Activated Carbon: Desalination Applications
The first in a five part series on the applications of activated carbon, desalination is discussed here. With certain parts of the world consistently suffering from a poor potable water supply, the concept of desalination is appealing. Contemporary methods for removing the salt from seawater are expensive and energy intensive. The use of activated carbon may help alleviate some of these concerns.
Current State Of The Art
Not only is it essential for the preservation of life, ensuring a good water supply is critical for the ongoing success and stability of a country. With global warming causing increasing temperatures leading to drought and therefore dehydration of humans and animals and crop failure, maintaining a steady supply of clean water has never been more important. Contemporary desalination plants use sea water (which is abundant) and through a process of filtration by size filters, membranes and others produce clean, potable supplies. Sea water can contain as much as 35 g L-1 of dissolved solids(1).
The leading type of desalination plant is the reverse osmosis plant. This forces clean water across a partially permeable membrane from a place of high salt concentration to a low one. This uses electrical energy. It also removes other ions and impurities from the salt water. Efficiency is lost, however, when these membranes become clogged or blocked by pollutants, organisms and microplastics for example. Modern reverse osmosis plants only operate at around 30% efficiency, and so the need for a pre membrane filtration regime is crucial.
Contemporary reverse osmosis installations employ stepwise filtration ahead of the permeable membrane to ensure an efficient process as possible. With the idea being to reduce solid build up at the membrane site, energy requirements can be reduced. Popular choices for filtration media include anthracite, sand and granular activated carbon. Such filtration pathways can allow for a 35% water recovery enhancement over the course of the year relative to no filtration. Such multi media filters can process up to 40 cubic metres of seawater per hour - oftentimes producing around 10 cubic metres of potable water(2).
Adding GAC As A Pre-Reverse Osmosis Filter
Just like with anthracite, deployment of GAC as a pre-osmosis filter is commonplace. Oftentimes these are used alongside each other in order to provide the most comprehensive filtration possible - advantageous to further protect the filter, practical because both GAC and anthracite are highly resilient materials. Such an example of using them together is where the overall system efficiency was improved by the GAC effectively removing dissolved organic materials (by up to 70%) and cTEP (by up to 90%) with overall efficiency enhancements coming by way of limited permeability decline(3). When used as part of an ultrafiltration set up, efficiency gains are greater still.
For the removal of bacterial residue, GAC and anthracite have been used in a competing study. It was found that both are highly effective as pretreatment for seawater desalination in cases where that seawater was bacteria rich. In a fixed bed column set up with a 1 m bed depth, both were as effective, but anthracite tended to permit more bacterial agglomeration at the top of the filter bed(4). In fact, the use of GAC as part of the pre membrane filtration system has long been considered effective against biological materials(5).
One of the major advantages to GAC as a pre-membrane filter is its ability to reduce the required dose of flocculants. GAC is able to remove in excess of 70% of low molecular weight compounds - both neutral and acidic - which reduces biofouling of the membrane. In conventional systems, a dose of ferric chloride and poly-ferric sulfate (3 and 2 mg L-1 respectively) are normally required to ensure removal of organic residues via flocculation. Using GAC as a filter significantly reduces the amount of ferric chloride and sulfate down to around 1 mg L-1 consistently - reducing cost and complexity(6).
Microbial activity in the GAC biofilter is not a concern. GAC reduces the amount of biofoulants through adsorption and biodegradation, with biological material being desirable to have on the filter as these can cause degradation of other pollutant molecules(7). GAC biofilters consistently result in the reduction in concentration of transparent exopolymer particles and assimilable organic carbon.
Even oil contaminated seawater is treatable with GAC. Naturally, it is more difficult and large deployments should consider using other materials to remove oil in addition, but research has shown that GAC filtration can remove up to 98% of dissolved organic carbon from weathered oil contaminated seawater(8). This represents an impressive enhancement over what is possible with iron chloride flocculation alone, which was only able to remove a quarter of dissolved organic carbon.
Overall, it can be said without doubt that GAC is an effective pre reverse osmosis membrane filter treatment pathway that ensures adequate to excellent removal of dissolved organic compounds, bacteria and others, in order to protect the membrane.
