ABSTRACT This study was conducted to compare the treatment performance and energy efficiency of a wastewater treatment system using a reciprocating membrane bioreactor rMBR and a convent
Since the development of the plastic industry, we have continuously used and released a large amount of non-biodegradable waste into the environment Products made from plastic that is improperly disposed of, after a period of time, under the influence of mechanical processes, oxidation, and biodegradation, form microplastics (MPs) Microplastics were first discovered and reported in the 1970s (Carpenter and Smith, 1972) They are plastic particles less than 5 mm in size (Talvitie et al., 2017) and can persist for thousands of years in the environment due to their chemical stability (Cózar et al., 2014) Microplastics are considered as a new pollutant by causing plastic pollution ± a global phenomenon that is of great concern to the world due to its adverse effects on aquatic animals and human health due to the absorption of microplastics in the food chain (Ma et al., 2019) Due to the inability to distinguish food sources, marine organisms can mistakenly ingest microplastics, causing physical phenomena inside the body (clogging, erosion, and injury from within the body), loss of energy, and death or death from exposure to chemicals from microplastics (stabilizers, plastics, etc.) affecting endocrine and reproductive activities of species (Challenges and Treatment of Microplastics in Water, 2018)
Despite extensive studies on microplastic occurrence, research on their removal remains scarce Proposed methods like plastic-to-energy incineration fail to reduce plastic output and contribute to air pollution Recycling, on the other hand, effectively reduces plastic waste and generates new materials To address microplastic accumulation, reuse practices can minimize waste and create resources, although their impact remains limited To increase adoption, public education and awareness campaigns are crucial.
Wastewater treatment systems are significant sources of microplastics due to their inability to effectively remove them Despite high removal rates in primary stages like oil-grease separation and settling tanks, a substantial number of microplastics are still discharged Advanced technologies such as A2O, SBR, and bioremediation processes have demonstrated high removal efficiencies, achieving over 98% Additionally, recent research has shown promising results in treating polyethylene, the most prevalent microplastic, using coagulants and ultrafiltration membranes.
The common point of the above studies is that they only evaluate the microplastic removal efficiency through the processes but have not given a specific and practical solution Using a membrane to remove microplastics remaining after the treatment process is considered a reasonable and highly effective solution (up to 99.9% efficiency)
Membrane technology is becoming increasingly popular in wastewater treatment due to its effectiveness However, conventional MBR technologies face limitations in simultaneous removal of nitrogen and organic matter, high energy consumption, and membrane fouling control Research on energy-saving membrane technology aims to address these challenges This study proposes a physical membrane cleaning method using a motor to induce reciprocal membrane movement, reducing floc deposition and operating costs significantly By replacing aeration with physical movement, the operating costs associated with membrane cleaning are substantially lowered.
Given the research gaps indicated above, this study was conducted to explore the following objectives: x Evaluate the organic matter and nutrient removal efficiencies of the rMBR system at different frequencies in comparison with the conventional MBR system; x Compare the membrane fouling control ability when changing membrane configuration and membrane fouling control method From there, evaluate the energy usage of both systems x 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 x In content 1, a reciprocating membrane system with a capacity of 1 m 3 /day was located near the BK food court, on the campus of University of
Technology Ho Chi Minh City Real wastewater was taken directly from the manhole of the food court and has prelimarity treatment Activated sludge was taken from the WWTP of Coopmart Ly Thuong Kiet supermarket with the same properties as the wastewater in this study and was used in both contents x In content 2, a lab-scale anoxic-MBR system with a capacity of 18 L/day was located in the Key Laboratory of Advanced Waste Treatment
Technology Synthetic wastewater was used to evaluate system performance.
