Filtration Media In Desalination Plants: Expanded Clay Vs Anthracite
Modern-day desalination is a process essential to provide water for both agriculture and potable supply in regions where adequate rainfall is rare or highly unpredictable. The contemporary desalination plant takes in sea water and subjects it to a reverse osmosis process. Reverse osmosis is the mechanism where a partially permeable membrane is used to remove ions and unwanted molecules from water.
Efficiency of the reverse osmosis (RO) process chiefly lies on the efficacy of the membrane itself and of the pretreatment of the source water. As such, the modern desalination plant employs various types of filters, often in sequence, to remove a large proportion of contaminants before they can reach the membrane. Sea water often has concentrations of dissolved solids in excess of 35 g per L(1).
Two such widely used filter technologies are Light Expanded Clay Aggregate (LECA, process expanded clay) and anthracite, and both will be discussed here. The basic idea is that sea water passes through these materials set up as filters before it reaches the membrane. When it reaches the membrane, it is largely devoid of dissolved solids, making the overall RO process more efficient.
Many of the causes of membrane fouling from untreated seawater are not easily visible to the human eye. Leading causes include dissolved silica compounds, adsorbed organic molecules, microorganisms, metallic oxides and various colloids of iron and aluminium(2). Most effective are dual media filtration set-ups, and it is these that are used most extensively worldwide(3). Direct filtration (i.e. a simple filtration through just sand or through a mesh filter) is regarded generally as ineffective, especially against organic contaminants(4).
Imperative for RO operations, due to the immense scale on which they operate, any filter needs to be reliable for long periods of time and relatively inexpensive. LECA and anthracite fit these requirements. Pretreatment of seawater is the most effective method of ensuring non-fouling of the RO membrane(5).
Light Expanded Clay Aggregate (LECA)
LECA is a lightweight aggregate formed by heating standard clay to ca. 1,200 °C in a rotary kiln. During the heating and rotating process, the clay dries out, forms itself into ball shapes and becomes highly porous. This porosity leads it to possessing good absorbent properties, having been utilised previously to remove heavy metals such as cadmium from paint factory waste streams(6).
In the water purification space, where research has shown that LECA is able to adsorb fluoride compounds from aqueous solution, either on its own or modified with magnesium chloride or hydrogen peroxide(7). Furthermore, dual media filters containing sand and MgCl2/H2O2-modified LECA are able to remove chromium compounds from water(8) - and these doped LECAs are able to be regenerated via treatment with dilute acid.
One of the most pernicious pollutants is arsenic. Should this get into the water system, fatalities could occur. Researchers have demonstrated the use of Fenton’s reagent-modified LECA has been effective at removing 99% of arsenite (As2+) and arsenate (As4+) ions from an aqueous solution at a concentration of 150 μg per litre, in 60 minutes(9). Fenton’s reagent is a mixture of hydrogen peroxide and iron, and is an inexpensive material to use at scale.
A viable comparison would be to activated molecular sieves, as used frequently in the chemistry laboratory, as LECAs can be regenerated, are effective at low ‘loadings’ and are themselves insoluble. These properties mean that LECA filtration is simple to work with.
Anthracite is one of the most ubiquitous forms of coal. Occurring naturally and easily mined, it is inexpensive. Having many uses aside from as fuel, its use as a filtration medium is widely known(10). One of the key environmental benefits of employing anthracite as a filter and not as a fuel is that as it is not combusted, there is no emission of carbon dioxide to the atmosphere. It should be noted, however, that anthracite is seldom used alone as a pre-RO filter. Rather, it is used as part of a dual media filter set-up alongside sand(11) or alongside sand and garnet as a mixed-media filter, providing superior coarse-to-fine filtration. Typically, anthracite is present to remove suspended solids and some dissolved organics(12).
Anthracite-led dual media filters have been used in tropical settings in RO plants, proving highly effective at treating raw, polluted seawater pre-RO, eventually producing in excess of 50 m3 per day (50,000 L) of drinking water that is fully compliant with WHO standards(13). This particular example provided over 35% water recovery over the course of one year. Alongside granular activated carbon (a material with which anthracite shares many similarities) anthracite was used as a highly effective component in a dual filter to remove biological material from a pre-RO experimental set up(14). This small scale set-up showed no decline in performance even after 55 days of constant activity, in excess of 5 metres per hour through a 20 cm filter. In both examples, minimal processing of the anthracite was required - limited to only grinding to suitable particle sizes. As part of a multi-media filtration set up, anthracite alongside greensand and activated charcoal was able to remove significant quantities of iron from aqueous solution prior to a RO process(15).
Across filtration applications for RO anthracite grain sizes may vary between 1mm to 2.4mm , with a minimum bed depth of 0.8 m. The most modern plants using anthracite/sand dual media filters can handle in excess of 40 m3 per hour(16). Most of anthracite’s filtration utility stems from its excellent size-exclusion functionality and porosity. After use, spent anthracite is typically discarded.
One study in northern Greece took anthracite and LECA filters for RO and ran them in parallel, aiming to find a discernible difference between the two approaches(17). The research - both on plant and laboratory scales - found that LECA and anthracite performed largely the same across testing for organics concentration in the filtrate, silt density and turbidity. Notable, however, is that both methods suffered performance wise in the colder winters, but not to an unacceptable level. The LECA material and the anthracite were in the size range of 1.2 to 2.4 mm.
Both types of filtration - when in a dual media filter set-up - are highly effective against the exposure of algal blooms to the RO membrane(18). Bacteria are much more difficult to remove using dual filter processes than solid contaminants and organic materials, however, dual media filters containing anthracite have been shown to remove some bacteria in one study, but the majority were removed by subsequent filtration processes(19). In a further study, LECA filters were shown to entrap - but also in some cases to develop - bacterial life(20).
Naturally, filtration is not the only weapon in the RO plant designer’s arsenal. Oftentimes, seawater prior to filtration will be chemically treated to precipitate out expected contaminants - these are then collected in the sand filter or are allowed to separate out with gravity. Sodium hexametaphosphate can be added to induce precipitation of calcium carbonate and calcium sulfate. Acidification (as seawater is slightly basic) can be performed and aluminium sulfate added to make filtration easier(21). Post-filtration treatments are not uncommon, depending on the scale of contamination.
Whilst both materials (clay and anthracite) are abundant and easily accessible in both mining and economical terms, anthracite holds the advantage of being able to be used ‘out of the box’ with only minimal processing required. Processing is typically limited to grinding the raw anthracite to suitable particle sizes. Clay on the other hand must first be heated in a rotary kiln, and then depending on application, doped/impregnated with another chemical substance - adding expense, time and complexity to the process. Anthracite is more cost effective as it requires less processing which also adds the benefit of being more environmentally friendly.
- Reverse osmosis is one of the processes by which drinking water can be produced from sea water
- RO relies on a partly permeable membrane to function, and if this becomes blocked or fouled, the process efficiency drops significantly
- Filtration, often using a dual media set-up, is thus used to ensure a better quality supply to the membrane to prevent fouling
- Anthracite and lightweight expanded clay aggregate are two leading materials used as filters
- With seas getting ever more polluted, and water scarcity is becoming more of an issue in some regions, effective RO processes must be established - such processes cannot work without efficient filtration from the likes of anthracite
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