Abstract
This study evaluates the performance of a membrane bioreactor (MBR) for treating wastewater from the laminated plywood industry. To this end, a pilot-scale MBR was operated for 60 days with a hydraulic retention time of 20 h and a solid retention time of 20 days. The reactor's performance was assessed based on the removal of chemical oxygen demand (COD), phenol, turbidity, and apparent color. Furthermore, we monitored the solids content, dissolved oxygen concentration, and pH of the mixed liquor, as well as the progression of the transmembrane pressure (TMP). The wastewater exhibited a COD/biochemical oxygen demand (BOD) ratio of 5.5, suggesting low biodegradability, usually when this ratio is higher than 4.0. Nevertheless, it was observed that the MBR's performance was stable and satisfactory, with average removal efficiencies of 98% for COD, 70% for phenol, 99% for turbidity, and 93% for true color. The evolution of TMP indicated gradual membrane fouling; however, the operational limit of 0.6 bar was not reached during the study period. In conclusion, the utilization of MBR presents a promising approach to mitigate the environmental impacts associated with wastewater from the laminated plywood industry.
HIGHLIGHTS
This work addresses a gap in knowledge by studying the applicability of MBRs in treating effluents from the laminated plywood industry, standing out as a significant reference for researchers and industry professionals.
The study employed a pilot-scale membrane bioreactor to treat the laminated plywood industry effluent, showcasing impressive removal efficiencies for COD, phenol, turbidity, and true color.
INTRODUCTION
Globally, the wood processing industry holds a prominent position, emerging as one of the key sectors in fulfilling societal needs. It is crucial to note, though, that there has been a gradual decline in the consumption of sawn wood in recent years, juxtaposed with an uptick in the utilization of plywood. This shift can be ascribed to several primary factors: the dwindling availability of economically feasible wood for harvesting; societal pressure toward minimizing forest degradation; rapid technological advancements in producing cost-effective, high-quality plywood; and the increasing market receptivity toward substituting traditional solid wood products with plywood (Vieira et al. 2023).
These materials are composed of overlaid wood veneers from logs that have been previously cooked and cut, bonded together using adhesives or resins under high temperatures and pressures. Subsequent final finishing involves machining and sanding.
Despite the economic relevance of this activity, during the manufacturing process of laminated plywood, significant quantities of wastewater are generated due to the cooking of logs, containing a high concentration of lignin, which is known to be a compound that is difficult to degrade microbiologically. Furthermore, various chemicals can also be used in other stages of the production process, with a focus on formaldehyde, phenol, sodium hydroxide, urea, and acetic or formic acids, which are common components of bonding resins. Consequently, wastewater from the laminated plywood industry exhibits high concentrations of organic substances, suspended solids, dissolved solids, and nutrients, especially nitrogen and phosphorus (Prasad et al. 2019).
Due to the aforementioned characteristics, the discharge of effluents from this type of industry into water bodies without proper treatment can lead to various adverse environmental impacts. Among these impacts, eutrophication stands out, which is caused by an increase in nutrient concentrations that create favorable conditions for the growth of algae, resulting in reduced light penetration and an increase in the availability of organic matter in the environment. The oxidation of organic matter carried out by the activity of aerobic heterotrophic bacteria can also be considered a problem, as it results in a reduction in the concentration of dissolved oxygen (DO) in the environment, directly affecting local biodiversity. Furthermore, the presence of suspended and dissolved solids in high concentrations can also have various detrimental impacts on the entire aquatic ecosystem. This is mainly due to increased water turbidity and color, as well as sedimentation in riverbeds or water sources, resulting in sedimentation and siltation (Metcalf et al. 2014).
Another significant environmental issue related to this effluent is the presence of phenolic compounds, which are found in the lignin molecules of logs and are dissociated into the aqueous environment when subjected to high temperatures. Phenolic compounds are recalcitrant and can bioaccumulate, and as a result, when discharged into water bodies, they can impact the entire trophic chain, potentially affecting humans (Prasad et al. 2019). Considering the aforementioned factors, it becomes evident that effective management of these effluents to remove pollutants is imperative before their discharge into natural water bodies.
