The plastic industry has significantly contributed to the accumulation of non-biodegradable waste in the environment, leading to the formation of microplastics (MPs) from improperly disposed plastic products First identified in the 1970s, microplastics are defined as plastic particles smaller than 5 mm, which can persist in the environment for thousands of years due to their chemical stability As a new pollutant, microplastics pose a global threat, adversely impacting aquatic life and human health through their entry into the food chain Marine organisms often mistake microplastics for food, resulting in physical harm, energy loss, and potential death, as well as chemical exposure that disrupts endocrine and reproductive functions.
Despite extensive studies on the appearance, origin, and distribution of microplastics, research on effective removal methods remains limited While some propose burning plastic for energy, this approach fails to decrease plastic production and contributes to air pollution Recycling plastic emerges as a more effective solution, as it not only reduces waste but also generates new materials However, the impact of recycling is diminished without broader awareness and education efforts to promote sustainable practices (Kershaw, 2016).
Wastewater treatment systems are significant contributors to microplastic pollution, primarily because they are not designed to effectively remove these small particles Research indicates that the particle size of microplastics poses a challenge for retention, leading to their release into the environment (Roex et al., 2013; Murphy et al.).
In 2016, research indicated that wastewater treatment plants achieved a microplastic removal rate of 98.41%, yet still released over 65 million microplastics daily The primary removal occurred during the oil and grease separation stage, with an efficiency of 44.59%, followed by the settling tank at 33.75% Further studies by Lee and Kim in 2018 confirmed that anaerobic-anaerobic-aerobic (A2O) systems, continuous batch reactors (SBR), and biological processes combined with microbial media demonstrated excellent microplastic removal efficiencies, exceeding 98% for all three technologies Recent investigations into polyethylene (PE), the most prevalent microplastic, revealed effective treatment using AlCl3.6H2O and FeCl3.6H2O in conjunction with ultrafiltration (UF) membranes, resulting in nearly complete removal of PE.
Recent studies primarily focus on assessing the efficiency of microplastic removal processes, yet they often lack specific, practical solutions Implementing a membrane filtration system to eliminate residual microplastics post-treatment emerges as a highly effective strategy, achieving removal efficiencies of up to 99.9%.
Membrane technology is increasingly utilized for effective wastewater treatment, yet conventional membrane bioreactor (MBR) systems face significant challenges, including the simultaneous removal of nitrogen and organic matter, high energy consumption, and membrane fouling control To address these issues, research into energy-efficient membrane technologies is essential for translating academic findings into practical solutions for wastewater treatment A major contributor to high energy costs is the aeration process used to mitigate membrane fouling This study proposes a novel approach by replacing aeration with a motor-driven physical movement system that facilitates reciprocal membrane motion, thereby minimizing floc deposition on the membrane surface This innovative installation significantly lowers the operating costs associated with membrane cleaning, enhancing the overall efficiency of the MBR process.
Given the research gaps indicated above, this study was conducted to explore the following objectives:
‚ Evaluate the organic matter and nutrient removal efficiencies of the rMBR system at different frequencies in comparison with the conventional MBR system;
‚ Compare the membrane fouling control ability when changing membrane configuration and membrane fouling control method From there, evaluate the energy usage of both systems
‚ Evaluate the ability to remove and metabolize microplastics in the system Evaluate the potential of microplastics in biological systems
This study was carried out with two main contents corresponding to two experimental systems with different scales
A reciprocating membrane system with a capacity of 1 m³/day was installed near the BK food court at the University of Technology Ho Chi Minh City Real wastewater, sourced directly from the food court's manhole, underwent preliminary treatment Activated sludge, obtained from the wastewater treatment plant of Coopmart Ly Thuong Kiet supermarket, was utilized due to its similar properties to the wastewater in this study.
A lab-scale anoxic membrane bioreactor (MBR) system with a daily capacity of 18 liters was established at the Key Laboratory of Advanced Waste Treatment Technology, where synthetic wastewater was utilized to assess the system's performance.
Microplastic
Plastic products play a crucial role in our daily lives and are widely used across various industries due to their convenience, durability, resistance to erosion, ease of processing, and affordability However, the rise of synthetic materials, particularly plastics, has led to significant environmental challenges, as vast amounts of non-biodegradable waste are continuously generated and released into the ecosystem Since their inception, global plastic production has surged dramatically, reaching 1.5 million tons, highlighting the urgent need for sustainable solutions to address plastic waste.
From 1950 to 2018, global plastic production surged from 1.5 million tons to 359 million tons, with projections estimating it will reach 500 million tons by 2025 (Bui et al 2020; Zhang et al 2020) Annually, over 240 million tons of plastic are consumed to satisfy consumer demands (Browne et al., 2011) A staggering estimate reveals that between 15 and 51 trillion microplastic particles, weighing between 93,000 and 236,000 tons, are present in the world's oceans (Ioakeimidis et al., 2016) Furthermore, a 2015 statistical report indicated that more than 8.3 million tons of plastic were produced, with 5.25 million tons of plastic particles entering the ocean.
Approximately 269,000 tons of plastic debris float on the ocean's surface, with about 4 million microplastics per square kilometer found on the seafloor (Parker, 2015; University of Georgia, 2017) While sunlight can decompose plastic, the cooling effect of seawater and microbial cover significantly prolongs this process (Gregory, 1999; Barnes et al., 2009) Improperly disposed plastic products eventually break down into microplastics (MPs), which are defined as plastic particles smaller than 5 mm (Talvitie et al., 2017; Kokalj et al., 2018) and can remain in the environment for thousands of years due to their chemical stability (Cozar et al., 2014) Microplastics are emerging as a major pollutant, contributing to global plastic pollution and raising significant concerns regarding their harmful effects on aquatic life and human health (Ma et al., 2019).
Microplastics are categorized into two types: primary and secondary Primary microplastics are extremely small particles, originating from sources such as raw plastic resins (nurdles), granular plastics (pellets), and the deterioration of plastic products, as well as emissions from industrial processes In contrast, secondary microplastics are larger fragments that result from the breakdown of plastic materials in both aquatic and terrestrial environments This fragmentation primarily occurs through decomposition processes influenced by solar radiation (UV light) and oxidation, with weathering also playing a significant role in their formation.
