. Similarly, during the operation of a modified MBR treating tannery wastewater, an increase in the OLR from 1.38 g COD/L·d to 2.05 g COD/L·d increased the hydraulic COD removal from 1.132 to 1.784 g COD/L·d, as observed by Giacobbo et al. [19]. Furthermore, in this study, specific COD removal loads increased from 0.179 to 0.336 g COD/g TSS∙d (0.262–0.464 g COD/g VSS∙d), which is better than the reported increase from 0.161 to 0.259 g COD/g VSS∙d. This means that there is an increase from 82% to 87% and removal loads from 161 to 259 mg COD/g VSS∙d; however, the average residual COD concentration increased from 413 to 522 mg/L. Despite the improvement in COD% removal and specific COD removal with increasing OLR, the residual concentration in the treated effluent increased from 34.2 to 63.3 and 76.5 mg/L during the second and third COD experiments. Similarly, increasing the OLR enhanced COD% removal, but the residual concentration in the treated effluent also increased [19]. Vergine et al. [27] reported a residual COD level of <50 mg/L during the treatment of saline canning wastewater in MBR, which matches these results. The residual COD in this study in the second and third experiments is higher than the value reported in [27], which could be attributed to the low F/M ratio (0.29) compared to the values of the current work (0.360 and 0.502) in the second and third OLRs.
The removal rates (Table 4) of ammonia and TKN in the current study were 80.2–96.1 and 65.8–92.2%, with higher values at a low OLR. Munz and his colleagues [31] achieved 99% ammonia removal during the treatment of tannery wastewater with a variable (2–5 g/L) salinity level at 70 h HRT. The higher removal rate reported by [35] could be attributed to the longer HRT of 70 h compared to 24 h in the current study. On the other hand, a lower removal rate of ammonia, between 15% and 50%, has been reported [28] during the treatment of saline pharmaceutical wastewater at 30 h HRT. These reported removal rates correspond to 10.8–35.9 gN/L∙d, which is comparable to the removal loads in the current study: 13.4, 20.9, and 33.7 mgN/L∙d. Furthermore, during the treatment of mixed residential and industrial wastewater with a salinity range of 1.6–3.2 g/L in MBR, Frank et al. [32] reported 24–88% ammonia removal, which is less than the removal rate in the current study. A decline in the ammonia removal rate and an increase in the residual concentration in the final treated effluent due to a rise in the OLR has also been reported before by Giacobbo et al. [19], who detected a decline in ammonia removal and an increase in its residual concentration with increasing OLR from 1.38 to 2.05 g COD/L. Following the same trend, a significant decline in ammonia removal and an increase in the residual concentration were reported [7] with increasing OLR when real tannery wastewater was treated in an MBR. Ammonia is mostly removed by the nitrification process, as seen in the high nitrate concentration in the final treated effluent. Increasing the OLR enhances heterotrophs, which potentially compete with autotrophs, enabling a rise in the residual ammonia concentration in the final treated effluent. The removal rate of total P was increased by increasing the ORL. The TP removal loads were 5.08, 5.0, and 7.8 mgP/L∙d, with the highest removal load occurring at the highest OLR. This could be attributed to the noticeably higher growth rate of the MLSS, with an average concentration of 3.97 g/L compared to 3.95 and 3.9 g/L at low and medium OLRs.
When saline synthetic wastewater was treated at 0.25 g COD/L·d, the system achieved 90% ammonia removal at a 3 g/L salinity level and 88.5% at a 5 g/L salinity level [36]. Similarly, at salinity levels of 5 g/L, an SBR system operated with an 8 h cycle achieved 83.8% ammonia removal, while TP declined by 74.3% [35]. Following the same trend, an MBR system for treating synthetic wastewater with 5 g/L salinity [37] achieved 84% ammonia removal and only 16% P removal. The better results of the current study could be attributed to the nature of the inoculum sludge obtained from a saline industrial wastewater treatment plant. This sludge might be adapted to the salinity level. A similar note was made by the authors of [38], who detected an increase in the treatment performance and COD removal from 77% to 92% due to sludge adaptation. Furthermore, the TN removal of the current study is better than the 30% removal efficiency that was reported in [39] during the treatment of urban saline wastewater at a salinity range of 3.0–3.5 g/L using an MB-MBR.
Despite the slight elevation in the salinity (Figure 3) of the influent wastewater during the three experiments, the nitrification process was stable. It provided a sufficient concentration of nitrate in the treated effluents. The higher salinity level was still below 1 g/L. Many authors report that a salinity level of 1 g/L is critical to the performance of the nitrification process, and a slight effect of salinity on nitrification could occur at a salinity of up to 1 g/L [4,40]. Even so, salinity above 1 g/L significantly affects microorganisms in general [12].