Wastewater from the wood industry, as a whole, can be treated using various processes depending on the characteristics and volume of the generated wastewater, primarily aiming to remove solids and organic materials. Among the most used treatment methods, primary clarification in settling tanks is prominent, followed by biological processes such as activated sludge systems and anaerobic reactors. Additionally, constructed wetlands, which combine physical filtration processes with biological degradation processes, have also been employed for this purpose (Zenaitis et al. 2002; Patel et al. 2021).
However, it should be noted that these alternatives usually do not provide satisfactory treatment, especially regarding the removal of recalcitrant and difficult-to-degrade compounds, such as phenol, necessitating the incorporation of a post-treatment stage (Klauson et al. 2015; Prasad et al. 2019). For this purpose, coagulation, flocculation, and sedimentation processes are commonly employed. Nevertheless, they also have disadvantages, as they involve the use of large quantities of inorganic chemicals and have high operational costs due to the treatment and final disposal of the generated sludge (Patel et al. 2021).
Given these constraints, there is a pressing need to explore new alternatives to tackle these challenges, particularly in the context of treating wastewater from the laminated plywood industry, for which literature on the subject remains sparse. In this context, one of the promising concepts for effluent treatment is membrane bioreactors (MBRs). Conventional MBRs are distinguished by their aerobic microorganism-based degradation system, coupled with an internal or external membrane module within the reactor. The use of membrane filtration processes allows for an increase in the microbial concentration within the bioreactor, as all of the biomass is retained within it. This technology provides operational simplicity and high-performance stability and this enhances the degradation processes of organic compounds and results in high removal efficiencies for various pollutants, such as organic matter, nutrients, and phenolic compounds (Judd & Judd 2011; Zhang et al. 2020).
Due to these advantages, MBRs have been successfully employed for the treatment of various types of effluents, including domestic wastewater, food processing, textile, tannery, landfill leachate, pharmaceutical, oily, and petrochemical wastewaters (Fazal et al. 2015; Park et al. 2015). According to Judd & Judd (2011), when comparing MBRs with conventional technologies in industrial wastewater treatment, the main advantages are greater pollutant removal, especially of solids and microorganisms due to retention by the membranes, a smaller footprint, and higher potential for reuse.
In the wood processing industry, particularly in pulp and paper production, the application of MBRs for wastewater treatment has been documented with notable efficacy in pollutant removal. Patel et al. (2021) reported excellent performance in this context. Additionally, Galil & Levinsky (2007) assessed MBR treatment of paper mill wastewater to achieve high-quality effluent suitable for reuse, observing reductions of 86% in chemical oxygen demand (COD) and 98% in biochemical oxygen demand (BOD). Furthermore, both total kjeldahl nitrogen (TKN) and ammonia levels decreased by 90%, with total suspended solids (TSS) consistently below 5 mg/L in the effluent. Erkan & Engin (2017) investigated wastewater treatment and activated sludge properties in a submerged MBR within the paper mill industry. Their findings indicated removal efficiencies of 98% for COD, 92.99% for NH3-N, and 96.36% for total phosphorus (TP), confirming MBR's effectiveness for this type of wastewater. However, the literature lacks reports on using this technology for treating wastewater specifically from the laminated plywood industry, underscoring the need for further research in this field.
Within this framework, this study seeks, in an innovative way, to assess the effectiveness and stability of an MBR when treating real wastewater from a laminated plywood manufacturing industry, with a focus on the removal of organic matter and phenol.
METHODS
Experimental setup and reactor operating conditions
Experimental setup. 1 – influent tank; 2 – permeate tank; 3 – air blower; 4 – filling pump; 5 – peristaltic pump; 6 – power supply; 7 – membrane; 8 – aerobic reactor; 9 – digital gauge.