Table 2.1 Overview of sources for primary and secondary microplastics present in the environment (Duis et al, 2016)
- Personal care products containing microplastics as exfoliants/abrasives;
- Specific medical applications (e.g., dentist tooth polish);
- Drilling fluids for oil and gas exploration;
- Preproduction plastics, production scrap, plastic degranulate: accidental losses, runoff from processing facilities
- Preproduction plastics, production scrap, plastic degranulate: accidental losses, runoff from processing facilities;
- General littering and dumping of plastic waste;
- Losses of plastic materials during natural disasters;
- Plastic mulching synthetic polymer particles used to improve soil quality and as composting additive;
- Abrasion/release of fibers from synthetic textiles;
- Release of fibers from hygiene products;
- Paints based on synthetic polymers;
Wastewater treatment plants (WWTPs) are significant contributors to microplastic pollution, as they can transform primary microplastics into secondary forms (Sun et al., 2019; Roex et al., 2013) The microplastics present in municipal wastewater often stem from everyday human activities, such as the shedding of polyester and polyamide fibers from clothing during washing (Napper and Thompson, 2016) and the introduction of microplastics through personal care products like toothpaste and shower gels (Magni et al., 2019) Unfortunately, untreated microplastics are frequently released from WWTPs into water bodies, leading to environmental accumulation (Carr et al., 2016) These treatment systems struggle to capture microplastics due to their small particle size and inherent design limitations.
2.1.2 Effect of microplastic on the environment, organisms and human heath Because of the widespread distribution of MPs, particularly in aquatic environments, marine life is threatened by exposure to MPs with varying impact levels depending on the possibility of toxic chemicals leaching from plastic additives and adsorbed pollutants such as metals, pesticides, or persistent organic pollutants (Fossi et al., 2014) MPs are not only toxic, but they can also act as reservoirs for pathogen transmission, endangering marine life (Kor and Mehdinia.,
Different types of microplastics (MP), such as polypropylene (PP) and polyethylene (PE), demonstrate greater toxicity compared to polyvinyl chloride (PVC) when assessing their impact on Daphnia Magna, as noted by Renzi et al (2019) While larger organisms, including fish, reptiles, birds, and mammals, are influenced by both micro and macroplastics, smaller organisms like zooplankton, worms, coral, crustaceans, mollusks, and small fish are predominantly impacted by microplastics.
Microplastics adversely affect vertebrates, including mammals, reptiles, and aquatic birds, by disrupting essential behaviors like swimming, breathing, and feeding, which ultimately reduces their survival rates and hinders growth and reproduction Research by Li et al (2016) highlighted the widespread presence of plastic particles in various seabirds, fish, and mammal species across tropical, temperate, and polar regions The ingestion of microplastics can lead to significant digestive system damage and reproductive issues in females due to blockages in the intestine and cloaca (Nelms et al., 2016) Notably, the death of marine mammals, such as manatees, has been linked to plastic obstruction in their digestive tracts (Li et al., 2016).
The consumption of microplastics by large animals can lead to secondary effects due to the leaching of harmful contaminants, including trace metals and persistent organic pollutants, into their digestive systems This contamination may result in significant developmental and reproductive abnormalities in these animals.
Plastics on beaches significantly impact the environment, contributing to decreased sand temperatures, which can alter the sex ratios of reptiles, such as turtles, that lay eggs on these shores (Nelms et al., 2016) The potential toxicity of microplastics is linked to three main pathways: ingestion stress from physical blockage and energy expenditure for egestion, leakage of harmful additives like plasticizers, and exposure to associated contaminants, including persistent organic pollutants (Anderson et al., 2016) Additionally, plastics affect abiotic factors by changing light penetration in the water column and altering sedimentation characteristics (Eerkes-Medrano et al., 2015).
Plastics often contain harmful additives like bisphenol A and phthalates, which can affect both molecular and whole-organism health (Cole et al., 2011) As humans consume significant amounts of seafood, they are also exposed to microplastics (MPs), with data from the FAO (2016) indicating that 11 out of more than 25 fish species in global sea fishing contain these pollutants Furthermore, research by Browne et al (2010) highlights that organisms in coastal food webs ingest more microplastics compared to those in offshore habitats.
Despite the established harm of microplastics (MPs) in the marine food chain, there is a lack of conclusive data on their effects on human health However, MPs have been detected in various human food items, including canned sardines, carp, sprats, salt, beer, honey, and sugar Research indicates that individuals may consume between 1 to 5 MPs particles annually from canned fish, with sea salt in some regions containing as much as 19,800 MPs particles per kilogram.
243 ẻ 684 particles/L were also discovered in drinking water (Eerkes-Medrano et al., 2019)
Figure 2.1.Classification, Origins and impact of microplastic (Elitza S Germanov et al., 2018; Raymond Mason, 2018)
2.1.3 Removal technologies for microplastic, advantages and disadvantage ơ Pre ẻ treatment
According to the study of Hidayaturrahman and Lee (2019), they found that pre ẻ treatment can remove 62.7% ẻ 64.4% of microplastics effectively
Grit removal is the initial phase of wastewater treatment plants, aimed at slowing sewage flow to eliminate inert materials like sand and eggshells The aerated grit chamber is the most effective type for removing microplastics, utilizing compressed air to facilitate surface skimming and sedimentation, thereby efficiently capturing these contaminants Research by Zhang et al (2020) and Bilgin et al highlights the effectiveness of this method in enhancing wastewater treatment processes.
Research indicates that aerated grit chambers can effectively eliminate 59% of microplastics, particularly larger films measuring 1-5mm (Zhang et al., 2020) Additionally, primary sedimentation is capable of removing suspended solids with flow densities ranging from 1.1g/mL to 1.5g/mL, specifically targeting small-sized but high-density plastics such as PET (0.96 to 1.45 g/cm³) and PES (1.24 to 2.3 g/cm³) (Ngo et al.).
Flotation technology is highly effective for removing microplastic (MP) particles from water systems, achieving up to 100% removal efficiency under specific conditions, such as an aeration volume of 5.4 mL/min and a frother dosage of 28 mg/L (Zhang et al., 2021) The study highlights that removal efficiency is influenced by the type and size of the plastic, with larger polystyrene (PS) and polyethylene (PE) particles showing a greater tendency to float than smaller ones Additionally, research by Pramanik et al (2021) indicates that air flotation can effectively remove 69% to 85% of polyethylene (PE), polyvinyl chloride (PVC), and polyethersulfone (PES) particles, further emphasizing the potential of flotation as a secondary treatment method for microplastic removal.
Membrane fouling and control methods
Fouling refers to the increase in filtration resistance in membrane bioreactors (MBRs), primarily due to the accumulation of decayed fractions from activated sludge on the membrane surface or within its pores over time This interference reduces the membrane's filtration efficiency, leading to decreased productivity and higher operational and maintenance costs, as more frequent cleaning and backwashing are required to maintain stable permeation conditions Fouling can occur on the membrane surface or inside the pores, and in some cases, both simultaneously It is categorized into reversible fouling, which can be physically removed through methods like air scouring and backwashing, and irreversible fouling, which results from the internal clogging of pores by adsorbed colloidal and dissolved materials, necessitating vigorous chemical cleaning for resolution.
