By Sakshi Kabra Malpani, Publishing Associate: Researcher and Writer at Save the Water | August 5, 2022 

Coal fly ash, a by-product of thermal power stations, has always been a potential threat to the environment. But nowadays, it can be converted into many meaningful products including membrane filters. Membrane filters are cheaper and eco-friendly, so they have replaced traditional water filtration techniques like distillation, evaporation, sedimentation, etc.

How is Coal Fly Ash a Threat to the Environment?

The world meets about 40% of its electricity demand by burning coal in thermal power plants. This coal burning produces huge amounts of coal fly ash which needs to be disposed of in eco-friendly ways. Coal fly ash has toxic heavy metals like cadmium, mercury, arsenic, chromium, nickel, and cobalt. These poisonous elements can leak out of it and contaminate the environment. They can also impact human health with skin and lung infections, cancer, nausea, vomiting, malfunctioning of vital organs, etc.

Trends in Recycling of Coal Fly Ash

In the past few years, researchers have worked very hard to develop new ways for recycling coal fly ash. Now, it has been used in adsorption, catalysis, wastewater treatment, ceramic materials, building materials, cement industries, horticulture, and more. Apart from these uses, costly metal and metal oxides can also be extracted from it.

How do Membrane Filters Work in Water Treatment?

Membranes are semi-permeable, porous sheets that can separate dissolved solids and contaminants from water when some driving force is applied. They are generally used to treat and process wastewater, groundwater, drinking water, rainwater, etc. Depending upon the water quality and types of impurities in it, a membrane filter is selected for water treatment. It helps in the removal of particles, microbes, micropollutants, harmful ions like fluoride, chloride, cyanides, and other fouling organic materials. Based on the pore size of membranes and the size of impurities to be filtered out, there are four common methods of membrane filtration in water treatment, namely, microfiltration, ultrafiltration, nanofiltration and reverse osmosis.

Different Types of Membrane Filters

There are several commonly used membrane filters:

• Synthetic organic polymers (like polysulfone, cellulose acetate, polyethylene, polypropylene) which are very porous, strong, flexible, lightweight, durable, and ultrathin. They could be used in water treatment, gas separation, nanofiltration of solvents, etc.
• Inorganic membranes like ceramics and metals. Ceramic membranes are microporous, thermally and chemically stable, and frequently used in microfiltration and nanofiltration. Metallic membranes are finely porous and normally composed of stainless steel. They are often used in gas separations but could also be used for wastewater treatment at high temperatures.

Coal Fly Ash as Membrane Filters

Despite various advantages associated with ceramic membranes, they are less frequently used in wastewater treatment because of their delicate nature and complicated manufacturing process. Raw materials used in their production like high purity alumina and silica are expensive. Coal fly ash is a rich source of silica and alumina and thus can be used to fabricate cheaper ceramic membranes. Such membranes also offer extra benefits like high pressure and mechanical stability, longer lifetime, better porosity, durability, resistance towards biological degradation. Coal fly ash based ceramic and geo-polymeric membranes have been used in the treatment of industrial wastewater, removal of dyes and heavy metals, seawater desalination, municipal wastewater treatment, and other processes. These membranes have a great number of pores, so they can withstand large amounts of water. Various studies have concluded that the use of such membrane filters has reasonably reduced turbidity, total dissolved solids, and oil content from wastewater.

Future perspectives

Many researchers have been working on improvement in filtering capacity and porous nature of coal fly ash based membrane filters and their use at large scale. We can also empower them by replacing traditional membrane filters in our vicinity with these eco-friendly substitutes.

By Lauren Hansen, Staff Researcher/Writer at Save the Water | September 23, 2021

Researchers at Tufts University have designed a synthetic polymer membrane that can separate fluoride from chloride and other small ions in water. This development could have a positive impact on the health of millions of people worldwide as well as on our environment. 

What do the new synthetic polymer membranes do?

The new synthetic polymer membranes mimic biological cell membranes. The research team at Tufts used biological membranes as inspiration because these membranes are highly selective. This means that they are good at (dis)allowing ions to come in and out of the cell. This new filtering method through synthetic membranes is twice as selective as other methods.

How is this new filtering method different from others?