Experimental setup. 1 – influent tank; 2 – permeate tank; 3 – air blower; 4 – filling pump; 5 – peristaltic pump; 6 – power supply; 7 – membrane; 8 – aerobic reactor; 9 – digital gauge.
Wastewater source and system acclimatization
This effluent, derived from the cooking process of Pinus logs and other production stages, was collected post the primary treatment phase that includes screening. To preserve its intrinsic properties, effluent collection was performed weekly, in an industry located in Irati/PR, Brazil.
For the initiation of the MBR, sludge from the aerobic pond of the facility's wastewater treatment plant was used. This sludge underwent a sedimentation process, followed by supernatant removal, to achieve a TSS concentration of approximately 5.0 g/L. Thereafter, the reactor was inoculated with this concentrated biomass. A 30-day acclimatization phase ensued, resulting in an initial TSS concentration of 3.0 g/L at the commencement of system monitoring.
Operation of the MBR
After acclimatization, the operation of the MBR was conducted for a period of 60 days. The treatment was carried out using continuous flow, facilitated by a peristaltic pump responsible for filtration, in addition to a float that allowed the inflow of raw effluent by gravity, ensuring a constant operational volume of the reactor. Despite being operated in continuous mode, filtration was performed intermittently to induce relaxation and, consequently, minimize the fouling process. The intermittency was controlled by an Arduino board, consisting of 8 min in filtration and 1 min in relaxation (Judd & Judd 2011).
In an effort to mitigate the effects of temperature variation on treatment performance, the MBR was installed in a temperature-controlled environment at 20 °C, aided by air conditioning equipment. The remaining operational parameters adopted are detailed in Table 1.
Operational conditions of the membrane bioreactor used
Operational parameters . | Operating condition . |
---|---|
Cell retention time (SRT) | 20 days |
Hydraulic retention time (HRT) | 20 h |
Permeate flow rate | 47.5 mL/min |
Filtration flux | 5.7 L/m2 h |
Membrane aeration rate | 3 m3/m2 h |
Operational parameters . | Operating condition . |
---|---|
Cell retention time (SRT) | 20 days |
Hydraulic retention time (HRT) | 20 h |
Permeate flow rate | 47.5 mL/min |
Filtration flux | 5.7 L/m2 h |
Membrane aeration rate | 3 m3/m2 h |
We adopted a cell retention time (SRT) of 20 days, in accordance with the recommendations provided by Meng et al. (2009). Consequently, the system operated for a period three times longer than the sludge age (60 days), a requirement for achieving a steady-state condition.
The selection of a hydraulic retention time (HRT) of 20 h was informed by a comprehensive review of prior MBR studies, where suggested values ranged from 10 to 40 h (Belli et al. 2017; Gavlak & Vidal 2022). The permeate flow rate, and by extension, the filtration flux, were carefully chosen to align with the selected HRT.
The membrane aeration rate (MAR) of 3 m3/m2 h was established through preliminary tests conducted within the system, although the specific data are not presented here. Air was evenly distributed among diffusers located at the reactor's base (0.48 m3/h) and at the membrane's base (1.5 m3/h). This arrangement was instrumental in enhancing liquid mixing and preserving aerobic conditions within the system.
Monitoring of the MBR
To assess the behavior and efficiency of the system throughout the operational period, monitoring was conducted at three distinct points through sample collection and analysis. The first sampling point corresponds to the raw influent, the second at the mixed liquor (aeration tank), and the third in the permeate reservoir after treatment. The collected samples were subjected to the analyses presented in Table 2, performed twice a week and in triplicate, following the methodologies outlined in the 23rd Edition of the Standard Methods for the Examination of Water and Wastewater (Baird & Bridgewater 2017).