Figure 2.2 Membrane fouling by sludge cake formation
Fouling in Membrane Bioreactor (MBR) processes occurs through three primary mechanisms: pore narrowing due to the sorption of soluble and micro-colloidal substances smaller than the membrane pore size, pore plugging from particles that match the pore size, and the formation of a cake layer on the membrane surface from deposited substances (Krzeminski et al., 2017) According to Judd et al (2011), three key groups of parameters influence membrane fouling in MBR systems: biomass characteristics (including mixed liquor suspended solids (MLSS) concentration, particle size distribution (PSD), soluble microbial products (SMP) concentration, and extracellular polymeric substances (EPS)), operating conditions (such as hydraulic retention time (HRT), sludge retention time (SRT), operating flux, and the type and frequency of backwashing and chemical cleaning), and the physicochemical properties of the membrane (including pore size, surface energy and charge, and hydrophobicity).
The interaction between mixed liquor suspended solids (MLSS) and membranes in membrane bioreactor (MBR) processes can significantly reduce productivity A study by Yigit et al (2008) investigated the causes of fouling by examining five different concentrations of MLSS.
The study revealed that as the Mixed Liquor Suspended Solids (MLSS) concentrations increased (4600, 6600, 8600, 10100, 12600 mg/L) alongside varying aeration velocities (0.067, 0.101, 0.201, 0.250 m/s), there was a corresponding rise in protein and carbohydrate concentrations Additionally, the distribution of particle sizes shifted towards smaller particles While aeration velocities positively influenced fouling control across all tested concentrations, their effectiveness diminished significantly with higher MLSS levels Furthermore, operating above the critical flux significantly impacted membrane fouling.
Reciprocating membrane
Membrane bioreactor (MBR) technology is advanced, merging the benefits of traditional activated sludge processes with superior mechanical features However, it faces a significant challenge known as membrane fouling, which negatively impacts permeate quality and quantity while increasing operational costs and energy consumption To address this issue, various methods have been employed, including chemical, biological, and particularly physical techniques such as air scouring, backwashing, and relaxing Air scouring is favored in many MBR systems for two primary reasons: it helps maintain sludge floc suspension and dissolved oxygen levels, and it generates physical interactions that dislodge particulates from the membrane surface Despite its advantages, air scouring's effectiveness is limited by biofouling from solutes and bacterial flocs, leading to insufficient shear stresses to sustain high flux rates Consequently, optimizing bubble size and aeration processes is essential, although it may significantly increase energy consumption, accounting for about 50% of total operational costs These challenges have spurred the development of more cost-effective and sustainable MBR configurations, such as reciprocating membranes.
The reciprocating membrane bioreactor (rMBR) is an advanced version of conventional membrane bioreactor (MBR) technology, designed to minimize fouling and reduce energy consumption while maintaining system performance Both rMBR and conventional MBR utilize membrane modules for wastewater treatment through solid-liquid separation; however, the key difference lies in the fouling control mechanism Instead of an air diffuser that generates bubbles to clean the membrane surface, the rMBR employs a motorized crank system that creates a slow reciprocating motion at frequencies below 0.5 Hz This motion generates inertial and shear forces that dislodge particles from the membrane fibers, significantly reducing the likelihood of suspended solids adhering to the membrane Consequently, the rMBR system enhances fouling control and treatment efficiency while decreasing energy usage by over 65% compared to traditional air scouring methods in MBR tanks, and by 20-25% for the entire system.
Fouling control method Air scouring Inertial force caused by reciprocation Mechanism of removing foulants on the membrane surface
Vertical impact by bubbles Horizontally impact by inertial force
DO in MBR tank High Low
Internal recirculation pump At least two pumps Only one or no need
While not as widely used as conventional membrane bioreactors (MBR), reciprocating membrane bioreactors (rMBR) have shown effectiveness in managing membrane fouling and improving treatment performance for parameters such as COD, BOD, TN, and TP Research directly addressing the fouling propensity of rMBR is limited, with most studies evaluating membrane fouling primarily through its correlation with energy consumption Therefore, this section will concentrate on the energy required to maintain a consistent flux in rMBR systems.
Research on reciprocating membrane bioreactors (rMBR) indicates that the specific energy demand (SED) for reciprocation motors is significantly lower than that of aeration systems operating at similar scales A study by Barillon et al revealed that energy consumption for membrane air scouring and aeration in full-scale wastewater treatment plants constitutes 60% to 80% of total electricity usage, with flat sheet membranes requiring approximately 60% of that energy (about 0.45 to 0.55 kWh/m³) compared to 23% to 49% for hollow fiber membranes, highlighting the high energy demands of flat sheet membrane air scouring Additionally, a 2010 pilot-scale study found that maintaining constant fluxes of 19 and 25 LMH necessitated electricity inputs of around 3.0 and 2.3 kWh/m³ for aeration, respectively, which were higher than those reported by Barillon et al., underscoring the economic limitations of pilot-scale wastewater systems compared to full-scale operations.
Table 2.5.Energy consumption for different MBR systems
System scale Wastewater Specification of membrane
MBR Municipal Flat sheet and hollow fiber -
Pilot MBR Municipal Flat sheet, pore size 0.4 àm
Hollow fiber, PVDF, pore size 0.1 àm
0.01 (for short-term operation) 0.003 ẻ 0.015 (for long-term operation)
Hollow fiber, PVDF, pore size 0.1 àm
Hollow fiber, PVDF, pore size 0.4 àm
Hollow fiber, PVDF, pore size 0.4 àm
(*) SED: Specific Energy Demand, only for aerator (conventional MBR) or for reciprocation motor (reciprocating MBR)
Replacing the air scouring system in membrane bioreactors (MBR) with a reciprocating motor maintains membrane fouling control efficiency, as indicated by consistent energy requirements for constant flux At a flux rate of 25 LHM, the specific energy demand (SED) for the reciprocating motor averages 0.01 kWh/m³ in short-term operations and ranges from 0.003 to 0.015 kWh/m³ in long-term operations (Bae et al., 2020; De Sotto et al., 2018) These energy consumptions are significantly lower than those of conventional MBR systems, which report SED values of 0.149, 0.135, and 0.072 kWh/m³ for fluxes of 20, 25, and 40 LHM, respectively (Ho et al., 2015) This innovative MBR configuration enhances wastewater treatment technology by leveraging the benefits of conventional activated sludge while substantially reducing energy consumption and addressing inherent drawbacks of traditional MBR systems.
Overall research content
This study aims to achieve its research objectives through two main components utilizing different scale systems The first component involves a pilot-scale system designed to assess the performance, membrane fouling control, and energy efficiency of the rMBR system In contrast, the second component focuses on a lab-scale system primarily dedicated to investigating microplastics.