3.3. Influence of OLRs during the Treatment Experiments on Membrane Fouling and TMP

During the experimental intensive flux monitoring, TMP was conducted to keep the flux rate constant at 10 L/m2∙h (Figure 4). The average flux values were 0.96, 10.0, and 9.91 L/m2∙h, showing no significant difference between the reactors due to the continuous readjustment of the suction pump and online control of membrane fouling. However, there was a significant impact of the OLR on the membrane fouling, since the average TMP values recorded during the treatment experiment were 4.04 ± 1.88, 8.38 ± 1.59, and 14.2 ± 3.63 Kpa for COD 800, COD 1400, and COD 2000, respectively. Like in this study, elevation of the OLR from 1.3 to 2.6 kg COD m3∙d significantly increased membrane fouling during the treatment of tannery wastewater [7].
In this experiment, the high OLR did not increase the TMP to 60–70 Kpa, as reported by Di Trapani et al. [16]. However, they operated the MBR at a lower flux rate (5 L/m2∙h) than was used in this study. The membrane fouling in the current study was mostly devoted to cake layer formation, which was easily removable by back washing, as indicated by a scanning electron microscope (Figure 5). The elevation of the TMP and membrane fouling with increasing OLR, especially at 2.0 g COD/L·d or influent with 2000 mg COD/L, could be attributed to an increase in the excretion of Extracellular Polymeric Substances (EPSs) and an increase in the Soluble Microbial Products (SMPs) due to a high F/M ratio.
This explanation could be confirmed via SEM imaging of the membrane during the treatment trials. The roughness of the membrane surface was detected during start-up (A) and at low OLRs (B and C). In contrast, at a high OLR (D), the membrane surface seemed smoother, which could be attributed to the accumulation of more Extracellular Polymeric Substances (EPSs) and Soluble Microbial Products (SMPs) in the cake layer. This agrees with the rise in the TMP at high OLRs in this study, which has been attributed to a high concentration of EPSs and SMPs [7]. Operation at a high F/M ratio and OLR promotes more SMPs and EPSs, which results in lesser sludge filterability and a high TMP [17,18]. The SMPs and EPSs contribute to the formation of microbial aggregates and condensed gel [17]. However, cake deposition on the membrane surface may reduce the pore fouling and blocking tendency [16]. The presence of a high concentration of EPSs enhances cake layer formation since it plays a key role in the deposition and sticking of the MLSS to the membrane surface.
The total coliform count declined from an average value of 1.5 × 107–4.1 × 107 cfu/100 mL to a residual number of colonies ranging between 1.1 and 1.6 cfu/100 mL, which is comparable to the reported data regarding the potential of MBR to remove bacterial contamination. The MBR system can produce treated effluent with undetectable levels of coliform bacteria [41], and this mostly depends on the pore size of the membrane. A 7-log reduction was reported for E. coli and Fecal coliform in MBR treating municipal wastewater [42].