Sampling points and their analyzed parameters
Sampling points . | Analized parameters . |
---|---|
Raw influent | Soluble chemical oxygen demand (sCOD), biochemical oxygen demand (BOD), phenol, turbidity, and apparent color |
Mixed liquor | Soluble chemical oxygen demand (sCOD), phenol, pH, dissolved oxygen (DO), total suspended solids (TSS), volatile suspended solids (VSS), and fixed suspended solids (FSS) |
Permeate reservoir | Soluble chemical oxygen demand (sCOD), phenol, turbidity, and apparent color |
Sampling points . | Analized parameters . |
---|---|
Raw influent | Soluble chemical oxygen demand (sCOD), biochemical oxygen demand (BOD), phenol, turbidity, and apparent color |
Mixed liquor | Soluble chemical oxygen demand (sCOD), phenol, pH, dissolved oxygen (DO), total suspended solids (TSS), volatile suspended solids (VSS), and fixed suspended solids (FSS) |
Permeate reservoir | Soluble chemical oxygen demand (sCOD), phenol, turbidity, and apparent color |
Prior to conducting the analyses for COD and phenol in the mixed liquor, the sample was filtered using a vacuum pump and 0.45 μm membranes. This step was taken to exclude overestimated values that might arise from the presence of solids in the sample, ensuring that only the soluble fraction was analyzed. The sCOD was determined by the dichromatic closed reflux method (5220D), and the total phenol was determined by the Folin–Ciocalteu spectrophotometric method, described by Singleton et al. (1999). For the TSS, volatile suspended solids (VSS), and fixed suspended solids (FSS), the sample was carried out by filtering using a 1.2 μm glass fiber filter. The main instruments used were the UV-VIS spectrophotometer DR 6000 by Hach, pH meter PG1800 by Gehaka and DO Orion3 Star pH Benchtop Meter by Thermo Scientific for measuring DO inside the reactor, which maintained values close to 5 mg/L.
In addition to these analyses, the transmembrane pressure (TMP) in the filtration module was measured daily with a vacuum gauge integrated with a digital sensor. This measurement aimed to track the membrane's pore fouling process during filtration.
Data referring to sampling points and their analyzed parameters were analyzed using descriptive statistics. In relation to the tests involving the membrane clogging processes (increase TMP), a Kendal correlation matrix was made in the Action 3.7 software, involving the variables CODs, total phenol, turbidity, and color with the TMP.
RESULTS AND DISCUSSION
The wastewater used in this study was characterized for different parameters, and the results are presented in Table 3.
Characterization of the wastewater generated in laminated plywood manufacturing
Parameters . | Average values . |
---|---|
COD (mg/L) | 4,003 ± 248.7 |
BOD (mg/L) | 730.8 ± 185.05 |
Phenol (mg/L) | 27.64 ± 3.1 |
Turbidity (uT) | 396 ± 5.3 |
Apparent color (uC) | 3,746.5 ± 118.9 |
pH | 7.7 ± 2.09 |
Total solids (mg/L) | 5,211 ± 188.7 |
Parameters . | Average values . |
---|---|
COD (mg/L) | 4,003 ± 248.7 |
BOD (mg/L) | 730.8 ± 185.05 |
Phenol (mg/L) | 27.64 ± 3.1 |
Turbidity (uT) | 396 ± 5.3 |
Apparent color (uC) | 3,746.5 ± 118.9 |
pH | 7.7 ± 2.09 |
Total solids (mg/L) | 5,211 ± 188.7 |
Effluents from laminated plywood manufacturing are typically characterized by high concentrations of COD and BOD. This is primarily due to the log cooking process, which releases various organic substances like lignin. The COD/BOD ratio of the analyzed effluent was 5.5, indicating low biodegradability wastewater (Metcalf et al. 2014). Given the high COD and BOD concentrations, effective treatment is imperative. Organic matter can pose significant environmental threats when released into water bodies, especially by reducing DO levels.
Phenol is another prevalent compound in this type of wastewater, primarily derived from lignin in wood and bark. Phenols are recalcitrant, bioaccumulative, and known to cause environmental and public health challenges due to their hydrophobic nature and lipophilic characteristics (Prasad et al. 2019).
