Plastic pollution in the aquatic environment poses an enormous impact on wildlife and human beings. Currently, wastewater treatment plants (WWTPs) are a main source of microplastic and nanoplastic in the water. WWTPs do no account for plastic removal in their design. This study reviews possible membrane technologies that can be applied to WWTPs to filter microplastic smaller than 100 nanometers. From this literature analysis, the researchers discovered the removal of microplastics with membrane technology is insufficient for the task at hand. Further technologies must be designed for efficient filtration and membrane bioreactors (MBRs) might be one potential method.
There has been a substantial increase in plastic production globally which causes large quantities of plastic to enter the aquatic environment. Such plastics include the entry of microfibers. These are reported to come from the washing of synthetic clothing. The health effects after ingestion of these microplastic particles by the marine environment is currently unknown as of this time of publication. A major problem besides the physical ingestion of these plastic particles is the chemicals that they tend to absorb while in the water. This causes the microplastic to potentially become carcinogenic and could cause the malformation of animals/humans, damaged reproductive activity, and suppressed immune response.
Microplastics (MP) are defined as being made of synthetic polymers that are less than 5mm in diameter that do not degrade naturally. Nanoplastics (NP) are defined as particles between 1 and 100nm. MP and NP can be defined as either primary or secondary based upon their origin. Primary MP or NP are specifically manufactured to be that size. Such as facial cleaners, industrial scrubbers, and toothpaste. Secondary MP or NP have achieved their sizes via degradation from larger pieces.
Currently, 98% of MP is retained from wastewater treatment plants (WWTPs) but any MP with a size smaller than 20 nanometers or NP is not retained. Treatment can be broken up into four main processes: preliminary treatment, primary treatment, secondary treatment, and tertiary treatment [final/advanced treatment]. Preliminary treatment aims to filter via physical methods in order to protect equipment used later. Primary treatment is typically chemical and removes large suspended organic solids and can remove 25% of MP. Secondary treatments use biological methods to clean the water and can remove an additional 75% of MP. Tertiary treatment is not always used, but could theoretically bring MP removal to 98%.
A literature analysis was done searching various databases. Due to a lack of a standardized report and testing method for MP and NP removal it was difficult or impossible to compare certain results. Varying accepted sizes, shape, and type of MP has affected comparability.
The most common MP shape is the fiber. These come from the domestic washing of synthetic clothes. It has also been shown that MP and NP tend to coexist and are separate or distinct in certain solutions. This is because the fragmentation of MP can create NP readily. This process can occur by hydrolysis, photodegradation, mechanical/physical degradation, thermooxidative degradation, and biodegradation. The reduced size allows them greater surface area to absorb more chemicals, more reactivity, and reduced size to pass through biological membranes. The amount of plastic waste is expected to increase by order of magnitude by 2025.
Ultrafiltration (UF) is a possible alternative for the treatment of wastewater. It is able to cheaply create drinking water of high quality with lower energy consumption, higher separation efficiency, and a compact plant size. It has a pore size of 1-100nm and can reject macromolecules such as proteins, fatty acids, bacteria, protozoa, viruses, and suspended solids. It is particularly effective at reducing organic matter by at least 95%. It exceeded regulatory standards of waterquality and can remove 90-100% of pathogens. It can be used to replace secondary water treatment. Despite its efficiency, it is not designed to remove MP. It can effectively remove 15% of polyethylene [a common MP]. However, upon adding Polyacrylamide (PAM) the MP coagulated and the removal of MP increased from 13 to 91%. The coagulation also helped to reduce how quickly the membrane fouled. However, larger pieces of plastic decreased the time before the filter was fouled. This offers a potential alternative to filter microplastic, but more research must be done on how different types, shapes, and sizes of MP/NP can affect fouling rate, filtration efficiency, and flux are affected.
Dynamic Membrane Technology
This technology utilizes the materials that are filtered out of solution to create a dynamic layer that is used to filter out further particulates. The thicker the layer of “cake” gets the less the flow through the membrane, but the more permeable it becomes. Therefore it must be monitored and adjusted as needed. This is an attractive technology for many reasons: it is low cost in comparison to traditional membranes, no extra chemicals are introduced, it is more compact, and it requires less energy to operate. Dynamic membranes have been tested on their ability to filter MP. It was moderately effective at filtering out particles, but required a higher initial concentration of particles to build up the dynamic layer it uses for filtration.
Reverse osmosis (RO) is typically used in municipal and industrial waste treatment systems. It uses non porous or nanofiltration membranes and removes salts, contaminants, heavy metals, and other impurities. It applies a high pressure to a water solution which forces the water through a semipermeable membrane. This leaves a highly concentrated solution on one side. It is used in food and beverage, biopharmaceutical, and power production. The largest issue with RO is its tendency to foul and thus lower performance. In order to extend the membrane's life a pretreatment stage is employed to minimize cleaning frequency and prolong its life. It requires either chemical or physical cleaning to achieve consistent performance.
Membrane Bioreactor (MBR)
Membrane Bioreactor (MBR) have been hailed as one of the most powerful technologies for municipal and industrial wastewater treatment around the world. It has enormous versatility and permits easy coupling to other processes along with being inline with green chemistry. It also offers quality products while being environmentally conscious. It has recently acquired sufficient interest around the globe or its various benefits over traditional systems. When tested for its ability to remove MP it removed 99%, with a higher quality end product with less steps than other systems. However, further studies indicate that using specific enzymes it is possible to degrade the MP from 31.5 to less than 1 micrometers. These enzymes can be integrated into prexisting MBR systems and show promise for the future of MP filtration and removal.
Polymeric Membranes as Source of Plastic Waste: Recent Advances in Their Reuse and Recycling
The industry for Membrane filtration will nearly double from 22019 to 2024. There have been huge strides made in recycling of old end of life membranes. Similarly, the creation of new membranes from old can save 85% to 95% compared to purchasing brand new ones. The entire industry is shifting towards recycled bio-based polymers as an alternative to the traditional petroleum based polymers.
The industry does not currently have the equipment or experience to remove MP from waste water. From the researchers literary analysis, it is apparent that tertiary treatment is needed to remove MP from water. The most promising tertiary step would be BMR in particular with a removal efficiency of 99.9%. It could also decrease the total number of steps in treatment due to its versatility. The researchers call for a more standardized protocol for testing so data can be compared. This should be applied to MP and NP research. The researchers also state that removal of plastic from water must be paired with reduction in single use plastic products and biodegradable plastics must be further explored.