Microplastic
Plastic products are an indispensable part of people's daily lives They are applied in various industries owing to their excellent advantages, such as convenience, durability, resistance to erosion, ease of processing, and low cost (Sharuddin et al., 2016; Wong et al., 2015) However, since the development of the industry for the production of synthetic materials such as plastic, we have continuously used and released a large amount of non-biodegradable waste into the environment Global plastic production increased rapidly from 1.5 million tons in
1950 to 359 million tons in 2018 and is estimated to reach 500 million tons in 2025 (Bui et al 2020; Zhang et al 2020) Each year, more than 240 million tons of plastic are used to meet consumer needs (Browne et al., 2011).It was estimated that there were between 15 and 51 trillion microplastic particles in the world's oceans with a weight between 93,000 and 236,000 tons (Ioakeimidis et al., 2016).Besides, according to a statistical report in 2015, more than 8.3 million tons of plastic were created, 5.25 million tons of plastic particles were released into the ocean, of which
269 thousand tons floated on the surface, about 4 million microplastics/km 2 were discharged into the seafloor deep (Parker, 2015; University of Georgia, 2017) Substances in the plastic can decompose under sunlight (UV rays), but the cooling effect of seawater and cover caused by microorganisms makes the time it takes for the plastic to decompose completely increased significantly (Gregory, 1999; Barnes et al., 2009) If products made from plastic that is improperly disposed of, after a period of time, under the influence of mechanical processes, oxidation, and biodegradation, form microplastics (MPs) They are plastic particles less than 5 mm in size (Talvitie et al., 2017; Kokalj et al., 2018) and can persist for thousands of years in the environment due to their chemical stability (Cozar et al., 2014) Microplastics are considered a new pollutant causing plastic pollution ± a global phenomenon of great concern to the world due to its adverse effects on aquatic animals and human health (Ma et al., 2019)
Microplastics are divided into two types which are primary and secondary microplastics Primary microplastics are supersmall microplastics based on their size Their sources include intermediate sources of raw materials for plastic products such as raw plastic resins (nurdles), granular plastics (pellets), maintenance and deterioration of plastic products, and by-products of emissions in industrial production Secondary microplastics are larger pieces of plastic fragmented from plastic materials in aquatic as well as terrestrial environments This type of microplastic is formed mainly by the decomposition process under solar radiation (UV light), leading to the decomposition of the bonds in the polymer matrix due to the influence of oxidation In addition, weather processes significantly influence the formation of this plastic (Barnes et al., 2009)
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;
- Abrasions from other plastic materials Wastewater treatment plants (WWTPs) are considered as one of the major sources of microplastics, as they are able to convert primary microplastics into secondary microplastics (Sun et al., 2019; Roex et al., 2013) Microplastics found in municipal wastewater are frequently the result of daily human activities Polyester and polyamide components, for example, are frequently shed from clothing during the washing process (Napper and Thompson., 2016), and personal care products such as toothpaste, cleanser, and shower gel enter WWTPs as a result of our daily use (Magni et al., 2019) Untreated microplastics have been shown to be commonly discharged from WWTPs, enter water bodies, and eventually accumulate in the environment (Carr et al., 2016) These wastewater treatment systems are not capable of capturing microplastics, partly because the particle size is so small, and partly because they are not designed to handle microplastics by their nature
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., 2020) MP types have varying effects on the ecosystem For example, PP and PE exhibited much higher toxicity than PVC when studying exposure to Daphnia Magna reported by Renzi et al (2019) Large organisms (such as large fish, reptiles, birds, and mammals) are affected by both micro and macroplastics, whereas smaller organisms (such as zooplankton, worms, coral, crustaceans, mollusks, and small fish) are primarily affected by microplastics
Microplastics have a negative impact on vertebrates such as mammals, reptiles, and water birds by interfering with their behaviors (such as swimming, breathing, and feeding), reducing survival capacity, and inhibiting growth and reproduction Li et al (2016) thoroughly reviewed the presence of plastic particles in many seabirds, fish, and mammal species from tropical, temperate, and polar regions The presence of microplastics in animals caused digestive system and obstruction damage, as well as a loss of female reproductive ability due to the intestine and cloaca blockage, respectively (Nelms et al., 2016) The death of sea mammals (such as the manatee) was thought to be the result of plastics clogging their digestive tracts (Li et al., 2016)