4. Conclusions

The utilization of MBR technology for treating moderately saline industrial wastewater has been demonstrated to be feasible, promising, and sustainable. The F/M ratio and OLR are the most important design parameters of an MBR treating moderate saline industrial wastewater. Moreover, MBR technology can be used to achieve Saudi disposal limits by adjusting and controlling the F/M ratio. Overall, this study provides clear and comprehensive guidelines for using an MBR to treat mixed saline industrial wastewater with moderate salinity. However, more research needs to be conducted in Saudi Arabia to evaluate MBR technology for saline industrial wastewater at different operating conditions, such as different salinity levels and sources of sludge inoculum. This research work is a valuable resource for industry professionals seeking effective solutions for managing saline wastewater and researchers investigating more operating parameters.

Author Contributions

Conceptualization, S.A.E.-S.; Methodology, M.A.; Software, S.A.E.-S.; Investigation, A.R. and F.M.; Writing—original draft, M.A.; Visualization, M.A.E.-S.; Supervision, F.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data Availability Statement

Data will be made available on request of authors.

Acknowledgments

The authors would like to express their sincere thanks to the Miahona company for its support during the operation and lab analyses.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The research was conducted independently of any commercial or financial involvement that could be construed as a potential conflict of interest.

References

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Water 16 02629 g001
Figure 1. A schematic diagram of the treatment unit.
Figure 1. A schematic diagram of the treatment unit.
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Figure 2. The COD and BOD loading rates during the experiment.
Figure 2. The COD and BOD loading rates during the experiment.
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Figure 3. Salinity, TDS, and EC during the COD 800, COD 1400, and COD 2000 experiments.
Figure 3. Salinity, TDS, and EC during the COD 800, COD 1400, and COD 2000 experiments.
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Figure 4. The flux rate and TMP during the COD 800, COD 1400, and COD 2000 experiments.
Figure 4. The flux rate and TMP during the COD 800, COD 1400, and COD 2000 experiments.
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Figure 5. SEM images showing the formation of the cake layer ((A): start-up; (B,C): samples with low and medium OLRs; and (D) samples with high OLRs).
Figure 5. SEM images showing the formation of the cake layer ((A): start-up; (B,C): samples with low and medium OLRs; and (D) samples with high OLRs).
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Table 1. The characteristics of wastewater from factories and residential areas reaching the Al-Hassa plant.
Table 1. The characteristics of wastewater from factories and residential areas reaching the Al-Hassa plant.
Facility TypeQuantity (m3/day)Total Dissolved Solids (TDSs) (mg/L)COD (mgO2/L)
Accommodation387585
Basic Metals2.342827278
Chemicals54.530071203
Coke and Refined Petroleum19.12101762
Computer, Electronic, and Optical3.712470580
Electrical0.37867240
Food11254381595
Furniture5.672420554
Machinery and Equipment37.21763714
Non-Metallic14.03234865
Paper26229001169
Pharmaceutical2.31200450
Printing14.560002400
Rubber and Plastics1215527926
Textiles0.491171473
Table 2. Influent wastewater before and after enrichment with glucose, nitrogen, and phosphorus *.
Table 2. Influent wastewater before and after enrichment with glucose, nitrogen, and phosphorus *.
ParameterUnitCOD 800COD1400COD 2000
Before After Before After Before After
Temperature°C28.628.625.826.822.122.7
pHpH Unit6.877.686.87.2
TurbidityNTU11713595145122165
TDSmg/L403360555430600074108455
CODmgO2/L30280040013803761976
TSSmg/L9311573.7122110155
BOD5mgO2/L120443.7117.17851101150
ECmS/cm4.526.295.646.227.828.57
AmmoniamgN/L11.2149.522.28.739.1
TKNmgN/L13.916.714.927.610.140.5
TPmgP/L5.85.84.25.73.28
Salinitymg/L355550554400500062146900
Total coliformMPN/100 mL3.9 × 1074.1 × 1071.4 × 1071.6 × 1071.4 × 1071.5 × 107
Notes: * nitrogen and phosphorus were provided as ammonium chloride and sodium dihydrogen phosphate, respectively, as needed to achieve a COD:N:P ratio of 250:5:1 [21,22].
Table 3. The residual concentrations of the treatment parameters and ANOVA test results.
Table 3. The residual concentrations of the treatment parameters and ANOVA test results.
Experimental COD 800COD 1400COD 2000
ParameterUnit
Temperature°C23.4 ± 1.5 a22.5 ± 1.2 a23.3 ± 0.6 a
CODmgO2/L34.2 ± 12.8 a63.3 ± 5.9 b76.5 ± 5.4 c
BODmgO2/L7.5 ± 1.1 a13.8 ± 1.2 b16.1 ± 2.57 b
TSSmgO/L1.84 ± 0.88 a1.82 ± 0.79 a2.40 ± 0.99 a
TurbidityNTU0.59 ± 0.24 a0.54 ± 0.29 a0.65 ± 0.33 a
Ammonia-NmgN/L0.59 ± 0.24 a1.29 ± 0.31 b5.4 ± 0.73 c
NitratemgN/L9.75 ± 2.53 a9.49 ± 4.49 a10.02 ± 3.30 a
NitritemgN/L0.01 ± 0.01 a0.01 ± 0.001 a0.02 ± 0.01 a
TKNmgN/L2.7 ± 0.57 a2.15 ± 0.21 b13.85 ± 0.92 c
TP *mg P/L0.72 ± 0.16 a0.70 ± 0.28 a0.20 ± 0.14 b
Notes: Analysis of Variance (ANOVA); values in the same row with different superscript letters are significantly different (p < 0.05).* Total Phosphate.
Table 4. The percentage removal rates of different parameters in the MBR system.
Table 4. The percentage removal rates of different parameters in the MBR system.
Experimental ParameterCOD 800COD 1400COD 2000
Unit
CODmgO2/L95.7 ± 1.6 a95.5 ± 0.4 a96.1 ± 0.3 a
BODmgO2/L98.3 ± 0.2 a99.8 ± 0.1 a98.5 ± 0.1 a
TSSmgO2/L98.4 ± 0.8 a98.5 ± 0.6 a98.5 ± 0.7 a
TurbidityNTU99.6 ± 0.2 a99.6 ± 0.2 a99.6 ± 0.2 a
Ammonia-Nmg N/L96.1 ± 0.5 a93.5 ± 1.6 b80.2 ± 0.9 c
TKNmg N/L83.8 ± 3.4 a92.2 ± 0.8 b65.8 ± 2.3 c
TPmg P/L87.1 ± 3.7 a87.7 ± 5.0 a97.5 ± 1.8 b
Note: the values in the same row with different superscript letters are significantly different.
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MDPI and ACS Style