Another deionisation method is membrane capacitive deionisation, which uses an electrolysis-like set up to deionise liquids on small scales. Whilst much focus is centred on the identities of the electrodes themselves, the identity of the material that separates them is not. Common materials are stainless steel fibres and filter paper. GAC outperforms these by a large margin in desalination. GAC is able to desalinate at a rate of 513.4 mg L-1 h-1, whilst values of 374.1 and 297.9 mg L-1 h-1 were observed for the traditional materials respectively(9).
Required Considerations
GAC is not a silver bullet, and is often used alongside other treatment methods for good reason. Studies have shown that while overwhelmingly effective as a pre-membrane treatment, GAC may not be as effective against contaminants such as low molecular weight humics-type molecules that are linked to polysaccharides and proteins(10). Thankfully, GAC is effective on most other lines, including and especially biodegradables and other organics which constitute most of the contaminants responsible for fouling anyway.
Real World Examples
Whilst extensive laboratory testing is useful, nothing can beat the experience gathered in the real world. GAC is used in several places around the world as part of a comprehensive desalination programme. On Curacao in the Netherlands Antilles, GAC is used to immobilise boron species after they have passed through the reverse osmosis membrane. As a Caribbean island, Curacao relies on reverse osmosis for a large proportion of its potable water requirements. The Curacao plant’s GAC is responsible for the removal of boron levels down to below the legal limit of 0.3 mg L-1, and this process provides 50% of the island’s water(11).
Brackish river water in the basins of the rivers Llobregat and Ter - near Barcelona - contains elevated levels of natural organic matter and trihalomethanes. GAC filtration is used in advance of an electrodialysis reversal desalination process. Unlike reverse osmosis, the main aim is not to protect a membrane per se, but the removal of haloalkanes and organic residues means that the electrolyser is able to perform in much more suitable conditions(12). The plant is able to produce 2.3 m3 s-1 of potable water for the 4.5 million residents that rely on it.
More applicable to the developing world is solar desalination. This is also not membrane based and can be described as a cruder - but more robust - method. Because a solar desalination system relies on boiling and condensing water, it is imperative that the only things that water contains are water and salt. Granular activated carbon is used as a broad range filter to remove a multitude of contaminants. This porous adsorbent filter leads to better solar performance, a lower cost of operation and fewer overall emissions too(13).
Summary
- Granular activated carbon is a widely used and useful filter for a variety of applications
- Based on its porosity and unique surface chemistry, it finds use in desalination applications as part of the pretreatment pathway, often alongside materials like anthracite and sand
- Desalination is one of the most studied and broadly used methods of ensuring clean water supplies
- As a pretreatment, GAC acts to protect the reverse osmosis filter membrane - ensuring it can operate unabated and undisturbed by molecules that aren’t salt and water
- Using a pretreatment process means less energy is required for reverse osmosis and the membrane lasts longer - with up to 35% more potable water being produced as a result
- The use of GAC as a pre-membrane filter is been successfully used today in the Netherlands Antilles for reverse osmosis and in Spain for reverse electrodialysis desalination methods
- It is also useful in membrane capacitive deionisation, outperforming traditional separation technologies
References
1 S.-H. Kim et al., Desalination and Water Treatment, 2011, 32, 339
2 Department of the Army, Water Desalination Technical Manual, Washington, D.C., 1986
3 S. Laborie et al., Desalination, 2016, 383, 1
4 G. Naidu et al., Ecolog. Eng., 2013, 60, 370
5 S. Vigneswaran et al., Desalination, 2009, 247, 77
6 S. Vigneswaran et al., Desalination, 2014, 354, 9
7 S. Vigneswaran et al., Desalination, 2013, 309, 254
8 W.-H. Kim and M. Okada, Kor. J. Water Env., 2004, 20, 447
9 P. Liang et al., Desalination, 2016, 381, 95
10 C. G. Dosoretz et al., Water Res., 2008, 42, 1595
11 V. Bonnélye et al., Desalination, 2007, 205, 200
12 F. Valero and R. Arbós, Desalination, 2010, 253, 170
13 G. B. Abdelaziz et al., Process Safety Environ. Protection, 2021, 147, 1052
You must be logged in to post a comment.