Saturday, 3 June 2017

Activated carbon

Activated carbon is a form of carbon which could be produced from any carbonaceous material such as coal, rice husk, paddy husk, coconut shell, wood etc.Due to their low ash content, high carbon content and natural pore structure, coconut shells are ideal for producing high quality activated carbon. Activated carbon manufactured from coconut shell is considered superior to those obtained from other sources mainly because of small macro-pores structure which renders it more effective for the adsorption of gas/vapor and for the removal of color and odour of compounds and as pre-filters in water purification because of the large internal surface area which makes it as a very good adsorbent for many contaminants in drinking water. The activated carbon is also used

    In food and non-food industries.
    In processing of cooking oil, sugar, and chemical matter purification
    Used in drinking water and waste-water treatment.
    Refining and bleaching of vegetable oils.
    Recovery of solvents and other vapours & gold.
    In gas mask for protection against toxic gases etc



Steam activation is the most widely used process because it is generally used to activate both coconut shell and coal based carbons. Steam activated carbons are produced in a two-stage process. Firstly the raw material, in the form of lumps, pre-sized material is carbonized by heating it in an inert atmosphere such as flue gas, so that de-hydration and devolatilization of the carbon occur. Carbonization reduces the volatile content of the source material to fewer than 20%. A coke is produced which has pores that are either small or too restricted to be used as an adsorbent. The second stage is the activation stage which enlarges the pore structure, increases the internal surface area and makes it more accessible. The carbonized product is activated with steam at a temperature between 900°C and 1100°C. The chemical reaction between the carbon and steam takes place at the internal surface of the carbon, removing carbon from the pore walls and thereby enlarging the pores. The steam activation process allows the pore size to be readily altered and carbons can be produced to suit specific end-sues. Steam Activation produce activated carbon in the form of 1mm to 3mm pieces, which are crushed and screened to remove fines and dust to meet the specifications for granular activated carbons. To produce powdered activated carbons, the carbon pieces are further grinded using a gentle pulverizing action.


The activation process generates a network of minute openings of different diameters on the carbon surface which become the path for water to access the extended internal surface created by activation.Carbon has a natural affinity for organic pollutants which bind to its surface. Once activated, the carbon forms little pores and pockets that increase its surface area. An imbalance of forces then exists on the activated carbon atoms at the pore wall surface. To neutralize this imbalance, molecules are physically adsorbed, i.e., drawn and held physically to the pore wall (Van der Waals forces). In other words, compounds like pesticides, chloroform, and contaminants slide into the holes of this honeycomb-like substance and hold fast through a process called adsorption.The porosity of activated carbons offers a vast surface on which this adsorption can take place. Adsorption occurs in pores slightly larger than the molecules that are being adsorbed, which is why it is very important to match the molecule you are trying to adsorb with the pore size of the activated carbon.

Backwashing of Industrial Media Filters


Water Filtration is to collect or gather impurities from the incoming water flow. When the pores of filters logged, it has to be cleaned by forcing out the accumulated particles by reversing the flow and increasing the velocity of water passing back through the filter. It is called backwashing. We can improve the quality of water by doing it at regular intervals.


First the filter is taken off line and the water is drained to a level that is above the surface of the filter bed. Next, compressed air is pushed up through the filter material causing the filter bed to expand, breaking up the compacted filter bed and forcing the accumulated particles into suspension for 20-30 minutes.

After the air scour cycle, clean backwash water is forced upwards through the filter bed, causing the filter bed to expand and carrying the particles in suspension into backwash troughs suspended above the filter surface. In some applications, air and water streams are simultaneously pushed upwards through the granular media. Backwashing continues for 30 minutes to one hour time, or until the turbidity of the backwash water is below an established value. A filter bed should have as much expansion as possible upto 30 to 50 percent of bed volume without loosing media or displacing the support gravel. At the end of the backwash cycle, the upward flow of water is terminated and the filter bed settles by gravity in its initial configuration.

After allowing sufficient time for the filter bed to settle by gravity, rinsing can be done with clear water for 30 minutes until the crystal clear water comes out as the output. Then water to be filtered is passed through the filter and the service cycle can be from 16hrs to 20hrs.


In order to measure the degree of the fouling problem, a measure called Silt Density Index (SDI) is used. Here filtration rates are calculated by exposing a 0.45-micron filter to the feed water under pressure. An SDI of less than 5 is typically considered acceptable for a reverse osmosis systems.(RO). If the value of SDI is higher than 6, stop passing the feed through RO membranes and the pretreatment cycle should be carried out to bring down the SDI levels to less than 4.

In any pretreatment system the SDI reduction in multi grade filters (MGF) should be brought down significantly. If the size of the impurities in the raw water is less than 25 microns, suitable coagulants/flocculants should be added in the raw water to increase the size of the particles (Impurities) and improve the filtering efficiency of MGF.

In carbon filters (CF) SDI reduction across the CF should be minimum. If a significant reduction in SDI is seen across the CF, it will foul very frequently. The remedy is to improve the particle removal efficiency of the MGF.


A turbidity spike can occur if the filter bed is not allowed to settle by gravity for a reasonable time. For small plants back washing can be carried out at the end of the day and allow the filter to settle by gravity at night time.


A carefully designed pretreatment plant aimed at reducing the SDI to as low as possible will go a longway in trouble free operation of the RO plants.



 In water treatment applications all the three varieties of carbon (Coconut , Coal & Wood) are used depending upon the nature of treating potable water and waste water. Hence there is a need for a comparative study of various types of carbon based on their specifications.

Choosing the best solution:

Porosity plays a vital role in choosing the right type of carbon. While Coconut activated carbon contains many micropores , Coal activated carbon contains mainly mesopores as well as as micropores whereas wood activated carbon contains mesopores and macropores only. If the molecular size of the impurities are less than 100 angstroms then coconut carbon can be preferred. Likewise if the molecular size of the impurities are between 100 and 1000 angstroms we can use coal carbon. And if the molecular size of the impurities are greater than 1000 angstroms Wood carbon can be considered.

In general:

    Wood activated carbon is most suitable for decolorization in powder form.

    Coal activated carbon is suitable for odor removal.

    Coconut activated carbon is suitable for dechlorination.

Cost wise coconut activated carbon is little bit expensive compared to coal & wood activated carbon. ”If we are to fill up a vessel of a given volume, the weight of coconut activated carbon required is more due to its high density”. But at the same time there is a negligible loss of material during back washing for coconut activated carbon due to its high hardness. Thus operation cost is low. Wettability is very high, and ash content is very low in Coconut activated carbon. Since it is a renewable source. It is preferable for drinking water treatment and RO water applications. Having said that Coconut activated carbon is best suited for drinking water products.

Coal activated carbon is used in drinking water projects. Apart from that, there are few other industrial applications like Effluent treatment and Waste water treatment. This type of carbon can be best suited for odor removal and cost effective applications.

Wood carbon is mainly used in POWDER applications where “decolorization” plays a vital role.


Choosing the right form of activated carbon mainly depends upon your applications and cost of operation. Professional guidance is the best way to ensure the right product for a particular application.