Applications for Anthracite Filter Media in Desalination Water Purification Processes
African Pegmatite mines, mills and supplies the highest quality anthracite milled to order for water purification applications, amongst many others.
In the process of removing impurities from water, the typical method is to use a filter medium in order to efficiently accomplish that goal. Anthracite has long been associated with water filtration applications and in some cases, the use of two media such as in applications involving anthracite and sand or gravel are involved.
The chemical composition of anthracite itself is largely responsible for its filtration ability, along with the ease of manufacturing to idealised grades and sizes that work best for filtration. More specifically, how anthracite interacts with impurities often found in water is of key importance.
Anthracite - Properties and Applications
What is anthracite?
Anthracite is a form of coal that is densely packed and has a high carbon content, relatively few impurities, and the second highest calorific content of any type of coal with only graphite beating it. Anthracite is not described as a bituminous coal, therefore it does not contain elevated levels of volatile organics or oily substances. The principle uses of anthracite in the industrial sector include power production as well as the fields of metallurgy and water purification, in addition to a burgeoning presence in the refractory materials space.
Representing only one percent of all of the coal in the world, anthracite is mined at scale in only a select number of countries such as China (the world’s leading producer), Russia, Vietnam, Canada, South Africa and the United States. Anthracite can often be distinguished from other types of coal due to its semi metallic appearance which reflects light in a manner similar to the mineraloid (and fellow ignite) jet, a substance used in jewellery products.
The high carbon content of anthracite makes it particularly suitable for filtration.
Physical Properties of Anthracite
Dry Bulk Density 800 kg m-3
Specific Gravity 1.6 ± 0.05 g cm-3
Effective Size 1.2 - 2.4 mm
Uniformity Coefficient 1.5 - 1.7
Bond Work Index 20 - 22 kWh T-1 (BBH)
Because this ensures that anthracite for water filtration is not impacted by the chemicals it encounters. In other words, it does not absorb or retain dangerous chemicals which can then be drained into runoff water that passes through the anthracite filter media. Further, the shape of anthracite itself helps to provide an effective means for preventing impurities from remaining in water.
Anthracite as a Water Filtration Mechanism
Anthracite As A Water Filtration Mechanism
How does a media filter such as anthracite work?
Anthracite coal filter media and others work through a combination of chemical composition as well as using the laws of gravity and physics to purify contaminated water.
A filter media is any kind of substance - such as crushed gravel, sand, anthracite, or even glass - which acts as a sieve from which solids are removed. The finer the grain of your filter media, the more granular it can get in removing these impurities.Whilst a small particle size of anthracite filter will result in better filtration of smaller contaminants, such small sizes can easily be compacted to a more solid bed, which can lead to overpressure situations. This can be avoided by the use of the correctly sized anthracite particles, a suitable operation flow rate/pressure and a well provisioned backwashing programme to release contaminants from the filter bed.
Because substances can readily dissolve in water but do not readily dissolve entirely, substances that are not completely dissolved in water can be “caught” in the filter media with only the water passing through at the end of the filter.
It is not hard to imagine that anthracite filters have myriad uses in the modern world from managing runoff from a storm to keeping water clean and drinkable. It is this last concern that has anthracite water treatments gaining traction in many arid parts of the world where access to fresh water is limited and the resource itself is scarce. In fact, anthracite filter media suppliers around the world are pushing it as a solution to many impending water crises in different locations across the globe.
The only question is: Are anthracite water treatment filter media effective at removing the impurities found in seawater which are myriad and microscopic?
Performance, Operating Characteristics And Backwashing
Compared to sand, anthracite is more tolerant of higher operating flow rates and backwashing rates. These are important characteristics when large volumes of liquid are to be purified. Across grain sizes of 1.4 to 2.5 mm, backwashing rates of 55 to 60 m3 h-1 can be achieved at 20 °C. Such high backwashing rates are testament to ground anthracite’s ability to be a reliable performer in filtration, with only minimal downtime. Furthermore, the superior hardness of anthracite means that physical abrasion is minimised and thus filter lifetime is further extended.
Is Using Anthracite for Water Filtration Effective at Removing Impurities in Salt Water?
While there is little question about anthracite’s effectiveness as a filter, it is not the only option when it comes to desalination. Many industrial water purification systems, be those for desalination or otherwise, rely on anthracite as a filtration medium. Not only is it an effective medium for desalination but is performatively on par with processed expanded clay (another popular material in the desalination space).
Research along the coast of northern Greece has shown that in a direct comparison of filtration using anthracite and using process expanded clay, in every case scenario, the two media performed at nearly identical levels, with optimal filtration performance achieved in the summer months because of what was attributed to higher aggregations of material caused by hotter temperatures.
The only differences in performance arose from the coagulant material used in each scenario.
How Anthracite Water Treatment Methods Work with Salt Water
Because the impurities in salt water are quite tiny indeed, filter media such as anthracite often work in tandem with a coagulant substance for optimal performance and filtration of water.
Coagulants are chemicals that aid in the creation of what are called aggregates or clumps of impurities that are larger in sum than they are in their parts. When a coagulant helps create larger aggregates, this makes the impurities easier to remove for filter media like sand and anthracite.