It is clear that the analyzed effluent contains high concentrations of total solids, likely originating from cooking and washing processes that dislodge materials from the plant matter. This abundant solid content elevates the effluent's color and turbidity, as reflected in the high values noted in this study. Releasing such effluent without adequate treatment into water bodies can be detrimental to the environment. It not only intensifies the color and turbidity of these water bodies but also obstructs photosynthetic processes and may promote siltation.
Biomass monitoring
Biomass behavior was monitored using parameters like DO, pH, and TSS in the mixed liquor, as shown in Table 4.
Average values of monitoring analyses of the membrane bioreactor throughout the system's operation
Monitoring parameters . | Mean values and standard deviation . |
---|---|
Dissolved oxygen (mgO2/L) | 5.45 ± 0.55 |
pH | 7.4 ± 0.35 |
Total suspended solids (TSS) (g/L) | 3.37 ± 0.25 |
Fixed suspended solids (FSS) (g/L) | 0.88 ± 0.05 |
Volatile suspended solids (VSS) (g/L) | 2.48 ± 0.25 |
Monitoring parameters . | Mean values and standard deviation . |
---|---|
Dissolved oxygen (mgO2/L) | 5.45 ± 0.55 |
pH | 7.4 ± 0.35 |
Total suspended solids (TSS) (g/L) | 3.37 ± 0.25 |
Fixed suspended solids (FSS) (g/L) | 0.88 ± 0.05 |
Volatile suspended solids (VSS) (g/L) | 2.48 ± 0.25 |
Throughout the operation, the average concentration of DO was found to be 5.45 mgO2/L, which aligns with the optimal concentrations for MBR operation (Judd & Judd 2011; Park et al. 2015). Ensuring the appropriate DO concentrations in the aerobic reactor is crucial, as it is utilized by aerobic microorganisms during the oxidation of organic matter and ammonium nitrogen.
The mean pH observed was 7.4, showing minimal fluctuations during the operational period. Von Sperling (2014) indicates that the optimal pH range for aerobic effluent treatment processes is close to neutral, between 6 and 8, which aligns with our findings. Von Sperling (2014) proposes that the ideal pH for aerobic biological systems ranges from 6.5 to 9. They emphasize that excessively low or high pH levels can be detrimental to the microorganisms.
The average concentrations of TSS, fixed, and volatile were 3.37 ± 0.25, 0.88 ± 0.05, and 2.48 ± 0.25 g/L, respectively. Additionally, due to the low standard deviation values calculated, it is evident that there was minimal variation in the solids content, indicating system stabilization, a characteristic of steady-state operation. In MBRs, the concentration of suspended solids can vary depending on the influent, primarily due to the applied organic load, as well as the operational conditions adopted, such as HRT or SRT. Damayanti et al. (2011) evaluated the effect of TSS concentration (between 5 and 20 g/L) on membrane fouling in an MBR. According to these authors, the membrane fouling rate (MFR) decreases with a reduction in TSS concentration. Similarly, Park et al. (2015) recommend that the TSS concentrations in aerobic MBRs are maintained generally below 15 g/L, in order to membrane fouling control.
In this context, it is noteworthy that MBRs offer a significant advantage over other biological processes, owing to the physical barrier imposed by the membrane, which provides complete solids retention. This enables operation with high biomass concentrations, resulting in a reduced amount of sludge to be disposed of and an increased capacity for degrading compounds present in the effluent (Judd & Judd 2011).
When analyzing the volatile suspended solids (VSS)/TSS ratio of the system, which indicates the degree of mineralization of the mixed liquor, an average value of 0.73 was observed. It is important to note that the obtained value is slightly lower than the typically recommended range for activated sludge or MBR operation, which usually falls between 0.8 and 0.9. According to Park et al. (2015), operating with a reduced organic load per unit of biomass is a common characteristic of MBRs, allowing these systems to operate near endogeneous conditions. Under such conditions, it is common for biomass to be stabilized with a lower content of volatile organic matter. Furthermore, the low VSS/TSS ratio observed in this study may also be related to the high concentration of total solids in the influent, as the fixed solids, which are not biologically degraded, tend to be retained in the mixed liquor due to the selectivity imposed by the membrane.