Current filtering membranes can only separate molecules that are significantly different from each other in size and charge. Furthermore, current membranes cannot distinguish between different single-atom ions. This is because single-atom ions are smaller and basically have the same level of charge.

The new membranes, however, can distinguish between small, single-atom ions that have very similar electric charges. They allow for finer precision in separating tiny ions. In addition, the new synthetic polymer membrane is also relatively inexpensive compared to other methods. Some of these other methods include high-pressure filtration and the process of completely purifying the water supply before subsequently remineralizing it. 

What are the potential applications of this synthetic polymer membrane?

The new membrane could have a positive effect in the areas of public health and industrial production. 

Since the synthetic polymer membrane can separate fluoride from chloride, it could reduce cases of fluorosis. Fluorosis is a disease caused by overexposure to naturally-high fluoride levels in the water supply. Those diagnosed with fluorosis may experience weakened teeth, calcified ligaments and tendons, and other bone deformities. Scientists are encouraged that applying this membrane to the public water supply may decrease cases of fluorosis around the world. 

This new membrane technology can also be scaled up for industrial use. For example, it can help clean agricultural water supply and clean up chemical waste in the water supply as well. It can also improve the yield on ore extraction of elements that we use in order to manufacture lithium ion batteries, for example.

What is the future of synthetic polymer membranes?

Researchers at Tufts and its funding sources will explore and test how this filtering method can be scaled up for industrial use. 

, Staff Writer and Researcher for Save The Water | April 17, 2016

Imagine a substance so thin it is considered two-dimensional. It may seem like something from a science fiction novel, but it’s real—and it’s changing the face of water purification technology as we know it.

What is graphene?

This miracle material, called graphene, is essentially a single layer of carbon atoms packed together in a hexagonal lattice structure, or “honeycomb” shapes (as shown above).1 At only one atom thick, it is the thinnest compound and lightest material.

Despite the fact that it naturally repels water, graphene allows water – and only water – to pass through when small pores are made in the sheet.2 In the past, scientists have created filters out of graphene that successfully purified water. However, the delicate properties and high cost of the material only allowed the filters to be made on a small scale.3

Recent breakthroughs and new applications

Recently, a new breakthrough was made in graphene-based water treatment when a research team at Monash University and University of Kentucky created a vicious form of graphene oxide that can be spread with a blade. It is also able to withstand harsh environments for extended periods of time. This, along with the capability for faster production, makes these filters valuable for a variety of larger-scale commercial applications.4

This filter is still likely just the beginning of graphene technology, but already a wide variety of applications can be imagined. One main use would be in desalination, which would allow seawater to be turned into tap water.5,6 This would be a major advance in mitigating water shortages worldwide, as most of the Earth’s water is found in oceans. Other uses could be for uses other than consumption, such as treating industrial wastewater or public utility water so it could be reused. If adopted, the filter could even purify other liquids, such as wine or milk.

As an organization that is involved in raising public awareness about issues related to water contamination, Save the Water can help educate people about the potential for graphene to be used in water filters. Informing more people about it may inspire more scientists to start working to apply these filters in the real world.

References

1. Abozar Akbari et al. March 10, 2016. “Revolutionary graphene filter could solve water crisis.” Phys.org. http://phys.org/news/2016-03-revolutionary-graphene-filter-crisis.html
2. Graphene-info. n.d. “Graphene and water treatment: introduction and market status.” http://www.graphene-info.com/graphene-water-treatment
3. Jesus de La Fuente. n.d. “Graphene – What Is It?” Graphenea. http://www.graphenea.com/pages/graphene#.VvhAYBIrJE4
4. Jesus de La Fuente. n.d. “Graphene Applications & Uses.” Graphenea. http://www.graphenea.com/pages/graphene-uses-applications#.VvhCOxIrJE4
5. Wei Gao, et al. 2011. “Engineered Graphite Oxide Materials for Application in Water Purification.” American Chemical Society. http://pubs.acs.org/doi/abs/10.1021/am200300u
6. Xylem Inc. n.d. “From Water Reuse to Water Desalination, Treating Water the World Around.” http://www.xylem.com/en-us/applications/water-treatment/Pages/default.aspx