To look at it a different way: using coagulants with anthracite makes anthracite more effective at removing the impurities in salt water by increasing their weight and surface area. Research findings show that not only was anthracite water treatment effective at desalination across a wide range of performance environments (temperature, etc.) but also that it was comparable to the clay material in building up water resistance or what is called hydrostatic head.
The process begins with a pretreatment process which is where untreated or “raw” seawater is drawn and algae and organic materials are removed from the water. This is accomplished by pumping the raw seawater into multimedia filter tanks consisting of a layer of anthracite, sand, and then gravel at the bottom. Water moves from the top of the tank out through the bottom, passing through the anthracite, sand and gravel layers as it does so. The resulting liquid is largely free of contaminants, including all at the macro scale, at this point.
The treated salt water that results from this process then goes through another treatment which is often called microfiltration. What microfiltration refers to is the removal of small matter in the treated seawater. At this point, the water is ready for a process referred to as reverse osmosis, as detailed below.
It should be noted that coagulants are not used before every example of reverse osmosis (RO). Anthracite filters, in more depth below, with sand are part of pre-RO treatment, without which the RO process would be inefficient.
Proposed as a solution to the problem that 4 billion people - or nearly half of the world’s population - will be short of potable water by 2030, anthracite filter media suppliers will be working in concert with others such as sand and gravel presents one among many possible solutions to this problem. Given its demonstrated efficacy in this area, the use of anthracite water treatment methods is expected to grow in the coming decades.
Anthracite In Reverse Osmosis Filtration For Water Treatment
Reverse osmosis is a process used for desalination whereby a partially permeable membrane is present across which dissolved ions (such as sodium, chloride, phosphate etc.) cannot pass. The resulting liquid is potable water, completely devoid of dissolved ions. As RO treatment of salt water is a physical rather than chemical phenomenon, it is crucial that the membrane does not get blocked, i.e. the maximum amount of water can reach it as possible. Blockage of the membrane by contaminants including - but not limited to - insoluble pollutants, microplastics and (micro)organisms will result in a severe loss of efficiency of the process. This is a particularly acute problem as RO systems tend to operate around the only 30% efficiency regime. FIltration is therefore an essential part of the RO system.
Anthracite is used as a pre-RO filter, often used as part of a dual media system alongside sand or alongside both sand and ground garnet in a mixed media filter(1). The sum effect of these filters as set up together is to provide effective coverage across the coarse to ultra-fine size range. In addition to conventional size exclusion filtration, anthracite excels in the removal of dissolved organic material (such as residual oils, a common feature in coastal sea water) as well as suspended solids(2).
The two major issues with RO for water production are energy requirement and overall water recovery. By reducing the amount of solid build up at the partially permeable membrane, overall energy requirements will be reduced. 35% water recovery over the course of one year can be achieved with RO plants that use anthracite as a component in their pre-RO filtration systems. This number is low, but would be significantly lower in the absence of adequate filtration such as that provided by anthracite.
Other co-filtration media include granular activated carbon (GAC), chromate and charcoal. When used with GAC, the combined filter is highly effective against biological materials(3). Chromate and activated charcoal can be combined with anthracite and collectively they are particularly astute at removing heavy metal ions such as iron from aqueous solution(4).
Anthracite for pre-RO filtration tends to be used in the 0.35 to 0.8 mm particle size regime, with bed depths typically no shorter than 0.8 m. Modern plants can easily process up to 40 m3 per hour of salt water when they have been suitably equipped with anthracite-containing multi media filters(5).
Research has shown that anthracite-containing dual and multi media filters are highly effective at treating raw, moderately polluted seawater in pre-RO applications in both tropical waters and in the eastern Mediterranean, producing WHO-compliant drinking water in volumes in excess of 50 m3 per day(6). Overall, it can be said that most of anthracite’s filtration utility in the pre-RO manifold is based on its excellent size exclusion properties, removal of organic materials and desirable porosity. Anthracite filters used in most RO settings are discarded after use rather than being recycled.
- Anthracite is one of the highest carbon forms of coal, occurring with high levels of natural purity
- Being almost chemically inert, anthracite has long been used in standard filtration applications
- With the use of a coagulant, anthracite filtration is highly effective for saltwater purification
- In a reverse osmosis set up, desalination is enhanced by adequate filtration by anthracite powder in advance of the osmosis membrane
- Anthracite is also used in many other high performance filtration environments, such as in solvent extraction-electrowinning
African Pegmatite is a leading miner, miller and supplier of high quality anthracite for filtration and desalination applications. Offering a wealth of experience and the broadest product range, African Pegmatite is the natural choice of industrial partner for any project.
1 S. Jeong and S. Vigneswaran, Chem. Eng. J., 2013, 228, 976
2 S. Vigneswaran et al., Separation and Purification Tech., 2016, 162, 171
3 S. Vigneswaran et al., Desalination, 2009, 247, 77
4 S. Chaturvedi and P. N. Dave, Desalination, 2012, 303, 15
5 Department of the Army, Water Desalination Technical Manual, Washington, D.C., 1986
6 C. P. Teo et al., Desalination and Water Treat., 2009, 3, 183