Assessment of organic matter removal
Concentrations of CODs in the raw effluent, mixed liquor and permeate, as well as the respective removal efficiencies obtained over the operational period.
Concentrations of CODs in the raw effluent, mixed liquor and permeate, as well as the respective removal efficiencies obtained over the operational period.
As previously elucidated, the effluent from the laminated plywood industry had a high average concentration of COD of 4,048 ± 248 mg/L, mainly due to log cooking processes. The average concentrations of COD in the mixed liquor and permeate were 111 ± 3 and 55 ± 3 mg/L, respectively. Thus, even though the effluent exhibited relatively low biodegradability (COD/BOD ratio of 0.73), the system achieved an average total efficiency of 98%. It is believed that the high efficiency obtained can be attributed primarily to the high biomass concentration in the system, which maximizes the biological degradation processes of organic matter (Judd & Judd 2011). Furthermore, by comparing the COD concentration results observed in the mixed liquor and permeate, it is evident that the selectivity imposed by the membrane also played a significant role in the additional removal of undegraded dissolved organic matter by microorganisms.
Prasad et al. (2019) conducted a study on effluent treatment from a plywood industry, employing chemical precipitation with lime, and achieved an approximate 40% reduction in COD. Woodhouse & Duff (2004), in their treatment of wood processing wastewater within an aerobic reactor, reported an average COD removal efficiency of 86% at 24 °C and 93% at 34 °C, highlighting effective biodegradation under both temperature conditions. Klauson et al. (2015) assessed various treatment methods for plywood production effluent, incorporating biological and physicochemical processes (coagulation, Fenton process, and ozonation). Their research revealed that pretreatment through aerobic biological degradation resulted in a 78% COD reduction. However, the optimal treatment approach involved a combination of biological pretreatment, chemical treatment with the Fenton reagent, and post-biological treatment, leading to the remarkable removal of up to 99% of the organic load.
Based on the findings presented, it becomes apparent that COD removal from such effluents poses a significant challenge, demanding a complex treatment approach. Notably, this study achieved satisfactory COD removal efficiency values, even when dealing with effluents characterized by high organic loads and limited biodegradability. These results underscore the potential viability of employing MBRs as an intriguing alternative for treating effluents originating from the plywood industry.
Evaluation of phenol removal
Concentrations of phenol in the raw influent, mixed liquor, and permeate, as well as the respective removal efficiencies obtained over the operational period.
Concentrations of phenol in the raw influent, mixed liquor, and permeate, as well as the respective removal efficiencies obtained over the operational period.
The high concentration of total phenols in this type of wastewater requires the use of treatment techniques capable of removing it due to its high pollution potential. In this context, it is evident that the treatment employed in this research allowed for an average removal efficiency of 70%, with average concentrations in the raw influent, mixed liquor, and permeate of 27.6 ± 2, 14.3 ± 0.6, and 8 ± 0.2 mg/L, respectively.
Total phenols are considered compounds that are difficult to remove through biological means, as the microorganisms present do not tolerate high concentrations of this pollutant. Therefore, the inhibitory nature of phenolic compounds sets a barrier to microbial metabolism, causing a decrease in their activity, which typically prevents the degradation of these compounds (Hsu et al. 2004). However, despite the significant challenge of phenol removal in conventional biological systems, MBRs can achieve satisfactory efficiencies (Fazal et al. 2015).
This behavior is explained because the presence of the membrane module allows for longer cell retention times, enabling microorganisms to have more contact time with this contaminant (Diez et al. 2002). Marrot et al. (2006) studied an MBR with different sludge ages for the treatment of effluents containing phenol with concentrations ranging from 3 to 10 g/L. According to these authors, longer sludge ages (120 days) provided better conditions for phenol removal, allowing for efficiencies of 80–100%.
