Technologies for removal of microbiological contaminants in drinking water, their limitations & the need for advanced materials
In this era of “new normal,” ensuring the supply of safe drinking water for every home is more important than ever to maintain the overall well-being of every individual. The WQP survey conducted in March to April 2020 indicated that COVID-19 would have a significant impact on the residential water filtration industry (Ref 1). This pandemic brings renewed attention to the methods for removal of microbial contaminants from drinking water, and the limitations associated with these methods. It is the need of the hour to address these concerns swiftly and efficiently to add to our capacity to fight against water-borne microorganisms.
The human body’s immune system is the best response to any virus until a vaccine comes along. As 60% of the human body consists of water, the role of clean and safe drinking water is even more important. It is the primary responsibility of the residential water filtration industry to be proactive and identify the needs to provide effective solutions for safe drinking water. The World Health Organization (WHO) released interim guidance on water, sanitation, hygiene and waste management for the COVID-19 virus (April 2020) that stated, “The presence of the COVID-19 virus in untreated drinking-water is possible” (Ref 2). Currently, there are limited studies on the survival of the COVID-19 virus in drinking water and nobody knows if the next generation of such deadly viruses may spread through drinking water. If such a virus enters wastewater from human secretion of people infected by COVID-19, it may not take long for it to come into the drinking water.
A Deeper Understanding
The United Nations International Children’s Fund (UNICEF) and the WHO estimated that one in 10 people (785 million) still lack basic drinking water services worldwide. It also stated that “mere access is not enough. If the water is not clean, is not safe to drink or is far away, then we’re not delivering for the world’s children” (Ref 2). Waterborne diseases are a major health concern due to the huge mortality and morbidity they cause. An estimated 3.4 million deaths a year are reported due to waterborne diseases.
Unfortunately, in some cases, waterborne intestinal illnesses either remain undetected or may not be identified as water-related due to lack of supporting data. Pathogens include viruses comprising DNA or RNA with a protein coating, bacteria, a single-celled organism, and protozoa-single-celled with a distinct membrane-bound nucleus. Viruses take over individual cells in their host and use them to reproduce themselves. The etiologic species (e.g. Legionella, G. intestinales, and Pseudomonas aeruginosa) can cause acute gastrointestinal illness, acute respiratory illness and hepatitis (Ref 3). The chemical coating on pipelines, break or leaks of pipes due to pressure changes may also allow pathogens to enter into the drinking water systems that may sometimes lead to outbreaks by Shigella, E.coli , Salmonella, Campylobacter, Cryptosporidium and Norovirus. Sometimes the changes in temperature can interfere and alter the dynamics of microbes on biofilms inside pipelines and may cause release of pathogens in water.
Technologies for Removal of Microbiological Contaminants & Their Limitations
Commonly used oxidative disinfectants for microorganisms today are chlorine-based such as chlorine gas, sodium hypochlorite, monochloramine and chlorine dioxide, but chlorine has limited activity against parasites like Cryptosporidium. Organisms such as microsporidia, Enterocytozoon bienusi, E. intestinalis and Encephalitozoon hellem have also shown certain resistance against chlorine.
Ozone is another oxidant used for effective killing of microorganisms by oxidizing the organic material in cell membranes which leads to their rupture. Ozonation byproducts are still under evaluation to check for their carcinogenic nature. Silver-based media has been used for a range of both gram-negative and gram-positive bacteria, but use of silver is expensive. The inactivation of pathogens is also done using heat by exceeding thermal tolerance limits of microbial cell membranes but some endospore-formers (e.g. C. botulinum) have higher tolerance for heat. Ultraviolet (UV) radiation efficiently kills cells by damaging their DNA, but certain viruses like Mycobacterium avium (a group of bacteria related to tuberculosis) are reported to have resistance to UV inactivation. This makes the removal of these pathogens from water more challenging. Membrane processes are also used for microbial removal, but they have concerns such as high maintenance cost, leakage and biofouling.
Apart from the technical limitations mentioned above, the common limitations with present technologies is they require infrastructure, cost, pressurized systems, and cannot be easily deployed to anywhere in the world. This keeps us where we are today, far away from realizing the goals of the WHO that states, “All people, whatever their stage of development and their social and economic conditions, have the right to have access to an adequate supply of safe drinking water,” where “safe” refers to a water supply that poses no significant health risk.