Operate the system at reciprocation stage
- No aeration in membrane tank
Evaluate the performance of treating pollutants and microplastics removal coupling with fouling control of MBR system
Evaluate the pollutant removal efficiency of MBR system
Evaluate membrane fouling at the different salt concentrations
Evaluate the characteristics and metabolism of microplastic in the system
Evaluate the performance and fouling control of the reciprocating MBR
Operate the system at adaptation stage:
- Resistance (Rt, Rm, Rf, Rc) (content 2)
Content 1: Evaluate the performance and fouling control of the
Content 1 aims to compare the differences between a conventional MBR system (use air scouring) and a reciprocating MBR system (use rotating motor) in the performance, membrane fouling control, and specific energy demand These two operating conditions were operated sequentially in one system and evaluated by the required parameters Figure 3.2 presents the schematic diagram and general working mechanism of the MBR system in this study
Figure 3.2 Schematic diagram of reciprocating MBR system ơ Technical description (general for both operating condition)
A pilot-scale Membrane Bioreactor (MBR) system with a capacity of 1 m³/day was utilized to evaluate two operational conditions for wastewater treatment The process was divided into two key stages: primary treatment, which involved an oil and grease tank and an equalization tank, and secondary treatment, comprising an anoxic, oxic, and membrane compartment Wastewater from the BK food court was channeled through a manhole to the oil and grease tank via a submersible pump To mitigate the risk of pump clogs caused by thick oil, grease, and large debris that could settle as floating solids in the biological tank, the influent pump was safeguarded with a protective cage.
The system managed to effectively process wastewater, along with a small quantity of oil and grease To enhance separation, a fine bubble diffuser was installed at the tank's bottom, which helped float the oil, grease, and surfactants to the surface for manual removal three times daily Subsequently, the wastewater was directed to an equalization tank, where aeration stabilized the flow rate and concentration before it was transferred to the biological tank for further treatment.
The biological tank consists of four compartments: anoxic, oxic, membrane, and treated wastewater In the anoxic compartment, a mixer operates at 30 rpm to ensure adequate contact between activated sludge and wastewater, maintaining a dissolved oxygen (DO) concentration below 0.5 mg/L, which is ideal for the denitrification process Wastewater then flows to the oxic compartment, where DO levels are maintained between 2 and 4 mg/L, supported by air blowers that help aerobic microorganisms decompose organic compounds into simpler inorganic substances like CO2 and water Subsequently, the treated wastewater overflows into the membrane compartment, with all operating conditions of the Membrane Bioreactor (MBR) system detailed in Table 3.1.
In the conventional MBR stage, continuous operation of the air diffuser enhances pollutant treatment and aids in removing cake layers from the membrane surface, thereby reducing fouling Conversely, the reciprocating MBR stage utilizes a rotating motor at the tank's top, with a fixed amplitude of 60 mm and adjustable frequencies of 0.3 and 0.46 Hz, generating inertial forces that dislodge foulants from the membrane This method effectively controls membrane fouling while minimizing energy consumption compared to conventional systems The membrane, with a pore size of 0.1 µm, acts as a barrier, allowing only clean water to pass through to the clean water container via a filtration pump with a permeate flux of 20 L/m².h An online pressure gauge monitors transmembrane pressure (TMP) to track fouling, while a backwashing pump operates on a cycle of 9 minutes filtration, 0.5 minutes backwash, and 0.5 minutes idle to alleviate surface cake layers Two electric valves automate the filtration and backwashing processes, and sludge is recirculated between compartments to sustain biomass concentration and enhance nitrogen treatment All pumps are connected to electric floaters to maintain stable water levels in each tank, with detailed equipment specifications provided in the Appendix.
Table 3.1 Operating condition of rMBR system
Filtration cycle 9 minutes on : 0.5 minutes backwash : 0.5 minutes idle
Backwashing flowrate (Q bw ) 1.5 Q (pressure < 0,6 kgf/cm 2 )
F/M 0.1 ẻ 0.2 kg COD/kg MLVSS.day
Frequency of rotating motor - 0.3 Hz 0.46 Hz
Amplitude of rotating motor - 60 mm
The wastewater used in this content was directly taken from the manhole of
The BK Food Court, situated on the campus of Ho Chi Minh City University of Technology, operates with a submersible pump to manage its wastewater This facility caters to the cooking and dining needs of lecturers, students, and staff, resulting in wastewater that is classified as domestic It is characterized by elevated levels of oil and grease, high chemical oxygen demand (COD), and significant nitrogen concentration.
Table 3.2 Characteristics of wastewater in content 1
The feed sludge for the biological system was activated sludge sourced from the membrane tank of the Coopmart Ly Thuong Kiet Supermarket's wastewater treatment plant, chosen due to its similar characteristics to the wastewater from the BK food court This similarity promotes the growth of aerobic bacteria and reduces the adaptation period of sludge in the rMBR system Initial measurements indicated an MLSS concentration of 1,955 mg/L and a pH of 7.6 upon the rMBR's operation Continuous operation of the activated sludge aimed to enhance biomass concentration, thereby improving treatment efficiency Additionally, the fouling behavior of the rMBR system was analyzed by assessing the relationship between sludge flocs and other materials present in the system.
This study utilized hollow fiber membranes with a pore size of 0.1 micrometers, constructed from PVDF material and produced by KOLON Industrial The membrane module comprised two identical elements, sharing the same characteristics, dimensions, and fiber count Detailed specifications of these membranes can be found in Table 3.3.
Table 3.3 Specification of membrane in content 1
5 Number of elements in one module element 2
6 Number of fibers in one element fibers 88
7 Number of fibers in one module fibers 176
9 Length of each fiber cm 100
10 Outer diameter of each fiber mm 2
12 Dimension of module frame cm D x R x C = 17×6×5
A reciprocating membrane bioreactor (rMBR) is an advanced version of conventional membrane bioreactor (MBR) technology designed to decrease fouling and energy use while maintaining system performance The key difference between rMBR and traditional MBR lies in the incorporation of a fouling control system In the rMBR setup, all equipment and motors are positioned atop the tank, replacing the air diffuser at the bottom with a rotating motor connected to a crank that facilitates slow reciprocal movement This movement, fixed at an amplitude of 60 mm and adjustable frequencies of 0.3 and 0.46 Hz, enhances fouling control by generating inertial and shear forces that dislodge particles from the membrane fibers, thereby reducing the likelihood of suspended solids adhering to the membrane surfaces (Kim et al., 2021).