The secondary effects of microplastics consumption in large animals would be related to the leaching of contaminants such as trace metals and other toxins (e.g., persistent organic pollutants) from the plastics into the digestive tracts, causing developmental and reproductive abnormalities in animals (Nelms et al., 2016) Plastics on the beach are also known as a factor contributing to a decrease in sand temperature, which strongly influences the alteration of sex ratios of reptiles (e.g., turtles) laying eggs on beaches (Nelms et al., 2016) The potential toxicity of microplastics appears to be linked to three pathways: I ingestion stress (physical blockage, energy expenditure for egestion), (ii) additive leakage from plastic (plasticizers), and (iii) exposure to contaminants associated with microplastics (e.g., persistent organic pollutants) (Anderson et al., 2016) On the other hand, Plastics may have an impact on abiotic qualities in the environment by altering light penetration into the water column and sedimentation characteristics (Eerkes- Medrano et al., 2015)
Plastics can contain a variety of additives, such as bisphenol A and phthalates, which have molecular and whole-organism effects in organisms (Cole et al., 2011) Humans, as previously stated, consume a lot of seafood, which contains a lot of MPs According to the FAO (2016) database, 11 out of over 25 species contain MPs in global sea fishing According to Browne et al (2010), the number of microplastics ingested by organisms from the coastal food web was greater than that of organisms from offshore habitats
Despite scientific evidence of the harm caused by MPs in the marine food chain, there is insufficient data to prove the subsequent effects of microplastics on human health On the other hand, MPs were discovered in a wide range of human food items, including canned sardines, carp, and sprats (Hanachi et al., 2019; Karami et al., 2018), salt (Zhang et al., 2020), beer (Kosuth et al., 2018; Liebezeit and Liebezeit., 2014), honey and sugar (Liebezeit and Liebezeit., 2013), etc According to Karami et al (2018), humans consume approximately 1 to 5 MPs particles per year from canned fish products According to Peixoto et al (2019), sea salt in various countries contains up to 19,800 MPs particles/kg MPs containing
243 ± 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
According to the study of Hidayaturrahman and Lee (2019), they found that pre ± treatment can remove 62.7% ± 64.4% of microplastics effectively
Grit is the first stage of the wastewater treatment plants that is designed to reduce the velocity of the flow of sewage to eliminate the girt materials such as sand, eggshells, and many inert materials inorganic in nature There are many types of grit, but the one most used in wastewater treatment to removed microplastics is aerated grit chamber (Zhang et al., 2020) Due to the spiral release of compressed air of the aerated by surface skimming and sedimentation grit chamber, most of the microplastics are removed at this stage Based on the study results of (Bilgin et al., 2020), can be seen that aerated grit chamber can remove 59% microplastics (large size films 1-5mm) From the result of (Zhang et al., 2020), primary sedimentation can remove suspended solids with flow density ( 1.1g/mL and 1.5g/mL) or small size but high density (1.5 g/mL) So this stage can only remove some high density plastics like PET (0.96 ± 1.45 g/cm 3 ), PES (1.24 ± 2.3 g/cm 3 ) (Ngo et al., 2019), etc
Flotation is an excellent technology to remove MPs particles from water system Recently, froth flotation can remove 100% MPs under aeration volume of 5.4mL/min and frother dosage of 28 mg/L (Zhang et al., 2021) In this study, they demonstrated that the removal efficiency depends on the type of plastic and its size Indeed, large PS and PE particles were more prone to float with bubble than small counterpart Similarly, the results of (Pramanik et al., 2021) show that the air flotation can remove 69 ± 85% of PE, PVC and PES
Based on the result of (He et al., 2021), the total amount of MPs in wastewater can reach to 3160 particals/L, and most of them are found to be transferred to sludge during wastewater process According to research paper of (He et al., 2021) the mechanism to remove microplastics in influents is to convert them from liquid phase to solid phase, or can be understood as the MPs are enriched in sludge rather than final removal And according to Alvim et al (2021) microplastics which correspond to plastic fragments smaller than 5 mm, are mostly retained in the activated sludge Therefore, removal microplastics in sludge is necessary It has been observed that activated sludge process is capable of removing MPs from range of 17 - 98% (Lee et al., 2018, Liu et al., 2019) Specifically, Liu et al (2019) evaluated the MPs removal with anaerobic-anoxic-oxic (A2O) process and observed a 17 particles/L removal during biological step Similarly, in the A2O process and the SBR process the concentration of microplastics in the influent was 29.9, 16.5, and 13.9 particles/L After treatment, the concentration of microplastics decreased to 0.44, 0.14 and 0.28 particles/L, respectively It can be seen that the treatment efficiency is about 98% (Lee et al., 2018) Talvitie et al., (2017) also describe similar results, in the secondary treatment in a wastewater treatment in Finland, around 87% of MPs were removal by activated sludge process Lares et al (2018) also investigated this performance of the CAS at the municipal wastewater treatment plant in Mikkeli, Finland After passing this stage, microplastics removal efficiency of approx 98.3% However, microplastics also cause adverse effects on sludge From to results of (He et al., 2021), it can be seen that PS content > 0.2g/L the methane productions were decreased by 17.9-19.3% Similar with PVC and PE, the methane production was decreased by 27.5 ± 90.6% when the microplastics increased to 20 ± 200 particals/g TS