Although the MBR showed a removal efficiency of 70% under the tested conditions, the concentration of total phenols in the permeate remained above the recommended standard (0.5 mg/L) set by Resolution 430/2011 of CONAMA (Brasil 2011), indicating the need for post-treatment to meet regulatory requirements. One alternative to consider is the use of chemical precipitation employing coagulants or the application of advanced oxidative processes. Prasad et al. (2019) studied wastewater treatment in a plywood industry using chemical precipitation with lime and achieved a phenol removal of approximately 38%. Klauson et al. (2015), on the other hand, assessed different biological and physicochemical methods for treating plywood production effluent. According to these authors, the combination of biological pretreatment, chemical treatment with Fenton reagent, and post-biological treatment enabled them to achieve up to 99% phenol removal.
Evaluation of the removal of apparent color and turbidity
Apparent color values in the raw effluent and permeate, as well as the respective removal efficiencies obtained during the operational period.
Apparent color values in the raw effluent and permeate, as well as the respective removal efficiencies obtained during the operational period.
Turbidity values in the raw effluent and permeate, as well as the respective removal efficiencies obtained during the operational period.
Turbidity values in the raw effluent and permeate, as well as the respective removal efficiencies obtained during the operational period.
It is observed that the average apparent color value in the raw effluent was 3,746 ± 118 uC, which was reduced to 227 ± 5 uC in the permeate. Regarding turbidity, the initial average value of 396 ± 4 was reduced to 0.3 ± 0.02 uT. This behavior allowed for the achievement of high removal of apparent color and turbidity throughout the operational period, reaching average efficiencies of 93 and 99%, respectively.
The high efficiencies obtained are related to the presence of the membrane filtration module, where most of the suspended solids and colloids present in the effluent are retained (Judd & Judd 2011). Zhang et al. (2009), when operating a microfiltration MBR in the treatment of effluent from the pulp and paper industry, using a 20-day sludge age and an 18-h HRT, achieved a turbidity removal efficiency of 99.24% in the effluent, resulting in a residual value in the permeate of 0.53 uT.
It is important to emphasize that the removal of color and turbidity holds paramount significance, particularly in water bodies like rivers and reservoirs, where elevated concentrations can give rise to esthetic concerns. Moreover, it leads to a reduction in the euphotic zone, hindering sunlight penetration, which, in turn, impedes photosynthesis and may indicate the presence of recalcitrant compounds (Von Sperling 2014). Consequently, the utilization of membrane filtration processes in effluent treatment has demonstrated substantial potential for clarification. Beyond mitigating pollution, these processes also enhance esthetic qualities, thereby reducing visual pollution (Park et al. 2015).
Membrane fouling
According to Figure 6, it can be observed that the TMP gradually increased over the course of the 60 operational days, exhibiting an average MFR of 8.9 mbar day−1. However, the TMP values remained below 0.6 bar under the operational conditions. Therefore, in accordance with the manufacturer's recommendations, there was no need to perform corrective membrane cleaning procedures.
It is noteworthy that the TMP profile presented here falls within the conditions of a subcritical flow regime, characterized by an initial slow increase in TMP, followed by a more pronounced growth (Pollice et al. 2005). Battistelli et al. (2018) assessed the mixed liquor characteristics and membrane fouling process in MBRs applied to wastewater treatment. They found that at the start of the operation, the MFR was 11.25 mBar/day, reaching 34.7 mBar/day after 30 days. Like the current study, these authors also indicated that the observed behavior resembles a subcritical flow condition during operation.
Table 5 presents a correlation matrix involving the response variables COD, total phenols, color, and turbidity with TMP, aiming to assess whether the response variables that are part of the effluent composition influence the increase in TMP.