Figure 3.3 A rotating motor of system in content 1
Content 2: Evaluate the performance of treating pollutants and
microplastics removal coupling with fouling control of MBR system
Content 2 was conducted with an AO-MBR system to evaluate its application ability in treating microplastic composition Accordingly, evaluating treatment performance, membrane fouling control ability and microplastic observation were required to give satisfactory results Essentially, the use of MBR system is similar to rMBR in terms of biological removal mechanism (except for the membrane fouling and energy consumption) Using the conventional AO-MBR system with synthetic wastewater source helped to minimize fluctuations in the influent closely control operating parameters In addition, with a lab-scale system, it helped to reduce sludge and plastic waste
Figure 3.4 Schematic diagram of Anoxic MBR system used for the Content 2
Figure 3.5 Picture of Anoxic MBR system under operation ơ Technical description
A lab-scale AO-MBR system with an 18 L/d capacity was established for experiments, featuring an anoxic tank, aeration tank, and membrane tank, all constructed from acrylic The system's working volumes were 12.5 L, 3.5 L, and 2.5 L, respectively Synthetic wastewater was mixed in a feed tank to prevent solids from settling and then pumped to the anoxic tank, where a mixer ensured optimal contact between pollutants and microorganisms while maintaining a DO concentration below 0.5 mg/L for denitrification The wastewater then overflowed into the aeration tank, equipped with an air diffuser to supply oxygen for nitrification Following aeration, the wastewater proceeded to the membrane tank, which utilized a 0.1 µm pore-size membrane to retain pollutants and sludge, with air scouring implemented to minimize fouling Treated wastewater was subsequently transferred to a designated tank via a peristaltic pump, with a pressure gauge monitoring transmembrane pressure to manage fouling A backwashing pump was employed to clean the membrane surface, operating on an 8-minute filtration and 2-minute backwash cycle to enhance performance and longevity Sludge from the membrane tank was recirculated to the anoxic tank to sustain biomass concentration and nitrogen treatment, while microplastics were introduced to the biological tanks to monitor their changes over time.
Table 3.4 Operating conditions of Anoxic MBR
Filtration cycle 8 minutes on : 2 minutes backwash
Table 3.5 Specification of membrane used for the content 2
Number of elements in one module element 1
Number of fibers in each element fibers 32
Length of each fiber cm 21.5
Outer diameter of each fiber mm 2
Dimension of module frame cm L×W×H = 7.5×1×21.5
To ensure stable experimental conditions, synthetic wastewater was utilized, with influent concentrations of COD at 600 mg/L, NH4+-N at 60 mg/L, and TP at 4 mg/L The specific components of the wastewater are detailed in Table 3.6 A concentrated stock medium was prepared at 100 times the required concentration (as illustrated in Figure 3.6) and stored in a refrigerator, with daily dilutions made to maintain the system's supply.
Feed sludge for the biological system was sourced from the MBR tank of Coopmart Ly Thuong Kiet Supermarket, as its characteristics closely resemble those of the wastewater in this system This approach aims to minimize the adaptation time required for microorganisms The initial pH of the sludge was measured at 7.1, with a mixed liquor suspended solids (MLSS) concentration of 3,276 mg/L.
Table 3.6 Components of synthetic wastewater used in the content 2
CuSO 2 5H 2 O 0.03 g/L; FeCl 3 6H 2 O 1.5 g/L; MnCl 2 2H 2 O 0.12 g/L; Na 2 Mo 4 O 24 2H 2 O 0.06 g/L; ZnSO 4 7H 2 O 0.12 g/L; KI 0.03 g/L
(*) can be changed demanding on pH of the system
In this research, fragment microplastics measuring 0.5 to 1.6 mm were introduced into a biological system (AO and MBR) at an initial concentration of 1,000 MPs/L to assess their metabolic effects after each unit The choice of fragment microplastics facilitates easier observation, and their size allows for effective examination under a microscope.
Methodology
Analyzing wastewater quality was conducted for both Content 1 and Content
The study aimed to identify the characteristics of wastewater and evaluate the treatment performance of two systems, while also assessing the impact of operating conditions such as the C:N:P ratio, F/M, and OLR Daily wastewater samples were collected and analyzed (excluding weekends) to ensure result accuracy In unavoidable circumstances, samples were stored in a refrigerator at temperatures below 4°C for subsequent analysis.
The pH, dissolved oxygen (DO), and total dissolved solids (TDS) levels were directly measured using specialized devices to ensure optimal conditions for microorganism growth Specifically, a pH meter HI 9813-6 was utilized for pH and TDS measurements, while a DO meter HI 9812-6 was employed to monitor dissolved oxygen levels.
Parameters for evaluating the quality of wastewater such as COD, NH4 +-N,
The analysis of NO2-N, NO3-N, TP, MLSS, and MLVSS was conducted in accordance with the Standard Methods for the Examination of Water and Wastewater (Baird, 2017) Detailed methodologies and the frequency of analysis for these parameters are presented in Tables 3.7 and 3.8 below.
Table 3.7 Methods for analyzing wastewater quality
No Parameters Methods Devices/Equipment
4 NO 3 - -N Colorimetric Stove, Spectrophotometer Hach DR5000
5 NO 2 - -N SMEWW 4500B Spectrophotometer Hach DR5000
6 TP Colorimetric Stove, Spectrophotometer Hach DR5000
7 MLSS SMEWW 2540D LabTech oven (105 0 C), Desiccator, 4-digit balance
8 MLVSS SMEWW 2540D Lenton furnace (550 0 C), Desiccator, 4-digit balance
3.4.2 Membrane fouling a) Transmembrane pressure observation
Transmembrane pressure (TMP) is the primary and simplest method for detecting fouling in membrane bioreactor (MBR) systems Due to variations in scale and installation, TMP values recorded in two different systems showed discrepancies Nevertheless, the goal and assessment of monitoring TMP values remained consistent across both systems.
In this study, TMP values were automatically logged every minute by a PLC and subsequently transferred to Microsoft Excel for analysis The daily average TMP values were calculated to assess the fouling process, which was deemed significant when TMP values reached 500 mbar.
In the small-scale system analyzed in Content 2, Transmembrane Pressure (TMP) values were manually recorded, with the most frequently observed TMP selected for analysis Fouling was identified when TMP values exceeded 50 kPa, prompting the consideration of chemical cleaning methods to address the issue.
As TMP increased and reached the fouling threshold, chemical cleaning became necessary to eliminate residual and irreversible fouling This study utilized a NaOCl solution for membrane cleaning.
Before chemically cleaning the membrane, the system's aeration was temporarily halted for approximately 30 minutes to allow sludge to settle Activated sludge from the membrane compartment was recirculated to the anoxic and oxic compartments using a sludge pump Tap water was then added to the membrane compartment, followed by the introduction of a 250 ppm NaOCl solution The cleaning process lasted 24 hours, after which the permeate pump discharged the solution The membrane compartment was filled with tap water again, and aeration was activated to dilute any residual chloride before discharging the mixture of tap water and leftover chemicals Finally, the system was restarted to initiate a new filtration cycle.
In contrast to content 1, the chemical cleaning process described in content 2 was more straightforward The system was temporarily halted, allowing for the membrane to be removed from the tank and immersed in a 0.5% NaOCl solution for four hours After this treatment, the membrane was thoroughly rinsed with tap water before being reinstalled in the membrane tank This process effectively addressed the removal of proteins (PN) and polysaccharides (PS).
