Water treatment is required for applications ranging from human consumption to industrial operations and is not a new concept, dating back to more than 2000 years ago. The Mayan population, for instance, employed zeolites and crystalline quartz to filter water at the urban site of Tikal, Guatemala, to remove microorganisms coming from corrential reservoirs along with mercury leaking from their wall pigments.
Nowadays, water treatment technologies are in continuous development. Here, we introduce some of the most recent and game changing high-tech applications. Specifically, this article focuses on technologies aimed at the detection and removal of micropollutants such as metals and persistent organic pollutants (POPs) along with microorganisms such as viruses and bacteria.
POPs, metals, and other micropollutants:
Water pollution is considered to be one of the major threats to the global environment by major organizations including the World Health Organization and the US Environmental Protection Agency. Perfluoroalkyl chemicals (PFAs), microplastics, pesticides, and antibiotics in water bodies represent the most dangerous threats to all living beings. The same is true of toxic metals such as lead, arsenic, and mercury. These pollutants mainly derive from mining activities, intensive farming and agriculture, industrial manufacturing activities and accidents, vehicle emissions, and disposal of toxins into water bodies.
Detection of micropollutants:
It would not be possible to address the issue of micropollutants without powerful technologies able to detect, identify, and locate the sources of contamination. In this respect, geospatial artificial intelligence (AI) is an emerging technology promoted by organizations such as NASA and the European Space Agency, along with innovative companies such as Rezatec and startups such as Gybe.
Geospatial AI combines earth observation science with neuronal networks; as a result it allows for remote monitoring of water quality and dynamic management of dams and pipeline networks. This can be achieved by real-time recording of satellite data based on parameters such as ground movement, vegetation color, and water stream behavior, followed by data elaboration and interpolation with global geophysical and hydrological parameters of water networks. Dedicated algorithms can then elaborate and extract meaningful and scalable information from the resulting datasets. By means of this technology, it is possible to estimate the water quality of reservoirs, locate and fix issues with water networks, and “close the circle” on micropollutants.
Next-gen sensing technologies:
In order to get data available for processing and elaboration, these should be first collected in the field. With this aim, sensing technologies play a crucial role in the complex detection of micropollutants in water. Concerning microplastics, for example, spectroscopy is considered as the analytical method of choice.
Infrared imaging systems:
Recent infrared imaging systems based on quantum cascade laser (QCL) technology, such as those developed by Agilent Technologies, reportedly outperform standard techniques used in the field like vibrational spectroscopy. These methods allow scientists to optically analyze samples in minutes and to determine size, shape, and chemical nature of all kinds of microplastics present in solution.
Molecularly imprinted polymer electrodes:
Similarly to microplastics, PFAs are inert materials, and thus they are very difficult to detect in water samples. Nevertheless, advanced electrochemical detection methods have been developed in order to reach adequate levels of selectivity and sensitivity.
Molecularly imprinted polymer electrodes able to detect PFAs even at nanomolar concentrations have been recently disclosed. The research teams developing these systems conveniently exploited the electrochemistry of oxygen – which is abundant in natural water – to detect perfluorooctanesulfonate (PFOS), the most ubiquitous of perfluorinated compounds. These compounds have been linked to immune system and liver damage in humans.
Heavy metals are particularly dangerous due to their tendency to accumulate over time in plants and animals (known as bioaccumulation). Standard laboratory detection techniques rely on spectrometry and voltammetry; but these require sophisticated equipment and timely operations. Alternatively, online, real-time analyses can be performed by means of biosensors.
A biosensor is able to convert biochemical signals deriving from different interactions of metals with some biological elements (e.g., certain proteins) into measurable electrical signals. Simplified and miniaturized sensors can also be integrated in detection kits like those commercialized by NanoAfix. These devices can be readily employed in everyday life to check that metal content in drinking water is below unsafe levels. By this approach, a microsensor immersed in the water provides user-friendly, real-time results via smartphone app.
Removal of micropollutants:
In order to remove non-biodegradable contaminants, oxidation is often the most effective treatment. A number of traditional methods are available to generate in situ ozone, and these techniques are generally referred to as advanced oxidation processes (AOPs). However, more recently, novel approaches to oxidative water treatment have emerged.
Plasma-based techniques are garnering interest from both academia and industry. Non-thermal plasma (NTP) is a strongly energetic state of matter characterized by a high density of radicals and radical ions. NTP can be generated in water at room temperature by application of strong electric fields. The oxygen derivatives, including the hydroxyl radical and ozone, are highly effective at removing contaminants that are present. This process takes place through mineralization reactions. While this can be considered a green chemical-free approach, it is important to note that NTP and other plasma techniques require high amounts of energy in order to be effective.
Photocatalytic water treatment:
The natural power of sunlight can also be conveniently exploited for AOPs. Photocatalytic water treatment is based on the chemical activity of a photocatalyst exploited to promote a chemical reaction. Despite photocatalysis being an older technology dating back to the 1950s, its true potential has not been fully disclosed yet. Modern metal-free visible light photocatalysts can provide a low-cost, environmentally friendly, sustainable, and renewable approach to water treatment. At present, the main technical barriers preventing the commercialization of these technologies result from the complex post-recovery of the catalyst particles after use.