Correlation matrix of TMP with COD, total phenols, color, and turbidity variables
Parameters . | TMP . | |
---|---|---|
rKendal . | p-value . | |
COD | 0.41 | 0.01* |
Total phenol | 0.35 | 0.04* |
Color | 0.15 | 0.41 |
Turbidity | − 0.12 | 0.46 |
Parameters . | TMP . | |
---|---|---|
rKendal . | p-value . | |
COD | 0.41 | 0.01* |
Total phenol | 0.35 | 0.04* |
Color | 0.15 | 0.41 |
Turbidity | − 0.12 | 0.46 |
(*) signifies a confidence interval of 95% and a p-value ≤ 0.05.
The variables that exhibited a significant correlation with the increase in TMP were COD (rKendall = 0.41; p = 0.01) and total phenols (rKendall = 0.41; p = 0.01). Wastewater from the wood industry typically contains a complex composition of organic substances, including carbohydrates, wood extractives (lipids, resins, and acids), and phenolic compounds, all of which play a crucial role in membrane fouling processes (Bokhary et al. 2018).
In general, it is observed that the high concentration of COD and total phenols influenced the increase in TMP. However, it is understood that the low fouling rate observed is related to the subcritical flow regime of the reactor's operation and the high efficiency of removal of these contaminants by the microorganisms present in the mixed liquor. Thus, the adsorption of contaminants and the growth of biofilm on the membrane surface were reduced.
Furthermore, it should be highlighted that the high flux air velocity (FAV) used in the current study (3.0 m3/m2 h) tends to contribute to fouling control due to the turbulence generated by air bubbles in the liquid medium. This turbulence promotes an increase in shear stress at the membrane surface, removing a portion of the accumulated fouling and minimizing the deposition of new particles. According to Park et al. (2015), in order to control fouling and promote proper oxygenation of the biological suspension with minimal energy costs, full-scale MBRs are typically operated with FAV values close to 1.0 m3/m2 h. Conversely, Ivanovic & Leiknes (2008) evaluated different aeration rates in a pilot-scale MBR without considering energy consumption and suggested that better operational conditions are achieved with FAV ranging between 1.7 and 3.4 m3/m2 h, which is close to the value adopted in the present study.
Finally, other factors that may have contributed to the low fouling rate were the high values of SRT (sludge retention time) and HRT used, which were 20 days and 20 h, respectively. Iorhemen et al. (2016), in an extensive literature review, emphasized that increasing these parameters significantly influences the reduction of fouling-related processes. In general, the adopted SRT and HRT can alter important characteristics of the mixed liquor, as they have a direct effect on the food-to-microorganism (F/M) ratio. Thus, higher values of HRT and SRT lead to reduced microbial activity due to substrate limitation, resulting in a reduction in the production of compounds responsible for membrane fouling (Qu et al. 2013).
In this context, Meng et al. (2009), in their literature review on MBRs, indicate that very low or very high sludge ages can impair the performance of this type of reactor. Therefore, they suggested optimal values within the range of 20–50 days for SRT. The HRT used, on the other hand, was based on various previous studies employing MBRs, with suggested optimal values ranging from 10 to 40 h (Belli et al. 2017; Gavlak & Vidal 2022).
CONCLUSIONS
It was observed that the treatment of plywood industry wastewater by an MBR demonstrated satisfactory efficiency in removing the studied parameters, with high removal efficiencies of 98% for COD, 99% for turbidity, and 93% for apparent color. Another factor to consider is the 70% removal efficiency of total phenolic compounds present in the effluent, highlighting the significant challenge in removing such compounds through biological treatment systems. Regarding membrane fouling-related processes, both COD and total phenols were found to be significantly correlated with the increase in TMP. However, it is important to highlight that the MBR operation remained stable, with a low fouling rate, and no chemical cleaning was required during the monitoring period. This behavior is believed to be associated with the low filtration flux, high FAV, and appropriate selection of SRT and HRT. Finally, this study provides a practical framework for applying MBR technology in other industrial contexts with similar wastewater challenges. In conclusion, the use of MBR can be considered a promising alternative for the treatment of plywood industry wastewater. In this way, the results obtained in the present study can support the replication on a real scale, contributing to the reduction of environmental impacts related to this industrial segment.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.