One of the key parameters for evaluating film fouling is the concentration of extracellular polymeric substances (EPS), primarily composed of proteins and polysaccharides Protein concentration (PN) was determined using the Lowry method, with absorbance readings taken at 750 nm Similarly, polysaccharide (PS) concentration was measured via the phenol sulfuric acid method, with absorbance assessed at 490 nm.
PN and PS analysis was presented in Figure 3.7 as below
Figure 3.7 Protein and polysaccharides analysis method
Mixture of bound EPS solution
The procedure for PS determination:
- Pipet sample & adjust volume with distilled water to 2 mL solution into tube
- Add 1 mL of phenol solution 5% & 5 mL of sulfuric acid
- Allow the tubes to stand in 10 mins
- Shake, place in water bath for 15 mins
- Tgcf" "?"6;0 after 2 mins but before 1h ồ sample concentration has A mg/L
The procedure for PN determination:
- Bring sample solution to 0.5 mL with distilled water
- Vortex & let stand at room temperature for 5-10 mins
- Add 0.25 mL of solution D (*) & vortex
Na 2 CO 3 + 4 g NaOH); Solution C: 1 mL of solution A + 50 mL of solution B; Solution D: 10 mL of Folin-Ciocalteu phenol reagent + 10 mL of deionized water
Heat at 80 0 C, 1h, leave it cool
Centrifuge 4000 rpm, 20 mins c) Resistance analysis
The resistance-in-series model effectively analyzes membrane fouling resistances, illustrating the relationship between permeate flux and transmembrane pressure (TMP) across the entire pressure range This model allows for a comprehensive understanding of how applied TMP influences permeate flux, as highlighted by Judd (2008).
禎抜眺 ồ ÄR"?"* ìRt)ìJ (2)
航: Dynamic viscosity of permeate
In the laboratory, tap water was utilized to assess the components of membrane resistance, employing a method that analyzes the variations in Transmembrane Pressure (TMP) during each cleaning step.
The self-resistance of the membrane (Rm) was assessed using a clean membrane prior to its operation with wastewater The membrane was tested with tap water, and the transmembrane pressure (TMP) was recorded against the flux A graph was plotted to illustrate this relationship, and linear regression was applied to derive the equation y = ax + b, where the coefficient 'a' represents Rm in relation to the viscosity (à) Consequently, Rm was calculated using the formula Rm = a/à.
R t , R c , R p , R a were determined after the end of 1 operating cycle:
To calculate the total resistance (Rt) of the membrane, it was observed that cake layers formed on the membrane surface after operation while submerged in clean water The membrane was then operated using tap water, following the same procedure The total resistance can be determined using the formula Rt = a/à.
To measure the resistance Rc, the initial resistance Rt was recorded before removing the membrane to clean the cake layers from its surface Subsequently, the membrane was rinsed with tap water, and the same procedure was repeated to calculate the total resistance R', which is the sum of the membrane resistance Rm and the resistance due to the cake layer Rp.
Ra The difference between the total resistance Rt and the membrane resistance after removing the cake layer R' is the value of Rc
Performance and fouling control of rMBR
In a biological system, maintaining optimal pH and dissolved oxygen (DO) levels is crucial for effective operation, as these parameters indicate a well-controlled environment that supports microorganism growth As outlined in the Materials and Methods section, pH and DO measurements were conducted at least three times weekly, with the results detailed below.
Figure 4.1 illustrates the fluctuations in pH values throughout the operation phases, including the influent from the equalization tank, the anoxic compartment, the aerobic compartment, and the permeate across stages I, II, and III The data indicates that pH levels experienced significant variations during each operational period.
During Stage I, the influent pH remained stable, fluctuating between 6.8 and 6.9 Similar stability was observed in the pH levels of the anoxic and aerobic compartments, as well as the permeate, with values ranging from 6.5 to 7.3, 6.7 to 7.3, and 7.0 to 7.3, respectively This stability indicates that Stage I serves as an acclimatization period, where operating conditions closely resemble those of a conventional biological system Additionally, the feed sludge used in this stage was sourced from the Coopmart Ly Thuong Kiet wastewater treatment plant.
Stage I Stage II Stage III supermarket has the same characteristics as the system of this study Thus, it did not take too much time to control the system and the adaptation time of microorganisms was shortened, leading not to fluctuate the pH values In addition, stable activities of the BK Food court (wastewater source) play an important role in supplying an abundant food source for microorganisms to grow up
During stage II, the pH of the influent fluctuated between 5.8 and 7.1, with a notable drop to 6.2 on day 33, followed by an increase to 6.8 by day 36, remaining stable until day 47 However, the pH gradually decreased to 5.8 by day 57, prompting the use of a 1N NaHCO3 solution to raise the pH above 6.5 before entering the biological system This adjustment was repeated on day 100 when the pH fell to 6.0 The infrequent pH drops during stage II, occurring only twice, were attributed to temporary changes in food court activity, necessitating pH adjustments to maintain optimal conditions for the biological system The permeate pH ranged from 6.5 to 7.1, with similar values in both the anoxic and aerobic compartments, measuring 6.7 to 7.4 and 6.6 to 7.3, respectively Generally, the anoxic compartment exhibited a higher pH than the aerobic compartment, which is consistent with the nitrification process in the aerobic compartment that releases H+ ions, lowering its pH, while denitrification in the anoxic tank produces OH- ions, raising the pH of the wastewater.
During stage III, the influent and permeate pH levels were recorded at 6.4 to 7.0 and 6.5 to 7.2, similar to stage II However, the influent pH experienced multiple drops to 6.1, prompting the addition of a 1N NaHCO3 solution to stabilize it Despite the low pH levels, the activities of microorganisms in the system remained unaffected, as evidenced by the stable pH in both the anoxic and aerobic compartments, which were conducive for microbial functions.
Figure 4.2 DO concentration during operation Figure 4.2 shows the changes in DO concentration at stage II and stage III
Dissolved oxygen (DO) plays a crucial role in the efficiency of nitrogen treatment in biological processes, necessitating daily control of DO levels in biological tanks The operation demonstrated effective management of DO, particularly during stages II and III, where DO in the anoxic compartment was consistently kept below 0.5 mg/L to facilitate the denitrification process Notably, the average DO in stage II was 2.5 times higher than in stage III, with recorded values of 0.47 ± 0.14 mg/L in stage II and 0.19 ± 0.16 mg/L in stage III.
During the aeration compartment's stage II, the dissolved oxygen (DO) levels exhibited instability, fluctuating between 2.19 and 2.94 mg/L for the first 86 days By day 91, DO decreased to 1.86 mg/L but rose to 2.16 mg/L by day 96, stabilizing thereafter The average DO in stage II was measured at 2.34 ± 0.32 mg/L, which is comparable to stage III's average of 2.24 ± 0.21 mg/L Despite the variability, these DO levels remain conducive to supporting essential biological processes.