The formation of biofilms generally involves microbial attachment, secretion of EPS and proliferation of microorganism (Tu et al., 2020) Biofilm process using adsorption to remove MPs After exhaustion, the biofilm can drop in film and flow out with treated water despite performed of backwashing The microplastics in the treated water and backwashed water then settle and stay in the sludge However, it is possible that the microplastics attached or fixed in the biofilm to be release into the wastewater generated during backwash (Zhang et al., 2020) Furthermore, there is also interest in the biofilm formation on the surface of microplastics Indeed, Tu et al (2020) used PE microplastics sample in stainless steel cage and released into the seawater at different depths of 2m, 6m and 12m Samples exposed for 30, 75 and 135 days were removed and analyzed to observe biofilm formation The results showed that the total amount of biofilm in PE exposed to seawater at depths of 2 m and 6 m was significantly higher than in water from 12 m depth within the first 75 days However, on day 135, the biofilm at a depth of 12m increased more than the depth of 2m and 6m It can be seen that the growth of biofilm depends on temperature, light intensity, organic matter, exposure time, pH, DO, etc They found that after 75 days of exposure, suggesting possible biodegradation effect occurring on the PE surface They indicated that biofilms and the dominant functional microbes attached to the microplastics surface will affect the migration, sinking, weathering, and degradation of microplastics in the environment
The pore size of MF and UF are 0.1 ߤm and 0.01 ߤm, respectively With such a small pore size, membrane bioreactors have slightly higher MPs removal efficiency than conventional activated sludge processes Indeed, many studies have shown high efficiency in the treatment of microplastics by membrane technology According to Talvitie et al (2017), 99.9% of microplastics are removed at this stage, however, the influent concentration of microplastics were quite low (6.9 േ 0.1MPs/L) In this study, Mirka et al were found that MBR process has a good effect on microplastics removal, the treatment efficiency reaches 99.4% (Lares et al., 2018)
Membrane fouling and control methods
Fouling is an increase of filtration resistance Membrane filtration in MBRs is mostly activated sludge mixed liquor, so over time the fractions decayed from the activated sludge accumulated on membrane surface or clog the membrane pores during filtration As the definition, fouling interferes with membrane filtration ability which lead to reduction of productivity and increase operation and maintenance cost by demanding more membrane cleaning, backwashing to maintain stable condition of permeation Fouling can manifest on membrane surface or inside the pores, in some case both on membrane and inside of pores may occur simultaneously These cases of fouling are categorized as reversible ones due to the deposition of particles is considered as a gel or cake layer and possible to eliminate physically by air scouring and backwashing Among the reversible fouling which can be easily eliminate by mentioned methods, there are also irreversible fouling which is more problematic This type of fouling caused by internal clogging of pores by adsorption of colloidal and dissolved materials and can only be dealt with by vigorous chemical cleaning
Figure 2.2 Membrane fouling by sludge cake formation
Fouling in MBR process may occurred in three main mechanisms: pore narrowing which is attributed to the sorption of soluble and micro-colloidal substances having size much smaller than membrane pore size; pore plugging due to deposition of particles having size equivalent to membrane pore size; cake layer formation on membrane surface from deposition of substances in the membrane surface (Krzeminski et al., 2017) Judd et al (2011) suggested three main group of parameters which impact on membrane fouling in MBR processes: biomass characteristics (MLSS concentration, particle size distribution (PSD), soluble microbial products (SMP) concentration and extracellular polymeric substances (EPS)), operating conditions (HRT, SRT, operating flux, type and frequency of backwashing and chemical cleaning) and membrane physicochemical characteristics (pore size, surface energy and charge, hydrophobicity)
Interaction between MLSS and membrane in MBR processes lead to a reduction of productivity Yigit et al (2008) studied the cause of fouling by conducting five different mixed liquor suspended solids (MLSS) concentration (4600, 6600, 8600, 10100, 12600 mg/L) at four individual aeration velocities (0.067, 0.101, 0.201, 0.250 m/s) at each MLSS concentration As a results,