Novel filtration technologies:
Compared to POPs, metals are more easily removed from water. In this context, classic separation techniques include: precipitation, adsorption, membrane filtration, reverse osmosis, and electrochemical treatment. At the same time, modern technological advances provide a continuous improvement to these methods. For example, bacterial cleaning systems known as “living filters” are currently under development. Such filtration methods exploit the strong and selective metal binding properties of specific proteins that are biosynthesized by bacteria.
Bacteria, viruses, and other microorganisms:
The current global pandemic has led to increased attention on the multi-faceted problem of microbiological water contamination. While an estimated 663 million people lack access to clean and safe water sources, it is important to note that water contamination is not limited solely to poor and underdeveloped countries. At the industrial level, microbial proliferation can lead to reduced process efficiency, loss of revenue, resistance to sanitation, microbially influenced corrosion, and equipment failure, among many other problems.
Detection of microorganisms:
The standard approach toward microbial control and bacteria detection in water is focused on liquid sampling and laboratory analysis, but the latter usually requires hours, days, or even weeks to furnish results. For this reason, innovation in this area is focused on online, real-time monitoring techniques that are able to automatically detect microorganisms in water. VWM Solutions has developed a technology for the automatic measurement of the specific enzymatic activity of microbes.
When it comes to bacterial proliferation, techniques monitoring free-floating microorganisms only provide a partial view of the problem since these microorganisms represent just 10% of the total bacterial population whereas 90% of bacteria live attached to internal surfaces of pipes, in a microbial layer known as “biofilm.” ALVIM Srl developed high-tech electrochemical sensors able to detect biofilm growth online and in real time. By means of these highly sensitive technologies, it is possible to optimize cleaning treatments, keep pipelines clean from biofilm, and improve process efficiency while also reducing the amount of chemicals used for sanitation – thus saving money and time.
For viruses, following the COVID-19 outbreak, the focus has been on the detection of viral RNA in wastewater. While research studies involving the advanced determination of viral contents in water are ongoing, Internet of Things (IoT) techniques are proving to be extremely useful during the current pandemic. Kando is able to examine the spread of coronavirus in water streams based on data collection via electrochemical and optical analyses, followed by machine learning and elaboration. Alerts and data projections resulting from these technologies allow utilities, cities, and policymakers to reduce expenses, negative environmental impacts, and wastewater-related problems.
Removal of microorganisms:
The standard approach toward water sanitation usually consists of physical pretreatments followed by, or combined with, chemical treatments. Ultrafiltration, reverse osmosis, ultraviolet, ultrasounds, and heat treatment – or combinations of these – can physically remove up to 99.99% of microorganisms present in water.
Biocides – chemical sanitizers such as hypochlorite, chlorine dioxide, bromine, and ozone – along with enzymatic formulations (usually referred to as biodispersants) are employed to kill residual microorganisms and prevent microbial regrowth as well as biofilm growth.
Oxidation methods coupled with membrane treatments:
Oxidation methods are frequently employed in water sanitation, and previously mentioned AOPs such as NTP and photocatalytic methods are also effective at the removal of microorganisms. Moreover, these techniques can be coupled with membrane treatments to increase their efficiency and prevent the complex problem of membrane microfouling.
The Korea Institute of Science and Technology (KIST), for example, developed a membrane material that is reported to self-clean microbiological contaminants away through irradiation by means of sunlight. Such surface-modified photocatalytic membranes can fully decompose contaminants accumulated on their surface when simply irradiated with visible light.
Electrofiltration with electrically conducting membranes:
Electrically driven physical-chemical phenomena, such as electrophoresis, can be coupled with membrane-based separation techniques in a process known as electrofiltration. These materials enable the efficient use of these electro-driven processes. Specifically, the development of polymeric and inorganic electrically conducting membranes (ECMs) have reduced the energy consumption of electrofiltration by using the membrane as an electrode included in an electrochemical circuit. In essence, a membrane-electrode allows for the concentrated delivery of electrical energy directly to the membrane-water interface, where the actual separation process takes place.
In-situ generation of chlorine dioxide:
While technological innovation mainly involves physical treatments, chemical sanitation and biocide formulation have also been implemented in recent years. In this respect, in-situ generation of chlorine dioxide is rapidly replacing old-fashioned chlorination treatments due to higher efficiency and lower impact in terms of corrosion and toxicity. Sigura Water developed a chemical formulation that can exponentially increase the biocidal effectiveness of simple chlorine solutions. Such formulations are often based on nitrogen-containing organocatalysts that are able to stabilize chlorine derivatives such as the hypochlorite anion, thus increasing its sanitation efficacy. While catalyst recovery and reuse are still the subjects of research and development, these catalysts are fully biodegradable and thus do not harm the environment.
Smart technological innovations in the field of water treatment are in rapid and constant development. While the examples reported in this article cannot be considered as exhaustive of the whole high-tech scenario, some of the most powerful techniques related to the detection and removal of micropollutants and microorganisms in different kinds of water sources have been introduced. While the COVID-19 pandemic accelerated and promoted innovation in the field of water treatment, further advances will surely be made by both industrial and academic researchers.
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