In the membrane compartment, the dissolved oxygen (DO) levels were consistently lower than in the aerobic compartment and higher than in the anoxic compartment This variation is attributed to the removal of the air diffuser from the bottom of the membrane compartment in stages II and III, which was replaced by a reciprocal motor at the top of the tank The membrane's movement facilitates a limited diffusion of oxygen from the air into the wastewater.
Stage I Stage II Stage III
Theoretically, an anoxic zone in the membrane compartment enhances nitrogen treatment; however, the higher dissolved oxygen (DO) levels from the aerobic compartment continuously flow into the membrane compartment, resulting in an increased DO concentration The average DO value in stage III (1.15 ± 0.46 mg/L) was slightly higher than in stage II (1.05 ± 0.25 mg/L), but this difference is negligible In this pilot-scale system, the small volumes of compartments make it challenging to control DO movement It is anticipated that better control of DO concentration will be achievable when the system is upscaled.
Figure 4.3 (A) Biomass change; (B) Average biomass concentration
Figure 4.3A illustrates the changes of biomass during operation time Overall, MLSS concentration had significant variations in three operation stages
Stage I Stage II Stage III
Stage I Stage II Stage III
The instability of the influent during three stages, primarily due to activities at the BK food court, hindered microorganism adaptation, negatively impacting the growth of activated sludge Consequently, the measured MLSS concentrations were lower than the expected range of 8,000 to 15,000 mg/L for the MBR tank In Stage I, despite the feed sludge resembling the wastewater characteristics, nutrient supply issues led to a significant biomass decrease, with concentrations dropping from an initial 1,955 mg/L to 1,100 mg/L by the second day However, biomass levels began to recover, reaching 1,776 mg/L by day 29 The implementation of bottom aeration and stable wastewater supply enhanced the efficiency of biological processes within the system.
During the first day of stage II, the MLSS concentration dropped to 883 mg/L due to changes in operating conditions, including the replacement of aeration in the membrane compartment with reciprocal membrane movement This adjustment halved the DO concentration, significantly altering the microbial environment After a 13-day acclimatization period, biomass concentration rose to 1,602 mg/L by day 49 but slightly decreased to 1,410 mg/L by day 56 On days 90 and 94, MLSS concentrations were again measured, showing a reduction to 728 and 761 mg/L, respectively From days 96 to 110, MLSS fluctuated between 591 and 1,103 mg/L, impacted by pipe issues that delayed repairs and affected biomass levels However, from day 110 until the end of stage II, the system stabilized, with biomass gradually increasing to 1,582 mg/L by day 153, while MLVSS remained stable, ranging from 281 to 622 mg/L throughout this period.
In stage III, following adjustments to the reciprocal stage and resolution of system issues, microbial growth led to a significant increase in biomass, as indicated by rising MLSS and MLVSS concentrations On day 190, the MLSS concentration was recorded at 1,896 mg/L, gradually peaking at 2,859 mg/L on day 224, before stabilizing for the next 20 days During this period, reducing the feed tank volume enhanced the COD entering the biological system, allowing microorganisms to effectively utilize these nutrients for biomass growth However, by day 250, MLSS experienced a sharp decline to 1,669 mg/L, which continued to drop to 1,173 mg/L by day 264.
During this period, the reduction in food supply to the biological system was attributed to the downsizing of the food court and the dilution of feed wastewater by rain Consequently, the concentration of Mixed Liquor Suspended Solids (MLSS) decreased over time due to a decline in available nutrients for biomass growth This fluctuation also affected the concentration of Mixed Liquor Volatile Suspended Solids (MLVSS), which ranged between 1,159 and 2,061 mg/L.
The analysis of biomass concentration and the MLVSS/MLSS ratio across three operational stages reveals significant variations in average MLSS levels In stage I, the average MLSS was recorded at 1,336 ± 349 mg/L, which decreased to 1,058 ± 289 mg/L in stage II The average MLVSS concentration during this period was 417 ± 88 mg/L, resulting in a MLVSS/MLSS ratio of 0.5, notably lower than the optimal range of 0.7 to 0.8 for effective activated sludge systems This low ratio indicates minimal microbial growth during the initial stages, as the microbes required time to adapt to the transition from aeration to mechanical membrane movement Additionally, the increased influent COD during stage II further challenged microbial adaptability However, by stage III, the MLVSS/MLSS ratio improved to 0.6, suggesting that the activated sludge system was nearing efficient operation, although it remained slightly below the standard range.
Figure 4.4 (A) Organic treatment efficiency; (B) Average removal rate
Performance of treating pollutants and microplastics removal coupling
with fouling control of MBR system
Figure 4.8 illustrates the pH variations during operation across different tanks in stages I and II, showing that pH levels remained relatively stable throughout In stage I, the influent pH exhibited low volatility, ranging from 6.7 to 7.3, due to the use of synthetic wastewater, which provided more consistent properties compared to real wastewater This stability in influent pH enhanced the efficiency of the biological processes In the anoxic tank, pH fluctuated more significantly, peaking at 8.7 on day 14 and dropping to a low of 6.3 on day 15 Similarly, the aerobic tank displayed pH values between 7.3 and 8.4, while the membrane tank consistently maintained a pH above 8.0.
14, pH reached the lowest value of 7.4
Similarly in stage I, the influent pH in stage II was quite stable, ranging from 6.9 ẻ 7.3.The pH in anoxic and aerobic tanks showed relatively slight fluctuations
Specifically, in anoxic tanks, pH ranges from 6.8 to 8.8 and aerobic tank from 7.5 to 8.3
The pH levels in the biological system remained stable throughout both stages of the process To enhance this stability, NaHCO3 buffer solution was added to the feed wastewater, preventing fluctuations in alkalinity that could negatively impact biological processes This stable pH environment is crucial for supporting the metabolism of microorganisms within the system.
Figure 4.9 illustrates the variations of dissolved oxygen (DO) within a biological system, highlighting its strict control throughout the operation for effective nitrogen treatment In the anoxic tank during stage I, DO was maintained between 0.25 and 0.50 mg/L, which is essential for optimal denitrification Conversely, the aerobic tank recorded DO levels ranging from 4.63 to 7.78 mg/L, while the membrane tank exhibited DO values between 5.43 and 7.73 mg/L The elevated DO levels in these tanks can be attributed to the small volume of the lab-scale system, allowing for efficient oxygen diffusion from the air blower Overall, the measured DO levels in the membrane bioreactor (MBR) were conducive for nitrification processes.
In stage II, the dissolved oxygen (DO) levels in the anoxic tank fluctuated more and were higher than in stage I, with concentrations ranging from 0.27 to 0.86 mg/L and an average of 0.51 ± 0.15 mg/L, compared to 0.39 ± 0.08 mg/L in stage I This increase was attributed to a significant rise in biomass concentration, making it challenging to maintain DO levels below 0.5 mg/L while ensuring even mixing To prevent sludge settling at the tank's bottom, the mixer speed was increased, leading to higher DO concentrations In contrast, the DO levels in the aerobic and membrane tanks remained relatively stable compared to stage I, with the aerobic tank showing DO levels between 5.14 and 6.88 mg/L, and the membrane tank ranging from 4.51 to 6.89 mg/L.
