When it comes to filtering – be it water, various chemicals or even the air around us – many commercial and domestic processes rely on carbon. Innocuous looking, carbon occupies position six on the periodic table between boron and nitrogen, and has a valency of four, meaning that it can accept four different atomic connections as covalent bonds. Carbon is also a useful material for the manufacture of water filters due to its ability to be formed into blocks capable of sifting water and removing a surprising number of unwanted additives.
Carbon has a property whereby it allows the medium being filtered to pass round it and crucially, filling pore areas with the extracted material and allowing the cleaned transport material – be it water, air, or some other low viscosity throughput – to exit in a much-refined state. This retention of filtration material is a process known as adsorption, and activated carbon has a high capacity for the collection of unwanted material. Furthermore, beyond the adsorption of organic material, carbon block can remove disinfectant material via the catalytic reduction process. The filtration material is commonly referred to as granular activated carbon (GAC).
GAC is highly effective at removing organic matter and disinfectant or apparent health additives such as chlorine and fluorine from water, and can dramatically improve taste as well as reducing health hazards. Activated carbon is a favoured technique for filter material because of its multifunctional nature that adds nothing detrimental to the water. In addition, much of the base material for the carbon can be derived from many natural sources, such as coal, wood and even discarded nutshells, and is surprisingly easy to prepare and form into filtration blocks. Coconut shells for instance, are hard and pure, and make a fantastic material for drinking water filters.
The actual performance – the ability to remove contaminants – is dependent upon a few factors, and in making purposeful products, carbon block manufacturers aim to create a balance of them. These factors include;
Molecular weight. Increasing molecular weight leads to a corresponding increase in adsorption as there is physically more carbon available, and less soluble in water. To be highly effective, the pore size and structure needs to be maximised and carbon blocks may be a mixture of high and low molecular weight in order to work effectively across all adsorption and catalytic ranges.
Particle size. Allied to molecular weight, carbon is usually available in a variety of mesh sizes, which dictates the average size of the resulting particles. Generally, a finer mesh gives a greater adsorption as there is an increases surface area for it to work on, but this may restrict the water flow because it represents a more torturous path.
pH. Based on the relative potential of hydrogen (hence pH), the acidity/alkalinity of the carbon will impact its effectiveness at removing material form water. Generally, additives such as chlorine are unaffected by pH, but organic matter is more effectively removed at slightly lower pH’s – making it more acidic in nature.
Flow rate. This is directly proportional to particle size, with smaller, denser particulate decreasing flow rate but increasing cleanliness of the final throughput. Flow rate needs to be appreciated when selecting a mesh size for your carbon block filter.
Temperature. Like pH, the temperature of operation can affect throughput as high temperatures will increase the activity of the carbon, and vice versa. Increased activity equals a more efficient removal process, but conversely, lower temperatures have been shown to aid adsorption in some cases. The temperature you set to will be dependent on what you are trying to remove from the water, and how fast you are trying to do it.
Carbon block filters are regarded as one of the simplest and most effective means of removing particulate and additives in water and, being relatively low cost, should be one of the first means of cleaning your flow. Furthermore, since they are radial flow, carbon block filters do not have any channel problems.