Alotaibi, M.; Refaat, A.; Munshi, F.; El-Said, M.A.; El-Shafai, S.A. Influence of Organic Loading Rates on the Treatment Performance of Membrane Bioreactors Treating Saline Industrial Wastewater. Water 2024, 16, 2629. https://doi.org/10.3390/w16182629

AMA Style

Alotaibi M, Refaat A, Munshi F, El-Said MA, El-Shafai SA. Influence of Organic Loading Rates on the Treatment Performance of Membrane Bioreactors Treating Saline Industrial Wastewater. Water. 2024; 16(18):2629. https://doi.org/10.3390/w16182629

Chicago/Turabian Style

Alotaibi, Majeb, Ashraf Refaat, Faris Munshi, Mohamed Ali El-Said, and Saber A. El-Shafai. 2024. "Influence of Organic Loading Rates on the Treatment Performance of Membrane Bioreactors Treating Saline Industrial Wastewater" Water 16, no. 18: 2629. https://doi.org/10.3390/w16182629

APA Style

Alotaibi, M., Refaat, A., Munshi, F., El-Said, M. A., & El-Shafai, S. A. (2024). Influence of Organic Loading Rates on the Treatment Performance of Membrane Bioreactors Treating Saline Industrial Wastewater. Water, 16(18), 2629. https://doi.org/10.3390/w16182629

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