Figure 4.10 Biomass growth during operation
Initially, the operating Mixed Liquor Suspended Solids (MLSS) concentration was lower than the starting level, measuring 2,557 mg/L compared to 3,276 mg/L By the eighth day of operation, MLSS further declined to 2,493 mg/L due to issues with the inverter system in the anoxic tank and the air diffuser in the aerobic tank Concurrently, the Mixed Liquor Volatile Suspended Solids (MLVSS) concentration dropped from 2,197 mg/L to 1,736 mg/L After resolving these issues, both MLSS and MLVSS stabilized and began to gradually increase, reaching a peak of 5,989 mg/L by day 37.
Stage I Stage II and 5,209 mg/L, respectively The sludge in the biological system is mainly concentrated in the membrane tank because the membrane pore size is very small (0.1 àm), so it retains all the sludge in the system With an SRT of 30 days, the sludge in the system was withdrawed in a corresponding amount to maintain a suitable biomass concentration As a result, the MLSS and MLVSS concentration gradually decreased and reached respectively 5,276 mg/L 4,295 mg/L on day 43 The concentration of biomass gradually increased and stabilized over time, showing that the microbes in the system was completely adapted and working very well This is relatively easy to understand, as pH and DO in the biological tanks were very strictly controlled and were always maintained within the optimal ranges for microbial growth In addition, the biomass synthesis took place very efficiently with the COD:N:P ratio always guaranteed at least 150:5:1
In stage II, biomass concentration increased significantly compared to stage
I MLSS continued to increase to 5782 mg/L at the first day of phase II corresponding to MLVSS concentration of 4562 mg/L From day 50, the biomass concentration in the tank increased significantly, reaching 7359 mg/L and 7519 mg/L on day 58 MLVSS concentrations were 4851 mg/L and 5381 mg/L, respectively This is mainly because the biomass concentration in the membrane tank was too high, leading to an increase in the MLSS of the whole system For this system in content 2, the membrane tank was designed to be relatively small (to ensure flux and organic loading rate) When the biomass concentration was low, the air diffuser system facilitated complete disturbance of the sludge in the tank However, when the biomass concentration was high, the mixing was no longer effective as expected, leading to a large amount of biomass deposited in the membrane tank and could not be recirculated to the anoxic tank
The MLVSS/MLSS ratios observed during the operational period fluctuated between 0.65 and 0.87, with the majority falling within the standard range of 0.7 to 0.8 A higher MLVSS/MLSS ratio signifies increased sludge activity, as noted by Kumar et al.
2014) The MLSS concentrations of membrane tanks and in the system were 8718±
In MBR applications, MLSS concentrations of 262 mg/L and 5722±266 mg/L significantly influence the formation of a cake layer on the membrane surface, leading to reduced membrane filtration duration (Wang et al., 2020).
Figure 4.11 Sludge and microorganisms under microscope on (A) day 7; (B) day
On days 7, 44, and 58, Figure 4.8 illustrates the presence of microorganisms in activated sludge, revealing a notable increase in mixed liquor suspended solids (MLSS) density over time Despite this increase, the flocs remain small and do not aggregate into larger blocks, which can be attributed to the limited volume of the aeration tanks and the effects of blowing and agitation.
The presence of abundant nutrients fosters a diverse growth of microorganisms within the system, including Ciliates, which signifies that the sludge forms floc, indicating young sludge that settles effectively (Madoni et al., 2011; Foissner et al., 2016).
Figure 4.12 illustrates the organic removal efficiency of the AO-MBR system, which operated using synthetic wastewater where the soluble COD concentration was considered equivalent to the total COD concentration During the initial stage (days 1 to 15), the influent dissolved COD was unstable, fluctuating between 160 and 320 mg/L, with an average of 241 ± 60 mg/L, resulting in an organic loading rate (OLR) of 0.7 kgCOD/m³.day and a food-to-microorganism ratio (F/M) of 0.2 kgCOD/kgMLSS.day This inconsistency was attributed to the use of CH3COONa, which contained impurities that prevented achieving the desired concentration However, from day 17 onward, the soluble COD improved significantly, ranging from 488 to 686 mg/L, due to the effective combination of CH3COONa and glucose, which not only increased the COD influent concentration but also stabilized the pH within an optimal range for the biological system.
Operation time, day sCODin Permeate Efficiency
Stage I Stage II average concentration in this period was 589 ± 66 mg/L, corresponding to the better OLR of 1.7 kgCOD/m 3 day and F/M of 0.4 kgCOD/kgMLSS.day The COD concentration was observed to be quite stable after being treated by the membrane Almost half of the analyzed data showed that COD was completely removed from the wastewater (day 21 ẻ 37) The remaining days, COD concentrations were relatively low, ranging from 7 ẻ 32 mg/L
In stage II of the wastewater treatment process, the COD concentration remained stable between 579 and 849 mg/L, with an average of 695 ± 75 mg/L, corresponding to an OLR of 2.0 kgCOD/m³.day and an F/M ratio of 0.3 kgCOD/kgMLSS.day These optimal conditions significantly enhanced microbial metabolism, resulting in an organic removal efficiency exceeding 99% The effectiveness of pollutant treatment underscores the importance of controlling initial conditions, particularly maintaining a stable influent pH above 6.8 Additionally, the presence of well-developed biomass contributed to a diverse and abundant microbial community, which efficiently converted wastewater pollutants into nutrients for further growth.
Figure 4.13 Ammonia removal of AO-MBR system
The AO-MBR system demonstrated effective nitrogen removal capabilities, particularly in the treatment of synthetic wastewater, where ammonia concentration was treated as total nitrogen The influent nitrogen levels remained stable between 52.64 to 78.40 mg/L during the initial stage In the aeration tank, ammonia was oxidized to nitrite by Nitrosomonas bacteria and subsequently to nitrate by Nitrobacter bacteria, facilitated by adequate dissolved oxygen levels, averaging 5.92 ± 0.66 mg/L in the aerobic tank and 6.39 ± 0.68 mg/L in the membrane tank The permeate showed low NH4+ concentrations, ranging from 0 to 3.36 mg/L, achieving at least 95% removal efficiency, with complete ammonia removal observed at 100% efficiency on days 21, 39, and 42.
Similar to phase I, the influent NH4 + concentration of stage II was a bit more stable with a value of 56.00 ẻ 75.04 mg/L The average value was 68.37 ± 5.37 mg/L with the lowest efficiency reaching 96%
Figure 4.14 Nitrogen balance for stage I and stage II