The WHO established the guidelines for drinking water that includes no detectable levels of Escherichia coli or coliform bacteria; however, despite a continuous effort over several years this aim has not been achieved in many countries. The cost and infrastructure requirements make it more difficult in developing world. Aesthetic quality of water changes based on the geographical location and hence specific solutions might be needed. The microbes that exist and grow in one part on the world may not survive in another part due to extreme temperature diversity and other factors. This brings our focus to develop simple, effective, low-cost and easily deployable solutions for microbial removal. The microbial removal technology that utilizes carbon-based materials for enhanced performance and multiple functionalities, could be one of the technology visions as it might not require power or infrastructure, and could enable affordable point-of-use (POU) microbial treatment for all, based on activated carbon block filtration and gravity-based granulated carbon filters.
Advanced Materials for Removal of Microbiological Contaminants
Carbon-based media—such as activated carbon (AC), carbon nanotubes and graphene—have high surface area to volume ratio, pores/voids and surface functional groups. They provide ample opportunities to improve the antimicrobial properties by surface modifications through impregnation and functionalization. Activated carbons impregnated with silver, iron and copper have shown higher bacterial inactivation efficiency, as well as improved viral and bacterial spore adsorption capacity than traditional adsorbents (Ref 4).
Activated carbon impregnated with Ag, ZnO and Ag doped ZnO (Ag/ZnO) nanoparticles were evaluated for antibacterial activities against gram-negative and gram-positive bacteria. Results indicated that Ag/ZnO–AC nanohybrid exhibited the highest zone of inhibition and killing kinetics among all. The order of antibacterial activity was found to be AC < ZnO–AC < Ag–AC < Ag/ZnO–AC. This performance of Ag/ZnO-AC is attributed to the generation of highly active free radicals that harmed the bacterial cells (Ref 5). AC spheres containing ZnO are reported to have greater antimicrobial activity towards gram-positive S. aureus than gram-negative E. coli bacteria. The antibacterial activity is caused by generation of highly reactive species such as OH-, H2O2 and O2- in the ZnO suspensions (Ref 6). The bimetal Cu/Zn–carbon fibres composite has been reported for inhibition of the growth of the E. coli and S. aureus (Ref 7). AC functionalized with quaternary ammonium groups is also reported to be very effective against E. coli (Ref 8).
Carbon nanotubes (CNTs) are also reported to have an inherent antimicrobial activity by physically compromising the cell envelope and neutralization of bacteria and viruses (Ref 4). Both single-walled and multi-walled CNTs are a potential material for size exclusion membranes that can block the transport of certain microorganism across the membrane. Polymer bound, vertically aligned CNTs have been reported for size exclusion of sub 5 nm materials. By controlling the amount of CNT loading, this approach can be used for development of advanced membranes for bacteria and virus filtration (Ref 4). Graphene, a 2D structure of sp2 hybridized carbon has emerged as a promising material for antimicrobial applications since its discovery in 2004. Graphene nanosheets were reported to induce degradation in cell membrane of E. coli by destructive phospholipids extraction (Ref 10). Silver nanoparticles attached on graphene oxide and deposited onto cellulose acetate membranes are reported to inhibit bacterial growth/biofilms. Graphene impregnated with metal ions and nanoparticles of ZnO/TiO2 showed enhanced antibacterial activity (Ref 4).
In a recent study, the minimum inhibitory concentration (MIC) of graphene nanosheets against pathogenic bacteria was evaluated. Results suggested that graphene nanosheets have predominant antibacterial activity compared to the standard antibiotic, kanamycin (Ref 9). Sharp edges of graphene nanoflakes are reported to cause the leakage of intracellular components, such as RNA/DNA, phospholipids and proteins upon physical contact with microorganisms (Ref 10). Removal of microorganisms has also been observed by engineered materials/nanomaterials based on iron oxide, silica and aluminium hydroxides. Metal-oxide nanoparticles like TiO2 photocatalysts produce highly reactive oxidants, such as OH radicals for disinfection of microorganisms. Amine-functionalized magnetic Fe3O4-SiO2-NH2 nanoparticles are also used for removal of both pathogenic bacteria and viruses (Ref 11). This makes advanced materials a suitable platform for next generation water purification with high performance and versatile capabilities.
Everyday there are emerging unknown pathogens (originating from animals or due to antigenic drift caused by random mutations) or sometimes the patterns of infections of existing pathogen are changing over time (Ref 12). More advanced detection and treatment methods are needed to monitor water quality and source tracking to achieve the goal of safe drinking water for all.
The development of new advanced materials and surface modifications of existing adsorbent materials, new disinfectants and disinfection technologies are required to prevent water-borne illnesses. Efforts will be needed to develop economically-viable materials for advanced membranes. Modified activated carbon, CNTs and graphene have great potential for synthesis of advanced multifunctional materials for microbial inactivation in the future.
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