Purifying Water: Removing Pollutants, Restoring Nature's Balance

Water pollution is a pressing issue, with around 90% of surface waters and over 60% of drinking water sources in urban China being contaminated. To address this, various methods are employed to remove pollutants from water, including physical, chemical, and biological processes. For instance, adsorption, ion exchange, electrocoagulation, filtration, and membrane separation techniques are commonly used. Additionally, innovative approaches like phytoremediation, electrochemical purification, and advanced oxidation processes are being explored to tackle challenging contaminants such as drugs, antibiotics, and industrial chemicals. These techniques aim to purify water, making it safe for human consumption and reducing negative environmental impacts.

Characteristics of Pollutant Removal from Water

Bioremediation

The process of bioremediation involves the use of microbial metabolism to remove pollutants from water. This can include the use of prokaryotes (microbial metabolism) or microbial bioremediation, which uses microscopic bacterial organisms that naturally exist in the environment to decompose, recycle, and rectify imbalanced groundwater and soil chemical conditions. These microorganisms exist naturally to help decompose and recycle contaminants, and the process can be enhanced with specialized equipment to accelerate the natural groundwater cleaning process.

One of the most successful examples of microbial bioremediation is sewage treatment, which has been used in various forms for thousands of years. Bioremediation can also be used to remove agricultural chemicals that leach from the soil into groundwater, as well as toxic metals and oxides such as selenium and arsenic compounds. For example, bacteria such as Pseudomonas aeruginosa can convert Hg+2 into the less toxic Hg0 through anaerobic respiration.

There are two main classifications of bioremediation: ex situ and in situ. Ex situ bioremediation involves removing contaminated material from one location and moving it to a remote treatment location, while in situ bioremediation occurs at the contamination site, either in polluted soil or contaminated groundwater that is remediated at its point of origin. In situ bioremediation is the preferred method as it requires less physical work and prevents the spreading of contaminants during transportation.

Phytoremediation

The process of phytoremediation can be enhanced by combining it with other techniques, such as bioremediation, which uses microorganisms to break down contaminants. Hydroponic systems, for example, can be coupled with bioremediation to improve the removal of water contaminants. Additionally, the root secretions of some plants can create an environment conducive to microbial reproduction, further contributing to the biodegradation of certain pollutants.

Electrochemical purification

Electrochemical water purification is an effective alternative to traditional water treatment methods, especially in areas with large volumes of wastewater and pollution of water bodies with toxic refractory compounds. Electrochemical methods are also useful for water purification systems in remote areas in the developing world, where pollution from pesticides, dyes, and other chemicals is a concern. These methods are characterised by advantageous mass/volume characteristics and compactness of equipment, making them ideal for cramped conditions.

There are several electrochemical systems for water purification, including electrodialysis (ED), which has been used for water desalination for decades. Other methods include EDI, shock ED, CDI, and Faradaic electrosorption. Electrocoagulation is another regularly used electrochemical process, particularly in industrial applications such as the removal of heavy metals, remediation of wastewater, and treatment of produced water. Electrocoagulation and electroflotation are two-step processes by which metal anodes are dissolved to induce the formation of flocs that trap contaminants for removal by settling, sedimentation, precipitation, or flotation. Electroflotation produces bubbles by water redox to transport lighter contaminants or flocs by flotation. Electrodeposition is based on the electrochemical deposition of metal ions in solution onto an electrode.

Electrochemical oxidation is another method that generates reactive oxidising agents called free radicals that interact with and degrade contaminants. Superoxide, hydroperoxyl, hydroxyl, and sulfate radicals are examples of reactive agents that can degrade organic and organometallic contaminants. Electrochemical technologies can also be combined with other approaches, such as filtration, sedimentation, and flotation, to achieve the desired degree of pollutant removal.

The development of new electrochemical water purification methods presents new challenges for researchers and engineers, including the need for new electrode materials, reactor designs, and the quantitative determination of kinetic regularities of purification processes. However, future electrochemical water purification technologies are expected to be energy-efficient and affordable, benefiting both the environment and society.

Adsorption

The use of adsorption techniques for water purification has several advantages. Firstly, it is cost-effective in terms of initial investment and ongoing operation, and it is easy to implement. Secondly, adsorption techniques can utilise a wide range of naturally occurring solid media, including natural materials, agricultural wastes, and industrial wastes, which can be utilised as low-cost adsorbents. Thirdly, the design of adsorption systems is simple and does not require a large amount of land. Fourthly, adsorption methods are highly effective in removing harmful contaminants, even at low concentrations. They can remove a broad range of pollutants, including heavy metals, organic compounds, dyes, nutrients, and pathogens.

In recent years, there has been a growing exploration of alternative materials for use in adsorption techniques, such as starch, chitosan, nanoscale structures like zeolites and metal-organic frameworks, magnetic materials, and carbon-based substances. These nanoscale materials offer better performance, specificity, and reusability than older methods. For example, carbon nanotubes can remove dyes and heavy metals from water, while metal-organic frameworks can selectively adsorb organic compounds and nutrients.

One disadvantage of using adsorption techniques for water purification is the challenge of dealing with complex variables, such as electrostatic, dispersive, and chemical interactions, intrinsic properties of the solute and adsorbent, solution properties, and the temperature of the system. Additionally, the regeneration and disposal of used adsorbent materials can be a challenge, as they may become contaminated with pollutants that require further treatment or disposal in landfills.

Filtration

Water filtration is a core stage of water and wastewater treatment. It is the process by which solid particles are removed from a fluid by passing it through a filter medium that catches the solid particles while allowing the fluid to pass through.

There are different types of water filters, such as strainers, precoat filters, slow sand filters, rapid gravity and pressure filters, membrane filters, and granular media filters. The type of filter selected depends on the water source (surface or sub-surface water) and the quality of the water. For example, water that contains gravel would require a different filter to water that contains bacteria.

Membrane filters utilize microfiltration and ultrafiltration, which can be effective in eliminating bacteria and/or act as pre-treatment before nanofiltration or reverse osmosis. Media filtration systems filter water through a prescribed filter medium, including metal screens and pressure filters.

Rapid sand filtration is a physical process that removes suspended solids from water. It is much more common than flow sand filtration because rapid sand filters have high flow rates and require little space to operate. During rapid sand filtration, the water flows at a rate of up to 20 meters per hour. The filters are cleaned twice per day with backwashing filters and are put back into operation immediately.

Another type of filtration is activated carbon filtration. This method can remove fuel, PCBs, dioxins, and radioactive wastes. The polluted water is sent through columns of activated carbon; the chemicals stick or sorb to the surface and within the pores of the granules, and the clean water flows through. Many people are familiar with activated carbon filters, as these comprise most tap water filters and fish tank filters.

Frequently asked questions